Lecture 4 - Simulation of Recycle Streams

Simulation of Recycle Streams

Outline  Sequential modular approach for

simulating a recycle system  Tips for converging recycle loops  Recycle systems modelling with HYSYS  Some notes for Recycle model  Heat exchanger network modelling

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The Onion model

Reactor Separation & recycle Heat exchange network Utilities H82CYS - Computer System

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(Linnhoff et al., 1982; Smith 1995, 2005) 3

Introduction

 Reasons why recycle stream(s) is

needed (Felder & Rousseau, 2000):  Unconsumed reactants can be reused to

minimise fresh intake (chemical reaction rarely proceeds to completion)  Catalyst recovery  Dilution of a process stream  Control of process variable  Circulation of a working fluid

 Recycling is often the cause of

unconverged flowsheet simulation. H82CYS - Computer System

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Types of recycle streams Material recycle

Heat recycle

Tube Cold inlet

Hot outlet

Shell

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Sequential modular (SM) approach Individual equipment blocks may require iterative solution algorithms

Overall process solution is sequential & not iterative (Turton et al., 1998)

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Simulation of recycling system with SM Guess a number for r1 Calculate r2 r1 and r2 must be the same! If not, try with another value again!! r

“Tear the recycle stream into two”

Tear recycle stream r2

1

Recycle stream

A

B

C

D

E

F

Unit operation in simulator (Turton et al., 1998) H82CYS - Computer System

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Simulation of recycling system with SM  Basic algorithms in handling a recycle stream:

 Before the Equipment C is solved, some

estimation of stream r must be made  a “tear stream” occurs.  Provided information is supplied about Stream r2, we can solve the flowsheet all the way to Stream r1 by using sequential modular approach.  Compare Streams r1 and r2.  If r1 & r2 agree within some specified tolerance  we have a converge solution  Or else, r2 is modified & simulation is repeated until convergence is obtained. H82CYS - Computer System

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Modelling of recycle system

Tutorial 5 – isomerisation process

 In an isomerisation process, component A is

converted to component B. No by product is formed.  The mixture from the reactor is separated into relatively pure A (which is recycled) & relatively pure product B.  No by-products are formed and the reactor performance can be characterised by its conversion.  The performance of the separator is characterised (Smith, 2005) by- Computer the recovery of A to theof recycled H82CYS System Simulation Recycle Streams stream (rA) and 10

Mass balance equations  Given the following variables:  mi ,j = molar flowrate of Component i in Stream j  X = reactor conversion (given by question)  ri = fractional recovery of Component i

 Mass balance equations for each unit may be

written as:

•mA,2 = mA,1 + mA,5  Mixer: •mB,2 = mB,1 + mB,5 •m = mA,2(1 – X) •mB,3 = mB,2 + XmA,2

A,3  Reactor:

•mA,4 = mA,3(1 – rA)  Separator: •mA,5 = rAmA,3 •mB,4 = rBmB,3 •mB,5 = mB,3(1 – rB)

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Strategy with SM approach

 Calculation sequence in SM: .  However, problem is encountered at the mixer, as

the flowrate & composition of the recycle are unknown.  Strategy using SM approach:

 Tear the recycle streams  Add a recycle convergence unit/solver in the tear stream.  Estimate the component molar flowrates of the tear stream.

This allows the material balance in the reactor and separator to be solved, & provide the molar flowrates for the recycle stream.  The calculated and estimated values of the tear(Smith, stream 2005) are 12 H82CYS - compared Computer System to test whether Simulation of Recycle Streams errors are within a specified

Data given

 Given the following values:  mA,1 = 100 kmol; mB,1 = 0 kmol  X = 0.7  rA = 0.95; rB = 0.95  Assume the flowrate of component A and B in the

recycled stream (stream 5) as follow:  mA,5 = 50 kmol  mB,5 = 5 kmol

 Setting at the recycle convergence unit/solver –

iteration stops when the scaled residue is smaller -5 than a specified tolerance (1 x 10 for this case). R1 R2 Calculated value estimated value Scaled residue is given as:

Scaled residue =

For an accurate answer. As small as possible!! Small difference between calculated and guess value!!! H82CYS - Computer System

Estimated value

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(Smith, 2005) 13

Recycle simulation with spreadsheet

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Time for exercise!

Strategy to converge recycle loops  Few simple steps to converge recycle systems faster & easier regardless of the no of equipment modules and streams: 1. Analyse your flowsheet 2. Provide estimates for recycle streams 3. Simplify your flowsheet 4. Avoid over-specifying mass balance 5. Check for trapped material 6. Increase number of iterations (WinSim, 2002)

 Let’s visit them one by one… H82CYS - Computer System

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1. Analyse the flowsheet Determine if any recycle stream exist.

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1. Analyse the flowsheet

Splitter

 The feed stream’s condition is given.  If we calculate the flowsheet straight

through (from Units 16), which stream(s) do we need to specify in order to complete the calculation?  What if we change the calculation sequence to start with Unit 4?

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2. Provide estimates for recycle  Once recycle streams streams (or tear streams) are determined, enter estimates for its T, P, flowrate & composition for each recycle stream.  Example 1: Stream 3 has the same composition & flowrate as the feed stream. We should have a good guess for its T & P, since it is the outlet from a heat exchanger. Example 1  Example 2: Instead of estimating the recycle stream, we may also guess the reactor inlet stream.

