04_ColumnDynamics

Column Dynamics

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Column Dynamics

Instructor: Eng. Ahmed Deyab Fares Mobile: 0127549943

1 © 2004 Aspen Technology. Technology. - All Rights Reserved.

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Column Dynamics

Workshop The 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 Objectives In this module you will learn to: • • • •

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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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Column Dynamics

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Expanding the DePropanizer Simulation Flowsheet 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. You may get a consistency error when you start the steady state solver. You need to resolve the error before you continue.

Add a Distillation Column Distillation Column icon

 Add a distillation column to the Flowsheet. Configure the distillation column with the following information: In this cell...

Enter...

Connections

HYSYS is equipped with a distillation column Input Expert option that allows you to have properly configured the column. The Use Input Expert option can be accessed from the Tools-PreferencesSimulation menu

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

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Column Dynamics

Adding Column Specifications 1. Open the Column Property view to the Monitor page on the Design tab. Notice that two specifications are Active, Reflux Ratio and Overhead Vapour Rate. Make the Overhead Vapour Rate Inactive, and the Degrees of Freedom goes to 1. 2. On the Specs page of the Design tab add a new specification with the following information and make it Active: In this cell...

Enter...

Column Specification Types

Column Component Fraction

Name

Lights

Stage

Condenser

Flow Basis

Mole Fraction

Phase

Vapour

Spec Value

0.95

Components

Ethane, Propane

3. Click the Run button 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.

Save your case!

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Column Dynamics

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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 ft3) and Reboiler Volume of 20 m3 (700 ft3).

Sizing the Column Tray Section If you do not know the dimensions of your process equipment, calculate the vessel size based on an appropriate residence time: • 10 minutes is typically a suitable residence for liquid phase hold ups. • 2 minutes is typically a suitable residence time for vapour phase hold ups.

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 t o estimate the size of the tray section before running dynamically. 7

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Column Dynamics

Some columns may have multiple tray sections. In an effort to balance model rigour with ease of model development only 1 tray section geometry is provided on the Rating tab. Averaged tray section information 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.

Running the Tray Sizing Utility To run the Tray Sizing utility, do the following: The HYSYS Dynamics Assistant is capable of performing the Tray Sizing automatically. However, the process is given here because the Assistant is deactivated.

1. From the Tools menu, select Utilities and from the Available Utilities view select Tray Sizing and click the Add Utility  button. 2. Click the Select TS button and choose the tray section to size. In this case select the DePropanizer Flowsheet and Main TS as the object. Click the OK  button. 3. Click the Add Section button. This adds the selected tray section to the utility, allowing the sizing calculations to be performed. 4. Move to the Performance tab and then select the Results page. The hydraulic calculations based on the recommended tray geometr y for 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 (1.4 psi) 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).

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Column Dynamics

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In dynamic mode, the pressure drop at each stage will be calculated based 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 t o 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 Section 1. From the Results page on the Performance tab of the Tray Sizing utility, fill in the appropriate tray dimensions in the table below. In this cell...

Enter...

Rating - Tray Sections Section Diameter Weir Height Weir Length Tray Spacing Tray Volume (calculated)

2. Open the Column Property view (by double-clicking on the column). If the Tray Sizing utility determines more than one section in the Column, there are two options: 1. Use the weighted average dimensions for the dynamic tray dimensions. 2. Configure the column with multiple tray sections inside the column sub-flowsheet and supply the dimensions as appropriate.

3. Enter the values shown in the table above into the appropriate fields on the Tray Sections page of the Rating tab. 4. Click on the Parameters tab and select the Profiles page. 5. Change the pressure of the Condenser and Reboiler as follows: Condenser Pressure

1925 kPa (280 psia)

Reboiler Pressure

1945 kPa (283 psia)

Save your case as: ClmSize.hsc.

Save your case! 9

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Column Dynamics

Making Pressure Flow and Dynamic Specifications 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 the Flowsheet that are not connected to conductance devices (i.e., heat exchangers, coolers, heaters)

Pressure Specifications

Place a pressure specification on all boundary streams (feed/product streams) within the Flowsheet.

Distillation Columns

Distillation columns with condensers require an extra specification around the condenser. Make a flow specification for the reflux f low.

Valves

Use the “pressure/flow relationship” as the dynamic specification for a valve.

