Multi-stage continuous system

Despite the advantages of continuous cultures, the technique has found little application in the fermentation industry. A multi-stage system is the most common continuous fermentation and has been used in the fermentation of glutamic add. The start-up of a multi-stage continuous system proceeds as follows. Initially, batch fermentation is commenced in each vessel. Fresh medium is introduced in the first vessel, and the outflow from this proceeds into the next vessel. The overall flow rate is then adjusted so that the substrate is completely consumed in the last vessel, and the intended product accumulated. The concentration of cells, products and substrate will then reach a steady state. The optimum number of vessels and rate of medium input can be calculated from simple batch experiments. 

In the laboratory batch ozonation is easy to apply, whereas multi-stage continuous-flow systems are difficult to handle (Method 1). However, mainly due to large liquid flow rates the inverse situation is valid for many full-scale applications. Often three oxidation reactors in series are found in waste water ozonation (cf. Table A 3-3). The advantage of a multi-stage CFSTR system - or even a batch system - lies in their faster reaction rates compared to a single CSTR due to the reduced axial/longitudinal mixing. 

Consideration of the basic differences between these scenarii lead to the present proposition that for a multi-stage continuous PHA-production process, the use of a plug-flow tubular reactor, or PFTR (in which Reynolds numbers are large), brings substantial increases in productivity when compared to a system consisting of CSTRs only. Support of this assertion comes firom the works of Levenspiel (29), where mean residence times for the two types of reactors are compared under the restriction of a desired goal ... 

Figure 2.5 Possible technological solutions to bioprocess problems a) Fed-batch culture b) Continuous product removal (eg dialysis, vacuum fermentation, solvent extraction, ion exchange etc) c) Two-phase system combined with extractive fermentation (liquid-impelled loop reactor) d) Continuous culture, internal multi-stage reactor e) Continuous culture, dual-stream multi-stage reactor f) Continuous culture with biomass feedback (cell recycling). (See text for further details).

The multi-stream multi-stage system is a valuable means for obtaining steady-state growth when, in a simple chemostat, the steady-state is unstable eg when the growth-limiting substrate is also a growth inhibitor. This system can also be used to achieve stable conditions with maximum growth rate, an achievement that is impossible in a simple chemostat (substrate-limited continuous culture). 

The entire production system is organized as a global multi-stage network with multi-purpose and continuously operated production resources... 

Tekmaheat Oy manufacture three kiln systems a compact chamber of 15-25m per batch, having an annual production capability of 1600-3000 m a drive-through chamber having a volume of 40-60 m, with an annual capacity of 5000-10 000 m and a multi-stage production line, in which the timber is pre-heated, dried, heat-treated and conditioned in a continuous throughput process, which has a capacity of 30 000-50 000 m per annum. 

Assess results Several methods can be applied to minimize the ozone consumption in combined processes of ozonation and biodegradation. For example, the different operational behavior of batch and continuous-flow systems, with the resulting differences in oxidation products, etc. has to be considered, as well as the fact that multi-stage systems for each treatment process may be of advantage over single stage systems. 

More complex multi-pass SWRO systems include the system used in Ashkelon, Israel, which uses four RO passes in series to treat seawater from an open water intake in the Mediterranean Sea (40,700 mg/1 TDS). The permeate must be produced with less than 0.4 mg/1 boron and 20 mg/1 chloride. Thus, a series of passes with changes in pH was necessary to obtain the required permeate water quaHty. The first pass has a recovery of 45% and is operated at neutral pH. Permeate from the feed end is collected as product, while permeate from the concentrate end is collected and flows to the second pass, which operates at 85% recovery and pH > 8.5 to achieve greater boron removal. The concentrate firom the second pass continues to the third stage, also operated at 85%, but at low pH. The objective of the third pass is to achieve higher recovery without salt precipitation. However, the boron removal in the third pass is minimal at low pH, and a fourth pass (high pH, 90% recovery) treats the third pass permeate for boron removal. Overall, the recovery is approximately 44%, and the plant uses 25,600 SWRO membranes and 15,100 BWRO membranes [3,51]. 

Multi-stage extraction is used to achieve a higher efficiency of separation, in which the product is almost completely removed from the raffinate. The solvent is split up into several portions and fed to a series of mixers and settlers. The disadvantage of this method is the need to use large volumes of solvent. A more complicated system, called countercurrent, multi-stage extraction, uses a series of mixers and settlers arranged as before, but the feed liquid and pure solvent are passed through the system in opposite directions, that is counter-currently. Continuous countercurrent operation may be carried out by means of spray columns, packed columns (similar to those used in distillation), plate columns, or, sometimes, bubble cap columns. 

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