Stirred continuous mode

Chapter 2 treated multiple and complex reactions in an ideal batch reactor. The reactor was ideal in the sense that mixing was assumed to be instantaneous and complete throughout the vessel. Real batch reactors will approximate ideal behavior when the characteristic time for mixing is short compared with the reaction half-life. Industrial batch reactors have inlet and outlet ports and an agitation system. The same hardware can be converted to continuous operation. To do this, just feed and discharge continuously. If the reactor is well mixed in the batch mode, it is likely to remain so in the continuous mode, as least for the same reaction. The assumption of instantaneous and perfect mixing remains a reasonable approximation, but the batch reactor has become a continuous-flow stirred tank. 

Crameri et al. (1997) have reported an asymmetric hydrogenation constituting an important step in the production of a new calcium antagonist, Mibefradil (POSICOR) (of Hoffmann-LaRoche). Pilot-scale synthesis of (S)-2-(4-flurophenyl)-3-methylbutanoic acid by the asymmetric hydrogenation of 2-(4-fluorophenyl)-3-methyl but-2-enoic acid with a [Ru (/ )-MeOBIPHEP)(OAc)2]-catalyst has been described. The hydrogenation was performed in a continuous mode in a cascade stirred-tank reactor system at a pressure of 270 bar. A large reduction in total reactor volume compared to the batch mode was realized. 

The esterification process can be carried out in either batch or continuous mode, the final decision depending most likely on the size of the flow rates involved. For most commercial sizes of 15 MM gal/yr or higher, the continuous process is probably more cost effective and for this option, two additional options are available continuous stirred tank reactor (CSTR) or a fixed-bed reactor (FBR). 

Stirred-tank reactors can be operated in batch, semi-batch, or continuous mode. In batch or semi-batch mode ... 

Bubble slurry column reactors (BSCR) and mechanically stirred slurry reactors (MSSR) are particular types of slurry catalytic reactors (Fig. 5.3-1), where the fine particles of solid catalyst are suspended in the liquid phase by a gas dispersed in the form of bubbles or by the agitator. The mixing of the slurry phase (solid and liquid) is also due to the gas flow. BSCR may be operated in batch or continuous modes. In contrast, MSSR are operated batchwise with gas recirculation. 

An attractive property of monolithic reactors is their flexibility of application in multiphase reactions. These can be classified according to operation in (semi)batch or continuous mode and as plug-flow or stirred-tank reactor or, according to the contacting mode, as co-, counter-, and crosscurrent. In view of the relatively high flow rates and fast responses in the monolith, transient operations also are among the possibilities. 

Agitated Stirred Tanks Stirred tanks are common gas-liquid reactors. Reaction requirements dictate whether the gas and liquid are in a batch or continuous mode. For a liquid-phase reaction with a long time constant, a batch mode may be used. The reactor is filled with liquid, and gas is continuously fed into the reactor to maintain pressure. If by-product gases form, these gases may need to be purged continuously, if gas solubility is limiting, a higher-purity gas may be continuously fed (and, if required, recycled). As the liquid residence time decreases, product may be continuously removed as well. A... 

The enzymatic system used for the continuous production of Mn3+-malonate is presented in Fig. 10.3. It is composed by a stirred tank reactor (200-mL working volume) operated in continuous mode coupled to a 10 kDa cutoff ultrafiltration membrane (Prep/Scale-TFF Millipore), which permits the recycling of the enzyme to the reaction vessel. The enzyme was recycled in a recycling feed flow ratio of 12 1. 

After specifying the energy form, the catalyst and the phases in contact, the next task is to decide whether to conduct the reaction in a batch or continuous mode. In the batch mode, the reactants are charged to a stirred-tank reactor (STR) and allowed to react for a specified time. After completing the reaction, the reactor is emptied to obtain the products. This operating mode is unsteady state. Other unsteady-state reactors are (1) continuous addition of one or more of the reactants with no product withdrawal, and (2) all the reactants added at the beginning with continuous withdrawal of product. At steady-state, reactants flow into and products flow out continuously without a change in concentration and temperature in the reactor. 

