Semi-continuous Mode

In this strategy the feed mixture is charged in the reboiler (at the beginning of the process) to its maximum capacity. For a given condenser vapour load Vc, if the reflux ratio R (which governs the distillate rate, LD, kmol/hr) and the solvent feed rate F (kmol/hr) are not carefully controlled the column will be flooded. To avoid column flooding Tran and Mujtaba (1997) developed the following necessary and sufficient condition  

Reprinted with permission from IChemE, UK. Full reference is at the end of the chapter. 

For a given value of F and assuming that the column design (Vc) is fixed the above inequality constraint on R (Equation 10.2) must be satisfied to avoid flooding. However, if the column operates using a reflux ratio below for some time, the reboiler will be able to accommodate extra solvent and the recovery or the productivity can be further improved. Mujtaba (1999) had considered time dependent (piecewise constant) solvent feed rate within a distillation Task. 

For a given initial feed charge (Bo), and operating at constant values (which can be optimised) of reflux ratio and solvent feed rate, the length of this period can be calculated using  

Beyond this time period the column operation should satisfy the condition given by Equation 10.2 to avoid flooding. 

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In some cases we may benefit from adopting a semi-continuous mode of operation, e.g. to a batch of one reactant we continuously feed the other reactant, while removing a volatile product continuously. An example where this is advantageous is the production of ethyl-4-pentenoate, CH2=(CH2)3(CO)OEt from allyl alcohol and triethyl orthoacetate, CHs-CfOEt). Continuous addition of allyl alcohol to a batch of triethyl orthoacetate and continuous removal of the produced ethylalcohol (and. some allyl alcohol) by distillation resulted in high yields of the dersired ester ethyl-4-pentenoate. By contrast, if allyl alcohol and triethyl orthoacetate were reacted in a batch-wise manner the product consisted of a 1 1 mixture of the desired ester and the undesired ester (Anderson, 2000, p 279 Bollyn and Wright, 1998). 

A major problem to be solved for multiproduct plants is the occurrence of disparities in the cycle times and size requirements for the different stages. In the following it will be assumed that the size factors as well as the cycle times of all units are independent on equipment size. This assumption is usually relaxed in further stages of the design. In case of batch heating and cooling, or reactors operated in semi-continuous mode, this is necessary in order to adopt the cycle times to the capacity of equipment, which is related to batch size. 

Study the semi-continuous mode of operation further, using different values of feed rate and feed concentration to the reactor. Determine the feeding conditions that are most beneficial in terms of selectivity. 

Santos, J.C., Silva, S.S., Mussatto, S.I., Carvalho, W. and Cunha, M.A.A., Immobilized cells cultivated in semi-continuous mode in a fluidized bed reactor for xylitol production from sugarcane bagasse, World. Microbiol. Biotech., 21 (2005) 531-535. 

The distillation process can be carried out in continuous, batch or in semi-batch (or semi-continuous) mode. 

Figure 1.3 shows a typical semi-batch (semi-continuous) distillation column. The operation of such columns is very similar to CBD columns except that a feed is introduced to the column in a continuous or semi-continuous mode. This type of column is suitable for extractive distillation, reactive distillation, etc. (Lang and coworkers, 1994, 1995 Mujtaba, 1999). Further details of semi-batch distillation in extractive mode of operation are provided in Chapter 10. 

Before true continuous reactor trains became common, many were operated in a semi-continuous mode. Typically, there were three or four reactors in series and the styrene would be polymerized to a certain degree of conversion and transferred to the next vessel. This would allow reactants to be transferred into the vacated vessel and batch polymerization begun. This scheme was successful in normal operation, but a surge vessel was needed in case there was a problem with any of the reactors in sequence. 

Recently a method has been described in the patent literature for the fractionation of essential oils using a fluorinated solvent.Oils studied included clove bud and bergamot, and fractionations could be performed in a semi-continuous mode. Fluorous solvents, as very non-polar media, offer an interesting alternative to the aqueous or alcoholic solvent approaches typically used in natural product extractions. However, it is unlikely that the technique will become widely used in this field. 

The fourth step in the direct ethanol process is considered to be key to the economic viability of the bioconversion of cellulose to chemicals. The pretreated cellulose slurry is simultaneously converted to glucose and the glucose to ethyl alcohol in the same vessel in a continuous or semi-continuous mode. The enzyme sample is the whole culture from the enzyme production vessel. The feedstock is a slurry of 7.5% to 15% cellulose. The yeast is either added as a cake or recycled as a cream. 

They modified an existing autoclave and a real-time digital microprocessor control system so that It could be operated In a semi-continuous mode. Various components of MSW were studied In order to obtain meaningful data, not confused by the different thermokinetics of different MSW components. Feedstocks Included wood flour, cardboard, newsprint and rice (starch), as well as the Important model compounds alpha cellulose and lignin. It was found that these MSW components could be converted to liquid oils and a high-heating value residual solid at temperatures of 325 0 to 400 0 and pressures of 1000 psl to 3000 psl. 

Application of New Types of Process Techniques. As will be further explored in Chap. 3, a great number of mixing and aeration systems have been developed that could be of industrial interest. All of these reactors can be operated in discontinuous, semicontinuous, or fully continuous modes. The semi-continuous mode of operation offers some advantages (Pickett, Topiwala, and Bazin, 1979). However, a fully continuous operation seems to be limited (infections, strain stability, continuous flow). 

The operational modes of bioprocess technology, especially the semi-continuous modes, can be categorized using a function describing the time dependence of the input. This is done in Fig. 3.31. The discontinuous and fully continuous processes are special cases with = 0 and = F, respectively through a constant reactor volume. In the case of a CSTR there are practical difficulties a constant, continuous input is not particularly easy to arrange. 

The blow tank is automatically taken through repeated cycles of filling, pressurizing and discharging. Since one third of the cycle time is used for filling the blow tank, a system required to give a mean delivery rate of 20 t/h must be able to deliver a peak rate of over 30 t/h. Dense phase transport is thus a batch operation because of the high pressures involved, whereas dilute phase transport can be continuous because of the relatively low pressures and the use of rotary valves. The dense phase system can be made to operate in semi-continuous mode by the use of two blow tanks in parallel. 

In a common semi-batch operation mode (sometimes called semi-continuous mode), some fraction of reactants, i.e. the initial charge, are initially charged into the reactor, and the rest of the formulation is continuously provided to feed over some period of time. In a fully semi-batch emulsion process, the polymerization starts with an aqueous solution only containing a surfactant and an initiator (micellar solution), and then the monomer feed is provided in very small drops with a given rate. 

From a manufacturing point of view spray-based processes are naturally continuous and can be operated in eiflier continuous mode or semi-continuous mode. 

Many of the batch mixers can be modified to operate in the semi-batch (semi-continuous) mode by feeding the material to be blended continuously or by withdrawing the blended material continuously. By the same token, many of the continuous mixers can be made to operate in the semi-continuous or semi-batch mode. 

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