Removal of VOCs from air by membrane-based absorption and stripping

j o u r n a l of MEMBRANE SCIENCE

ELSEVIER

Journal of Membrane Science 120 (1996) 221-237

Removal of VOCs from air by membrane-based absorption and stripping Tarun K. Poddar, Sudipto Majumdar, Kamalesh K. Sirkar * Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark. NJ 07102, USA

Received 9 January 1996; accepted 6 May 1996

Abstract Atmospheric emission of volatile organic compounds (VOCs) such as toluene, xylene, acetone etc. from industrial facilities causes serious environmental problems and financial losses. Existing technologies for VOC emission abatement have many strengths as well as considerable limitations. A regenerative absorption-based process for removal of VOCs from N 2 in an inert, nonvolatile, organic liquid flowing in compact hollow fiber devices has been studied here. These devices eliminate flooding, loading and entrainment encountered in conventional absorption units. Detailed experimental results and theoretical analyses for absorption studies were communicated elsewhere. The overall performance of the combined absorption-stripping process is described here; it appears to be controlled by stripping due to the low temperature and the lower membrane surface area in the stripper. The difference between only absorption and combined absorptionstripping results was more pronounced for VOC-absorbent systems having higher Henry's law constant and diffusivity. A theoretical model has been developed from first principles to simulate the behavior of the membrane stripper; this has been combined with the model for the membrane absorber to determine the overall process performance. Simulated results obtained from the mathematical models agree well with the experimental results for combined absorption-stripping. Simulation results suggest that higher stripping temperature and larger stripper area enhance the performance considerably. Keywords." VOC removal; Nondispersive regenerative absorption and stripping; Microporous and porous membranes; Fiber membranes; Composite membrane

1. Introduction Atmospheric discharge of VOC-contaminated N 2 (purge) streams in chemical plants and air streams in chemical processes poses a serious environmental problem and entails large financial loss. Such emissions may be reduced by: (i) activated carbon adsorption; (ii) absorption in a liquid; (iii) incineration

* Corresponding author.

or thermal oxidation (usually without energy recovery) or catalytic oxidation. These techniques have their pluses and minuses. There is therefore a continuing search for techniques which do not suffer from their limitations [1-4]. Another emerging technique involves the use of a nonporous permselective rubbery membrane having a high selectivity for VOCs [5]. In such a technique, the vapor-laden air is usually at atmospheric pressure; the permeate side is maintained under vacuum. The feed VOC concentrations are generally low; thus the partial pressure driving force for VOC perme-

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T.K. Poddar et al./Journal of Membrane Science 120 (1996) 221-237

222

ation is small and decreases along the membrane length as the feed VOC concentration is depleted by permeation. It is uneconomic to bring down the VOC concentration below about 100-200 ppmv. We have recently proposed [6] highly efficient compact hydrophobic hollow-fiber absorbers to remove VOCs from N2/air; the VOCs are recovered next for recycle by applying vacuum in a hollow-fiber membrane stripper and subsequent condensation to prevent air pollution. The many advantages of these hollow fiber devices resembling shell-and-tube type heat exchangers are: very high gas-liquid interfacial area per unit volume; independent variation of gas and liquid flow rates without problems like flooding, loading, weeping etc.; no entrainment of drops in the exiting gas thus eliminating demisters; much higher volumetric mass transfer coefficient than conventional contactors; known gas-liquid interfacial area and modular nature. They have been used to absorb gases in aqueous solutions [7,8] or strip volatile species from water [9,10]. The absorbents studied here are, however, inert, nontoxic, essentially nonvolatile and water-insoluble organic solvents. This paper focuses on VOC stripping from the inert absorbent and the overall performances of the combined absorption-stripping process. Before such a process, its model and detailed results are considered, it is important to indicate briefly the nature of the membrane absorption process [6]. In the proposed scheme for VOC absorption, contaminated air

flows in the fiber lumen and a suitable nonvolatile and inert absorbent having a high VOC solubility is pumped countercurrently over the outside of the fibers (Fig. 1). The VOCs are effectively removed from N2/air and concentrated in the absorbent. They are removed next from the spent absorbent which is regenerated by applying vacuum (and/or by heating the spent absorbent). The solutes and the absorbent can be reused since the latter is essentially nonvolatile. Stripping is carried out in a separate hollow fiber module. Studies were carried out by Poddar et al. [6] for VOC absorption with fresh absorbent liquid. Combined absorption-stripping was achieved here by recycling the absorbent liquid after VOC stripping. The present technique will make ultimate destruction of VOCs much more efficient than direct incineration of VOCs in air. First, a much smaller volume of liquid is to be incinerated. Secondly, the recovered VOCs will eliminate supplemental fuel-firing. Of the two novel types of membrane absorption utilized, the first type uses microporous hydrophobic membranes and the pores are spontaneously wetted by the absorbing organic solvent contacting the N2/air. The absorbent pressure is equal to or lower than that of the gas stream in the fiber bore to achieve nondispersive gas absorption; the gas-liquid contacting interface is at the pore mouth on the gas side of the fiber. The partial pressure profile of a VOC being absorbed in this configuration is illus-

voc enriched air

Absorber L

4~'t

NY

VOC lean air

Heater I

ooler

Stripper

Condenser • Vacuu~~m Pump

Solvent

Noneondensables Fig. 1. Schematicprocessdiagramfor VOC removalfromair by absorptionand stripping in hollowfibermodules.

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

(a)

~/eUq~d Microporous memhr~

..... ~

Absorbent

liquid inside the

,

pore

~

~

-

-

-

=

I [_ . . . . . .

-

~

--'--'--~~-----~-[-~---~-~------C W/J///////////~-_--_--_--_-~_----------

Nitrogen

boorben,

~_---_---_---~:--_----

r I I

ri

.

ro re

(b)

1liquid ~

Pgas

I Gas filled pore ' j

" ,

Micro~orous ~' I/ membrane

Silicone skin

_::=6::~_:__-~:__-=:_

~-~-__-! with VOC

]

- Z - Z - ~

- _

............

