Lecture 4 - Simulation of Recycle Streams

Simulation of Recycle Streams

Simulation of Recycle Streams

H82CYS - Computer System Simulation of Recycle Streams 2

OutlineOutlineSequential modular approach for

simulating a recycle systemTips for converging recycle loopsRecycle systems modelling with

HYSYSSome notes for Recycle modelHeat exchanger network modelling


The Onion modelThe Onion model

Reactor

Separation & recycle

Heat exchange network

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


IntroductionIntroductionReasons 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 variableCirculation of a working fluid

Recycling is often the cause of unconverged flowsheet simulation.


Types of recycle streamsTypes of recycle streamsMaterial recycle

Heat recycle Tube

Shell

Cold inlet Hot outlet


Sequential modular (SM) approach Sequential modular (SM) approach

Individual equipment blocks may require iterative solution algorithms

Overall process solution is sequential & not iterative

(Turton et al., 1998)


Simulation of recycling system with SMSimulation of recycling system with SM

A B C D E F

Recycle stream

Unit operation in simulator

Tear recycle stream

r1 r2


Guess a number for r1Calculate r2

r1 and r2 must be the same!If not, try with another value

again!!

“Tear the recycle stream into two”


Simulation of recycling system with SMSimulation of recycling system with SMBasic 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 solutionOr else, r2 is modified & simulation is

repeated until convergence is obtained.

Modelling of Modelling of recycle systemrecycle system


Tutorial 5 – isomerisation processTutorial 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 by the recovery of A to the recycled stream (rA) and recovery of B to the product (rB). (Smith, 2005)


Mass balance equationsMass 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: Mixer:

Reactor:

Separator:

•mA,2 = mA,1 + mA,5

•mB,2 = mB,1 + mB,5

•mA,3 = mA,2(1 – X) •mB,3 = mB,2 + XmA,2

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


Strategy with SM approachStrategy 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 stream are compared to test whether errors are within a specified tolerance.

(Smith, 2005)


Data givenData 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 than a specified tolerance (1 x 10-5 for this case). Scaled residue is given as:

(Smith, 2005)

valueEstimated

valueestimated- valueCalculatedresidue Scaled

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

R2 R1


Recycle simulation with spreadsheetRecycle simulation with spreadsheet

Time for exercise!Time for exercise!


Strategy to converge recycle loopsStrategy 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 flowsheet2. Provide estimates for recycle streams3. Simplify your flowsheet4. Avoid over-specifying mass balance5. Check for trapped material6. Increase number of iterations

Let’s visit them one by one…(WinSim, 2002)


1. Analyse the flowsheet

1. Analyse the flowsheetDetermine if any

recycle stream exist.


1. Analyse the flowsheet1. Analyse the flowsheet

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

through (from Units 16), 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?

Splitter


2. Provide estimates for recycle streams 2. Provide estimates for recycle streams Once recycle 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 2: Instead of estimating the recycle stream, we may also guess the reactor inlet stream.

Example 1

Example 2


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

3. Simplify the flowsheet 3. Simplify the flowsheet


In the 1st trial to determine if a process is 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.

3. Simplify the flowsheet 3. Simplify the flowsheet


4. Avoid over-specifying mass balance 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 make a lucky guess.


Which is the best option ?Which is the best option ?

Set the flowrate of the recycle stream (S9)Set the flow fraction of the recycle stream

(S9)Set the flow fraction of the product stream

(S8)

GOODBETTER

BEST


4. Avoid over-specifying mass balance4. 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.


5. Check for trapped material 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.

GAS PHASE


5. Check for trapped material5. Check for trapped materialWhen 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 conditions

or change the location of 1 or more product draw-offs.


6. Too few iterations6. Too few iterationsMany 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.

Simulation of Simulation of recycle system recycle system

with Aspen HYSYSwith Aspen HYSYS


Tutorial 6 (from Tutorial 3)Tutorial 6 (from Tutorial 3)

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



Main product (n-

octane)

This should be recycled

to the reactor

Unconverted raw

material


Adding recycle & purge streamsAdding recycle & purge streams

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”.

Stream splitter

model: Tee

90% recovery

Question: why a purge is needed?


Adjusting the stream pressureAdjusting the stream pressure

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 & “Q-103” for energy streams.4.Double click stream 8 & specify the outlet pressure as 20 psia.

20 psia

15 psia

Compressor


Adjusting for stream temperatureAdjusting for stream temperature

95.6ºC

93ºC

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.

Cooler


Add a recycle unitAdd a recycle unit

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.

Recycle unit – this serves as the

convergence unit that was demonstrated in

the earlier tutorial


Add a Mixer to connect the recycleAdd a Mixer to connect the recycle

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.

Mixer

Double click the Reactor, select Streams “11” for inlet.


Simulation resultsSimulation results

Product streams

Working sessionWorking session1. Add a splitter for recycle &

purge2. Adjust the stream T & P3. Add a recycle model to

connect the recycle stream


Some notes about Recycle modelSome notes about Recycle modelMost 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.

(Seider et al., 2003)


Convergence setting in Recycle modelConvergence setting in Recycle model

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 <10 are more stringent; i.e., the smaller the multiplier,

the tighter the convergence tolerance.

