Volume Reactors

The choice of reactor temperature depends on many factors. Generally, the higher the rate of reaction, the smaller the reactor volume. Practical upper limits are set by safety considerations, materials-of-construction limitations, or maximum operating temperature for the catalyst. Whether the reaction system involves single or multiple reactions, and whether the reactions are reversible, also affects the choice of reactor temperature, as we shall now discuss. 

Increasing the pressure of irreversible vapor-phase reactions increases the rate of reaction and hence decreases reactor volume both by decreasing the residence time required for a given reactor conversion and increasing the vapor density. In general, pressure has little effect on the rate of liquid-phase reactions. 

Multiple reactions producing byproducts. The arguments presented for the effect of pressure on single vapor-phase reactions can be used for the primary reaction when dealing with multiple reactions. Again, selectivity is likely to be more important than reactor volume for a given conversion. 

For liquid-phase reactions, the effect of pressure on the selectivity and reactor volume is less pronounced, and the pressure is likely to be chosen to... 

Clearly, in the liquid phase much higher concentrations of Cfeed (kmol m ) can be maintained than in the gas phase. This makes liquid-phase reactions in general more rapid and hence leads to smaller reactor volumes for liquid-phase reactors. 

However, a note of caution should be added. In many multiphase reaction systems, rates of mass transfer between different phases can be just as important or more important than reaction kinetics in determining the reactor volume. Mass transfer rates are generally higher in gas-phase than liquid-phase systems. In such situations, it is not so easy to judge whether gas or liquid phase is preferred. 

Catalytic gas-phase reactions play an important role in many bulk chemical processes, such as in the production of methanol, ammonia, sulfuric acid, and nitric acid. In most processes, the effective area of the catalyst is critically important. Since these reactions take place at surfaces through processes of adsorption and desorption, any alteration of surface area naturally causes a change in the rate of reaction. Industrial catalysts are usually supported on porous materials, since this results in a much larger active area per unit of reactor volume. 

Also, although there are no selectivity data for the reaction, the selectivity losses would be expected to increase with increasing conversion. Complete conversion would tend to produce unacceptable selectivity losses. Finally, the reactor volume required to give a complete conversion would be extremely large. 

As the reactor conversion increases, the reactor volume increases and hence reactor capital cost increases. At the same time, the amount of unconverted feed needing to be separated decreases and hence the cost of recycling unconverted feed decreases, as shown in Fig. 8.1. Combining the reactor and recycle costs into a total cost indicates that there is an optimal reactor conversion. 

In 1994 Stamicarbon introduced a pool condenser in the synthesis section (see Figs. 4 and 5). This allowed a 34% decrease in reactor volume and a 45% decrease in carbamate heat-exchange area, thus reducing costs considerably for equipment, stmctural steel, and constmction. 

High purity acetaldehyde is desirable for oxidation. The aldehyde is diluted with solvent to moderate oxidation and to permit safer operation. In the hquid take-off process, acetaldehyde is maintained at 30—40 wt % and when a vapor product is taken, no more than 6 wt % aldehyde is in the reactor solvent. A considerable recycle stream is returned to the oxidation reactor to increase selectivity. Recycle air, chiefly nitrogen, is added to the air introducted to the reactor at 4000—4500 times the reactor volume per hour. The customary catalyst is a mixture of three parts copper acetate to one part cobalt acetate by weight. Either salt alone is less effective than the mixture. Copper acetate may be as high as 2 wt % in the reaction solvent, but cobalt acetate ought not rise above 0.5 wt %. The reaction is carried out at 45—60°C under 100—300 kPa (15—44 psi). The reaction solvent is far above the boiling point of acetaldehyde, but the reaction is so fast that Httle escapes unoxidized. This temperature helps oxygen absorption, reduces acetaldehyde losses, and inhibits anhydride hydrolysis. 

Ca.ta.lysts, Catalyst performance is the most important factor in the economics of an oxidation process. It is measured by activity (conversion of reactant), selectivity (conversion of reactant to desked product), rate of production (production of desked product per unit of reactor volume per unit of time), and catalyst life (effective time on-stream before significant loss of activity or selectivity). 

