Semicontinuous production processe

Semicontinuous production processes can be described in a similar way as batch production processes by adding a duration information to a material flow. This makes it possible to handle the overlapping of independent process orders over multiple stages in campaign planning and semicontinuous production planning. 

In the semicontinuous production process of polydimethylphenylsiloxane varnish (Fig. 70) agitator 5 is filled with raw phenyl and dimethyldi-chlorosilane from the batch boxes. The mixture is agitated for 0.5-1 hours and sampled for chloroions. Then, the apparatus is filled with a necessary amount of toluene and the reactive mixture is agitated for 0.5-1 hour. 

The release of sanguinarine from the cells into the medium after elicitation was used as the basis for the design of an industrial process for production of the alkaloid 72,99,487,492,493). As the cells of P. somniferum remain viable after a 72-hr exposure to a fungal elicitor, it is possible to recycle the cells after induction of alkaloid biosynthesis. Because about 40-60% of the alkaloid is released to the medium, a simple change of medium is sufficient to collect the alkaloid and to recover viable cells. By a repeating sequence of elicitation and medium replenishment, a semicontinuous production process was obtained. Such a process was also developed for alkaloid production in Catharanthus roseus, Ruta grave-olens, and P. somniferum cell cultures using different elicitors (99). 

In some industry branches semicontinuous production occurs together with both multistage joint production (often with multiple side products per stage that have to be processed further just like the main product) and the opportunity to vary the throughputs of the production devices. 

Due to this overdetermination problem there exists a strong prejudice in the chemical industry that production planning for batch production is completely different from production planning for continuous production. This leads to the ineffective separation between process engineering and business data processing in continuous and semicontinuous production. 

In continuous processes the reactants are fed to the reactor and the products withdrawn continuously the reactor operates under steady-state conditions. Continuous production will normally give lower production costs than batch production, but it lacks the flexibility of batch production. Continuous reactors will usually be selected for large-scale production. Processes that do not fit the definition of batch or continuous are often referred to as semicontinuous or semibatch. In a semibatch reactor, some of the reactants may be added or some of the products withdrawn as the reaction proceeds. A semicontinuous process can be one which is interrupted periodically for some purpose, for instance, for the regeneration of catalyst. 

The semicontinuous production of -amylase and cellulase has been studied in PEG-Dextran systems using Bacillus subtilis and Tricoderma reesel, respectively (21, 22). Some improvements in the yields have been observed in both the cases. In case of cellulase production, the economics of the process could be improved by using a cheap, lignocellulosic waste as the substrate, and replacing the fractionated dextran with a crude one. 

The semicontinuous SAS processes share the wide applicability and the performance of the batch processes in terms of particle size, and improve the production capacity. Its main... 

Semicontinuous crystallization processes often eombine the best features of both batch and continuous operation. For example, a rapid mixer with a short residence time (possibly an on-line deviee, e.g. Figure 9.14) can discharge its product slurry into an agitated residenee tank. In many eases, a series of tanks may have to be installed, whieh can then be operated as individual units or arranged in cascade (section 9.1.2). 

Semicontinuous rectification processes are of practical importance for vapor mixtures generated in a discontinuous chemical reaction. These then have to be separated into a component continuously fed back to the reactor and a discharging fraction. For example, the main task during the transesterification of dimethyl teraphthalate with ethylene glycol is to continuously separate methanol from the reaction mixture. The reaction equilibrium is then shifted toward the product. Methanol is the top product of the column which is connected directly to the transesterification reactor. The high-boiling, or heavy, bottom product. 

Many variations of these two basic processes are described in process patents (20,21), including the use of co-solvents and azeotropic removal of water to facilitate the reactions and to minimize undesirable by-products such as insoluble pol5uners. The original batch methods have been modified to allow for continuous or semicontinuous production. New developments have been focused on improving manufacturing yield and resin purity. 

The two procedures primarily used for continuous nitration are the semicontinuous method developed by Bofors-Nobel Chematur of Sweden and the continuous method of Hercules Powder Co. in the United States. The latter process, which uses a multiple cascade system for nitration and a continuous wringing operation, increases safety, reduces the personnel involved, provides a substantial reduction in pollutants, and increases the uniformity of the product. The cellulose is automatically and continuously fed into the first of a series of pots at a controlled rate. It falls into the slurry of acid and nitrocellulose and is submerged immediately by a turbine-type agitator. The acid is deflvered to the pots from tanks at a rate controlled by appropriate instmmentation based on the desired acid to cellulose ratio. The slurry flows successively by gravity from the first to the last of the nitration vessels through under- and overflow weirs to ensure adequate retention time during nitration. The overflow from the last pot is fully nitrated cellulose. 

Hydrothermal Synthesis Systems. Of the unit operations depicted in Figure 1, the pressurized sections from reactor inlet to pressure letdown ate key to hydrothermal process design. In consideration of scale-up of a hydrothermal process for high performance materials, several criteria must be considered. First, the mode of operation, which can be either continuous, semicontinuous, or batch, must be determined. Factors to consider ate the operating conditions, the manufacturing demand, the composition of the product mix (single or multiple products), the amount of waste that can be tolerated, and the materials of constmction requirements. Criteria for the selection of hydrothermal reactor design maybe summarized as... 

Ton-exchange systems in process appHcations may be batch, semicontinuous, or continuous. Batch operations are not common but, where used, involve a ketde with mechanical agitation. Injecting with air or an inert gas is an alternative. A screened siphon or drain valve is requited to prevent resin from leaving with the product stream. 

In the Bnchamp process, nitro compounds are reduced to amines in the presence of iron and an acid. This is the oldest commercial process for preparing amines, but in more recent years it has been largely replaced by catalytic hydrogenation. Nevertheless, the Bnchamp reduction is still used in the dyestuff industry for the production of small volume amines and for the manufacture of iron oxide pigments aniline is produced as a by-product. The Bnchamp reduction is generally mn as a batch process however, it can also be mn as a continuous (48) or semicontinuous process (49). 

Chisso-Asahi uses a spouted bed process for the production of their coated materials (12). A 12,000 t/yr faciHty is located in Japan. The semicontinuous process consists of two batch fluid-bed coaters. A dilute polymer solution is prepared by dissolving 5% polymer and release controlling agent into a chlorinated hydrocarbon solvent such as trichloroethylene. The solution is metered into the spouted bed where it is appHed to the fertilizer core. Hot air, used to fluidize the granules, evaporates the solvent which is recovered and reintroduced into the process. Mineral talc, when used, is either slurried into the polymer solution or introduced into the fluidizing air. 

The fine chemicals business is characterized by a small volume of products manufactured. Therefore, batch production predominates and small-scale reactors are used. The need to implement fine chemistry processes into existing multiproduct plants often forces the choice of batch reactors. However, safety considerations may lead to the choice of continuous processing in spite of the small scale of operation. The inventory of hazardous materials must be kept low and this is achieved only in smaller continuous reactors. Thermal mnaways are less probable in continuous equipment as proven by statistics of accidents in the chemical industries. For short reaction times, continuous or semicontinuous operation is preferred. 

The size requirements of a semicontinuous unit k for product i are characterized by duty factors Dik whose physical meaning is similar to the size factors, and the processing rates Rk the duty factor is related to the processing time for semicontinuous equipment as follows ... 

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