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Developments in Chemical Engineering & Mineral Processing.

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Developments in Chemical
Engineering & Mineral Processing
Volume 3,Number 314, 1995
Guest Editorial
Special Issue on Advances in Membrane Separation
This issue of Developments in Chemical Engineering & Mineral Processing is
devoted to the important topic, “Membmne S e p d o n and A&oq&n’’.
There is
little doubt that both adsorption and membrane separation pmcessa axe going to be
inmasingly important in the chemical and mineral pmcesshg indust&.
Conventional separation processes involve introduction of either an en=
separating agent @A), e.g. in distillation, entailing a heavy expendim of energy,
or a mass separating agent (MSA), e.g. in extraction, which must eventually be
removed, fiequently with an ESA. In contrast, the rather facile separation of the
MSA (adsorbent) in adsorption and of that (the membrane banier itself) in
membrane separation renders these p c e s s e ~energy ef6cient, and accoullts for their
numerous commercial applications. For adsorption, these include separation of
water and ethanol, xylene isomers,fructose and glucos, permanent gases,colored or
odifennrs impurities. and VOCs fmm gases. For mcmbraae sepamms
- ,thelean
many wamples involving revcrsc osmosis, dtrafiitration, microfiltration, dialysis,
Pervaporation, and separationof VOCs from gases.
There also are, however, reasons for their slow rate of further proliferation.
The chief limitations of adsorption are: (1) a limited variety of adsorbent matetjalS
available, currently mainly restricted to molecular sieves, activated carbon, silica gel,
and activated alumina;(2) a lack of clear understauding of the &&teadmrbent
interactions, now mainly classified as hydrophobicity or hydrophilicity, that impedes
rational selection and design of adsorbents; and (3) process and mechanical
complexitiesdue to its batch nature. In membrane separations also, the applications
wouldbenefitby a better comprehensionofthe pexmeaut-membrane inmactions and
122
transport mechanism, as well as development of inexpensive, robust, and enduring
materiais. Although, bejIlg continuous. membrane processes do not suffer from the
pmcess complexities of adsorption, the problem of concentration polarizaton and
membrane fouling that plagues practically all liquid-phase membrane pmesses is no
less debilitating.
The future of these technologies is bright. A large assortment of available
mate&& coupled with a better understanding should spawn new applidons
including the separation of closely related and complex mixtures, eg. chiral isomers,
fermesltor broth, and organic Separations, as well as in pollution control and
consumer and biomedical applications. Further, both membranes and adsorbents
willbeincnasinglylltilizedinnactar-~.I n d y I i C ~ - ~ , t h e
sorptive ability of the catalyst suppcut can also be exploited Natme, of come,
abounds witb examples of the use of membranes for compartmentalization and
selective permeation, and cau provide important cues fur developing new
applications.
The papers published in this special issue reflect Various practical and
theoretical aspects of membrane sepsrations and -tion
prpcesses. Thus,
Ruth@udttaL discuss gas and surface difbion of gases in the pms of activated
carbon pellets by the time lag method. Such data is useful in predicting
bmkthrougb c~yes.llieyalsoraise a question about the validity ofthe common
assumption of helium as a non-adsorbing gas.
MiknUsek and Filmulioprf present interesting results on the use of
millimeter-sized fluidized particles inside vertical tubular ceramic membranes to
enhancepermeam flux m microfiltration by a reduction of concentration polarizatian
and membrane fouling. De er aL have developed practical conelations far the
pndiction of the mass transfer coeflkients in d m o n by modifying the
Sherwood conelation valid fop non-porous channels. The simple film t k x y
approach based on reliable mass transfer coefficient conelations is useful in the
engineering design of ultrafilmtionsystems.
Kofirnrr et ol. present experimental results on the separation of van&& and
chromate ions through an anion-exchange membrane and two suppmcd liquid
membrants, one containing a tertiary mine and the other containing a quaternary
ammaninmsatt The results are important in thecolltextdreclaimingrnesespecies
b m waste streams. Lung rurd Lin present a diffusion model fur extraction by an
123
emulsion liquid membrane in a continuous unit. Bwgoyne et OL discuss a coupled
heat and mass transfer model fur membrane distillation and experiments to validate
the theory. This approach should be useful in the design and scale-up of membrane
distillation units.
G o b h et al. describe a theo~ticalanalysis for evaluating the pexformance a€
a dense (eg. metallic) as well as a microporous composite membrane in high
temperature reactor-separators of potential use in dehydrogenation reactions. They
use the dehydrogenation of ethylbenzene as a model reaction in their simulations.
Rigorous modeling of transport, Sorption, and reaction in such sysms based on
more accurate models, e.g. the dusty-fluidmodel,remains a challenge.
F?ofessorRavindraDaaa
Dept of Chemical & BiochemicalEngineering
The Univmity of Iowa, Iowa City
Iowa 52241-1219, USA
124
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