K value

Use the “overall K value” as the dynamic specification for coolers, heaters, and heat exchangers and LNG exchangers.

Pressure gradients

Be sure to account for pressure gradients throughout the Flowsheet. Moreover, be sure to specify reasonable pressure drops/rises in the Flowsheet. Pressure differentials are the driving force for flow through the process Flowsheet.

Tray Sizing

Use the tray sizing utility to estimate the column geometry and pressure profile.

Dynamic specifications can only be modified when the integrator is stopped. Once the integrator is started the value of the dynamic specification can be changed (its value appears in blue), but the choice of dynamic specification can not b e changed.

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Column Dynamics

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Add Valves to the Boundary Feed and Product Streams 1. In the expanded Flowsheet, two boundary streams have been added and 1 boundary stream has been removed. The two new boundary streams are Propane and C4+. The boundary stream that has been removed is ColFeed. 2. Remembering Rule one, add a valve to all boundary streams, so add a valve downstream of the Propane and C4+ streams in the Column Sub-flowsheet. 3. Specify a pressure drop of 70 kPa (10 psi) and size the valves as you did in Module 2. What C v  value does HYSYS calculate for the Propane valve? _____________  And for the C4+ valve? _______________________________________________

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 Connections page of the Design tab of the Column Property view.

Make the Appropriate Pressure-Flow Specifications 6. On the Dynamics tab of streams Propane and C4+ select the Pressure Specification by checking the checkbox. Again, make sure only the Pressure Specification is Active. Propane and C4+ are boundary streams in the main Flowsheet. Remember to deactivate the Pressure Specification of the ColFeed stream.

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Column Dynamics

 Figure 1

7. On the Dynamics tab of the Reflux stream (Column Sub-flowsheet) make the Molar Flow specification Active.  Figure 2

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Column Dynamics

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8. Run the Dynamics Assistant to see if there is something else that needs to be changed. What recommendations does the Dynamics Assistant make?  ____________________________________________________________________

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

Save your case as: Clm-Specs.hsc.

Save your case! Click the Dynamics Mode icon 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. Integrator Start icon

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Column Dynamics

Adding Strip Charts and Controllers  Add the following Controllers to your case for Column Control: 1. Add the Column Controllers in the Column Environment. Installing the controllers in the Column Environment eliminates the need for the controllers to cross the Flowsheet boundary. Pressure Control Overhead Condenser In this cell... Control Valves should be sized with a maximum of twice the Steady-State value.

Enter...

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

In this cell...

Enter...

Connections Don’t forget to size the control valve properly.

Name

Cond-LC

Process Variable Source

Condenser, Liquid Percent Level

Output Target Object

Reflux

Parameters - Configuration Action

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Direct

PV Minimum

0%

PV Maximum

100%

Mode

Auto

Kc

2.0

TI

10.0 minutes

SP

50%

Column Dynamics

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Reboiler Duty Control In this cell...

Enter...

Connections Name

Tray23-TC

Process Variable Source

Main TS, Stage Temp., Stage 23

Output Target Object

Q-Reb

Parameters - Configuration Action

Reverse

PV Minimum

100°C (200°F)

PV Maximum

200°C (400°F)

Mode

Auto

Kc

2.0

TI

10.0 minutes

SP

140°C (270°F) Reboiler Level Control

In this cell...

Enter...

Connections Name

Reb-LC

Process Variable Source

Reboiler, Liq Percent Level

Output Target Object

C4+ Valve

Parameters - Configuration Action

Direct

PV Minimum

0%

PV Maximum

100%

Mode

Auto

Kc

2.0

TI

10.0 minutes

SP

50%

Save your case as: Clm-Control.hsc.

Save your case!

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Column Dynamics

Exercise 1 Create 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. Does the simulation reach a steady state if given enough time?  ____________________________________________________________________

Exercise 2 Change the Output Variable for the Condenser-LC to Cond-Q, from Reflux. Make sure that you choose reasonable values for t he maximum and minimum cooling duty. Change the molar flow specification for the Reflux stream to 1620 kgmole/h (3560 lbmole/hr). 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 3 Change 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 Propane concentration back to its original level. Does the temperature have to be lowered or raised? ___________________________________________________

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Column Dynamics