Direct-contact transfer

Utilization of coal and oil shale to produce liquid and gaseous synfuels results in the generation of many hazardous sub-tances. Workers in these synfuel plants are likely to be exposed to potentially carcinogenic materials present in coal tars and oils. Among the various pathways of exposure, skin contamination by direct contact transfer or by adsorption of vapors and particulates into the skin presents a serious occupational health hazard. The skin irritant and potential carcinogenic properties of raw syncrudes and their distillate fractions have been reported (1. 2, 3). 

Given the presence of such a wide variety of surface groups, several kinds of interaction mechanisms are possible. One we shall consider might involve the direct contact transfer of electrically neutral acid groups in response to a thermodynamic drive. A second would be the exchange of a proton if Bronsted character predominates, or thirdly, an electron transfer between Lewis groups. 

One disadvantage of fluidized heds is that attrition of the catalyst can cause the generation of catalyst flnes, which are then carried over from the hed and lost from the system. This carryover of catalyst flnes sometimes necessitates cooling the reactor effluent through direct-contact heat transfer hy mixing with a cold fluid, since the fines tend to foul conventional heat exchangers. 

Heat transfer. Once the basic reactor type and conditions have been chosen, heat transfer can be a major problem. Figure 2.11 summarizes the basic decisions which must be made regarding heat transfer. If the reactor product is to be cooled by direct contact with a cold fluid, then use of extraneous materials should be avoided. 

The final restriction of simple columns stated earlier was that they should have a reboiler and a total condenser. It is possible to use materials fiow to provide some of the necessary heat transfer by direct contact. This transfer of heat via direct contact is known as thermal coupling. 

While a superstructure based on the structure in Fig. 16.26 allows for many structural options, it is not comprehensive. Wood, Wilcox, and Grossmanr showed how direct contact heat transfer by mixing at unequal temperatures can be used to decrease the number of units in a heat exchanger network. Floudas, Ciric, and Grossman showed how such features can be included in a heat exchanger network superstructure. Figure 16.27 shows the structure from Fig. 16.26 with possibilities for direct contact heat transfer included. In the... 

Figure 16.27 Possibilities for direct contact heat transfer can be added to the superstructure.

Other Models for Mass Transfer. In contrast to the film theory, other approaches assume that transfer of material does not occur by steady-state diffusion. Rather there are large fluid motions which constantiy bring fresh masses of bulk material into direct contact with the interface. According to the penetration theory (33), diffusion proceeds from the interface into the particular element of fluid in contact with the interface. This is an unsteady state, transient process where the rate decreases with time. After a while, the element is replaced by a fresh one brought to the interface by the relative movements of gas and Uquid, and the process is repeated. In order to evaluate a constant average contact time T for the individual fluid elements is assumed (33). This leads to relations such as... 

If condensation requires gas stream cooling of more than 40—50°C, the rate of heat transfer may appreciably exceed the rate of mass transfer and a condensate fog may form. Fog seldom occurs in direct-contact condensers because of the close proximity of the bulk of the gas to the cold-Hquid droplets. When fog formation is unavoidable, it may be removed with a high efficiency mist collector designed for 0.5—5-p.m droplets. Collectors using Brownian diffusion are usually quite economical. If atmospheric condensation and a visible plume are to be avoided, the condenser must cool the gas sufftciendy to preclude further condensation in the atmosphere. 

Direct Contact Heat Exchangers. In a direct contact exchanger, two fluid streams come into direct contact, exchange heat and maybe also mass, and then separate. Very high heat-transfer rates, practically no fouling, lower capital costs, and lower approach temperatures are the principal advantages. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

External coils spaced away from the tank wall exhibit a coefficient of around 5.7 W/(m -°C) [1 Btu/(h-ft of coil surface-°F)j. Direct contact with the tank wall produces higher coefficients, but these are difficult to predict since they are strongly dependent upon the degree of contact. The use of beat-transfer cements does improve performance. These puttylike materials of high thermal conductivity are troweled or camked into the space between the coil and the tank or pipe surface. 

Contactive (Direct) Heat Transfer Contactive heat-transfer equipment is so constructed that the particulate burden in solid phase is directly exposed to and permeated by the heating or cooling medium (Sec. 20). The carrier may either heat or cool the solids. A large amount of the industrial heat processing of sohds is effected by this mechanism. Physically, these can be classified into packed beds and various degrees of agitated beds from dilute to dense fluidized beds. 

This section describes equipment for heat transfer to or from solids by the indirect mode. Such equipment is so constructed that the solids load (burden) is separated from the heat-carrier medium by a wall the two phases are never in direct contact. Heat transfer is by conduction based on diffusion laws. Equipment in which the phases are in direct contact is covered in other sections of this Handbook, principally in Sec. 20. 

Spray Dryers A spray diyer consists of a large cyhndrical and usu ly vertical chamber into which material to be dried is sprayed in the form of small droplets and into which is fed a large volume of hot gas sufficient to supply the heat necessary to complete evaporation of the liquid. Heat transfer and mass transfer are accomphshed by direct contact of the hot gas with the dispersed droplets. After completion of diying, the cooled gas and solids are separated. This may be accomplished partially at the bottom of the diying chamber by classification and separation of the coarse dried particles. Fine particles are separated from the gas in external cyclones or bag collectors. When only the coarse-particle fraction is desired for fini ed product, fines may be recovered in wet scrubbers the scrubber liquid is concentrated and returned as feed to the diyer. Horizontal spray chambers are manufactured with a longitudinal screw conveyor in the bottom of the diying chamber for continuous removal of settled coarse particles. 

Introduction Insoluble hquids may be brought into direct contact to cause transfer of dissolved substances, to allow transfer of heat, and to promote chemical reaction. This subsection concerns the design and selection of equipment used for conduc ting this type of liquid-hquid contact operation. 

Cooling or heating a liquid hy direct contact with another Although liquid-hquid-contact operations have not been used widely for heat transfer alone, this technique is one of increasing interest. Applications also include cases in which chemical reaction or hquid extraction occurs simultaneously... 

Heat is transferred by direct contact with solids that have been preheated by combustion gases. The process is a cycle of alternate heating and reactingperiods. The Wulf process for acetylene by pyrolysis of natural gas utilizes a heated brick checker work on a 4-min cycle of heating and reacting. The temperature play is 15°C (59°F), peak temperature is 1,200°C (2,192°F), residence time is 0.1 s of wmich 0.03 s is near the peak (Faith, Keyes, and Clark, Industrial Chemicals, vol. 27, Wiley, 1975). 

Mass-Transfer Contact Section Where there is a strong possi-bihty that not all of the incoming vapors will be condensed in the pool, a direct-contact mass-transfer section is superimposed on the quench tank. This can be a baffle-tray section (as shown in Fig. 26-21) or a packed column sec tiou. 

The design of direct-contact mass-transfer columns is discussed in detail by Scheimau (Petro Chemical Engr. 37(3) 29-33, 1965 ibid. 37(4) 75, 78-79) and Fair Chem. Eng., June 12, 1972). 

Miscellaneous Systems Many other systems have been proposed for transferring heat regeneratively, including the use of high-temperature hquids and fluidized beds for direct contact with gases, but other problems which hmit industrial application are encountered. These svstems are covered by methods described in Secs. 11 and 12 of this handbook. 

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