Experimental and Numerical Investigation of ...

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here was developed using equilibrium data provided by the W.R Grace &. Company. (8). Nitrogen adsorption was represented with the Langmuir expression:.
Experimental

and Numerical

Adsorption/Desorption

Investigation

in Packed

under Ideal and Non-Ideal

of

Sorption

Beds

Flows

H. Mohamadinejad Boeing Aerospace Huntsville,

Company

AL

J. C. Knox Marshall

Space Flight Center

National

Aeronautics

Huntsville,

James.

and Space Administration

AL

E. Smith,

Department

Ph.D

of Chemical

University

of Alabama

Huntsville,

Engineering in Huntsville

AL ABSTRACT

The importance nitrogen,

and water on molecular

dimensional diameter partial

of the wall effect

computer

cylindrical differential

equations

column

(submitted

are solved

to Separatuin

breakthrough

information

Science

results

are presented

measurements

linear driving

and Tchnology) Additional

effects of carrier

and desorption

cm in radius is examined

using finite differences

non-isothermal,

concentration.

on the interactive

Experimental

with concentration

data with the axial-dispersed, model

beds in the adsorption

sieve 5A of 0.127

simulations.

column

on packed

gas coadsorption

experimentally

for a 22.5-cm

taken at various

radial

and Newmans's

method.

force model

is required

comparisons

of carbon

of

suggests

for accurate test

data

and with onelong

by

locations.

4.5-cm

The set of

Comparison

of test

that a two-dimensional

simulation with

dioxide,

the

of the average model

with CO2, as well as CO2-H:O

provided

interactions.

Introduction

In a packed and therefore increase

bed, the presence

heat and mass transfer

in porosity

particle

diameter

infinite porous governing

of the confining

walls can have a significant

in the packed

near the wall can have opposing

ratio, R, less than about

medium

influence.

(1,2,3).

Between

bed. The viscous

shear

effects

permeability.

for an R value greater

force

has been shown

of 7 and 30, variations

For R values above 30, the influence

it was found to be important

on overall

7, the permeability R values

influence

is generally

on the axial flow

along

For values

to increase

in bed packing

considered

than 100 in an adsorption

the wall and the of tube to

above that of an may determine

insignificant

the

(3), although

study by Tobis and Vortmeyer

(4). Wall effects on packed

bed processes

the value of R is frequently a heater packed

matrix

results

within this sensitivity

in small rectangular

range.

columns

in heat and mass transfer

Examples (CDRA

are thermal

refs),

driven

processes,

swing adsorption

wall-cooled

catalytic

since

beds where

reactors

(5) and

bed energy storage units (6).

In this work, we examine desorption nitrogen

in a packed carder

thermal

model

balances

adsorption

The experimental

information

results

for carbon

on the sensitivity

diameter

was developed

(radial

for the pellet, and axial)

and desorption

dioxide

of the adsorption

ratio, R, of 15. Adsorption

during

and water

adsorption

and

adsorption

to wall effects

breakthrough

in a

in a column

data is presented

of permeability

gas

Station

on the finite-differencing

in a flow-through

and canister,

model

was also

and is described

The test flow rate was scaled Space

based

and desorption

Experimental

International

on mass and heat transfer

at the bed exit, which can be used as a measure

gas adsorption

dimensional

of wall effects

or channeling

for

along

wall.

A computer simulates

column.

tube to particle

radial locations

the column

the influence

gas provides

with an intermediate various

can be significant

and

the momentum

developed

to predict

technique.

and multicomponent equation the effect

The model adsorption;

are included. of flow

A two-

channeling

on

in a later paper (7).

Apparatus

and Results Adsorption

to give gas velocities

Carbon

bed. Single

numerical

Dioxide

Removal

Test System

similar to that in the proposed Assembly.

Instrumentation

flight beds for the

of the packed

column

includes temperature probesandsampling tubesformeasurements atsorbent material endpoints andone intermediate point.To attainapproximately adiabatic bedconditions, thecolumnwasfirstcovered with Mansfield Q-fiberfeltinsulation, thenwrapped witha thermal blanketof Mansfield Min-Kmaterial.For adsorption runs,anadditional jacketfabricated of STSExternal Tankinsulating foamwasattached. The insulation, approximately 3 inches thickinall,wasusedwithsatisfactory results. Instrumentation wasprovidedfor continuous measurement of packedcolumnoutletCO2 andH20 concentration. Thelocationofsensors andotherequipment comprising theadsorbing apparatus areshown inFigure1.Thecolumnbypass isused priortotheteststarttoensure stable columninletconditions. Thegaschromatograph, a Shimadzu GC-14AwithCR601integrator, wasusedto sampleat three sample portlocations to determine gasconstituent volumetric fractionsduringtheadsorption runs.The probedepthof thesampling tubeat thecolumnexit wasadjusted to obtaina radialprofileof theexit concentration duringaseries ofidentical CO2adsorption tests. Column A small packed column 2) located sorbent.

at Marshall A 10-inch

with approximate

Space Flight Center

packing

was used

Dynamics

Test Bed

dimensions

of 2 inches

was used. The column

in this study for quicker

Placing 4.75 inches of glass beads at the two ends of the column

in diameter and 20 inches long (Figure may be packed

results

with up to 20 inches of

and reduced

eliminated

thermal

end effects.

the end effects.

