Transcript
Page 1: Pinch Technology by Shubham

• Introduction • Techniques available for process

integration• Pinch technology• Features and Benefits of Pinch• Where pinch technology is used?• Concept of pinch technology• Phases of pinch technology• A Retrofit Project

Contents

Page 2: Pinch Technology by Shubham

PROBLEMS OF PROCESS INDUSTRY RELATED TO ENERGY

ENERGY MANAGEMENT THROUGH PROCESS INTEGRATION - A REALITY

• Problems of Indian industries can be solved by using techniques that minimize energy consumption with minimum investment.

• Process integration is one such technique.

• Borrowed, often obsolete technology

• Energy consumption per unit production much higher than in western industries

• No concept of process integration

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PROCESS INTEGRATION

The Process Integration is defined as “Systematic and general methods for designing integrated production systems, ranging from individual processes to total sites, with special emphasis on the efficient use of energy and reducing environmental effects”.

Process Integration is a part of Process Intensification (PI).

Ramshaw, 1995, defined PI as: “A strategy for making dramatic reductions in the size of a chemical plant so as to reach a given production objective”.

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Techniques available for process Integration

Pinch Technology Approach

MILP/MINLP Approach

State-Space Approach

Genetic Algorithm Approach

Process Graph Theory Approach

Supertargeting Approach

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Methods in Process Integration

The three major features of Process Integration methods are:

• The use of heuristics (insight)• The use of thermodynamics • The use of optimization techniques.

Pinch Analysis is a method with a particular focus on Thermodynamics. Hierarchical Analysis and Knowledge Based Systems are rule-based approaches with the ability to handle qualitative (or fuzzy) knowledge. Finally, Optimization techniques can be divided into deterministic (Mathematical Programming) and non-deterministic methods (stochastic search methods such as Simulated Annealing and Genetic Algorithms).

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HierarchicalAnalysis

HeuristicRules

KnowledgeBased Systems

Thermodynamic Methods

Optimization Methods

QUALITATIVE

QUANTITATIVE

INTERACTIVEAUTOMATIC

Fig. 1 One possible Classification of Process Integration Methods

One possible classification of Process Integration methods is to use the two-dimensional (automatic vs. interactive and quantitative vs. qualitative) representation in figure 1.

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Pinch technology reveals all the possible savings and their corresponding financial benefits.

• It defines the maximum possible savings.• It looks at the overall site.• It does not bench-mark but takes into account all specific mill

factors, age, location, process equipment, operating preferences, product, etc.

• It reveals the maximum cogeneration potential

PINCH TECHNOLOGY

Pinch Technology was introduced by Linnhoff in 1978 to solve heat exchange problems as an energy saving tool. Pinch Technology forms the essence of optimization of processes by energy and resource analysis (OPERA).

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Features and Benefits of Pinch…

• Targets for minimum heating & cooling.

• Quantifies scope for heat recovery.

• Analysis Includes the process unit or the whole site, as appropriate:

• Design tools define appropriate project.

• Shows what to do with low-grade waste heat.

• Combined Heat and Power (CHP).

• Practical application brings real benefits.

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WHERE PINCH TECHNOLOGY IS USED?

Heat integration Distillation column targeting Cogeneration & total site targeting Batch process targeting Emission targeting Mass exchange network ( Water & waste water

management & recovery of valuable materials) Hydrogen management in refineries

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CONCEPT OF PINCH TECHNOLOGY

ONION DIAGRAMFig. 2. The process design hierarchy can be represented by “onion diagram” as shown below.

Reactor

SeparatorHeat exchange network

Utilities

The heat and material balance is at this boundary

Site-Wide Utilities

Fig. 2 Onion Diagram

1

2

34

Main points from onion diagram

Design of a process starts with the reactors.

Separator can be designed for known feeds, products, recycle concentrations and flow rates.

For heat and material balance, heat exchange network (HEN) can be designed.

For remaining heating and cooling duties, the utility system is designed.

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PROBLEM ADDRESSED Generally two types of problem are addressed:

• Creating New Designs

This is related to the design of HEN for a new plant, which is in design stage.

• Retrofit – Revamping Existing Designs

This is related to the retrofitting of an already existing HEN in a plant to improve its exchange efficiency.

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PHASES OF PINCH TECHNOLOGY

There are four phases of pinch analysis in the design of heat recovery systems for both new and existing processes:

DATA EXTRACTION

It relates to the extraction of information required for pinch technology from a given process heat and material balance. PERFORMANCE TARGETSTargeting provides a fundamental insight into heat recovery options in a process. It does this by giving a system-wide view of the heating and cooling requirements at different temperature levels.

NETWORK DESIGNINGIn design the user will typically work with an incomplete network and try to follow the pinch design rules.

NETWORK OPTIMIZATIONHeat exchange network for maximum energy recovery established by pinch design method, should only be regarded as initial designs and some final optimization is required.

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Graphical Representation

Composite curve

Fig. 6. The HCC and CCC show the heat availability and heat requirement for the overall process

0

50

100

150

200

0 1000 2000 3000 4000 5000

Heat Content Q (kW)

T (

C)

HCC

CCC

Region of heat recovery by process to process exchange

QHmin

QCmin

Tmin

Above pinch

Below pinch

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Analytical ProcedureProblem Table Algorithm (PTA)

For the calculation of energy targets, only the inlet temperatures, outlet temperature and heat capacity flow rates are required.

The steps involved in PTA are:

• Determination of temperature intervals

• Calculation of net MCP in each interval

MCp,int = MCp,c – MCp.h for each interval

• Calculation of net enthalpy in each interval

• Calculation of cascaded heat

• Revision of cascaded heat

• Determination of energy targets

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Implementation of Problem Table Algorithm

Interval i.

