Aspen Hysys Three Phase Distillation

Three Phase Distillation

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Introduction In previous modules, we have installed distillation columns design for two phases. However, these columns will not work for three phase systems. When dealing with three phase systems, it is necessary to use a different type of column that uses a different Input Expert, and a different default solver. Due to the addition of a second liquid phase in the column, it is very difficult to model this type of column. For this reason, we must use the Sparse Continuation Solver to solve these systems. The three phase system in this example consists of a 2-butanol, water, n-butyl-acetate system. The feed to the column is saturated liquid at atmospheric pressure. The liquid product from the condenser is 98% pure water, and the liquid product from the reboiler is a mixture of the two other components (2-butanol and n-butyl-acetate).

Learning Objectives In this module, you will learn:

• How to use the three phase distillation column in HYSYS. • When to use the Dynamic Initial Estimate Generator (IEG). • Why the Sparse Continuation Solver must be used to solve three phase columns.

Prerequisites Before beginning this module, you should have some experience with installing distillation columns in HYSYS. Previous experience with normal two phase systems will help you understand the principals behind the operation of three phase systems.

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Background The three phase stream that will be distilled here will consists of three components: water, n-butyl-acetate, and 2-butanol. The ternary diagram for these three components at 25 oC (77 oF) and 101.3 kPa (14.9 psia) looks something like this:

This plot was generated using the DISTIL software package produced by AEA Technology Engineering Software. The composition of the feed stream is given in the following table: Component

Mole Fraction

Water

0.35

n-Butyl-Acetate

0.40

2-Butanol

0.25

As you can see (using the ternary graph above), this stream will lie within the two liquid phase region.

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The Differences Between Two and Three Phase Distillation With a normal two phase distillation setup, it would be impossible to distil the stream described on the previous page. Fortunately, HYSYS is able to model the distillation of streams like this by using a different column setup routine.

Different Solvers With two phase distillation the most common solver that is used is the "HYSIM Inside-Out" solver. This solver is good for most hydrocarbon systems, but it can not handle highly non ideal chemical systems, including three phase systems. The default solver for three phase columns is the "Sparse Continuation" solver. This more advanced solver is able to handle three phase, non ideal chemical systems, that other solvers can not calculate.

Different Input Experts The second major difference between two and three phase distillation is the different input experts that each uses. You have already seen the input expert used by the "normal" two phase distillation column. The input expert used by the three phase column is similar though slightly more complex. You will use this interface when we install the three phase column later in this module.

Different Column Specifications Finally, the third major difference between the two column types is the additional column specification that are available when using the three phase column operation. Due to the complications that a second liquid phase adds to the column operation, additional specifications are required to help the column converge. Despite these apparent differences, setting up columns to handle three phase streams is not that much more complicated than setting them up to handle two phase streams. You already have experience with two phase systems, and you will find that setting up the three phases system is not that much more complicated.

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Process Overview


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Column PDF

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Building the Simulation The Basis Environment In this case, we will use the NRTL Activity model with the UNIFAC VLE estimated interaction parameters for the unknown binaries. Begin a new HYSYS case and select the NRTL Activity model as the Property Package. The three required components are:

• Water • 2-Butanol • n-B-Acetate On the Binary Coefficients tab, press the Unknowns Only button. Enter the Simulation Environment.

The Simulation Environment Adding the Feed Stream Create a new stream and define it with the following information:

The mass densities of the phases can be found on the Properties page. You will have to stretch the window in the horizontal direction in order to view all the information.

In This Cell...

Enter...

Name

Feed

Vapour Fraction

0.0

Pressure

101.3 kPa (14.9 psia)

Molar Flow

50 kgmole/h (110 lbmole/hr)

Mole Fraction - Water

0.35

Mole Fraction - 2-Butanol

0.25

Mole Fraction - n-B-Acetate

0.40

What is the Mass Density of the Liquid Phase? __________ and the Aqueous Phase? __________ Which phase is the "heavy" phase? ______________

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Adding the Distillation Column Add a Three Phase Distillation Column operation to the simulation using one of the available methods. The Three Phase Distillation Column icon.

Note: Make sure that you install the Three Phase column operation, not the two phase one. As mentioned before, the Input Expert for the Three Phase column is slightly different than the one used for an ordinary Two Phase column. For this reason, each page will be shown here. The input expert for the three-phase distillation column is different from the input expert that was seen previously. The first page of the expert allows you to select the type of column that you want to add to the model. Select the Distillation radio button and press the Next button.

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On the next page, set the number of stages at 6, and set the "Two Liquid Phase Check" on the top five stages.

On the third page, enter names for the streams around the condenser. For this column, the Reflux Stream will be the Lights only; therefore, select the appropriate radio button in this group box.

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On the fourth page, set the vapour rate to 0.0 and the Light Reflux Fraction to 1.0. The degrees of freedom should read 0, once both of these values are entered.

On the fifth page, attach the Feed stream to stage 1. Also, define the reboiler energy stream and attach a liquid product stream to the reboiler. From now on, the screens will look familiar to the input expert screens that you have seen previously.

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On the next page, set the condenser pressure at 101 kPa (14.9 psia), and the reboiler pressure at 105 kPa (15.5 psia).

On the next page, you are asked to supply optional temperature estimates. Enter values of 85, 90, and 95 oC (185, 195, and 205 oF) for the condenser, top stage and reboiler temperature estimates, respectively.

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We are now done. Press the Done button to close the input expert. Note that if the formation of azeotropes were a concern in this column, the Azeotropic Initialization button would have been pressed, and HYSYS would have checked for possible azeotropes among the selected components. You are automatically placed on the Column’s property view; move to the Monitor page of the Design tab. How many Degrees of Freedom are shown here? __________ Enter a Bot Prod Rate of 33 kgmole/h (75 lbmole/hr). Press the Run button to converge the column; the column should start to solve, and will reach a converged solution quickly. If the column does not solve in the first attempt, increase the Bot Rate Spec value to 35 kgmole/h (80 lbmole/hr) and try again. Once the column converges, decrease the spec back to its original value.

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Aspen Hysys Three Phase Distillation

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