Batch operations

In modeling a batch reactor, we have assumed that there is no inflow or outflow of material and that the reactor is well mixed. For most liquid-phase reactions, the density change with reaction is usually small and can be neglected (i.e., V = Vq). In addition, for gas-phases reactions in which the batch reactot 

Generally, when analyzing laboratory experiments, it is be.st to process the data in terms of the measured variable. Because concentration is the measured variable for most liquid-phase reactions, the general mole balance equation applied to reactions in which there is no solume change becomes 

This is the form we will use in analyzing reaction rate data in Chapter 5. 

Let s calculate the time necessary to achieve a giwn conversion X for the irreversible second-order reaction 

The mole balance on a constant-volume, V = batch reactor is 

The batch operation of fermentors is much more common than continuous operation, although theories for continuous operation are well established (as will be indicated later in this section). The reasons for this are  

In the fed-batch operation of fermentors (which is also commonly practiced), the feed is added either continuously or intermittently to the fermentor, without any product withdrawal, the aim being to avoid any excessive fluctuations of oxygen demand, substrate consumption, and other variable operating conditions. 

The kinetics of cell growth was discussed in Chapter 4. By combining Equation 4.2 and Equation 4.6, we obtain  

This equation provides the rate of cell growth as a function of the substrate concentration Cs. In case the cell mass is the required product (an example is baker s yeast), the cell yield with respect to the substrate, Yxs, as defined by Equation 4.4, is of interest. 

In the case where a product (e.g., ethanol) is required, then the rate of product formation, rp (kmol h-1 (unit mass of cell)-1), and the product yield with respect to the substrate Yps, as well as the cell yield, should be of interest. Then, for unit volume of the fermentor, 

In this chapter batch and semi-batch operation modes to produce copolymers in emulsion polymerisation will be discussed. 

A batch reactor is a closed system, that is, no materials enter or leave the reactor during the polymerisation reaction, in which the time is the only independent variable. The batch operation can be used for some small production of homopolymers from monomers with a relatively small heat of polymerisation. However, the drawbacks associated to this type of operation limit its industrial use. These drawbacks are  

Batch reactors are commonly employed in research laboratories because of their simplicity and low cost of operation. 

The composition of the copolymers produced in batch reactors wiU be dictated by the reactivity ratios of the monomers, r see Equation 4.6, as well as by the mole fractions of the monomers, in the polymer particles, see Equation 4.7. The instantaneous composition can then be predicted by the terminal model, see Equation 4.5. Most of the common monomers employed in emulsion polymerisation recipes present different reactivities, and a consequence of this is the compositional drift (non-constant copolymer composition) produced in batch operation. The compositional drift can be easily calculated by computing the instantaneous, see Equation 4.2, and cumulative copolymer compositions. 

In order to calculate the rate of polymerisation, the monomer concentration in the particles, [M,]p, the average number of radicals per particle, n, the number of particles, Np, as well as the reactivity ratios should be available. The calculation of [M,]p has been described in Section 4.2. The calculation of n and Np can be found in Chapters 2 and 3 of this book. Note that to calculate the time evolution of the instantaneous copolymer composition, the time history of the variables [M,]p, n and Np must also be known. The unreacted or free monomer present in the reactor can be computed from the general macroscopic material balance for a perfectly mixed stirred tank reactor by omitting inlet and outlet streams  

Let us look at the cooling requirements for batch operation. The flow sheet in Fig. 2.17b now comprises a cell, a reservoir, a circulating pump, and of course a power supply. 

Batch operation is essentially unsteady-state, so the energy balance is drawn up with respect to conditions obtaining at some instant t. If the reactor is run isothermally the cooling requirements at t will be given by  

89) we have taken the value of H2 — as zero. This is true when we take a datum line for the enthalpies. However, under those circumstances the values of and r2AHj 2 st be those for the 

The average cooling rate of the batch Q will be given by integration with respect to time  

EXAMPLE 2.14. Estimation of the Heat Load on the Heat Exchangers of an Organic Electrosynthetic Plant 

In this chapter, continuous operation has been implicitly assumed. However, batch operation is very often used in both the fine chemical and pharmaceutical industries. Analysing and designing batch equipment is generally different from the equivalent continuous units, in that 

Seader and E.J. Henley, Separation Process Principles, Wiley, New York, 1998. 

