Facilitated emulsion liquid membrane separation of complex hydrocarbon mixtures

Journal of Membrane Science, 56 (1991) 239-246 Elsevier Science Publishers B.V., Amsterdam

239

Facilitated emulsion liquid membrane separation of complex hydrocarbon mixtures N. Garti Casali Institute of Applied Chemsstry, School of Applied Science and Technology, The Hebrew University of Jerusalem, 91904 Jerusalem (Israel)

A. Kovacs* Hungarian Hydrocarbon Institute, H-2443, Pf.32, Szazhalombatta (Hungary) (Received August 15,1989; accepted in revised form June 18,199O)

Abstract An attempt was made to separate non-polar components from the polar fractions in kerosene using modified environments including the addition of polar solvents and surfactants. The solvent extraction process was based on the formation of an emulsion liquid membrane (ELM) which facilitates the separation. It was demonstrated that even a complex hydrocarbon mixture could be separated by ELM techniques if a facilitating agent (N-methyl pyrrolidone, NMP) was used in conjunction with high HLB surfactants (Tween 80). Low HLB value surfactants acted against the separation of polar compounds but participated in separation of apolar compounds. Keywords: emulsion liquid membranes; kerosene fractionation; facilitated transport, liquid membranes; liquid permeability and separations; extraction; N-methylpyrrolidone

Introduction

Few papers have been published concerning the use of the emulsion liquid membrane separation (ELM) technique for separation of hydrocarbons [ 11. In the majority of studies, simple binary mixtures such as n-hexane-benzene, l-methylnaphtalene-dodecane n-heptane-toluene, n-heptane-benzene, [ 1,2,5,7] were separated into the original components. In other studies impurities or low concentrations of additives were separated by this method. In an attempt to separate non-polar components from the polar fractions in kerosene, we applied the techniques found in literature without great success. In order to separate such a complex hydrocarbon mixture, the mass transfer environment needed to be considered and changed. The modified environment included the addition of a polar solvent to the emulsion membrane-forming *To whom correspondence should be addressed

0376-7388/91/$03.50

0 1991-

Elsevier Science Publishers B.V.

240

components [ 41. The solvent extraction processes, in the presence of polar solvents, improved the separation selectivity of the polar components from the non-polar fractions of the hydrocarbons. In this study some aspects of ELM formation and the presence of separation (facilitating) agent and surfactants in a complex hydrocarbon mixture are discussed and evaluated. Experimental Materials

A heavy kerosene fraction served as the complex hydrocarbon mixture (see Table 1). The characteristics of the surfactants are listed in Table 2 (ICI, USA). The mass transfer facilitating agent was the universal solvent N-methyl2-pyrrolidone (NMP). The receiving phase was n-hexane. TABLE

1

Kerosene

(base stock)

Distillation Temp (“C)

characteristics

range 196 -

217 10

Refractive index (RI) (at 2O’C) El&ion chromatography

233 50

243 70

260 90

278 final bp

I,4551

Non-polar yield (% ) Polar yield (% )

TABLE

227 30

77.4 22.6

RZ(20”C) RI(20”C)

1.4386 1.5120

2

Characteristics

of surfactants

Trade name

Chemical name

HLB”

Viscosity, CP

Solubility

(20ZC) Tween 20

ethoxylated (20) sorbitan monolaurate

16.7

400

water

Triton X-100

ethoxylated octylphenol

13.5

240

water

Tween 85

ethoxylated (20) sorbitan trioleate

11.0

300

oil

Span 80

sorbitan monooleate

4.3

1000

oil

“HLB = hydrophilic

lypophilic

balance.

