Experimental investigation of a new supercritical fluid-inorganic membrane separation process

j o u r n a l of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 116 (1996) 293-299

Rapid communication

Experimental investigation of a new supercritical fluid-inorganic membrane separation process G. A f r a n e a E.H. C h i m o w i t z b,, a Department of Chemical Engineering University of Science and Technology Kumasi, Ghana b Department of Chemical Engineering University of Rochester, Rochester, NY 14627, USA

Received 25 September 1995; accepted 30 January 1996

Abstract In this short paper we present data illustrating the use of a novel continuous scheme for the separation of a binary non-volatile organic solute mixture. The solutes to be separated are initially extracted into a supercritical solvent, and the resulting dilute ternary mixture is led into a high pressure vessel divided into two chambers by a porous barrier constructed with alumina particles supported in a porous steel container. Solvent flow rates can be independently adjusted on both sides of the barrier and the separation occurs through a process of adsorption and subsequent transport through the barrier. In principle this process will allow for continuous refinement of a solute by adsorption-diffusion through a series of porous barriers. In this study we present some initial selectivity data using a single barrier for a system consisting of the solutes 2,3-dimethylnaphthalene and naphthalene dissolved in supercritical carbon dioxide. This system was chosen because previous work from this group has investigated adsorption of these solutes at supercritical conditions and a thermodynamic model is available for interpreting the data given here. The data show that quite significant selectivities can be achieved in this system, especially in the solvents near-critical regime. Keywords: Composite membranes; Supercritical fluids; Diffusion; Separation

1. Introduction The use of supercritical solvents to effect the separation of non-volatile organic solid species from multicomponent mixtures has been an objective of research in this field for the past decade. The processes most often proposed have involved cascade processing ideas whereby solutes are extracted and deposited in a series of stages with each stage occurring at a different set of thermodynamic conditions; at each step the process objective is to facilitate a * Corresponding author. E-mail: [email protected].

degree of purification and the success of such a process largely depends upon the difference in selectivity between each extraction-deposition stage. While this is conceptually straightforward such processes rely heavily upon strong differences in selectivity occurring at successive stage in the cascade. Most available solubility data in multicomponent solute systems though shows these selectivity differences to be small [1], even over large pressure regimes, thus making for inefficient separation schemes. In response to this limitation another staged processing approach, termed the crossover process, was proposed by Chimowitz and Pennisi [2]. This

0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S0376-73 8 8(96)00049-X

294

G. Afrane, E.H. Chimowitz / Journal of Membrane Science 116 (1996) 293-299

process relies upon the differences in retrograde behavior of solute species in near-critical solvents and when these effects are present they lead to effective separations [3]. However, the optimal use of this process requires the solutes to have widely separated crossover pressure regions which tend to diminish with solutes of similar molecular structure [4]. The adsorption-diffusion concept proposed here exploits molecular differences in the near-critical solvent region without the strictures of the previously described process. Adsorption from near-critical fluids has been widely investigated using data acquired by supercritical fluid chromatography [5-8]. Chromatographic processes, however, are most useful for analytical purposes and do not lend themselves easily to continuous chemical separations processing, a characteristic of the separation scheme discussed. The mixture of solutes to be separated is first extracted into a supercritical fluid, and the outlet of the extractor used as the feed into one side of an adsorbent-partitioned high pressure vessel at a temperature and pressure in the supercritical region of the solvent gas. Given the large differences in solute adsorption behavior found in this region, it seems reasonable to expect the solute concentrations on the stationary partition to be quite different from one another. Therefore, if an "outlet" through the partition is available, i.e. it acts as a porous membrane, the resultant differences in flux through the porous adsorbent should lead to enrichment of a given component on the other side of the barrier where the solutes can be removed by another solvent stream. The process lends itself to continuous operation and depends for its efficacy upon two effects in series: the solute adsorption step followed by diffusion through the porous barrier. To our current knowledge an investigation of this type of process has not previously been published. Hence our objective at this point has been to gain some basic insight into the process before pursuing a detailed modeling effort. Our purpose here therefore is to present some initial experimental data on a prototypical system together with some qualitative observations related to the data. In several recent experimental studies [5-8], it has been observed that adsorption coefficients for dilute solutes from a near-critical solvent mixture show

20.0

15.0

~Toll \ uene

10.0

5.0

. . . .

i

. . . .

i

. . . .

i

. . . .

i

. . . .

