Downstream solid-liquid separation

The parallel product flow configuration for a two-effect evaporative forced-circulation crystallizer is illustrated in Figure 9.16. The feed rate is maintained at a constant rate by feedback flow control. The level is controlled by varying the mother liquor return from downstream solid-liquid separation to the suction of a centrifugal pump. 

It is also possible to pre-blend limonite and saprolite slurries. However as the examples in Table 4 demonstrate, recoveries fi om the sequential limonite followed by saprolite leach (JAL) are at least equivalent and arguably superior, while producing end solutions of lower acidity and iron content [28]. This has material implications for the quantity of limestone required for neutralization and equipment sizing for downstream solid-liquid separation and residue storage. 

Crystallization is an important separation proeess that purifies fluids by forming solids. Crystallization is also a partiele formation proeess by whieh moleeules in solution or vapour are transformed into a solid phase of regular lattiee strueture, whieh is refleeted on the external faees. Crystallization may be further deseribed as a self-assembly moleeular building proeess. Crystallo-graphie and moleeular faetors are thus very important in affeeting the shape (habit), purity and strueture of erystals, as eonsidered in detail by, for example, Mullin (2001) and Myerson (1999). In this ehapter the internal erystal strueture and external partiele eharaeteristies of size and shape are eonsidered, whieh are important indieators of produet quality and ean affeet downstream proeessing, sueh as solid-liquid separation markedly. Larger partieles separate out from fluids more quiekly than fines and are less prone to dust formation whilst smaller partieles dissolve more rapidly. 

Thus, methods are now becoming available such that process systems can be designed to manufacture crystal products of desired chemical and physical properties and characteristics under optimal conditions. In this chapter, the essential features of methods for the analysis of particulate crystal formation and subsequent solid-liquid separation operations discussed in Chapters 3 and 4 will be recapitulated. The interaction between crystallization and downstream processing will be illustrated by practical examples and problems highlighted. Procedures for industrial crystallization process analysis, synthesis and optimization will then be considered and aspects of process simulation, control and sustainable manufacture reviewed. 

After biological reactions have generated a product of interest, it is necessary to recover this product from a liquid mixture that typically contains several undesirable components. The treatment of any culture broth after bioreactor cultivation is known as downstream processing. Downstream processing can account for 60-80%) of the total production cost, particularly in the production of modern recombinant proteins and monoclonal antibodies. A typical downstream process requires several steps in the areas of solid-liquid separation, cell rupture, product recovery, and product purification. It is important to minimize the number of downstream processing steps required because significant product losses inevitably occur during each step.f ... 

The product from a desired fermentation is either secreted from the cells into the fermentation broth or is entrapped within the cells themselves. In either case, the first step in downstream processing is to separate the cells from the fermentation broth. This step is typically accomplished by filtration or centrifugation and is known as solid-liquid separation. 

Solid-liquid separations are important operations in chemical engineering ranging from upstream raw material purification, to downstream product separation and waste managements. They are usually low cost, and low energy consumption. A successful operation of a SLS system upstream a major chemical engineering unit such as a catalyst bed will help to improve the efficiency the catalyst, and decrease overall cost of the whole process. 

Level. Level control using a control valve to throttle the exit flow rate presents a potential for plugging at the constricting point. Attrition of the crystals from the shear in the valve will generate additional nuclei, especially if the crystallizer head is insufficient to supply slurry to the downstream process and a centrifugal pump is required upstream of the valve. If the slurry is sent to solid-liquid separation equipment, the nuclei are inconsequential because they... 

In the following section, important downstream processes as solid-liquid separation, washing, and reslurrying are discussed briefly with respect to their impact on final product purity. 

Figure 7.14 Simplified process scheme of aspirin production. The downstream processes consisting of solid—liquid separation, reslurrying, and washing steps are indication. (Reproduced with permission from Ref [13].)...

The unit operations of solid-liquid separation and of drying are closely linked to crystallization. A solid-liquid separation process can best be improved by changes in the habit and particle size distribution obtained in the crystallization step. The same holds for the drying process. Furthermore, both the solid-liquid separation and the drying step can have a pronounced influence on the crop, with respect to chemical purity, crystal size and degree of agglomeration, and solid-state form. Thus, crystallization and the downstream processes have a mutual interference. 

In the SCWO process, the feed stream (aqueous waste) to the reactor is heated, pressurized, mixed with oxidant, and pumped through a flow reactor at supercritical conditions designed to provide the required residence time (typically, several seconds to a minute). Heat produced by the reaction can be recovered (or must be removed) based on the heating value of the feed stream. If the heating value is too low to heat the reactor, supplemental fuel can be added. Downstream of the reactor, the system is depressurized, either before or after cooling. Solids produced from oxidation reactions can be recovered prior to or following pressure letdown. The effluent is then passed through gas/liquid separators, and the gas stream and aqueous streams can be treated further. 

Compartmentalization is a concept that can be used widely in multienzymatic processes whereby different parts of the reactor operate imder different conditions (e.g., two-liquid phase biocatalysis) or catalysts are separated (e.g., by immobilization). The two compartments may be separated by a phase boundary (most likely solid-liquid or liquid-liquid). The compartments will selectively contain enzymes and reaction components such that not all enzymes and components are present in all parts of the reactor at the same concentration at a given time. This has benefits not only for the reaction itself (e.g., reducing product inhibition) but also downstream processing (e.g., separation of enzymes). 

From the gaseous product, HCl is recovered by absorption in water. The other gases (CO, propane, ethane) are incinerated and released. The liquid phase is separated into an organic condensate and an aqueous condensate. Solutions containing HCl can be reused in the downstream separation process. The solid phase... 

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