che451_chemical process design and economics

5/20/2018 CHE451_Chemical Process Design and Economics

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KWAME NKRUMAH UNIVERSITY OF SCIENCE AND

TECHNOLOGY, KUMASI

INSTITUTE OF DISTANCE LEARNING

CHE 451

Chemical Process Design & Economics

(Credits: 4)

Benjamin Afotey, PhD

Chemical Engineering Department

August, 2014


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Course IntroductionDesign is the synthesis of ideas to achieve a desired goal (product). The designer starts with

an idea and proceeds to develop several alternative designs that he evaluates and finallysettles on the one that satisfies his objective (goal).

The search for alternative designs is important to the economic viability of the design project.CHE 451 is a fourth year core course offered in first semester in BSc. Chemical Engineering.

Course OverviewMethodology of the Design Process: Constraints on a Design Problem; Fixed/Rigidconstraints, Less rigid constraints, The design process; Design objectives, Data collection,

Generation of possible designs,Selection, Chemical manufacturing processes, Continuous

and batch processes, Organization of a chemical engineering design project.

Codes and Standards, Design Factors, Variable & Mathematical Representation of DesignProblems: Codes and standards, Design factors, Systems of units, Mathematical

representation of the design problem, Selection of design variables. Optimization and Batch

Production Process: Introduction, Simple models, Multiple variable systems, Methods ofanalysis, Other optimization methods, Batch production process.

Process Synthesis: Introduction, Raw materials and chemical reactions, Summary of process

design heuristics, Heuristics in equipment design. Process Simulation: Introduction, Processsimulator, Types of process simulation, Units operation solvers, Uncertainty and sensitivity

issues. Flow sheeting, Piping and Instrumentation: Introduction, Piping & instrumentation

diagrams, Valve selection, Pumps, Classification of pumps, Factors to consider in pump

selection, Centrifugal pumps, Effective characteristics curves, Design parameters of

centrifugal pumps, Operating point, Choice of rotational speed,. Process Economics: Costestimation, Cash flow for industrial operation, Factors affecting investment and production

costs, Capital investment, Estimation of capital investment, Types of capital cost estimates,cost indices, Methods of estimating capital investment, Turnover ratio, Estimation of total

product cost, Break-even point. Process Economics: Depreciation and profitability analysis,

Service life, Salvage value, Present value, Methods for determining depreciation,Profitability standards, Basis for evaluating project profitability, Mathematical methods for

profitability evaluation, Rate of return on investment, Discounted cash flow, Capitalized cost,

Pay-back time, Sensitivity analysis.

Course ObjectiveThe following is the main course Objective

1. To enable students understand and acknowledge the importance of economic analysis

in chemical process design, by way of efficiently maximizing profit.


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Course OutlineThe course outline is divided into eight units. Each of the eight units is broken into subtopics.

Each unit addresses one or more of the course objectives.

Unit 1: Methodology of the Design Process

Unit 2: Codes and Standards, Design Factors, Variable &Mathematical Representation ofDesign Problems

Unit 3: Optimization and Batch Production Process

Unit 4: Process Synthesis

Unit 5: Process SimulationUnit 6: Flow Sheeting, Piping and Instrumentation

Unit 7: Process Economics: Cost Estimation

Unit 8: Process Economics: Depreciation and Profitability Analysis

GradingContinuous Assessment: 30%

End of Semester Examination: 70%

Reading List/Recommendation Textbooks/Websites/CDs1. Plant Design and Economics for Chemical Engineers: By Peters and Timmerhaus

2. Coulson & Richardsons Chemical Engineering Design: By Sinnot, Volume 6

Course Writer

Dr. Benjamin Afotey received his BSc. Degree in Chemical Engineering in 2000 at KwameNkrumah University of Science and Technology, Kumasi. He received his MSc. and PhD

Degrees at the University of Texas, Arlington, and U.S.A in 2003 and 2008 respectively.

He worked with the Texas Commission on Environmental Quality, U.S.A between 2008 and

2009 and joined the Chemical Engineering Department in 2010.

AcknowledgementI wish to thank the Almighty God for His guidance throughout the write up. I would also like

to thank my colleagues who provided encouragement of any kind. Finally, I acknowledge the

effort of my Teaching Assistant who contributed in a way to the successful completion of thematerial.


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TABLE OF CONTENT1.0 METHODOLOGY OF THE DESIGN PROCESS 1

SESSION 1-1 : 1

1-1.1 Introduction 1

1-1.2.Constraint on a Design Problem 11-1.2.1 Fixed/Rigid constraint 1

1-1.2.2 Less rigid constraint 11-1.3 The Design Process 2

1-1.3.1 The design objective 3

1-1.3.2 Data collection 41-1.3.3 Generation of possible designs 4

1-1.3.4 Selection 5

1-1.4 Chemical Manufacturing Processes 5

1-1.5 Continuous and Batch Processes 71-1.5.1 Choice of continuous verses Batch processes 7

1-1.5.2 Organization of a chemical engineering design project 72.0 CODES AND STANDARDS, DESIGN FACTORS, VARIABLE &MATHEMATICAL REPRESENTATION OF DESIGN PROBLEMS 11

SESSION 2-1 11

2-1.1 Codes and Standards 112-1.2 Factors of Safety 12

2-1.3 System of Units 13

2-1.4 Mathematical Representation of the Design Problem 13

2-1.5 Selection of Design Variables 183.0 OPTIMIZATION AND BATCH PRODUCTION PROCESS 20

SEESION 3-1 20

3-1.1 Introduction 203-1.2 Simple Methods 21

3-1.3 Multiple Variable Systems 22

3-1.4 Methods of Analysis 23

3-1.5 Other Optimization Methods 24

3-1.6 Batch Production Process 24

4-0 PROCESS SYNTHESIS 26

SESSION 4-1 26

4-1.1Introduction 264-1.2 Raw Materials and Chemical Reactions 264-1.3 Summary of Process Design Heuristics 34

4-1.4 Heuristics in Equipment Design 345.0 PROCESS SIMULATION 38SESSION 5-1 38

5-1.1 Introduction 38

5-1.2 Process Simulator 395-1.3 Types of Process Simulation 39

5-1.4 Unit Operation Solvers 40

5-1.5 Uncertainty and Sensitivity Issues 40


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6.0 FLOW SHEETING, PIPING AND INSTRUMENTATION 42

SESSION 6-1 426-1.1 Introduction 42

6-1.2 Piping and Instrumentation Diagrams 43

6-1.3 Valve Selection 44

6-1.3.1 Gate valves 456-1.3.2 Globe valves 46

SESSION 6-2 466-2.1 Introduction of Pumps 46

6-2.2 Classification of Pumps 46

6-2.3 Factors to Consider in Pump Selection 476-2.4 Centrifugal Pumps 48

6-2.4.1 Effective characteristics curves 48

6-2.4.2 Design parameters of centrifugal pumps 49

6-2.4.3 Operating point 496-2.4.4 Q/H Curve versus Technical choices 51

6-2.4.5 Choice of rotation speed 526-2.4.6 Suction Conditions: Concept of NPSH 537.0 PROCESS ECONOMICS: COST ESIMATION 57

SESSION 7-1 57

7-1.1 Cost Estimation 577-1.2 Cash Flow for Industrial Operations 58

7-1.3 Factors affecting Investment and Production Costs 59

7-1.4 Capital Investment 59

7-1.5 Estimation of Capital Investment 617-1.5.1 Introduction 61

7-1.5.2 Types of capital cost estimates 61

7-1.5.3 Cost indexes 627-1.5.4 Methods for estimating capital investment 63

7-1.5.4.1 Power factor applied to plant-capacity ratio 63

7-1.5.4.2 Detailed item estimate 667-1.5.4.3 Other Methods for Estimating Equipment or Capital Investment 67

7-1.5.5 Turn-over ratio 68

SESSION 7-2 68

7-2.1 Estimation of Total Product Cost 687-2.2 Break-even Point 70

8.0 PROCESS ECONOMICS: DEPRECIATION AND PROFITABILITY ANALYSIS 75

SESSION 8-1 75

8-1.1 Depreciation 758-1.2 Service Life 76

8-1.3 Salvage Value 768-1.4 Present Value 76

8-1.5 Methods for Determining Depreciation 77

8-1.5.1 Straight-line method 778-1.5.2 Decliningbalance ( or fixed percentage) method 77