0.0

250

300

350

400

450

500

Temperature, K Fig. 1. Experimental capacity factors of benzene and toluene vs. temperature in ethane (30%)-carbon dioxide mixture at 75 bar.

unusual behavior as the critical point of the supercritical phase is approached. In this regime the concentration of solutes on the adsorbent phase tends to be much higher than at conditions further removed from the critical region. We illustrate these observations with a typical set of adsorption data shown in Fig. 1, which were acquired using a chromatographic technique [6] in which the adsorbing solutes capacity factors (defined as the ratio of the number of moles of solute in the stationary and mobile phases respectively) were directly measured. The capacity factor itself is directly related to the adsorption coefficient of the species at the given conditions [6,7]. The data shown in Fig. 1 are for a dilute mixture of toluene and benzene adsorbing from a supercritical mixture of carbon dioxide and ethane (30%) in contact with a hydrocarbon bonded octadecylsilica stationary phase called ODS-2. The temperature maxima in the respective adsorption coefficients for these species at the given pressure are quite evident and the shape of these adsorption isotherms appears to be universally characteristic of near-critical adsorption systems [7]. In chromatographic separations, for example, the region of the maxima is where optimal resolution between species is to be expected [7]. This may be inferred from the bell-shaped nature of the data in Fig. 1, where the largest difference in the data of the two species occurs at the respective summits along the adsorption isobars. The differences observed are quite significant considering the fact that the struc-

G. Afrane, E.H. Chimowitz/ Journal of Membrane Science 116 (1996) 293-299

tures of these two solute species are reasonably similar. This behavior of near-critical solute adsorption has its origins in the critical divergence of the solutes partial molar volume (and enthalpy) as the critical point of the system is approached. This behavior is analogous to retrograde solubility behavior in supercritical systems as discussed in detail in previous work from our group [5,6]. Prior to presenting data for the experiments we have carried out we provide a brief, mostly qualitative description of the characteristics of the proposed process. Solute partitioning between the fluid and stationary phases in supercritical adsorption systems has usually been treated thermodynamically. In this approach equilibrium between the fluid and stationary phases is assumed, with the stationary phase concentration of a given component Cis, expressed as (1)

Cis = K i C i b

where K i is its adsorption coefficient, which as described above will show behavior in the near-critical region qualitatively similar to the data in Fig. 1. Here fib is the solute's fluid phase concentration. The flux N i of species i through the barrier depends, amongst other things, upon the stationary phase concentration of the species. This flux can be expressed using the Fick's law approach as

dNi-

Deft

6 [(gisCis)l-(gisCis)2]da

(2)

where Deff is an effective diffusivity through the porous membrane, 6 is the thickness of the membrane, A is the membrane area, and the subscripts 1 and 2 refer to the two sides of the membrane. Experiments have indicated that when the membrane is an adsorbent of high surface area and small pore size, the mode of transport is predominantly surface diffusion [9,10]. The surface diffusivity relationship for a given component is often considered to be an activated process, following an Arrhenius-type relationship with an activation energy related to the heats of adsorption of the component AHi,ads, a s [10,11]. Ds = D Oexp

(

RT

t

AHi,ads for a solute adsorbing from a high pressure supercritical phase was studied extensively in a recent paper from our group [6] where the term AHi,ads was found from the following thermodynamic approximation relating the temperature variation of the solutes Henry's constant in the stationary phase with temperature 01n H i ]

= _ AHi,ads

J

D O is a pre-exponential term, and q is a fraction which accounts for surface coverage. The property

(4)

m 2

Here o- denotes a property taken along the saturation envelope. Since AHi,ad s is a condensed phase property, its temperature and pressure dependences in the stationary phase are quite weak as shown in our previous work [6]. Therefore, a plot of Henry's law constant for the solute in the stationary phase versus temperature yields AHi,ad s a s the slope [6]. These results in conjunction with Eq. (3) suggest that the surface diffusion coefficients for the species in this study should monotonically increase with temperature. The other parameters in Eq. (3) may be estimated from the work of Sladek et al. [11], thus the diffusion coefficients of the solutes used in this study can be estimated with these results given in Fig. 2. Adsorption coefficients, however, show a more complicated relationship with temperature as indicated by the bi-directional behavior portrayed in Fig. 1. How these quantities interact with one another is a key feature of the process and will determine the extent of the separation achieved. Given this back-

-5.0 -5.2

~---...Naphthalene

~-5.4 © ~

-5.6

(3

" - . 2,3 DMN -5.8 -6.o

(3)

295

-.. ....

2.95

~ ....

~ ....

