unit operation in process simulators

Unit Operation in Process Simulators

Maurizio [email protected]

Department of Engineering & Architecture

University of Trieste

mailto:[email protected]

Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 2

Agenda

Unit operations model types◼ Single unit operation

◼ User models

Heat exchangers◼ HEATX

◼ Energy analysis

◼ Utilities

◼ Heat curves

Rewiewing Results


Unit Operations Model Types

Mixers/SplittersSeparatorsExchangersColumnsReactorsPressure ChangersManipulatorsSolidsSolids SeparatorsUser ModelsBatch Models

Remember:the use of specific models is described in the Help!


Mixers/SplittersMixer – Stream Mixer: combines multiple streams into a single one. • Adiabatic. • Pressure can change. • Energy streams can be used

FSplit – Stream Splitter: separates one (or more) streams into a number of outlets. • Give flow or fraction for each outlet.• Composition does not change.

SSplit – SubStream Splitter: separates one (or more) streams into a number of outlets. • Give flow or fraction for each sub-stream

in the outlet.


Separators

Flash/ Decanter: • Give two operating conditions (e.g. T,P, Enthalpy, …) uses phase equilibrium

to calculate output streams. • The differences between them are the number of phases involved in each

system that can be assigned to specific streams

Sep & Sep2 – Component Separators: Separates one (or more) streams into a number of outlets. • Give flow or fraction for each

component in each outlet.


Heat Exchangers

Heater: one-sided heat exchanger (cooling as well as heating).• Specify the outlet conditions, e.g. T, Duty, P, …• Pressure Drop can be specified, using a negative

value in the pressure field.

MHeatX: • multi-stream heat exchanger. Exchange heat

between any number of streams.Used in Air separation, LNG processes, Olefins, …

HeatX: two-sided heat exchanger. Two options can be specified:• Simple (user specifies performance)• Rigorous (Shell&Tube, Air-cooled, Plate, Kettle

Reboiler,…)


Columns

• Other columns: Extract for L-L extraction,

• ScFrac & MultiFrac for multiple columns solved simultaneously, • PetroFrac for refinery columns, • Consep for conceptual design of distillation columns

DSTWU, Distl, RadFrac: two shortcut models and a rigorous model for representing, desing and rating a distillation column. • Used for distillation, absorption, stripping, extractive and

azeotropic distillation, reactive distillation. • The rate-based option is available for separations that are

known to be mass transfer and/or heat transfer rate limited


Reactors

REquil & RGibbs: assumes chemical equilibrium based on Gibbs Free Energy minimization. • With REquil reactions are specified, • With RGibbs they are not. • Rgibbs is often used for combustion and solids

reactions

RStoic: stoichiometric reactor. • Define the reactions and the molar extent or

fractional conversion

RYield: specify the outlet composition. • Outlet mass flow matches inlet one. • It is used for decomposition of coal


Reactors

Fluidbed: models fluidized bed unit operations where reactions involve solids.• Located on the Solids tab of the Model Palette

RCSTR, Rplug & RBatch: kinetic-based reactors where users give correlations of kinetic data.• RCSTR: Continuous Stirred Tank Reactor• RPlug: Plug Flow Reactor or Packed Bed

Reactor• RBatch: Batch Reactor


Pressure Changers

Pump: pumps and hydraulic turbines

Compr: compressors or Expanders using simple specifications or performance curves

MCompr: multi-stage compressor

Valve: valve uses either simple specification, or pick from list of manufacturer’s valves

Pipe or Pipeline: Multi-phase pipe model. • Pipe is a single segment, i.e. one diameter and one

elevation change. • Pipeline has multiple segments connected together.


Manipulators

Mult: multiply streams flow by a user supplied factor for flowsheet scale-up or scale-down

Dupl: copy a stream to any number of outlet streams with the same flowrate

CIChng: link section or blocks that use different stream classes

Analyzer: report requested stream properties to be used in EO mode

Selector: switch between different inlet streams to test different flowsheet scenarios


Manipulators

Measurement: obtain a value of a variable for EO solution

Design-Spec: allows user to set the value of a calculated flowsheet quantity to a particular value by manipulating a specified input variable

Calculator: allows user to write equations in a Microsoft Excel spreadsheet or in Fortran syntax to be executed in Aspen Plus

Transfer: allows user to copy the values of flowsheet variables from one part of the flowsheet to another


Solids

• Solids Models: Models for solid-solid interactions

• Solids handling, comminution and classifications models• Solids formation: crystallization, granulation and agglomeration

• Solid separator Models: Fluid-solid separation models with different techniques


User Models

Users Models can be added to Aspen Plus using: ◼ An Excel spreadsheet

◼ Fortran Interface

◼ Cape Open Interface (C++, VB)

◼ User models from Aspen Custom Modeler

Hierarchy: ◼ The hierarchy user model allows the user to group part of the flowsheet

into a sub-flowsheet, aiming to simplify the visualization of the whole process.

