Lecture 4 - Simulation of Recycle Streams

(1)

Simulation of

(2)

Outline



Sequential modular approach for

simulating a recycle system



Tips for converging recycle loops



Recycle systems modelling with

HYSYS



Some notes for Recycle model

(3)

H82CYS - Computer System Simulation of Recycle Streams 3

The Onion model

Reactor

Separation &

recycle

Heat exchange

network

Utilities

_{(Linnhoff et al., 1982;}

(4)

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

(5)

Types of recycle

streams

Material

recycle

Heat

recycle

_Tube

Shell

(6)

Sequential modular

(SM) approach

Individual equipment blocks may

require iterative solution algorithms

Overall process solution is sequential & not

iterative

(Turton et al.,

1998)

(7)

Simulation of recycling

system with SM

A

B

C

D

E

F

Recycle stream

Unit operation

in simulator

Tear

recycle stream

r

₁

r

₂

(Turton et al., 1998)

Guess a number for r1

Calculate r2

r1 and r2 must be the same!

If not, try with another value

again!!

“Tear the recycle stream

into two”

(8)

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 r

₂

, we can solve the flowsheet all

the way to Stream r

₁

by using sequential

modular approach.



Compare Streams r

₁

and r

₂

.



If r

₁

& r

₂

agree within some specified

tolerance  we have a converge solution



Or else, r

₂

is modified & simulation is

(9)

Modelling of

recycle system

(10)

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

(11)

Mass balance

equations



Given the following variables:



m

_{i ,j}

= molar flowrate of Component i in Stream j



X = reactor conversion (

given by question

)



r

_i

= fractional recovery of Component i



Mass balance equations

for each unit may be

written as:



Mixer:



Reactor:



Separator:

•m

_A,2

= m

_A,1

+ m

_A,5

•m

_B,2

= m

_B,1

+ m

_B,5

•m

_A,3

= m

_A,2

(1 – X)

•m

_B,3

= m

_B,2

+ Xm

_A,2

•m

_A,4

= m

_A,3

(1 – r

_A

)

•m

_A,5

= r

_A

m

_A,3

•m

_B,4

= r

_B

m

_B,3

•m

_B,5

= m

_B,3

(1 – r

_B

)

(12)

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 stream are

compared to test whether errors are within a specified

(13)

Data given



Given the following values:



m

_A,1

= 100 kmol; m

_B,1

= 0 kmol



X = 0.7



r

_A

= 0.95; r

_B

= 0.95



Assume the flowrate of component A and B in the

recycled stream

(stream 5) as follow:



m

_A,5

= 50 kmol



m

_B,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)

value

Estimated

value

estimated

value

Calculated

residue

Scaled

=

For an accurate answer. As small as

possible!! Small difference between

calculated and guess value!!!

(14)

Recycle simulation

with spreadsheet

(15)

Time for exercise!

(16)

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



Let’s visit them one by one…

(17)

1. Analyse the

flowsheet

_{Determine if any}

(18)

1. Analyse the

flowsheet



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

?

(19)

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

(20)



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

(21)



In the 1

st

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

(22)

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.

(23)

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)

GOOD

BETTER

(24)

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.

(25)

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

(26)

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

(27)

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.

(28)

Simulation of

recycle system

with Aspen HYSYS

(29)

Tutorial 6 (from

Tutorial 3)

Let’s standardise

the specification

for key

components:

•Ethylene in

bottom:

0.0015

•n-octane in

distillate:

0.2800

(30)

Tutorial 6 (from

Tutorial 3)

Main

product

(n-octane)

This should

be recycled

to the

reactor

Unconverte

d raw

material

(31)

Adding 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?

(32)

Adjusting 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

Compresso

r

(33)

Adjusting for stream

temperature

95.6º

C

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

.

(34)

Add 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

(35)

Add 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

.

(36)

Simulation results

Product

streams

(37)

Working session

1. Add a splitter for recycle &

purge

2. Adjust the stream T & P

3. Add a recycle model to

(38)

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.

(Seider et al.,

2003)

(39)

Convergence 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

0.01 Temperature

0.01 C

Pressure

0.01 kPa

Flow

0.001 kmol/s

(relative tolerance)

Enthalpy

1.00 kJ/s

Composition

0.0001

Entropy

0.01

(40)

Nested vs.

simultaneous options



Nested

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

(41)

Common convergence

methods

Direct

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}

(42)

Wegstein 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 line

G(x

_k

) & G(x

_k-1

) = calculated values

for iteration k & k-1; x

_k

& x

_k-1

=

estimated values for iteration k &

k-1.



The intersection is required with the equation: G(x

k-1

) = x

k-1



Substitute & rearrange the equations yield:



Substitute Q = a/(a – 1) gives:

x

_k-1

= Qx

_k

+ (1 – Q) G(x

_k

)

( ) ( )

1 1 − −

−

=

k k k k

x

G

x

G

( )

k

G

x

a

x

a

x

1

1 −

−

=

−

(Smith, 2005)

(43)

Wegstein 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

(44)

Wegstein acceleration



HYSYS determines the actual

acceleration (Q) to apply based on the

amount of change between successive

iterations. The values for Q

_max

& Q

_min

set

bounds on the amount of acceleration

applied.



Tips: If the recycle is oscillating, a

slightly larger value for Q

_max

can be

(45)

Example from Tutorial

5 

If Wegstein method is applied after 2

iterations:

(

1 ) ( )

0 .

3986

(

42 .

75 ) (

1

0 .

3986

)(

40 .

6838

)

39.8602

kmol

3986

.

0

1 =

+

−

=

−

+

−

=

−

k k

Q

G

x

Qx

a

___

__________

_________

slope

1

=

+ k

x

Q

a

42.507500−42−.407500.6838

=

0 .

2850

(46)

Simulation of heat

exchanger

network

(47)

The Onion model

Reactor

Separation &

recycle

Heat exchange

network

Utilities

_{(Linnhoff et al., 1982;}

(48)

Tutorial 7 (from

Tutorial 6)

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

(49)

Heat 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

(50)

The final heat

integrated flowsheet

Simulation

starts from

here…

However,

both

streams are

unknown

!

Can we solve this without a Recycle convergence

unit?

(51)

Remember what we

have learnt before

A

B

C

D

E

F

Recycle stream

Unit operation

in simulator

Tear

recycle stream

r

₁

r

₂

(52)

Tear 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

(53)

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

(54)

Final flowsheet

1. Remove the imaginary

inlet stream of the heater.

2. Connect the shell outlet of

the heat exchanger to the

heater.

(55)

Time for exercise!

(56)

Tutorial 8: flash separator

(self learning)

(57)

Tutorial 8: flash separator

(self learning)



Consider the flash separation process

in the figure, with 3 simulation cases

(different % bottom).



Thermodynamic model in HYSYS: SRK



Tasks:



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.