Procedure Experiments Nitrogen bypass

were performed

on the insulated

is the carrier gas. Experiments

as it proceeded

Figure

The sorbent

is a 5A zeolite.

begin with the column at ambient room temperature.

was switched

before exposure

during the test to follow

The column

of the column to the

the sorbate mass transfer

wave

down the bed.

Results 3 shows

the water breakthrough

of gas mixed

gas breaks through process

rig at MSFC.

shown in Figure 1 was used to obtain intended inlet conditions

inlet gas. The GC sampling location

pressure

fixed-bed

efficiency

by passing

before

through

and Discussion

at the column

glass beads downstream

the gas at the centerline,

for the two-inch

diameter

midpoint

column.

indicating

and outlet. Also shown is the HzO partial of the sorbent

that channeling

material.

Note the mixed

has a significant

effect on the

To providea measure of theradialvariationin axialflow, adsorption testsof CO2andN2were conducted repeatedly withtheexitsampling tubein differentradialpositions. Thetestresults, shownin Figure4, showa clearrelationship between concentration andradialposition.As observed for H20 breakthrough results, these resultsindicate greater permeability nearthecolumn wall.Sincetheheater core of the4BMS(FourBedMolecularSievs)sorbent bedconsists of channels roughlyone-halfinchin diameter, thechanneling effecton CO2removalwill likelybe significant.Theresultsof thetwodimensional modelpresented in partII alsoconfirmthatchanneling hasa significant effecton this adsorption process. Mathematical

Gas/Solid

The Company

model

discussed

here

(8). Nitrogen

was developed

adsorption

*

Equilibrium

using

was represented

Model

equilibrium

with the Langmuir

data

provided

by the W.R

Grace

&

expression:

qmiBiPi

(1)

qi = l+BiPi Carbon dioxide

and water adsorptions



qmiBiPi

qi -

Two approaches

Prasusnitz,

by an iterative

for multiple

procedure

PY i

i=1

t 7rAni

(RTqmiJ

(9):

(2)

1-11 n--_

]

of gases adsorbed

components.

with the ideal

(10). For the case of the Langmuir-Freundlich

exp(

equation

ni

were used to solve for the amounts

i=n

by the Langmuir-Freundlich

l+BiPini

with the gas phase concentrations was obtained

were correlated

in the solid phase in equilibrium

For simulations

adsorption

solution

single component

with

and CO 2, the solution

N 2

theory isotherm,

(IAST)

of Myers

one gets:

1=0

(3)

and

Knowing estimated H20,

the total pressure

and solution

will be obtained

and CO2 the approach

based

found to give a more accurate process

for submision

towards

in details

correlations

predicts

adsorption

is under predicted,

systems

was

of co2/h2o, a paper is in the

the use of IAST and Langmuir-Freundlich

and test results

for components

multicomponent

equilibrium adsorption

as discussed

model.

equilibria

on the adsorption

of

OuC i

Ox 2

_x

For

adsorptivities. example,

indicates

low sensitivity

However,

at low

work

humidity

continues

levels,

for CO 2 and H20. At higher humidity

to

levels,

IAST CO z

by Finn (11).

In the bulk stream of the gas within the bed, the material

_2C i

using these correlations

with differing

Mass Balance

Boundary

with N 2,

(4)

gas/solid

accurately

- Dl

for multicomponent

rc is

ni

of simulation

a comprehensive

Ot

equation

pressure

For the simulations

l+_Bjpjnj J

agreement

_C i

for the spreading

until Eq. 3 is satisfied.

on the Langmuir-Freundlich

qmiBiPi

qi

of these

by iteration

a value

on the 5a :



limitations

temperature,

results than lAST model based on the non-ideality

to discuss

CO2/Nz and H20/CO2/N2

The favorable

and the solid

1-e

Equation

balance

for the adsorbate

concentration

d _"

is

(5)

e

and initial conditions: att O,

%._=O

x< L (9)

forx=O for x= L

OX

Note that Tw is calculated

in Eq 12.

Solid Phase The energy

equation for the solid phase includes

phase plus heat generation

Energy

Equation

the term for heat flux from the solid phase to the gas

due to adsorption.

m

aT s

a2Ts

p sC ps -'_-t = k s 7

Boundary

n

d qi

+ hsa s (Tg - Ts ) - i=1 ]_ AHi _

(10)

and initial conditions:

att O,

Ts=T

att>-O,--_-=O

i

x