Column Sizing

Column Sizing

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Column Sizing

© 1999 - AEA Technology plc - All Rights Reserved ADV 6_2.pdf

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Column Sizing

Workshop HYSYS contains a Shortcut Distillation tool that allows users to quickly design a distillation column with a minimum number of specifications. Users familiar with simulating a distillation column with HYSYS know that a regular column requires a good deal of information before it will converge. The Shortcut tool will estimate most of the physical parameters of a distillation column with only a very basic understanding of the process. It is a very valuable tool to use in the preliminary design of a distillation column. HYSYS also allows users to estimate flow rates inside the column and it will calculate the the appropriate tray dimensions.

Learning Objectives After completion of this module, module, you will be able to:

• Use the the Shortcut Shortcut Distill Distillatio ation n tool in in the design design of a column. column. • Build a distilla distillation tion column column in in HYSYS HYSYS and add add different different specifications. • Perform Perform Tray Tray Sizing Sizing and Ratin Rating g calculat calculations. ions.

Prerequisites Before beginning this module, you should be able to:

• Add stream streams s and and un unit it operations operations to the the PFD. PFD. • Navigate Navigate the PFD and Workboo Workbook k Enviro Environmen nments. ts.

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Shortcut Column Overview

Distillation Column Overview

Distillation Column Overview

Column Sizing

Information on Shortcut Distillation The Shortcut distillation tool allows users to estimate several column parameters without knowing a great deal of information about the specifics of the column. The Shortcut column requires much less information in order to solve than a regular column. There are five pieces of required information: 2 component specifications, 2 pressure specifications, and the Reflux ratio. A fully defined feed stream is also required in order to solve the column. The two component specifications are Light Key in Bottoms and Bottoms and Heavy Key in Distillate. Distillate. The Light Key is defined as the most volatile compound to appear at the bottom of the column. The Heavy Key is

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Column Sizing

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Information on Shortcut Distillation The Shortcut distillation tool allows users to estimate several column parameters without knowing a great deal of information about the specifics of the column. The Shortcut column requires much less information in order to solve than a regular column. There are five pieces of required information: 2 component specifications, 2 pressure specifications, and the Reflux ratio. A fully defined feed stream is also required in order to solve the column.

The Light Key and Heavy Key often have adjacent volatilities.

The two component specifications are Light Key in Bottoms and Bottoms and Heavy Key in Distillate. Distillate. The Light Key is defined as the most volatile compound to appear at the bottom of the column. The Heavy Key is defined as the least volatile compound to appear at the top of the column. Typically, the Light and Heavy Keys define the operation of the column. For example, example, the column that will be simulated in this module is a depropanizer. That is to say, it separates C1, C2 and C3 from heavier molecules like the C4’s and C5’s. The split occurs between propane and i-butane. Therefore, propane is the Light Key and i-butane is the Heavy Key. The concentrations of the Light and Heavy Keys must also be specified. Setting these values too low will result result in a large (tall) column with a high Reflux, while choosing high values may result in poor product quality and a weak separation. The estimates for Condenser and Reboiler pressure have a direct relationship relationship on the te mperatures mperatures that HYSYS will estimate in these locations. Until Until the user is comfortable with making estimates about these values, they may have to make an initial guess and check that the temperature estimates are reasonable. Of course, the Reboiler pressure should be greater than the Condenser pressure. The Reflux ratio must also be specified. After the other four specifactions are entered, HYSYS will calculate the Minimum Reflux Ratio. Ratio. The estimated, or External, Reflux ratio must be greater than this value. Once all of the required data is entered, HYSYS will calculate the remaining column parameters. parameters. These values can be found on the Performance tab. Performance tab. The user should check the temperature estimates on this page and, if necessary, adjust the pressures. pressures.

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Column Sizing

Building the Simulation As mentioned before, the Shortcut Distillation Distillation operation is an effective tool for preliminary design work on a column. In this module we will use this tool to design a distillation column.

Defining the Fluid Package Once again, the first step in any simulation is defining the fluid package. Start a New Case and choose the Peng Robinson EOS Robinson EOS package. The components C1 to n-C9 must n-C9 must be added to the simulation.

Installing the Feed Stream Install a new stream in the PFD and PFD and enter the following information: information:

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In This Cell...

Enter...

