absorber

Design Procedure for an Absorption Unit onthe AspenPlus Software

Author: Brigitte McNames

This manual presents all steps necessary to design an absorber using the AspenPlussimulation software. The manual also includes useful tips, recommendations, andexplanations throughout the design procedure. The following example will be used:

Example 1Problem Statement: Absorption of Acetone in a Packed TowerAcetone is being absorbed by water in a packed tower having a diameter of 0.4866 m at293 K and 101.32 kPa (1 atm). The inlet air contains 2.6 mol% acetone and outlet 0.5mol% acetone. The total gas inlet flow rate is 14.0148 kmol/h. The pure water inlet flowis 45.36 kmol/h. (This example is EXAMPLE 10.6-2 taken from reference 1).

Schematic:

AbsorberT = 293 KP = 1 atm

Gas Outletxacetone = 0.005

Pure Water InletF = 45.36 kmol/h

Gas Inletxair = 0.974xacetone = 0.026F = 14.014 kmol/h

Liquid Outletxacetone = 0.00648

Absorber Design Procedure SDSM&T 2/15

Procedure

Logon to the AspenPlus system and start a blank simulation. The flowsheet area shouldappear. (Refer to “AspenPlus Setup for a Flow Simulation” if you need help.)

Shown above is the Columns subdirectory. Choose the RateFrac block from thesubdirectory by clicking on it. If you click on the down arrow next to the RateFrac block,a set of icons will pop up. These icons represent the same calculation procedure and arefor different schematical purposes only. Choose the block that best represents the processthat you are designing. For our example we will use the RATEFRAC rectangular blockat the top left corner.

RateFrac is a rate-based nonequilibrium model for simulating all types of multistagevapor-liquid operations such as absorption, stripping, and distillation. RateFrac simulatesactual tray and packed columns, rather than the idealized representation of equilibriumstages.

A column consists of segments (see schematic for a packedcolumn at right). Segments refer to a portion of packing ina packed column or one or more trays in a tray column.RateFrac performs an initialization calculation where allsegments are modeled as equilibrium stages. The resultsfrom the initialization step are used to perform the rate-based nonequilibrium calculations. To learn more aboutRateFrac and its applications refer to the “RateFrac” helppages.

Packed segment n-1

Packed segment n

Packed segment n+1

Absorber Design Procedure SDSM&T 3/15

First, create a schematic similar to the one above using the RateFrac block. Refer to“AspenPlus Setup for a Flow Simulation” if you need help. Attach the liquid inlet andgas inlet streams to the feed port. Attach the gas outlet to the vapor distillate port and theliquid outlet to the bottoms port. Once the flow sheet is complete, click the “Next”button ( →→ ) and the title screen should appear (see below). Give the example a title andchange the units from English to Metric on the same screen. Click the →→ button.

Absorber Design Procedure SDSM&T 4/15

The components screen should appear next. Enter in the species used in the example (seeabove). The “Find” button on the bottom of the screen enables you to quickly search forcomponents in the databanks by formula, name, CAS registry number, molecular weight,and normal boiling point. The “Elec Wizard” button can be used to generate electrolytecomponents and reactions for electrolyte applications from components you entered. Acustom component that is not found in the databanks can be created using the “UserDefined” button. The “Reorder” button will simply reorder the components that arealready defined on the selection sheet. When all components have been entered, click the

→→ button.

On the next screen,choose a PropertyMethod from the list bypressing on the downbutton to the right of thebox. If you need helprefer to “AspenPlus Setupfor a Flow Simulation.”This example will useNRTL. Then click the

→→ button.

Absorber Design Procedure SDSM&T 5/15

The input sheet for the gas inlet stream should appear (see above). Enter the values fromthe problem statement. If values are unknown, leave the respective boxes blank. Whenfinished, click the →→ button. The input sheet for the liquid inlet stream should appear.Once again, enter the respective values from the problem statement, and click the →→button.

The input sheet for the absorber block will appear next. A column consists of segmentsthat are used to evaluate mass and heat transfer rates between contacting phases. Asegment refers to a portion of packing in a packed column or a series of trays in a traycolumn. Enter the number of segments. As a rule of thumb, there should be one segmentper foot of column height. However, more segments could be used to increase theaccuracy. The height of the segment should not be less than the average size of thepacking used. This example will use ten. Also on this screen, you can select thecondenser and reboiler type. Since we are modeling an absorber, select “none” forcondenser and reboiler. Click the →→ button.

