1.2.5 column operations 4

Column Operations 1

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Column Operations

© 2000 AEA Technology plc - All Rights Reserved.Chem 5_4.pdf

2 Column Operations

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WorkshopIn this module, you will simulate an Ethanol Plant. You will get more practice with the Column unit operation of HYSYS by:

• modeling columns with side draws • adding a column with real trays.

Typically an ethanol fermentation process produces mainly ethanol plus several by-products in small quantities: methanol, 1-propanol, 2-propanol, 1-butanol, 3-methyl-1-butanol, 2-pentanol, acetic acid, and CO2.

The CO2 produced in the fermentation vessel carries some ethanol. This CO2 stream is washed with water in a vessel (CO2 Wash) to recover the ethanol, which is recycled to the fermenter.

The ethanol rich product stream from the fermenter is sent to a concentration (Conc) tower. An absorber with a side vapour draw can be used to represent this tower. This vapour draw is taken from Stage 2 so as to have an azeotropic ethanol product with less methanol contamination. The top vapour is fed to a light purification tower (Lights) where most of the remaining CO2 and some methanol is vented.

The feed to the Rectifier (Rect) is the bottoms product of the Lights purification tower and the vapour draw from the concentration tower. The Rectifier is operated as a conventional distillation tower.

Methanol concentrates towards the top stages, so a small distillate draw is provided at the condenser. Also, a small vent for CO2 is provided at the condenser.

Another interesting point is the concentration of heavy alcohols in the interior of the Rectifier. These alcohols are normally referred to as Fusel oils. Fusel oils are a mixture of propanols, butanols and pentanols, with a potential value superior to that of ethanol. Accumulation of fusel oils in the Rectification Tower can cause the formation of a second liquid phase and subsequent deterioration of performance for these trays, so small side liquid draws of fusel oils are installed on the rectifier to avoid this problem.

Column Operations 3

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Learning ObjectivesOnce you have completed this section, you will be able to:

• Model a distillation column with side draws• Add specifications to a column• Add efficiencies to a column

PrerequisitesBefore beginning this section, you need to be able to:

• Add streams, operations and columns.

Process Overview

Column Operations 5

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Column OverviewsCO2 Wash

Concentrator

Lights

6 Column Operations

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Rectifier

Column Operations 7

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Building the Simulation

Defining the Simulation BasisAny activity model (except Wilson, which cannot predict two liquid phases) can be used to solve this problem.

1. Start a new case and select NRTL as the Property Package.

2. Use the following components: Ethanol, H2O, CO2, Methanol, Acetic Acid, 1-Propanol, 2-Propanol, 1-Butanol, 3-M-1-C4ol, 2-Pentanol and Glycerol.

3. On the Binary Coeffs tab of the Fluid Package use UNIFAC VLE and press the Unknowns Only button to estimate the missing interaction parameters.

Adding Streams and Unit Operations

Input the material streams required for the flowsheet:

In This Cell... Enter...

Conditions

Stream Name Wash H2O

Temperature 25°C (77°F)

Pressure 102 kPa (15 psia)

Mass Flow 2340 kg/h (5165 lb/hr)

Composition - Mass Fraction

H2O 100%

8 Column Operations

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Conditions

Stream Name FromFerm



Mass Flow 46 720 kg/h (1.03e+05 lb/hr)


Ethanol 0.0637

H2O 0.8759

CO2 0.0601

Methanol 4.433e-5

Acetic Acid 1.026e-5

1-Propanol 2.802e-5

2-Propanol 2.808e-5

1-Butanol 2.505e-5

3-M-1-C4ol 9.727e-5

2-Pentanol 2.457e-5

Glycerol 3.141e-5

Conditions

Stream Name Steam A



Mass Flow 11 000 kg/h (24,250 lb/hr)


H2O 100%

Column Operations 9

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CO2 Vent Separator

The CO2 Vent Separator separates products from the Fermentor. The liquid bottoms of the separator are sent to the distillation section of the plant (Concentrator Tower), while the overhead vapour goes to the CO2Wash Tower.

Install a Separator and make the connections shown here:


Conditions

Name CO2 Vent

Inlets FromFerm

Vapour Outlet To CO2 Wash

Liquid Outlet Beer

10 Column Operations

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The Column Operation

Column SubFlowsheet

A SubFlowsheet contains equipment and streams and exchanges information with the Parent Flowsheet through the connected streams. From the Main Environment, the Column appears as a single, multi-feed multi-product operation. In many cases, you can treat the column in exactly that manner.

You can enter the Column SubFlowsheet by pressing the Column Environment button on the Column Property View. Once inside the Column Environment you can return to the Parent Environment by pressing either the Parent Environment button on the Column Runner view or the Parent Simulation Environment button in the Button Bar.