8-1.5.3 Other methods for determining depreciation 78


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SESSION 8-2 82

8-2.1 Profitability Analysis 828-2.2 Profitability Standards 82

8-2.3 Basis for Evaluating Project Profitability 83

8-2.4 Mathematical Methods for Profitability Evaluation 84

8-2.4.1 Rate of return on investment 848-2.4.2 Discounted cash flow 86

8-2.4.3 Capitalized cost 888-2.4.4 Payout period (or Pay-back time) 90

8-2.4.5 Sensitivity Analysis 91


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LIST OF FIGURESFigure 1-1.1 Design Constraints 2

Figure 1-1.2 The Design Process 3

Figure 1-1.3 Anatomy of a Chemical Process 5

Figure 1-1.4 The Structure of a Chemical Engineering Project 8Figure 1-1.5 Project Organisation 9

Figure 3-1.1 Effect of Constraints on the Optimum of a Function 23Figure 3-1.2 Yield as a Function of Reactor Temperature and Pressure 24

Figure 6-1.1 Flow-sheet of Simplified Nitric Acid Production Process 42

Figure 6-1.2 Polymer Production Diagram 43Figure 6-1.3 Commonly used Valves 45

Figure 6-2.1 Approximate Range of Operation for the Three Main Types of Pump 48

Figure 6-2.2 Basic Curves Characterizing a Centrifugal Pump 49

Figure 6-2.3 Curve Characteristic of the System 50Figure 6-2.4 Variation of Operating Point by means of a Valve 50

Figure 6-2.5 Variation in Specific Speed versus the Type of Impeller used 51Figure 6-2.6 Different Types of Characteristic Curves 52Figure 6-2.7 Variation in the Operating Point versus the Rotation Speed 53

Figure 7-1.1 Cash Flow for an Overall Industrial Operation 58

Figure 7-2.1 Cost Involved in Total Product Cost for a Typical Chemical Process Plant 70Figure 7-2.2 Break-even Chart for Chemical Processing Plant 71

Figure 7-2.3: Project Cash Flow Diagram 72


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LIST OF TABLES

Table 6-2.1 Main Types of Pumps 47Table 7-1.1: Cost indexes as Annual Averages 63

Table 7-1.2 Typical Exponents for Equipment Cost vs. Capacity 64


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UNIT 1METHODOLOGY OF THE DESIGN PROCESS

IntroductionThis unit discusses the methodology of the design process and its application to thedesign of chemical manufacturing processes.

Learning ObjectivesAfter reading this unit, you should be able to:

1. Explain chemical process design2. Distinguish between fixed and less rigid constraints3. Know what constitutes the design process4. List the basic components of a chemical manufacturing process

SESSION 1-1 In this session we shall discuss what a chemical process design is,

the constraints of the design problem, the components of the chemical manufacturing

process and the difference between a continuous and a batch process.

1-1.1 Introduction: Design is the synthesis of ideas to achieve a desired goal (product).

The designer starts with an idea and proceeds to develop several alternative designs that

he evaluates and finally settles on the one that satisfies his objective (goal).

The search for alternatives: this step becomes necessary because the designer will be

constrained by several factors.

1-1.2 Constraints on a Design Problem

1-1.2.1 Fixed/Rigid constraints: these are constraints the designer must live with outside

his influence. E.g. physical laws, government regulations and standards. The fixed

constraints define the outer boundary of all possible designs.

1-1.2.2 Less rigid constraints: these are constraints the designer can manipulate inorder

to arrive at the best design. E.g. materrials of construction, time. These are interrnal

constraints over which the designer has some control.

In summarry we have the following diagramatic sketch.


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Figure 1-1.1: Design Constraints

1-1.3 The Design Process

The design process can be shown in schematic form in Figure 1-1.2.


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Figure 1-1.2: The Design Process

The diagram shows the design process as an iterative procedure because as the design

proceeds the designer will be looking for information and ideas to refine the design.

1-1.3.1 The Design objective

In the particular case of a chemical process plant, the objective/goal is to satisfy the

public need for a product. In large commercial organizations, this need is identified by

the sales/ marketing department. Before starting to work, the designer should obtain

complete information/background on the need for the product and its application

areas/uses.


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1-1.3.2 Data collection

To proceed with the design, the designer must assemble all the relevant facts and data

required. For process design, the information should include process alternatives,

equipment performances, physical property data. In large design companies, they have

in house manuals containing all the process know how on which the design is based

and preferred methods and data for the frequently used design procedures.

1-1.3.3 Generation of possible designs

At this stage the designer must come up with all possible solutions for analysis,

evaluation and selection. To do this, he must rely on his own experience or that of others,

using tried or tested methods.

Chemical engineering projects can be divided into 3 types:

1. Modification, additions to existing plant often undertaken by the plant design

group.

2. New production capacity to meet growing sales demand, and the sale of

established processes by contractors. Repetition of existing designs, with only

minor design changes.

3. New processes, developed from laboratory research, through pilot plant, to a

commercial process. Here most of the unit operations and process equipment willuse established designs.


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1-1.3.4 Selection

The selection process can follow the following screening stages:

Possible designs (credible)within the external constraints

Plausible designs (feasible)- within the internal constraints

Probable designslikely candidates

Best designs (optimum)judged the best solution to the problem

To select the best design from the probable designs, detailed design work and costing will

be necessary.

1-1.4 Chemical Manufacturing ProcessesThe basic components of a typical chemical process can be shown using the block

diagram below.

Figure 1-1.3: Anatomy of a Chemical Process

Each block represents a stage in the overall process for producing a product from the raw

materials. Each stage is a collection of equipment required to accomplish a defined task.


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Stage 1: Raw Material Storage

Unless the raw materials are supplied as intermediate products from a neighboring plant,

storage space is needed to hold several days or months supply. Types of storage required

will depend on the nature of the raw materials, and the methods of delivery.

Stage 2: Feed Preparation

Some purification of the raw materials will be necessary to render them in a form

required for feed to the reaction stage.

Stage 3: Reactor

In the reactor the raw materials are brought together under conditions that promote the

production of the desired product. However by-products and unwanted compounds/

impurities will also be formed.

Stage 4: Product Separation

After the reactor, the products and by-products are separated from any unreacted

material. If in sufficient quantity, the unreacted material will be recycled to either the

reactor directly or to the feed purification and preparation stage.

Stage 5: Purification

Before sale, the main product is purified to meet product specification. If the by-product

is also produced in sufficiently large quantities, it must also be purified for sale.

Stage 6: Product Storage

Provision for product packaging and transport will be required. Besides some inventory

of finished product must be held to match production with sales

Ancillary Process

In addition to the main process stages/ units, provision will have to be made for the

supply of utility services, process water, cooling water, compressed air, steam. Facilities

are required for maintenance, firefighting, offices, laboratories and accommodation.


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1-1.5 Continuous and Batch Processes

Continuous processes are designed to operate 24 hours a day, 7 days a week, throughout

the year (365 days). However down time is allowed for maintenance and process catalyst

regeneration.

Plant attainment: this is the percentage of the available hours in a year that the plant

operates. This is usually 90-95%.

1-1.5.1 Choice of continuous versus batch production

The choice will not be clear - cut, however one can use as a guide the following rules:

Continuous

1. Production rate greater than 5*106 kg/h (5000tonnes/h)

2. Single product

3. No severe fouling

4. Good catalyst life

5. Proven process design

6. Established market

7. Cost can be reduced

8. Less laborBatch

1. Production rate less than 5*106 kg/h (5000tonnes/h)

2. A range of products or product specifications

3. Severe fouling

4. Short catalyst life

5. New product

6. Uncertain design

1-1.5.2 Organization of a chemical engineering design project

The design work required in the engineering of a chemical manufacturing process plant

can be divided into two broad phases:


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Phase 1.Process Design

This covers the steps from the initial selection of the process to be used, through to the

issuing of the process flow-sheets; and includes the selection, specification, and chemical

engineering design of equipment. In any organization, this phase is handled by the

process design group composed of chemical engineers. The group is also responsible for

the preparation of piping and instrumentation diagrams.

Organization of a project group is shown in Figure 1-1.4 below.

Figure 1-1.4: The Structure of a Chemical Engineering Project


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Phase 2. The detailed mechanical design of equipment; the structural, civil and electrical

design; specification and design of ancillary services. Other specialist groups will be

responsible for cost estimation, and the purchase and procurement of equipment and

materials.