Shown at the left is theinput page for theair/acetone inlet gas.Enter all the data fromthe problem statementfor temperature,pressure, flow rate,and composition.Make sure that unitsand definitionscorrespond to thevalues that you areentering.

Absorber Design Procedure SDSM&T 6/15

The button will automatically bring you to the tray specification sheet. Since our columnconsists of packing rather than trays, choose “PackSpecs” from the data browser at theleft. You will be brought to the packing specification sheet. Choose “New” to createyour packing specification for the tower. Start “pack segment number” at 1, which is thetop packed section in the column. The screen shown below will appear for enteringpacking specifications. Enter a value for the ending segment. For our example, enter tensince it is the last segment of packing in our column. For this example, I have arbitrarilychosen 1.5-in ceramic raschig rings as packing since the packing type was not specifiedin the problem statement. Guess a packing height that may give us the separation weneed to get our final gas and liquid concentrations. Since I have already tackled thisproblem, I know that the required height of packing necessary to achieve the separationwe need is 1.94 m. Click the →→ button to continue.

The view selector thatappears on the nextscreen allows you toselect the type ofpressure specificationthat you want to enter.Choose Top/Bottomand enter 1 atmpressure from theproblem statement forsegment 1. Segment1 will refer to the firstsegment at the top ofthe tower. Click the

→→ button.

Absorber Design Procedure SDSM&T 7/15

The next sheet will ask you to enter the value for the column diameter. Since the columndiameter is given in the problem statement, enter the value of 0.4866 m. Click the →→button. The next screen shown below asks you to specify the location of feed inlets andoutlets. Notice that the number eleven was entered for the gas inlet stream. This isbecause the convention for stream location is “above segment”. Click the →→ buttonafter entering all necessary information.

All required input is now entered and the simulation can be run. The Results screen isshown below. You can browse through the results by clicking on the double arrow nextto the “Results” header.

Notice that the heightand packingspecifications that weentered gave us theseparation specifiedin the problemstatement. From theproblem statement,the mole fraction ofacetone leaving in theliquid phase is equalto 0.00648, and themole fraction ofacetone leaving in thegas phase is equal to0.005.

Absorber Design Procedure SDSM&T 8/15

Additional RateFrac Features on AspenPlus:

Additional RateFrac features can be explored and utilized by viewing the data browser.The data browser refers to the column at the left side of the AspenPlus screen. It gives anoutline view of the available simulation input, results, and objects that have been defined.Following are additional, but not all, of the commonly used RateFrac features. Thesefeatures can be utilized through the data browser.

Report Options: Report Options is found under the Setup menu of the data browser.This feature will allow you to specify report options and data toinclude or suppress in the standard AspenPlus report. The reportdocuments all of the input data and defaults used in an AspenPlus runas well as the results of the simulation. This feature allows youcontrol over final information displayed for general information,flowsheets, blocks, and streams.

Column Parameters : There are many options available to simulate all different types ofcolumns. Following is a list of options available and their primaryfunctions:Setup – Enter the number of segments, specify the condenser and thereboiler, and column operating specifications.TraySpecs and PackSpecs – Define the trayed or packed section,tray or packing type, and other parameters.Reactions – Enter starting and ending segments for a reactions, aswell as reaction, chemistry, and user reactions IDs.

Estimates – Provide initial liquid and vapor temperature estimates for segments in thecolumn. If you do not enter an estimate, RateFrac generates an initial profile based onthe initialization option selected.Equilibrium Segments – Specify optional groups of equilibrium segments.Heaters Coolers – Enter side heater segment numbers and duties.

DesignSpecs: Design specifications can be created if you have a final design value inreach. You enter the specification type and the target value that youwould like to obtain. In the process, the stream type, components,segments, and other design information must be identified.AspenPlus will use this information to conform to your design. Thenumber of design specifications must equal the number ofmanipulated variables. Use the RateFrac Vary Form to specifymanipulated variables for the design mode.

Convergence: RateFrac usually performs the initialization calculations only up to arelatively relaxed tolerance. In certain situations, you might need totighten the tolerance on these calculations to generate a good startingpoint for the rate-based nonequilibrium calculations. When thecolumn has many segments, you might need to loosen the tolerance.

