Continuous flow processes

Pasteurization may be carried out by batch- or continuous-flow processes. In the batch process, each particle of milk must be heated to at least 63°C and held continuously at this temperature for at least 30 min. In the continuous process, milk is heated to at least 72°C for at least 15 s ia what is known as high temperature—short time (HTST) pasteurization, the primary method used for fluid milk. For milk products having a fat content above that of milk or that contain added sweeteners, 66°C is requited for the batch process and 75°C for the HTST process. For either method, foUowiag pasteurization the product should be cooled quickly to <7.2° C. Time—temperature relationships have been estabHshed for other products including ice cream mix, which is heated to 78°C for 15 s, and eggnog, which must be pasteurized at 69°C for 30 min or 80°C for 25 s. 

High Temperature—Short Time Pasteurizers. The principal continuous-flow process is the high temperature—short time (HTST) method. The product is heated to at least 72°C and held at that temperature for not less than 15 s. Other features are similar to the batch hoi ding method. 

Electrodialysis. In electro dialysis (ED), the saline solution is placed between two membranes, one permeable to cations only and the other to anions only. A direct electrical current is passed across this system by means of two electrodes, causiag the cations ia the saline solution to move toward the cathode, and the anions to the anode. As shown ia Figure 15, the anions can only leave one compartment ia their travel to the anode, because a membrane separating them from the anode is permeable to them. Cations are both excluded from one compartment and concentrated ia the compartment toward the cathode. This reduces the salt concentration ia some compartments, and iacreases it ia others. Tens to hundreds of such compartments are stacked together ia practical ED plants, lea ding to the creation of alternating compartments of fresh and salt-concentrated water. ED is a continuous-flow process, where saline feed is continuously fed iato all compartments and the product water and concentrated brine flow out of alternate compartments. 

Tubular reactors have been the main tools to study continuous flow processes for vapor or gas-phase reactions. These are also used for reaction in tv o flowing phases over a solid catalyst. When the catalyst is in a fixed bed, the contact between the liquid on the outside surface of the particulate is uncertain. For slurry-type solid catalyst the residence time of the catalyst or the quantity in the reactor volume can be undefined. 

C. Draw-off of the mixed liquids influences tlie location of the impeller. For blending, the preferred impeller location in a Z/T =1.0 Vessel is at the midpoint of liquid depth for a continuous flow process, but may not be for a batch system. 

Differential and Integral Balances. Two types of material balances, differential and integral, are applied in analyzing chemical processes. The differential mass balance is valid at any instant in time, with each term representing a rate (i.e., mass per unit time). A general differential material balance may be written on any material involved in any transient process, including semibatch and unsteady-state continuous flow processes ... 

The design of production plants for the manufacture of the three categories of product varies considerably. Fine chemicals are usually produced in batch reactors, which may also be used for the production of a variety of similar products. Fine chemicals usually have demanding product quality specifications and, consequently, a significant fraction of the production costs are involved in product purification and testing. Intermediate volume chemicals have less rigorous quality specifications than fine chemicals and are usually manufactured in product-specific-plants, either as batch or continuous flow processes. Bulk chemical production plants usually operate continuous flow processes... 

Separation. The proeess by which the bacterial cells are separated from the culture fluid. Centrifugation using either a batch or continuous flow process is commonly used, but preeipitation of the cells by reducing the pH is an alternative. In the case of vaeeines prepared fiom eells, the fluid is discarded and the cells are resuspended in a saline mixture where vaeeines are made fiom a constituent of the fluid, the cells are discarded. 

One of the most far reaching analyzes along these lines of thought was given by Commenge [114] in the context of gas-phase reactions in continuous-flow processes. Specifically, he analyzed four different aspects of micro reaction devices, namely the expenditure in mechanical energy, the residence-time distribution, safety in operation, and the potential for size reduction when the efficiency is kept fixed. 

Kirschning A, Jas G (2004) Applications of immobilized catalysts in continuous flow processes. Topics in Curr Chem 209-239... 

The continuous flow process shown in Figure 4—2 has a mixture of benzene, cyclohexane, and hydrogen being heated to about 4Q0°F, pressured to about 400 psi, and pumped through the first reactor. The proportions of each feed depend on the type of catalyst being used. On a once-thro ugh basis, about 95% of the benzene is converted to cyclohexane. 

Their adaptability to continuous flow processes and hence use in automated... 

Similarly Silica-Bound Co(salen) 37 (Scheme 10) [69] was also effectively used in the HKR of styrene oxide (Scheme 11) and 4-hydroxy-1-butene oxide (Scheme 12). The immobilized catalysts were adapted to a continuous flow process for the generation of reaction products in high yield and ee, requiring only very simple techniques for product purification (Scheme 13). 

Kirschning, A. and Jas, G. (2004). Applications of Immobilized Catalysts in Continuous Flow Processes. Topics in Current Chemistry, 242, 209-239. 

Marquie, J. and Salmoria, G. and Poux, M. and Laporterie, A. and Dubac, J. and Roques, N. (2001). Acylation and Related Reactions under Microwaves. 5. Development to Large Laboratory Scale with a Continuous-Flow Process. Ind. Eng. Chem. Res., 40, 4485-4490. 

Scheme 8 Microwave-assisted continuous flow process to form quinolinones [25]...

Baumann M, Baxendale IR, Ley SV et al (2008) A modular flow reactor for perfor-mingCurtius rearrangements as a continuous flow process. Org Biomol Chem 6(9) 1577-1586... 

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