The sequence of steps in the design, construction and start up of a chemical process plant

is shown diagrammatically in Figure 1-1.5.

.

Figure 1-1.5: Project Organization

Project manager; a chemical engineer by training is responsible for the coordination of

the project.


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SELF ASSESSMENT 1-1

(a) Distinguish between Fixed and less rigid constraints

(b) List the components of a basic chemical manufacturing process and in just two

sentences explain the importance of each in the manufacturing process.

(c) In a tabular form, list six differences between a continuous process and a batch

process.


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UNIT 2CODES AND STANDARDS, DESIGN FACTORS, VARIABLE &

MATHEMATICAL REPRESENTATION OF DESIGN PROBLEMS

IntroductionThis unit discusses the codes & standards, design factors and variables and mathematicalrepresentation of the design problems.


1. Distinguish between codes and standards2. Understand the importance of design factors as a margin of safety

in meeting design specifications3. Appreciate the importance of mathematical representation of the

design problem

SESSION 2-1 In this session we shall study the difference between codes and standards,

design factors and their relation to equipment safety and the mathematical representation

of the design problem.

2-1.1 Codes and Standards

The terms CODE and STANDARD are used interchangeably, though CODE should be

reserved for a code of practices. That is a recommended design or operating procedure.STANDARD on the other hand refers to preferred sizes, eg. pipes, composition etc.

In modern engineering practice we have standards and codes that cover various functions

e.g

1. Materials, properties and composition

2. Testing procedures for performance, composition and quality

3. Preferred sizes, eg tubes, plates, sections

4. Design methods, inspection, fabrication

5. Codes of practice, for plant operation and safety

All developed countries have national organizations responsible for the issue and

maintenance of standards for the manufacturing industries and for the protection of

consumers.


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In the U.K:; British Standards Institution

In the U.S; National Bureau of Standards

They are responsible for coordinating information on standards. Standards are issued by

the Federal State and various commercial organizations. The major ones of interest to

chemical engineers are:

American National Standards Institute (ANSI)

American Petroleum Institute (API)

American Society for Testing Materials (ASTM)

American Society of Mechanical Engineers (ASME)

International Organization for Standardisation (ISO) coordinates the publication of

International Standards.

In Ghana, there is no national standards organization to coordinate local standards for

industries. However there are national standard organizations with standards for the

protection of consumers eg. Standards Boards, Food and Drug Administration (FDA),

and the Environmental Protection Agency (EPA).

Equipment manufactures work together to produce standardized designs and size ranges

for commonly used items; electric motors, pumps, pipes, and pipe fittings.

2-1.2 Factors of Safety (Design Factors)

Design is an inexact act; errors and uncertainties can arise in the design data available

and in the approximations necessary in the design calculations. To meet design

specifications, factors are included to give a margin of safety in the design so that the

equipment will not fail to perform satisfactorily, and that it will operate safely.

e.g. in a mechanical and structural design, the magnitude of design factors used to allow

for uncertainties in material properties, design methods fabrication and operating loads

are well established. In process design, design factors are used to give tolerances in the

design e.g. process stream average flows calculated from material balances are often

increased by a factor of 10%, to give some flexibility in process operation. This factor

then sets the maximum flows for equipment, instrumentation and piping design.


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2-1.3 System of Units

Chemical engineering uses a diversity of units from American and British engineering

Systems, CGS (grain, centimeter, second)

MKS( kilogram, meter, seconds)

English and Americanpound mass (lb), foot, second or hours, pound force.

If working in S. I units is preferred, data expressed in the American and British

engineering systems can be converted to S.I units. Conversions factors are available in

the literature.

2-1.4 Mathematical Representation of the Design Problem

A process unit e.g. distillation unit in a chemical process plant performs some operation

on the inlet material stream to produce the desired outlet stream.

Inlet stream Outlet stream

In the design of such a unit, the design calculations model the operation of the unit. Thus

the flow of materials is replaced by flow of information into the unit and flow of derivedinformation out of the unit.

Input Output

Information Information

Process

Unit

Calculationmethod


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Information flows are the values of the variables which are involved in the design.

Example:

Input stream Output stream

variables variables

Flow rate same as the input variables.

Composition

Temperature

Pressure

Enthalpy

Composition, temperature, pressure are called intensive variables, i.e. independent

of quantity of material flow (flow rate)(Nv).

The constraints on the design will place restrictions on the possible values that

these variables can take; for example the values of some of the variables will be

fixed directly by process specification. The values of other variables will be

determined by design relationships arising from constraints.

Some of the design relationships will be in the form of explicit mathematicalequations (design equations): such as those arising from material and energy

balances, thermodynamic relationships, and equipment performance parameters.

Other relationships will be less precise such as those arising from the use of

standards and preferred sizes and safety considerations (Nr).

The difference between the number of variables in the design and the number of

design relationships is called the number of degrees of freedom.

If Nvthe number of possible variables in a design problem

Nrthe number of design relationships

Nd = NvNr

Nd = number of degrees of freedom


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Case 1.

If Nv=Nr implies Nd = 0; this implies that there is only one unique solution to the

problem. The problem is not a true design problem and no optimization is possible.

Case 2

Nv is less than Nr, Nd is less than 0; this implies that the problem is over defined, and

only the trivial solution is possible.

Case 3

Nv >Nr, Nd > 0; implies there is an infinite number of possible solutions. However for a

practical problem, there will be only a limited number of feasible solutions. Nd represents

the number of variables which the designer must assign values to solve the problem.

EXAMPLE 2-1.1: Consider a single phase stream (liquid/vapour) containing C

components.

Input OutputProcess Unit


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Note

(1) The sum of the mass/mol fractions equal 1.

(2) The enthalpy is a function of stream composition, temperature and pressure.

Therefore, Degrees of Freedom, Nd = NvNr =(C+4)(2) = C+2

Specifying C+2 variables completely defines the stream.

EXAMPLE 2-1.2: Flash distillation / Equilibrium distillation

In this process unit, a feed is passed into a still (fractionating column), where part is

vaporized, and the vapour remaining in contact with the liquid. The mixture of vapor and

liquid leaves the still and is separated so that the vapour is in equilibrium with the liquid.

Where, F= Stream flow rate, P= pressure, T=temperature, xi= concentration of

component i, q = heat input.

Surfixes, 1= inlet; 2=outlet vapor; 3 =outlet liquid


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Note: 1. given the temperature and pressure, the concentration of any component in the

vapor phase can be obtained from the concentration in the liquid phase ( v-l-e data)

2. An equilibrium separation implies that the outlet streams and the still are the

same pressure and temperature.

This implies that P2 = P (1) T2 = T (3)

P3 = P (2) T3 = T (4)

Gives 4 equations.Degrees of freedomNd(No of degrees of freedom)=Nv - Nr= (3C + 9) - (2C + 5) = C + 4

Though the total degrees of freedom calculated is (C+4), some of the variables will be

fixed by the process conditions and will not be free for the designer to select as design

variables. For example: the flash distillation unit will normally be one unit in a process

system and the feed composition and feed conditions will be fixed by the upstream

process. Hence defining the feed, fixes (C+2) variables and the designer is left with,

(C+4)-(C+2) = 2


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2-1.5 Selection of Design Variables

To solve a design problem, the designer has to decide which variables are to be chosen as

design variables, i.e the ones he can manipulate to produce the best design. This choice is

crucial to enable the simplification of the calculations.

Example: Flash distillation problem in the previous example.

For a binary mixture, C=2; this implies Nd=C+4=6

If the feed stream flow, composition, temperature, pressure are fixed by upstream

conditions, then the number of design variables is Nd=6-(C+2)=6-4=2. This implies, the

designer is free to select 2 variables from the remaining variables to proceed with the

calculation of the outlet stream composition and flows.

Scenerio 1

Suppose you select the still pressure implies for a binary system, vapour-liquid

equilibrium (V-L-e) relationship is determined, and one outlet stream flow rate, then the

outlet compositions can be calculated by the simultaneous solution of mass-balance and

(v-l-e) relations.


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Scenerio 2

If you select the still pressure and the liquid outlet stream composition, then the

simultaneous solution of the mass balance and the v-l-e relationship will not be

necessary. Following the procedure below, one can calculate the stream compositions.

1. Specify P determines the v-l-e curve from experimental data

2. Knowing the outlet liquid composition, the outlet vapor composition can be

calculated from the v-l-e.