Absorber Design Procedure SDSM&T 9/15

Oftentimes, the value of the diameter is not given in the problem statement. Thefollowing problem is a modification of example one, where percent of flooding is givenrather than diameter. The following example will utilize Design Specs to design anabsorber under these conditions:

Example 2Problem Statement: Absorption of Acetone in a Packed TowerAcetone is being absorbed by water in a packed tower having a percent flooding of 0.8 at293 K and 101.32 kPa (1 atm). The inlet air contains 2.6 mol% acetone and outlet 0.5mol% acetone. The total gas inlet flow rate is 14.0148 kmol/h. The pure water inlet flowis 45.36 kmol/h. (This example is a modification of EXAMPLE 10.6-2 taken fromreference 1).

AbsorberT = 293 KP = 1 atm

Gas Outletxacetone = 0.005

Pure Water InletF = 45.36 kmol/h

Gas Inletxair = 0.974xacetone = 0.026F = 14.014 kmol/h

Liquid Outletxacetone = 0.00648

Absorber Design Procedure SDSM&T 10/15

Procedure

Logon to the AspenPlus system just as in the first example. Create an identicalflowsheet, and enter all data exactly the same as in Example one, until you get to packingspecifications. This time, we do not know the packing height, so we must make anestimate. For this example, we will estimate a packing height of 1 meter. Click the →→button.

The diameter input screen should appear next. In this example, the value of the diameteris not known. AspenPlus can calculate the diameter based upon the percent flooding ofthe column. Choose “Use calculated diameter” and enter the values as shown below. Anestimate must also be entered for the diameter. This example will use 1 meter.

A value for thetotal packingheight must beentered in orderfor the AspenPlussimulation to run.This estimatedvalue will beoverridden in theDesign Specs areaof the program,where the heightwill be varied inorder to satisfyentered molefraction values.

Absorber Design Procedure SDSM&T 11/15

Now, you must enter the Design Specification information. We are going to vary thepacking height in the column in order to satisfy the mole fraction of acetone leaving thecolumn in the gas outlet stream. Scroll down the data browser and choose “FlowsheetingOptions” and then “Design Specs.” Click the New button on the screen that appears.The design spec of DS-1 will appear and choose okay to accept. The screen for theFortran variable will appear. Choose new. The screen shown below will appear.

The screen for the manipulated variable will appear next. For the manipulated variabletype, choose Block-Var (the variable that we are manipulating is a block variable). Fill inthe block name and the variable name. In the area labeled ID1 enter the number one.This refers to the column number. In the area labeled ID2, enter the number one. Thisrefers to the starting segment number. Choose values for the lower and uppermanipulated variable limits. For our example, we will use a packing height between 1and 10 meters. Click the →→ button.

In the area labeled“Spec,” enter thevariable name that weknow. In this case, itis the mole fraction ofacetone leaving thecolumn in the gasoutlet stream. In ourexample we willarbitrarily call thisvariable CONC. Onthe next line, enter thetarget value of thevariable, and finallyset the tolerance.Click the →→ button.

A list of manipulatedvariables can be accessedby clicking on the downbutton to the left of thevariable blank. Shortdescriptions of thevariable abbreviationsare given as each of thevariable names ishighlighted.Whenever you are unsurewhat information you aresupposed to enter,highlight that area andrefer to the dialogue box.It may contain adescription of the data.

Absorber Design Procedure SDSM&T 12/15

All required input has now been entered and the simulation can be run. The resultsscreen are shown below.

The value of packing height that satisfied our design conditions was 1.94 meters. Thisvalue is equal to the 1.94 meters calculated by reference one.

REFERENCES

1. Geankopolis, C.J. Transport Processes and Unit Operations. 3rd ed., Prentice Hall,1993.

2. Help pages. AspenPlus Software.

Mole fraction valuesfor the gas outletand liquid outletstream closelymatch the valuesthat we were tryingto obtain in theproblem statement.

Absorber Design Procedure SDSM&T 13/15

Example Input Summary File:

;Input Summary created by ASPEN PLUS Rel. 10.0-1 at 13:38:59 Tue Mar 7, 2000;Directory C:\My Documents\ASPEN-INTEGRATED\Absorbers Filename C:\MyDocuments\ASPEN-INTEGRATED\Absorbers\absorberdesign.inp;

TITLE 'Absorption of Acetone in a Packed Tower'

IN-UNITS MET

DEF-STREAMS CONVEN ALL

DATABANKS PURE10 / AQUEOUS / SOLIDS / INORGANIC / & NOASPENPCD

PROP-SOURCES PURE10 / AQUEOUS / SOLIDS / INORGANIC

COMPONENTS ACETONE C3H6O-1 ACETONE / WATER H2O WATER / AIR AIR AIR

FLOWSHEET BLOCK ABSORBER IN=LIQ-IN GAS-IN OUT=GAS-OUT LIQ-OUT

PROPERTIES NRTL

PROP-DATA NRTL-1 IN-UNITS MET PROP-LIST NRTL BPVAL ACETONE WATER 6.398100000 -1808.991000 .3000000000 & 0.0 0.0 0.0 293.1500000 368.2500000 BPVAL WATER ACETONE .0544000000 419.9716000 .3000000000 0.0 & 0.0 0.0 293.1500000 368.2500000

