Circulation, external

One design for a low temperature convection furnace shown in Figure 4 utilizes an external circulating fan, heating chamber, and duct system. The fan draws air (or a protective atmosphere) from the furnace and passes through the external heating chamber and back into the furnace past the work. This system minimizes the chance that the work receives any direct heat radiation. In theory it is less efficient because the external blower, heating chamber, and ductwork add external surfaces that are subject to heat losses. 

The reaction takes place at low temperature (40-60 °C), without any solvent, in two (or more, up to four) well-mixed reactors in series. The pressure is sufficient to maintain the reactants in the liquid phase (no gas phase). Mixing and heat removal are ensured by an external circulation loop. The two components of the catalytic system are injected separately into this reaction loop with precise flow control. The residence time could be between 5 and 10 hours. At the output of the reaction section, the effluent containing the catalyst is chemically neutralized and the catalyst residue is separated from the products by aqueous washing. The catalyst components are not recycled. Unconverted olefin and inert hydrocarbons are separated from the octenes by distillation columns. The catalytic system is sensitive to impurities that can coordinate strongly to the nickel metal center or can react with the alkylaluminium derivative (polyunsaturated hydrocarbons and polar compounds such as water). 

The cooling duty can be provided by either making the draught tube an internal heat exchanger or with a heat exchanger in an external circulation loop. The mass transfer coefficient for external loop airlift Fermenter is estimated as 8... 

Table I provides an overview of general reactor designs used with PS and HIPS processes on the basis of reactor function. The polymer concentrations characterizing the mass polymerizations are approximate there could be some overlapping of agitator types with solids level beyond that shown in the tcd>le. Polymer concentration limits on HIPS will be lower because of increased viscosity. There are also additional applications. Tubular reactors, for example, in effect, often exist as the transfer lines between reactors and in external circulating loops associated with continuous reactors.

The 3He evaporated in the still is pumped by an external circulation pump. The still temperature is kept between 0.6 and 0.8 K by means of a heater. Changing the temperature within this range allows to control the 3He vapour pressure, while keeping a low 4He concentration in the vapour. In fact, the DR efficiency critically depends on the 3He/4He ratio in the circulating mixture. 

The process is run in a CSTR without solvent at 50 °C. The reaction is highly exothermic and heat is removed via external circulation. To ensure high... 

Submarine gel chamber with an external circulation through heat exchangers to cool the electrophoresis buffer. 

High-shear stirrers can be a useful way of mixing components, but they often draw in air and can destroy added emulsions. Mixing can also be done through an external circulating loop with an in-line pump or emulsifying mixer. All systems should ideally be connected to a clean-in-place (c.i.p.) system. 

The p-jump unit produced by Hi-Tech Limited (PJ-55 pressure-jump) is based on a design by Davis and Gutfreund (1976) and is shown in Fig. 4.7, with a schematic representation in Fig. 4.8. A mechanical pressure release valve permits observation after 100 /us. There is no upper limit to observation time. Changes in turbidity, light absorption, and fluorescence emission can be measured in the range of 200-850 nm. The PJ-55 is thermostated by circulating water from an external circulator through the base of the module. The temperature in the cell is continuously monitored with a thermocouple probe. A hydraulic pump assembly is used to build up a pressure of up to 40.4 MPa. A mechanical valve release causes the pressure build-up to be applied to the solution in the observation cell. The instrument has a dead time of 100 /us. A fast response UV/fluorescence... 

The liquid-phase oxidation of toluene with molecular oxygen is another example of a well established process (Table 4, entry 40). A cobalt catalyst is used in the process and the reaction proceeds via a free-radical chain mechanism. Heat of reaction is removed by external circulation of the reactor content and both bubble columns or stirred tanks are employed. It is important to note that air distribution is critical to prevent the danger of a runaway. Another example of direct oxidation is the commercial production of nitrobenzoic acid by oxidation of 4-nitrotoluene with oxygen (Table 4, entry 41). 

Figures Degradation rate (mmol g -cat min ) of benzoic acid for different catalyst loading without external circulation. Experimental conditions Cso = 0.2mM, pH = 3.7 3.9,7 = 303 K, / = 8.0 mW cm and O2 saturated (Mehrotra et al., 2005).

For fast exothermic reactions, temperature control can be a problem. This is often solved by external circulation of part of Ihe reactor content through a heat exchanger, or by adding an internal heat exchange area. Alternatively, semi-batch operation can be applied, i.e., part of a reactant can be fed steadily over time or at certain intervals. Ihis also minimizes the occurrence of unwanted side reactions. 

The H-Oil reactor (Fig. 21) is rather unique and is called an ebullated bed catalytic reactor. A recycle pump, located either internally or externally, circulates the reactor fluids down through a central downcomer and then upward through a distributor plate and into the ebullated catalyst bed. The reactor is usually well insulated and operated adiabatically. Frequently, the reactor-mixing pattern is defined as backmixed, but this is not strictly true. A better description of the flow pattern is dispersed plug flow with recycle. Thus, the reactor equations for the axial dispersion model are modified appropriately to account for recycle conditions. 

Situation 3 Measurements were taken at the channel with the same height as in situation 2, yet with applying external circulation, which implies that much greater air velocities have been realized again. 

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