Sour Water Stripper with Aspen HYSYS

 Welcome to the Chemical Engineering Blog!

Hello friends, in this post we will explore the world of simulation in the petrochemical industry. To be precise, we will talk about "Sour Water Stripping Simulation with the Help of Aspen HYSYS." Don't worry if these terms still sound unfamiliar, as we will be breaking them down in simple language. Let's get started!

 What is Sour Water Stripping?

Sour water stripping is a process used in factories to remove acidic compounds and toxic gases from water used in various stages of the process. This sour water comes from crude oil and gas separation units and contains substances that need to be removed before the water can be released back into the environment.

column stripper

Why Do We Need Simulation?

Before we go any further, let's imagine if we had to try out various physical experiments in a factory to understand how this process works. That would be very inconvenient and expensive, right? Well, this is where simulation comes to the rescue! Simulation is a virtual way to model real processes on our computers so that we can understand how things work without having to build physical experiments.

 Why do we need Aspen HYSYS?

Aspen HYSYS is a very powerful simulation software. With its help, we can create computer models that represent all aspects of the acid water stripping process. From sour water composition to temperature and pressure, we can input all this information into the computer and see how a change in one parameter can affect the entire process.

 Sour water is commonly generated in many refinery operations. Any refinery process water that contains sulfides is considered sour water. Sour water typically contains ammonia and hydrogen sulfide, which must be removed before the water can be reused or sent to the wastewater system. In this lesson, we will simulate this process as well as analyze the effect of feed temperature on the column.

 Case study example

A sour water stream containing mass fractions of 0.988 water, 0.005 ammonia, and 0.007 hydrogen sulfide is produced from a crude oil tower. This stream is at 37.78°C, 2.758 bar, and has a mass flow of 328,900 kg/hr. Our goal is to produce pure water with a maximum of 0.00005 mol% ammonia while recovering 99% of the water in the feed stream.

 

Aspen HYSYS Setup

Open Aspen HYSYS and create a New simulation

Enter the components that will be involved in the case above. In the Component list folder select Add, add Water, Ammonia, and Hydrogen Sulfide to the component list.

Next, we determine the property package that we use. In the Fluid Packages folder select Add. Select Sour PR as the property package. The Sour PR model combines the Peng-Robinson and API-Sour equations of state from the Wilson Model to handle sour water systems.

Click on Simulation located at the bottom left of the screen

Add Material Stream, this stream will serve as the sour water feed. Double-click on the material stream. Change the name to SOUR WATER. Enter the values of Temperature 37.78 ^oC, Pressure 2.758 bar, and Mass Flow 328,900 kg/h.

In the Composition section located under the Worksheet tab, enter Mass Fraction values of 0.988 for H₂O, 0.007 for H₂S, and 0.005 for Ammonia. The flow should now be completed.

Add Heater. The heater serves as a heat for the SOUR WATER stream before entering the column.

Double-click heater (E-100). Select SOUR WATER stream as Inlet, define Outlet stream as StripperFEED, and Energy stream as Q-Heat.

In the Parameters section located under the Design tab, enter a Delta P value of 0.6895 bar.

Enter the outlet Temperature value of 100^oC on the Worksheet tab. Then the heater will be completed

Add Distillation Column Sub-Flowsheet from Model Palette

Double-click on the column (T-10). Then Distillation Column Input Expert will open. On page 1 enter the information as follows.

On page 2 leave the default then click Next

On page 3 enter the value of Condensor Pressure 1.979 bar and Reboiler Pressure 2.255 bar. Then click Next

On page 4 let it remain default then click Next. On the last page let it remain default too and click Done to complete the column.

The Column T-10 window will automatically open. Go to the Spech section under the Design tab to specify the column. In this step, we will specify the mole fraction of ammonia in the reboiler. Click Add and select Column Component Fraction. Select Reboiler for Stage and enter 0.00005 in Spec Value. Select Ammonia in the Component section

We will also recover 99% of the water from the feed stream. create a specification by clicking Add and selecting Column Component Recovery. Select Water from Draw enter 0.99 for Spec Value, and select H₂O for Component.

Go to Specs Summary and make sure that only Comp Fraction and Comp Recovery are active. Once these two specs are made active, the column will try to solve them. If the solver fails to converge, you may need to look at the Damping Factor.

Go to the Solver section under the Parameters tab. You will see that the default Damping Factor is 1. The damping factor serves to reduce the amplitude of oscillations that occur in the solver. Many times convergence can become cyclical, which can prevent the solver from finding a solution. This is where the damping factor becomes useful. If you click on the Troubleshooting icon in the ribbon section below Get Started and search for 'Damping Factor', you will see the following guide

We are working on an acidic water stripper, therefore, the recommended damping factor is between 0.25 and 0.5. On the Solver form under the Parameters tab, enter a Fixed Damping Factor of 0.4. After clicking Run, the column should complete

Save this file first before we continue 

If we look at the Water stream coming out of the reboiler, we will realize that this stream contains superheated water. We will utilize the energy from this stream to heat the feed stream thus lowering the energy requirement needed in this process. Remove the heater block (E-10) and place the Heat Exchanger block onto the flowsheet. Note that you can right-click the Heat Exchanger block and select Change Icon to change the icon displayed.

Double-click on the Heat Exchanger (E-10). Select SOUR WATER stream as Tube Side Inlet, StripperFEED as Tube Side Outlet, Water as Shall Side Inlet, and create a stream with the name Water Cool on Shall Side Outlet.

In the Parameters section under the Design tab enter a Pressure Drop of 0.6895 bar for both the Shell and Tube sections. Also, change the number of Tube Passes to 1. Then the Heat Exchanger should be solved.

Now we will do a case study to determine the optimum column feed temperature. In the Navigation Pane click on the Case Studies folder and select Add in Case Study 1, Add StripperFEED Temperature and Heat Flows from the Q-Cond and Q-Reb energy streams.

For Independent Variable (StripperFEED temperature), enter Low Bound 80^oC High Bound 115^oC, and Step Size 2^oC.

Click Run to start the calculation. To view the results go to Results or Plots tab.

From the case study, you can see that at higher feed temperatures, we have lower reboiler duty but higher condenser duty. Since the cost of steam is generally higher than the cost of cooling water, we had to increase the column feed stream temperature to 115°C.

 Conclusion

 In this lesson, we have learned how to simulate the sour water stripping process. We configured the distillation column as well as the heat exchanger. Through the use of case studies, it was determined.d that a higher column feed higher temperature would lead to lower energy costs for this separation

 

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