241

Procedure 1. Emulsification An oil-water (o/w) emulsion of kerosene (the oil phase) in water and NMP (the aqueous phase) was prepared. The inner oil phase concentration was kept constant (121~ 0.1). Different surfactants were used to form the emulsion and to stabilize the oil droplets. Constant shear rate and time (Silverson laboratory homogenizer) were applied to the emulsion. The emulsions were characterized by measuring their viscosity (Brookfield, 20” C ) and droplet size distribution (Coulter counter, Coulter Electronics, model TA-II). 2. &fuss transfer The o/w emulsion was dispersed in n-hexane. Multiple phase emulsions were formed, consisting of an o/w emulsion as the inner phase dispersed in the outer oil phase (n-hexane). The outer interface did not contain any emulsifier to stabilize its droplets. Therefore, at this stage a selective migration of components from the inner phase took place. Those components, upon reaching the outer hexane phase, dissolved in it. 3. Separation The separation of the phases was achieved in a thermostated mixer-settler at a stirring rate of 300-350 rpm for 10 min. This contact time was selected arbitrarily in order to obtain good, comparable results. It should be noted that the present ELM process has only limited commercial potential for recovery of products of similar composition. The mass transfer was estimated by quantitative mass balance carried out on the different ingredients (using a refractive index calibration curve ) . Results and discussion 1. Characterization of the inner phase All the emulsions were oil-in-water emulsions. The inner oil phase volume was kept constant at every level of the added NMP. The outer aqueous phase consisted of water and increasing concentrations of NMP. The emulsifier concentration, in weight percent, was calculated on the basis of total emulsion weight. The NMP content is given in wt.% of the aqueous phase. All the emulsions were stable enough (no strong creaming or oil separation) to be mixed with the outer continuous receiving phase, prior to performing the separation. Viscosity The concentrations of water-soluble surfactant (high HLB ) had only a limited effect on the viscosity of the emulsion (Triton X-100, Tween 20). At al-

242

50-

A

40-

yy----y

30::

20

c-c AA

A .

0 r, l

‘:-I’--r,, I_, 0.2

0.5

1.0

0.2

0.5

1.0

wt % Emulsifier Fig. 1. Viscosity characteristics of kerosene in water/NMP emulsions prepared with constant shear rates and time with different surfactants and various NMP concentrations (wt.% of the water); emulsions prepared with (A) Tween 20; (B) Triton X-100; (c) Tween 85; (D) Span 80. For details see text.

most every NMP concentration straight lines were obtained at low and high surfactant concentrations (see Fig. 1) . However, the surfactants with a more lipophilic nature (lower HLB ), which have considerable solubility in oil, did affect the viscosity of the emulsions. For Span 80, which is mostly lipophilic, the emulsion viscosity increased with the emulsifier concentration. The Tween 85 (HLB = 11.0) had confusing effects. At low NMP concentrations it increased the viscosity, while at high NMP concentrations it reduced the viscosity of the emulsions. This effect might be related to its solubility partition coefficient between the two phases. As the NMP concentration increased in the water phase more of the surfactant partitioned into the outer phase. NMP, as expected, behaved as a viscosity builder throughout the range of surfactant concentrations, except in emulsions prepared with the hydrophobic surfactant, where high concentrations led to a decrease in the viscosity. 2. Droplet size distribution In Fig. 2 a demonstration of droplet size distribution of several emulsions prepared with the four sets of emulsifiers, at 0.1 wt.% level, is given for several NMP concentrations added to the outer aqueous continuous phase. The more concentrated systems showed similar trends. The effect of NMP was obvious at concentration levels of O-30%, independent of the nature of the surfactant.

243

30 c r

’

40rB

30

D r

diam,p Fig. 2. Droplet size distribution (DSD) of emulsions prepared with different surfactants and various NMP levels. Measurements were carried out a few minutes after preparation and immediately prior to the addition of the receiving solvent. (A) Tween 20, 0.1 wt.%; (B) Triton X-100, 0.1 wt.%; (C) Tween 85,0.1 wt.%; (D) Span 80,O.l wt.%.

Addition of a polar component, a “water-breaker” (a water soluble agent which eases the disaggregation of the continuous phase by competing in hydrogen bonding of water and breaking up part of the hydrogen bonds), into the aqueous phase and the interface improved the emulsion stability and maintained smaller droplets (less energy was required to shear the drops into smaller droplets). At high NMP concentrations (50-70% ) the opposite effect was observed. The droplets were much larger and the emulsion quality deteriorated. The 30-50% level is probably most ideal from the stability point of view. It is also clear from these curves that the hydrophilic surfactants will lead to smaller droplets and more stable emulsions then the hydrophobic surfactants. With the latter the droplets are large enough to form unstable emulsions that will be easy to break at a later stage (upon adding hexane). 3. Characterization of thepermeationprocess Evaluation of the permeability (mass transfer) of the hydrocarbon components was done by calculating the degree of polarity (dp) as wt.% of NMP used in each ELM process (see Figs. 3 and 4). The change in polarity (dp) was calculated as follows: dp=

Rl(permeate) -Rl(base stock) x100= AR1 x 100 0.000735 Rl(polars) -Rl(nonpolars)

It can be seen that the hydrophilic surfactants, with high HLB values, enabled permeation of mainly polar compounds, while surfactants with low HLB

