Rate semi-batch reactor

Thermal runaway reactions are the results of chemical reactions in batch or semi-batch reactors. A thermal runaway commences when the heat generated by a chemical reaction exceeds the heat that can be removed to the surroundings as shown in Figure 12-5. The surplus heat increases the temperature of the reaction mass, which causes the reaction rate to increase, and subsequently accelerates the rate of heat production. Thermal runaway occurs as follows as the temperature rises, the rate of heat loss to the surroundings increases approximately linearly with temperature. However, the rate of reaction, and thus the... 

The catalytic degradation of PS was carried out in a semi-batch reactor where nitrogen is continuously passed with a flow rate of 30 mL/min. A mixture of 3.0 g of PS and 0.3 g of the catalyst was loaded inside a Pyrex vessel of 30 mL and heated at a rate of 30 C/min up to the desired temperature. The distillate from the reactor was collected in a cold trap(-10 °C) over a period of 2 h. The degradation of the plastic gave off gases, liquids and residues. The residue means the carbonaceous compounds remaining in the reactor and deposited on the wall of the reactor. The condensed liquid samples were analyzed by a GC (HP6890) with a capillary column (HP-IMS). 

Optimization sequence (experimental data, arbitrary units) Runs 1 and 2 are initial experiments. From run 3 to run 6 the amounts of A, B, G, and feed rate of G are fixed. These constraints are relaxed for runs 7 and 8. (Reprinted from Marchal-Brassely et al. (1992), Optimal operation of a semi-batch reactor by self-adaptive models for temperature and feed profiles . Copyright (1992), with permission from Elsevier Science). 

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 choice of a reactor is usually based on several factors such as the desired production rate, the chemical and physical characteristics of the chemical process, and the risk of hazards for each type of reactor. In general, small production requirements suggest batch or semi-batch reactors, while large production rates are better accommodated in continuous reactors, either plug flow or continuous stirred tank reactors (CSTR). The chemical and physical features that determine the optimum reactor are treated in books on reaction engineering and thus are not considered here. 

It should be noted that there are cases in which some selectivity will be lost in choosing a semi-batch mode over a simple batch reactor. If the desired product decomposes by a consecutive reaction, the yield will be higher in the batch reactor [177]. If, on the other hand, the reactants are producing by-products by a parallel reaction, the semi-batch process will give the higher yield. In any case, if the heat production rate per unit mass is very high, the reaction can then be run safely under control only in a semi-batch reactor. 

The factors that can affect the rate of heat transfer within a reactor are the speed and type of agitation, the type of heat transfer surface (coil or jacket), the nature of the reaction fluids (Newtonian or non-Newtonian), and the geometry of the vessel. Baffles are essential in agitated batch or semi-batch reactors to increase turbulence which affects the heat transfer rate as well as the reaction rates. For Reynolds numbers less than 1000, the presence of baffles may increase the heat transfer rate up to 35% [180]. 

Semi-Batch Reactor (SBR) a type of batch reactor from which at least one reactant is withheld and then added at a controlled rate, usually to control the rate of heat generation or gas evolution both heat generation and concentrations vary during the reaction process products are removed from the reactor only upon conclusion of the reaction process. 

Keller (1998) describes the semi-continuous reaction process of a vinyl ketone K with lithium acetylide LA to yield lithium ethinolate LE an intermediate in the vitamin production. In an undesired side reaction an oligomer byproduct BP is produced. During the process, reactant K is fed to the semi-batch reactor at a rate to maximize the selectivity for LE. 

In addition to these three principal types of reactor, there is also the semi-batch reactor in which one reactant is added gradually to the others. This is a convenient manner of operation for some highly exothermic reactions since the temperature can be controlled by adjusting the rate of addition. 

Ridelhoover and Seagrave [57] studied the behaviour of these same reactions in a semi-batch reactor. Here, feed is pumped into the reactor while chemical reaction is occurring. After the reactor is filled, the reaction mixture is assumed to remain at constant volume for a period of time the reactor is then emptied to a specified level and the cycle of operation is repeated. In some respects, this can be regarded as providing mixing effects similcir to those obtained with a recycle reactor. Circumstances could be chosen so that the operational procedure could be characterised by two independent parameters the rate coefficients were specified separately. It was found that, with certain combinations of operational variables, it was possible to obtain yields of B higher than those expected from the ideal reactor types. It was necessary to use numerical procedures to solve the equations derived from material balances. 

A kinetic study in a well-stirred semi-batch reactor was conducted to determine the rate of methane as shown in Figure 4 (Lee et al., 2005b). As seen, the system with TBME has the shortest nucleation time and fastest hydrate growth rate followed by NH and MCH. This trend... 

