real world of indu~trialc hemi/ t ry
edited by
K. E. KOLB Bradley University Peoria. IL 61625
HAROLD WITTCOFF Chem Systems, international. Ltd. 28 St. James Square London S W l Y 4JH. England
Vinyl Chloride in Asia Roy H. King and Bryan G. Reuben South Balik Polytechnic, London SE1 OAA, United Kingdom Vinyl chloride (chloroethene in the systematic nomenclature, but the traditional name is still common in industry) can be polymerized to poly(viny1 chloride) (PVC) very easily in the laboratory. Apart from recognizing the need t o carry out the reaction in a fume cupboard, the chemist scarcely needs t o exercise his or her skills. If the process has to he scaled up to tens of thousands of tons per year, however, i t becomes much more difficult. If i t has to be carried out in a Third World country with few technically qualified staff and a shortage of capital for plant improvement, then the problems may be insuperable. This article is a case study of a consulting visit by one of us (RHK) to an Asian country. I t illustrates what chemical engineers do, how their methods change, and the scale of the problems facing the industrialization of Third World countries. The visit was made under the auspices of BESO-the British Executive Service Overseas-which identifies areas of technology in Third World countries that people areplanning or needing to move into but where they lack the expertise. Free Radlcal PolymerIration The company seeking help owned a small plant designed to produce PVC from vinyl chloride monomer. This is straightforward t o the laboratory chemist. Vinyl chloride polymerizes readily by a free radical mechanism. The process is initiated by thermally unstable free radical generators such as peroxides, peroxycarbonates, or peroxyesters. Benzoyl peroxide is typical and gives benzoyl and phenyl radicals:
The propagation process continues until the chain is terminated. This can come about via radical cou~line.dis~rooor-
Chain transfer is not a true termination process. A growing polymer radical may extract a hydrogen atom from a finished polymer chain. This "finished" polymer chain now becomes a radical and starts to grow again. If the hydrogen atom is extracted from the end of the chain, the new chain simply continues to grow linearly. But if, as is more probable statistically, the hydrogen atom is extracted from the body of the chain, then further propagation occurs a t right angles to the original polymer chain and a branch forms to give a polymer that may have quite different properties: Chain transfer may also occur to monomer or other molecules to give variable molecular weight products. -CH2-CH-
I CI
+ -CH'
-
+ --€HZ
-cH~-C-
LI "Finished" polymer Growing chain chain
I
Cl
A1 ~L!Hx
-CH2-CCI
The radicals then add t o vinyl chloride monomer to give more free radicals, and the polymer chain builds up:
/ \
(CH2CHCI),CH2CHCI.
nCH2CHCI
-CHz-CI
\
If the vinyl chloride is kept as a liquid under pressure (it boils a t -13 "C), the reaction, which is exothermic, goes smoothly on the laboratory scale. It is called "hulk" or "mass" polymerization. If one attempts to scale up the process, however, there is a problem in that the polymer dissolves in the monomer to give a viscous mass. Heat cannot be removed rapidly enough from the mixture, which may char, develop voids, or even explode. This problem can he overcome; nonetheless, hulk polymerization of vinyl chloride is rarely used in industry.
480
Journal of Chemical Education
Suspension Polymerlzatlon The most straightforward method of coping with the heat transfer problem is suspension polymerization. The monomer and initiator are suspended as droplets in water. The droplets have a high surface-to-volumeratio, so heat transfer to the water is rapid. The water also serves t o transfer the heat to the cooled walls of the reaction vessel and as a fluidizing medium to eliminate the viscosity problem. The suspension is maintained by vigorous agitation and by the addition of traces of various stabilizers. These include water-soluble polymers such as methylcellulose, gelatin, and poly(viny1alcohol), which increase the viscosity of the water, and finely divided inorganic materials such as clay, talc, aluminum oxide, and magnesium carbonate, which also stabilize the suspension. Initiation occurs within the monomer droplets, and, hefore much of the monomer has reacted, the polymer precipitates, leading to monomer droplets that contain a larger number of growing polymer particles. As polymerization continues, the particles within each droplet agglomerate. I t is this agglomerated structure that makes the finished resin porous and ahle to absorb large amounts of plasticizer (see below). The Asian Plant Suspension polymerization was employed a t the Asian plant. The set-up is shown in Figure 1. Water, suspension stabilizers, and vinyl chloride monomer were added to conventional, jacketed, agitated batch reactors. The initiator was then added, and polymerization continued for about 15 h. Unreacted vinyl chloride was removed under vacuum and returned to the monomer storage tank. The slurry of PVC in water was dropped into a holding tank for further degassing. The water was then centrifuged off, and the polymer-now in the form of a fine powder-was dried and bagged.
