Exothermic reactor conditions

In some situations it is very important to be able to increase the flow rate above the design conditions (for example, the cooling water to an exothermic reactor may have to be doubled or tripled to handle dynamic upsets). In other cases this is not as important (for example, the feed flow rate to a unit). Therefore it is logical to base the design of the control valve and the pump on having a process that can attain both the maximum and the minimum flow conditions. The design flow conditions are only used to get the pressure drop over the heat exchanger (or fixed resistance part of the process). 

The synthetic production of benzene generally involves lire de-alkylation of toluene. In one non-catalyfic process, a hydrogen-rich gas is mixed with liquid toluene feed and preheated prior to charging to the reactor. Toluene reacts with Hie hydrogen to form benzene and methane. The reaction is exothermic Operating conditions approximate 500 1000 psi and 595-760 T. The process provides about 98% yield of benzene. The toluene is recycled. 

The entropy production of a sulfur dioxide oxidation (exothermic) reactor with heat exchangers was minimised in two different cases.Case 1 was a four-bed reactor with intermediate heat exchangers of a given total area, see Figure 8. The entropy production rate was calculated from the entropy balance over the system. All inlet and outlet flow conditions were kept constant, except the pressure at the outlet. Tlie... 

The adiabatic instruments described so far were originally developed to accurately determine the onset of exothermicity under conditions which allow its extrapolation to full-scale plant. The rates of pressure and temperature rise obtained from such equipment can, after suitable corrections for Phi, be used for sizing the emergency vent required to deal with a runaway exotherm in a reactor. 

Figure 13.1a shows two possible thermal profiles for exothermic plug-fiow reactors. If the rate of heat removal is low and/or the heat of reaction is high, then the temperature of the reacting stream will increase along the length of the reactor. If the rate of heat removal is high and/or the heat of reaction is low, then the temperature will fall. Under conditions between the two profiles shown in Fig. 13.1a, a maximum can occur in the temperature at an intermediate point between the reactor inlet and exit. 

A typical flow diagram for pentaerythritol production is shown in Figure 2. The main concern in mixing is to avoid loss of temperature control in this exothermic reaction, which can lead to excessive by-product formation and/or reduced yields of pentaerythritol (55,58,59). The reaction time depends on the reaction temperature and may vary from about 0.5 to 4 h at final temperatures of about 65 and 35°C, respectively. The reactor product, neutralized with acetic or formic acid, is then stripped of excess formaldehyde and water to produce a highly concentrated solution of pentaerythritol reaction products. This is then cooled under carefully controlled crystallization conditions so that the crystals can be readily separated from the Hquors by subsequent filtration. 

Oxidation of cumene to cumene hydroperoxide is usually achieved in three to four oxidizers in series, where the fractional conversion is about the same for each reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are operated at low to moderate pressure. Due to the exothermic nature of the oxidation reaction, heat is generated and must be removed by external cooling. A portion of cumene reacts to form dimethylbenzyl alcohol and acetophenone. Methanol is formed in the acetophenone reaction and is further oxidized to formaldehyde and formic acid. A small amount of water is also formed by the various reactions. The selectivity of the oxidation reaction is a function of oxidation conditions temperature, conversion level, residence time, and oxygen partial pressure. Typical commercial yield of cumene hydroperoxide is about 95 mol % in the oxidizers. The reaction effluent is stripped off unreacted cumene which is then recycled as feedstock. Spent air from the oxidizers is treated to recover 99.99% of the cumene and other volatile organic compounds. 

The methanation reaction is carried out over a catalyst at operating conditions of 503—723 K, 0.1—10 MPa (1—100 atm), and space velocities of 500—25,000 h . Although many catalysts are suitable for effecting the conversion of synthesis gas to methane, nickel-based catalysts are are used almost exclusively for industrial appHcations. Methanation is extremely exothermic (AT/ qq = —214.6 kJ or —51.3 kcal), and heat must be removed efficiently to minimise loss of catalyst activity from metal sintering or reactor plugging by nickel carbide formation. 