STREAM GAS-IN SUBSTREAM MIXED TEMP=293. <K> PRES=1. MOLE-FLOW=14.014 MOLE-FRAC ACETONE 0.026 / WATER 0. / AIR 0.974

STREAM LIQ-IN SUBSTREAM MIXED TEMP=293. <K> PRES=1. MOLE-FLOW=45.36 MOLE-FRAC WATER 1.

Absorber Design Procedure SDSM&T 14/15

BLOCK ABSORBER RATEFRAC PARAM NCOL=1 TOT-SEGMENT=10 COL-CONFIG 1 10 CONDENSER=NO REBOILER=NO PACK-SPECS 1 1 10 HTPACK=1. <meter> PACK-ARRANGE=RANDOM & PACK-TYPE=RASCHIG PACK-MAT=CERAMIC PACK-DIM="1.5-IN" & PACK-SIZE=.0381000 SPAREA=1.213900 PACK-FACTOR=3.116760 & PACK-TENSION=61.00000 DIAM-EST=1. <meter> BASE-SEGMENT=10 & VOID-FRACTIO=0.73 FEEDS LIQ-IN 1 1 / GAS-IN 1 11 PRODUCTS LIQ-OUT 1 10 L / GAS-OUT 1 1 V P-SPEC 1 1 1. COL-SPECS 1 MOLE-RDV=1.0 Q1=0.0 QN=0.0

DESIGN-SPEC DS-1 DEFINE CONC MOLE-FRAC STREAM=GAS-OUT SUBSTREAM=MIXED & COMPONENT=ACETONE SPEC "CONC" TO "0.005" TOL-SPEC ".001" VARY BLOCK-VAR BLOCK=ABSORBER VARIABLE=HTPACK & SENTENCE=PACK-SPECS ID1=1 ID2=1 LIMITS "1" "10"

STREAM-REPOR MOLEFLOW MOLEFRAC;;;;;

Absorber Design Procedure SDSM&T 15/15

ABSORBERLIQ-IN

GAS-IN

LIQ-OUT

GAS-OUT

Absorption of Acetone in a Packed TowerStream ID GAS-IN GAS-OUT LIQ-IN LIQ-OUTFrom ABSORBER ABSORBERT o ABSORBER ABSORBERPhase VAPOR VAPOR LIQUID LIQUID

Substream: MIXED Mole Flow KMOL/HR ACETONE .3643640 .0761541 0.0 .2882099 WATER 0.0 .3171725 45.36000 45.04283 AIR 13.64964 13.59127 0.0 .0583660Mole Frac ACETONE .0260000 5.44557E-3 0.0 6.34972E-3 WATER 0.0 .0226801 1.000000 .9923644

AIR .9740000 .9718743 0.0 1.28590E-3Total Flow KMOL/HR 14.01400 13.98460 45.36000 45.38940Total Flow KG/HR 416.3316 403.6166 817.1731 829.8881Total Flow L/MIN 5615.509 5611.735 13.64048 13.92747Temperature K 293.0000 293.4187 293.0000 292.2765Pressure ATM 1.000000 1.000000 1.000000 1.000000Vapor Frac 1.000000 1.000000 0.0 0.0

Liquid Frac 0.0 0.0 1.000000 1.000000Solid Frac 0.0 0.0 0.0 0.0Enthalpy CAL/MOL -1376.595 -1623.627 -68319.40 -68199.92Enthalpy CAL/GM -46.33711 -56.25579 -3792.303 -3730.086Enthalpy CAL/SEC -5358.779 -6307.159 -8.6082E+5 -8.5988E+5Entropy CAL/MOL-K -1.227476 -.3513392 -39.14193 -39.35059Entropy CAL/GM-K -.0413176 -.0121732 -2.172707 -2.152218Density MOL/CC 4.15931E-5 4.15338E-5 .0554232 .0543163

Density GM/CC 1.23566E-3 1.19873E-3 .9984660 .9931067Average MW 29.70827 28.86151 18.01528 18.28374Liq Vol 60F L/MIN 7.937254 7.645211 13.64580 13.93784