Shot reactions

The reactivity of aliphatic diisocyanates is low in comparison to aromatic isocyanates. It is a problem in the manufacturing stage when using the prepolymer route. Quasiprepolymers and one-shot reactions require the correct choice of curative and catalyst for the system to work. 

Reaction Products. No mercaptan or tetrahydrothiophene intermediates were found in the products of shot reactions over either catalyst. Instead thiophene reacted by rejecting its hydrocarbon portion either as a butadiene-butene mixture (from chromia) or a butene-butane mixture without butadiene (from cobalt molybdate). In both cases, the sulfur was left on the catalyst as adsorbed H2S. No H2S peak was observed following any such reaction and subsequent experiments with H2S showed that the rate of desorption of H2S was negligible until much higher concentrations on the catalyst surface had been reached. 

Figure 3. Effect of temperature on conversion for flow and single-shot reactions...

C4 peak on the shot reaction chromatogram, making measurement difficult. The estimated probable error has been indicated by bars in Figure 3. (A typical peak is shown in Figure 5, d the dashed line shows the peak width for a shot passed through the analytical column alone.) There was also a loss of material in the shot reactions—i.e., the difference between the initial and final quantities of thiophene (upper curve, Figure 3) was greater than the C4 produced (middle curve). Presumably the loss was due to irreversible adsorption of either butene or thiophene, possibly as a polymer. 

The flow reaction over cobalt molybdate catalyst had an apparent activation energy of 25 kcal. per mole. Shot reactions showed comparatively high conversions at low temperatures, due possibly to the absence of blocked sites. 

Cold shot or hot shot. Injection of cold fresh feed for exothermic reactions or preheated feed for endothermic reactions to inter-... 

In both the sulfuric and nitric acid processes, the dorn metal must be in shot form prior to treatment to secure a reasonably rapid reaction. A number of steps also may be required in processing the dorne metal to remove miscellaneous impurities, particularly in treating material from copper-anode slime (31). 

Economic Aspects. Lithium metal is available commercially in ingots, special shapes, shot, and dispersions. Ingots are sold in 0.11-, 0.23-, 0.45-, and 0.91-kg sizes. Special shapes include foil, wire, and rod. Lithium is available in hermetically sealed copper cartridges and in sealed copper tubes for use in treating molten copper and copper-base alloys. Shot is sold in 1.19—4.76 mm (16—4 mesh) sizes. Lithium dispersions (30% in mineral oil) of 10—50-p.m particle size are used primarily in organic chemical reactions. Dispersions in other solvents and of other size fractions can be suppHed. 

Nonisotnermal Operation Some degree of temperature control of a reaction may be necessary. Figures 23-1 and 23-2 show some of the ways that may be applicable to homogeneous liquids. More complex modes of temperature control employ internal surfaces, recycles, split flows, cold shots, and so on. Each of these, of course, requires an individual design effort. 

In recent years there has been some substitution of TDI by MDI derivatives. One-shot polyether processes became feasible with the advent of sufficiently powerful catalysts. For many years tertiary amines had been used with both polyesters and the newer polyethers. Examples included alkyl morpholines and triethylamine. Catalysts such as triethylenediamine ( Dabco ) and 4-dimethyla-minopyridine were rather more powerful but not satisfactory on their own. In the late 1950s organo-tin catalysts such as dibutyl tin dilaurate and stannous octoate were found to be powerful catalysts for the chain extension reactions. It was found that by use of varying combinations of a tin catayst with a tertiary amine... 

Because of the high cross-link density of polyisocyanurates as prepared above, the resultant foams are brittle, so that there has been a move towards polyisocyanurate-polyurethane combinations. For example, isocyanurate-con-taining polyurethane foams have been prepared by trimerisation isocyanate-tipped TDI-based prepolymers. The isocyanurate trimerising reaction has also been carried out in the presence of polyols of molecular weight less than 300 to give foams by both one-shot and prepolymer methods. 

The basic RIM process is illustrated in Fig. 4.47. A range of plastics lend themselves to the type of fast polymerisation reaction which is required in this process - polyesters, epoxies, nylons and vinyl monomers. However, by far the most commonly used material is polyurethane. The components A and B are an isocyanate and a polyol and these are kept circulating in their separate systems until an injection shot is required. At this point the two reactants are brought together in the mixing head and injected into the mould. 

The macroazoinimer obtained by the end capping reaction of polyazoester with a diizocyanate and hy-droxyethyl methacrylate was used in wood impregnation leading to the one-shot polymerization of styrene thermally [50]. 

A motorist fills his tank at a Union 76 service station where two out of three grades of gasoline cost more than 2 per gallon (March 7, 2000). West Coast fuel prices on the spot market shot up in reaction to the highest oil prices in nearly a decade. (Corbis-Bettmann)... 

Essential modelling for scale-up relates to heat production (ref.4), and the universally applied calculation relates to the disaster calculation where the runaway instant temperature rise is always calculated for any one-shot exothermic reaction. In addition, the normal heat production rate is calculated to determine optimum feed rates, safety margins on cooling coil and condensers, etc. Increasingly, kinetic models are used as these become available. 

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