Using Aspen HYSYS Dynamics With Columns

Jump Start: Using Aspen HYS HYSYS YS ® Dynamics with Columns A Brief Tutorial Tutorial (and supplement supplement to training training and online documentation) documentation) Nicholas Brownrigg, Product Marketing, Aspen Technology, Inc. Ajay Lakshmanan, Lakshmanan, Product Product Management, Aspen Technology Technology,, Inc. Zachary Peers, Product Management, Aspen Technology, Inc. Alex Rao, Product Product Management, Management, Aspen Technology Technology,, Inc.

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Jump Start: Using Aspen HYSYS ® Dynamics with Columns

Table of o f Contents Conte nts Introduction Introduc tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Preparing Prepa ring a Steady-State Steady-State Model Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Debutanizer Debutaniz er Column Specifics Specifics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Development Developm ent of a Control Control Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Preparation Prepa ration of Flowsheet Flowsheet for for Dynamic Simulation Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Implementing Implementin g and Sizing Control Control Valves Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Activating Activat ing Dynamic Specifications Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Column Equipment Equipment Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Adding and Specifying Specifying Controllers Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Strip Charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Execution of of Dynamic Simulation Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Implementing Implementin g Disturbances Disturbances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Additional Resourc Resources es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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CVb


Introduction Columns are an integral part of most processes. They are used to separate components in mixtures where the material exiting columns often have stringent purity and flow constraints to maintain. It is also important to maintain flow through columns to ensure safety. For these reasons—and more, control schemes are usually implemented on columns in order to ensure that variables such as temperature, pressure, and flow at critical points throughout the column remain constant. Control schemes also help to maintain product purity and flow, ensuring that acceptable materials exit the column. In order to obtain working simulation for a column in steady-state operation, Aspen HYSYS can be used. To obtain a simulation of a column with an implemented control scheme, Aspen HYSYS Dynamics should be utilized. Using both of these programs in concert provides a comprehensive summary of how a column will perform under varying plant conditions and perturbations to the column’s normal steady-state operation. This guide will begin with a brief walkthrough of the process for setting up a steady-state column model. The steps required towards developing and implementing a working control scheme, and studying column dynamic response using Aspen HYSYS Dynamics, will then be outlined. Four Aspen HYSYS files come compressed with this guide. The file “Debutanizer – SS Starter.hsc” is the steady-state simulation for the debutanizer column. “Debutanizer Solution – RefluxBoilup1 Control Case.hsc” is a dynamics-ready file. This guide will show the steps necessary to add the control equipment to the steady-state debutanizer file that is present in the LV-1 control case. In addition, two other alternative control scheme HYSYS files are included. These files are “Debutanizer Solution – RefluxBottoms Control Case.hsc” and “Debutanizer Solution – DistillateBoilup Control Case.hsc”. The Aspen HYSYS flowsheet for each of these files and a short description of the control schemes are included in the conclusion section of this guide. This document is not meant to be used as a stand-alone reference document. We recommend that a range of other resources be called upon to give the user a comprehensive view of how to use Aspen HYSYS Dynamics. These may include: • AspenTech support website (support.aspentech.com) – this website has a wealth of information on the use of AspenTech products and provides answers to frequently asked questions. • AspenTech courseware available in on-line and in-person versions • AspenTech business consultants This document will show how to prepare a column and analyze its response to varying conditions using Aspen HYSYS Dynamics. It assumes that the user has Aspen HYSYS V8.0 or higher installed on his or her computer and a functional process design completed, as well as a very basic knowledge of dynamic simulation using Aspen HYSYS Dynamics.

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Preparing a Steady-State Model In order to properly use Aspen HYSYS Dynamics, a working steady-state process simulation model must first be obtained in Aspen HYSYS. For more information about using Aspen HYSYS, please refer to the separate Aspen HYSYS Jump Start Guide available at www.aspentech.com/JumpStart_HYSYSV8/. For the purposes of this Jump Start Guide, a complete dynamic simulation of a column will be demonstrated utilizing a previously completed steady-state Aspen HYSYS process involving a debutanizing column. The process developed is shown in Figure 1.

Vent SS Specs

Cond Duty Butanes

Feed1

Feed2 Reb Duty Debutanizer C5+

Figure 1. Steady-State Process Simulation with Debutanizer Column

Debutanizer Column Specifics It is important to appropriately design and rate the column that is going to be the focal point of the dynamic simulation by double-clicking the column model block on the flowsheet. The parameters in Figure 2 were specified for the debutanizer, including 15 separation stages, a feed on stage 8, a condenser pressure of 13.12 barg, and a reboiler pressure of 13.47 barg.