3. Knowing the feed and outlet compositions, and the feed flow rate, the outlet

stream flows can be calculated from a material balance.

4. An enthalpy balance gives the heat input required.

SELF ASSESSMENT 2-1

(a) List 3 American organizations responsible for coordinating information on standards.

(b) List 3 national standard organizations with standards for the protection of consumers.


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UNIT 3OPTIMIZATION AND BATCH PRODUCTION PROCESS

Introduction

This unit discusses batch production processes and the importance of optimization.

Learning ObjectivesAfter reading this unit, you should:

1. Appreciate the importance of optimization in plant design2. Understand the process of batch production

SESSION 3-1 In this session we shall discuss the steps involved in optimizing the design

of a chemical process plant. Further, we shall

3-1.1 Introduction

Optimizing the design of a chemical process plant is a foreboding task. This can be

achieved by subdividing the plant into subunits and optimizing each subunit. However

this does not result in the optimal design of the whole plant because the optimization of

each subunit is at the expense of the other.

The general procedure for optimizing process units and equipment design:

Step1: the first step is to clearly define the objective. i.e the criteria to be used for

measuring the performance of the system. For a chemical process plant, the overall

objective is tomaximize profit.

This overall objective can be broken down into sub objectives such as; minimize

operating cost, minimize capital investment, maximize yield of the product, reduce labour

requirements, reduce maintenance, operate safely.

Step 2: the second step is to determine the objective function; the system of equations

and other relationships, which relate the objectives with the variables to be manipulated

to optimize the function.

Step 3: this step is to find values of the variables that give the optimum value of objective

function i.e maximum or minimum. The best technique to be used will depend on the

complexity of the system and the type of mathematical model used to represent it.


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3-1.2 Simple Models

If the objective function can be expressed as a function of one variable (single degree of

freedom), the function can be differentiated or plotted to find the maximum or minimum.

This situation arises only for exceptional cases. In most practical situations, the numbers

of variables exceed the number of relationships.

Example of a simple model: Determine the optimum proportions for a closed cylindrical

container.

D

L

The surface area A in terms of the dimensions is:

( )

VV4VLabove(2)inDngsubstituti

VD

VDD

D

4V

zerotoequationabovethesetweDoptimumthefinding

(3)DD

4V

(D)fatingdifferenti

2

D

D

4Vf(D)

haveweD,variableoneoftermsinfunctionobjectivethegres

(2)D

4VLL

4

DV

have,we(V)volumegivenafor

(1)2

DLDLD,fisfunctionobjectivethe

DDLDDLA

3

2

2

2

2

31

32

31

'

22

44

440

,

;

sinexp

242

=

=

=

==+

+

=

+

=

=

=

+=

+=

+=


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For a cylindrical container, the minimum surface area to enclose a given volume is

obtained when the length (height) is made equal to the diameter.

In practice, when the cost is the objective function; L=2D; this is because the cost must

include that of forming the vessel, making the joints in addition to the cost of the

material.

3-1.3 Multiple Variable Systems

The general optimization problem can be represented mathematically as:

variablesthearevvvvandfunctionobjectivefwhere

vvvvff

n

n

,.......,,

),.......,,(

321

321

=

In a design situation, there will be constraints on the possible values of the objective

function due to constraints on the variables.

- Equality constraints are expressed by equations of the form

0),.......,,( 321 == nmm vvvv

- Inequality constraints are expressed by equations of the form

pnpp Pvvvv = ),.......,,( 321

Optimization of the problem involves finding values for the variables nvvvv ,.......,, 321

that will optimize the objective function ( i.e give maximum or minimum values within

the constraints).

3-1.4 Methods of Analysis:

a. Analytical method: objective functions can be expressed as a mathematical function;

use the methods of calculations to find maximum or minimum values.

- For practical situations where the values of the variables are subject to constraints,

the optimum of the constrained objective function will not necessarily occur

where the partial derivatives of the objective function are zero.

e.g.


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Figure 3-1.1: Effect of Constraints on the Optimum of a Function

- The method of Lagranges undetermined multipliers is a useful analytical

technique for dealing with problems that have equality constraints (fixed designvalues).

b. Search Methods: Relationships between variables and constraints that arise in

practical design problems are such that analytical methods are not feasible; hence the use

of search methods.

For single variable problems where the objective function is unimodal, the simplest

approach is to calculate the value of the objective function at uniformly spaced values of

the variable until a maximum or minimum is obtained.

Figure 3-1.2 Yield as a Function of Reactor Temperature and Pressure


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3-1.5 Other Optimization Methods

- Linear programming; a technique used when the objective function and constraints can

be expressed as a linear function of the variables.

- Dynamic programming; used for the optimization of large systems.

3-1.6 Batch Production Process

- Productive period: this is the period when product is being produced

- Nonproductive period; this is the period when the product is discharged and

equipment prepared for the next batch.

- Total batch time: productive period + nonproductive period

- The rate of production is determined by the total batch time as follows:

Batches per year = 8760 x Plant attainment

Total batch time (batch cycle time)

Annual production rate = (quantities produced per batch)x(batches per year)

Cost per unit of production= annual cost of production

Annual production rate

SELF ASSESSMENT 3-1

A rectangular tank with a square base is constructed from 5 mm steel plates. If thecapacity required is eight cubic meters. Determine the optimum dimensions if the tank

has a closed top.


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UNIT 4PROCESS SYNTHESIS

IntroductionThis unit discusses the synthesis of chemical processes.


1. Explain process synthesis2. Define and explain heuristics

SESSION 4-1 In this session we shall discuss process synthesis and the importance of

heuristics in chemical process optimization.

4-1.1 Introduction: process synthesis aims at the optimization of the logical structure of

a chemical process; specifically the sequence of steps; reaction, separation (distillation,

extraction etc), the source and destination of recycle streams.

The logical structure of a chemical process: Given the following;

- Raw materials, required products, allowed byproducts, a set of unit operations for

consideration, cost factors for materials and unit operations required to generate

and rank in order of preference and feasible chemical plant flow sheets.

Approach 1. Combinatorial algorithms are used to find all feasible flow sheets contained

in a toolkit of raw materials and operating steps. The flow sheets are then reviewed and

optimized based on performance, economic and safety criteria.

Approach 2. Heuristic rules based on experiences that are used for the selection and

positioning of processing operations as flow sheets are assembled.

4-1.2 Raw Materials and Chemical Reactions

Heuristic 1: select raw materials and chemical reactions to avoid or reduce the

handling and storage ofhazardous and toxic chemicals.e.x. Manufacture of ethylene glycol

(2)OHCHHOCHOHOHC

(1)OHCOHC

++

++

22242

42242 21


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- Both reactions are extremely exothermic, therefore they need to be controlled

carefully. Such processes are designed with two reaction steps with storage of the

intermediate, to enable continuous production, even when maintenance problems

shut down the first reaction operation.

Alternative to the 2-step example process:

1. Use chlorine and caustic in a single reaction step, to avoid the intermediate:

NaClOHCHHOCHaqNaOHClCHCH 2)(2 22222 +++=

2. Use the 2-step reaction with the following modifications:

- As ethylene-oxide is formed, react it with carbon dioxide to form ethylene

carbonate, a much less active intermediate that can be stored safely. This can then

be hydrolysed to form the required ethylene glycol product

2222343

343242

42242 21

COOHCHHOCHOHOHC

OHCCOOHC

OHCOHC

++

+

++

Heuristic 2: Distribution of Chemicals

Use an excess of one chemical reactant in a reaction to completely consume a second

valuable, toxic or hazardous reactant.

e.x. use an excess of ethylene in the production of Dichloroethane.

C2H4

Cl C2H4Cl2 +C2H4

C2H4


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Heuristic 3: when nearly pure products are required, eliminate the inert species before

the reaction operations, when the separations are easily accomplished, or when the

catalyst is adversely affected by the inert.

