04_columndynamics

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Column Dynamics 1

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Column Dynamics

2004Aspen Technology. - All Rights Reserved.

Instructor:

Eng. Ahmed Deyab Fares

Mobile: 0127549943


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2 Column Dynamics

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WorkshopThe Column Dynamics module introduces you to the process for setting

up a distillation column with HYSYS Dynamics. Some of the concepts

you have learned in the previous modules are applied here. However the

distillation column is one of the most complex unit operations in

HYSYS. As such, it deserves special attention. Starting with the Feed

Heater Train, you will expand the Flowsheet by adding the

DePropanizer column.

Learning ObjectivesIn this module you will learn to:

Configure a distillation column to meet purity specifications

Prepare the distillation column for dynamic simulation analyses

Develop a control strategy

Test the model


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ProcessOve

rview


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ColumnOve

rview


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Column Dynamics 5

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Expanding the DePropanizer SimulationFlowsheet

Load the simulation case that you created and saved in the

Transitioning from Steady State to Dynamics module (FHT-Dyn1.hsc).

Click the Steady State Mode button to return to Steady State in order to

add a column.

Add a Distillation Column

Add a distillation column to the Flowsheet. Configure the distillation

column with the following information:

You may get a consistency error when you start the steady state solver.

You need to resolve the error before you continue.

In this cell... Enter...

Connections

Name DePropanizer

Number of Stages 24

Inlet Stream ColFeed

Inlet Stage 12

Condenser Type Full Reflux

Condenser Energy Stream Q-Cond

Overhead Vapour Outlet Propane

Reboiler Energy Stream Q-Reb

Bottoms Liquid Outlet C4+

Pressure Profile

Condenser Pressure 1925 kPa (280 psia)

Reboiler Pressure 2070 kPa (300 psia)

Condenser Pressure Drop 0 kPa (0 psi)Specification

Reflux Ratio 2

Distillation Column icon

HYSYS is equipped with adistillation column InputExpert option that allowsyou to have properlyconfigured the column. TheUse Input Expert option canbe accessed from the

Tools-Preferences-Simulationmenu


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6 Column Dynamics

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Adding Column Specifications1. Open the Column Property view to the Monitorpage on the Design

tab. Notice that two specifications are Active, Reflux Ratio andOverhead Vapour Rate. Make the Overhead Vapour Rate Inactive,and the Degrees of Freedom goes to 1.

2. On the Specs page of the Designtab add a new specification withthe following information and make it Active:

3. Click the Runbutton to converge the column.

The steady state model of the DePropanizer process is now complete.

All unit operations, material and energy streams are solved.

Save your case as Column_SS.hsc.


Column Specification Types Column Component Fraction

Name LightsStage Condenser

Flow Basis Mole Fraction

Phase Vapour

Spec Value 0.95

Components Ethane, Propane

Save your case!


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Column Dynamics 7

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Preparing for Dynamic Simulation Analyses

Equipment Sizing

On the Rating tab of the Column Property View, you can input the sizing

information for the Distillation Column. The information required to

size a distillation column is the Condenser Volume, the Reboiler Volume

and the Column Tray Section dimensions. Additionally, on the Rating

tab there exists the facility to size other equipment that is in the column

sub-flowsheet.

Vessels

Appropriate vessel sizing is important for dynamic simulation analyses.

The vessel hold-up will affect the system's transient response during

dynamic analyses as you move from one operating regime to the next.

In addition, the vessel size affects the pressure calculations that are

associated with the unit operation.

1. Input a Condenser Volume of 10 m

3

(350 ft

3

) and Reboiler Volumeof 20 m3(700 ft3).

Sizing the Column Tray Section

In steady state mode you are free to specify the column pressure profile

as you desire. In fact, we just specified a column pressure drop of 145

kPa (20 psi). In dynamics mode, the column pressure profile is

calculated by the hydraulic calculations on each stage. Thus the

calculated pressure drop on each tray section is a function of the tray

geometry (diameter, weir height, weir length and tray spacing).

The dynamic column pressure profile of each column in your Flowsheet

can be estimated with the Tray Sizing utility. The Tray Sizing utility

performs the hydraulic calculations on each stage of the column. If you

do not know the actual dimensions of your column tray section the tray

section geometry should be used to estimate the size of the tray section

before running dynamically.

If you do not know thedimensions of your processequipment, calculate thevessel size based on anappropriate residence time:

10 minutes is typicallya suitable residence for

liquid phase hold ups. 2 minutes is typically asuitable residence timefor vapour phase holdups.


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8 Column Dynamics

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Running the Tray Sizing Utility

To run the Tray Sizing utility, do the following:

1. From the Tools menu, select Utilitiesand from the AvailableUtilities view select Tray Sizingand click theAdd Utilitybutton.

