Union Carbide process

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

In the low-pressure systems a shot of material is injected into the mould which, if it did not expand, would give a short shot. However, the expanding gas causes the polymer to fill the mould cavity. One important form of the low-pressure process is the Union Carbide process in which the polymer is fed to and melted in an extruder. It is blended with nitrogen which is fed directly into the extruder. The extruder then feeds the polymer melt into an accumulator which holds it under pressure (14-35 MPa) to prevent premature expansion until a predetermined shot builds up. When this has been obtained a valve opens and the accumulator plunger rams the melt into the mould. At this point the mould is only partially filled but the pressurised gas within the melt allows it to expand. 

Linear low-density polyethylene (LLDPE) is produced in the gas phase under low pressure. Catalysts used are either Ziegler type or new generation metallocenes. The Union Carbide process used to produce HDPE could be used to produce the two polymer grades. Terminal olefins (C4-C6) are the usual comonomers to effect branching. 

Union Carbide Patents. During World War II, Union Carbide and Carbon Corporation patented a process (24) having some features in common with the German process described above. The Union Carbide process as patented is illustrated in Figure 15. The continuous process consists essentially of two reaction zones followed by a milling-pumping zone. 

In 1951 Robert Banks and Paul Hogan of Phillips Petroleum discovered that ethylene could be polymerized under rather mild conditions of temperature and pressure to afford high molecular weight polyethylene using chromium trioxide as the catalyst. This invention laid the foundation for both the Phillips and Union Carbide processes for ethylene polymerization (both use heterogeneous chromium catalysts). 

Table 10.2 Some Operating Conditions for the Union Carbide Process... 

Figure 5 Flow diagram of the Union Carbide Process for hydroformylation of higher olefins catalysed by Rhltppms in a single phase with biphasic catalyst separation.

Trichlorosilane can be dismutated in the presence of basic catalysts into SiCLt and SiH2Cl2, SiH3Cl and SihLt. This is the basis of the Union Carbide process to produce pure silicon via SibLj. 

The linearity of the aldehyde product increases with the concentration of triphenyl phosphine. This is being exploited in the Union Carbide process for the hydroformylation of propene in which linearities >90% are obtained. The rate, however, drops to lower values and the most likely explanation for the higher linearities in this system would seem to be the steric congestion around the rhodium atom at high phosphine concentrations, which kinetically and thermodynamically favours the formation of linear alkyl rhodium complexes relative to branched alkyl rhodium complexes. Product linearity decreases with the number of triphenyl phosphines present in the series of precursor complexes ... 

Figure 6 Union-Carbide process, a) catalyst hopper b) fluidized-bed reactor c) cyclone ...

Ethylbenzene separation by distillation. Union Carbide process. 

The metal values associated with other minerals but not amenable to separation and concentration by the standard beneficiation techniques such as flotation can usually be recovered by chemical methods. The Climax Molybdenum process to recover the molybdenum oxide values associated with the sulfide ore and the Union Carbide process for the separation of molybdenum from tungsten in a floatation concentrate are in this category. 

An interesting use for the anhydrous reagent, at least that prepared by the Union Carbide process, is for generation of hydroxyl radicals. Prior to publication of the method by Heywood, Phillips, and Stansbury, Schleyer and Nicholas reported an experiment in which Dr. Heywood of Union Carbide carried out the oxidation and they worked up the reaction mixture. A mixture of 150 g. of adamantane... 

Silicon of exceptional purity for semiconductor and photovoltaic industries is obtained from metallurgical-grade silicon or liquid silicon tetrachloride. At least four of six steps involve fluidized beds. In Osaka Titanium s process, step 1 (obtaining SiHCls from Si and HCl) takes place at 300°C, whereas in the Union Carbide process, SiHCls is obtained by hydrogenating gaseous SiCU over CUCI2 catalyst at 500°C. Other... 

The Union Carbide process for trichlorosilane production is an alternative. Tetrachlorosilane is hydrogenated through a bed of granular MG silicon, using one of the reactions commonly used for the recycling of the byproduct SiCU in the Siemens process ... 

Figure 1.4 Simplified flow chart of the Bhopal Union Carbide process.

Figure 1.5 Methyl isocyanate (MIC storage) tank used in the Bhopal Union Carbide process. Source adapted from Reference [31].

Figure 1.7 Schematic diagram of the emergency relief effluent treatment system that includes a scrubber and flare tower in series in the Bhopal Union Carbide process.

Homogeneous catalysts based on cobalt carbonyl compounds were previously used, but have been largely replaced by the Union-Carbide process with [(Ph3P)2Rh(CO)Cl] as the catalyst. 

A key property of catalytic processes is selectivity. Catalysis has revolutionized process chemistry by replacement of wasteful, unselective (i.e. multiple-product-forming) reactions with efficient, selective (i.e. one-product-dominating) ones. For example, selective catalytic methanol carbonylation (practiced by BP, BASF Monsanto, Eastman) has to a large extent substituted unselective non-catalytic n-butane oxidation (Celanese, and Union Carbide processes). 

The Union Carbide process starting with cydohexane, which is first converted to cyclohexanone, which is in tnm oxidized to caprolactooe. This compound is then converted to caprolactam by the action of ammonia, at 4O0°C, under 17.106 Pa absolute pressure. Cyclohexanone is oxidized around 50 C, at normal pressure, by means of peracetic add, in the presence of acetaldehyde, which is oxidized to acetic add. The plant built by Union Carbide was shut down shortly after its construction. Variants have been proposed by Degussa, Toa Gosei and Ugine Kuhlmann without industrial success. 

The most important point is the complete separation of the catalyst and the products. The Union Carbide process proposal for the hydroformylation of higher olefins solves this problem [36, 37]. The catalyst leaching is lower than 20 ppb of... 

Figure 2.1 2 Union Carbide process for gas phase polymerisation of ethylene.

---