A fixed-site carrier composite for NHJN, separation
membrane
S.K. Shukla and K.-V. Peinemann GKSS Research Centre, 2054,
Geesthacht, Germany
The use of a composite membrane for NH3 separation is presented. A polyethylene/polyetherimide/ carrier composite is shown to permeate NH, selectively from NH3/N2 mixture at ambient conditions, attaining a maximum selectivity of 480 at 1% NH3 feed concentration. Experimental data indicate facilitated-transport-mode membrane function. Keywords: facilitated transpoti; fixed-site carrier; composite membrane; ammonia separation
Introduction A very important step in ammonia synthesis. with regard to overall process efficiency. is the recovery of ammonia from the product stream. At 35 ~01% NH, for 800-1000 bar’. reaction output is low. More efficient removal ofNH, would favour ammonia yield by driving the synthetic step in the mass-law equation. It is this consideration that makes membrane application to the problem important. A monomer in the gas phase, NH3 is one of the best known ligands with a lone electron pair on the nitrogen atom capable of forming labile coordination complexes with compounds like zinc halides’. That fact immediately suggests the possibility of facilitated transport. The presence of a carrier would raise both permeability and selectivity. not only by the carrier-permeant interaction yielding higher NH> flux, but possibly also by the additional barrier effect of the carrier for the non-reacting Nz! Brubaker and Kammermeyer4 first suggested a permselective membrane for separating ammonia. A number of reports have appeared since. In one, immobilized molten salts were used to achieve separation5. An ionexchange membrane was used in liquid ammonia extraction in another’. Laciak eraI.’ reported a multilayer polymeric membrane with poly(vinylammonium thiocyanate) as a carrier, which shows high selectivity, but polymer gelation at 2 bar feed pressure spoilt the development. Instability is a considerable drawback where a membrane occurs as an immobilized fluid. In contrast. a composite membrane offers its own advantages. A membrane thickness of l-2 pm, crucial to flux, is obtained and liquid-membrane instability problems are avoided. The limit to carrier loading of ionomer is lifted and the barrier effect in achieving higher selectivity could be exploited, together with the ease and simplicity of membrane fabrication. Relying on our continuing research on alkenelalkane separation with fixed-site carrier membranes. we have made preliminary tests on NHj/N, separation. Results indicate the feasibility of using a composite membrane at ambient conditions. 0950-4214/92/020079-03 0 1992 Butterworth-Heinemann Ltd
The membrane function yet to be addressed.
at real process temperatures
has
Experimental Preparation of the carrier NH,SCN was used as obtained from Merck. The zinciodide-ammonia complex was prepared as follows: to 8 g ZnI?. dissolved in 2 g H,O (pH 5). was added 25% ammonia solution, dropwise. There results a precipitate initially. which redissolves upon further addition of ammonia. This complex solution. (Zn(NH,),)‘+ type (pH 12). was used in preparing the composite. Membrane
preparation
A microporous polyetherimide (PEI) membrane” served as support. The casting solution consisted of polyethylene dispersion (PE) diluted 1:l with water, I wt% glycerine and the zinc-iodide-ammonia complex or the NH,SCN solution in weight proportions as required by the carrier/ polymer ratio in the membrane. The support was dipcoated and dried in air. Selectivity measurement An NH,/N, mixture ranging from I to 30~01%. from Messer Griessheim. Liibeck. served as feed. Selectivity was directly determined with a mass spectrometer by using the relation NH.&
selectivity = (c”,,,/c”,,)/(c’,,,/c’~,)
where C” and C’ denote component partial pressure in permeate and feed-stream. respectively. The conditions of the experiment were: I bar feed pressure: 0.1 mbar permeate pressure: and 303 K temperature. Nitrogen flux was measured against pressure rise in known permeate volume as a function of time and its value computed from a steady-state curve.
