Jump Start: Using Aspen HYSYS ® Dynamics with Columns A Brief Tutorial (and supplement supplement to training and online documentation) Ajay Lakshmanan, Product Product Management, Aspen Technology, Technology, Inc. Alex Rao, Product Management, Aspen Technology, Technology, Inc.
Jump Start: Using Aspen HYSYS ® Dynamics with Columns
Table of Contents Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Preparing a St Steady-State Mo Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 .2 Debutanizer Column Specifics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 .2 Development of of a Co Control Sc Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Preparation of Flowsheet for Dynamic Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Implementing and Sizing Control Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Activating Dynamic Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Column Equipment Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Adding and Specifying Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 .9 Strip Charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Execution of Dynamic Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Implementing Disturbances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 12 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 15 Additional Re Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 17
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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
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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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
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 debutanizer 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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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
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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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
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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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
Figure 4. Column Profile for Debutanizer
In order to develop an appropriate control scheme, the differentials in temperature between stages were studied under the column profiles. Stage 5 through stage 11 all have high temperature differentials. For the purposes of this guide, stage 6 was chosen for implementation of the control scheme described in the following section.
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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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
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 VLV-100, VLV-101, VLV-102 and VLV-103 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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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
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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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
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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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
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. After configuring all of the controllers based on the table below, be sure to switch the controller action from manual to auto.
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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
Controller
Process Variable Source
Output Target Object
Tuning Parameters
Action
Range
Feed1 FIC
Feed1 Mass Flow
VLV-100 Actuator Desired Position
Kc = 0.5 Ti = 1.0
Reverse
0 lb/hr 30,000lb/hr
Feed2 FIC
Feed2 Mass Flow
VLV-101 Actuator Desired Position
Kc = 0.5 Ti = 1.0
Reverse
0 lb/hr 30,000lb/hr
Cond PC
Condenser Vessel Pressure
Condenser Duty Control Valve
Kc = 1.0 Ti = 2.0
Direct
180 psia - 220 psia
Cond LC
Condenser Liquid Volume Percent
VLV-102 Actuator Desired Position
Kc = 2.0 Ti = 5.0
Direct
0% - 100%
Column TC
Column Stage 6 Temperature
Reboiler Duty Control Valve
Kc = 2.0 Ti = 5.0
Reverse
210°F 260°F
Reboiler LC
Reboiler Liquid Volume Percent
VLV-103 Actuator Desired Position
Kc = 2.0 Ti = 5.0
Direct
0% - 100%
Table 2. Controller Connections and Variables Controlled
After implementing the control scheme from the above table, the flowsheet should then appear as Figure 10 displays below. For Cond PC and Column TC the control valves on the duty streams need energy ranges specified. To do this, open the form for the controller and then click on the button labeled “Control Valve...” at the bottom right of the form. For the condenser duty, choose a “Direct Q” duty source instead of “From Utility Fluid”. For both duty streams’ control valves, specify a minimum flow of 1 btu/hr and a maximum flow of 1*10 7 btu/hr.
Cond PC Column TC Cond LC Reboiler LC
SS Specs Vent Cond Duty
Feed1 FIC Butanes To Feed1
VLV-100
Feed1
VLV-102
Butane Product
Feed2 To Feed2
VLV-101
Reb Duty C5+
Debutanizer Feed2 FIC
Figure 10. Flowsheet with Controllers Implemented
10
VLV-103
Liquid Product
Jump Start: Using Aspen HYSYS ® Dynamics with Columns
Strip Charts Strip charts help users to view the results of dynamic simulation to disturbances. Four strip charts are automatically available under the “Dynamics” tab once the control scheme is implemented. These strip charts show the liquid percent level in the condenser and reboiler versus time, the two feed mass flows versus time, condenser pressure and column stage 6 temperature versus time, and the composition in the Butanes product stream versus time. These strip charts can be found by clicking the “Strip Charts” button under the “Dynamics” header in the ribbon, then selecting the desired graph, shown in Figure 11.