Heuristic 4: introduce liquid or vapour purge streams to provide exit for species that;

- Enter the process as impurities in the feed

- Produced by irreversible side reactions

e.x. Ammonia, NH3 synthesis loop


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Heuristic 5: Do not purge valuable species or species that are toxic and hazardous, even

in small concentrations

- Add separators to recover valuable species

- Add reactors to eliminate toxic and hazardous species

e.g. catalytic converter in car exhaust

Heuristic 6: For competing series or parallel reactions, adjust temperature, pressure, and

catalyst to obtain high yields of desired products. In the initial distribution of chemicals,

assume that these conditions can be satisfied; obtain kinetic data, and check this

assumption before developing a base-case design.

e.x. Manufacture of alkyl-chloride

HClClCHCHCHClClCHClCHCH

kk

Cl

HClClCHCHCHClCHCHCH

2

k

+=

++=+=

223

3

2

222321

Dichloropropane dichloropropene

This is a series/parallel reaction;

- for each reaction, obtainR

EKH oR ,,


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For each reaction, obtain kinetic data and examine the dependency of reaction rate on

temperature; implies RTE

oekk

=

Since for multiple reactions, high temperature favors the reaction of higher activation

energy and vice versa.

Heuristic 7: for reversible reactions, consider conducting them in a separation device

capable of removing products, and hence driving the reaction to the right.

e.g. manufacture of ethyl-acetate (ethyl ethanoate) using reactive distillation

Conventionally, this will call for the reaction:

EtOH + HOAc EtOAc +H2O

followed by separation of products using a sequence of separation of towers, using areactive distillation:


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Heuristic 8: Separations : separate liquid mixtures using distillation, stripping towers

and liquid-liquid extractors.

Heuristic 9: attempt to condense vapour mixtures with cooling water, then use heuristic

8.


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Heuristic 10: Heat Transfer in reactors: to remove highly exothermic heat of reaction,

consider the use of excess reactant, an inert diliuent.

Heuristic 11: For less exothermic heat of reaction, circulate reactor fluid to an external

cooler, or use a jacketed vessel or cooling coils.


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Heuristic 12: Pumping and Compression: To increase the pressure of a stream, pump a

liquid rather than compress a gas: i.e condense a vapor as long as refrigeration ( and

compression) is not needed before pumping.

Instead of:

Compressors have large capital cost and consume a lot of power.


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4-1.3 Summary of Process Design Heuristics

The discussion focused on the following:

- understanding the importance of selecting reaction paths that do not involve toxic

or hazardous chemicals.

- Be able to distribute the chemicals in a process flowsheet, to account for the

presence of inert species, to purge species that would otherwise build up to

unacceptable concentrations, to achieve a high selectivity of the desired products.

- Apply heuristics in in selecting separation processes to separate liquids, vapours,

and vapour-liquid mixtures.

- Understand the advantages of pumping a liquid rather than compressing a vapor.

4-1.4 Heuristics in Equipment Design

1. Equipment Size

Need information on the required throughput to determine vessel size

- General guidelines for vessel size

o Height: 2-10 m

o L/D: 2-5m

- Towers/ Columns

o

Height: 2-50mo L/D: 2-30m

Note: Do not specify units outside these ranges

2. Heat Exchangers

Several kinds are used

- Area: 10-1000m2

- For shell and tube (tubular)

o Tube diameter: 1-2 cm

o Tube length: 2-6m

o Shell diameter:0.3-1m


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- Plate and frame

Newer technology- thin, gasketed plates separate hot and cold fluids.

Advantage: more compact and very efficient

3. Heat transfer considerations

- Minimum temperature approach: for fluids 10oC and for refrigerants, 5

oC

- Cooling water in: 30oC; Cooling water out: 45

oC

- Equipment heat transfer(overall) coefficient in decreasing order of magnitude

Reboiler>Condenser>Liquid-to-Liquid>gas-to-gas

- Heat Exchangers:tlm < 100oC

4. Towers/ Columns

Tower operating pressure is usually determined by the temperature of condensing

medium or maximum allowable reboiler temperature

Sequencing multiple towers: typically

- Do easy separation first and leave difficult ones for last

- If relative volatilities of all species are close , remove one-by-one from overhead

- When volatilities are close but feed concentrations vary, remove high

concentrations first. Distillation operating conditions

- Economically reflux ratio is typically 1.2-1.5 times Lmin

- Optimum number of theoretical trays trays is typically 2 times Nmin

- Find Nmin from Fenske-Underwood equation

Tower design

- Tray spacing are typically 20-24

- Pressure drop typically 0.1psi per tray

- Tray efficiencies: 60-90% for gas absorption and stripping


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5. Process Conditions: General Guidelines

(a) High Pressure

To achieve a high pressure in a process:

- For a liquid use a pump; for a vapour first condense it and pump it into an

evaporatior

- For a gas compression; P/Po


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(c) Low Temperature

Operate at low temperature to achieve the following:

o Slow reaction rate

o Prevent thermal sensitive species from degradation

o Change thermodynamics

Achieve a high temperature in a process by the following means:

o Using a heat exchanger with the following fluids( cold steam)

Cooling tower water

Chilled water

Active refrigeration

SELF ASSESMENT 4-1

In the manufacture of ethylene glycol, a two step reaction process is used; the first step isthe formation of the intermediate ethylene oxide which can be stored, followed by itshydrolysis to form the ethylene glycol. Since both reactions are extremely exothermicand explosive, show with balanced chemical equations a safer two step approach after theinitial production of ethylene oxide.


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UNIT 5PROCESS SIMULATION

IntroductionThis unit discusses the simulation of a chemical process.


1. Explain what process simulation is.2. Identify the types of process simulations

SESSION 5-1 In this session we shall discuss the importance of process simulations, the

various types of simulations available and the importance of mathematical models in

process simulation.

5-1.1 Introduction

- Process simulation is the act of representing some aspects of the real world by

numbers or symbols that may be easily manipulated to facilitate their study.

- The important step in process simulation is the representation of that aspect of the

real world to be studied in terms of a mathematical model.

- With respect to chemical engineering, the real world is a chemical process

described by a process flow sheet. Therefore process simulation is used to solve

problems related to the process flow sheet, i.e process design, process analysis,

process control etc.

- The mathematical model that simulates an aspect of the process needs an

appropriate method of solution.

- A process simulation is a computer program developed to solve the model and

study its behavior by manipulating the model parameters.


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5-1.2 Process Simulator

Computational package that enables predictions of the process behavior

Inputs for the package are:

Basic engineering relationships such as:

- Mass and energy balances

- Phase and chemical equilibrium

- Flowsheet topography

Uses are:

- For design of new plants

- Increase profitability in existing plants

- Run many possible cases

- Sensitivity studies, and optimization

Accuracy depends on the use of

- Reliable thermodynamic and property data

- Realistic operating conditions

- Rigorous equipment models

5-1.3 Types of Process Simulation

There are several commercial packages. E.g Aspen Plus, Chemcad, Hysis Pro/II.Common features of all these are:

1. Database of hundredsthousands( a large number) of compounds

2. A parameter library to compute /estimate properties of these compounds

3. Flowsheet builder- graphical interface that enables units to be defined and

connected.

4. Thermo Solver; computes phase equilibrium and thermodynamic properties given

the database

5. Unit operations Solver: short cut and rigorous solvers for equipment

6. Overall flowsheet solver; mathematical convergence of the flowsheet


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5-1.4 Unit Operations Solvers; e.g Reactors

- specifies the reaction stoichiometry, temperature, pressure and conversion

- when the reaction is equilibrium limited, specifies the stoichiometry , the approach to

equilibrium and equilibrium constants

- for the reaction involved, specifies the kinetic expression, reactor type and enables the

reactor size calculation.

- distillation columns

- Uses the Feuske, Underwood methods to provide a good initial guess for subsequent

calculations

- for rigorous plate-by-plate approach, it solves simultaneously, material and energy

balance equations, and VLE relations for each plate.

5-1.5. Uncertainty and Sensitivity Issues

It is important to be able to quantify the uncertainty of results:

- Determine the probability of accuracy of results

- Determine what part of results obtained is most likely to be incorrect, and

estimate error range

- Sensitivity issues; in cost estimation and profitability studies, estimate the

sensitivity of the results to variations in capital cost, operating cost etc.Other causes of uncertainty of results using a simulator.

- Thermo model used

- Physical properties data

- Convergence tolerance

- Simulation method ( simulator)

Because there are disturbing variations between different process simulators using

the same models.


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- What can we do?

o Estimate uncertainties by performing simulations using a range of

parameters, different models.

o Determine the sensitivity of results

o Use statistical methods to design experiments over ranges of parameters

and the results will provide confidence limits

o Ultimately, final designs should be tested on pilot studies.


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UNIT 6FLOW SHEETING, PIPING AND INSTRUMENTATION

IntroductionThis unit discusses flow sheeting, piping and instrumentation in chemical process design.