The Column SubFlowsheet provides a number of advantages:

Independent Fluid Package

HYSYS allows you to specify a unique Fluid Package for the Column SubFlowsheet, as the Fluid Package in the Main Flowsheet may not necessarily be the best one in which to run the Column. That is the case if a Column does not use all the components of the Main Flowsheet, or if different Interaction Parameters are best suited to the Column conditions.

Isolation of Column Solver

When you are in the Column runner, the main simulation can be “checked” by unchecking the Update Outlets box. All aspects of the Main Environment downstream of the Column will be paused until you are satisfied with the behaviour of the Column. To update the rest of the Flowsheet, check the box again. This allows you to make changes and focus on the Column without re-calculating the entire Flowsheet. Once you have made the necessary changes, simply run the Column again to produce a new converged solution. In the Column runner, you are free to view profiles, stage summaries, and other data, as well as make changes to Column Specifications, parameters, equipment, efficiencies, reactions, and so on.

Parent Simulation Environment button

Column Operations 11

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Use of Simultaneous Solution Algorithm

The Column SubFlowsheet does not use the standard non-sequential solver, but a simultaneous one whereby all operations within the SubFlowsheet are solved simultaneously. The simultaneous solver permits in particular the user to install multiple unit operations within the SubFlowsheet without the need for Recycle blocks.

Once a Column is converged, if changes are made to the parent Flowsheet, the Column will automatically be up-dated and re-run.

Column Types

HYSYS has several basic Column Templates (pre-constructed column configurations) which may be used for installing a new Column, allowing HYSYS to model several different separation processes.

This module will introduce the Absorber. Subsequent modules will present different columns so that, by the end of this course, most types of columns will have been used.

Initial Estimates

In order to calculate, the Column solver in HYSYS requires user-supplied estimates ranging from condition parameters to feed and draw locations. These data can be supplied in several ways. The first interface when you select a Column requires values for the Number of Trays, Reflux, Pressure, and so on. In the Column Solver, on the Work Sheet tab, Conditions page, data can also be supplied. On the Design tab, Specs page, HYSYS allows you to choose from a library of specifications gathered in the Column Specification Types window, activated when you press the Add button. Specification Types can be supplied as Estimates or as Active specifications. Only Active specifications fill the Degree of Freedom of the Column which must be zero for the solver to calculate. Inactive specifications (Estimates) are used only as initial estimates for the convergence algorithm and never use a degree of freedom. Alternatively, Estimates can be supplied on the Estimates page under the Parameters tab. However, they will not appear on the Monitor, and hence cannot be set as Active specifications.

If the user does not have initial estimates HYSYS generates them. However for chemical systems, it is recommended to use the HYSYS Estimate Generator tool. In the Column Environment, on the Parameters tab, Solver page, there is the Initial Estimate Generator


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Parameters box. By checking the box, the IEG will perform iterative flash calculations to provide the initial estimates for the temperature and composition profiles. No user estimates are required when the Iterative IEG check box is activated.

CO2 Wash Tower

The CO2 Rejection Tower is a simple Absorber. Water is used to strip any ethanol entrained in the off gas mixture, this produces an overhead of virtually pure CO2. The bottoms product from the tower is recycled to the Fermentor (however the recycle is not a concern in this example).

The Input Expert will guide you through the installation of the column.

1. Add an Absorber with the information shown here on the first Connections page. Absorber button


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2. On the Pressure Profile page, enter the following values. If using field units, the values for the Top and Bottom stage pressures will be 14 and 15 psia, respectively.

3. Press the Done button to complete the column installation.

4. Press the Run button on the column property view to converge the column.

Save your case!


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Concentrator

This tower removes most of the Methanol from the Fermentor products.

The Concentrator is an Absorber with a side vapour draw.

By default, whenever a side draw stream is added to a column, HYSYS automatically creates a Draw Rate specification for that stream. This eliminates the additional DOF that adding the side draw stream would normally produce.

However, in this case, we do not need a Draw Rate specification for this stream; therefore, we need to replace the Draw Rate specification that HYSYS added automatically with one that we will define to meet our simulation needs.


Connections

Column Name Conc

No. of Stages 17

Feed Beer (Top Stage)

Steam A (Bottom Stage)

Ovhd Vapour To Light

Bottoms Liquid Stillage A

Vapour Side Draw Rect Feed, Stage 6

Pressure

Top Stage Pressure 100 kPa (14.5 psia)

Bottom Stage Pressure 102 kPa (15 psia)

Temperatures

Stage 1 Temp Estimate 90°C (195°F)

Stage 17 Temp Estimate 110°C (230°F)


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1. Go to the Specs page on the Design tab of the column property view.

2. Press the Add button in the Column Specifications group to create a new specification.

3. Select the specification you want from the list that appears.In this case, we want to add a Column Component Recovery specification.

