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Two-Stage Compression 1

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Two-Stage Compression

2003 AspenTech. All Rights Reserved.EA1031.31.0507 Two-Stage Compression.pdf


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2 Two-Stage Compression

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WorkshopThis example demonstrates a typical application of the recycleoperation. Feed gas enters the compressor station at 35C (95F) and1725 kPa (250 psia). The gas is to be delivered at 6900 kPa, and it is to becompressed in two stages. Each stage consists of a knockout drum, acompressor, and a cooler. Liquids from each separator are recycled backto the previous stage, after the pressure has been reduced.

Learning ObjectivesOnce you have completed this section, you will be able to:

Use the recycle operation in HYSYS Choose suitable tear locations for recycles

PrerequisitesBefore beginning this module you need to know how to add streams andoperations.


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P r o c e s s

O v e r v

i e w


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Building the Simulation

Defining the Simulation BasisFor this case, you will be using the Peng Robinson EOS with thefollowing components: N2, CO2, C1, C2, C3, i-C 4, n-C 4, i-C 5, n-C 5, C6.

Starting the Simulation

Add a new Material stream with the following values:

In this cell... Enter...

Name To Compression

Temperature, C 35C (95F)

Pressure 1725 kPa (250 psia)

Molar Flow 5000 kgmole/h (11,000 lbmole/hr)

Component Mole Fraction

N2 0.0075

CO2 0.0147

C1 0.5069

C2 0.1451C3 0.0725

i-C4 0.0652

n-C4 0.0578

i-C5 0.0515

n-C5 0.0431

C6 0.0357


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Build the Simulation without the Recycle1. Install a Mixer with the following values:

2. Add a Separator with the information shown below:

3. Add a Compressor with the following values:


Connections

Name Mixer 1

Inlet To Compression

Outlet To LP Sep

Parameters

Pressure Assignment Equalize All


Connections

Name LP Sep

Inlets To LP Sep

Vapour Outlet LP Sep Vap

Liquid Outlet LP Sep Liq


Connections

Name Stage 1

Inlet LP Sep Vap

Outlet Stage 1 Out

Energy Stage 1 HP

Parameters

Adiabatic Efficiency 75% (default)

Worksheet

Stage 1 Out, Pressure 3450 kPa (500 psia)


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4. Install a Cooler with the values listed in the following table:

5. Add a Mixer operation to your case.

6. Install a Separator with the following information:

7. Add a Valve with the following information.


Connections

Name Stage 1 Cooler

Inlet Stage 1 Out

Outlet Cooler 1 Out

Energy Stage 1 Q

Parameters

Pressure Drop 0 kPa

Worksheet

Cooler 1 Out, Temperature 50C (125F)


Connections

Name Mixer 2

Inlet Cooler 1 Out

Outlet To IS Sep

Parameters

Pressure Assignment Equalize All


Connections

Name IS Sep

Inlet To IS Sep

Vapour Outlet IS Sep Vap

Liquid Outlet IS Sep Liq


ConnectionsName LetDown 1

Inlet IS Sep Liq

Outlet LD1 Out

Worksheet

LD1 Out, Pressure 1725 kPa (250 psia)


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8. Install a Compressor with the values shown below:

9. Install a Cooler operation to the case.

10. Add a Separator with the following values:


Connections

Name Stage 2

Inlet IS Sep Vap

Outlet Stage 2 Out

Energy Stage 2 HP

Parameters

Adiabatic Efficiency 75% (default)

Worksheet

Stage 2 Out, Pressure 6900 kPa (1000 psia)


Connections

Name Stage 2 Cooler

Inlet Stage 2 Out

Outlet Cooler 2 Out

Energy Stage 2 Q

Parameters

Pressure Drop 0 kPa

Worksheet

Cooler 2 Out, Temperature 50C (125F)


Connections

Name HP Sep

Inlet Cooler 2 Out

Vapour Outlet HP Sep Vap

Liquid Outlet HP Sep Liq


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11. Install a Valve with the following information:

The only operations missing at this point are the Recycles.

Installing the Recycles A recycle operation is a mathematical unit operation and is installed asany other. It has an inlet (calculated) stream and an outlet (assumed)stream. The operation is called/calculated whenever changes to theinlet stream fall outside of the converged tolerance.

The Recycle installs a theoretical block in the process stream. The feedinto the block is termed the calculated recycle stream, and the productis the assumed recycle stream. The following steps take place during theconvergence process

HYSYS uses the conditions of the assumed stream (outlet) andsolves the flowsheet up to the calculated stream (inlet).

HYSYS then compares the values of the calculated stream tothose in the assumed stream.

Based on the difference between the values, HYSYS modifies thevalues in the calculated stream and passes the modified values to

theassumed

stream.


Connections

Name LetDown 2

Inlet HP Sep Liq

Outlet LD2 Out

Worksheet

LD2 Out, Pressure 3450 kPa (500 psia)

Save your case!

In general, a Recycleoperation is required formaterial transfer and not forthermal recycles.

Always supply a guess orstarting point for the outletstream of the Recycle, neverthe inlet . A guess close to thesolution will result in a fasterconvergence time.


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The calculation process repeats until the values in the calculated stream match those in the assumed stream within specifiedtolerances.

Notice that both LetDown Valve outlets (also the Recycle Inlets) areknown. This will be a good starting point for the Assumed stream (orguess of the Recycle).

1. Add the first Recycle by double-clicking on the Recycle icon in theObject Palette.

Connections Page

2. Supply the Name, Feed and Product information as shown in thefigure:

Examine streams LD1 Out and LD2 Out. Can these be used as the guess forthe Recycle outlet?____________________________________________________

LD1 Out LD2 Out

Temperature

Pressure

Flow

Is CompositionKnown?