Tube

ro rc re

Abso:bent

~::-~::::::

,I

Shell

i I i

Fig. 2. (a) Local partial pressure and concentration profiles of VOC being absorbed in a microporous/porous hollow fiber module. (b) Local partial pressure and concentration profiles of VOC being absorbed in microporous/porous hollow fibers having a nonporous silicone skin on the outer surface.

trated in Fig. 2a. This is quite distinct from conventional membrane-based gas-liquid contacting [11] where the pore is usually gas-filled, the absorbent does not spontaneously wet the hydrophobic fibers, the absorbent is at a pressure higher than that of the gas and the gas-liquid interface at the pore mouth is on the liquid side of the fiber [7,8]. Jansen et al. [12] have attempted VOC recovery from air in micro-

223

porous membrane contactors using organic solvents which wet the membrane pores. Successful stabilization of gas-liquid interface at the pore mouth on the gas-side of the membrane could not be achieved due to a lack of proper pressure differential conditions. In the second type of membrane absorption, there is an ultrathin ( ~ 1 /~m) but highly VOC-permeable plasma polymerized nonporous silicone skin on the outside surface of the hydrophobic hollow fiber. The phase pressure conditions are quite different to ensure nondispersive VOC absorption. One can reduce by orders of magnitude any trace contamination of the feed gas by the essentially nonvolatile absorbent since the thin nonporous skin is a significant barrier to permeation of the higher molecular weight absorbent molecules. The absorbent flow may be in the fiber lumen or the shell side. Poddar et ah [6] primarily studied shell-side absorbent flow to eliminate liquid phase mass transfer resistance in the pores of the microporous support beneath the nonporous skin and avoid excessive pressure drop the liquid phase would encounter while flowing through the fiber lumen. Fig. 2b illustrates the partial pressure profile of a VOC being absorbed through such a skinned membrane. For shell-side organic absorbent flow and tubeside gas flow, the liquid and the gas pressures should be essentially equal or the liquid pressure should be higher for the skinned membrane (Fig. 2b). Otherwise, if the gas pressure is higher, gases like 0 2, N2, etc. will permeate easily through the silicone skin and bubble through the flowing organic solvent making the process dispersive. Further, these air or N~ bubbles will be highly contaminated with the VOCs and would have to be recycled to the feed after disengagement from the organic absorbent at the module end. That will significantly reduce the gas scrubbing throughput [6]. The skinned fibers used here are intrinsically different from conventional coated fibers. The skin developed on the microporous hollow fiber substrate by plasma polymerization forms a much stronger bond with the substrate. It has much greater resistance to delamination and solvent swelling than conventional silicone coated fibers which are likely to develop leaks and detachment from the porous substrate; this may lead to performance instability and deterioration [13]. A module based on such fibers

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

224

Microporous membrane

Paratherm TM, a heat transfer oil; both are inert, nontoxic, essentially nonvolatile and water insoluble. The VOCs investigated, acetone, methanol, dichloromethane and toluene, represent broad classes of VOCs emitted. Nitrogen was used as carrier gas. Low feed VOC concentrations (1000 ppmv maximum) were employed to study absorption as a polishing technology in competition with activated carbon adsorption. This paper presents experimental studies as well as mathematical models. A stripping model was developed from first principles. Solutions were obtained locally in analytical form and then employed in a numerical technique to obtain the simulated results. The overall performance of the combined absorption-stripping process was simulated by combining the individual absorption model from Poddar et al. [6] and the stripping model developed here.

[

Silicones~n F/-r_-_-_-_-_------:._-_-_-_------

Absorbent liquid

v.

. . . .

~ubo ~

~i=----~:---:

~bell

rc

Fig. 3. Local partial pressure and concentration profiles of VOC being stripped from absorbent in a hollow fiber module.

was also used for stripping in the simultaneous absorption-stripping process. VOC-contaminated absorbent liquid is passed through the shell side of the module and vacuum is applied through the tube side. VOC gets desorbed from the liquid absorbent phase and permeates through the highly VOC-permeable silicone skin and gets condensed in the condenser. Since the solubility of N2/O 2 in the absorbent is extremely small, the rate of production of noncondensables from the condenser is extremely low. Fig. 3 illustrates the profiles of concentration and partial pressure of a VOC being stripped from the absorbent through the silicone skin. The absorbents used were: silicone oil 50 cs and

2. Experimental 2.1. Membranes and modules

Two types of hollow fibers were used in the experimental investigations; the first one was microporous hydrophobic Celgard TM X-10 made of polypropylene (Hoechst Celanese, Charlotte, NC). The second one was a microporous polypropylene hollow fiber having an ultrathin ( ~ 1 /~m) plasmapolymerized nonporous silicone coating on the outside surface (AMT, Inc., Minnetonka, MN). For both

Table 1 Geometrical characteristics of hollow fiber modules Module no.

1 2 3

a b c d

Type of fiber

Celgard a X-10 Celgard X-10 Celgard b with a silicone skin

Fiber i.d.

Fiber o.d.

Effective length

Shell i.d.

(cm)

(cm)

(cm)

(cm)

0.01 0.01 0.024

0.015 0.015 0.030

35.7 31.0 20.5

0.60 0.37 0.80

No. of fibers

600 102 300

Fiber tortuosity: 2.5; surface porosity: 0.3 [17]. Hoechst Celanese SPD, Charlotte, NC. A M T Inc., Minnetonka, MN. Based on outside diameter. Based on inside diameter.

Void fraction (%)

62.5 83.2 57.8

Mass transfer area (cm z)

Mass transfer area per unit volume ( c m 2 / cm 3)

c

d

c

d

1009 149 580

673 99 464

100.0 44.7 56.2

66.7 29.8 45.0

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

kinds of fibers, parallel flow modules were made by inserting the appropriate number of fibers within a metallic shell and potting the ends with epoxies [14]. Table 1 provides details of the geometrical characteristics of the modules, the fiber properties and the fiber surface area.

2.2. Chemicals The absorbents used were silicone oil 200 fluid (Dow Coming, Midland, MI) and Paratherm NF ® (Paratherm Corp., Conshohocken, PA). Paratherm NF oil is mineral-oil-based, very stable and possesses an extremely low vapor pressure (0.001 mmHg at 100°F). The VOC-N 2 mixture for each VOC (993 ppmv acetone, 999 ppmv dichloromethane, 514 ppmv methanol and 236 ppmv toluene (certified standard)) was obtained in primary standard cylinders (Matheson, E. Rutherford, N J). Other gas compositions needed for gas chromatograph (GC) calibration were prepared by blending such a mixture and N2-zero gas (99.99% N 2) in predetermined ratios via two electronic mass flow transducer-controllers.

2.3. Experimental procedures The schematic diagram of the experimental setup for combined absorption-stripping is shown in Fig.