In slurry reactors (Fig. 8.8) small catalyst particles (10-100 pm) are suspended in a liquid phase by mechanical mixing. Stirring also improves the contact between the gas bubbles and the liquid phase and heat exchange with the surroundings. Slurry reactors can be operated in a batch, semi-batch or continuous mode. In the semi-batch mode often the gas-phase is supplied continuously. 

Stirred tank reactor systems can also operate in a continuous mode. In this configuration, fresh medium is continually supplied to the reactor and the desired products are continuously removed in the course of production. A continuous system is referred to as a chemostat when the flow rate is set to a constant value. It is further known as a turbidostat when the flow rate is set to maintain a constant turbidity or cellular concentration.f Continuous reactor systems are commonly abbreviated as CSTR or CSTF and they refer to continuous stirred tank reactor and continuous stirred tank fermenter, respectively. 

The stirred-tank reactor consists of a box or tank with a stirrer in it. The solid medium in the tank is stirred in batch or continuous mode to facilitate oxygen mass transfer. Figure 8 shows the rectangular pilot reactor (2 x 0.8 x 2.3 m) designed by Durand and Chereau [70] to culture T. viride with sugar beet pulp as raw material. The thickness of the substrate layer in the reactor is 1 m. Three vertical screws are bound to a conveyor with dual direction movement. The linear movement speed of the conveyor and the rotating rate of the screws are... 

Commercial industrial processes can be operated in a either batch or a continuous mode. Batch processes are suitable for small plants, while for larger plants (>100 000 ty ) continuous process tend to be more economical. In the ESTERFIP batch process (IFF license), transesterification occurs in a single stirred-tank reactor. Continuous transesterification processes include the Ballestra, Connemann CD and... 

A third type of bioremediation involves the use of a bioreactor in a dedicated treatment area. The contaminated soil is excavated, slurried with water, and treated in the reactor. The horizontal drum and airlift-type reactors are usually operated in the batch mode but may also be operated in a continuous mode. Because there is considerable control over the operating conditions, treatment often is quick and effective. Contaminated groundwater and effluent also may be treated in either fixed-film or stirred-tank bioreactors. However, bioreactors are still in the developmental stages and further research is required to optimize their efficiency and cost effectiveness (Wilson and Jones 1993). 

FIGURE 8.4 Effect of solvent removal and the stepwise addition of fructose on the percent conversion of saccharide (a) and on the solubility of fructose (b) for lipase-catalyzed fructose-oleic acid esterification. ( ) Fructose was added to 50 mmol oleic acid and 13.4 g tert-butanol in 5 mmol increments for 2-h cycles until the net amount of fructose added was 25 mmol tert-butanol underwent free evaporation for up to 10 h, at which time, the reaction was stopped, solvent was removed completely by rotary evaporation, then the reaction was continued. (A) Fructose (25 mmol) was added at time 0 to 50 mmol of oleic acid and 13.4 g (46.9 wt.% fructose-free basis) tert-butanol solvent freely evaporated away throughout at a rate of 0.47 g per h. Both reactions were operated in stirred batch mode using approximately 0.5 g immobilized C. antarctica lipase, (Chirazyme L-2, s.-f, c2, Lyo., Boehringer-Mannheim, Indianapolis, IN), a stir rate of 450 rev min and a reaction temperature of 65°C. (From Walde, P. et al., Chem. Phys. Lipids, 53, 265-288, 1990. With permission.)... 

Reactions were also conducted in a stirred slurry reactor operating in continuous mode with the solution injected by means of a pump and the catalyst retained by a filter at the liquid outlet [19,21]. A fixed-bed reactor was used by Kimura [71,72,74] for the oxidation of glycerol and derivatives. 

Figure 1.19. Stirred tank reactors in a) batch, b) continuous mode.