Generally the oxidation of compounds with ozone is considered to be second order, which means first order with respect to the oxidant (03 or OH°) and to the pollutant M (Hoigne and Bader, 1983 a, b). A requirement for the experimental determination of the reaction order with respect to the pollutant is that the ozone concentration in the bulk liquid remains constant. A further requirement for determining kinetic parameters in general, is that the reaction rate should be independent of the mass transfer rate. These are easy to achieve for (very) slow reactions by using a continuously sparged semi-batch reactor. Such a reaction... 

For the determination of reaction parameters, as well as for the assessment of thermal safety, several thermokinetic methods have been developed such as differential scanning calorimetry (DSC), differential thermal analysis (DTA), accelerating rate calorimetry (ARC) and reaction calorimetry. Here, the discussion will be restricted to reaction calorimeters which resemble the later production-scale reactors of the corresponding industrial processes (batch or semi-batch reactors). We shall not discuss thermal analysis devices such as DSC or other micro-calorimetric devices which differ significantly from the production-scale reactor. 

This effect is especially important in the semi-batch reactor. If the temperature difference between reactor and feed is important and/or the feed rate is high, this term may play a dominant role, the sensible heat significantly contributing to reactor cooling. In such cases, when the feed is stopped, it may result in an abrupt increase of the reactor temperature. This term is also important in calorimetric measurements, where the appropriate correction must be performed. 

Figure 7.3 Concentrations in molkg as a function of time in a semi-batch reactor with the fast addition reaction. Compound B is fed at constant rate within 4 hours in a stoichiometric excess of 25%. B is fed in stoichiometric amounts.

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]. 

A catalytic hydrogenation is performed at constant pressure in a semi-batch reactor. The reaction temperature is 80 °C. Under these conditions, the reaction rate is lOmmolT s-1 and the reaction may be considered to follow a zero-order rate law. The enthalpy of the reaction is 540 kj moT1. The charge volume is 5 m3 and the heat exchange area of the reactor 10 m2. The specific heat capacity of water is 4.2kJkg 1K 1. 

The semi-batch reactor produces 5 m3 of product solution every 2 hours. Therefore, in a continuous reactor, the flow rate must be 2.5m3h 1 or 6.94-10-4 mV1. 

A fast exothermal reaction is to be performed in a semi-batch reactor. In order to control the temperature course of the reaction, one of the reactants is added at a constant rate, producing a constant heat flow. The reactor is cooled with water from a river (at 15 °C in winter). The cooling water should not be rejected at a temperature higher than 30 °C. 

Emergency measures, such as pressure relief or containment, must be taken to mitigate the consequences of a runaway that can no longer be avoided. Nevertheless, by far a better measure is to redesign the process to reduce the MTSR to a level below Tm4. This may be achieved, for example, by using a semi-batch reactor instead of a batch reactor and ensuring that the feed rate is properly limited and... 

The IAD I(a,t) in a chemical reactor is specially interesting and it does not seem that sufficient attention has been paid to the possibilities offered by this function. For instance, let us consider a semi-batch reactor, and let Q(t) be the feed flowrate of an incompressible fluid. The instantaneous fluid volume is V = /q Q(tf)dtf, from which the IAD is written I(a,t) = Q(t-a)/V. I(a,t) can be used to calculate the chemical conversion in different segregation states. Consider a species of concentration C produced with the rate t. If the mixture is assumed to be well mixed at the molecular scale, one obtains the familiar mass balance equation... 

Results obtained in a constant-pressure semi-batch reactor in which synthesis gas was supplied on demand are listed in Table 5.2. The analytical accuracy is said to be within 0.002 M. [Before alcohol analysis the samples were hydrogenated to convert any residual aldehydes to the corresponding alcohols.] At the resulting constant partial pressures of CO and H2 in the reactor and with very good gas-liquid mass transfer, the CO and H2 concentrations in the liquid remain constant and the rate depends only on the propene concentration (see pseudo-orders, Section 3.3.2). 

All the methods mentioned above use a mathematical model of the copolymerisation process in one way or another to arrive at a control policy for the production of compositionally homogeneous products. In this work use is made of a dynamic model of the process to control the feed rate of the more reactive monomer to a semi-batch reactor. Feedback from the process comes from an off-line model. The method is a general one and can be readily extended to accomodate feedback loops using on-line measurement devices with an experimental reactor. 

Experimental System The copolymerisation of styrene with methyl acrylate in toluene using azo-bis-iso- butyronitrile (AIBN) was selected as the model experimental system because the overall rate of reaction is relatively fast, copolymer analysis is relatively simple using a variety of techniques and the appropriate kinetic and physical constants are available in the literature. This monomer combination also has suitable reactivity ratios (i = 0.76 and r4 =0.175 at 80 C),(18) making control action essential for many different values if compositionally homogeneous polymers are to be prepared at higher conversions in a semi-batch reactor. 

Figure 6. Feed flow-rate profile with time to a semi-batch reactor.

Figure 10. Dynamic feed flow-rate profile in response to step change in monomer ratio in a controlled semi-batch reactor.

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