2. Click the Select TSbutton and choose the tray section to size. In thiscase select the DePropanizer Flowsheet and Main TS as the object.Click the OKbutton.

3. Click theAdd Sectionbutton. This adds the selected tray section tothe utility, allowing the sizing calculations to be performed.

4. Move to the Performancetab and then select the Resultspage. Thehydraulic calculations based on the recommended tray geometryfor the column are displayed.

5. This page also displays the estimated Tray Section Pressure Drop.Notice the Tray Sizing utility estimates approximately a 10 kPa (1.4psi) pressure drop through the 24 stages of the column.

When we initially designed our column in Steady State mode, we

inputted a pressure drop of 145 kPa (20 psi) through the column. The

condenser pressure being specified as operating at 1925 kPa (280 psia)

and the reboiler pressure being specified at 2070 kPa (300 psia).

Additionally, the ColFeed pressure was calculated to be operating at

2000 kPa (290 psia).

Some columns may have multiple tray sections. In an effort to balancemodel rigour with ease of model development only 1 tray sectiongeometry is provided on the Rating tab. Averaged tray sectioninformation should be specified on this tab.

If different tray sections are required for more accurate modelling,multiple tray sections can be configured and linked together.

The HYSYS DynamicsAssistant is capable ofperforming the Tray Sizingautomatically. However, theprocess is given herebecause the Assistant isdeactivated.


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In dynamic mode, the pressure drop at each stage will be calculatedbased on the tray geometry. Moreover, the Condenser, Reboiler and

ColFeed pressures will be based on this dynamic pressure profile. If the

steady state pressure profile does not match the dynamic pressure

profile the column tray pressures and flow rates will oscillate until an

equilibrium pressure profile is established. As the column tries to adjust

to an equilibrium pressure profile, the column can possibly go unstable.

Thus a proper column pressure profile based on the hydraulic

calculations should be entered for the column before moving to

dynamic simulation analysis.

Size the Column Tray Section1. From the Resultspage on the Performancetab of the Tray Sizing

utility, fill in the appropriate tray dimensions in the table below.

2. Open the Column Property view (by double-clicking on thecolumn).

3. Enter the values shown in the table above into the appropriate fieldson the Tray Sectionspage of the Rating tab.

4. Click on the Parameterstab and select the Profilespage.

5. Change the pressure of the Condenser and Reboiler as follows:

Save your case as: ClmSize.hsc.


Rating - Tray Sections

Section Diameter

Weir Height

Weir Length

Tray Spacing

Tray Volume (calculated)

Condenser Pressure 1925 kPa (280 psia)

Reboiler Pressure 1945 kPa (283 psia)

If the Tray Sizing utilitydetermines more than onesection in the Column, thereare two options:

1. Use the weighted

average dimensions for

the dynamic tray

dimensions.

2. Configure the columnwith multiple tray

sections inside the

column sub-flowsheet

and supply the

dimensions as

appropriate. Save your case!


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10 Column Dynamics

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Making Pressure Flow and DynamicSpecifications

Refer to the Rules to Remember Outlined in the Transitioning from

Steady State to Dynamics module.

For your ease of reference, we have included the first seven rules here:

Dynamics Specifications

Boundary Streams Insert a valve on all boundary streams(feed/product streams) within theFlowsheet that are not connected toconductance devices (i.e., heatexchangers, coolers, heaters)

Pressure Specifications Place a pressure specification on allboundary streams (feed/product streams)within the Flowsheet.

Distillation Columns Distillation columns with condensersrequire an extra specification around thecondenser. Make a flow specification forthe reflux f low.

Valves Use the pressure/flow relationship asthe dynamic specification for a valve.

K value Use the overall K value as the dynamic

specification for coolers, heaters, andheat exchangers and LNG exchangers.

Pressure gradients Be sure to account for pressure gradientsthroughout the Flowsheet. Moreover, besure to specify reasonable pressuredrops/rises in the Flowsheet. Pressuredifferentials are the driving force for flowthrough the process Flowsheet.

Tray Sizing Use the tray sizing utility to estimate thecolumn geometry and pressure profile.

Dynamic specifications can only be modified when the integrator is stopped. Oncethe integrator is started the value of the dynamic specification can be changed (itsvalue appears in blue), but the choice of dynamic specification can not bechanged.


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Add Valves to the Boundary Feed and Product Streams1. In the expanded Flowsheet, two boundary streams have been added

and 1 boundary stream has been removed. The two new boundarystreams are Propane and C4+. The boundary stream that has beenremoved is ColFeed.

2. Remembering Rule one, add a valve to all boundary streams, so adda valve downstream of the Propane and C4+ streams in the ColumnSub-flowsheet.

