Reactors Continuous plug-flow

The alternative to batch mode operation is continuous operation. In the continuous mode there is a continuous flow of medium into the fermentor and of product stream out of the fermentor. Continuous bioprocesses often use homogenously mixed whole cell suspensions. However, immobilised cell or enzyme processes generally operate in continuous plug flow reactors, without mixing (see Figure 2.1, packed-bed reactors). 

Biochemical reactors can be operated either batchwise or continuously, as noted in Section 1.5. Figure 7.1 shows, in schematic form, four modes of operation with two types of reactors for chemical and/or biochemical reactions in Uquid phases, with or without suspended solid particles, such as catalyst particles or microbial cells. The modes of operation include stirred batch stirred semi-batch continuous stirred and continuous plug flow reactors (PFRs). In the first three types, the contents of the tanks arc completely stirred and uniform in composition. 

Figure 7.1 Modes of reactor operation (a) batch reactor, (b) semi-batch reactor, (c) continuous stirred-tank reactor, and (d) continuous plug flow reactor.

Fig. 11.9 Types of linear continuous-flow reactors (LCFRs). (a) Continuous plug flow reactor (CPFR) resembling a batch reactor (BR) with the axial distance z being equivalent to time spent in a BR. (b) A tabular flow reactor (TFR) with (tq) miscible thin disk of reactive component deformed and distributed (somewhat) by the shear field over the volume, and (b2) immiscible thin disk is deformed and stretched and broken up into droplets in a region of sufficiently high shear stresses, (c) SSE reactor with (cj) showing laminar distributive mixing of a miscible reactive component initially placed at z = 0 as a thin slab, stretched into a flat coiled strip at z L, and (c2) showing dispersive mixing of an immiscible reactive component initially placed at z — 0 as a thin slab, stretched and broken up into droplets at z — L.

The application of the equations to chemical reactions requires the proper definition of the above quantities as well as correctly defining the transition probabilities pjj and pjk this is established in the following. It should also be noted that the models derived below for numerous chemical reactions, are applicable to chemical reaction occurring in a perfectly-mixed batch reactor or in a single continuous plug-flow reactor. Other flow systems accompanied with a chemical reaction will be considered in next chapters. 

Deflnitions. The basic elements of Markov chains associated with Eq.(2-24) are the system, the state space, the initial state vector and the one-step transition probability matrix. Considering refs.[26-30], each of the elements will be defined in the following with special emphasize to chemical reactions occurring in a batch perfectly-mixed reactor or in a single continuous plug-flow reactor. In the latter case, which may simulated by perfectly-mixed reactors in series, all species reside in the reactor the same time. 

When cross-linked crystals of thermolysin were applied in peptide synthesis in ethyl acetate, they were stable for several hundred hours at amazingly low enzyme consiunption, whereas a soluble enzyme preparation became inactive within a short period of time. Again it is worthwhile to consider the quality of the soluble enzyme preparation. When soluble thermolysin was stored in mixed aqueous-organic solutions, it lost about 50% of its activity within the first day of incubation only to be then quite stable for the next 15 days. It is possible that the initial inactivation was caused by an unstable fraction of thermolysin and that crystals of thermolysin no longer contained this unstable fraction [118]. Productivity comparable to that of crystals was achieved with thermolysin adsorbed on Amberlite XAD-7 resin which was employed in continuous plug flow reactors with tert-amyl alcohol as solvent [119]. 

Sutherland et al. [37] observed in a continuous plug flow reactor that other ethers (ETBE, DIPE, and TAME) are more efficiently removed than MTBE. Only tBA removal proved to be less efficient. These investigations resulted in lower calculated unit treatment costs for the alternative ethers (up to 64%) but higher costs for the treatment of fBA. 

Active immobilized enzyme used in a continuous plug-flow reactor for the conversion of riboflavin into riboflavin 5 -phosphate... 

In continuous plug flow reactors there is no longitudinal mixing of fluids [1]. Therefore, the appropriate approximation of the reactor parameters is the plug flow model. This model is characterised by the following points ... 

Continuous plug flow reactors are also unsuitable for these purposes because it is usually impossible to obtain an isothermic mode in such reactors, even for reactions with a relatively low rate of reaction. Plug flow reactors usually operate in adiabatic or intermediate modes, which are far from isothermic even with an external heat removal modification. In can be stated that almost all industrial reactors employed for fast processes are not optimally designed and are therefore ineffective. The quality of products is also far from optimal and the processes are generally not perfect from an engineering, economical, or social point of view (decrease of final product yield and quality, increase of nonrecyclable wastes, excessively high consumption of raw materials and low energy efficiency). 

Oeff.L [m s ] in case of continuous plug flow reactor (CPFR)... 

The ideal continuous plug flow reactor (CPFR) has no profile at any point of the tube in the steady state. The process, however, advances along the tube, and so shows a longitudinal concentration variation. The profile of a CPFR in space is identical to the profile of a DCSTR in time in case of a constant volume process this fact is of great importance for process design ( kinetic similarity ). 

Figure 3.30. Basic reactor concept and concentration-versus>time and concentration-versus-space profiles. DCSTR, discontinuous stirred tank reactor SCSTR, semicon-tinuous stirred tank reactor CSTR, continuous stirred tank reactor CPFR, continuous plug flow reactor NCSTR, a cascade of N stirred vessels.

Model 4 The Ideal Continuous Plug Flow Reactor (CPFR) or Tubular Reactor... 

Figure 6.36. Plot of the dimensionless concentration of cell mass x and substrate s for a continuous culture as a function of the dimensionless mean residence time I as in Fig. 6.1b with Xq > 0 Calculated comparison between a CSTR with maximum mixing (ST m) or one with total segregation (ST J and a continuous plug flow reactor (PF), assuming Monod kinetics with a death rate (Tsai et al., 1969).

Figure 6.57. Summary diagram of work flow in the systematic development of a bioprocess of the presented integrating strategy. The diagram is based on the interaction between kinetics (Chap. 5) and transport (Chap. 3) processes, which are clarified during a kinetic analysis (Chap. 4). As a special situation, the design and utilization of new types of reactors are shown (discontinuous stirred vessel, DCSTR bubble column, BC semicontinuous stirred vessel, SCSTR recycle reactor, RR continuous stirred vessel, CSTR continuous cascade, NCSTR tower reactor, TR continuous plug flow reactor, CPFR fixed and fluidized bed reactor, FBR).

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