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Figure 2. Column Design Parameters

Additional required column specifications of the reflux ratio, butane recovery from the condenser, and C5 exiting the reboiler can be made in the “Specs” window. For the particular debutanizer column in this guide, the butane recovery is 96.25% and the C5+ in the condenser is set at 2.5%, which makes the percentage of C5+ in the bottoms 97.5%. From these parameters, Aspen HYSYS calculates a reflux ratio of 3.697 and a molar reflux flow of 777.0 lbmole/hr. Figure 3 shows the “Specs” window and the setting up of the butane recovery in the condenser.


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Figure 3. Setting Column Specifications

Once a steady-state column has been solved in Aspen HYSYS, the user can then continue to develop a control scheme and add dynamic equipment to the flowsheet in order to begin a dynamic simulation using Aspen HYSYS Dynamics.

Development of a Control Scheme To develop a control scheme for the column, the column’s response to feed changes should be studied. Initially, for the simulation set up in Figure 1, Feed 1 has a flowrate of 18,000 lb/hr, while Feed 2 has a flowrate of 9,000 lb/hr. Using the “Column Profiles” window under the “Performance” tab for the column, it can be seen that the current feed flow scheme results in the stage parameters shown in Figure 4.


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Figure 4. Column Profile for Debutanizer

In order to develop an appropriate control scheme, the feed flow rates should be changed to discover which variables within the column vary most. The following table shows feed flow changes made to the column and the corresponding changes to column parameters that displayed the most variance with the feed change.

Feed1 Flow (lb/hr)

Feed2 Flow (lb/hr)

Tray 6 Temperature (°F)

Mass Fraction i-C5 in Butanes Stream

Condenser Duty (Btu/hr)

Reboiler Duty (Btu/hr)

18,000

9,000

218.1

.0210

6.563e6

5.631e6

9,000

18,000

213.1

.0193

6.860e6

7.113e6

0

27,000

210.9

.0206

7.171e6

8.653e6

27,000

0

231.5

.0402

6.331e6

4.252e6

Table 1. Changes in Column Performance with Feed Changes

The temperature on tray 6 in the debutanizer increased and decreased according to a respective increase or decrease of the flowrate of the Feed1 stream. Also, with an increase in the flowrate of Feed1, an increase in the i-C5 mass fraction and decrease of condenser and reboiler duty was observed. For these reasons, the control scheme described in the following section should be implemented.


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Preparation of Flowsheet for Dynamic Simulation (Note that the Dynamic Assistant can be used to guide the user in preparing a flowsheet for dynamic simulation. The Dynamic Assistant will suggest all the steps covered in this section.)

Implementing and Sizing Control Valves Dynamic simulation requires the proper equipment to be modeled on the flowsheet in order to work properly. The first pieces of equipment that should be added are valves. For the case being used in this guide, four valves will be necessary based on the control scheme identified. The valves should be connected to inlet streams Feed 1 and Feed 2 and outlet streams Butanes and C5+, as depicted below in Figure 5. All valves should have a pressure drop of 7 psig.

Vent SS Specs

To Feed1

To Feed2

VLV-100

Feed1

VLV-101

Feed2

Cond Duty

Butanes

VLV-102

Butane Product

Reb Duty C5+

Debutanizer

VLV-103

Liquid Product

Figure 5. Flowsheet with Valves Added

Valves 100 and 101 need to be sized. This is done by clicking on the “Rating” tab in the valve window and then clicking the “Size Valve” button in the bottom left of the window.


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Figure 6. Sizing a Valve

Activating Dynamic Specifications The next step in moving towards dynamic simulation is to activate the pressure specifications under the “Dynamics” tab for streams “To Feed1”, “To Feed2”, “Butane Product”, and “Liquid Product”, by checking the box shown in Figure 7.


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Figure 7. Activating Dynamic Parameters

In a similar fashion, check the flow specification box for the streams “Vent” and “Reflux”. “Reflux” is located within the column subflowsheet environment. Also ensure that no dynamic specifications are checked for streams “Feed1” and “Feed2”.

Column Equipment Sizing Next, the reboiler, condenser, and tray section must be given sizes. In order to define the reboiler and condenser volumes, open the column window and move to the “Rating” tab and click “Vessels” in the navigation pane, shown in Figure 8. Enter 530 ft3 for both the reboiler and condenser for the purposes of this guide.

Figure 8. Sizing Reboiler and Condenser


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To size the trays for the column, open the “Tray Section” window and then double click the named Tray/Packed Section, shown in Figure 9. This will open the sizing form for that tray section.

Figure 9. Sizing Tray Section

Enter a tray diameter of 4.5 ft, a tray spacing of 1.8 ft, a Weir height of 0.15 ft, and a Weir length of 4.0 ft to complete tray sizing.

Adding and Specifying Controllers Six controllers should be added to the flowsheet for process control. The process variables, output targets, and acceptable tuning parameters for each valve are listed in Table 2.


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Using Aspen HYSYS Dynamics With Columns

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