SEMI-CONTINUOUS

Batch polymerisations are often performed in screening experiments on the laboratory-scale level. However, batch polymerisations are used less often in large-scale, commercial produchon processes than semi-continuous polymerisations because of the inherent limitations in heat transfer and copolymer composition control. 

In copolymerisations (70, 408), the copolymer composition may be controlled by the relative rates of monomer addition. In this way, any large differences in the comonomer reactivity ratios or water solubilities can be overcome to produce a copolymer with uniform composition. In order to maintain control of the monomer concentration in the polymer particles, the polymerisation may have to be performed under monomer-starved conditions. This means that the polymer particles are not saturated with monomer, but are being polymerised at an instantaneous conversion of 90% or greater. If the monomer addition rate is greater than the polymerisation rate, the reactor will be operating under flooded conditions, and control over the copolymer composition is lost. 

In many cases, the copolymer composition is controlled to adjust the glass transition temperature (Tg) of the copolymer. An example of this would be varying the ratio of vinyl acetate to butyl acrylate in order to produce a copolymer with a Tg near room temperature for latex paint applications. In other cases, the water-resistance of the polymer is improved by the copolymerisation of hydrophobic monomers, as in copolymerisation of styrene with butadiene for nonwoven textile applications. 

In the feeding stage, additional monomer (often in the form of an emulsion to promote mass transfer and reduce the possibility of monomer pooling) is continuously pumped to the reactor to supply monomer to the polymerising particles. By controlling the rate of monomer addition, the rate of heat generation may be controlled. Initiator may be continually added over the course of the polymerisation to control the radical flux, polymerisation rate, and the rate of heat 

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Delayed Coking", is a semi-continuous process, developed at the end of the 1930 s. The reaction is conducted at 450-500°C under relatively low pressure, four atmospheres, maximum. 

The design techniques used include both stagewise and continuous-contac ting methods and can be applied to batch, continuous, and semi-continuous operations. 

Fig. 6. Micrograph of fixed catalyst VGCF showing the fibers are semi-aligned and semi-continuous.

This formulation applies both to the use of a semi-continuous cycle with an expansion valve and to a discontinuous cycle (such as that in the solar refrigerator) using a flooded evaporator in which the warm condensate must first cool itself before it can cool the load. 

Mathews and Rawlings (1998) successfully applied model-based control using solids hold-up and liquid density measurements to control the filtrability of a photochemical product. Togkalidou etal. (2001) report results of a factorial design approach to investigate relative effects of operating conditions on the filtration resistance of slurry produced in a semi-continuous batch crystallizer using various empirical chemometric methods. This method is proposed as an alternative approach to the development of first principle mathematical models of crystallization for application to non-ideal crystals shapes such as needles found in many pharmaceutical crystals. 

There are three important processes for preparing PVAc latexes in the presence of PVA as a protective colloid batch, semi-continuous, and delayed addition of monomer [10]. In this Chapter, the effects of the addition of VAc and initiators on the properties of PVAc latexes are discussed using the three methods under the same charge of ingredients for polymerization as shown in Fig. 1 [1,11]. 

Centrifugation can be operated on a semi-continuous or continuous basis and there are several different types of centrifuges. Large scale tests have to be performed to choose the proper centrifuge (unloading speed, capacity, separation performance etc). 

With semi-continuous (more properly, semi-batch) reactors only part of the charge is added at the beginning of the cycle. Usually some reaction time is allowed to pass before the remaining part of the charge is added in a controlled manner. Sometimes... 

The influence of inhibitor on the performance of a semi-continuous reactor can be, in some ways, similar to both batch and continuous systems. A dead time is usually observed upon addition of the initial charge. When the secondary stream flow is started after some reaction of the initial charge, additional inhibitor flows into the reactor and the initiation rate drops. When this programmed addition is stopped the initiation rate increases sometimes enough to cause temperature control problems. 

Semi-continuous reactors can be used to produce very narrow or quite broad particle size distributions depending on the nature of the secondary feed stream and how it is added to the reactor. 

Compositional drift in continuous reactor trains can be altered by introducing feed streams of the more reactive monomer between reactors. This procedure is equivalent to programmed addition of the more reactive monomer in a semi-continuous system. 

The proceeding discussion of polymer composition was based on the assumption that essentially all polymer is formed in the organic phases of the reaction mixture. If a water-soluble monomer, such as some of the functional monomers, is used, the reactions taking place in the aqueous phase can contribute to variation in polymer composition. In fact, in extreme cases, water soluble polymer can be formed in the aqueous phase. This can happen in batch, semi-continuous or continuous reactors. The fate of functional monomers could be considerably different among the different reactor types, but detailed studies on this phenomenon have not been reported. 

Recipe additions can also be important with semi-continuous reactors. Addition rates influence reactor performance, and incorrect addition location can lead to non-uniform reaction within the reactor, localized flocculation, and reactor short-circuiting. 

Additionally, the influence of the life time of the micro-structured devices as well as the expenditure of the peripheral equipment was estimated in order to obtain insight into the ecological hot-spots of the system. A conventional macro-scaled semi-continuously operated batch process was chosen as a reference process. The comparison of both technological systems was performed by means of the two-step synthesis of m-anisaldehyde serving as model reaction. 

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). 

Loonkar and Robinson (1972) extended the design problem to the optimal selection of semi-continuous equipment for pre-selected batch equipment that was fixed by the recipe. 

The batch plant shown in Fig. 7.4-6 is to be optimized. The required production capacity is 11070 m per year. The cost coefficients (see Eqn. 7.3-4) are given in Table 7.4-7. The fixed processing times in the batch units, /i. r, are given in Table 7.4-8 together with initial values of processing times in the semi-continuous units, 04-( and those found by optimization. The total batch times, volumes, and costs are also given in this table. 

The minimal cost of equipment was used as the criterion in the design of the plant, which was to be operated in a non-overlapping mode. For a plant consisting of MB true batch units (MS = 3) and MS semi-continuous units (MS = 5) which are grouped in MST semi-continuous trains (MST = 3), the cycle time is given by Eqn. (7.4-10). Combining this expression with Eqn. (7.4-22) and rearranging yields ... 

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. 

Rk - processing rate of semi-continuous equipment unit k kg/h... 

Qik - processing time in semi-continuous unit k for product i h... 

Let us assume an adiabatic, semi-continuous reactor (see Sec. 3.2.4) with negligible input of mechanical energy (Fig. 1.22). 

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