Flexible batch, processes, control

Flexible Batch Processing Control System (Logic Block)... 

Development of automated batch process control systems has lagged behind that of continuous process control. Flexible factory scale commercial systems have only begun to appear in the last five years (1-4). Increases in the performance/price ratio of small computers are now making automation of laboratory scale batch processes more practical. 

With feed and bleed or batch recycle systems some or all of the water that has already been processed by the stack is mixed with feed and returned via a recirculation pump. Recycle is inherently less efficient than once-through flow in both stack utilization and in energy consumptions since the same solution must be pumped and desalted repeatedly and then remixed with a more concentrated solution. However, the increased flexibility in process control makes recycle systems attractive for small-scale operations where stacks are over-sized to handle varying loads. Feed and bleed is useful for operations where the enriching stream needs to be as concentrated as possible. Ten-fold enrichment or 90% recovery of some feed waters as diluate can be achieved by concentrate recirculation if solubility limits of the dissolved substances are not exceeded in the concentrate stream. 

Despite their flaws, batch processes have stood the test of time for a number of reasons, the most important of which is the flexibihty it brings to the manufacturer in terms of the range of products that the plant can produce, the feedstocks used to produce them, and the speed at which they can be brought to market with very limited information on physical properties, reaction kinetics, and so on (very few, if any, Michelin-starred chefs have ever measured the rheology or kinetics of their latest culinary creation). This flexibility, however, has a price which comes in the form of lower efficiencies in terms of production, energy, labor, and so on, and ultimately efficiency equates to cost However, one should never underestimate the pull of flexibility particularly, as discussed earlier in the examples of fermentation, where control of important parameters is difficult to achieve. 

On the other hand, SMB requires strict process control and is less versatile than normal elution chromatography. In that sense, SMB should be viewed predominately as a very powerful tool for production plants, while batch chromatography with its higher flexibility is equally well suited for development purposes. The fact that efficient simulation software is needed to set up an SMB, while an empirical approach is often sufficient for success in batch chromatography points in the same direction. 

Several advantages accrue to this type of electrolytic process higher yield, avoidance of recycle of the lead metal, and potential flexibility of product production in the electrolysis step. However, the process requires pelletization of the lead metal, it consumes magnesium instead of the less costly sodium in the conventional batch alloy process, and it requires very precise process control as compared with the batch alloy process. 

There are a number of drawbacks to using continuous processes. Resources are needed to develop the process the appropriate residence time to reach a level of suitable reaction completion must be determined under the desired conditions of temperature, flow rate, and any other critical parameters. The reaction system may have limited flexibility for running other reactions. Pressure drops occur when using tubular flow reactors, and these can be calculated [18]. Once the conditions have been developed, time is necessary to reach steady-state conditions. What happens to material produced while the conditions are approaching steady state Such material is not produced under the desired conditions and hence is atypical of the majority of the batch. Effective control equipment is mandatory for large-scale operations otherwise expensive material is at risk and may need to be reworked. 

Lack of flexibility in controlling the reaction and product properties is a disadvantage of tubular reactors, especially compared to semi-batch processes. Multiple feed points along the tube length would need to be employed with potential mixing problems. Hence tubular reactors are not widely utilized for commercial production. 

Batchwise centrifuges have retained a considerable share of the market because of their operational flexibility, which permits qualitatively effective process control. In continuous centrifuges, relatively limited and only slightly variable times are available for main filtration, product washing, and dry centrifuging. All operations are carried out at a constant drum speed. In batch centrifuges, the duration and the drum speed can be adjusted for each operation. This facilitates the qualitative optimization of the processed products and adjustment to varying prodnct parameters. 

The major drawbacks of continuous operation are (i) the use of pumps for circulating the cell broths (in suspension systans) that can damage sensitive cells and (ii) elaborate process control for dissolved oxygen, CO, pH, and temperature that is more crucial as compared to the relatively flexible fed-batch systems (Catapano et al. 2009). 

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