Batch ultrafiltration process

Figure 6.17 Flow schematic of a batch ultrafiltration process...

Ultrafiltration, which uses selective membranes to separate materials on the basis of different molecular sizes, has become a valuable separation tool for a wide variety of industrial processes, particularly in the separation of dispersed colloids or suspended solids. In many cases where a high degree of separation is desired, a batch ultrafiltration process is used because it is the most economical in terms of membrane area. 

The schematic of a typical batch ultrafiltration process used for separating suspended solids is shown in Figure 1. 

Optimal Design of Batch Ultrafiltration-Diafiltration Process... 

The slope, m, and the intercept, b, are constant for a given ultrafiltration process. Since the product of solids concentration and total volume in a given batch is always a constant, the flux can also be expressed as... 

The ultrafiltration process is operated in a batch mode at a temperature of about 50 C. Ceramic membranes with 0.1 or 0.2 pm pore diameter enable processing of this highly viscous and concentrated raw or pasteurized whole milk due to their inherent mechanical strength. The viscosity of the concentrate has been found to increase exponentially with the rise of protein content in the precheese. Polymeric membranes have also been considered not suitable for this process in view of their structural compaction under pressure and their difficulty of cleaning. 

A typical batch ultrafiltration plant is represented in Figure 1. It consists of a processing tank, a feed pump, a circulation pump and a membrane unit with a number of modules in parallel. A permeate is obtained from the membranes while the retentate is recirculated until the desired concentration in the processing tank is reached. Then, a cleaning procedure is performed and the system is ready for the next batch. 

This work addresses the optimal design and operation of a batch protein ultrafiltration plant. The dynamic optimisation procedure adopted identifies simultaneously the optimal design parameters and operating policy of the installation. It should be emphasised that the approach can be directly applied to other ultrafiltration processes. This is the first work in which a formal dynamic optimisation methodology is applied to batch ultrafiltration. 

Ultrafiltration is a membrane process in which a solution containing a valuable solute such as a protein is concentrated by applying pressure to it and forcing the solvent across a semipermeable membrane, i.e., a membrane more permeable to the solvent than it is to the solute. Some of the latter will usually leak through as well that is, the process is not 100% efficient. Efficiency is here defined as 1 -where Cj, is the concentration at any instant of the solution passing through — the permeate — and Q denotes the concentration in the enriched solution left behind, termed the retentate. In a test nm of a batch ultrafiltration unit to determine leakage, it was found that the retentate concentration had doubled after 53.7% of the solu-... 

There are two other aspects of importance in practical ultrafiltration processing time and the membrane area. A brief treatment of processing time with a view to minimizing it will be considered now. The processing time depends on a variety of factors the total volume to be processed the concentration of retained solids, especially if it leads to gel polarized operation (equation (6.3.143)) the mode of operation (continuous, batch, etc.) (Cherytm, 1986). One approach suggested by Ng et al. (1976) employs the gel-polarized condition and perfect rejection of the protein i for continuous diafiltration operation ... 

Fig. 1. Schematic of a process for batch suspension culture of mammalian cells, where UF is ultrafiltration and DF is diafiltration.

Aim of this work was to optimise enzymatic depolymerization of pectins to valuable oligomers using commercial mixtures of pectolytic enzymes. Results of experiments in continuous and batch reactor configurations are presented which give some preliminary indications helpful to process optimisation. The use of continuous reactors equipped with ultrafiltration membranes, which assure removal of the reaction products, allows to identify possible operation policy for the improvement of the reaction yield. 

A first application using ferroceneboronic acid as mediator [45] was described for the transformation of p-hydroxy toluene to p-hydroxy benzaldehyde which is catalyzed by the enzyme p-cresolmethyl hydroxylase (PCMH) from Pseudomonas putida. This enzyme is a flavocytochrome containing two FAD and two cytochrome c prosthetic groups. To develop a continuous process using ultrafiltration membranes to retain the enzyme and the mediator, water soluble polymer-bound ferrocenes [50] such as compounds 3-7 have been applied as redox catalysts for the application in batch electrolyses (Fig. 12) or in combination with an electrochemical enzyme membrane reactor (Fig. 13) [46, 50] with excellent results. 

Just like chemical processes, biocatalytic reactions are performed most simply in batch reactors (Figure 5.5a). On a lab scale and in the case of inexpensive or rapidly deactivating biocatalysts, this is the optimal solution. If the biocatalyst is to be recycled, but the mode of repeated batches is to be maintained, a batch reactor with subsequent ultrafiltration is recommended (batch-UF reactor Figure 5.5b). The residence times of catalyst and reactants are identical in all batch reactor configurations. 

The simplest type of ultrafiltration system is a batch unit, shown in Figure 6.17. In such a unit, a limited volume of feed solution is circulated through a module at a high flow rate. The process continues until the required separation is achieved, after which the concentrate solution is drained from the feed tank, and the unit is ready to treat a second batch of solution. Batch processes are particularly suited to the small-scale operations common in the biotechnology and pharmaceutical industries. Such systems can be adapted to continuous use but this requires automatic controls, which are expensive and can be unreliable. 

For treating water containing VOCs with separation factors of more than 500, for which concentration polarization is a serious problem, feed-and-bleed systems similar to those described in the chapter on ultrafiltration can be used. For small feed volumes a batch process as illustrated in Figure 9.16 is more suitable. In a batch system, feed solution is accumulated in a surge tank. A portion of this solution is then transferred to the feed tank and circulated at high velocity through the pervaporation modules until the VOC concentration reaches the desired level. At this time, the treated water is removed from the feed tank, the tank is loaded with a new batch of untreated solution, and the cycle is repeated. 

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