Heat Exchangers


Heater Model

Mixes multiple inlet streams to produce a single outlet stream at a specified thermodynamic state

Can be used to represent heaters as well as coolers

May be used to set the thermodynamic conditions of a stream, e.g. as a transition between different property methods

Vapor Fraction of 1 means dew point condition, 0 means bubble point


HeatX Model

Performs a mass and heat balance for both hot and cold streams

Calculation and Type settings select the mode of calculation performed by the model:◼ Shortcut: specified outlet condition for one of the streams

Design: calculates the required area using fixed values for Heat Transfer Coefficient minimizing the economical expenditures

Rating: determines if specified area is over or under surfaced

Simulation: determines outlet condition based on given area or heat transfer coefficient, performing calculation of heat transfer

Maximum Fouling: determines the maximum fouling

◼ Rigorous: use Exchanger Design and Rating (EDR) to design a new or rate an existing heat exchanger

◼ A Utility can be used for the hot or cold side instead of a process stream


HeatX Shortcut Results


HeatX Model as Interface to EDR

Information related to the heat exchanger configuration and geometry is entered through the individual programs on EDR Browser form


Sizing using EDR Sizing Console

Obtain results using the shortcut method, then move to rigorous approach

Select Shell&Tube, Kettle Reboiler, Thermosyphon, Air Cooled, or Plate to open EDR window for a rigorous design


Layout of EDR Sizing Console


Activated Analysis Dashboard

Activated Exchanger Analysis is an Activated Analysis Tool ◼ located on the Activated Analysis Dashboard,

◼ with Economics Analysis Tool (for economic aspects) and Energy Analysis Tools for energy optimization and savings.

Activated Exchanger Analysis enables the user to use EDR within Aspen +

Heat Exchangers are designed for process constraints while minimizing capital costs


Activated Analysis Workflow


Activated Analysys Results

Model status reports information for rigorous exchangers on the flowsheet◼ as geometry, setting plan,

surface area, …

Exchange Summary Table provides a list of all heat exchangers in the flowsheet◼ including simple and rigorous

models.

◼ provides a list of risks related to each equipment


MHeatX Model

MHeatX can be used to represent heat transfer between multiple hot and cold streams, as in LNG Exchanger◼ Detailed, rigorous internal zone analysis can be performed to determine

pinch points

MHeatX uses multiple Heater blocks and heat streams to enhance flowsheet convergence◼ Two-stream heat exchangers also can be modeled using MHeatX. No

connection with EDR, therefore it is not able to estimate geometry


Heat Exchanger Convergence Tips

If you get a temperature crossover message, check that streams are connected to the correct ports (HOT to HOT PORT and COLD to COLD PORT)

Run HeatX model in shortcut mode to rule out flow rate and physical property errors

If there are flash failures, switch to rating with an outlet temperature specification; it is more stable

Use zone analysis to diagnose problems:◼ Go to Home and click on Report. Then select the exchanger and produce a

txt with zone info

Run the hot and cold stream trough the MHeatX block and plot a composite heating/cooling curve to check for pinches and temperature crossovers

Change to vapor-liquid-liquid phases if there is a possibility of two-liquid phases


Heat Streams (Inlet)

Any number of inlet heat streams can be specified for a Heater.

Heater uses the sum of the inlet heat streams

When a heat stream is an inlet to a block, you only need one thermodynamic specification (e.g. pressure or pressure drop)


Heat Streams (Outlet)

One heat stream can be connected as outlet

It represents the net heat load from a Heater, i.e. the sum of the inlet heat streams minus the actual (calculated) heat duty

Heat streams flow in the direction that information (not heat) flows

Q > 0: cooling duty («take heat out»)Q < 0: heating duty («give heat back»)


HXFlux Model

Used to describe the Convective or Radiant heat transfer across a surface:◼ For convective heat transfer, the standard equation is:

Q=UA*LMTD

Co-current or Counter-current operation

No inlet or outlet material streams◼ reference to stream temperature or EO variables

