Heat exchangers liquid residence time

Tucker, G. and Heydon, G Trans.I.Chem.E. 76 Part C (1998) 208. Food particle residence time measurement for the design of commercial tubular heat exchangers suitable for processing suspensions of solids im liquids,... 

In the first step the chlorine from the tail gas and chlorine feed reacts with the caustic in the jet-loop reactor. The advantage of the jet-loop reactor is that it also acts as a suction device for the gas stream. The residence time of the liquid in step one is dependent on the capacity of the hypochlorite production and liquid level in the tank and varies between 1 and 4 h. A heat exchanger in the loop controls the temperatures in steps one and two. The amount of caustic in the feed-tank of step two is the back-up for failure of chlorine liquefaction. 

Taking into account typical numbers for a and D, this underlines that the channel width should be considerably smaller than 1 mm (1000 pm) in order to achieve short residence times. Actually, heat exchangers of such small dimensions are not completely new, because liquid cooled microchannel heat sinks for electronic applications allowing heat fluxes of 790 watts/cm2 were already known in 1981 [46]. About 9 years later a 1 cm3 cross flow heat exchanger with a high aspect ratio and channel widths between 80 and 100 pm was fabricated by KFK [10, 47]. The overall heat transport for this system was reported to be 20 kW. This concept of multiple, parallel channels of short length to obtain small pressure drops has also been realized by other workers, e.g. by PNNL and IMM. IMM has reported a counter-current flow heat exchanger with heat transfer coefficients of up to 2.4 kW/m2 K [45] (see Fig. 3). 

After ion exchange, the material is overlimed. First, the liquid pH is increased to 2 by addition of sulfuric acid (90 kg/h). Then, lime is added (68.5 kg/h) to raise the pH to 10, and the mixture is heated by steam injection to 50 °C, at a residence time of lh. The liquid is then adjusted to pH 4.5 and held for 4h. In this way, large gypsum crystals are formed, which can be separated by means of hydrocyclone and rotary drum filtration. The amount of gypsum produced is about 200 kg/ h, in which the solid content is 80%. 

Monolithic Loop Reactor A novel MLR was developed af Air Products and Chemicals (Figure 17) (144). The reactor contains a monolithic catalyst operating under cocurrent downflow condifions. Because the residence time in the monolith is short and the heat of reaction has to be removed, the liquid is continually circulated via an external heat exchanger until the desired conversion is reached. The concept was patented for the hydrogenation of dinifrofoluene fo give toluenediamine (37). 

F is used as the heating medium and flows at a rate of 6000 ib/hr. An overall heat transfer coefficient of 40 Btu/(hr)(fF)(°F) may be assumed use Newton s law of heating. The liquid is circulated at a rate of 6000 Ib/hr, and the specific heat of the liquid is the same as that of water (1.0). Assume that the residence time of the liquid in the external heat exchanger is very small and that there is essentially no holdup of liquid in this circuit. 

Shell-and-tube exchanger with reactants and catalyst inside the tubes, 250 to 400 m /m. Tube diameter <50 mm. Gas with fixed bed of catalyst use high mass gas velocity to improve heat transfer kg/s m > 1.35. To ensure good gas distribution and negligible backmixing, Pe > 2 height/catalyst particle diameter H/D > 100 and D/D < 0.10. Gas velocity 3 to 10 m/s residence time 0.6 to 2 s. Heat transfer coefficient U = 0.05 kW/m K. For fast reactions, catalyst pore diffusion mass transfer may control if catalyst diameter is >1.5 mm. Liquids with fixed bed of catalyst to minimize backmixing, Pe> use UD > 200 and D/D <0.10. Liquid velocity 1 to 2 m/s residence time 2 to 6 s. Heat transfer coefficient U = 0.5 kW/m -K. 

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