4. Press the Add Spec(s) button.


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5. Complete the specification’s property view as shown below.

This specification will set the ratio of ethanol recovered in the specified stream compared to the amount of ethanol fed to the column. Here, we have set this ratio at 0.95, meaning that 95% of the ethanol supplied to the column is recovered in the Rect Feed stream.

We are not concerned about where the other 5% goes, although it must exit the column in one of the other product streams.

6. With the column’s DOF (degrees of freedom) at 0, and we need to set which specifications should be active and which HYSYS can use as estimates only. On the Monitor page of the Design tab, ensure that the Ethanol Recovery specification is active and that all others are inactive.

7. We can now start the column runner and allow HYSYS to find a solution for this column. Press the Run button now to begin the column solver.

8. Save your case.

Save your case!


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Lights

The Lights Tower is a purification tower.

Add the Refluxed Absorber column and enter the following data.

Refluxed Absorber buttonIn this cell... Enter...

Connections (Input Expert Page 1)

Column Name Lights

No. of Stages 5

Bottom Stage Inlet To Light

Condenser Type Partial

Bottoms Liquid To Rect

Condenser Energy CondDuty

Overhead Outlets Light Vent, 2nd EtOH

Pressure (Input Expert Page 2)

Delta P 0 kPa (0 psi)

Condenser Pressure 97 kPa (14 psia)

Bottom Stage Pressure 100 kPa (14.5 psia)

Temperatures (Input Expert Page 3)

Not Required

Specifications (Input Expert Page 4)

Vapour Flow 1.6 kgmole/h (3.5 lbmole/hr)

Reflux Ratio 5


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The Lights column requires 1 more specification; currently the DOF = 0, but we do not want to use these specifications to converge the column. The Reflux Ratio of 5 is only an estimate, and we will need to add another specifications that HYSYS will use to solve the column.

Add a Column Component Fraction specification with the following information:

The Active specifications for this column should be changed to:

• Vap Prod Rate• Ethanol Purity

So, deactivate the Reflux Ratio specification and activate the Ethanol Purity specification.

Press the Run button to converge the column.

In this Cell... Enter...

Name Ethanol Purity

Stage Condenser

Flow Basis Mass Fraction

Phase Liquid (default)

Spec Value 0.88

Component Ethanol


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Rectifier

The primary product from a plant such as this would be the azeotropic mixture of ethanol and water. The Rectifier serves to concentrate the water/ethanol mixture to near azeotropic composition. The Rectifier is operated as a conventional distillation tower. It contains a partial condenser as well as a reboiler.

Add a Distillation Column and enter the data shown here.Distillation column button


Connections

Column Name Rect

No. of Stages 59

Inlet Streams ToRect, Stage 38

RectFeed, Stage 44

Condenser Type Partial

Overhead Vapour RectVap

Overhead Liquid RectDist

Bottoms Liquid Stillage B

Condenser Energy RectCond Q

Reboiler Energy Rect Reb Q

Optional Side Draws

Stream 1st Prod, Type L, Stage 2

Fusel, Type L, Stage 37

Pressures

Condenser 100 kPa (14.5 psia)

Reboiler 105 kPa (15 psia)

Specifications

Reflux Ratio 7100

Ovhd Vap Rate 4.3 kg/h (9.5 lb/hr)

Distillate Rate 2.0 kg/h (4.4 lb/hr)


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Adding Additional Specifications

This column will require 5 active specifications in order to solve. We need to add at least two more; however, we will add three, with the third one acting only as an estimate.

We will add two Column Draw Rate specifications and one Component Fraction specification.

1. Remember that when we added the two side draw streams to the column, HYSYS automatically created Draw Rate specifications for those streams. On the Monitor page, enter the following values for these two Draw Rate specifications.

2. Add one Column Component Fraction specification with the following information:

3. Make following specifications active and all others inactive:

• Reflux Ratio• Ovhd Vap Rate• Distillate Rate• Fusel Rate• Product Purity

Specification 1 Specification 2

Name 1st Prod Rate Fusel Rate

Spec Value 3000 kg/h

(6500 lb/hr)

2 kg/h (5 lb/hr)


Name Product Purity

Target Type Stream (radio button)

Draw 1st Prod

Basis Mass Fraction

Spec Value 0.95

Component Ethanol


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4. On the Efficiencies page of the Parameters tab, add Stage Efficiencies of 0.55 for all the stages except feed and product stages (2, 37, 38, 44, and 59), where it must remain at 1. You can save time during this step by entering the value (0.55) in the Eff. Multi. Spec cell, selecting a number of cells on the right, and pressing the Specify button.

5. On the Solver page of the Parameters tab, set the Damping Factor to Adaptive and ensure that the Azeotropic check box is checked. Due to the azeotropic nature of ethanol and water, we need to have this box checked so that HYSYS is able to handle this situation. Setting the damping factor at adaptive allows HYSYS to adjust this parameter to help ensure that the solver can reach a solution.

