Mixing stirred tanks

In special cases, the departure from ideal single stirred-tank mixing behaviour is intended, e.g., when a series of stirred tanks are used to approach that of plug-flow, as shown in Fig. 3.26. 

In a stirred tank mixing system, perhaps the most fundamental measurement one can make is that of the power draw of the system, as many scale-up rules depend heavily on the specific power input. The power draw, P, of an impeller is characterized by its power number, Po (also denoted Np). The following expressions can be used to calculate the power draw of an impeller ... 

Next we consider the equipment that is used to implement control strategies. For the stirred-tank mixing system under feedback control in Fig. 1.4, the exit concentration X is controlled and the flow rate iV2 of pure species A is adjusted using proportional control. To consider how this feedback control strategy could be implemented, a block diagram for the stirred-tank control system is shown in Fig. 1.6. Operation of the concentration control system can be summarized for the key hardware components as follows ... 

By contrast, if the reactor is continuous well-mixed, then the reactor is isothermal. This behavior is typical of stirred tanks used for liquid-phase reactions or fluidized-bed reactors used for gas-phase reactions. The mixing causes the temperature in the reactor to be effectively uniform. 

Wet Oxidation Reactor Design. Several types of reactor designs have been employed for wet oxidation processes. Zimpro, the largest manufacturer of wet oxidation systems, typically uses a tower reactor system. The reactor is a bubble tower where air is introduced at the bottom to achieve plug flow with controlled back-mixing. Residence time is typically under one hour. A horizontal, stirred tank reactor system, known as the Wetox process, was initially developed by Barber-Cohnan, and is also offered by Zimpro. 

Processes. Toluene is nitrated ia two stages. Mononitration occurs ia mixed acid, 30% HNO and 55% H2SO4, at 30—70°C ia a series of continuous stirred-tank reactors. Heat is Hberated and must be removed. The isomer distribution is approximately 58% o-nitrotoluene 38% -nitrotoluene, and 4% y -nitrotoluene (Fig. 1). 

Copolymers are typically manufactured using weU-mixed continuous-stirred tank reactor (cstr) processes, where the lack of composition drift does not cause loss of transparency. SAN copolymers prepared in batch or continuous plug-flow processes, on the other hand, are typically hazy on account of composition drift. SAN copolymers with as Httle as 4% by wt difference in acrylonitrile composition are immiscible (44). SAN is extremely incompatible with PS as Httle as 50 ppm of PS contamination in SAN causes haze. Copolymers with over 30 wt % acrylonitrile are available and have good barrier properties. If the acrylonitrile content of the copolymer is increased to >40 wt %, the copolymer becomes ductile. These copolymers also constitute the rigid matrix phase of the ABS engineering plastics. 

FIG. 6-39 Typical stirred tank configurations, showing time-averaged flow patterns for axial flow and radial flow impellers. From Oldshue, Fluid Mixing Technology, McGraw-Hill, New Yo7 k, 1983.)... 

Real reactors deviate more or less from these ideal behaviors. Deviations may be detected with re.sidence time distributions (RTD) obtained with the aid of tracer tests. In other cases a mechanism may be postulated and its parameters checked against test data. The commonest models are combinations of CSTRs and PFRs in series and/or parallel. Thus, a stirred tank may be assumed completely mixed in the vicinity of the impeller and in plug flow near the outlet. 

Experimental confirmations of these mixing mechanisms are scarce. One study was with a 50-gal stirred tank reactor (Worrell and... 

Three basic fluid contacting patterns describe the majority of gas-liquid mixing operations. These are (1) mixed gas/mixed liquid - a stirred tank with continuous in and out gas and liquid flow (2) mixed gas/batch mixed liquid - a stirred tank with continuous in and out gas flow only (3) concurrent plug flow of gas and liquid - an inline mixer with continuous in and out flow. For these cases the material balance/rate expressions and resulting performance equations can be formalized as ... 

A continuous flow stirred tank reactor (CFSTR) differs from the batch reactor in that the feed mixture continuously enters and the outlet mixture is continuously withdrawn. There is intense mixing in the reactor to destroy any concentration and temperature differences. Heat transfer must be extremely efficient to keep the temperature of the reaction mixture equal to the temperature of the heat transfer medium. The CFSTR can either be used alone or as part of a series of battery CFSTRs as shown in Figure 4-5. If several vessels are used in series, the net effect is partial backmixing. 

The eontents of a eontinuously operated stirred tank are assumed to be perfeetly mixed, so that the properties (e.g., eoneentration, temperature) of the reaetion mixture are uniform in all parts of the system. Therefore, the eonditions throughout the tank are the same and equal to the eonditions at the outlet. This means that the volume element ean be taken as the volume, V, of the entire eontents. Additionally, the eomposition and temperature at whieh the reaetion oeeurs are the same as the eomposition and temperature of any exit stream. A eontinuous flow stiiTed tank reaetor as shown in Figure 5-21 assumes that the fluid is perfeetly well mixed. 

Source Holland, F. A., and Chapman, F. S. Liquid Mixing and Processing in Stirred Tanks,... 

TIME-DEPENDENT TURBULENT MIXING AND CHEMICAL REACTION IN STIRRED TANKS ... 

The objeetive of the following model is to investigate the extent to whieh Computational Fluid Mixing (CFM) models ean be used as a tool in the design of industrial reaetors. The eommereially available program. Fluent , is used to ealeulate the flow pattern and the transport and reaetion of ehemieal speeies in stirred tanks. The blend time predietions are eompared with a literature eonelation for blend time. The produet distribution for a pair of eompeting ehemieal reaetions is eompared with experimental data from the literature. 

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