Resume
Reactive separation processes (RSP) have demonstrated their decisive
advantages over more conventional processes. Running a chemical reaction in the
same place and at the same time with some physical process of separation in the
resultant reaction zone is an effective way to increase chemical processing
rates. Reaction and the diffusion step here affect each other rather strongly.
Through utilization of these and other effects associated with heterogenization
in the system, dramatic improvements can be attained in the rates of many
chemical processes together with much higher yields and enhanced selectivity.
This means improved purity of products, reduced pollution and conserved energy
and materials. Improved performance can be achieved via parallel
reaction-separation even with conventional processes. It's necessary to pay
attention on other advanced but not enough learnt RSP variant, when reaction and
separation occur simultaneously in integrated reactor/desorbers (RDP). The
article is devoted to: RSPs classification, algorithm of
production purity increasing on the base of system approach with RSPs
using, RSPs mañrokinetics, increasing of production
purity when using RSPs (on the example of classification production).
Keywords:
Reactive separation processes (RSP); heterogenization; reactor/desorbers;
makrokinetics; classification; classification.
Introduction
Reactive
separation processes (RSP) were introduced in chemical engineering relatively
recently, about 30 years ago, and have demonstrated their decisive advantages
over more conventional processes, together with some demerits that considerably
reduce their performance [1, 2]. Reactive distillation is the most famous
process among them. It's a unit operation, which combines a chemical reaction
with a multistage distillation in one step simultaneously. As EPA experts
believe this technique has two important advantages compared with conventional
reaction and distillation processes: (1) energy savings and (2) reduction of
capital investment. With reactive distillation, the heat generated by chemical
reaction can be utilized directly for the separation of products. Another
benefit of this process is to reduce both hardware investment and operation
costs. Combining the reactor and distillation column in one vessel, one process
step is eliminated along with its associated pumps, piping, and instrumentation.
In some situations, this elimination results a 30 - 40% reduction of hardware
investment.
Usually
reactive zone and distillation zone is the separate zone in similar units. But
we will have a lot of additional effects if these zones will be combined in
joint volume. Then in the reactive-separation zone not only the reaction heat
will cause additional mass transfer between vapor and liquid phases but it will
be increase a rate of the chemical process. Running a chemical reaction in the
same place and at the same time with some physical process of separation in the
resultant reaction zone is an effective way to increase chemical processing
rates. The reason is that removal of reaction products as they form promotes
reversible processes in accordance with Le Chatelier principle, and irreversible
processes in accordance with the law of mass action, because reagent
concentrations in the reaction zone are increased as the products are removed.
With increasing rates of reaction and mass transfer, the interface tension and
phase densities change more rapidly, resulting in more vigorous surface
turbulence. This promotes mass transfer, namely the removal of products from the
liquid into the gas phase, which, in its turn, increases the rate of reaction in
the liquid. Clearly, the reaction and the diffusion step here affect each other
rather strongly [3].
It's
necessary to pay attention on other advanced but not enough learnt RSP variant,
when reaction and separation occur simultaneously in integrated
reactor/desorbers (RDP). The separation of product(s) from reactant(s) enables
equilibrium limited reactions to be carried to higher conversions and
selectivity than would be possible in conventional non-separate reactors. The
RDP is also capable of improving yields of intrinsically low conversion
processes.
Through utilization of these and other effects associated with
heterogenization in the system, dramatic improvements can be attained in the
rates of many chemical processes together with much higher yields and enhanced
selectivity. This means improved purity of products, reduced pollution and
conserved energy and materials. Improved performance can be achieved via
parallel reaction-separation even with conventional processes. Thus it is
possible to realize CP concepts by use of parallel reaction-separation
processes.
That parallel reaction and separation offer a promising and fairly
versatile approach to perfecting processes was proved by laboratory and/or
industrial tests of the methodology in application to conventional synthesis of
monocarboxylic amides, organometallic compounds, some finished products or
semiproducts of organic and inorganic synthesis, solvents, organohalogen
compounds, organic acids, amino acids etc.
For example, an upgraded version of two-stage synthesis of
dimethylformamide from formic acid and dimethylamine via dimethylamine formiate
(a process currently in use both in Germany and the USA) embodied in a pilot
apparatus showed following improvements RSP over the conventional one [4]:
-
60-times increase in the reactor intensity, from
7.4 to 430 kg/(m3 h);
-
80-times decrease in processing time, from 48 to
0.6 h;
-
4% decrease in pollutant discharge level;
-
conversion rise to more than 95%;
-
2-times decrease in steam consumption rate;
-
2.5-times decrease in electric power consumption
rate;
-
2-times decrease in water consumption rate.
Laboratory and industrial research revealed that the reaction-separation
mode is well-suited for acylation, amidation, amination, condensation,
cyclization, dehydration, etherification, halogenation, hydrolysis, oxidation
and other chemical reactions.
On the other hand, many processes employed in industrially developed
countries like the USA, Japan, Germany etc. have not been upgraded to a combined
reaction-separation mode. Examples are condensation and polycondensation in the
manufacture of paints and polymers, disproportionation in making semiproducts
for semiconductor materials, depolymerization of cyclic compounds etc. For these
processes to be embodied in a combined mode, thorough investigations into their
macrokinetics (especially for catalytic synthesis) and optimum instrumentation
and process engineering are desirable.
A new comprehensive approach to perfecting chemical processes in
gas-liquid, liquid-liquid, gas-liquid-solid systems is suggested. It uses
parallel spatially overlapping chemical and separating processes performed in
flexible and possibly adaptive modular apparatuses involving forced
heterogenization of the reaction system, and multifunctional catalytic mass
transfer elements.
It is anticipated that the new processes will be competitive at high
technology markets because the general approach behind them enables raising
levels of technology even for conventional processes, improving their technical,
economic and environmental performance.
The new parallel reaction-separation processes may be used for
manufacture of solvents, oils, synthetic lubricants, detergents, paints, polymer
materials, semiproducts of organic synthesis, ultrapure substances for
microelectronics and fiber optics, bioengineering products etc.
References:
1.
O. Levenspiel, Chemical Reaction Engineering.
John Wiley & Sons, New York - London. 1965.
2.
G. Astarita, Mass Transfer with Chemical
Reaction. Elsevier Publishing Company. Amsterdam-London-New York. 1967
3.
US Patent 4,232,117, Int. Cl. C 07 C 1/20,
Catalytic Distillation Process/Smith L.A. Chemical Research & Licensing
Company, 21.02.79
4. W.M. Zadorsky, Increasing Reaction Rates in Chemical Processing via Systems Approach. Kiev. «Technika». 1989. 208 p. (In Russian)
DSc., Prof. William Zadorsky,
Academician of the Ukrainian Ecological Academy,
Ukrainian State University of Chemical Engineering.
Pridneprovie Cleaner Production Center
Tel: +(380) 567 440210
Tel/fax: +(380) 562 470813
http://www.crosswinds.net/~usuce/index.html
http://www.incubator.f2s.com
http://zadorsky.com