1. Appreciate the importance of flow sheet in chemical process design2. Understand why instrumentation in plant design is critical3. Acknowledge the processes involved in valves and pumps selection

SESSION 6-1 In this session we shall study the essence of flow sheets in process design.

We shall also look at piping and instrumentation diagram and the types of valves used in

a process plant design.

6-1.1 Introduction

Industrial equipment are always arranged and interconnected in a certain fashion. The

process flow sheet gives a pictorial representation of the various equipment selected to

carry out the process. The process flow sheet always convey information on the operating

conditions, stream flow-rates and composition. The information included in a typical

flow sheet is either essential or optional. An example flow sheet is shown in Figure 6-1.1

Figure 6-1.1: Flow-sheet of Simplified Nitric Acid Production Process


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The essential information included in the flow-sheet are;

1. Stream composition, either:

The flow-rate of each individual component, kg/h, which is preferred, or

The stream composition as a weight fraction.

2. Total stream flow-rate, kg/h.

3. Stream temperature, degrees Celsius preferred.

4. Nominal operating pressure (the required operating pressure).

Optional information

1. Molar percentages composition.

2. Physical property data, mean values for the stream, such as:

Density

Viscosity

3. Stream name, a brief, one or two-word, description of the nature of the stream

4. Stream enthalpy, kJ/h.

Figure 1 below shows the information included in a polymer production process.

6-1.2 Piping and Instrumentation Diagram (P&ID)The piping and instrumentation diagram shows the engineering details of the equipment,

instruments, piping, valves and fittings; and their arrangement.

Figure 6-1.2: Polymer Production Diagram


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In the polymer production flow sheet above, all the following information should be

included in the P&ID:

1. All process equipment identified by an equipment number. The equipment should be

drawn roughly in proportion, and the location of nozzles shown.

2. All pipes, identified by a line number. The pipe size and material of construction

should be shown. The material may be included as part of the line identification number.

3. All valves control and block valves, with an identification number. The type and size

should be shown. The type may be shown by the symbol used for the valve or included in

the code used for the valve number.

4. Ancillary fittings that are part of the piping system, such as inline sight-glasses,

strainers and steam traps; with an identification number.

5. Pumps, identified by a suitable code number.

6. All control loops and instruments, with an identification number.

For simple processes, the utility (service) lines can be shown on the P and I diagram.

For complex processes, separate diagrams should be used to show the service lines, so the

information can be shown clearly, without cluttering up the diagram. The service

connections to each unit should, however, be shown on the P and I diagram.

6-1.3 Valve Selection

Control of flow in lines and provision for isolation of equipment when needed are

accomplished with valves. Depending on primary function, valves are classified as:

1. Shut-off valves (block valves), whose purpose is to close off the flow.

2. Control valves, both manual and automatic, used to regulate flow.

Figure 6-3 shows the valves commonly encountered in industry.


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Figure 6-1.3: Commonly used Valves

The main types of valves used are:

Gate Figure 6-1.3a

Plug Figure 6-1.3b

Ball Figure 6-1.3c

Globe Figure 6-1.3d

Diaphragm Figure 6-1.3e

6-1.3.1 Gate valves

In gate valves, the flow is straight through and is regulated by raising or lowering the

gate. The majority of valves in the plant are of this type. In the wide open position they

cause little pressure drop.


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6-1.3.2 Globe valves

In globe valves, the flow changes direction and results in appreciable friction even in the

wide open position. This kind of valve is essential when tight shutoff is needed,

particularly of gas flow.

The ball and plug valves are also frequently used for the purpose of flow shutoff.

Butterfly valves are often used for the control of gas and vapor flows. Automatic control

valves are basically globe valves with special trim design.

SESSION 6-2: In this session we shall discuss the classification of pumps and factors to

consider in pump selection.

6-2.1 Introduction of Pumps

They are mechanical devices used to increase the energy of a liquid stream flowing in a

closed conduit or pipe. This energy may be used to increase the velocity (move the fluid),

the pressure or the elevation of the fluid.

6-2.2 Classification of PumpsPumps are generally classified into two main categories namely:

Centrifugal pumps

Positive displacement pumps


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Table 6-2.1: Main Types of Pumps

Category Types Structure

Centrifugal Single-stage

Multi-stage

Volute

Diffuser

Regenerative

Vertical

Hellico-centrifugal

Axial flow

Positive

displacement

Rotary Gear

Screw

Vane

Reciprocating Piston

Diaphragm

Plunger

There are many subgroups of pumps as indicated in Table 6-2.1.

6-2.3 Factors to Consider in Pump Selection

Capacity (flow rate in m3

/h)

The pressure head that is generated by the pump

The type of liquid pumped (its viscosity and vapour pressure under inlet

conditions)

An initial selection is generally made based on the first two criteria mentioned, i.e. the

capacity and the pressure generated.

The centrifugal pump is often the only possible choice for high capacities whereas

positive displacement pumps are better suited to generating high pressure heads.

Other criteria such as the viscosity of the liquid can modify this initial choice. A positive

displacement pump is generally recommended to pump liquids with a viscosity higher

than 2000 cP. Figure 6-2.1shows the approximate range of operation generally covered

by the main types of pump mentioned above.


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Figure 6-2.1: Approximate Range of Operation for the Three Main Types of Pump

6-2.4 Centrifugal Pumps

Basically consists of an impeller equipped with radial vanes rotating inside a shell called

the pump casing. It works by the transfer of centrifugal force of the rotating impeller into

kinetic energy of the liquid. This energy is then converted into pressure when the fluid

velocity decreases.

6-2.4.1 Effective characteristic curves

A centrifugal pump is characterized by 4 basic curves, all of which are expressed versus

the flow rate as shown in Figure 6-2.2.

A. The head generated

B. The mechanical/hydraulic conversion efficiency

C. The mechanical power input consumed at the shaft

D. The pump suction capacity or NPSH.


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Figure 6-2.2: Basic Curves Characterizing a Centrifugal Pump.

6-2.4.2 Design parameters of centrifugal pumps

- The rotation speed

- The number of impellers

- The impeller diameter

- The impeller design

6-2.4.3 Operating point

The pressure head HA required by the installation is represented by the system curve

versus flow rate, Q. It is the sum of the static and dynamic heads of the installation as

shown in Figure 6-2.3. The static heads are independent of the flow rate and include

differences in height and pressure between the unit inlet and outlet. The dynamic heads

correspond to pressure drops and are proportional to the square of the flow rate.


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Figure 6-2.3: Curve Characteristic of the System.

A centrifugal pump adjusts itself on an operating point B, corresponding to the

intersection between the Q/H curve of the pump and the HA curve of the system (Figure

6-2.4). A variation in the operating point (and therefore in the flow rate and the head) can

be obtained by a physical modification in the pump, but also by modifying its speed or

the system curve, usually by means of a valve.

Figure 6-2.4: Variation of Operating Point by means of a Valve.


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6-2.4.4 Q/H Curve versus Technical Choices

1. Basic choices; concept of specific speed

The number Nq, called the specific speed, allows all centrifugal pumps to be compared

with one another. It is calculated from the following expression:

43

60H

QNNq =

With:

N = rotational speed in rpm

Q= flow rate the best efficiency in m3/h (through one eye if double-suction impeller)

H= head in m generated at the best efficiency (for one stage)

For the same specific speed, the hydraulic designs will be similar on varying scales. The

choice of Nq is a major parameter in impeller hydraulic design (Figure 6-2.5). The

specific speed also considerably influences the best efficiency achievable by a pump. If

high pressure is required, a compromise will have to be found between a reasonable

number of stages and an acceptable number efficiency.

Figure 6-2.5: Variation in Specific Speed versus the Type of Impeller used.

2. Choice of Design: Concept of Q/H Curve Slope

For a given hydraulic choice, the different design parameters, in particular the number of

vanes can modify the curve shape downward, flat or bell-shaped (Figure 6-2.6). These

curves should be considered according to the requirements of the resisting network. Bell-

shaped curves in particular, should be avoided for pumps that have to work in parallel

(risk of instability).


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Figure 6-2.6: Different Types of Characteristic Curves

6-2.4.5 Choice of Rotation Speed

The rotation speed is a dominant parameter for the characteristic curve of a centrifugal

pump. Figure 6-2.7 shows how the operating point varies with speed. The flow rate varies

linearly with speed:

1

212N

NQQ =

The head generated varies with the square of the speed:

2

1

212

=

N

NHH

As a result, the power absorbed varies with the cube of the speed

3

1

212

=

N

NPP


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Figure 6-2.7: Variation in the Operating Point versus the Rotation Speed.