CPR

PC

! ~ PG

4. Regenerated absorbent liquid was pumped by means of an electronic metering pump (10818M, LMI, Milton Roy, Acton, MA) from a small glass container to the shell side of the absorber module for absorbing the VOC from VOC-N 2 feed gas mixture flowing through the tube side of the module countercurrently with respect to the absorbent flow. The absorbent exiting the absorber was connected to the shell side of the stripper hollow fiber module. A vacuum pump (HyVac 2, 9130808-010; HyVac ® Products, Norristown, PA) was connected to the tube side of the stripping module via a condenser. The purified absorbent liquid from the stripper was continuously recycled back to the absorbent circulation vessel. This vessel was closed tightly to avoid any VOC escape from the holdup liquid. The gas outlet from the absorber was connected to a gas chromatograph (GC) (3400 STAR, Varian, Sugarland, TX) via a back pressure regulator (10BP, 10132BPJT; Fairchild Industrial Products, Winston-Salem, NC). The GC outlet was connected to a bubble flow meter for manual measurement of gas flow rate. Finally the exit gas was vented out through a laboratory fume hood. Before starting an experiment, the shell side of the absorber module was first filled with the absorbent to wet the membrane in the case of porous membranes without a nonporous coating (for a mem-

Po

FNC

~

]

c°ntaminantl

HFM

CV BPR

HFM

with

225

TO GC

with

[ s k i n n e d fibers

I Condenser

ASV A b s o r b e n t S t o r a g e Vessel BPR Back P r e s s u r e R e g u l a t o r CPR Cylinder P r e s s u r e R e g u l a t o r CV Check Valve FNC Feed Nitrogen Cylinder GC Gas C h r o m a t o g r a p h HFM Hollow Fiber Module MFC Mass Flow C o n t r o l l e r

~r.~ ~ ~ I . VP Recovered s ol ve nt

To MP exhaust

ASV

MP PG

Metering P u m p P r e s s u r e Gauge

RM VP

Rotameter Vacuum Pump

Fig. 4. Schematic diagram of experimental setup.

226

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

brane having a nonporous coating, there is no need to fill the shell side with the absorbent liquid beforehand). Then the V O C - N 2 gas mixture flow was switched on at a predetermined rate through the tube side of the absorber module. The constant gas flow rate was maintained and monitored by an electronic mass flow transducer (8141, Matheson, E. Rutherford, NJ) and controller (8209, Matheson, E. Rutherford, NJ). The tube side gas pressure at the outlet of the absorber was maintained at 3 psig (122.01 kPa) above the liquid phase pressure for porous membranes by adjusting the back pressure regulator. Liquid phase pressure was essentially atmospheric. Absorbent circulation pump was then started. First the liquid flow rate was measured manually by collecting the liquid in a measuring cylinder over a definite period of time from the outlet of the stripper module. Once the liquid flow rate was set, the stripper outlet liquid line was connected to the absorbent circulation vessel. The liquid volume inside this vessel was in excess over the amount required to fill the pump suction line and the hold-up volume of the setup. The volume of the circulation liquid was kept at the lowest possible level to reduce the time required to achieve a steady state. The VOC concentration of exit gas was monitored in the GC every hour. Time taken to attain a steady VOC composition at the absorber outlet was found to be approximately 7 to 8 h. The experiments were repeated for different gas flow rates as well as liquid flow rates. Continuous runs as long as 120 h were carried out without any problem.

2.4. VOC concentration measurement VOC concentrations in the gas phase were measured in the Varian 3400-STAR GC using a flame ionization detector (FID). The response from FID was recorded in a built-in integrator. Certified standard (toluene-N 2) and primary standard (acetoneN 2, dichloromethane-N 2, and methanol-N 2) gas mixtures from Matheson (E. Rutherford, NJ) were used for calibration. The gas mixture was injected into the GC column through a 6-port gas sampling valve (Valco, Houston, TX). Dry air at 80 psig pressure was used to drive the solenoid valve in the actuator of the 6-port valve in the GC. A 6' × 1/8" column (Varian Analytical Instruments, Sunnyvale,

Vacuum

qg,ou~ Purified nitrogen

Cil,in

c~ Az

2 e dz absorbent. v Cil,out = 6~11,in

lontaminat

AZ

__ z= L

(o) Recycled a b s o r b e n t

liquid (purified)

ig,in VOC laden nitrogen

Fig. 5. Schematic diagram for simulation of simultaneous absorption-stripping.

CA) having 0.3% carbowax 20 M on a Carbopack C support was used for analysis.

3. Theoretical Considerations To describe the performance of the simultaneous absorption-stripping process, one needs to have a model for absorption and another model for stripping and then couple them together. A detailed description of the absorption model based on fresh absorbent flow has been provided in Poddar et al. [6]. We provide here a similar treatment for the stripping process• To facilitate an easy understanding of this stripping model, some basic results of the absorption model are provided here from Poddar et al. [6]. For countercurrent VOC absorption in membrane devices of length L built of porous hollow fibers with or without an ultrathin nonporous silicone skin, a generalized absorption model has been developed [6]. The governing equations in such a model are solved analytically to determine the incoming gas and exiting liquid phase concentrations for assumed module exiting gas phase concentration in a small segment of length Az ( = L / n ) at z = 0 in the membrane device where n is a large number (Fig. 5). Such analytical expressions are then employed stepwise to reach the gas inlet at z = L; then an iterative solution procedure is employed for the split boundary value problem to be solved [6].

227

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

segment of length A z located at the module top (z = 0), < 4}ig > , can be expressed in terms of that at the outlet ~ig.out as

To model the complex shell-side flow, Happel's free surface model [8,15,16] is employed. Hollow fibers are assumed to be distributed in a regular fashion; every fiber is surrounded by an equal volume of liquid envelope whose boundary radius, r e , is the free surface (Fig. 6) across which no mass, momentum or energy transfer takes place:

re = rd~/Nf

- 2~rAig P (~l~iml~:o -- Hil (/}ig]sc= 1) na

< ~ig > =

X ('7~t) ] ref "}- qSig,out

(1)

Similarly, the nondimensionalized bulk liquid-phase concentration at the outlet of the first segment located at z = 0, < qS~l> , can be obtained in terms of that at the inlet &iUn as

Analysis based on a single fiber may then be extended to the whole shell side of radius r S containing Nf fibers.

