2 Stage Compression

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2 Stage Compression 1

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2 Stage Compression

1999 AEA Technology plc - All Rights Reserved.

ADV-2 1.pdf


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2 2 Stage Compression

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WorkshopThis example demonstrates a typical application of the recycleoperation. Product gas from a glycol dehydration plant enters thecompressor station at 32.35 oC (90.2 oF) and 62.35 bar. The gas is to bedelivered at 100 bar, and it is to be compressed in two stages. Each stageconsists of a knockout drum, a compressor, and a cooler. Liquids fromeach separator are recycled back to the previous stage, after thepressure has been reduced.

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

Use the Recycle operation in HYSYS


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Process Overview


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Building the SimulationDefining the Simulation Basis

For this case, you will be using the Peng Robinson EOS with thefollowing components: N2, CO2, H2S, C1, C2, C3, i-C4, n-C 4, i-C5, n-C 5,C6, and H2O.

Starting the Simulation

Add a new Material stream with the following values:

In This Cell... Enter...

Name Gas from Glycol Plant

Temperature, C 32.35 oC (90.2 oF)

Pressure 1725 kPa (250 psia)

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

Component Mole Fraction

N2 0.0010

CO2 0.0284

H2S 0.0154

C1 0.8989

C2 0.0310

C3 0.0148

i-C4 0.0059

n-C4 0.0030

i-C5 0.0010

n-C5 0.0005C6 0.0000

H2O 0.0001


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Build the Simulation without theRecycle Operations.1. 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 Gas from Glycol Plant

Outlet To LP Sep

Parameters

Pressure Assignment Equalize All


Connections

Name LP Sep

Feed To LP Sep

Vapour Outlet LP Sep Vap

Liquid Outlet LP Sep Liq


Connections

Name Stage 1 Compressor

Inlet LP Sep Vap

Outlet Stage 1 Out

Energy Stage 1 HP

Parameters

Adiabatic Efficiency 75%


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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:


Connections

Name Stage 1 Cooler

Inlet Stage 1 Out

Outlet Cooler 1 Out

Energy Stage 1 Q

Parameters

Pressure Drop 70 kPa

Work Sheet

Cooler 1 Out Temperature 50C (125F)

Cooler 1 Out Pressure 80 bar


Connections

Name Mixer 2

Inlet Cooler 1 Out

Outlet To IS Sep

Parameters

Pressure Assignment Equalize All


Connections

Name IS Sep

Feed To IS Sep

Vapour Outlet IS Sep Vap

Liquid Outlet IS Sep Liq


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7. Add a Valve with the following information.

8. Install a Compressor with the values shown below:


Connections

Name LetDown 1

Inlet IS Sep Liq

Outlet LD1 Out

Work Sheet

LD1 Out Pressure 62.35 bar


Connections

Name Stage 2

Inlet IS Sep Vap

Outlet Stage 2 Out

Energy Stage 2 HP

Parameters

Adiabatic Efficiency 75%


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9. Install a Cooler operation to the case.

10. Add a Separator with the following values:

11. Install a Valve with the following information:


Connections

Name Stage 2 Cooler

Inlet Stage 2 Out

Outlet Cooler 2 Out

Energy Stage 2 Q

Parameters

Pressure Drop 0 kPa

Work Sheet

Cooler 2 Out Temperature 50C (125F)

Cooler 2 Out Pressure 100 bar


Connections

Name HP Sep

Feed Cooler 2 Out

Vapour Outlet HP Gas

Liquid Outlet HP Sep Liq


Connections

Name LetDown 2

Inlet HP Sep Liq

Outlet LD2 Out

Work Sheet

LD2 Out Pressure 80 bar


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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. In has an inlet (calculated) stream and an outlet (assumed)stream. The operation is called/calculated whenever changes to the

inlet 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 duringthe convergence process

HYSYS uses the conditions of the assumed stream (outlet)and solves 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 modifiesthe values in the calculated stream and passes the modifiedvalues to the assumed stream.

The calculation process repeats until the values in thecalculated stream match those in the assumed stream withinspecified tolerances.

Save your case!

In general, a Recycle operationis required for materialtransfer and not for thermalrecycles.

Always supply a guess orstarting point for the outletstream of the Recycle, neverthe inlet . A guess close to the

solution will result in a fasterconvergence time.


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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 clicking on the Recycle button in theObject Palette .

Connections Page

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

Recycle Button


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Parameters Tab

Tolerance Page

HYSYS allows you to set the convergence criteria or tolerance for eachof the Recycle variables. In this example, leave everything at the default.

The smaller the tolerance

value, the tighter thetolerance. Generally it is a good idea to start with thedefault tolerance until youhave a converged solution andthen tighten the tolerance.


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Numerical Page

This page contains the options for the two types of Recycles, Nestedand Simultaneous.

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 notconnected.

Simultaneous - all recycles set at Simultaneous will be calledat the same time. Use this option if your Flowsheet has multipleinter-connected recycles.

In this case, we will use Nested Recycles.

Monitor Tab

This page displays convergence information as the calculations areperformed. Any variable that changes between iterations is displayed inthis table.

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 was connected, the known Inlet conditions were automatically passedto the Outlet stream to serve as the starting guess.


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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:


Connections

Inlet LD2 Out

Outlet RCY 2 Out

Save your case!

Convert your case to a template

and save!


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Analysing 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 Convergence tab. How many iterationsdid each Recycle need to converge.

Look at the Worksheet tab for each Recycle. How close are the Inletand Outlet stream variables?

Are the vapour fractions identical for the Inlet and Outlet?

If the vapour fractions are slightly different, tighten thecomposition tolerance (change the tolerance from 10.0 to 1.0).Does this make any difference?

RCY 1 RCY 2

Inlet Vf

Outlet Vf


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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 theNumber of Recycles

Reducing the number of locations where the iterative process is

required will save on the total convergence time. Choosing the locationof the Recycle will depend on the Flowsheet topology. Attempt tochoose a point such that specifying the assumed stream will define asmany streams downstream as possible. It generally occurs downstreamof gathering points (mixers) and upstream of distribution points (tees,separators, and columns).

Choose a Tear Location to Minimize theNumber of Recycle Variables

Variables include vapour fraction, temperature, pressure, flow, enthalpyand composition. Choose the tear stream so that as many variables as

possible are fixed, thus effectively eliminating them as variables andincreasing convergence stability. Good choices for these locations areat separator inlets, compressor after cooler outlets and trim heateroutlets.

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 amain stream, instead of the physical recycle, the effect of f luctuations will be reduced. The importance of this factor depends on theconvergence algorithm. It is more significant when successive

substitution is used.

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


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Exploring with the SimulationExercise 1

A. Where should the Recycle be placed in this flowsheet and why? Assume that you know the following information:

Temperature and Vapour Fraction of "Cond Out". Pressure drop and Duty of "Chiller" operation. Pressure of "Chiller Out" stream. Pressure drop of "Condenser" Operation. The Mixer is set to "equalize all."


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Flowsheet 2 Where should the Recycle be placed in this flowsheet and why?

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

Flowsheet 3

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

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 are known.


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Flowsheet 4 Where should the Recycle be placed in this flowsheet and why?

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

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2 Stage Compression