3.05

~ ....

t ....

3.15

~....'~,.,

3.25

1000/T

Fig. 2. Surface diffusivity of 2,3-dimethylnaphthalene and naphthalene over an ODS surface.

296

G. Afrane, E.H. Chimowitz / Journal of Membrane Science 116 (1996) 293-299

ground we now describe a set of experiments in the system described above.

mass flowmeter, accurate for stream pressures of up to 310 bar, and connected to a model DPF64 rotameter, was installed on the pure solvent line to measure the flowrate of that stream when required. A 1 / 2 in stainless steel flexible hose was used for the inlet solute-laden stream to facilitate movement and prevent choking. The stainless steel separation vessel consisted of two half steel chambers bolted together by 22 3.0 in bolts which went completely through one piece and tapered into the other. It was designed for a maximum operating pressure of 105 bar. Heating mats (5 × 9 in; Cole Parmer model 3122-71) were glued onto the outside of the pieces with Dow Coming 732 RTV sealant and plugged into the same electrical heating controller, a Thermolyne model CN45515 from VWR. A schematic of this vessel with its dimensions is given in the inset of Fig. 3. Four Omega J iron-constant thermocouples connected to a DP462-TC digital panel were installed through holes drilled towards the entry and exit ports of the chamber to monitor the temperature change across the vessel. The porous barrier was constructed in the form of a tray made out of porous stainless steel of 20 /xm pore size which was purchased from the Purolator company. The tray was packed as tightly as possible with Sigma type WA-1 basic chromatographic alumina particles(size range 50-200 /xm) and was bolted into one of the two halves of the

2. Description of the experimental setup Although the use of inorganic membranes for gas separations has often been discussed in the literature there is little evidence of their use for high pressure fluid separations of the kind being studied here. In fact a review of the literature in this area showed that there were no materials commercially available in sheet or wafer form that could easily sustain the mechanical stresses induced by pressure variations in a vessel of the type used here. Thus we had to conceive of an experimental system where the concepts described could be reasonably investigated, given the current state of inorganic membrane technology and our requirements. A schematic diagram of the experimental system and separation unit is shown in Fig. 3. Carbon dioxide, the solvent gas used in this process, was pumped by means of a Haskel AG30C gas booster from a cylinder to a point where it could be split into two streams - one going into the extractor and the other going straight into the separation vessel. Valves placed on both the mixture and pure gas lines allowed for control of the input streams to the system. An Omega FMA-8507

Extractor CollectiOnvessel ~

I

Back Pressure Regulator

DryR°tameterTest Flowmeter

.......

,/

Vessel

t/

X

Vent Metering Valves

~;~-----~

Rotameter

DryTest Flowmeter

V2__cCZ-Ill

Collection I I I IH

I Compressed

Vent

Gas

st

Filters

Fig. 3. Schematicdiagramof the supercriticalfluid-inorganic membraneseparation setup, and the pressure vessel (inset).

G. Afrane, E.H. Chimowitz / Journal of Membrane Science 116 (1996) 293-299

stainless steel vessel at the points a - a in the inset of Fig. 3. A neoprene gasket was used between the tray and the vessel piece. The two exit streams from the separator vessel were led into stainless steel pressure vessels for capturing the solutes for subsequent weighing and composition analysis. Downstream of the separator vessel Autoclave metering and regular valves were used to maintain the upstream pressure and allow downstream stepdown to atmospheric pressure for collection of the precipitating solids in U-tubes immersed in ice baths. All the lines from the separator vessel, including the valves, were heated to prevent premature solute precipitation. The pressure inside the vessel was monitored by connecting the pure gas input line to an Ashcroft pressure gauge with a range of 0-2000 psig. Two Cole-Parmer rotameters and two American Meter Company model DTM-200A dry test meters were connected to both exit lines to determine their flowrates after going through the U-tubes.

3. Experimental procedure For these experiments the solutes chosen were naphthalene and 2,3-dimethylnaphthalene in supercritical carbon dioxide since relevant thermodynamic measurements and models were available from our prior work with these species. The extractor vessel was packed with a mixture of naphthalene and 2,3dimethylnaphthalene. The solid mixture was blended with 3 mm borosilicate glass balls of equal volume to avoid caking. The entrance and exit of the extractor were plugged with glass wool (VWR catalog number: 32848) to prevent solid particles from entering the lines. In addition a 10 /xm Supelco frit was placed at the outlet. An inch-wide electrical heating element was wound around the extractor to provide the necessary heating. Each run began with a purge of the system with pure gas until no solids could be collected in the U-tubes. It also was important to open up the vessel after every run and thoroughly clean it of deposited material prior to subsequent runs. The lines also needed to be purged after each run and depending on the amount of material deposited in them in the previous run, this process could take up to 4 h. During this time, the pressure vessel and the downstream lines were also heated.

