Fed-Batch Hydrogenation Reactor

The subject of biochemical reactors is discussed in more detail in a textbook by Bequette.4 

Hydrogenation reactions are frequently run in fed-batch reactors. The chemical component to be hydrogenated is charged to the reactor vessel. The hydrogen is then fed into the vessel on pressure control. The temperature of the reactor is controlled by manipulating the flowrate of coolant to the jacket, coil, or external heat exchanger. Thus this system has two manipulated variables (the flowrate of hydrogen and the flowrate of coolant) and two controlled variables (pressure and temperature). 

Since hydrogenation reactions are very exothermic, the situation often arises where the heat removal capacity cannot maintain the desired temperature with the normal operating hydrogen pressure. This usually occurs early in the batch when the concentration of the other reactant is high because it has not yet been diluted by the formation of the product compound. This situation requires that the flowrate of hydrogen be restricted so that temperature control is maintained. Thus pressure control should be temporarily abandoned. 

There are several ways to achieve this variable structure control strategy. The elegant approach is to use model predictive control (MPC). The simple approach is to use override control. The latter technique is demonstrated in this section. 

As a specific numerical example, we use the reaction of aniline with hydrogen to form cyclohexylamine (CHA)  

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Figure 4.40 Matlab program for fed-batch hydrogenation reactor.

Prior to the first hydrogenation batches, the supported ruthenium catalysts were reduced in the autoclave under hydrogen flow at 200°C for 2 hours (10 bar H2, heating/cooling rate 5°C/min). As the catalyst had been reduced, a lactose solution saturated with hydrogen was fed into the reactor rapidly and the hydrogen pressure and reactor temperature were immediately adjusted to the experimental conditions. Simultaneously, the impeller was switched on. This moment was considered as the initial starting point of the experiment. No notable lactose conversion was observed before the impeller was switched on. 

For the semi-batch stirred tank reactor, the model was based on the following assumptions the reactor is well agitated, so no concentration differences appear in the bulk of the liquid gas-liquid and liquid-solid mass transfer resistances can prevail and finally, the liquid phase is in batch, while hydrogen is continuously fed into the reactor. The hydrogen pressure is maintained constant. The liquid and gas volumes inside the reactor vessel can be regarded as constant, since the changes of the fluid properties due to reaction are minor. The total pressure of the gas phase (P) as well as the reactor temperature were continuously monitored and stored on a PC. The partial pressure of hydrogen (pnz) was calculated from the vapour pressure of the solvent (pvp) obtained from Antoine s equation (pvpo) and Raoult s law ... 

Theoretical Oxygen The moles (batch) or molar flow rate (continuous) of O2 needed for complete combustion of all the fuel fed to the reactor, assuming that all carbon in the fuel is oxidized to CO2 and all the hydrogen is oxidized to H2O. 

A fairly elaborate hydrofluoric acid recovery system is installed, adapted to batch operation. The concentration of hydrogen fluoride gas fed to the reactor is progressively increased as the reaction tends to slow down. The... 

Kinetic Model Discrimination. To discriminate between the kinetic models, semibatch reactors were set up for the measurement of reaction rates. The semi-batch terminology is used because hydrogen is fed to a batch reactor to maintain a constant hydrogen pressme. This kind of semi-batch reactor can be treated as a bateh reactor with a constant hydrogen pressme. The governing equations for a bateh reactor, using the product formation rate for three possible scenarios, were derived, as described in reference (12) with the following results ... 

The liquid hydrocarbon stream to be treated may be a crude oil, heavy crude oil, bitumen, or a refined fraction of the crude oil. The hydrogen gas stream is added to the mixture of the hydrocarbon stream with the organic solvent. The reactor, which is fed upflow, is a packed bed of biocatalyst dispersed on a support and is operated at about 74°C. Alternatively, the reactor can also be a batch reactor under stirring conditions. 

Reactions were carried out in liquid phase in a well-stirred (1000 rpm) high-pressure reactor (Parr Instruments, 300 mL) at 30 bar and 150°C. The reaction mixture consisted of 61 g of ADPA (Acros Chemicals), 53 g MIBK (Acros Chemicals) and 370 mg of catalyst. The test procedures used here is similar to that described earlier by Bartels et al. (7). The reactor was operated at a constant pressure with the liquid phase in batch mode and the hydrogen fed in at a rate proportional to its consumption. The reaction was monitored by hydrogen uptake and the product yield was determined from gas chromatographic (Agilent Technologies, 6890N) analysis. 

Gulf Research and Development Co. Two of the coals were processed in the Gulf continuous flow reactor, fed at the rate of about 1.5 kg coal/hr for 15-18 hrs. The third coal was processed in a conventional batch autoclave run. In all three runs, the coal was processed at about 400 °C and 3000 psi pressure of hydrogen using a proprietary catalyst. In the continuous runs, distillate from previous experiments was used as vehicle while in the autoclave experiment, partly hydrogenated phenanthrene was used. The vehicle-to-coal ratio was 2 1. In each case the reaction products were filtered on a steam-heated Buchner funnel. 

Methyl dichloride is obtained by the interaction of phosphorus thiotri-chloride with methyl alcohol. The process is carried out in an excess of alcohol (2 moles of CH3OH per 1 mole of PSCI3), which serves to absorb released hydrogen chloride. Phosphorus thiotrichloride from batch box 2 and methyl alcohol from batch box 3 are pumped with batching pumps through siphons into reactor 1. The synthesis occurs when the components are fed simultaneously at 0-5 °C. To withdraw the heat and maintain the temperature in reactor 1, the coil and jacket of the reactor are filled with -15 °C salt solution. The products of the reaction, which include dichloride, unreacted... 

Surface aeration is usually employed for slow reactions or for batch processes. It can be used in semicontinuous systems when it is desirable to recirculate the gas from the headspace. This is frequently the case in hydrogenation and is referred to as dead-end hydrogenation. In this system, gas is fed continuously to the reactor at the rate at which hydrogen is being consumed no compression costs to overcome the static head of liquid or external recirculation is needed. Feeding gas from the headspace may be preferred when there is a possibility of plugging sparger holes with reaction products. Surface aerators are also extensively used for waste-water treatment. There are two types of surface aerators the brush aerator, and the most commonly used turbine aerator. 

We studied the hydrogenation of acetophenone, where a great deal of side-reactions and production of intermediaries take place in a three-phase reactor, with liquid batch and gas continuously fed, at constant pressure and temperature. The catalyst is Rhodium (3%) over activated carbon. The solvent is Cyclohexane. Samples are taken at different instants and analysed by gas chromatography. The species for which measures are available are acetophenone, AC, phenyl-ethanol, PE, methyl-cyclohexyl-ketone, ethyl-benzene, EB, ethyl-cyclohexane, cyclohexenyl-ethanol, CNE, methyl-cyclohexenyl-ketone and cyclo-hexyl-ethanol, CE. 

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