Hansenula polymorpha and Saccharomyces cerevisiae were cultivated on synthetic medium with 1% glucose in fed-batch and continuous mode, respectively, in the absence of antifoam agents. For the nutrient preparation, sterilization and storage, 300-, 600-, 1000- and 5000-1 stirred tank vessels were used. The nutrient salt medium was steriUzed without glucose. The glucose solution was autoclaved separately and was added to the cold, sterilized nutrient medium. The flotation column was operated in continuous mode. 

There were published several reports about the hyoscyamine production by hairy roots grown in bioreactors [65]. As fare as we are aware there is not enough information about the scopolamine and particularly about 6P-hydroxyhyoscyamine production in these systems. Among them, Hilton and Rhodes [67] studied the hyoscyamine production by D. stramonium in a modified 14 L stirred tank reactor operated imder different conditions in batch and continuous mode. The 35 day culture produced 5.2 mg/g DW and 3.3 mg/g DW of hyoscyamine in Gamborg B5/2 and B5 medium, respectively [67]. B. Candida hairy roots produced a slightly higher amoimt of hyoscyamine. Specifically, the process carried out in the modified stirred tank produced 7.0 1.3 mg/g DW of hyoscyamine at the harvest time (Table 2) [28]. Hilton and Rhodes [67] also reported a low release of the alkaloid into the culture medium. The biomass productivity attained in this work was 0.24 g DW/l/d which is very similar to that reported here for B. Candida hairy root processes (Table 2). 

Mainly homogeneous type (. g., stirred tanks) in batch or continuous mode of operation... 

Thus, recycle reactors represent an advantageous device for experimental verification of mixing models, as emphasized here. Recycle reactors can be operated in either batch or continuous mode for this purpose, and they also exhibit the property of short cycle time distribution (CTD), which is very favorable compared with the CTD of normal stirred tanks. 

If a normal chromatographic column, as shown in Fig. 4,is employed a uniform distribution of samples throughout the voids will take an unpractically long time. Two different solutions for this problem have been offered. The first of them employs a construction similar to the stirred batch enzyme reactors used in the analysis of immobilized enzymes (ref. 35,37). In that mode, an open chromatographic column containing appropriate gel slurry is stirred continuously (ref. 35). The apparent advantages of this method are its simple construction, short equilibration and lack of need of special sanpling port. Equilibration times of about 10 minutes are sufficient with the apparatus (ref. 

In addition to processes involving gas-liquid reactions, stirred-tank reactors can also be used for single (liquid)-phase reactions. Moreover, their operation is not limited to the continuous mode, and they can be easily adapted for use in semibatch and batch modes. The absence of a gas phase does not pose important structural and operational differences from those stated earlier for multiphase systems. However, in the case of single-phase operation, the aspect ratio is usually kept lower ( 1) to ensure well mixing of the reactive liquid. Regardless of the number of phases involved, stirred-tank reactors can approach their ideal states if perfect mixing is established. Under such conditions, it is assumed that reaction takes place immediately just... 

A similar approach, starting with a material balance, can be used for the characterization of bioreactors operating in the continuous mode. Thus, for a perfectly mixed reactor, or continuous stirred tank reactor (CSTR), where the term of accumulation is zero at steady state and the liquid composition is uniform, the material balance for substrate A is given by Equation 7.7 ... 

Continuous mode of operation in well-mixed stirred tanks often starts as a fed-batch process at constant feed rate under a given substrate concentration Cjo, so that D = fi until an outlet valve is opened. This is performed in order to allow the effluent to be continuously recovered from the reactor at an outlet flow rate equal to the inlet flow so that the volume of the medium in the fermenter stays constant and steady state is achieved. 

Copredpitation Batch mode vs. continuous mode Flow rate Droplet size distribution Mixer design (shear and energy, e.g., stir bar, vortex, propeller, homogenizer, rotor-stator) Solvent to antisolvent ratio Temperature Processing times (scale dependent)... 

---