3. Specify a pressure drop of 70 kPa (10 psi)and size the valves as youdid in Module 2.

4. Return to the Main Environment.

5. Connect the boundary subflowsheet streams Propane Out and C4+Out to the Main Flowsheet streams Propane and C4+, respectively.This is done on the Connectionspage of the Designtab of theColumn Property view.

Make the Appropriate Pressure-Flow Specifications

6. On theDynamicstab of streams Propane and C4+ select thePressure Specification by checking the checkbox. Again, make sureonly the Pressure Specification is Active. Propane and C4+ areboundary streams in the main Flowsheet. Remember to deactivatethe Pressure Specification of the ColFeed stream.

What Cvvalue does HYSYS calculate for the Propane valve? _____________

And for the C4+ valve? _______________________________________________


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7. On the Dynamicstab of the Reflux stream (Column Sub-flowsheet)make the Molar Flow specification Active.

Figure 1

Figure 2


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8. Run the Dynamics Assistant to see if there is something else thatneeds to be changed.

Save your case as: Clm-Specs.hsc.

Click the Dynamics Modeicon to enter dynamics, and start the

integrator as the model is now ready to run in dynamics. Run the

integrator for a few minutes of integrator time.

What recommendations does the Dynamics Assistant make?

____________________________________________________________________

Dynamic specifications can only be modified when the integrator isstopped. Once the integrator is started the value of the dynamicspecification can be changed (its value appears in blue), but the choiceof dynamic specification can not be changed.

Save your case!

Integrator Start icon


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14 Column Dynamics

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Adding Strip Charts and ControllersAdd the following Controllers to your case for Column Control:

1. Add the Column Controllers in the Column Environment. Installingthe controllers in the Column Environment eliminates the need forthe controllers to cross the Flowsheet boundary.

Pressure Control Overhead Condenser


Connections

Name Cond-PC

Process Variable Source Condenser, Vessel Pressure

Output Target Object C3 Valve

Parameters - Configuration

Action Direct

PV Minimum 1725 kPa (250 psia)

PV Maximum 2070 kPa (300 psia)

Mode Auto

Kc 2.0

TI 2.0 minutes

SP 1925 kPa (280 psia)

Condenser Level Control


Connections

Name Cond-LC

Process Variable Source Condenser, Liquid Percent Level

Output Target Object Reflux


Action Direct

PV Minimum 0%

PV Maximum 100%

Mode Auto

Kc 2.0

TI 10.0 minutes

SP 50%

Control Valves should besized with a maximum oftwice the Steady-Statevalue.

Dont forget to size thecontrol valve properly.


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Column Dynamics 15

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Save your case as: Clm-Control.hsc.

Reboiler Duty Control


Connections

Name Tray23-TC

Process Variable Source Main TS, Stage Temp., Stage 23

Output Target Object Q-Reb


Action Reverse

PV Minimum 100C (200F)

PV Maximum 200C (400F)

Mode Auto

Kc 2.0

TI 10.0 minutes

SP 140C (270F)

Reboiler Level Control


Connections

Name Reb-LC

Process Variable Source Reboiler, Liq Percent Level

Output Target Object C4+ Valve


Action Direct

PV Minimum 0%

PV Maximum 100%

Mode Auto

Kc 2.0

TI 10.0 minutes

SP 50%

Save your case!


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Exercise 1Create a Strip Chart named Column and enter the following variables:

ColFeed Mass Flow

Condenser Vessel Pressure

C4+ Mass Flow

Propane Mass Flow

Run the Integrator and watch the variables change over time.

Exercise 2

Change the Output Variable for the Condenser-LC to Cond-Q, from

Reflux. Make sure that you choose reasonable values for the maximum

and minimum cooling duty. Change the molar flow specification for the

Reflux stream to 1620 kgmole/h (3560 lbmole/hr).

Does the simulation reach a steady state if given enough time?

____________________________________________________________________

Does this control strategy provide better control over the De-Propanizer?

____________________________________________________________________

Can you think of another control strategy for the Condenser Liquid level?

____________________________________________________________________


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Exercise 3Change the NGL-Feed composition as follows:

Component Mole Fraction

Ethane 0.01

Propane 0.25

i-Butane 0.08

n-Butane 0.15

i-Pentane 0.07

n-Pentane 0.07

Hexane 0.37

How does this change affect the Propane concentration in the C4+ stream?

____________________________________________________________________

How long does it take for the change to occur?

____________________________________________________________________

Try changing the Tray23-TC set-point in order to return the Propaneconcentration back to its original level. Does the temperature have to be

lowered or raised? ___________________________________________________


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18 Column Dynamics

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