297

Before the carbon dioxide was allowed to go through the extractor and on to the vessel, all the downstream valves were closed to allow the requisite pressure to build up. It was important to heat up the vessel to the required operating temperature before allowing in the gas, otherwise the vessel pressure rose too quickly if this was done simultaneously. The setting of the dry test meters, as well as that of a time-keeping device, were also noted before and after each experimental run. Depending on the pressure and temperature of operation, sample collection times lasted from 10 to 30 min. Lower pressures and temperature required longer times because of the lower solute solubility at these conditions. The U-tubes were weighed before and after the experiments to determine the weight of solids deposited. These solids were then dissolved in methylene chloride and their composition analyzed by a supercritical fluid chromatographic technique. The output of the UV detector of this unit was connected to a computer for the integration of the peak areas from which the mole fraction of each component was obtained. The unit was designed so that the two streams, the feed with the extracted solute mixture and the pure purge stream on the other side of the porous barrier, could be run in either a co- or countercurrent manner. However, for this set of initial experiments the pure gas inlet line (dashed line leading to the vessel) was shut off which meant that the feed input stream pressurized the entire vessel with concomitant solute transport across the barrier. The experiments were conducted at 35°C and 60°C, with pressures ranging from 60 to 105 bar. Flowrates were in the range of 0.057 to 0.071 m3/h. We observed that the pressure and temperature quickly equalized on both sides of the separation vessel.

4. Discussion of results In addition to the relationship between the component surface diffusivities and temperature shown in Fig. 2, the ratio of the adsorption coefficients of the two species at the two temperatures of operation are shown in Fig. 4. Both figures represent calculations done with the adsorption thermodynamic model discussed in our earlier work [6]. These results show that the surface diffusivity for naphthalene is higher

G. Afrane, E.H. Chimowitz / Journal of Membrane Science 116 (1996) 293-299

298

than that of 2,3-dimethylnaphthalene (2,3-DMN) at all temperatures. Fig. 4 shows that at 35°C, the adsorption coefficient of naphthalene switches from being the lesser of the two of subcritical pressures, to the greater at a pressure of roughly 80 bar. At this temperature, according to these previous calculations, naphthalene has both the higher diffusivity and adsorption coefficient, except at lower pressures. Adsorption and transport effects reinforce each other with the expectation that naphthalene should enrich itself in the purge (permeate) stream. This was observed in the experiments carried out as can be seen in the data shown in Fig. 5; the single point at 61 bars though suggests that the transport effect remains dominant at this pressure. Fig. 4 also shows that the adsorption coefficient of 2,3-DMN is larger than that of naphthalene at 60°C over the entire pressure range studied. At this temperature the effect of adsorption is countered by that of diffusion for the 2,3-DMN. One may hypothesize that if the adsorption coefficient (capacity factor) is large enough this effect could dominate leading to the enrichment of 2,3-DMN in the purge stream. At this temperature the data shows this to be the case as given in the results of Fig. 6. Consistent with this hypothesis would be the decrease in selectivity observed at higher pressures (at this temperature) - as pressure increases the ratio of capacity factors decreases with the separation becoming less pronounced. The most pronounced separation in both

1.0

° 0.9 .~

[]

~ o.8 z T = 3 5 °C ©

"~

0.7

fd

-~ ....

I ....

0.6 60

[]

Naph. in Feed I

•

Naph. in Purge

I ....

I ....

70

I ....

I ....

80

I ....

I,,,

90

i00

Pressure, bar Fig. 5. Solvent-free mole fraction of naphthalene in the feed and purge streams at 35°C.

cases, is obtained by operating in the vicinity of the critical point of the solvent. The experimental data presented on a solvent-free basis at both temperatures represent the measured solid concentrations from the feed - exit and permeate streams at the

1.0

0.80

O •

o

"6

2.5

.~= Z

0.60

20 eq O

t",l

_M

0.40

~1.5

T=60°C

O -r.~

0.20 ].0

. . . . . . .

I

o

0.5 . . . . ~. . . . i . . . . ~. . . . ~. . . . ~. . . . 60 70 80 90 100 110 120

Pressure, bar Fig. 4. Ratio of 2,3-dimethylnaphthalene and naphthalene capacity factors at 60 and 35°C on an alumina surface.

ill

0.0 65

....