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Results
Figure 1 shows the effect of feed NH, concentration on membrane selectivity, for a membrane in which NH,SCN is the carrier, the ca~er/polymer ratio being one. The significant information it provides is that the membrane shows higher selectivity at lower feed NH3 concentration. Moreover, selectivity falls with increasing feed NH3 fraction. Both these features are in good accordance with a facilitated transport mechanism of higher gas flux at lower permeant concentration in the feed, the fall at higher concentrations being ascribed to carrier saturation’. There was no significant effect on membrane selectivity following an increase in NH,SCN/PE ratio to two. However, its effect on N2 flux (Figure 2) is very telling. There is an inverse relation between the two, indicating that the salt acts as a barrier to N, transport. Results with zinc-iodide-ammonia complex are presented in Figure 3. The highest NH& selectivity occurs, with 5 ~01% feed NH,, at the lowest carrier/PE ratio. The expected rise in selectivity with increasing carrier content does not ensue. The data, however, are inadequate to explain this observation. It may be that the binding polymer phase in the blend is unable to take up a higher salt concentration. First results with a new
NH,,SCN/polyethylene
wt ratio
Figure 2 N, flux as a function of membrane expressed as carrier/~ivethvlene weight ratio
carrier content
polymer, a poly(methylacrylate) dispersion (PMA), are encouraging. A relatively high NH3/N2 selectivity of 360 is obtained for a PMA/PEI composite, in itself a considerable enhancement over the value of 50 for a comparable PE/PEI composite, and certainly a precedent for the carrier membrane.
u Feed
I
I
10
20
NH3
concentration
30 (vol %I
Membrane selectivity as a function of NH, fraction in Figure 1 the feed NH/N2 binary mixture
80
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Purification 1992
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Data presented the conclusion a composite separates NH, N,. Its is indicative facilitated transport. certain barrier is also A search a suitable that can higher carrier and yet an integral layer as as ~thstand temperatures is which should light on competitive a membrane could to other technologies, and worthwhile in experimental that could useful in the phenomenon fixed-site carrier
~orn~osit~ rn~rn~ra~e for ~~~/~~
separation: SK
Shukla and K.-V. Peineman~
References Bakemeier, H., G&sling, H. and Krabetz, R. Ammoniak. Ulbnann Enzyklopstdie der technischen Chemie 4 Auflage, Band 7 (Eds H.B. Meisenheimer. J. Frenzel and R. Pfefferkornf Verlag Chemie. WeinheimiBergstr. (1974) 444-513 Jorgenson, K. Inorganic Comp1exe.s Academic Press. London (1963) ch. 4.54-80
Shukla, S.K. and Peinemann, K.-V. ~-Butene/n-butane separation using fixed carrier membranes Prepr 6th Int Symp $vnthetic Membranes in Sci Ind University of Tiibingen. Tiibingen (1989) 83-86 Brubaker, D.W. and Kammermeyer, K. Separation of gases by plastic membranes Ind Eng Chem (1954) 46 733 Pez, G.P. and Laciak, D.V. USParent (9 August 1988) Langevin, D., Metayer, M., Hankaoui, M. and Pallet, B. Carrier facilitated transport and extraction through ionexchange membranes illustratedwith ammonia. acetic acid and boric acids Membranes and Membrane Processes (Eds. E. Drioli and M. Nakagaki) Plenum Press (1986) 309 Laciak, D.V., Quinn, R., Pez, G.P., Appleby, J.B. and Puri, P.S. Selective permeation of ammonia and carbon dioxide by novel membranes Sep Sci Tech& (1990) 25 1295-1305 Reinema~n, K.-V.,Gh~ro~e,K., Wind&and Behfing, R.D. Pol~vetbetimidmembra nen fiir die Gastrennung Sonderdruck aus GKSS Jahresbericht (1987) Way, J.D., Noble, R.D., Reed, D.L. and Ginley, G.M. Facilitated transpoft ofC0, in ion exchange membranesA~~hE I (1987) 33(3) 480-487
i
I
I
Zinc-lodlde-ammonwm
t
2
1 complex/PE
wt ratlo
Figure 3 Membrane selectivity as a function of carrier content expressed as zinc-iodide-ammonium/polyethylene weight ratio
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