Variables Internal tolerance

Vapour Fraction 0.01Temperature 0.01 CPressure 0.01 kPa Flow 0.001 kmol/s

(relative tolerance)Enthalpy 1.00 kJ/sComposition 0.0001Entropy 0.01


Nested vs. simultaneous optionsNested vs. simultaneous optionsNested option

(default): Recycle being called

whenever it is encountered during the calculations.

Use when there is single recycle operation, or multiple recycles which are not connected.

Simultaneous option: All recycles to be invoked at the same time

once all recycle streams have been calculated. Use when there are multiple inter-connected

recycles.


Common convergence methodsCommon convergence methodsDirect

substitution (approach used in Tutorial 5)

Wegstein method

All recycle convergence in simulators implement direct substitution & Wegstein methods.

Direct substitution – an initial value is estimated, the calculated value then becomes the value for next iteration.

Wegstein method accelerates the convergence of iteration.


Wegstein accelerationWegstein acceleration The direct substitution iterations are

linearised. A straight line equation is written for

2 iterations:G(x) = ax + b

where a = slope of the lineG(xk) & G(xk-1) = calculated values for iteration k & k-1; xk & xk-1 = estimated values for iteration k & k-1.

The intersection is required with the equation: G(xk-1) = xk-1

Substitute & rearrange the equations yield:

Substitute Q = a/(a – 1) gives: xk-1 = Qxk + (1 – Q) G(xk)

1

1

kk

kk

xx

xGxG

kkk xGa

xa

ax

1

1

11

(Smith, 2005)


Wegstein accelerationWegstein acceleration Significant of Q:

Q = 0, direct substitution is used.

Q < 0, acceleration is used

0 < Q < 1, damping occurs.

Typically, Q is bound between -20 & 0 to ensure stability & reasonable rate of convergence.

Other acceleration methods may be used when equations being solved are highly non-linear & inter-dependent, e.g. dominant-eigencvalue, Newton-Raphson, Broyden’s quasi-Newton methods.



Wegstein accelerationWegstein accelerationHYSYS determines the actual

acceleration (Q) to apply based on the amount of change between successive iterations. The values for Qmax & Qmin set bounds on the amount of acceleration applied.

Tips: If the recycle is oscillating, a slightly larger value for Qmax can be used to damp the direct replacement.


Example from Tutorial 5Example from Tutorial 5

If Wegstein method is applied after 2 iterations:

kmol 39.86026838.403986.0175.423986.01

3986.01

kk xGQQx

a

a

_____________

_________

_________slope

1

kx

Q

a 2850.07500.4250

6838.407500.42

Simulation of heat Simulation of heat exchanger exchanger networknetwork


The Onion modelThe Onion model

Reactor

Separation & recycle

Heat exchange network

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



1. Let’s standardise the specification for key components: •Ethylene in bottom: 0.0015•n-octane in distillate: 0.3500

2. Set the inlet stream temp to 30ºC. 3. Disconnect the stream from the mixer (right click & select Break Connection).


Heat recovery potentialHeat recovery potential

2. Add a Heater & rotate it by 90º.

3. Connect Stream 1 & energy stream Q-105 to the heater. Connect its outlet to the mixer.

4. Set the heater outlet temp to 93ºC & P to 0.

5. Observe the heat load needed.

Heater

5. Heat removed from the cooler (~27 MJ/h) can be matched to the energy needed by the heater (~131 MJ/h).

1.Move the fresh feed stream here


The final heat integrated flowsheetThe final heat integrated flowsheet

Simulation starts from

here…

However, both streams are unknown!

Can we solve this without a Recycle convergence unit?


Remember what we have learnt beforeRemember what we have learnt before

A B C D E F

Recycle stream

Unit operation in simulator

Tear recycle stream

r1 r2



Tear streamTear stream

1. Delete the cooler (E-100) & its energy stream. Replace it

with a Heat Exchanger (rotate it by choosing “Mirror

about Y”) & reconnect the recycle stream to the tube

side. Set P = 0 for both shell & tube sides.

2. Disconnect raw material stream from the heater.

Connect it to the shell side of the heat exchanger (add an

outlet stream too).

3. Add a new imaginary inlet stream

to the heater.

Heat exchang

er


Tear stream Tear stream

Specify the imaginary stream to match the specification of

Stream 1 via “Define from Other Stream”. Note: pressure & composition are more critical

than temp (due to the existance of the heater)

The imaginary stream is

exactly the same as the inlet stream


Final flowsheetFinal flowsheet

1. Remove the imaginary inlet stream of the heater.

2. Connect the shell outlet of the heat exchanger to the heater.

Time for exercise!Time for exercise!


Tutorial 8: flash separator (self learning)Tutorial 8: flash separator (self learning)



Tutorial 8: flash separator (self learning)Tutorial 8: flash separator (self learning)Consider the flash separation process

in the figure, with 3 simulation cases (different % bottom).

Thermodynamic model in HYSYS: SRKTasks:

Compare & discuss the flowrates & compositions for overhead stream by each of the 3 cases.

Modify Case 3 of to determine the flash temperature necessary to obtain 850 lb/hr of overhead vapor.

Documents

Lecture 4 - Simulation of Recycle Streams