3.1. Results of generalized absorption model

r so=--;

ri

t'=-

fig =

<&">

n(V,>[( A~d~c )

=

," 41il =

Here VOC concentrations of porous membrane-phase at r = r o and gas-phase at r = r i respectively. The expressions for these two interfacial concentrations were obtained by solving the basic equations of continuity [6] as

d%ml& = {&,g,ou,[ H,,QY + ( H~IHI2P/a)] + ~i,,in(1 + 4PXHi,)}

_ _ .

fig,in

z

/{QY+Hi2(1 + P / a + 4 P X H i , ) }

+ 4PXq~il,in}

The nondimensionalized bulk or cup-mixing gasphase VOC concentration at the inlet of the first

/{Or+Hi2(1 +P/a+aPXH~)}

£ Free

/

surface

/

E--_>__-->--__-)£-_, f_

_-_-_

r e

_ _

-~-I---S£--

<<-_--_--_--~_-_-_---<-;

X-----

S--X----5<--£-d-i<-i-i---<-i<-

,~_- -_-_- -_-_-_-_-/

(5)

4,i~ l e=-, = {IJ)ig,out(QY + Hi2 + Hi2 P/a)

(2)

L

- B]

X ((/)irnl~]o- Hil(/}ig[~ =1 ) -1- {/}il,in (4) ~im[~o and ~igl~- 1 are the nondimensionalized

Cil

C~g,m

2 rr LDil Qe'

-

A generalized model for VOC absorption, applicable to both porous fiber as well as skinned fiber was developed to predict the gas-phase and liquidphase concentrations of VOC at the module exit. Detailed model development has been communicated elsewhere [6]. The analytical results of the absorption model and the overall numerical strategy are outlined below. The nondimensional quantities used in this analysis are defined as

Cim ~im = Cig,i-"~ ; ~ig

(3)

-

.

-

.

-

_

-

.

-

.

-

.

-

_

-

-

-

-

~_-_-

-

-

-_-_

re

/

Silicone

skin

Microporous membrane w a l l

Fig. 6. Absorbent-filled annular space associated with a single hollow fiber.

(6)

228

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

Here a, P, Q, X, and Y are functions of gas diffusivity, liquid diffusivity, effective membrane diffusivity, skin permeance, tortuosity, porosity and geometrical properties of the module; their expressions are available in the Appendix. The effective diffusivity in the membrane phase is given by [17] Dim = Dil E/q"

(7)

where D~ is the diffusivity of VOC in the absorbent liquid inside the pores. Hi~ is dimensionless Henry's law constant of species i between the gas phase and the membrane phase. Hi2 is dimensionless Henry's law constant of VOC between membrane phase and liquid absorbent. The porous fiber has no coating; therefore, ~c=~o, ~ - - 0 and a = ~ [Eq. (A1)]. Absorbent liquid wets the fiber and fills pores (Fig. 2a). The pore fluid is identical to that in the shell side. Hence, Hi2 = 1. Hi1 here is simply H i, the Henry's law constant between the gas and the liquid phase.The nondimensional gas-phase VOC concentration at the exit (z = 0) of the module (O, which is essentially (Dig,out for the top most segment) is calculated as follows: I. A value of • was assumed as (Dig,out and < (Dig> and < (D~l> were calculated for the first segment of the module having length A z (Fig. 5). < Vt > was calculated at the average gas pressure of the segment. 2. (Dig,out and (Dil,in were replaced by < (Dig > and < (D~l> respectively and < Vt> was calculated at the new average pressure computed for the next segment. 3. Steps 1 and 2 were repeated till the gas entrance location of the module was reached. 4. < (D~g> calculated for the last segment was compared with the known value of (D~e,in"If the two values matched within the given tolerance, then the assumed value of qb will be the required dimensionless VOC concentration at the exiting gas stream. Otherwise, steps 1 to 4 were repeated with a different value of qb. The convergency of the computation was achieved by the simple bisection method. 3.2. Model f o r the stripper

The VOC stripping operation was carried out in the skinned fiber module. The VOC-contaminated

absorbent liquid flow was on the shell side and vacuum was applied through the tube side (Fig. 3). To analyze the stripping process, a simpler approach has been adopted. The VOC concentration in the tube side would be extremely low due to the extremely low absolute pressure. Although there will be variations in VOC concentration along the module length, due to the high vacuum, a constant average VOC concentration in the tube side could be reasonably assumed. The shell-side analysis for the liquid phase is done via Happel's free surface model [15]. Other assumptions made in obtaining the governing equations of species balance and boundary conditions are: steady state, isothermal conditions, constant viscosity of the absorbent liquid, constant diffusivity of VOC in the liquid absorbent, fully developed concentration profiles in the shell side, no axial diffusion in the shell side, no convective transport of VOC in the gas-filled membrane phase, negligible pressure drop for the absorbent liquid flowing through the shell side, negligible N 2 solubility in the absorbent liquid, an overall q o / 8 o for the composite membrane (porous membrane plus nonporous skin), and Henry's law is applicable. Let us introduce new nondimensional variables for liquid phase concentrations in the stripper. Other nondimensional variables used here are the same as in Eq. (2): s

(Dig

C~

C~

Cil.in ,s

(~i~ = Ci Is.in

(8)

The bulk liquid phase concentration at the outlet of the stripper segment of length A z at z = L, < (D~ > , is obtained by equating the convective rate of molar transfer of species i across the segment volume of length A z with the diffusional rate of mole transfer of the same species across the radial boundaries: •

il .in

27r(r°)lmLsq°((Di]l'c n ~o

((DiSg))

(9)

Hi

(Di][£c, the wall concentration at ~:= ~c, can be obtained by solving the following mass conservation equation for species i in the shell side 1 0 [ 06~ l ~ O(DSl

D,,? f-V)=

0-7-

(10)

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T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

For fully developed laminar flow through the cylindrical annular space (Fig. 6), the velocity profile in the positive z direction is given by [15]

= 2(V~s)[1- ( ~ : J ~)] ~

2

~

( ~:/ ¢) - ( J~:~)" + 21n( s~o/s~ )

X

3 q- ( ~ c / / ~ e ) 4 -- 4 ( ~ c / S C ~ )

<(~i~)=(~sil.in

~: = ~e, ( O h 3 / @ ) --- 0

(19)

(12)

The constant average concentration of VOC in the tube side under vacuum can be related to the absolute partial pressure p~ as

(bi~))

(13)

H---7a, = % ( r o ) m / 6 o Oil ,~,,

(14)

[18]: ,~¢/az Di,-'-~ - < ) = f£c( b~l.in - bi]) yes( ~:) r2~d ~

(15)

Here bi].~n is the nondimensional concentration of VOC in the liquid phase at the stripper inlet. Eq. (10) can be solved with the boundary conditions, Eqs. (12), (13) and (15) and the solution can be written at

as (16)

Here Ns = a s

Hi

From Eqs. (16) and (17), the following expression for wall concentration is obtained: bi~[< =

bi],io + a~Y(, big ) a, 1+ Y

(18)

p;

s

(big)

RTCi] ,in

(20)

From Eqs. (19) and (20), the following condition for effective stripping can be obtained:

nip b~l ,in

A simple mass balance for the segment of length Az at z = L can be used as the third boundary condition

bi~ll¢~ = bi~,in -- N, y

2 7r ( Sr °o() V~ Ln L) s q ° ( & isl"-H] in - -+--~ H i ( Yb ig )