I ....

75

© • I ....

2,3 D M N i n F e e d 2,3 D M N in P u r g e I ....

I ....

85

I , , , J i , , ,

95

105

Pressure, bar Fig. 6. Solvent-free mole fraction of 2,3-dimethylnaphthalene in the feed and purge streams at 60°C.

G. Afrane, E.H. Chimowitz / Journal of Membrane Science 116 (1996) 293-299

attempt at studying this process and further investigation would be required to ascertain the potential of this approach as a viable scheme for high value, non-volatile organic separations.

Table 1 Experimental conditions for separation at 35 and 60°C T = 35°C Pressure (bar) 61.0 Mass flowrate ( g / h ) Feed side 0.24 Purge side 0.09

78.0 0.35 0.24

87.0 0.49 0.32

299

97.0 0.55 0.42

Acknowledgements T = 60°C Pressure (bar) 69.0 Mass flowrate ( g / h ) Feed side 0.20 Purge side 0.15

76.5 0.45 0.44

85.5 0.45 0.45

93.0 0.58 0.55

100.0 0.61 0.68

conclusion of an experimental run. The ratio of the selectivity factor for 2,3-DMN at a temperature of 60°C and pressure of 69 bar is about 2.65, which is quite significant given that in any multi-membrane system this enrichment factor multiplies in a geometric fashion. The selectivity here is defined as the ratio of solute concentration in the permeate stream to its value in the feed-exit stream on a solvent-free basis. In Table 1 the experimental flow conditions for the separations are summarized.

5. Conclusions Experimental results have been presented for a prototypical continuous separation process combining supercritical fluid extraction and diffusion through an inorganic porous barrier specially constructed for this series of measurements. The data acquired for the 2,3-DMN-naphthalene-carbon dioxide system have been qualitatively interpreted with the use of model calculations showing how adsorption coefficients and diffusion coefficients for the two solute species vary with temperature and pressure within the range of conditions studied. Selectivities in the range of 1.2-2.6 have been measured in this system and relative adsorption strength of the two solutes appears to play the most significant role in determining which species enriches itself over the barrier. These results are a preliminary

The authors would like to acknowledge the National Science Foundation for partial financial support of this work through grant no. CBT-9213276.

References [1] R.T. Kumik and R.C. Reid, Solubility of solid mixtures in supercritical fluids, Fluid Phase Equil., 8 (1982) 93. [2] E.H. Chimowitz and K.J. Pennisi, Process synthesis concepts for supercritical gas extraction in the crossover region, AIChE J., 32 (1986) 1665. [3] F. Van Duyvelde, P. Van Rompay and E.H. Chimowitz, Optimal control of molecular resolution in supercritical-fluid chromatography, J. Supercritical Fluids, 5 (1992) 227. [4] K.P. Johnston, S.E. Barry, N.K. Read and T.R. Holcomb, Separation of isomers using retrograde crystallization from supercritical fluid, Ind. Chem. Res., 26 (1987) 2372. [5] F. Recasens, E. Velo, M.A. Larrayoz and J. Puiggene, Endothermic character of toluene adsorption from supercritical carbon dioxide on activated carbon at low coverage, Fluid Phase Equil., 90 (1993) 265. [6] G. Afrane and E.H. Chimowitz, Adsorption in near-critical binary solvent mixtures: Thermodynamic analysis and data, Fluid Phase Equil., (1995) in press. [7] F.D. Kelley and E.H. Chimowitz, Near-critical phenomena and resolution in supercritical fluid chromatography, AIChE J., 36 (1990) 1163. [8] J.-J. Shim and K.P. Johnston, Phase equilibria, partial molar enthalpies and partial molar volumes determined by supercritical fluid chromatography, J. Phys. Chem. 95 (1991) 353. [9] R.M. Barrer, Surface flow and mixture separation in microporous media, AIChE-I.Chem. E. Symp. Ser., 1 (1965) (London, Institute of Chemical Engineers). [10] A. Yamasaki and H. Inoue, Surface diffusion of organic vapor mixtures through porous glass, J. Membrane Sci., 59 (1991) 233. [11] K.J. Sladek, E.R. Gilliland and R.F. Baddour, Diffusion on Surfaces II: Correlation of diffusivities of physically and chemically adsorbed species, Ind. Eng. Chem. Fundam., 13 (1974) 100.