244

%NMP Fig. 3. Permeation philic surfactants: amountsofNMP.

characteristics [ dp (% ), yield (% ) ] of the extraction process with two hydro(A) Tween 80; (B) Triton X-100, at 4 levels of surfactant and increasing (O)l.O%; (A)0.5%; (0)0.2%; (A)O.l%.

values allowed permeation of either polar or nonpolar components, depending on the conditions applied. Water-soluble

surfactants

Surfactants, as expected, adsorbed on the oil interface and served as emulsion-stabilizing agents. Therefore, some supression in the mass transfer of certain components was observed with the increase in surfactant concentration. However, when NMP was added, in increasing concentrations, the system (Tween 20) behaved like a regular liquid-liquid extraction process; the higher the polarity of the permeate stream, the lower was its yield. The surfactant and the polar facilitating agent were acting in parallel during the separation process. Triton X-100, having a slightly lower HLB value, re-

245

20 ,” - IO 0 8

H 0

20

40

60 %NMP

Fig. 4. Permeation characteristics [dp (% ), yield (% ) ] of the extraction process with the hydrophobic surfactants (A) Tween 85; (B) Span 80, at 4 surfactant concentrations and increasing amountsof NMP. (O)l.O%; (n)0.5%; (0)0.2%; (A)O.l%.

quired a given amount of facilitating agent to start the transport ( N 10 wt.% ). At low surfactant concentrations and medium NMP levels the extraction reached a maximum and dropped slightly with the increase in NMP level. At higher surfactant concentrations ( > 0.5% ), the addition of NMP was most beneficial and increasing transport levels were observed. These results can be correlated to the data from droplet size distribution (DSD) curves. The destabilization effect of NMP cosurfactants (referred to above as “water breaker”) at given levels can be of considerable help to the migration of selected components through the barrier water layer and to better extraction characteristics. Oil soluble surfactants The hydrophobic surfactants, in the absence of NMP, behaved as poor emulsifiers, and the extraction was due to nonselective emulsion breaking. As expected, a small amount of the facilitating solvent enhanced the membrane characteristics promoting permeation of polar components. However, the yields were very low. Addition of more surfactant and more polar solvent would allow dominant migration of non polar compounds. Span 80, the surfactant with the lowest

246

HLB value, showed complex behaviour with two contradictory effects. One could be attributed to the ability of Span 80 to promote transportation of the non-polar components of the hydrocarbon dissolved in NMP to the inner phase; the other effect was related to its ability to stabilize the o/w emulsion, thereby retarding the transport effect. As a result, at high surfactant concentrations and high NMP levels, no permeation occurred. The n-hexane migrated to the inner phase. The emulsion characteristics were again a good indicator of the expected transport phenomena. Conclusions

Even complex hydrocarbon mixtures can be separated by ELM techniques if a facilitating agent is used. To separate polar compounds, a high HLB value is recommended, with the addition of a highly polar selective solvent (for example NMP) to the water phase. The combination of high HLB value surfactant-polar solvent can be evaluated as a conventional liquid-liquid extraction process. Low HLB value surfactants act against the separation of polar compounds but participate in the separation of apolar compounds.

References A.N. Goswami and B.S. Rawat, Studies on the permeation of aromatic hydrocarbons through liquid surfactant membranes. J. Membrane Sci., 24 (1985) 145-168. S.-K. Ihm, Y.-H. Jeong and Y.-S. Won, Experimental investigation of the oil/water/oil liquid-membrane separation of toluene and n-heptane. I. Batch test of a mixer-settler, J. Membrane Sci., 32 (1987) 31-46. N.N. Li, Separation of hydrocarbons by liquid membrane permeation, Ind. Eng. Chem. Process Des. Dev., 10 (1971) 215-221. S. Kato and J. Kawasaki, Enhancement of hydrocarbon permeation by polar additives in liquid membrane permeation, J. Chem. Eng. Jpn., 20 (1988) 585-590. S. Kato and J. Kawasaki, Separation of aromatic hydrocarbons contained in naptha by liquid membrane permeation, Sekiyu Gakkaishi, 29 (1986) 404-411. D. Rautenbach and 0. Machhammer, Modeling of liquid membrane separation processes, J. Membrane Sci., 36 (1988) 425-444. A. Sharma, A.N. Goswami, B.S. Rawat and R. Krishma, Effect of surfactant type on selectivity for the separation of 1-methyl-naphtalene from dodecane using liquid membranes, J. Membrane Sci., 32 (1987) 19-30.