Example 2 H82CYS - Computer System

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3. Simplify the flowsheet  Substitute Short Cut Distillation for

rigorous distillation columns.  If a rigorous distillation column is in the flowsheet, converge it as a stand-alone unit first.  Decouple heat recycle(s) – use utility exchanger to simplify the problem first

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3. Simplify the  flowsheet In the 1 trial to determine if a process is st

feasible, there is no need to include every valve, utility stream flowrates, etc.  A flash unit with recycle requires multiple iterations before it is solved  simplified to get the same answer with no recycle.

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4. Avoid over-specifying mass balance

 Stream splitting model is frequently

used to set the rate of a purge/recycle stream.  Example: setting a flowrate for Stream 8 may prevent the recycle from converging unless you happen to

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Which is the best option ?

 Set the flowrate of the recycle stream GOOD

(S9)  Set the flow fraction of the recycle stream (S9)  Set the flow fraction of the product stream (S8) H82CYS - Computer System Simulation of Recycle Streams

BETTER BEST

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4. Avoid over-specifying mass balance  In a distillation train, specifying product rate for each columns may be over constraining the overall mass balance for the flowsheet.

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5. Check for trapped material  Components in the

middle boiling range are building up in the system (does not exit the flowsheet).  In the example flowsheet, water is trapped.

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GAS PHASE

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5. Check for trapped material  When you have an unconverged recycle loop, check the material balance summary first to see which components have the largest error.  Which direction is the error – making more flow or less leaving the process than entering?  Review the recycle convergence history for the last few iterations:  Are the flowrates and errors oscillating?  Is there a steady increase/decrease of the

unconverged components?  It may be necessary to change process H82CYS - conditions Computer System Simulationthe of Recycle Streams or change location of 1 or more26

6. Too few iterations

 Many flowsheets will converge easily

within 5 to 10 iterations.  If you have a recycle loop, which is unconverged after 10 iterations but is approaching convergence, be sure to update the recycle stream guesses for T, P, flowrate and composition.

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Simulation of recycle system with Aspen HYSYS

Tutorial 6 (from Tutorial 3)

Let’s standardise the specification for key components: •Ethylene in bottom: 0.0015 •n-octane in distillate: 0.2800

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Tutorial 6 (from Tutorial 3) Unconverte d raw material Main product (noctane)

This should be recycled to the reactor

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Adding recycle & purge streams 90% recovery

Procedure: 1.Add a stream splitting model (Tee) 2.Right click Tee, select “Transform/ Rotate by 270” 3.Double click Tee, select Stream “4” for inlet; and enter “6” & “7” for outlet streams. 4.In the “Parameters” page, set 0.9 for the flow ratio of stream 6. 5.Change the direction of stream 6 by: right click/Transform/ Mirror about Y” 6.Save file as “Tutorial 5”. H82CYS - Computer System

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Question: why a purge is needed?

Stream splitter model: Tee

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Adjusting the stream pressure 15 psia

20 psia

Compresso r

Procedure: 1.Add a Compressor. 2.Change the direction of the Compressor: right click/Transform/ Mirror about Y” 3.Double click the Compressor, select Stream “6” for inlet; and enter “8” for outlet & “Q103” for energy streams. 4.Double click stream 8 & specify the outlet pressure as 20 psia. H82CYS - Computer System

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Adjusting for stream temperature 95.6º C

93ºC

Cooler Procedure: 1.Add a Cooler. 2.Change the direction of the Cooler: right click/Transform/ Rotate by 180” 3.Double click the Cooler, select Stream “8” for inlet; and enter “9” for outlet & “Q-104” for energy streams. 4.In Parameter page, set Delta P as 0. 5.Double click stream 9 & specify the outlet temperature as 93ºC.

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Add a recycle unit

Recycle unit – this serves as the convergence unit that was demonstrated in the earlier tutorial

Procedure: 1.Add a Recycle unit. 2.Change the direction of the Recycle: right click/Transform/ Rotate by 270” 3.Double click the Recycle, select Stream “9” for inlet; and enter “10” for outlet.

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Add a Mixer to connect the recycle Mixer

Double click the Reactor, select Streams “11” for inlet. Procedure: 1.Right click Stream 1 & choose “Break connection” 2.Add a Mixer. 3.Double click the Mixer, select Streams “10” & “1” for inlet; enter “11” for outlet. H82CYS - Computer System

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Simulation results

Product streams

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Working session 1. Add a splitter for recycle & purge 2. Adjust the stream T & P 3. Add a recycle model to connect the recycle stream

Some notes about Recycle model  Most simulators (e.g. Aspen Plus, ChemCad, DESIGN II, PRO/II) will not show the convergence unit in the flowsheet. However, the tear stream concept applies in all sequential modular softwares.  Exceptional for HYSYS, where recycle convergence unit(s) are positioned by the user and appear explicitly in the flowsheet. H82CYS - Computer System

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(Seider et al., 2003) 38

Convergence setting in Recycle model Variables

Vapour Fraction Temperature Pressure Flow Enthalpy Composition Entropy

Internal tolerance 0.01 0.01 C 0.01 kPa 0.001 kmol/s (relative tolerance) 1.00 kJ/s 0.0001 0.01

 The sensitivities values (that the users enter) serve as

a multiplier for HYSYS internal convergence tolerances (default setting).  Example: the internal tolerance for T is 0.01 and the default multiplier is 10  absolute tolerance used by the Recycle convergence algorithm = 0.01 x 10 = 0.1. Therefore, the assumed T and the calculated T must be within 0.1°C of each other if the Recycle is to converge.  A multiplier of 10 is recommended for most calculations.  Values i.e., the smaller the 39 H82CYS - Computer