Batch or Continuous Operation

Another consideration is whether the crystallization should be carried out on a batch or continuous basis. The present tendency in most processing plants is to use continuous equipment wherever possible. Continuous equipment permits adjusting the operating conditions to a relatively fine degree to get the best results in terms of overall energy usage and product characteristics. It permits the use of a smaller operating labor force and results in a continuous 

Batch handling of wet or semidry crystalline materials presents considerable difficulties as compared with the storing and handling of dried crystalline materials. On the other hand, a train of continuous processing equipment, such as just described, has economic application only on relatively large production rates from approximately 50 tons of product per day and upward. 

Another area of application is when the final liquor temperature is so low that it requires the use of extensive vapor compression equipment. In such systems, the use of batch equipment is frequently more economical than anything else, other than a relatively large number of multiple-stage series crystallizers. 

The use of multistage crystallization equipment, while not analogous to multistage evaporation equipment, nevertheless permits certain economics in operation. It is normally employed when a 

---

The sulfuric acid hydrolysis may be performed as a batch or continuous operation. Acrylonitrile is converted to acrylamide sulfate by treatment with a small excess of 85% sulfuric acid at 80—100°C. A hold-time of about 1 h provides complete conversion of the acrylonitrile. The reaction mixture may be hydrolyzed and the aqueous acryhc acid recovered by extraction and purified as described under the propylene oxidation process prior to esterification. Alternatively, after reaction with excess alcohol, a mixture of acryhc ester and alcohol is distilled and excess alcohol is recovered by aqueous extractive distillation. The ester in both cases is purified by distillation. 

Strengthening by cold rolling is accompanied by decreased ductUity. A softening heat treatment is needed when ductUity is lowered to below levels required by subsequent processing. Annealing treatments are done interchangeably as batch or continuous operations. 

Indirect type, batch or continuous operation for pharmaceuticals such as peniciUin and blood plasma. Expensive. Used on beat-sensitive and readily oxidized materials under Liquids under Liquids under Liquids used on pharmaceuticals and related products which cannot be dried successfuby by other means. Applicable to fine chemicals under Granular solids cial cases such as emulsion-coated films under Granular solids... 

Infrai ed. Batch or continuous operation Only for thin films See comments under Liquids See comments under Liquids (only for thin layers) Only for thin layers... 

Figure 4-4 shows a semi-batch reactor with outside circulation and the addition of one reactant through the pump. Semi-batch reactors have some reactants that are charged into the reactor at time zero, while other reactants are added during the reaction. The reactor has no outlet stream. Some reactions are unsuited to either batch or continuous operation in a stirred vessel because the heat liberated during the reaction may cause dangerous conditions. Under these... 

The product crystals were agglomerates of needles or dendrites. Loose floes of dendroid strontium carbonate are compacted by agitation, which is an important factor in controlling the habit of product particles. Semi-batch operation produces larger particles compared to batch or continuous operation. 

The choice between batch or continuous operation will not be clear cut, but the following rules can be used as a guide. 

Select the operating conditions batch or continuous operating pressure. 

Once a group of possible dryers has been selected, the choice may be narrowed by deciding whether batch or continuous operation is to be employed and, in addition to restraints imposed by the nature of the material, whether heating by contact with a solid surface or directly by convection and radiation is preferred. 

For the sepn of solids from liquids, there are two types of centrifuges a)Solid wall(such as imperforate basket type) centrifuge, in which sepn or concn is by subsidence or flotation and b)Perforated wall(such as perforated basket type) centrifuge in which the solid phase is supported on a permeable surface thru which the fluid passes. A 3rd type is a combination of a) b) above. Here the primary concn is achieved by subsidence, followed by drainage of die liquid phase from the solid phase. These centrifuges can be either of batch or continuous operations. For sepn of liquids, there are bottle and tubular centrifiiges(Ref 2, 8,10,12,14,16,19,20,21,23,25,26 27)... 

In semi-batch or continuous operations, the feed rate controls the reaction course. 

Because of the need to avoid mutations and maintain the superior qualities of the genetically developed strain, batch or fed-batch operations are used in most applications. Continuous culture operations, however, provide a time-invariant environment that facilitates greatly the study of a biological process in research laboratories. Moreover, some industrial operations employ continuous reactors, such as the single-cell protein facility of ICI in Billingham, England (total reactor volume of about 2,300 m3), all waste treatment processes, and others. It should be noted that it is relatively common to follow a batch process with a period of fed-batch or continuous operation. Also, in most cases batch cultivation is the optimal start-up procedure for continuous or fed-batch cultivation (Yamane et al, 1977). 

Centrifugal force, rather than pressure differential, is used to throw the solid phase against the filter medium and force liquid through the filter medium and cake. The filter medium may be wire mesh, perforated metal or cloth and is not usually used in a precoat manner. Centrifugal filters can be either batch or continuous operation. 

The presentation of the cells has been organized according to the following criteria two- or three-dimensional electrodes, batch or continuous operation, and absence or presence of a diaphragm. 

Because of their predictive capabilities, models are also essential tools in modem biochemical engineering for the design of processes and the optimization of media and reactor operational parameters in batch or continuous operation. They can also serve in the development of software sensors to estimate on-line the time variation of the medium composition. 

Infrared. Batch or continuous operation. Electric heating or gas-fired Only for thin films. Can be used in combination with other dryers such as drum. See comments under Liquids. See comments under Liquids (only for thin layers). Only for thin layers Primarily suited to drying surface moisture. Not suited for thick layers Specially suited for drying and baking paint and enamels Useful when space is limited. Usually used in conjunction with other methods, e.g., in drying paper coatings Useful for laboratory work or in conjunction with other methods... 

Crystal growth is a layer-by-layer process, and the retention time required in most commercial equipment to produce crystals of the size normally desired is often on the order of 2 to 6 h. Growth rates are usually limited to less than 1 to 2 pm/min. On the other hand, nucle-ation in a supersaturated solution can be generated in a fraction of a second. The influence of any upsets in operating conditions, in terms of the excess nuclei produced, is very short-term in comparison with the total growth period of the product removed from the crystallizer. A worst-case scenario for batch or continuous operation occurs when the explosion of nuclei is so severe that it is impossible to grow an acceptable crystal size distribution, requiring redesolution or washout of the system. In a practical sense, this means that steadiness of operation is much more important in crystallization equipment than it is in many other types of process equipment. 

When liquids are to be evaporated on a small scale, the operation is often accomplished in some form of jacketed tank or kettle. This may be a batch or continuous operation The rate ofheat transfer is generally lower than for other types of evaporators and only a limited heat transfer area is available. The kettles may or may not be agitated. 

The starting point in determining how to dry certain products is first to ascertain whether the process will be a batch or continuous operation. 

---