6-2.4.6 Suction Conditions: Concept of NPSHVapour pressure

For a given temperature, each liquid has a specific boiling pressure, called the vapour

pressure Tv. if the pressure at one point in the liquid becomes less than T v, the liquid

vaporizes instantly.

Cavitation

The lowest static pressure inside a centrifugal pump is located at the impeller inlet. If

vaporization begins at this point, the liquid will be repressurized nearby downstream. The

bubbles formed condense by collapsing suddenly, most often near a wall. This very noisy

phenomenon is called cavitation. The head generated by the pump and the absorbed

power then drop, the vibrations and noise increase and erosion can be observed, mainly in

the impeller, in the form of characteristic pits. If the pump is kept working under these

conditions, permanent damage may occur.

Required NPSH

To prevent cavitation, the total liquid pressure at the inlet must be such that no

vaporization can occur. The minimum value depends on the pump design

Available NPSH

It is the pressure available to force a given flow through the suction piping into the pump.

This is a function of the system. It is easily calculated with the formula below.


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=

+ ( )

EXAMPLE 6-1.1: Pressure drop calculation

A pipeline connecting two tanks contains four standard elbows, a plug valve that is fully

open and a gate valve that is half open. The line is commercial steel pipe, 25mm internal

diameter, length120m. The properties of the fluid are: viscosity 0.99mNm-2s, density

998kg/m3.Calculate the total pressure drop due to friction when the flow rate is 3500

kg/h.

SOLUTION 6-1.1

Cross-sectional area of pipe = ( ) 2323 10491.010254

m =

Fluid velocity, u = sm/98.1998

1

10491.0

1

3600

35003

=

Reynolds number, Re = ( )

4

3

109.9

102598.1998

4105900,49 ==

Absolute roughness commercial steel pipe = 0.046mm

Relative roughness = 0018.01025

046.03=

, round to 0.002

From friction factor chart, f = 0.0032

Fitting/valve Number of velocity

heads, K

Equivalent

pipe diameters

Entry

Elbows

Globe valve, open

Gate valve, open

Exit

0.5

(0.84)

6.0

4.0

1.0

25

(404)

300

200

50

Total 14.7 735


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Method 1, velocity heads

A velocity head mg

u20.0

8.92

98.1

2

22

=

= of liquid,

Head loss = 0.20 14.7 = 2.94m

As pressure = 2.94 998 9.8 = 28,754 N/m2

Friction loss in pipe,2

98.1998

1025

1200032.08

2

3

= fP

= 240,388 N/m2

Total pressure = 28,754 + 240,388 = 269,142 N/m2 = 270 kN/m2

Method 2, equivalent pipe diameters

Extra length of pipe to allow for miscellaneous losses

= 735 25 10-3

= 18.4 m

So total length for P calculation = 120 + 18.4 m

2

98.1998

1025

1380032.08

2

3

= fP

= 277 kN/m2

Note: the two methods will not give the same results. The method of velocity heads is the

more fundamentally correct approach. But the use of equivalent diameters is easier to

apply and sufficiently accurate for use in design calculations.


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SELF ASSESSMENT 6-1

A process fluid is pumped from the bottom of one distillation column to another, using acentrifugal pump. The line is standard commercial steel pipe 75 mm internal diameter.From the column to the pump inlet the line is 25 m long and contains six standard elbowsand a fully open gate valve. From the pump outlet to the second column the line is 250 m

long and contains ten standard elbows, four gate valves (operated fully open) and a flow-control valve. The fluid level in the first column is 4 m above the pump inlet. The feedpoint of the second column is 6 m above the pump inlet. The operating pressure in thefirst column is 1.05 bara and that of the second column 0.3 barg. Determine the operatingpoint on the pump characteristic curve when the flow is such that the pressure drop acrossthe control valve is 35 kN/m

2. The physical properties of the fluid are: density 875 kg/m

3,

viscosity 1.46 mNm-2s.Pump characteristicFlow-rate, m3/h 0.0 18.2 27.3 36.3 45.4 54.5 63.6Head, m of liquid 32.0 31.4 30.8 29.0 26.5 23.2 18.3


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UNIT 7PROCESS ECONOMICS: COST ESIMATION

Introduction

This unit discusses the estimation of capital investment, total product cost and thedetermination of breakeven point.


1. Define total capital investment2. Apply the Marshall & Swift cost index and capacity ratio to estimate

equipment cost3. Distinguish between fixed capital and working capital4. Define turnover ratio

SESSION 7-1 In this session we shall discuss the estimation of capital investment.

Further we shall discuss fixed capital investment, working capital, cost indices and turn-

over ratio.

7-1.1 Cost Estimation

An acceptable plant design must present a process that is capable of operating under

conditions which will yield a profit. Since net profit equals total income minus all

expenses, it is essential that the chemical engineer be aware of the many different types

of cost involved in manufacturing process. Capital must be allocated for direct plant

expenses, such as those for raw materials, labor and equipment. Besides direct expenses,

many other indirect expenses are incurred and these must be included if a complete

analysis of the total cost is to be obtained. Examples of these indirect expenses are

administrative salaries, product-distribution cost, and cost for interplant communications.

A capital investment is required for any industrial process, and determination of the

necessary investment is an important part of a plant-design project. The total investmentfor any process consist offixed-capital investment for physical equipment and facilities

in the plant plus working capital which must be available to pay salaries, keep raw

materials and products on hand and handle other special items requiring a direct cash

outlay.


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Thus in an analysis of cost in industrial processes, capital investment costs,

manufacturing costs, and general expenses including income taxes must be taken into

consideration.

7-1.2 Cash Flow for Industrial Operations

Figure 7-1.1 shows the concept of cash flow for an overall industrial operation.

Figure 7-1.1: Cash Flow for an Overall Industrial Operation.


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7-1.3 Factors affecting Investment and Production Costs

Sources of equipment

Price fluctuations

Company policies

Operating time and rate of production

Governmental policies

7-1.4 Capital Investment

Before an industrial plant can be put into operation, a large sum of money must be

supplied to purchase and install the necessary machinery and equipment. Land and

service facilities must be obtained, and the plant must be erected complete with all

piping, controls, and services. In addition it is necessary to have money available for the

payment of expenses involved in the plant operation.

- Fixed capital investment

The capital needed to supply the needed manufacturing and plant facilities.

- Working capital

The capital necessary for the operation of the plant

- Total capital investment = Fixed capital investment + Working capital

- Fixed capital investment= Manufacturing fixed capital investment + Non-

manufacturing fixed capital Investment.

- Manufacturing fixed capital investment: the fixed capital necessary for the

installed process equipment with all auxiliaries that are needed for complete

process operation. These include expenses for piping, instruments, insulations,

foundations, and site preparation.


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- Non- manufacturing fixed capital investment: the fixed capital required for

construction overhead and for all plant components that are not directly related to

the process operation. These plant components include the land, processing

buildings, administrative and other offices, warehouses, laboratories,

transportation, shipping and receiving facilities, utilities and waste disposal

facilities, shops and other permanent parts of the plant.

- Construction overhead cost: consists of field-office and supervision expenses,

home-office expenses, engineering expenses, miscellaneous construction costs,

contactors fees and contingencies. In most cases, construction overhead is

proportioned between manufacturing and non-manufacturing fixed-capital

investment.

- Working capital: the working capital for an industrial plant consist of the total

amount of money invested in

o Raw materials and supplies carried in stock

o Finished products in stock and semi-finished products in process of being

manufactured

o Accounts receivable

o Cash kept on hand for monthly payment of operating expenses , such as

salaries, wages, and raw materials purchases

o Accounts payables

o Taxes payables

The ratio of working capital to total capital investment varies with different

companies. Most chemical plants use an initial working capital of 10 to 20 percent

of the total capital investment. It is 50% or more for companies producing

products of seasonal demand because of the large inventories which must be

maintained for appreciable period of time.