Name

Feed

Temperature, oC (oF)

10 (50)

Pressure, kPa (psia)

50 0 ( 75 )

Molar Molar Flow Flow,, kgmol kgmole/h e/hrr (lbmo (lbmole/ le/hr hr))

50 (110) (110)

Molar Composition - C1

0.32


0.16


0.11

Molar Composition - n-C4

0.11

Molar Composition - i-C4

0.11


0.05

Molar Composition - i-C5

0.05


0.03


0.03


0.02


0.01

Column Sizing

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Adding the Shortcut Distillation Now Now we can add the Shortcut Distillation operation. Shortcut Distillation button

1.

Double cl click on the Shortcut Distillation button to enter it into the PFD.

2.

Enter Enter the the follo followin wing g inform informati ation on into into the the appro appropri priate ate box boxes es..

In This Cell...

Enter...

Feed

Feed

Top Product Phase

Vapour

Overhead Vapour

OVHD Product

Conden ser Duty

Condenser Q

Bottoms

Bttm Product

Reboiler Duty

Reboiler Q

3.

On the Parameters page, Parameters page, enter the information as shown below. The Field unit values for Condenser Pressure and Reboiler Pressure are 43.5 psia and and 46.5 psia , respectively.

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Column Sizing

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Comp Complet lete e the the follo followin wing g table table with with infor informat mation ion from from the the Performance tab. Performance tab.

Minimum Number of Trays Trays Actual Number of Trays Trays These values are estimates only. We We will use an actual distillation column to refine the simulation.

Optimal Feed Stage Condenser Temperature Reboiler Temperature Rectify Vapour Flow Rectify Liquid Flow

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Column Sizing

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Building the Distillation Column Distillation Column button

Remember that these are the same values that were used in the Shortcut column.

From the estimates generated by the Shortcut Distillation operation, a distillation column will be simulated. 1.

Add a Distillation Column to Column to the PFD.

2.

Break Break the the conn connecti ection on betwe between en the the Feed Feed Str Stream eam and and the the Short Shortcut cut Distillation operation.

3.

Compl omplet ete e the the firs firstt pag page e of of the the Input Expert to Expert to look like this:

4.

Ente Enterr the the foll follo owing wing dat data a on the the next next pag page e of the the Input Expert: Expert:

In This Cell...

Enter...

Condenser Pr Pressure, kP kPa (p (psia)

300 (4 (43.5)

Cond Condens enser er Pre Press ssur ure e Drop Drop,, kPa kPa (psi (psia) a)

0

Reboiler Pressure, kPa (psia)

320 (46.5)

5.

Enter Enter the the tempe temperat ratur ures es that that the the Shortc Shortcut ut Disti Distilla llatio tion n operat operation ion estimated on the next screen.

6.

Enter ter a Reflux flux Ratio tio of 2.0 on the final screen on the Input Expert. Expert. Press the Done button, button, and you will be placed on the Column Property View screen. screen.

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Column Sizing

Changing the Column Specifications Note that the column can not converge now because the Overhead Vapour Vapour rate, rate, a default specification, specification, has has not been specified. Fortunately Fortunately,, HYSYS offers a wide variety of specifications that can be used to solve a column. To complete this exercise, we will use a composition specification similar to the Heavy and Li ght Key specifications used in the Shortcut column.

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1.

Go to the Specs page Specs page (on the Design tab) Design tab) of the Column Property View .

2.

Press the Add button. button. Choose Column Component Fraction from Fraction from the list that appears. Press the Add the Add Specs button. button.

3.

Enter Enter the the infor informat mation ion as show shown n below below;; ensur ensure e that that the Target arget Type is selected as Stream. Stream.

4.

Add another Column Component Fraction specification Fraction specification with following information:

Column Sizing

Specifications are designated active and deactive depending on whether the box in the active column is checked or not.

5.

Return to the Monitor page and make sure that the active specifications are Reflux Ratio and Ratio and Light Key .

6.

Converge th the Co Column.

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This column can have only two active specifications. The Reflux Ratio, Light Key, and Heavy Key cannot all be active simultaneously.

What is the current value of the Heavy Key specification?_________

Change the Active specifications to Light Key and and Heavy Key .

What is the current Reflux Ratio? ____________

Note that the values calculated by the Distillation Column are different from the values found by the Shortcut Column. This is because the Shortcut column only estimates the values, while the Distillation Column performs rigorous calculations. calculations.

