Additional cooling capacity

Additional Cooling Capacity by Means of an External Heat Exchanger... 

During polymerization, a certain amount of polymer evaporates. One design option giving additional cooling capacity is to condense, in an external heat exchanger, the polymer from the vapor phase and to return the liquid to the reactor bulk. We assume that, under the most demanding conditions, 30% of the heat generated by the polymerization reaction can be removed in the condenser. Under these conditions, the minimum temperature difference becomes ... 

Watei-filled. Pressurised Containment Passive Additional cooling capacity for severe accidents... 

Continuous polymerization systems offer the possibiUty of several advantages including better heat transfer and cooling capacity, reduction in downtime, more uniform products, and less raw material handling (59,60). In some continuous emulsion homopolymerization processes, materials are added continuously to a first ketde and partially polymerized, then passed into a second reactor where, with additional initiator, the reaction is concluded. Continuous emulsion copolymerizations of vinyl acetate with ethylene have been described (61—64). Recirculating loop reactors which have high heat-transfer rates have found use for the manufacture of latexes for paint appHcations (59). 

Impurities or the delayed addition of a catalyst causes inhibition or delayed initiation resulting in accumulation in the reactors. The major hazard from accumulation of the reactants is due to a potentially rapid reaction and consequent high heat output that occurs when the reaction finally starts. If the heat output is greater than the cooling capacity of the plant, the reaction will run away. The reaction might commence if an agitator is restarted after it has stopped, a catalyst is added suddenly, or because the desired reaction is slow to start. 

If the main fractionator bottoms temperature is limited to 690°F, adding a pool quench can provide additional LCO product recovery. Assuming there is no penalty for the bottoms product quality and there is available cooling capacity in the upper section of the fractionator, this incremental LCO yield is valuable. 

Gaseous chlorine, under room temperature, was bubbled into liquid bromine maintained at -5°C. Excess chlorine left the reactor through a vent into an absorption column. The chlorine addition rate was adjusted to the reactor s cooling capacity, to prevent the temperature from rising above 0°C. 

Factors of importance in preventing such thermal runaway reactions are mainly related to the control of reaction velocity and temperature within suitable limits. These may involve such considerations as adequate heating and particularly cooling capacity in both liquid and vapour phases of a reaction system proportions of reactants and rates of addition (allowing for an induction period) use of solvents as diluents and to reduce viscosity of the reaction medium adequate agitation and mixing in the reactor control of reaction or distillation pressure use of an inert atmosphere. 

Two sources to obtain this necessary information are the use of data bases and through experimental determinations. Enthalpies of reaction, for example, can be estimated by computer programs such as CHETAH [26, 27] as outlined in Chapter 2. The required cooling capacity for the desired reactor can depend on the reactant addition rate. The effect of the addition rate can be calculated by using models assuming different reaction orders and reaction rates. However, in practice, reactions do not generally follow the optimum route, which makes experimental verification of data and the determination of potential constraints necessary. 

For exothermal reactions, the addition controls the heat production rate and therefore adjusts the reaction rate to the cooling capacity of the reactor. 

This is the most common mode of addition. For safety or selectivity critical reactions, it is important to guarantee the feed rate by a control system. Here instruments such as orifice, volumetric pumps, control valves, and more sophisticated systems based on weight (of the reactor and/or of the feed tank) are commonly used. The feed rate is an essential parameter in the design of a semi-batch reactor. It may affect the chemical selectivity, and certainly affects the temperature control, the safety, and of course the economy of the process. The effect of feed rate on heat release rate and accumulation is shown in the example of an irreversible second-order reaction in Figure 7.8. The measurements made in a reaction calorimeter show the effect of three different feed rates on the heat release rate and on the accumulation of non-converted reactant computed on the basis of the thermal conversion. For such a case, the feed rate may be adapted to both safety constraints the maximum heat release rate must be lower than the cooling capacity of the industrial reactor and the maximum accumulation should remain below the maximum allowed accumulation with respect to MTSR. Thus, reaction calorimetry is a powerful tool for optimizing the feed rate for scale-up purposes [3, 11]. 

The third step checks if natural convection is sufficient to maintain heat losses, to provide a sufficient cooling capacity. The additional data required in this step comprises the variation of density as a function of temperature (P), the viscosity, and the thermal conductivity. This again is only meaningful as long as the reacting mass has a low viscosity allowing for buoyancy. If this data set is not sufficient or the natural convection cannot be established, for example, as for solids, the system must be considered as purely conductive. 

Even with these improvements the heat-removal capacity of the reactor vessel is not sufficient when using fast initiators. By the addition of an external condenser the cooling capacity may increase by about 30% to 50% [1]. 

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