Figure 11. Opening Strip Charts
Execution of Dynamic Simulation After following the steps towards setting up a steady-state flowsheet for dynamic simulation, the dynamic simulation can be run. To accomplish this, click the “Dynamics” tab on the main ribbon in Aspen HYSYS, shown below. Alternatively, hitting F7 with Aspen HYSYS open will automatically enter dynamics mode. Once the “Dynamics” tab has been opened, activate Dynamics Mode by clicking the appropriate button, shown in Figure 12. Then, to run a dynamic simulation, either click the “Run” button, or press F9.
Figure 12. Navigating to the Dynamics Tab from the Main Ribbon and Running a Dynamic Simulation
If the steps in this guide are followed, the Dynamics Assistant will indicate that there are changes suggested before running the dynamic simulation. The suggested changes would revert some of the set up steps listed in this guide. Simply press “No” when prompted to run the dynamic simulation.
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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
Implementing Disturbances Some process modification suggestions to view dynamic response for the control scheme implemented include: • Change the feed flowrate • Change the feed composition • Change the temperature setpoints • Change the pressure setpoints • Change both temperature and pressure setpoints To change the composition or feed flowrates, once the dynamic simulation has been initialized, the feed stream’s definition worksheet can be opened by double clicking the appropriate stream. Then, the stream’s flow or composition can be modified. Additionally, flow controller setpoints can be modified to initiate disturbance in the simulation. To change the setpoints for either temperature or pressure, the controller’s “Parameters” tab can be used or the face plate for a controller can be opened by double clicking the appropriate controller and selecting the “Face Plate…” option, shown in Figure 13.
Figure 13. Changing the Setpoint and Opening a Face Plate for a Controller
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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
In the “Parameters” tab, the setpoint can be manually typed to the desired value. If the face plate is used, the setpoint can be modified by dragging the red arrow highlighted in Figure 14.
Figure 14. Face Plate with Highlighted Setpoint Control
To test the implemented control scheme, a dynamic simulation was run. After letting the process come to steady-state operation, the flowrate of the stream “To Feed1” was increased from 18,000 lb/hr to 28,000 lb/hr. The control response was evident in the Feed Flows strip chart, shown below in Figure 15.
Figure 15. Feed Flows Strip Chart from Dynamic Simulation of Debutanizer
The increased flow to 28,000 lb/hr to the column can be seen, as well as a small perturbation to the Feed 2 stream from the steady-state value of 9000 lb/hr.
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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
Additional strip charts showing the dynamic response for the simulation can be generated for the temperature on tray 6 of the column and condenser vessel pressure. This strip chart is shown in Figure 16.
Figure 16. Strip Chart Showing Tray 6 Temperature and Condenser Pressure
It can be observed that both the temperature and condenser pressure show fluctuations when the column feed experienced disturbance—before each parameter returned to its original value due to the control response. Figure 17 shows another strip chart for the liquid level percent present in the reboiler and condenser.
Figure 17. Strip Chart Showing Liquid Percent Level in Condenser and Reboiler
For this strip chart, neither the liquid level percent in the reboiler nor the condenser fully reaches its steady-state value of 50% before the feed flow disturbance is activated. However, upon control response, the liquid levels both move towards their steady-state values.
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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
Conclusion Dynamic simulation is a very powerful tool that allows users to view how processes will behave when deviations from steady-state operation occur. Aspen HYSYS Dynamics is the premier dynamic simulator, combining the simulation power of Aspen HYSYS with the ability to view rigorous dynamic process response. Aspen HYSYS Dynamics is especially effective for its use in viewing column dynamic response and exploring various control schemes to limit steady-state operation deviations in columns. The control scheme shown in this guide is a reflux-boilup control. Reflux-boilup control responds well to feed disturbances. The reflux flow rate controls the distillate composition while the heat input to the reboiler controls the bottoms composition. Other suggestions for control schemes to be implemented on the debutanizer column, as well as the scheme shown in this guide, are included in Table 3.