(11)

At the outside surface of the nonporous silicone coating, diffusional mass transfer rate is equal to the permeation rate through the composite membrane of thickness 8o: (b~jl¢~

--

2 + 41n(~c/SC~)

At the free surface, mass flux is equal to zero:

[ 0b~l]

The expressions for A, B and Y are given in the Appendix. Substituting Eq. (18) into Eq. (9), the expression for the bulk liquid phase concentration, < b~ > , at the exit of the segment volume at z = L can be obtained as

-

-

RTCi] ,in

>0

(21)

Where the liquid enters the stripper, &~l,in= 1. Assuming partial pressure of VOC species i in the vacuum side to be the absolute pressure, the following condition can be obtained from Eq. (21) and can be used to calculate the approximate vacuum level to be maintained for stripping:

RTCi] ,in p~< - Hi

(22)

Following are the steps employed to calculate the liquid phase VOC concentration at the exit of the stripper module. 1. From the known parametric information of the module, quantities A, B, D, a', b', c', d', e', S and a s for the stripper module were calculated from the relevant equations in the Appendix. 2. For the small segment of length Az at z = L, the value of Y was calculated. 3. < b~l > was estimated from the known value of bi].in (for the first segment at z = L, biSl,in = 1). 4. b il,in ~ value was replaced by < b~ > for the next segment and step 3 was repeated till the exit of the module (z = 0) was reached. 5. < bi~ > obtained for the last segment is the outlet nondimensionalized concentration of species i at the exit of the stripper module.

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T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

6. Actual outlet concentration can be obtained by multiplying < ~b] > for the last segment with the actual inlet concentration. 3.3. Simulation steps f o r combined absorption and stripping

1000

4. Results and discussion The results for simultaneous VOC absorption and stripping using silicone oil are provided in Fig. 7 in terms of the cleaned gas composition as a function of the feed gas flow rate for a given absorbent recirculation rate. Full vacuum was applied to the bore of the fibers in module no. 3 to remove the VOC from the absorbent flowing on the shell side on a continuous basis. The actual level of pressure could not be monitored due to the sensitivity limitations of the vacuum gauge. In module no. 2, the absorber, the flow arrangement was similar to simple absorption experiments [6], namely, gas through the fiber bore and the absorbent on the shell side (and in the pores) flowing countercurrent to the gas flow direction. It was observed that the feed gas containing 999 ppmv of dichloromethane was brought down to around 20 ppmv on a continuous basis when the feed gas flow rate was less than 0.1 c m 3 / m i n / f i b e r . Results of simultaneous absorption/stripping experiments for different VOCs (dichloromethane, methanol and

O

E 8OO e" @

700 600

e"

Numerical simulation for combined absorption and stripping can be easily carried out by combining individual absorption and stripping models developed earlier. The steps are as follows. 1. For an assumed value of ~bil,in (at the arbitrary point O in the recycle line, Fig. 5), t~ig,out and (~il,out were estimated for a given (~ig,in and < Vt > ref for the absorber. 2. Equating (~il,out to (Di~,in, (~i~,out was calculated for given < Vs > (Fig. 5). 3. Calculated ~b~,out was compared with the assumed value of ~biLin at the point O (see Fig. 5). If the two values matched, then ~ig,out obtained in step 1 would be the required (big,out for the combined absorption-stripping process. 4. If two values did not match, then steps 1 to 3 were repeated with t~il,in : ~Sl,out.

Absorption (module # 2) Stripping (module # 3)

900

0

5OO

0

400

o v v

300 O

200 ©

v

t00 o

o

oN o

I 20

A

zx

tx

4JO

610

I 100

810

I 120

140

Gas flowrate (cm3/min) Fig. 7. Steady state outlet concentration vs. gas flow rate for simultaneous absorption and stripping of different VOCs: ~ and [3, dichloromethane999 ppmv; A, toluene 214 ppmv; ~7, methanol 514 ppmv. Absorbent circulation rate: 5.2 ml/min (O, Zk and xT: silicone oil; ©: Paratherm). toluene) and silicone oil absorbent are also shown in Fig. 7. Results of dichloromethane removal by Paratherm oil are also provided in this figure. The relative removal performances of different VOCs are similar to that observed in simple absorption experiments [6]. Fig. 8 illustrates the variation of the gas phase outlet concentration with the absorbent liquid flow 500 450 Absorber : module # 2 Stripper : module # 3

400 ~'~ ¢. 350 @ "~ 300 o

250

o

200

e~

150

o

o

100 ©

50 0 0

I

I

I

I

2

4

6

8

ll0

112

14

1L6

]1

20

3

Liquid flowrate (era/min) Fig. 8. Steady state outlet concentration vs. liquid flow rate for simultaneous absorption and stripping of acetone. Gas flow rate: l0 cm3/min; feed concentration: 993 ppmv; temperature: 25°C

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

rate for acetone removal by simultaneous absorption-stripping for a fixed gas flow rate. The exit gas phase concentration of VOC was found to be virtually independent of the liquid circulation rate. This behavior may be explained as follows. The particular 500 ~-

(a)

Absorption: module# 2 Stripping : module# 3

E

400 t

© >

300 ~-

e~

200

8 100

•

©

D

0

2O

r~ I 40

610

8L0

I 100

I 120

Gas flow rate (cm3/min) 500

(b)

~ Absorption: module# 2 Stripping : module# 3

&

400

© >

~ 30o Q

200, o

©

lOOi 0

0

•

20

410

610

810

I

I

100

120

Gas flow rate (cm3/min) Fig. 9. (a) Comparison between dichloromethane removal results by only absorption and combined absorption-stripping. Absorbent: silicone oil. Feed concentration: 999 ppmv. D: only absorption (absorbent flow rate: 3.7 ml/min; temperature: 22°C); I1: absorption-stripping (absorbent flow rate: 5.2 ml/min; temperature: 22.5°C). (b) Comparison between methanol removal results by only absorption and combined absorption-stripping. Absorbent: silicone oil. Feed concentration: 514 ppmv. A: only absorption (absorbent flow rate: 3.7 ml/min; temperature: 22.2°C); - : absorption-stripping (absorbent flow rate: 4.5 ml/min; temperature: 25°C).