  Col. ATint

Col. BMCp,int

Col. CQint

Col. DQcas

Col. ERcas

 

0123456

streamMCp

        

16512211555503530

01025-15251020

0430175-900125150100

0-430-60529517020-80

605175

0900775625525

H1 H2 C3 C410 40 20 15

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The composite curve gives the information as:

Minimum hot utility (QHmin) = 605 kW

Minimum cold utility (QCmin) = 525 kW

Hot pinch temperature = 125 ˚C

Cold pinch temperature = 105 ˚C

Tmin is known as the “pinch” and once the pinch is recognized it is possible to consider the process as two separate systems: one above the pinch and one below the pinch. The system above the pinch requires a heat input and is therefore a net heat sink. Below the pinch, the system rejects heat and so is a net heat source.

Concept of Pinch

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Concept Of Multiple Utility

The energy requirement for a process is supplied via several utility levels e.g. steam levels, refrigeration levels, hot oil, furnace flue gas etc.

The general objective is to maximize the use of the cheaper utility levels and minimize the use of the expensive utility levels.

The composite curve provide overall energy targets but do not clearly indicate how much energy needs to be supplied by different utility levels. For this purpose, the grand composite curve is used.

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Grand Composite Curve

Fig. 7. GCC shows the multiple utilities

0

20

40

60

80

100

120

140

160

180

0 100 200 300 400 500 600 700 800 900 1000

Heat flow Q (kW)

T (

*C

)

Pinch

High temperature process sink profile

Low temperature process source profile

HU1

CU1

CU2

HU2

Process to process heat exchange

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The balanced composite curve is generated to estimate the targeted area.

Fig.8. The balenced composite curves

020406080

100120140160180200

0 1000 2000 3000 4000 5000Heat Content Q (kW)

T (C

) BHCC

BCCC

Interval i. Th,i.-1

Tc,i.-1

Th,i.

Tc,i.

BALANCED COMPOSITE CURVES

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Fig. 4: Potential energy savings in some major industrial sectors

Fig. 5: Potential water consumption savings in some major industrial sectors

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A RETROFIT PROJECT

Even in a simple process made up of two unit operations (Fig. 6 & 7), a reactor and separator, with a recycle stream, pinch technology has something to offer. In this case, a pinched design (right) reduces steam consumption by 38%, eliminating the need for external water cooling, cutting the number of heat exchangers needed from six to four, and reducing heat transfer surface area requirements from 629 to 533 m2.

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Reactors

Unpinched

Steam

Steam

Recycle

Separator

Feed

Cooling mater

Product

Reactors

Pinched process

Steam

Recycle

Separator

Feed Product

Figure.6 Figure.7

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RETROFIT PROJECT

The pinch technology principle is applied on various projects. One such project, Fig. 8, consists of a complex refining system. The process is already highly integrated, with various streams being heat-exchanged to reduce overall energy requirements. Application of pinch technology in this project results a minimum energy target of about 31 MW for the hot utility, which is steam. The original process consumed nearly 39 MW of steam. Thus, the “scope” for improvement is 8 MW or 20% of original demand. A brief analysis of the existing flow sheet uncovered the specific reasons for current utility requirements being greater than the target:1. One instance of utility heating below the pinch.2. One instance of utility cooling above the pinch.These violations totaled 8MW. If eliminated, they would bring the utility requirements to target. The problem was, it would take six new exchangers and a substantial new investment to accomplish this.

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Steam

Reactor

Recycle

3 1 2 4 5

17,000 lb/hr Steam

70psi

Feed and recycle Recycle

Flash Stripper

Preheater

20 psi

Fig. 8 Process prior to retrofit.

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At this stage, one should always look for “process modifications” that included the following:

1. Decrease the pressure of column No. 2 by 5 psi (34 kPa).2. Decrease the pressure of column No. 5 by 10 psi (69 kPa).

The effect of these modifications resulted the energy target for the process so modified was 27 MW representing another 15 % potential savings. The problem was, it would still take six new exchangers and a substantial new investment to accomplish this.At this stage, one should look for “second order” process modification aimed at simplifying the necessary hardware changes. Modify the process slightly so that its enthalpy changes fit as many of the existing exchangers as possible? These major adjustments led to the sacrifice of 1.3 MW but helped to save four exchangers reducing the number of new exchangers from six to two.The flow sheet for the process finally recommended is shown in Fig. 9. Process modifications and two new exchangers combined give 28 % energy savings at six months payback.

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Steam

Reactor

3 1 2 4 5

11,000 lb/hr Steam

60psi

Feed and recycles

Recycle

Flash

Stripper Preheater

15 psi

Fig. 9 The process after retrofit.

New exchangers

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CONCLUSIONS

With all of the tools that pinch analysis provides, one of the most important challenges before is to properly integrate pinch tools into the conceptual process design phase. Decisions made in the phase of planning affect the entire life cycle of a process facility. Using pinch technology tools & understanding the process doesn’t ensure the desired results. These tools must be applied at the right point in the process design phase. Just as it be incorrect to conduct a pinch analysis after completion of the process design phase, wherein critical process parameters have been fixed, it is just as incorrect to conduct a pinch analysis without a direct interaction with the process specialists & downstream engineering disciplines. It is Pinch Technology’s role to identify “what might be”. However, input from other engineering disciplines ultimately determines “what can be”.

 

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REFERENCES

1. Plant Design And Economics For Chemical EngineersMax S Peter,Ronal E West 5th Edition,McGraw Hill

2. Richardson & Colson Vol.6

3. Genaral Process Improvement Through Pinch Technology B.linnoff , G.T pollen


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