6. Press the Run button to converge the column. If the column does not converge quickly, stop the solver and increase the Fusel draw rate spec value to 10 kg/h (20 lb/hr), and try again. Once the column has converge, the Fusel draw rate can be returned to its original level.

Draw Stream Location

Theoretical trays assume that the liquid and vapour products are in thermodynamic equilibrium. In reality, columns can never achieve this perfect mixing and separation. There are two ways of accounting for less than ideal stages in HYSYS. An overall efficiency can be applied when setting up the column or efficiencies can be specified for specific trays in the column.

Applying an overall efficiency is the most straightforward, and in most situations, the recommended approach for modelling any tower. Simply taking the actual number of trays and multiplying by the efficiency less than 1.0 generates the theoretical stages. Feed, draws and equipment must be located appropriately.

In HYSYS, efficiencies are defaulted to 1.0. They can be user-modified in the appropriate tab of the Column Property view. Efficiencies are applied to individual stages calculation using Murphree’s formula.1

The side liquid draw, Fusel, is added at stage 37 of Rect. To determine if this is an appropriate stage to recover the heavy alcohols, you can view

What is the mass fraction of Ethanol in the "1st Prod" stream when the column is converged? ____________

1. Murphree, E.V., Ind. Eng. Chem., 17, 747, 1925.

The stage efficiencies for all feed and product stages must remain at 1. The HYSYS column solver is not able to handle non-ideal feed and draw stages at this time.

Due to the fact that HYSYS calculation of non-ideal trays does not take into account side-streams, efficiencies on stages with feeds, draws (reboilers and condensers included) and equipment connected to columns, have to be left at their default value of 1.0.


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the stage by stage composition profile:

1. Move to the Performance tab in the Column Runner.

2. Go to the Profiles page. Highlight Composition in the Tray by Tray Properties group

3. Press the View Graph button. In this view we can see the compositions on each tray.

We wish to view the 1-Propanol composition on Tray 37. The initial graph will not contain this component. To modify the components in this view, you must press the Properties button. This will open the Properties View:

1. Check the 1-Propanol box in the Components group.

2. Close this view to return to the graph.

Stage 37 has the highest concentration of 1-Propanol (which has the greatest concentration among the heavy alcohols). Therefore, we have selected the appropriate stage for the Fusel draw.

Save your case!


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Optional ChallengeReplacing the Kettle Reboiler with a Thermosyphon Reboiler

In this portion of the module, we will replace the default kettle type reboiler on the Rectifier column with a "Thermosyphon" type reboiler.

Thermosyphon reboilers are commonly used in this type of application, and it is often desired to use HYSYS to simulate the operation of the column with this type of reboiler as it will provide a more accurate simulation of the actual physical set-up of the equipment.

In order to change the kettle reboiler to a thermosyphon, we will have to add one additional stage to the column. This additional stage functions as a liquid sump that allows liquid to be drawn from the column and liquid to be fed to the column from the reboiler. With thermosyphon reboilers, both liquid and vapour are returned to the column rather than just the vapour that is returned to the column with standard kettle reboilers.

Following the steps below will allow you to replace the standard kettle reboiler with a thermosyphon type reboiler.

1. Change the number of stages in the column to 60. This value can be accessed from the Connections page of the column’s property viewer.

2. Enter the Column Environment by pressing the Column Environment button on the property viewer.

3. Disconnect the "Stillage B" stream from the reboiler and reattach it as a material withdraw stream from stage 60.

4. Disconnect the "Boilup" stream from stage 60 and reattach it to stage 59. You will need to expand the trays shown by the column icon in order to attach the stream to this tray. Right-click on the tray section, and select Show Trays. Use the Radio button to select Full Expansion, or scroll down and check to Show box for stage 59.

5. Attach the reboiler’s liquid product outlet (formerly the location of "Stillage B" stream) to the inlet point on stage 60.


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6. The reboiler should now be setup as follows:

7. Adding another Side Draw to the column (Stillage B) means that HYSYS has created another Draw Rate specification and the DOF remains at 0.

8. Add a Column Vapour Fraction specification to the column with the following information:

9. Activate this new specification, and deactivate the Draw Rate specification that HYSYS created. The column should resolve automatically; if it does not, press the Run button to allow the column to converge.

Note that adding the additional stage and modifying the reboiler has not changed the operating behaviour of the column. The bottom stage of the column (#60) does not function like a true equilibrium stage (there is no contacting vapour from the bottom). The boil-up provided by the thermosyphon reboiler will be the same as for the kettle reboiler. This is independent of the reboiler type.


Name Reboiler V.F.

Stage Reboiler

Spec Value 0.9

Save your case!

1.2.5 column operations 4

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