231

stripping module has a finite membrane area and hence a finite VOC removal capacity. Increase of liquid circulation rate decreases the liquid film resistance and makes the resistance of the silicone skin controlling. However, limited stripping capacity leads to a finite residual acetone concentration in the regenerated absorbent. This residual acetone concentration at the absorber liquid inlet limits the outlet gas composition via a pinching condition at the gas outlet end of the absorber module. A comparison of the removal performance of dichloromethane and methanol between only absorption with fresh absorbent and closed loop absorption-stripping experiments is shown in Figs. 9a and 9b. For dichloromethane, simultaneous absorptionstripping experiments exhibit somewhat poorer performance than absorption alone employing fresh absorbent (Fig. 9a). This difference is due to a lack of sufficient regeneration of the circulating absorbent in the stripper used. Stripping at a higher temperature and/or larger membrane area would substantially reduce this gap. On the other hand, in case of methanol, the performances in terms of the outlet treated gas phase concentrations in the two processes are almost identical (Fig. 9b). It was easy to strip methanol from the absorbent oil due to a much lower H value. This indicates that the higher is the H value of a VOC-absorbent system, the higher will be the difference between results in absorption based on fresh absorbent and combined absorption-stripping process under similar process conditions where stripping is limiting. This observation is further supported by the simulation results provided at the very end of results and discussion. We now focus on how well the model describes the observed results. To that end, we discuss calculations or experiments to obtain various quantities. The value of @ was calculated from the experimental data for each VOC outlet composition. In the absorption model development, a quantity appears in the denominator in the dimensionless expression of averaged VOC concentration [Eq. (3)] as ( < Vt > / D i g L)re f. This quantity, a dimensionless number known as the Graetz number, is generally expressed as [19] 7rr = / (Vt) / ( NGz)ref = ~-{ ( NRe Ns¢)ref [ D-~g L }ref

(23)

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

232

Table 2 Diffusivity and permeance data for various VOCs VOC

Acetone Methanol Dichloromethane Toluene

Silicone oil (50 cs)

Paratherm

Temperature (°C)

Diffusivity (cm2/s)

Temperature (°C)

Diffusivity (cmZ/s)

24 26 25 26

2.81 4.71 4.30 7.63

24.5 25.5 24.6 25

3.60 X 1.86 X 8.50 X 4.80 X

X X X X

10 -6 10 -6 10 -6 10 -6

Temperature (°C)

For a particular run, (NGz)re f remains constant and varies only with gas flow rates for different runs. Dimensionless gas phase VOC concentration at the absorber module exit (q~) was calculated for different gas flow rates from the simulation for the combined absorption-stripping process. The steps for such simulations have been discussed earlier. From the independent variable, gas flow rate per fiber (< Vt>ref) , (NGz)ref was then obtained using the value of (Dig)ref calculated from the following equation [20]: Dig = 27.3051 X 10 -3

XT3/2 [ (Mi + MN2)/MiMN2]1/2 P O . i 2 J'-2D

(24)

Here p is the absolute pressure in psi unit. The expressions for criy2 and J2 D are given in the above reference. Different parameters, e.g. Henry's law constant for different combinations of VOCs and absorbent, diffusivity of VOCs in the absorbent liquids, and permeability of different VOCs through the nonporous silicone skin of skinned fibers are needed as

10 7 10 -7 10- 7 10 6

20.4 19.5 22.4 21.0

Permeance ( c m / s ) Composite membrane

Silicone skin

(qo/6o)

(qc/6~.)

3.2X 10 -3 2.89 X 10 -3 5.01 x 10 -3 11.48)<10 3

3.22 X 2.91 X 5,06 X 11.75 X

input to the mathematical models to obtain simulated results. None are available in open literature or can be predicted with certainty. They were determined via independent experiments [14]. Henry's law constants were measured by the variable volume headspace-GC technique. Diffusivity of a VOC in the absorbent was measured indirectly by carrying out sweep gas-driven VOC permeation from nitrogen through an absorbent-filled immobilized porous membrane. Permeability values of VOCs through the nonporous silicone skin were estimated by simple standalone permeation experiments for each individual VOC with the skinned fiber. The values of VOC diffusivity in the absorbent liquid and permeability through the silicone skin are given in Table 2. The Henry's law constant was expressed as a function of temperature by the following equation [21,22]: H i = e x p ( TBH' -AH~)

(25)

The constants AH~ and B/4i depend on the solventsolute combination and are provided in Table 3. They were determined by fitting Eq. (25) and experimentally obtained Henry's law constant data at dif-

Table 3 Parameters of temperature dependent Henry's law constant in Eq. (25) VOC

Acetone Methanol Dichloromethene Toluene

Silicone oil

Paratherm oil

A14~

B.~

5.878 3.240 3.982 2.080

2948.8 1802.3 2504.4 2375.0

10 -3 10 -3 10 -3 10-3

AM~ 0.016 0.525 0.551 5.42

BH~ 1119.4 449.7 1444.6 3203.4

233

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

!

1.0 ] 0.9 ~-

0.7 ~ VOC : methylene chloride, 999 ppmv

Absorption (module # 2) Stripping (module # 3)

i

[

0.6 0"8tl ~ --[ ~,\ -0.7 ~ ~ • ~ uk v 0.6 [_ L \~ 0

Model simulation (Paratherm) Model simulation (silicone oil) methylene chloride (Paratherm) methanol (silicone oil) methylene chloride (silic . . . . il)

J

0.5

NGz=000544

0.4 e

0.4

0.3

0.3

0.2

0.2

Absorbent : silicone oil

Noz 000407 NGz=0"00272~ ! I!

NGz=0.00136

0.1 t_

0.1 0.0

;__1

5

I

I

I

I

I

I

0.01 0

I

10 15 20 25 30 35 40 45 50 (DigL/)refl0"2

Fig. 10. Ratio o f steady state outlet to inlet gas phase concentration of V O C s as a function o f inverse of Graetz n u m b e r for continuous absorption-stripping process: [] and •, d i c h l o r o m e t h a n e 9 9 9 ppmv" V , methanol 514 ppmv.

ferent temperatures. A tortuosity value of 2.5 [8,17] and surface porosity 0.3 [8] were used for the model simulation. Dimensionless gas phase VOC concentrations at the absorber outlet (q~) were plotted in Fig. 10 as a function of the inverse of gas phase Graetz numbers for removal of three different VOCs by the combined absorption-stripping process. Experimental results were compared with simulation results; all VOCs except methanol show a very good fit between the two results. It has been discussed earlier that the performance of the membrane-based overall absorption-stripping process is ultimately controlled by the limitations of the stripping process. Efficient regeneration of a good absorbent liquid for a given VOC will always be a difficult task unless a signifcant amount of energy is spent. To increase stripping efficiency it is required either to operate the stripper at a higher temperature (within the tolerance of the materials of construction) and/or to provide a higher stripper membrane area. The effects of stripping temperature and stripper area on the performance of overall absorption-stripping process were also simulated. Fig. 11 provides plots of @ vs. stripping temperatures at different Graetz numbers (gas flow rates). At a lower gas flow rate (0.25 c c / m i n / f i b e r , NGz = 0.0014), the drop in outlet VOC concentration is

lI0

2k0

310

410

5~0

610

Stripping temperature (°C) Fig. 1 1. Simulation plots of d i m e n s i o n l e s s outlet gas phase concentration vs. stripping temperature at different Graetz numbers. Absorbent circulation rate: 5 ml/min. Absorption temperature: 20°C.