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Column Sizing

Column Sizing HYSYS contains a Tray Sizing utility that greatly simplifies the mechanical design of a distillation column. A user can size full towers, or sections of towers, by specifying information related to the trays, tower internals, the downcomers, and the weirs. The most common use of the Tray Sizing utility is to identify a tray section, have HYSYS size the tower into tray sections based on your input and if desired, perform a rating analysis on the column. While HYSYS is able to size size and rate tray sections, sections, the values that it provides are only rough estimates and should be treated as such.

Column Sizing in Design Mode 1.

Change the Feed flowrate Feed flowrate to 300 kgmole/hr (660 lbmole/hr). lbmole/hr) .

The column will converge without difficulty; with the current set of specifications, the Feed flow rate does not affect convergence. 2.

Select Tools/Utilities from Tools/Utilities from the Main Menu bar, or press the hot key . .

3.

Select Tray Sizing from Sizing from the list of available utilities and press the Add Utility button.

4.

Press the Select TS... button, TS... button, and select T-101 as T-101 as the Flowsheet Flowsheet and Main TS as TS as the Object.

HYSYS allows users to select Tray Sections and not entire columns so that users may size Side Strippers independently from the Main Column. 5.

Press the Add Section button. Section button. Accept all of the default values that are presented.

HYSYS will calculate the dimensions of the column using preset values for the column internals and for the various parameters.

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Column Sizing

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On the Results tab, Results tab, a summary of the calculations are presented. A brief explanation of of the terms follows: follows:

• Number of Flow Paths : The number of times liquid crosses the tray, tray, most trays are single-pass or have NFP of 1. • Maximum Downcomer Backup: Represents Represents the maximum amount of liquid hold-up in the downcomer that can be tolerated by the column before flooding occurs. • Maximum Weir Loading : Measures the amount of liquid flowing over the weir. • Pressure Drops : Estimates the total pressure drop over the section and the maximum pressure drop per tray. The geometry of the trays in the section can be seen by selecting the Geometry radio button from the Trayed Section Results group Results group on the Results tab. Results tab. The information presented here includes tray diameter, tray area, tray spacing, section height, weir height and weir length. What is the maximum pressure drop per tray in the Distillation Column? ____________ Over which tray does this drop occur? ____________ What is the diameter of the trays inside the column? __________ What is the total section height? __________

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Column Sizing

Column Sizing in Rating Mode In rating mode, HYSYS allows you to perform rating calculations based on a specified tower diameter and fixed tray configuration. If desired, some of the tray dimensions may be left unspecified and HYSYS will automatically automatically calculate design values for them.

Exercise Head office is desperate to build a distillation column, but the fabrication mill is working overtime and there are very lengthy delays for special orders. The mill has the following trays in stock: Diameter, m (f (ft.) t.)

NFP (P (Passes)

0.75 (2.5 )

1

1.0 (3.3)

1

1.25 (4.1)

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Obtain the flow parameters and pressure drops for a column that uses the trays as given above. The following requirements must be met in order for the column to be constructed.

• • • •

Maximu Maxi mum% m% Floo Flood d=8 85 5 Maxi Ma ximu mum m weir weir loa loadi ding ng = 80 80 m3 /h-m (860 ftft3 /h-ft) Maximum Maximum pressur pressure e drop drop over column column = 10 10 kPa (1.5 (1.5 psi) psi) Maximu Maximum m downc downcome omerr backu backup p = 50% 50%

For each case, follow these steps: 1. 2.

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On the Specs page, Specs page, set the Calculation Mode to Rating . Rating . For For each each case, case, set set the the diamet diameter er and and numb number er of flo flow w paths, paths, press press the Calculate button button and move to the Results page. Results page.

Column Sizing

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Comp Complet lete e this this table table with with the the inform informati ation on pro provid vided ed by by HYSYS HYSYS..

Case #

1

2

3

Diameter, m (ft)

0.75 (2.5)

1.0 (3.3)

1.25 (4.1)

NFP

1

1

1

Weir Weir Load Flood DC Back Up Total delta P

Compare the table above with the specifications on the previous page; which set of trays will best meet the restrictions? Remember that smaller trays will be less expensive. __________ Note that the answer to this this question will depend on which which set of specifications you used to converge the column.

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Column Sizing

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