Semi-batch mode

Whether the reactor operates in batch or semi-batch mode... 

On an industrial level, the reaction is carried out in semi-batch mode with a yield of 70% [61, 62,127]. First steps towards continuous production with a mixer and a cooler increased the yield to 80-85% however, the reaction occurred partly in the mixer, thereby increasing the temperature in this unit The reaction is quenched by dilution with water, also having a cooling function. 

Stirred-tank reactors can be operated in batch, semi-batch, or continuous mode. In batch or semi-batch mode ... 

Nickel containing MCM-36 zeolite was used as new catalyst in the ethylene oligomerization reaction performed in slurry semi-batch mode. This catalyst, with micro-mesoporous structure, mild acidity and well balanced Ni2+/acid sites ratio, showed good activity (46 g of oligomers/gcataLh) and selectivity (100% olefins with even number of carbon atoms). The NiMCM-36 behaviour was compared to those obtained with NiMCM-22, NiY, NiMCM-41 and NiMCM-48 catalysts. 

The catalytic ethylene oligomerization was performed in a 0.3 L well-mixed three-phase reactor operating in semi-batch mode, at constant temperature (70 or 150 °C) and pressure (4 MPa of ethylene) in 68 g of n-heptane (solvent). Prior to each experiment, the catalyst was successively pretreated, firstly in a tubular electrical furnace (550 °C, 8 h) and then in the oligomerization autoclave (200 °C, 3 h), under nitrogen flow at atmospheric pressure. After 30 min of reaction, the autoclave was cooled at -20 °C and the products were collected, weighted and analyzed by GC (FID, DB-1 60 m capillary column). 

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 process is run in a semi-batch mode, and multiple reactors are used. There are several possible causes for a loss of control such as insufficient heat removal and loss of agitation. Overpressurization leading to the bursting of rupture discs takes place several times per year, indicating both the clear need for containment but also a need to consider design and control improvements. The reference describes the autoclave rupture disc assembly, procedures for replacement of the discs, the cleaning of the containment vessels, and the routine maintenance procedures for the containment vessels. 

It may be possible to minimise the time spent on relief system assessment and design if the basis of safety for a multi-purpose reactor can be changed to prevention rather than emergency pressure relief. For example, if it can be arranged for all the reactions to operate in semi-batch mode with no significant reactant accumulation, then the use of a trip system of sufficient integrity may provide a suitable basis of safety. (This may not always be possible.)... 

Figure 1-2 Operating parameters necessary for ozone mass balance(s) on a continuous-flow stirred tank reactor (for operation in semi-batch mode Ol - 0).

Using it in the semi-batch mode of operation this reactor type has been used in the determination of the reaction rate constants of fast direct reactions of ozone with certain waste water pollutants, e. g. phenol or azo-dyes (Beltran and Alvarez, 1996). Beltran and coworkers have also successfully studied the reaction kinetics of various fast reacting substances using semi-batch mode STRs (see further references of Beltran, Benitez or Sotelo et al. in Chapters B 3 and B 4). 

For example, Beltran and Alvarez (1996) successfully applied a semi-batch agitated cell for the determination of kL k,a, and the rate constants of synthetic dyes, which react very fast with molecular ozone (direct reaction, kD = 5 105 to 1 108 L mol-1 s l). In conventional stirred tank reactors operated in the semi-batch mode the mass transfer coefficient for ozone kLa(03) was determined from an instantaneous reaction of ozone and 4-nitrophenol (Beltran et al., 1992 a) as well as ozone and resorchinol (l,3-c//hydroxybenzene) or phloroglucinol... 

If we compare the sparged (BSCR) and mechanically stirred (MSSR) reactors, we can stress that the slurry bubble column may be operated in a semi-batch mode, does not use any moving parts, and requires minimal maintenance, smaller floor space and has lower power consumption. 

To ameliorate the effects of substrate inhibition, running the reactor in semi-batch mode, i.e., with continuous feeding of substrate, is always an option. 

A condensation reaction is to be in a stirred tank reactor in the semi-batch mode. The solvent is acetone, the industrial charge (final reaction mass) is 2500 kg, and... 

An exothermal reaction is to be performed in the semi-batch mode at 80 °C in a 16 m3 water cooled stainless steel reactor with heat transfer coefficient U = 300 Wm"2 K . The reaction is known to be a bimolecular reaction of second order and follows the scheme A + B —> P. The industrial process intends to initially charge 15 000 kg of A into the reactor, which is heated to 80 °C. Then 3000 kg of B are fed at constant rate during 2 hours. This represents a stoichiometric excess of 10%.The reaction was performed under these conditions in a reaction calorimeter. The maximum heat release rate of 30Wkg 1 was reached after 45 minutes, then the measured power depleted to reach asymptotically zero after 8 hours. The reaction is exothermal with an energy of 250 kj kg-1 of final reaction mass. The specific heat capacity is 1.7kJ kg 1 K 1. After 1.8 hours the conversion is 62% and 65% at end of the feed time. The thermal stability of the final reaction mass imposes a maximum allowed temperature of 125 °C The boiling point of the reaction mass (MTT) is 180 °C, its freezing point is 50 °C. 