about 46.7% for a rise in stripping temperature from 20 to 60°C. For a higher gas flow rate (1 cc/min/fiber, N6z = 0.0054), the corresponding drop is 22.7%. Fig. 12 illustrates the variation of 4) with stripper area at different gas phase Graetz numbers. A 78.5% drop in outlet VOC concentration was observed for a 5 fold increase in stripper area, at a gas flow rate of 0.25 c c / m i n / f b e r (NGz -= 0.0013). 1.0 VOC : methylene chloride, 999 ppmv Absorbent: silicone oil

0.9 0.8 0.7

e

N6z=0.0053 0.6 Ncz=00039 ~ 0.5 0.4 !Gz 0.002 0.30.2Naz 0

0

0

1

~

~

0.l0.0

0

i 200

L 400

i 600

I ~ 12100 14100 800 1000 Stripper area (cm2)

Fig. 12. Simulation plots o f d i m e n s i o n l e s s outlet gas phase concentration vs. stripper area at different Graetz numbers. Absorbent circulation rate: 5 ml/min. Temperature: 25°C.

234

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237 1.0

I

I

i

I

t

50

100

150

200

250

0.9 0.8: 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

0

300

Hi Fig. 13. Comparison of gas phase VOC concentration at the absorber exit as a function of Henry's law constant for two different stripper areas. O: only absorption; 0 : absorption-stripping. Gas flow rate: 100 cm3/min; absorbent circulation rate: 5 m l / m i n ; temperature: 25°C; Dig: 0.1 cm2//s; Dil: 4 . 3 × 1 0 .6 cm2/s.

Simulated VOC concentration at the absorber module gas phase exit (q~) was plotted as a function of Henry's law constant for only absorption (with fresh absorbent) and combined absorption-stripping in Fig. 13 with both operation carried out at 25°C. As observed experimentally, the difference in the outlet concentrations between the two processes increases as H i increases and became essentially constant at higher H i values. The gap between the two results reduces significantly when the ratio of the stripper area to the absorber area is two times that used in experimental setup. The gap between the two will also be considerably reduced if stripping is carried out at a higher temperature than that in absorption. As discussed before lower Hi at a higher temperature increases the stripping performance. Note, however, that Fig. 13 is an illustrative plot where changes in Henry's law constant are not associated with changes in Dil and Dig.

5. Concluding remarks VOCs were efficiently removed from a nitrogen stream by an inert organic nonvolatile absorbent to

potentially very low levels in a hollow-fiber-based continuous absorption and stripping process. The spent absorbent liquid was regenerated in a separate coated hollow fiber stripper by applying vacuum through the tube side and recycled back to the absorber. The present work was carried out with absorption in a porous fiber module and stripping in a coated fiber module. However, absorption can also be accomplished in the coated fiber module [6]. For a given V O C - a b s o r b e n t system, c o m b i n e d absorption-stripping achieved somewhat lower gas cleanup level (in terms of gas stream composition at the absorber outlet) than that for absorption experiments with fresh absorbent. This is because of the partial regeneration of the absorbent liquid in the stripper. Experimental results plotted as the dimensionless concentration of the treated gas stream VOC level (obtained from the combined absorption-stripping process) against the inverse of the Graetz number in the absorption module demonstrated good agreement with predictions from the mathematical model. Simulation results for combined absorptionstripping process show that a higher stripping temperature and a larger stripper area will improve the VOC removal efficiency considerably.

6. List of symbols ai a~ a' A A a, A s A& Am

b' B B~ c' Ci

defined in Eq. (A1) defined in Eq. (14) defined in Eq. (A12) defined in Eq. (A7) mass transfer area for absorber and stripper respectively (cm 2) constant in Eq. (25) membrane area based on inside diameter (cm 2) in case of porous fiber and based on outside diameter in case of skinned fiber (cm 2) defined in Eq. (A13) defined in Eq. (A8) Constant in Eq. (25) defined in Eq. (A14) local concentration of species i (g m o l / c m 3)

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

average concentration of species i (g m o l / c m 3) dr defined in Eq. (A15) inside diameter of the hollow fiber (cm) di defined in Eq. (A9) (cm2/s) D diffusivity of species i in gas phase (cm2/s) Dig diffusivity of species i in liquid phase O~ (cm2/s) diffusivity of species i in absorbent-filled Dim membrane phase (cm2/s) e! defined in Eq. (A16) dimensionless Henry's law constant of species i, defined as the ratio of liquid phase concentration (g m o l / c m 3) over gas phase concentration (g mol/cm3). effective length of the module (cm) L molecular weight of gaseous species i (g Mi m / g mol) molecular weight of N 2 MN, number of segments in z direction H Ne number of hollow fibers in a module defined in Eq. (17) Us N~z Graetz number defined in Eq. (23) pressure (atm, unless otherwise mentioned P in the text) partial pressure of VOC species i (atm) Pi defined in Eq. (A2) P permeability of VOC through the silicone qc skin (cm2/s) overall permeability of VOC through comqo posite membrane (cm2/s) Q defined in Eq. (A3) inside radius of the fiber (cm) ri outside radius of the coated fiber (cm) rc outside radius of hypothetical free surface re defined in Eq. (1) (cm) outside radius of the porous substrate (cm) ro inside radius of the shell (cm) rs (re)ira logarithmic mean of r c and r o (cm) logarithmic mean of r c and r i (cm) (ro)l m universal gas constant (cm 3 a t m / g mol K) R Defined in Eq. (A11) S Temperature (K) T < Vzt > average velocity of gas inside the tube per fiber (cm/s) average velocity of liquid in the shell side < Uzs per fiber ( c m / s ) volumetric flow rate per fiber (cm3/s)

X y, Y z AZ

235

defined in Eq. (A4) defined in Eq. (A6) and Eq. (A5), respectively z coordinate L/n, length of a small segment volume (cm)

6.1. Greek symbols 6 Ai~ e th~ < hi > K A q~ r ~"

thickness of the membrane (cm) Dig/(Dig)ref

surface porosity of porous membrane dimensionless concentration of species i average dimensionless concentration of species i defined in Eq. (A10) ref= < gt> / < V t > r e f

dimensionless radius, Eq. (2) ratio of gas phase VOC concentration at the module exit to that at the module entrance tortuosity of the porous membrane dimensionless z coordinate, Eq. (2)

6.2. Subscripts i in g 1 o m c out ref s t

VOC species, inside of the fiber inlet gas liquid outside of the porous fiber, overall composite membrane porous membrane nonporous silicone skin outlet ambient temperature and atmospheric pressure as reference condition shell side tube side

6.3. Superscript s

stripping

Acknowledgements The financial support of the Hazardous Substance Management Research Center at New Jersey Insti-

236

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

tute of Technology, Newark, NJ is gratefully acknowledged. We also thank Hoechst Celanese Separations Products, Charlotte, NC and Applied Membrane Technology Inc., Minnetonka, MN for generously supplying us with the Celgard fibers and the coated fibers, respectively.