The conditions for the carbon bum step are typically less than about 1.0 mol % oxygen, 400°C inlet temperature, 455°C maximum oudet temperature, which is controlled by adjusting the oxygen content of the circulating gas, and 0.45 to 2.2 MPa. The carbon bum is considered to be complete when no exotherm is observed for several hours. The oxygen concentration at all reactor inlets and outlets should be equal at this point. 

Reaction Conditions. Typical iadustrial practice of this reaction involves mixing vapor-phase propylene and vapor-phase chlorine in a static mixer, foEowed immediately by passing the admixed reactants into a reactor vessel that operates at 69—240 kPa (10—35 psig) and permits virtual complete chlorine conversion, which requires 1—4 s residence time. The overaE reactions are aE highly exothermic and as the reaction proceeds, usuaEy adiabaticaEy, the temperature rises. OptimaEy, the reaction temperature should not exceed 510°C since, above this temperature, pyrolysis of the chlorinated hydrocarbons results in decreased yield and excessive coke formation (27). 

Although fluidized sand or alumina can also be used in the jacket of these somewhat larger reactors, the size makes the jacket design a problem in itself, hence these reactors are seldom used. An advantage of the jacketed reactor is that several—usually four—parallel tubes can be placed in the same jacket. These must be operated at the same temperature, but otherwise all four tubes can have different conditions if needed. This type of arrangement saves time and space in long-lasting catalyst life studies. Jacketed tubular reactors come close, but still cannot reproduce industrial conditions as needed for reliable scale-up. Thermosiphon reactors can be used on all but the most exothermic and fast reactions. 

Most chemical reactions are greatly affected by temperature. The previous chapters discussed reactions at isothermal condition, however, industrial reactors often operate under non-isothermal condition. This is because chemical reactions strongly depend on temperature, either absorbing (i.e., endothermic) or generating (i.e., exothermic) a large amount of heat. 

Adesina [14] considered the four main types of reactions for variable density conditions. It was shown that if the sums of the orders of the reactants and products are the same, then the OTP path is independent of the density parameter, implying that the ideal reactor size would be the same as no change in density. The optimal rate behavior with respect to T and the optimal temperature progression (T p ) have important roles in the design and operation of reactors performing reversible, exothermic reactions. Examples include the oxidation of SO2 to SO3 and the synthesis of NH3 and methanol CH3OH. 

Some batch reactions have the potential for very high energy levels. If all the reactants (and sometimes catalysts) are put into a kettle before the reaction is initiated, some exothermic reactions may result in a runaway. The use of continuous or semi-batch reactors to limit the energy present and to reduce the risk of a runaway should be considered. The term semi-batch refers to a system where one reactant and, if necessary, a catalyst is initially charged to a batch reactor. A second reactant is subsequently fed to the reactor under conditions such that an upset in reacting conditions can be detected and the flow of the reactant stopped, thus limiting the total amount of potential energy in the reactor. 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

Reaction conditions are generally mild, but they differ from one process to another. In the newer Unipol process (Eigure 12-1) used to produce both HDPE and LLDPE, the reaction occurs in the gas phase. Ethylene and the comonomers (propene, 1-butene, etc.) are fed to the reactor containing a fluidized bed of growing polymer particles. Operation temperature and pressure are approximately 100°C and 20 atmospheres. A single-stage centrifugal compressor circulates unreacted ethylene. The circulated gas fluidizes the bed and removes some of the exothermic reaction heat. The product from the reactor is mixed with additives and then pelletized. New modifications for gas-phase processes have been reviewed by Sinclair. ... 

The heats of these reactions (2, 3) (Figure 1) indicate that all the reactions are exothermic over the cited range of conditions. For example, the heat liberated under typical reaction conditions for the conversion of CO to methane is 52,730 cal/mole CO that for carbon dioxide is 43,680 cal/mole. Such high heats of reaction cannot be absorbed by the process stream in an adiabatic reactor unless the CO and/or C02 conversion is limited to less than about 2.5 moles/100 moles feed gas. Since... 

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