Coulson, J.F. Richardson, J.R. Backhurst and J.H. Harker, Coulson Richardson s Chemical Engineering, Vol. 2 (Particle Technology and Separation Processes), 4th edn (revised), Pergamon Press, Oxford, 1993. 

McCabe, J.C. Smith and P. Harriott, Unit Operations of Chemical Engineering, 5th edn, McGraw-Hill, Singapore, 1993. 

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Using a batch reactor, a constant concentration of sulfuric acid can be maintained by adding concentrated sulfuric acid as the reaction progresses, i.e., semi-batch operation. Good temperature control of such systems can be maintained, as we shall discuss later. 

Given the choice of a batch rather than continuous process, does this need a different approach to the synthesis of the reaction and separation and recycle system In fact, a different approach is not needed. We start by assuming the process to be continuous and then, if choosing to use batch operation, replace continuous steps by batch steps. It is simpler to start with continuous process operation... 

Solution Having synthesized the continuous flowsheet shown in Fig. 4.136, let us now convert this into batch operation. 

Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

Batch processes can be synthesized by first synthesizing a continuous process and then converting it to batch operation. The process yield is an important measure of both raw materials efficiency and environmental impact. 

Clearly, the potential hazard from runaway reactions is reduced by reducing the inventory of material in the reactor. Batch operation requires a larger inventory than the corresponding continuous reactor. Thus there may be a safety incentive to change from batch to continuous operation. Alternatively, the batch operation can be... 

Although most appHcations of fixed bed have multiple adsorber beds to treat continuous streams, batch operation using a single adsorber bed is an alternative. For purification appHcations, where one vessel can contain enough adsorbent to provide treatment for days, weeks, or even months, the cost savings and simplicity often justify the inconvenience of stopping adsorption treatment periodically for a short regeneration. 

The main features in which the Radford process differs from the batch operation are in thermal dehydration and compounding. Water-wet nitrocellulose on a continuous vacuum belt filter is vacuum-dried followed by hot air transfusion (80°C) to reduce the moisture to less than 2%. After cooling, alcohol is sprayed on the nitrocellulose to a concentration of 15—20%. The alcohol-wet nitrocellulose is then transferred from a surge feeder to a compounder by a continuous weigh-belt along with the other ingredients of the composition, which are also weighed and added automatically. 

The trend in the use of deep bed filters in water treatment is to eliminate conventional flocculators and sedimentation tanks, and to employ the filter as a flocculation reactor for direct filtration of low turbidity waters. The constraints of batch operation can be removed by using one of the available continuous filters which provide continuous backwashing of a portion of the medium. Such systems include moving bed filters, radial flow filters, or traveling backwash filters. Further development of continuous deep bed filters is likely. Besides clarification of Hquids, which is the most frequent use, deep bed filters can also be used to concentrate soflds into a much smaller volume of backwash, or even to wash the soflds by using a different Hquid for the backwash. Deep bed filtration has a much more limited use in the chemical industry than cake filtration (see Water, Industrial water treatment Water, Municipal WATERTREATiffiNT Water Water, pollution and Water, reuse). 

Vacuum filters are available in a variety of types, and are usually classified as either batch operated or continuous. An important distinguishing feature is the position of the filtration area with respect to gravity, ie, horizontal or non-horizontal filtering surface. 

The pressure versions of the nutsche filter, which falls into this category, are either simple pressurized filter boxes or more sophisticated agitated nutsches, much the same in design as the enclosed agitated vacuum filters described eadier. These are extremely versatile, batch-operated filters, used in many industries, eg, agrochemistry, pharmaceuticals, or dyestuff production. 

High process temperatures generally not achievable by other means are possible when induction heating of a graphite susceptor is combined with the use of low conductivity high temperature insulation such as flake carbon interposed between the coil and the susceptor. Temperatures of 3000°C are routine for both batch or continuous production. Processes include purification, graphitization, chemical vapor deposition, or carbon vapor deposition to produce components for the aircraft and defense industry. Figure 7 illustrates a furnace suitable for the production of aerospace brake components in a batch operation. 

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. 

Reverberator Furnace. Using a reverberatory furnace, a fine particle feed can be used, the antimony content can be controlled, and batch operations can be carried out when the supply of scrap material is limited. However, the antimony-rich slags formed must be reduced in a blast furnace to recover the contained antimony and lead. For treating battery scrap, the reverberatory furnace serves as a large melting faciUty where the metallic components are hquefted and the oxides and sulfate in the filler material are concurrently reduced to lead metal and the antimony is oxidized. The furnace products are antimony-rich (5 to 9%) slag and low antimony (less than 1%) lead. 

