Semicontinuous mixture

Table 1 Three Phase Flash-Calculation Results for a Semicontinuous Mixture at 49 C and 164.8bar...

Figure 1 Calculted and experimental distribution of heavies for a semicontinuous mixtures...

Chou, G. F., and J. M. Prausnitz Adiabatic flash calculations for continuous or semicontinuous mixtures using an equation of state. Fluid Phase Equilibria, 30,75-82(1986). 

In this treatment, presented previously, all chemical species of the mixtures are considered to be similar. Thus, they all are described by one distribution function. In a generalized version the occurrence of several ensembles of very similar species (e.g. paralBnic and aromatic hydrocarbons or polymer blends) in the mixture may be accounted for by describing each ensemble with its own distribution function. Furthermore, some individual components may also be present e.g. in a polymer solution), that need to be included into the formalism too. Polydisperse mixtures of this kind are often called semicontinuous mixtures. There are no fundamental difficulties in generalizing the simple version of continuous thermodynamics discussed previously, but the equations become somewhat more complex. Here, the principles of generalization are to be presented only by some examples (for a more detailed treatment the reader should refer to ref. 48 for instance). 

As an analogy, the human body performs a mixture of batch, semicontinuous and continuous processes in order to operate effectively. Sohds-handhng operations are conducted batchwise for transfer and semicontinuously for processing, whereas heat exchange (HEx), filtration, pumping, and so on, are performed continuously. 

Results of regression are given in Table 7.4-4. The cycle time is size-dependent whereby this dependency is stronger for the semicontinuous process at these particular process conditions. The exponent p is almost the. same for both processes and is approximately 1/3. Clearly, the physical properties of the reaction mixture and thermokinetic data are needed to evaluate processing times and cycle times at a large scale. 

Semicontinuous polymerization experiments were carried out in a stainless steel enclosure (dry box) under a dry nitrogen atmosphere in three neck flasks (11) equipped with overhead stirrer and an inlet for the continuous introduction of precooled inifer/isobutylene/ solvent feeds to stirred, dilute BCI3/solvent charges. Experiments with binifer were performed at -80°C by the use of CH3Cl/n-hexane solvent mixtures (80/20 v/v), with trinifer at -40°C using CH3CI... 

Semicontinuous Binifer Runs -80°C, CH3Cl/n-hexane solvent mixture ... 

In the fed-batch (semicontinuous) operation mode, substrates are fed into the reactor but no material is removed from the reactor. Therefore, the total volume of the material within the reactor increases as a function of time. For this reactor type the mass balance for each component of the reaction mixture is given by... 

Submerged-culture oxidizers are usually operated on a semicontinuous basis. In most cases, ca half the liquid in the tank is removed every 1-2 d, when the alcohol concentration has dropped to 0.1-0.2 vol %. The removed vinegar is replaced with wine or mash of richer ethanol and lower acetic acid concentration, giving a mixture in the tank of 5-6 vol % ethanol and 6-8 vol % of acetic acid. These arc the optimum conditions for Acetobacter gr owth. 

The semicontinuous polymerization was carried out in a 500-ml four-neck flask immersed in a constant temperature bath at 60° C, and equipped with a reflux condenser, a two-bladed stainless steel stirrer and a graduated dropping funnel. The water and surfactant were introduced into the flask, then nitrogen gas was bubbled with agitation for at least 20 minutes. The initiator solution was added, and after 5 minutes the mixture of monomer addition started and continued for a total of three hours. The polymerization was continued for at least one hour past this point. Under these conditions the rate of monomers addition was less than 1/10 of their maximum rate of polymerization (Rpmax). 

Some brands of polydimethylphenylsiloxane varnishes can be produced from raw phenyl rather than phenyltrichlorosilane, a nonrectified mixture of phenylchlorosilanes (the phenyltrichlorosilane content is at least 77-80%, the chlorine content is 43-47%), using the semicontinuous technique. 

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. 

This has the advantage of being invariant under any distortion of the x scale, and it makes life easier when the mixture contains, in addition to the distributed components, a small number of discrete components, and it is particularly useful in some thermodynamic analysis, where a semicontinuous description is often used. It is also useful to note that, since c x) is invariant with respect to rescaling the label X, the conceptual problem discussed in Appendix B is alleviated if one uses the logarithm of c x). 

A Euclidean norm declares two functions that differ only on a countable infinity of isolated points as being close. This is not too much of a difficulty for the problems we consider, but there is another difficulty. If we want to consider distributions that include one or more discrete components (a semicontinuous distribution), s(x) may well contain some delta functions. This implies, first, that all integrals have to be interpreted as a Stjieltjies ones but even so one has a problem with the right-hand side of Eq. (177), because the delta function is not Stjieltjies square-integrable. One could be a bit cavalier here and say that we agree that 5 (j ) = 5(x), but it is perhaps preferable to keep continuous and discrete components separate. Let, for instance, the mole distribution be i, ri2,.. ., /v, n(x) in a mixture with N discrete components and a distributed spectrum. One can now define the scalar product as the ordinary one over the discrete components, plus... 

So far, the homologation reaction has reached only the pilot-plant scale [58, 61], Little information is available about the reaction in continuous operation. The only cobalt-catalyzed continuously conducted reaction led to a mixture of 20 different products. The yield of ethanol is low (16 mol%) [59]. By activation with iodine and variation of the space-velocity, the overall yield has been improved and the ratio of acetaldehyde/ethanol could be varied between 13 18 and 2 17 [60]. BP has described continuous homologation with the Co/l/PPh3 catalyst system. The yield of ethanol reached only 25 mol % [11]. Semicontinuous work on the homologation reaction has been reported by the former Ruhrchemie AG [61]. 

The most extensively used method is the carbothermic reaction of UO2 with carbon under vacuum at 1600-2000°C. The pelletized mixture of UO2 and carbon is heated in vacuum in a furnace that allows semicontinuous production (Fig. 1) The optimum reduction temperature is 1600°C for material that is to be pressed and sintered to form pellets. The residual oxygen contents are in the range 0.35-0.63%. Higher temperatures result in a less sinterable product, while lower temperatures give unacceptably high residual oxygen contents. 

In a semicontinuous or batch flow process, one or more ingredient is charged to the reactor, and the remaining components are added gradually. None of the reaction mixture is displaced through overflow. The total volume of a vessel usually increases as the reaction progresses, and the product is isolated at the end of the operation cycle. This type of operation is useful for exothermic reactions in vessels with limited heat transfer capacity. 

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