Unit operations crystallization

A crystallization process has to be designed to meet a number of requirements, for example, particle properties such as particle size and purity. As concerns the purification, a continuously operated crystallization has to deal with the accumulation of impurities in the mother liquor. The drain point for these impurities has to be carefully chosen. It is one of the most underestimated aspect of the process design activity and the source of many errors. Moreover, the process yield is strongly connected with the planning of the process drain points, as is the specific energy consumption and, thus, the success of the process. However, die separation attribute of the unit operation crystallization may not be the best available and one should remain open for competing alternatives. That could be recommendable, for example, in cases of solid solutions, especially with distribution coefficients close to 1, which are not feasible to separate by crystallization at all. 

In these process alternatives that are completely dedicated to the unit operation crystallization, one can find remarkably often, though, some other unit operations used to intensify the crystallization in separating components and to make the entire process more feasible (process intensification). Best examples are the discoloration with activated carbon or the use of a washing thickener for replacing the mother liquor (Figure 16.4). In the case of washing thickener, one uses the countercurrent washing principle to treat the suspension of the crystals from second crop with the purest solution available in this system, which is the feed solution. Even a solution prepared by redissolution of product could be taken. 

The unit operation crystallization belongs to the oldest technologies used by humans. Quite a number of early civilizations were formed in coastal areas with... 

Crystallization is one of the oldest unit operations in the portfoho of industrial and/or laboratory separations. Almost all separation techniques involve formation of a second phase from a feed, and processing conditions must be selected that allow relatively easy segregation of the two or more resulting phases. This is a requirement for crystallization also, and there are a variety of other properties of the sohd product that must be considered in the design and operation of a crystallizer. Interactions among process, function, product, and phenomena important in crystallization ate iRustrated in Figure 1. 

The SIMULAR, developed by Hazard Evaluation Laboratory Ltd., is a chemical reactor control and data acquisition system. It can also perform calorimetry measurements and be employed to investigate chemical reaction and unit operations such as mixing, blending, crystallization, and distillation. Ligure 12-24 shows a schematic detail of the SIMULAR, and Ligure 12-25 illustrates the SIMULAR reaction calorimeter with computer controlled solids addition. 

Similarly, different unit operations are available to accomplish the same processing objective. For example, a filter, a centrifuge, or a decanter could be used to separate a solid from a liquid. Crystallization or distillation could also be used for purification. 

As mentioned above, the unit operation of crystallization rarely exists in isolation but is normally part of a wider particulate processing system as illustrated schematically, in a very simplified form, in Figure 9.2. A particular feature of such processes is the variety of unit operations and the range of equipment types that may be employed at any stage. 

Rossiter and Douglas (1986) state that the first step in process design is to generate a basic structure for the flowsheet i.e. the choice of unit operations and interconnections which can be analysed, refined and costed, and then compared to alternatives. Thus, the generation of an industrial crystallization flowsheet gives rise to a number of optimization problems for which a systematic hierarchical decision process for particulate systems was proposed ... 

Definition of process objectives Generation of separation core structure Selection of separation sequence and unit operations Addition of further units to the process structure Selection of crystallizer type... 

Crystallization Process Systems brings together essential aspects of the concepts, information and techniques for the design, operation and scale up of particulate crystallization processes as integrated crystal formation and solid-liquid separation systems. The focus of the book, however, is on crystallization only dealing with related unit operations as far as is necessary. It is therefore... 

It should therefore not be surprising that for relatively small-scale operations involving solids handling within the fine and intermediate chemicals industry, batch operation is preferred. Similarly, continuous processes that involve precipitation or crystallization, a common unit operation in fine chemicals, are rare. Small-scale examples are known, for instance, a continuous crystallization process was used by Bristol-Myres Squibb in order to improve dissolution rates and bioavailability of the product [12]. The above does indicate that not all process or parts thereof are suited for conversion from B2C, given the current technology. 

The unit operations involved are quite different, less reaction and separation, more mixing, crystallization and preservation. 

Unit operations Traditional - distillation, crystallization, extraction, absorption, adsorption, etc. Unconventional -granulation, milling, coating, etching, etc. 

Unit operations are almost entirely physical in nature - for example, distillation, filtration, grinding, crystallization, etc. Chemical unit processes on the other hand, are the common standardized processes used in the chemical industry - for example, oxidation, reduction, halogenation, hydration, nitration, esterification, etc. 

Most operations scaled up in the pharmaceutical industry use semibatch (and batch) processing in the general-purpose equipment. Such operations allow for fine control of slow unit operations, for example, reactions needing hours to complete, fermentation, and crystallization, and such fine control may be necessary to ensure high quality and productivity. 

Tanks are used in innumerable ways in the chemical process industry, not only to store every conceivable liquid, vapor, or solid, but also in a number of processing applications. For example, as well as reactors, tanks have served as the vessels for various unit operations such as settling, mixing, crystallization (qv), phase separation, and heat exchange. Herein the main focus is on the use of tanks as liquid storage vessels. The principles outlined, however, can generally be applied to tanks in other applications as well as to other pressure-containing equipment. 

An alternative approach is taken in the production of monosodium glutamate (MSG) which, unlike interferon, is secreted into the fermentation broth. The stages of downstream processing for MSG are shown in Figure 14.3. Again, a variety of unit operations, including centrifugation, crystallization, vaporization, and fixed-bed adsorption, are used in this process. 

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