Control Configuration Name
Manipulated Variable for Condenser LC
Manipulated Variable for Reboiler LC
Manipulated Variable for Primary Composition Control (Stage 6 Temperature)
Manipulated Variable for Secondary Composition Control (Fixed in Base Case)
Manipulated Variable for Pressure Control
RefluxBoilup 1
Distillate Flow Rate
Bottoms Flow Rate
Reboiler Duty
Reflux Flow Rate
Condenser Duty
RefluxBoilup 2
Distillate Flow Rate
Bottoms Flow Rate
Reflux Flow Rate
Reboiler Duty
Condenser Duty
DistillateBoilup 1
Reflux Flow Rate
Bottoms Flow Rate
Reboiler Duty
Distillate Flow Rate
Condenser Duty
DistillateBoilup 2
Reflux Flow Rate
Bottoms Flow Rate
Distillate Flow Rate
Reboiler Duty
Condenser Duty
RefluxBottoms 1
Distillate Flow Rate
Reboiler Duty
Bottoms Flow Rate
Reflux Flow Rate
Condenser Duty
RefluxBottoms 2
Distillate Flow Rate
Reboiler Duty
Reflux Flow Rate
Bottoms Flow Rate
Condenser Duty
Table 3. Alternate Control Scheme Configurations
Figure 18 shows the flowsheet of Aspen HYSYS file “Debutanizer Solution – DistillateBoilup Control Case.hsc”, which is an example of Distillate-Boilup control. This Aspen HYSYS file was downloaded along with this guide and is available for examination and modification inside of Aspen HYSYS and Aspen HYSYS Dynamics.
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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
To Feed 2 P/F Specs Pressure
Pressure
Vent
Cond PC
198.5
psig
Cond LC
To Feed 1 P/F Specs Pressure
P/F Specs
Flow
Molar Flow
0.0
Column TC
Pressure 199.2
psig
Key Compositions
Reboiler LC
Vent Feed 2 FC
Butane Product P/F Specs
Cond Duty Butanes To Feed2
To Feed1
VLV-100
VLV-101
Feed2
Butane VLV-102 Product
Pressure
Pressure 182.6
psig
Liquid Product P/F Specs
Feed1
Pressure
Pressure 186.9
psig
Reb Duty Feed1 FC
C5+ Debutanizer
VLV-103
Liquid Product
Figure 18. Distillate-Boilup Control Case Flowsheet
Distillate-Boilup control is effective for columns operating at high reflux. The distillate flow rate controls the distillate composition while the heat input to the reboiler controls the bottoms composition. Figure 19 shows the flowsheet of the Aspen HYSYS file “Debutanizer Solution – RefluxBottoms Control Case.hsc”, which is an example of Reflux-Bottoms control. This file was also downloaded in conjunction with this guide. Reflux-Bottoms control is effective when the boilup ratio of a column is high. The reflux controls distillate composition while the bottoms flow controls the bottoms composition.
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Jump Start: Using Aspen HYSYS ® Dynamics with Columns
To Feed 2 P/F Specs Pressure
Vent
Cond PC
Pressure 198.5
psig
Cond LC
To Feed 1 P/F Specs Pressure
P/F Specs
Flow
Molar Flow
0.0
Column TC
Pressure 199.2
psig
Key Compositions
Reboiler LC
Vent Feed2 FC
Butane Product P/F Specs
Cond Duty Butanes To Feed2
To Feed1
VLV-100
VLV-101
Feed2
Butane VLV-102 Product
Pressure
Pressure 182.6
psig
Liquid Product P/F Specs
Feed1 Reb Duty
Feed 1 FC
C5+ Debutanizer
Pressure
VLV-103
Pressure 186.9
psig
Liquid Product
Figure 19. LB Control Case Flowsheet
In-depth exploration of these control schemes, as well as others that would be appropriate for a user’s specific process, ultimately leads to safer and more profitable column operation.
Additional Resources Public Website: www.aspentech.com/products/aspen-hysys-dynamics.aspx
Online Training: www.aspentech.com/products/aspen-online-training
AspenTech YouTube Channel: www.youtube.com/user/aspentechnologyinc
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About AspenTech AspenTech is a leading supplier of software that optimizes process manufacturing—for energy, chemicals, pharmaceuticals, engineering and construction, and other industries that manufacture and produce products from a chemical process. With integrated aspenONE ® solutions, process manufacturers can implement best practices for optimizing their engineering, manufacturing, and supply chain operations. As a result, AspenTech customers are better able to increase capacity, improve margins, reduce costs, and become more energy efficient. To see how the world’s leading process manufacturers rely on AspenTech to achieve their operational excellence goals, visit www.aspentech.com.
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