Heat Duty may be specified in inlet heat stream or as reference to a heat stream


Utilities

A Utility is an option in Aspen Plus that can be used to calculate:◼ Energy usage

◼ Utility usage

◼ Energy/utility cost

◼ Greenhouse gas generation (optional)

It is possible to assign a utility to any block where Duty or Power is either specified or calculated (except MHeatX)


Utilities

To calculate the required utility flow for a given process, no physical changes to flowsheet are necessary:◼ From the Block Specification form, select the Utility sheet

◼ Choose the <New> from the Utility ID dropdown list

◼ Enter a name for the Utility or select a predefined one

◼ Click the Next button to go to Utilities folder

◼ Choose the Utility type from eight selections provided

◼ For Utility cost calculations, enter either the Purchase price or Energy price

◼ Set the Calculation option as Specify heating/cooling value, or Specify inlet/outlet conditions (set values on State Variables form) and supply related parameters

◼ Choose to calculate CO2 emissions if appropriate on the Carbon Tracking sheet


Utilities Demo

Modify the simulation of Exercise settling tank (water – chlorobenzene):◼ FLASH3 unit with water, CLBZ, CO2, N2 at 1 atm)

◼ Change the temperature of the settling tank to 15 °C (from 25°C).

◼ Create a new utility to cool down the mixture in the settling tank:

◼ Cooling water @ atmospheric pressure: Inlet Temperature 5°C

Outlet Temperature 10°C

How much cooling water is needed? ◼ Report in Models of Results Summary

◼ Perform a sensitivity study vary: inlet T of utility cooling water (from 5 to 9)

Tabulate: water flow rate in kg/h


Heat Curves

Heat Curves are used by design programs, including EDR, to evaluate exchanger behavior

Many unit operation models in A+ are able to calculate Heat Curves

Tables can be generated for various independent variables (typically duty or temperature) for any property that A+ can generate

These tables can be printed, plotted, or exported


Heat Curves

With an increase of the Heat flux:

◼ Enthalpy increases

◼ Density decreases


Heat Exchanger Demo

Compare the simulation of a heat exchanger that uses water to cool of 10 °C a hydrocarbon mixture using three methods:

- Two heaters connected with a Heat stream

- A Heater using Utility

- A rigorous HeatX


Data

Streams:◼ Hydrocarbon stream: 200 °C, 4 bar, 10000 kg/hr

◼ 50% wt benzene, 20% styrene, 20% ethylbenzene, 10% water

Cooling water: 20 °C, 10 bar, 60000 kg/hr

Property methods:◼ NRTL-RK for process side

◼ STEAMNBS for water side

Unit operations◼ For Heaters:

Hydrocarbon outlet stream degrees subcooling 10 °C

Zero pressure drop

Use property set HXDESIGN

Show the temperature of the streams on the flowsheet


Procedure

HeatX Block:◼ First run as a Shortcut model with:

Hydrocarbon stream exit degrees subcooling 10 °C

No pressure drop in either stream

◼ Next, run as a Rigorous Design type using EDR Sizing Console for a Shell&Tube heat exchanger: What is the calculated actual heat transfer area?

What is the cost of the heat exchanger?

What Operational Risk Type produce a warning?

Utility:◼ Use the cooling water with following specifications:

Inlet conditions: 20°C, 10 bar; Outlet conditions: 35 °C, 10 bar

Price: 0.0001 $/kg

◼ How much cooling water is required?

Plot Heat curves of Vapor mass flow and T versus Duty


Reviewing results

History file or Control Panel Messages contains any generated errors or warnings

Report: printable text file oh input data and simulation results

Model summary grid is a concise view of unit operation variables, design specification data, utilities, etc.

Accessed from Home Ribbon Summary group or Results Summary folder in Navigation Pane

Key input operation data is condensed into a single form◼ Input variables can me modified

◼ Data is organized by unit operations model type


Exporting results

Data in Aspen Plus Model Summary can be exported to MS Excel in two formats:◼ Not linked one-time transfer

◼ Linked to Aspen Plus simulation using Aspen Plus Simulation Workbook


Exporting results


Custom Tables

Custom Table allows the construction of customized tables of any Aspen Plus inputs or outputs

Custom Tables can be placed on the flowsheet:◼ Full Table view

◼ Table Icon, useful for minimizing flowsheet space. Double-click on it to display the table

Click to create a Custom Table

Custom Tables can be exported to Excel (static data or linked data)

Add variables by:

Copy and Paste from input/output forms into Custom Tables

Drag and Drop from input/output forms into Custom Tables

unit operation in process simulators

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