---~3SCe + Into--

a = qc( rc)lm/ac Dig e = [Dim/Dig ] / l n

S~o

Q = [Dim/Di,]/'~¢ln '~o

(A13) 12See 21['c6(ln~:c-6)-~:e6(ln'e-1)]

(,: ln, 1)[(l)

(A1)

+

(A2)

-':(in'c- 4tl

(A3)

X - 9611 2nATrAig (DigL/(Vt))ref

~4)

D [ s~4(In s~e- 1 ) - s~ (ln ~:c- 1 ) 1

c'=

Appendix A

~-2](~4-

8s~e2

- (ln

(14)

4

8

~4 In ~:e-

4 so)2s~c/4

(A14)

d, = - _4;e _ [ ~:4 (ln 'e -- 1 ) -- ' 4 (ln ~c -- 1 ) ]

Y=Y/( A '~e'~c-B) ~d D~c2 Y= -----~ 16~e + T

LDi~ + Knr---7

(15) +

~¢2 2 ln{~c-A~eln {~c

S e'

(A6)

A = (3See/4) - ( s¢~2/2S~e)+ s~eln(scc/~:e)

(AS)

D = 1 + In ~ - 0.5( ~c/~)2

(A9)

K=

2(Vzs)[1-(re~G) 2]

3 + (re~re) 4 - 4(re~re) 2 + 41n(rc/re) (All)

S= a' + b' + c' + d'

,:_,: (,n,c 128~--e4 + 48s~2 1

48,:[':0n'e-

96s~4 (See6 - S~c6) 1

e'=

,c4

3 ~e2 -~- 4--~e2 __ ~c2 "l'- ~e21n

(A15)

"~e

(A16)

References

(A10)

ap

2 ln e--

- S¢c2(ln ~c) 2]

(A7)

B = (~c/2) - ( SCc3/g~e2)

-a eln c-A

1

(112)

[1] B.L. Armand, H,B. Uddholm and P.T. Vikstrom, Absorption method to clean solvent-contaminated process air, Ind. Eng. Chem. Res., 19 (1990) 436. [2] R.W. Baker, C.M. Bell and H. Wijmans, On membrane vapor separation versus carbon absorption, presented at the AIChE Annual Meeting, in San Francisco, CA, 1989, paper 174d. [3] A.L. Kohl and F.C. Riesenfeld, Gas Purification, 3rd edn., Gulf Publishing Company, Houston, TX, 1979. [4] N. Mukhopadhyay and E.C. Moretti, Current and potential future industrial practices for reducing and controlling volatile organic compounds, A Publication by Center for Waste Reduction Technologies, AIChE, 1993. [5] K.V. Peinemann, J.M. Mohr and R.W. Baker, The separation

T.K. Poddar et al. / Journal of Membrane Science 120 (1996) 221-237

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

of organic vapors from air, AIChE. Symp. Ser., 82 (250) (1986) 19. T.K. Poddar, S. Majumdar and K.K. Sirkar, Membrane-based absorption of VOCs from a gas stream, AIChE J., (1996) accepted. Q. Zhang and E.L. Cussler, Microporous hollow fibers for gas absorption. I. Mass transfer in the liquid, J. Membrane. Sci., 23 (1985) 321. S. Karoor and K.K. Sirkar, Gas absorption studies in microporous hollow fiber membrane modules, Ind. Eng. Chem. Res., 32 (1993) 674. Q. Zhang and E.L. Cussler, Microporous hollow fibers for gas absorption. II. Mass transfer across the membrane, J. Membrane. Sci., 23 (1985) 333. M.J. Semmens, R. Qin and A. Zander, Using a microporous hollow fiber membrane to separate VOCs from water, J. Am. WWA, April (1989) 162. K.K. Sirkar, Other new membrane processes, in W.S. Winston Ho and K.K. Sirkar (Eds.), Membrane Handbook, Chapman and Hall, New York, 1992, p. 885. A.E. Jansen, P.H.M. Feron, J.J. Akkerhuis and B.P.T. Meulen, Vapor recovery from air with selective membrane absorption, paper presented at ICOM'93, Heidelberg, Germany, September 1993. Kvaerner Engineering, Sanderfjord, Norway, Letter to K.K. Sirkar, 15 November 1993.

237

[14] T.K. Poddar, Removal of VOCs From Air by Absorption and Stripping in Hollow Fiber Devices, Ph.D. thesis, New Jersey Institute of Technology, 1995. [15] J. Happel, Viscous flow relative to arrays of cylinders, AIChE J., 5(2) (1959) 174. [16] W.N. Gill and B. Bansal, Hollow fiber reverse osmosis systems analysis and design, AIChE J., 19(4) (1973) 823. [17] R. Prasad and K.K. Sirkar, Dispersion-free solvent extraction with microporous hollow fiber modules, AIChE J., 34(2) (1988) 177. [18] R.B. Bird, W.E. Stewart and E.N. Lightfoot, Transport Phenomena, Wiley, New York, 1960, p. 296. [19] R.H. Perry and D.W. Green, Perry's Chemical Engineers Handbook, 6th edn., 1984. [20] R.C. Reid, J.M. Prausnitz, and T.K. Sherwood, The Properties of Gases and Liquids, 3rd edn., McGraw-Hill, New York, 1977. [21] G.A. Robbins, S. Wang and J.D. Stuart, Using the static headspace method to determine Henry's law constants, Anal. Chem., 65 (1993) 3113. [22] J.C. Hutter, G.F. Vandegrift, L.N. Nunez and D.H. Redfield, Removal of VOCs from groundwater using membrane-assisted solvent extraction, AIChE J., 40(1) (1994) 166.