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. 

The SCISR can also be operated continuously, and the product so prepared has essentially the same size and size distribution as that obtained by operation in semi-batch mode under the seme conditions. 

The results of the comparative experiments operated In semi -batch mode indicate that the product prepared with the SCISR is finer and with a narrower distribution than that from the STR of 0.6x10-6 m3 in effective volume, suggesting that the performance of the SCfSR is superior to that of the STR. 

Figure 1.15 The biocatalytic synthesis of acrylamide from acrylonitrile is performed in Japan on a scale of 10000 tons per year. The bacterial cells are immobilized in a poly(acrylamide) gel, and the process is run at pH 8.0-8.5 in semi-batch mode, keeping the substrate concentration below 3%.

These factors are attributed to batch and semi-batch processes rather than continuous processes. However, the use of continuous processes on fine chemical manufacturing sites is limited. It is often preferable to use the semi-batch mode as opposed to batch processes. The Appendix lists hazards of pertinent chemical reactions for toxic and reactive hazards chemicals. Information concerning the safety of various chemicals (e.g., ammonia and others) can be readily obtained from the World Wide Web. Table 12-1 shows how to access a material safety data sheet at the Vermont Safety Information (VIRI) site on the Internet. 

In slurry reactors (Fig. 8.8) small catalyst particles (10-100 pm) are suspended in a liquid phase by mechanical mixing. Stirring also improves the contact between the gas bubbles and the liquid phase and heat exchange with the surroundings. Slurry reactors can be operated in a batch, semi-batch or continuous mode. In the semi-batch mode often the gas-phase is supplied continuously. 

For homogeneous systems, the kinetic parameters k and tq are commonly determined by lab-scale batch experiments in which all reagents are combined at the start of the reaction. Following the concentration of all components (reactants and products) over the course of the reaction then allows for the estimation of the kinetic parameters. Since water has limited solubility in the reaction mixture at the start, conventional kinetic batch experiments could result in erroneous calculation of kj and tq if the limits for homogeneity are crossed. To ensure reaction homogeneity and reliable kinetic measurements, the gradual and continuous addition of water was selected as a suitable method for experimentation (semi-batch mode). The kinetic parameters were then recovered using an appropriate mathematical model with parameter estimation module. 

In order to avoid a pronounced shift of the phase behaviour with the progress of the reaction one reactant can be fed to the reactor in a semi-batch mode. With this concept shift in phase behaviour may be compensated and the optimal state of the system is maintained during the whole reaction time. After phase separation at the most suitable temperature conventional product isolation follows. This sequence of process steps is also possible in a continuously operating process, as illustrated in Fig. 5.17 for the synthesis of 1-phenoxyoctane in a microemulsion stabilised by Triton X-100 [28],... 

An agitated vessel may be operated in either a continuous, batch, or semi-batch mode. In continuous operations, the typical heat-transfer requirement is to maintain a set process temperature by either adding or removing heat, depending on the chemical reaction involved. In batch operations, the heat-transfer process can have a number of different functions at different stages of the operation. Examples include the ... 

Catalytic experiments were performed in a semi-batch mode. After introducing... 

An often used gas-liquid reactor is the bubble column. The gas is usually fed from the bottom through a sparger and the liquid flows either cocurrently or counter-currently. Counter-current operation is more efficient than co-current, but for certain types of parallel reactions, cocurrent operation can give better selectivity. Bubble columns are often operated in semi-batch mode the gas bubbles through the liquid. This mode of operation is attractive in the production of fine chemicals which are produced in small quantities - especially in the case of slow reactions. The flow patterns can vary a lot in a bubble column. Generally, as a rule of thumb, the liquid phase is more back-mixed than the gas phase. The plug flow model is suitable for the gas phase whereas the liquid phase can be modelled with the backmixed, dispersion, or plug flow model. 

Which experimental method to choose depends on whether the process is to be operated in a batch or semi-batch mode. The choice of mode can be justified experimentally, but the initial decision is made either on the basis of past experience and considerations of product quality, yield and throughput, or from consideration of dimensionless groups (see Section 4.4,2 below). The approach using dimensionless groups requires numerical data from kinetic ( , AD k) and heat transfer studies (t/, Cp). 

Continuous or semi-batch processes are usually inherently safer than batch processes. The applications of continuous processes are limited, but almost all processes can be designed to operate in a semi-batch mode. 

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