Germany, Bitterfeld 1920 two-stage rotary kilns heated internally using intermediate grinding of roast oxidation completed within 3—4 h cylindrical monopolar ceUs, 4 m volume undivided con-centric Ni anodes, rod-shaped Fe cathodes unfiltered electrolyte batch operation KMnO crystallizes in ceU electrolysis energy consumption about 700 kWh/1 4,000 27,113... 

Manganate(VI) formed in the initial oxidation process must first be dissolved in a dilute solution of potassium hydroxide. The concentrations depend on the type of electrolytic cell employed. For example, the continuous Cams cell uses 120 150 g/L KOH and 50 60 g/L K MnO the batch-operated Bitterfeld cell starts out with KOH concentrations of 150 160 g/L KOH and 200 220 g/L K MnO. These concentration parameters minimize the disproportionation of the K MnO and control the solubiUty of the KMnO formed in the course of electrolysis. 

Gas Combustion Retort. The continuous gas combustion retort (GCR) has been modeled after the earlier batch-operation NTU retort. Although the term "gas combustion" has been appHed to this process, it is a misnomer in that, in a weU-designed and properly operated system, the residual char on the retorted shale suppHes much of the fuel for this process. The GCR is the foremnner of most continuous AGR processes (Table 7). 

Most aroma chemicals are relatively high boiling (80—160°C at 0.4 kPa = 3 mm Hg) Hquids and therefore are subject to purification by vacuum distillation. Because small amounts of decomposition may lead to unacceptable odor contamination, thermal stabiUty of products and by-products is an issue. Important advances have been made in distillation techniques and equipment to allow routine production of 5000 kg or larger batches of various products. In order to make optimal use of equipment and to standardize conditions for distillations and reactions, computer control has been instituted. This is particulady well suited to the multipurpose batch operations encountered in most aroma chemical plants. In some instances, on-line analytical capabihty is being developed to work in conjunction with computer controls. 

Distillation. This is the point at which refining begins and was the first method by which petroleum was refined. Originally, distillation (qv) involved a batch operation in which the stiU was a cast-iron vessel mounted on brickwork over a fire and the volatile materials were passed through a pipe or gooseneck which led from the top of the stiU to a condenser. The latter was a coil of pipe, or a "worm" (hence the expression worm end products), immersed in a tank of miming water. 

In this representation the FeCl2 which takes part in the first step of the reaction is not a tme catalyst, but is continuously formed from HQ. and iron. This is a highly exothermic process with a heat of reaction of 546 kj /mol (130 kcal/mol) for the combined charging and reaction steps (50). Despite the complexity of the Bnchamp process, yields of 90—98% are often obtained. One of the major advantages of the Bnchamp process over catalytic hydrogenation is that it can be mn at atmospheric pressure. This eliminates the need for expensive high pressure equipment and makes it practical for use in small batch operations. The Bnchamp process can also be used in the laboratory for the synthesis of amines when catalytic hydrogenation caimot be used (51). 

Enerco, Inc. (Yardley, Pennsylvania) has a 600 tine/d demonstration pyrolysis plant located in Indiana, Pennsylvania. The faciUty operated 8 h/d, 5 d/wk for six months. The process involves pyrolysis in a 5.4 t/d batch-operated retort chamber. The heated tines are broken down to cmde oil, noncondensable gases, pyrolytic filter, steel (qv), and fabric waste. In this process, hot gases are fed direcdy to the mbber rather than using indirect heating as in most other pyrolyses. The pyrolysis plant was not operating as of early 1996. 

Synthetic Organic Chemical Manufacturers Association, 1100 New York Ave., N.W., Washington, D.C., 20005, (202) 414-4100, which offers information about regulatory impact on the chemical industry, particularly small and batch operations. 

Sedimentation equipment can be divided into batch-operated settling tanks and continuously operated thickeners or clarifiers. The operation of the former is simple. Whereas use has diminished, these are employed when small quantities of Hquids are to be treated, for example in the cleaning and reclamation of lubricating oil (see Recycling, oil). Most sedimentation processes are operated in continuous units. 

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