Figure 1

Recycle icon


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Parameters Tab

Tolerance Page

HYSYS allows you to set the convergence criteria or tolerance for each ofthe Recycle variables. In addition, the direction of transfer ofinformation can be set to Forwards or Backwards, or Not Transferred at

all. In general you will want to leave the transfer as Forwards. In thisexample, leave everything at the default.

Numerical Page

This page contains the options for the two types of Recycles, Nested andSimultaneous.

Nested . This type of recycle gets called whenever it isencountered during calculations. Use this type if you have asingle Recycle or if you have multiple recycles which are not

connected. Simultaneous . All recycles set at Simultaneous will be called at

the same time. Use this option if your Flowsheet has multipleinter-connected recycles.

In this case, we will use Nested Recycles.

Figure 2

The smaller the tolerancevalue, the tighter thetolerance. Generally it is agood idea to start with thedefault tolerance until youhave a converged solutionand then tighten thetolerance.


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Monitor Tab

This page displays convergence information as the calculations areperformed. Any variable that changes between iterations is displayed inthis table. In order to view plots of variables as they change during thecalculation of the recycle, you must select the variables to view on the

Setup page.

Worksheet Tab

The Recycle Worksheet page displays the Inlet and Outlet streaminformation. In this instance, notice that the Inlet and Outlet streamshave the same values. This is because before we installed the Recycle,the Inlet stream was already calculated by HYSYS. When the Recycle wasconnected, the known Inlet conditions were automatically passed to theOutlet stream to serve as the starting guess.

Figure 3


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3. Install the second Recycle with the following information:

The final step to solving the flowsheet is to connect the Recycle Outletsas Inlets to Mixer1 and Mixer2. Once this is done, the flowsheet willbegin solving.

4. Attach RCY 1 Out as a feed to Mixer 1 .

5. Attach RCY 2 Out as a feed to Mixer 2 .

Optional

Analyzing the ResultsIf you saved your case as a template, close the template and open thesaved case.

Examine the convergence process for the Recycles. Open the Recycleproperty view and look at the Monitor tab. How many iterations dideach Recycle need to converge.


Connections

Inlet LD2 Out

Outlet RCY 2 Out

Look at the Worksheet tab for each Recycle. How close are the Inlet andOutlet stream variables?______________________________________________

Are the vapour fractions identical for the Inlet andOutlet?______________________________________________________________

Save your case!

Convert your case to a templateand save!


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Advanced ModellingBecause the Recycle operation is a mathematical representation of aphysical process, its location in a simulation is a particularly importantone. The location of the tear stream can often determine success orfailure to converge a recycle.

Choose a Tear Location to Minimize the Number

Reducing the number of locations where the iterative process is required will save on the total convergence time. Choosing the location of theRecycle will depend on the Flowsheet topology. Attempt to choose apoint such that specifying the assumed stream will define as manystreams downstream as possible. It generally occurs downstream ofgathering points (mixers) and upstream of distribution points (tees,separators, and columns).

Choose a Tear Location to Minimize the NumberVariables

Variables include vapour fraction, temperature, pressure, flow, enthalpyand composition. Choose the tear stream so that as many variables aspossible are fixed, thus effectively eliminating them as variables andincreasing convergence stability. Good choices for these locations are atseparator inlets, compressor after cooler outlets and trim heater outlets.

RCY 1 RCY 2

Inlet Vf

Outlet Vf

If the vapour fractions are slightly different, tighten the compositiontolerance (change the tolerance from 10.0 to 1.0). Does this make anydifference?__________________________________________________________

A very poor choice of a tearstream is a stream with anAdjust operation controllingone of its variables.


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Choose a Stable Tear Location

The tear locations can be chosen such that fluctuations in the recyclestream have a minimal effect. For example, by placing the tear in a mainstream, instead of the physical recycle, the effect of fluctuations will bereduced. The importance of this factor depends on the convergencealgorithm. It is more significant when successive substitution is used.

Exploring the Simulation

Exercise 1: Adding a RecycleWhere should the Recycle be placed in this flowsheet and why?____________________________________________________________________________________________________________________________________________________________________________________________________________

Figure 4


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Assume that the Feed is fully defined, Shell and Tube Side pressuredrops are known, as well as the Column Feed temperature.

Assume the Feed is completely defined, shell and tube side pressure

drops for E-100 and E-101, and the temperatures of streams 3 and 4 areknown.

Where should the Recycle be placed in this flowsheet and why?____________

Figure 5

Where should the Recycle be placed in this flowsheet and why? _____

Figure 6


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Assume the Feed is completely defined, and the shell and tube sidepressure drop for E-100 are known.

Exercise 2: Compressor Outlet Pressure After finishing the simulation, your boss provides you with a set ofCompressor Curves for the Stage 2 Compressor which now exists in thefield. Will the existing Compressor be able to achieve the outlet pressure you want (i.e., 6900 kPa), or will you have to add a third stage ofcompression?

Where should the Recycle be placed in this flowsheet and why?_____

Figure 7

Flow, ACT [m3/min (ACFM)] Head [m (ft)] Polytropic Efficiency

30 (1000) 6000 (19670) 74

55 (2000) 5950 (19500) 7595 (3400) 5800 (19000) 75.25

140 (5000) 5500 (18000) 74.5

175 (6200) 4800 (15700) 72

205 (7300) 3360 (11000) 68


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Challenge You complain to your boss, Polly Sim that your simulation takes too longto converge on your computer because of the recycles and use thisopportunity to petition for a new high speed computer. Polly has a lookat your simulation and denies your request on the basis that you canreduce your convergence time by eliminating one of the recycles in thesimulation. Is your request justified or is Polly correct?


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