Semi-batch plants

One final note While the techniques used here were applied to control temperature In large, semi-batch polymerization reactors, they are by no means limited to such processes. The Ideas employed here --designing pilot plant control trials to be scalable, calculating transfer functions by time series analysis, and determining the stochastic control algorithm appropriate to the process -- can be applied In a variety of chemical and polymerization process applications. 

The batch plant shown in Fig. 7.4-6 is to be optimized. The required production capacity is 11070 m per year. The cost coefficients (see Eqn. 7.3-4) are given in Table 7.4-7. The fixed processing times in the batch units, /i. r, are given in Table 7.4-8 together with initial values of processing times in the semi-continuous units, 04-( and those found by optimization. The total batch times, volumes, and costs are also given in this table. 

During the development of a chemical process, a choice must be made regarding the type of reactor to be used on a plant scale. Some theoretical considerations and their practical impact on reactor issues are presented here. Choosing the right type of reactor can indeed improve the safety of the process. The considerations are reflected as well in the mode of operation. Reactors are characterized by type of operation (i.e., batch, semi-batch, and continuous). 

The RC1 is an automated laboratory batch/semi-batch reactor for calorimetric studies which has proven precision. The calorimetric principle used and the physical design of the system are sound. The application of the RC1 extends from process safety assessments including calorimetric measurements, to chemical research, to process development, and to optimization. The ability of the RC1 to generate accurate and reproducible data under simulated plant scale operating conditions may result in considerably reduced testing time and fewer small scale pilot plant runs. 

There are a number of different types of adiabatic calorimeters. Dewar calorimetry is one of the simplest calorimetric techniques. Although simple, it produces accurate data on the rate and quantity of heat evolved in an essentially adiabatic process. Dewar calorimeters use a vacuum-jacketed vessel. The apparatus is readily adaptable to simulate plant configurations. They are useful for investigating isothermal semi-batch and batch reactions, and they can be used to study ... 

J Singh, "Relief Sizing for Semi-batch Exothermic Processes", 1996 Process Plant Safety Symposium, South Texas Section of the AlChEy Volume 2,... 

The continuous-flow set-up can use either a reaction, similar to semi-batch, to remove the oxygen transferred or two reactors in series. The ozone or oxygen is removed from the liquid in the first reactor by stripping or vacuum degassing and then it flows into the absorber. After having passed through the absorber, the liquid can be recycled or discharged. More information on full-scale application of this method in municipal waste water treatment plants can be found in Redmon (1983) and ASCE (1991). 

Quantitative failure frequency data are difficult to obtain for multipurpose batch plants in the way that they are often used in the fine chemicals and pharmaceutical industries. Moreover, a quantitative assessment requires detailed knowledge of the control instruments, which may not be available during process development Therefore, a semi-quantitative approach is proposed, providing the required reliability for future plant equipment. 

The condensation of an aromatic nitro compound with a second reactant should have been performed in an aqueous solution with DMSO in the semi-batch mode. The nitro-compound is initially charged into the reactor with water and DMSO as solvent. Before the progressive addition of the second reactant had been started, the initial mixture was heated to the process temperatures of 60-70 °C. Then a failure of the cooling water system of the plant occurred. It was decided to interrupt the process at this stage and to maintain the mixture under stirring until the failure had been repaired. The feed of the second reactant was postponed and the jacket of the reactor had been emptied. 

To handle the volume of solution (about 30,000 L) necessary in the plant operation, a semi-batch denitration was necessary. Slow evaporation during product accumulation reduced the volume to <12,000 L, but increased the nitric acid concentration to about 11M. Experiments indicated that for a semi-batch denitration mode, a projected nitric acid concentration of 2M was an excellent stopping point, because no residual formic acid remains through the reflux and evaporation steps. Additional high nitric acid solution can then be added to the evaporated-denitrated solution without auto-initiation of a formic acid-nitric acid reaction. After all the Am-bearing solution had been transferred to the denitration evaporator and denitrated to <2M, the solution could be evaporated to 2500 L and denitrated to a residual free-acid concentration of 0.5 to 0.8M. In actual practice, the final 2500 L of solution was denitrated to 0.25M HNO3. 

Features common to many operating units are batch or semi-batch operations multi-purpose plant ... 

In the event of an alarm, an automatic sequence of actions should make the plant safe without the intervention of an operator. The sequence is likely to be simplest for continuous or semi-batch processes typically it may involve no more than stopping the reactant feeds, provided there are no problems with accumulation of reactants. Batch reactors are more difficult, particularly if they contain large amounts of unreacted material and are more likely to require the provision of protective measures such as emergency relief venting, or the provision of dump tanks with drown out facilities. 

The simplicity of the batch and semi-batch reactors allows us to respond quickly to evolving market opportunities. As soon as we have successfully determined the reaction or process conditions in a laboratory batch or semi-batch reactor and established the heat transfer requirements of the reaction or process, we can move to pilot plant-sized or commercial-sized equipment. If the heat transfer requirements are met by currently available equipment, then we can quickly produce product for market sampling. Such rapid response is important in today s global... 

We must know the process cycle time for a batch or semi-batch process in order to estimate the number of reactors required for a given plant capacity and to estimate the staffing requirement for that plant. By process cycle time, we mean the time between initiating one reaction and the next, subsequent reaction. One process cycle for a batch or semi-batch process involves... 

In this way, the reaction rate constants k, to can be found by matching semi-batch product trajectories to experimental results. This kinetic information can then be utilised to evaluate various forms of reactor design, particularly for plant scale continuous flow reactors. 

The temperature for deodorization and deacidification is dependent on the vapour pressure of the compounds to be removed. Thus for low-molecular-weight fatty acids as found in palmkernel oil a temperature of 185 °C in batch equipment and 200 °C in continuous or semi-continuous plant will suffice for deodorization. Although, as stated earlier, carotene is substantially destroyed and removed at 240 °C, in practice temperatures of up to 270 °C are required to break down compounds formed by oxidation during storage. Temperatures should, however, be kept as low as possible, firstly to reduce losses and secondly to reduce the possibility of isomerization and other thermochemical reactions (Rossell et al. 1981 Eder, 1982). It should also be remembered that it is in the interests of the refiner to minimize the loss from the oil of natural antioxidants, tocopherols and tocotrienols. There is a point at which the advantage to shelf-life of removal of aldehydes and ketones is cancelled out by the loss of these antioxidants. 

The modern batch deodorizer has a capacity from 5 to 30 tonnes. It is still frequently constructed of mild steel, though stainless steel is preferred. It is heated by high-pressure steam or, in some cases, by Dowtherm or thermal oil, and its vaporization efficiency has been improved by the inclusion of mammoth pumps and effective head space insulation to prevent refluxing. A vapour scrubber can be installed to recover oil and fatty acids from the distillate vapour. The complete batch cycle time is between 6 and 10 h depending on the feedstock. The unit is not, however, suitable for physical refining and it is expensive to operate compared with continuous and semi-continuous plant. 

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