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7-1.5 Estimation of Capital Investment

7-1.5.1 Introduction

Of the many factors which contribute to poor estimates of capital investments, the most

significant one is usually traceable to sizeable omissions of equipment, services, or

auxiliary facilities rather than to gross errors in costing. A check list of items covering a

new facility is an invaluable aid in making a complete estimation of the fixed capital

investment. A typical list of items for estimating fixed capital investment is:

Direct cost

1. Purchased equipment; all equipments listed on a flow sheet

2. Purchased-equipment installation

3. Instrumentation and controls

4. Piping

5. Electrical equipment and materials

6. Buildings(including services)

7. Yard improvements

8. Service facilities

9. land

Indirect cost1. Engineering and supervision

2. Construction expenses

3. Contractors fees

4. Contingencies

7-1.5.2 Types of capital cost estimates

An estimate of the capital investment for a process may vary from a predesign estimate

based on little information except the size of the proposed project to a detailed estimate

prepared from complete drawings and specifications. The following five categories

represent the accuracy range and designation normally used for design purposes:

1. Order-of-magnitude estimate(ration estimate) based on similar previous cost data:

probable accuracy of estimate over30 percent


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2. Study estimate (factored estimate) based on knowledge of major items of

equipment; probable accuracy of estimate up to30 percent

3. Preliminary estimate (budget authorization estimate; scope estimate) based on

sufficient data to permit the estimate to be budgeted; probable accuracy of

estimate within20 percent.

4. Definitive estimate(project control estimate) based on almost complete data but

before completion of drawings and specifications; probable accuracy of estimate

within10 percent

5. Detailed estimate (contractors estimate) based on complete engineering

drawings, specifications and site surveys; probable accuracy of estimate within

5 percent.

7-1.5.3 Cost indexes

Most cost data which are available for immediate use in a preliminary or predesign

estimate are based on conditions at some time in the past. Because prices may change

considerably with time due to changes in economic conditions, some methods must

be used for updating cost data applicable at a past date to costs that are representative

of conditions at a later time. This is done using a cost index.

Acost index is hence an index value for a given point in time showing the cost ofanequipment or plant at that time relative to a certain base time.

=

obtainedwascostoriginaltimeatvalueindex

timepresentatvalueindexcostOriginalcostesentPr

Cost indexes can be used to give a general estimate, but no index can take into

account all factors, such as special technological advancements or local conditions.

The common indexes permit fairly accurate estimates if the time period involved is

less than 10 years.


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Some indexes are used fo

labor, construction, mate

The most common of the

process-industry equipm

the Nelson refinery const

index. This is shown in T

Table

7-1.5.4 Methods for Estima

7-1.5.4.1 Power factor appl

( )nn RCFeC =

Where:

R = size (or capacity) ratio o

62

r estimating equipment cost, others apply speci

ials etc.

se indexes are theMarshall and Swift all-indust

nt indexes, the Engineering New-Record const

ruction index and the Chemical Engineering pl

ble 7-1.1.

7-1.1: Cost indexes as Annual Averages

ing Capital Investment

ed to plant-capacity ratio:

equipment=plaorequipmentoldofcapacity

plaorequipmentnewofcapacity

ically to

ry and

uction index,

nt cost

t

t


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Fe = equipment cost index ra

n = equipment size (or capac

C = purchased cost of old eq

Cn = purchased cost of new e

Table 7-1.2 shows typical ex

Table 7-1.2 Typi

63

tio=obwascostoriginaltimetheatvalueindex

timepresentinvalueindex

ity) exponent

ipment

quipment

ponents for equipment cost vs. capacity

al Exponents for Equipment Cost vs. Capaci

tained

ty


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EXAMPLE 7-1.1

If a process plant was erected in Kumasi at a fixed capital investment of $436,000 in

1970, determine what the capital investment will be in 1975 for a similar process plant

located near Dansoman in Accra with twice the process capacity but with an equal

number of process equipment.

SOLUTION 7-1.1

( )nn RCFeC =

Where:

R = size ( or capacity) ratio of equipment

Fe = equipment cost index ratio

n = equipment size exponent

C = purchased cost of equipment in 1970

Cn = cost of equipment in 1975

From Table 7-1.1

M & S index, 1970 =303,

M & S index 1975 = 444,

From Table 7-1.2

Equipment size exponent, n = 0.6

( ) 378,968$2303

444000,436

6.0 ==nC


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7-1.5.4.2 Detailed item estimate

EXAMPLE 7-1.2

Initial design work was done for a chemical plant to revamp the process in order to

recover valuable product from an effluent gas stream. The gas will be scrubbed with a

solvent in a packed column and the recovered product and solvent separated by

distillation. The solvent will then be cooled and recycled. The major items of equipment

required and their purchased costs in cedis are:

Absorption column

Column purchased cost = 19,800

Cost of column packing = 6,786

Recovery column

Column purchased cost = 45,000

Cost of 30 sieve trays = 8,670

Reboiler

Cost of reboiler = 7,600

Condenser

Purchased cost = 4,800

Recycle solvent cooler

Cost of cooler = 2,550Storage tank

Cost of tank = 7,790

The relevant component factors for this processing plant are;

f1: equipment erection = 0.40

f2: piping = 0.70

f3: Instrumentation = 0.2

f4: electrical = 0.10

f10: design and engineering = 0.30

f12: contingencies = 0.20

Estimate the total capital investment for the project, if the working capital can be taken as

10 % of the fixed capital cost.


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SOLUTION 7-1.2

Total capital investment, TCI = Fixed capital investment, FCI Working capital, WC

Total purchased cost of equipment, PCE

Absorption column = 19,800 + 6,786 = 26,586

Recovery column = 45,000 + 8,670 = 53,670

Reboiler = 7,600

Condenser = 4,800

Recycle solvent cooler = 2,550

Solvent and product storage tank = 7,790

Total purchased cost of major equipment = 102,996

Equipment erection = 102,996 (0.4) = 41,198

Piping cost = 103,996 (0.70) = 72,097

Instrumentation cost = 102,996 (0.2) = 20,599

Electrical = 102,996 (0.10) = 10,300

Total physical plant cost (Direct cost) = 10,300 + 20,599 + 72,097 + 41,198 + 102,996

= 247,190

Indirect cost

Design and engineering, f10 = 0.30 (247,190) = 74,157

Contingencies = 0.20 (247,190) = 49,436Total indirect cost = 123,595

Fixed capital investment (Direct cost + Indirect cost) = 247,190 + 123,595 = 370,785

Working capital = 0.10 (370,785) = 37,079

Total capital investment = 370,785 + 37,079 = 407,864

7-1.5.4.3 Other Methods for Estimating Equipment or Capital Investment

(i) Unit cost Estimate

(ii) Percentage of delivered-equipment Cost

(iii) Lang factors for approximation of capital Investment


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7-1.5.5.Turnover Ratios:

A rapid evaluation method suitable for order-of-magnitude estimates is known as the

turnover ratio method.

Turnover ratio is defined as the ratio of gross annual sales to the fixed capital

investment

tinvestimencapitalfixed

salesannualgrosstioTurnoverra =

Where the product of the annual production rate and the average selling price of the

commodities produced is the gross annual sales figure.

ratioTurnoverratioinvestmentorratioCapital

1=

Turnover ratio ranges between 0.2 to 5. For chemical industries, as a very rough rule of

thumb, the ratio can be approximated to 1.

SESSION 7-2: In this session we shall study the estimation of total product cost and

determination of breakeven point.

7-2.1 Estimation of Total Product Cost

Manufacturing cost: all expenses directly connected with the manufacturing

operation or the physical equipment of a process plant itself.

There are 3 classifications of these expenses:

Direct production cost: expenses directly associated with the

manufacturing operation. E.g. expenses for raw materials ( including

transportation, unloading)


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Fixed charges: expenses that remains practically constant from year to

year. Do not vary widely with changes in production rate. E.g. property

taxes, insurance.

Plant overhead cost: are for medical and hospital services; general plant

maintenance and overhead; safety services; social security

General Expenses:

Administrative expenses

Distribution and marketing expenses

Research and development

Financing expenses: extra costs involved in procuring the money

necessary for the capital investment.

Gross earnings expenses

Total Product Cost= Manufacturing cost + General Expenses.

Gross earnings (Gross Profit)= The total incomethe total product cost.

Net annual earnings = Gross annual earningsincome taxes.

Figure 7-2.1 shows the components constituting the estimation of total product cost


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Figure 7-2.1: Cost Involved in Total Product Cost for a Typical Chemical

Process Plant

7-2.2 Break-even Point

(a) Break-even point occurs (or is the percentage of plant capacity) when the total

annual product cost equals the total annual sales.

i.e: Total Annual Product Co

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