Fines (very low particle size product), which came over with the waste water, were dumped into pits whence they were dug out by local families who used them for "cottage" plastics industries, especially for making cheap shoes. There were several difficulties with the process. Impurities in the water and the recvcled vinvl chloride stream led to excessive fouling on the reactor wkls. The problem was exacerbated by a poor desim of agitator that caused excessive splashing:~s-aresult, after every run, each reactor had to be purged with nitrogen, evacuated, and flushed with water. I t then had to he opened and thoroughly cleaned manually by the operators. This dram-out procedure led to low productivity, hut, worse, it exposed the operators to what is, by modern standards, an undue concentration of vinyl chloride monomer, in spite of the checks and safe working procedures instituted by the company. ~
Toxlclty end CharacterlzatlonProblems The technology had been licensed 15 years earlier, which meant it was now 20-25 years old. It became obsolete in the
early 1970's with the realization that inhalation of vinyl chloride vapor even in very low concentrations can lead to angioaarccma of the liver, a rare form of liver cancer. The Asian plant was designed to meet the pre-1973 exposure limits for vinyl chloride operatives of 200 ppm. The post1974 limits are 5 ppm over a 15-min period with a 1 ppm time-weighted average measured overan 8-h day. ~ i ~ ; r 2e shows the stages by which these targets were achieved. Modern technoloav aims to reduce emission of vinvI chloride vapor and to mjnimize fouling of the reactor walls so that workers need enter the vessels to clean them only rarely. Most of the time, the plant is sealed. The new technology also aims a t better control of molecular weight and molecular weight distribution of the polymer, together with particle size and particle size distribution. Thus the modern product is much hetter characterized, and
.
~~
lrnproved m y 1 chlorlde recovery
Steam srnppmg! activated carbon adsorptlord gas traps
J
Dry polymet
Figure 1. me Asian plant.
~
F i e 2. Reduction of vinyl chloride emissions. SOUICB:S h ~ iBriefing i Service.
Volume 68
Number 6
June 1991
481
lank for
initial deaassina and tor &erge;lcy dumping Figure 3. Simplified flowsheet of a m m n plant.
this, in turn, leads to savings in processing. For example, PVC is frequently plasticized; that is, it is mixed with nonvolatile esters such as di(2-ethylhexyl) phthalate to give a soft leathery polymer used for synthetic leather, purses, and shoes. A well-characterized PVC absorbs plasticizer more uniformly, and less power is required for making the PVCI plasticizer mixture. Control of molecular weight and molecular weight distribution requires close control of all feed rnaterids,kspecially theorocess water used to surround the polymer dropletsand the;ecycled vinyl chloride monomer: which may contain chlorinated hydrocarbon impurities. The reaction is particularly affected by dissolvedchloride ions, which promote chain transfer reactions (see above) and thereby reduce the polymer molecular weight. The water available to the Asian plant from the public supply contained variable but usually high chloride ion concentrations, hence a deionizing plant was essential. Clean-Wall Technology
The modern process involves so-called "clean-wall technology", which effectively eliminates fouling. Part of the setu p is shown in Figure 3. Fouling is partly eliminated by purification of recycled vinyl chloride and control of other feed materials, but equally important is the design of the agitator. In the modern plant the agitator shaft enters at the base of the reaemr instead of at the top. The advantage is that .-- -.the .- - - shaft - - - - eland is in contact with liouid slurrv instead of gaseous monomer, so the risk of vinyl ciloride leakage to the atmosphere is effectively eliminated.
482
Journal of Chemical Educatlon
The polymer goes first to a blow-down tank, where the initial degassing takes place and which serves for dumping material from the reactor in an emergency. Further deeassine takes olace in the stream stripoina column, and a polymer slur& emerges a t the hot& of the column to be centrifuged and dried. The residual vinyl chloride passes out of the-top of the column mixed with-steam. This stream is condensed and separates into vinyl chloride and aqueous layers. The two ark separated and the vinyl chloride distilled in a further column to purify it ready for recycle. Impurities are incinerated. Will the company be able to upgrade? I t is uncertain. Vinyl chloride polymerization has changed from low to high technology, and established companies guard their knowhow, which gives them an advantage in a competitive market. The technology can be acquired under license, but the price is high, and royalties have to be paid. For the small independent company in a highly competitive market, the oroiect mav simolv be uneconomic. 'fhird world EoLpanies in general are short of expertise. Onlv the managing director of this company had a degree and-that was in-mechanical rather than ihemical engineering. T o upmade a 20-year old process to meet modern standaids of safety and p;oductiv%y will also require a substantial input of capital, and that too is scarce. What will probably happen in the end is that, if the market is sufficiently attractive, a multinational chemical company from the developed world will move in with its own plant and technology. This may ensure better compliance with safetv standards but. on the other hand.. orovides little . spin-off for the bust country and does little to improveThird World standards of technology and education.
