Recirculation, with batch processing

Industrial-scale adsorption processes can be classified as batch or continuous. In a batch process, die adsorbent bed is saturated and regenerated in a cyclic, operation. In a continuous process, a countercurrent staged contact between lire adsorbent and die feed and desorbent is established by cidier a true or a simulated recirculation of die adsorbent. The efficiency of an adsorption process is significantly higher in a eoiuinuous mode of operation than in a cyclic batch mode. For difficult separations, batch operation may require 25 times more adsorbent inventory and twice die desorbent circulation rate than does a continuous operation. In addition, in a batch mode, the four functions of adsorption, purification, desorption, and displacement of the desorbent from the adsorbent are inflexibly linked, wtiereas a continuous mode allows mure degrees of freedom with respect to these functions, and thus a better overall operation. 

It has been observed that enantioselective polymer-bound catalysts prepared by copolymerization produce in some cases better asymmetric inductions than systems prepared by grafting [175]. After much optimization, a monolithic polymer catalyst 51 suitable for a titanium-TADDOLate catalyzed Diels-Alder reaction was developed (Scheme 4.77). The monolith was applied in a flow system both under one pass and 24 h recirculation conditions, the latter producing the best yield (55%) and ee (23%) however, this contrasts poorly with the homogeneous batch reaction although the ee is comparable with the heterogeneous batch process. The reversal of topicity was also... 

Surface aeration is usually employed for slow reactions or for batch processes. It can be used in semicontinuous systems when it is desirable to recirculate the gas from the headspace. This is frequently the case in hydrogenation and is referred to as dead-end hydrogenation. In this system, gas is fed continuously to the reactor at the rate at which hydrogen is being consumed no compression costs to overcome the static head of liquid or external recirculation is needed. Feeding gas from the headspace may be preferred when there is a possibility of plugging sparger holes with reaction products. Surface aerators are also extensively used for waste-water treatment. There are two types of surface aerators the brush aerator, and the most commonly used turbine aerator. 

Compared to batch processes, continuous processes often show a higher space-time yield. Reaction conditions may be kept within certain limits more easily. For easier scale-up of some enzyme-catalyzed reactions, the Enzyme Membrane Reactor (EMR) has been developed. The principle is shown in Fig. 7-26 A. The difference in size between a biocatalyst and the reactants enables continuous homogeneous catalysis to be achieved while retaining the catalyst in the vessel. For this purpose, commercially available ultrafiltration membranes are used. When continuously operated, the EMR behaves as a continuous stirred tank reactor (CSTR) with complete backmixing. For large-scale membrane reactors, hollow-fiber membranes or stacked flat membranes are used 129. To prevent concentration polarization on the membrane, the reaction mixture is circulated along the membrane surface by a low-shear recirculation pump (Fig. 7-26 B). 

In most cases, the flux is too low to operate in the single-pass mode the recovery of permeate in a single pass is a small percentage of the feed (very little concentration of retained species). Recirculation of the process stream across the membrane is necessary to obtain the desired concentration or recovery. Typically the ratio of recirculation rate to permeate flux is 10 to 100-fold. Recirculation can be accomplished with either a batch concentration or with a feed and bleed operating mode. 

It is unwise to use the feed and bleed mode to concentrate a batch of solution. Once steady state is achieved, the membrane filter at the final concentration for the duration of the run. In a batch process, a higher average flux is achieved due to the gradually increasing concentration. Thermodynamically, the feed and bleed mode is less efficient due to increase in entropy upon mixing the low concentration feed with the high concentration recirculating retentate. 

Process Unit or Batch Unit A process unit is a collection of processing equipment that can, at least at certain times, be operated in a manner completely independent from the remainder of the plant. A process unit normally provides a specific function in the production of a batch of product . For example, a process unit might be a reactor complete with all associated equipment (jacket, recirculation pump, reflux condenser, and so on). However, each feed preparation tank is usually a separate process unit. With this separation, preparation of the feed for the next batch can be started as soon as the feed tank is emptied for the current batch. 

The filtrate from this first batch will comprise a solution of 180 to 270 kg of unprecipitated acetylsalicylic acid (1.0 to 1.5 mols), 510 kg of acetic anhydrice (5.0 mols), 600 kg of acetic acid (10.0 mols) (obtained as a by-product in the acetylation step) and 1,200 kg of the diluent toluene. Into this filtrate, at a temperature of 15° to 25°C, ketene gas is now passed through a sparger tube or diffuser plate, with good agitation, until a weight increase of 420.5 kg of ketene (10 mols) occurs. The reaction mixture wiil now contain 180-270 kg of unprecipitated acetylsalicylic acid (1.0-1.5 mols) and 1,532 kg of acetic anhydride (15 mols) in 1,200 kg of toluene. This mother liquor is recycled to the first step of the process for reaction with another batch of 1,382 kg of salicylic acid. On recirculating the mother liquor, the yield of pure acetylsalicylic acid is 1,780 to 1,795 kg per batch. 

Figure 16.13. MWB (Metallwerk Buchs) batch recirculating crystallizer, with freezing on and melting off insides of thin film heat exchanger tubes adaptable to multistage processing without external solids handling [Miitzenberg and Saxer, 1971).

A type of processing equipment in which the entire amount of material to be used is put into the mixer and mixed for a definite period, with multiple recirculation of material through the mixing zone, in contrast to what happens in a continuous mixer. After the mixing period the whole amount of material is removed from the mixer. In oil production or processing, the process in which emulsion is collected in a tank and then broken in a batch. This process is used as opposed to continuous or flow-line treating of emulsions. 

FIGURE 8 Schematic diagram showing the two basic modes of operating an adsorption separation process (a) cyclic batch two-bed system (b) continuous countercurrent system with adsorbent recirculation. Concentration profiles through the adsorbent bed are indicated. Component A is more strongly adsorbed than B. (Reprinted with permission from Ruthven, D. M. (1984). Principles of Adsorption and Adsorption Processes, copyright John Wiley Sons, New York.)... 

After the raw stock is prepared and the equipment is checked for her-meticity, the etherification of silicon tetrachloride is started. The process is carried out in etherificator 6, which is a cast iron enameled apparatus with a jacket. From batch boxes 2 and 3 the etherificator is simultaneously filled with anhydrous ethyl alcohol and silicon tetrachloride. Apart from anhydrous alcohol, the etherificator receives recirculating ethyl alcohol from batch box 4. In certain volume ratios (usually from 1 2.2 to 1 2.3) silicon tetrachloride and alcohol enter through siphons the lower part of the etherificator. The temperature of the process (30-40°C) is maintained by regulating the supply of the components. The pressure in the apparatus should not exceed 0.015-0.016 MPa. 

Many plants have been designed and operated on a batch system with various stirring systems and recirculation loops since the early 1950s but the latest thinking for bulk production is probably the plants that Davy Process Technology has developed for alkoxylation. At least 10 plants of its design have been put into beneficial operation since they were first introduced in 1990 and are based on the Buss Loop Reactor technology. 

A pilot plant scale starch solution, approximately 300 l, was prepared in a stainless steel container equipped with a paddle wheel electric stirrer, steam-heated coils, steam injection, an external gear pump with recirculation capability, insulation surrounding the container, a container lid, and a vent. The starch solution was prepared with the same percentage composition and duration of heating as described for the laboratory scale process. The first batch was pimped into the "sizing box" of the slasher and utilized immediately hot as a conventional yarn treatment. 

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