Polymerization in Hquid monomer was pioneered by RexaH Dmg and Chemical and Phillips Petroleum (United States). In the RexaH process, Hquid propylene is polymerized in a stirred reactor to form a polymer slurry. This suspension is transferred to a cyclone to separate the polymer from gaseous monomer under atmospheric pressure. The gaseous monomer is then compressed, condensed, and recycled to the polymerizer (123). In the Phillips process, polymerization occurs in loop reactors, increasing the ratio of available heat-transfer surface to reactor volume (124). In both of these processes, high catalyst residues necessitate post-reactor treatment of the polymer. 

Catalyst Development. Traditional slurry polypropylene homopolymer processes suffered from formation of excessive amounts of low grade amorphous polymer and catalyst residues. Introduction of catalysts with up to 30-fold higher activity together with better temperature control have almost eliminated these problems (7). Although low reactor volume and available heat-transfer surfaces ultimately limit further productivity increases, these limitations are less restrictive with the introduction of more finely suspended metallocene catalysts and the emergence of industrial gas-phase fluid-bed polymerization processes. 

Fig. 7. Residence time distributions where U = velocity, V = reactor volume, t = time, = UtjV, Cj = tracer concentration to initial concentration and Q = reactor volume (a) output responses to step changes (b) output responses to pulse inputs.

Porosity and Pore Size. The support porosity is the volume of the support occupied by void space and usually is described in units of cm /g. This value represents the maximum amount of Hquid that may be absorbed into the pore stmcture, which is an especially important consideration for deposition of metal salts or other active materials on the support surface by Hquid impregnation techniques. The concentration of active material to be used in the impregnating solution is deterrnined by the support porosity and the desired level of active material loading on the catalyst. If the porosity is too low, inefficient use of the support material and reactor volume may result. If the porosity is too high, the support body may not contain sufficient soHd material to provide the strength necessary to survive catalyst manufacture and handling. 

Catalyst Function. Automobile exhaust catalysts are perfect examples of materials that accelerate a chemical reaction but are not consumed. Reactions are completed on the catalyst surface and the products leave. Thus the catalyst performs its function over and over again. The catalyst also permits reactions to occur at considerably lower temperatures. For instance, CO reacts with oxygen above 700°C at a substantial rate. An automobile exhaust catalyst enables the reaction to occur at a temperature of about 250°C and at a much faster rate and in a smaller reactor volume. This is also the case for the combustion of hydrocarbons. 

The participant A is identified by the subscript a. Thus, the concentration is C the number of mols is n -, the frac tional conversion is the partial pressure is p and the rate of decomposition is /. Capital letters are also used to represent concentration on occasion thus, A instead of C. The flow rate in mol is n but the prime ( ) is left off when the meaning is clear from the context. The volumetric flow rate is V reactor volume is or simply V of batch reac tors the total pressure is 7C and the temperature is T. The concentration is = n /V or n IV. ... 

Flow reactors usually operate at nearly constant pressure, and thus at variable density when there is a change of moles of gas or of temperature. An appai ent l e.sidence time is the ratio of reactor volume and the inlet volumetric flow rate. 

CSTBs—minimum volume of battery, maximum yield, optimal temperature for reversible reaction, minimum total cost, reactor volume with recycle, maximum profit for reversible reaction with recycle, and heat loss... 

Minimum reactor volumes of isothermal and nonisothermal cascades by dynamic programming... 

A stirred tank sometimes can be modeled as having a fraction Ot in bypass and a fraction of the reactor volume stagnant. The material balance then is made up of... 

One quantitative measure of reactor efficiency at a conversion level x is the ratio of the mean residence time or the reactor volume in a plug flow reactor to that of the reactor in question,... 

Catalytic Incinerators Catalytic incinerators are an alternative to thermal incinerators. For simple reactions, the effect of the presence of a catalyst is to (1) increase the rate of the reaction, (2) permit the reaction to occur at a lower temperature, and (3) reduce the reactor volume. 

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