Direct contact heat exchangers have been used by heat transfer practitioners for more than one hundred years. In fact, the success of the industrial revolution has much to do with their initial use by James Watt in creating the needed vacuum for efficient steam engines. In 1900, Hausbrand's book, "Evaporation, Condensing and Cooling Apparatus," published information dealing with several types of direct contactors including barometric condensers. Despite this early start, the development of a true understanding of their nature lagged and still lags behind the understanding of surface-type heat exchangers. Nevertheless, they are widely used as open-feed water heaters in power plants, open-evaporative cooling towers, barometric condensers throughout the petroleum industry, and in gas (air) separation plants. Still another use is in absorption refrigeration plants. Other applications are in rotary retorts, drying processes, etc. Thus, knowledge of them as alternatives to conventional regenerators or recuperators is necessary to economically optimize systems that include heat exchange.
Direct contact heat exchange takes place between two process streams. The streams can include combinations such as gas-solid, gas-liquid, liquid-liquid, liquid-solid, or solid-solid streams. For obvious reasons, gas-gas systems cannot be achieved directly; however, two direct contactors can be used in series where a third stream extracts heat from one gas stream and transfers it to another. Thus, direct contactors can be used for almost all systems; but, the complexity of multiple component systems may overcome their economic advantage over surface type heat exchangers.
The exchange of heat between two fluid streams can, in general, be accomplished using either direct contact or surface-type heat exchangers. There are, however, several limitations to the use of direct contactors. First, if two fluid streams are placed in direct contact, they will mix, unless the streams are immiscible. Thus, stream contamination will occur depending on the degree of miscibility. The two streams must also be at the same pressure in a direct contactor, which could lead to additional costs. The advantages in utilizing a direct contactor include the lack of surfaces to corrode or foul, or otherwise degrade the heat transfer performance. Other advantages include the potentially superior heat transfer for a given volume of heat exchanger due to the larger heat transfer surface area achievable and the ability to transfer heat at much lower temperature differences between the two streams. Still another advantage is the much lower pressure drop associated with direct contactors as compared to their tubular counterparts. A final advantage is the much lower capital cost as direct contact heat exchangers can be constructed out of little more than a pressure vessel, inlet nozzles for the fluid streams, and exit ports. Of course, it is sometimes advantageous to provide internals, as will be discussed later.
A typical direct contactor provides heat transfer between two fluid streams. The processes include the simple heating or cooling of one fluid by the other; cooling with the vaporization of the coolant; cooling of a gas-vapor mixture with partial condensation; cooling of a vapor or vapor mixture with total condensation; and cooling of a liquid with partial or complete solidification. Most of the direct-contact applications can be accomplished with the following devices: a) Spray columns , b) Baffle tray columns , c) Sieve tray or bubble tray columns , d) Packed columns, e) Pipeline contactors, and f) Mechanically agitated contactors.
Figures 1-6 illustrate the general configurations of a) through f), respectively. Except for the turbulent pipe contactor, all of the devices are countercurrent devices and depend upon the relative buoyancy of the dispersed phase through a continuous phase. While the figures illustrate a less-dense dispersed phase being introduced at the bottom of the column, it is possible for the dispersed phase to be denser and introduced at the top, with the configuration internals appropriately revised.
Figure 1. Schematics of spray columns for evaporation and for sensible heating of the dispersed lighter phase.
Figure 2. Schematic of a disk and donut baffle tray column for use as a steam condenser [Jacobs and Nadig (1987)].
Figure 3. Schematic of a sieve tray column used for extracting heat from geothermal brine [Jacobs and Eden (1986)].
Figure 4. A possible configuration of a packed bed condenser [Jacobs and Eden (1986)].
Figure 5. Turbulent pipe contactor.
Figure 6. Typical mechanically agitated towers [Treybal, (1966)].
The turbulent pipe contactor is a parallel-flow device and has the limits of efficiency of all such systems, whether they be direct contact or surface-type heat exchangers. That is, the maximum temperature achieved by the cool stream is that of the mixing cup temperature.
The size of the turbulent pipe contactor is dictated by the relative mass flow rate and the nature of the turbulence. Turbulence promoters can be installed to enhance the turbulence and, thereby, reduce the length of contactor necessary to essentially obtain the mixing cup temperature. If separation of the streams is desired, the contactor must be followed by a separation device such as a settler, a cyclone separator, or other mechanisms. While the turbulent pipe conductor is very inexpensive, if separation is desired, the cost of the settler will in all probability dictate the economics of the process.
The remaining apparatus all have the heat transfer take place between a continuous phase and a clearly defined disperse phase in the form of drops, bubbles, jets, sheets, or thin supported films in the case of packed beds. Heat exchangers with mechanical agitators (Figure 6), while often superior as heat or mass transfer equipment, are more difficult to design as the dispersed phase may have a wide range or drop or bubble sizes. Thus, empirical data from the manufacturer to establish performance is necessary. Further, problems may result in seals at the penetration point of the drive shafts. Special designs may therefore be necessary.
Figure 7. Drop characterization map [Grace (1983)].
Figure 8. Effective thermal diffusivity to molecular diffusivity as a function of drop Peclet number [Jacobs and Eden (1986)].
Figure 9. Schematic of a tray in a sieve tray column [Jacobs and Eden (1986)].
Baffle tray columns may have similar problems in defining the nature of the curtain of the dispersed phase. Depending on flow rates and battle design, the dispersed phase may be a sheet, a series of rivulets or defined streams, which can break up into drops. If the baffles are, in fact, trays with serrated or notched rims, the dispersed phase can be designed to be a series of well-defined streams and the heat transfer is more easily analyzed. The baffles/trays then result in mixing of the dispersed phase and enhance the internal-to-the-dispersed phase mixing.
The spray column shown in Figure 1 is an open column whose only internals are the inlet nozzles for the dispersed and continuous phase. Ideally, such columns are capable of pure counterflow operation, with the dispersed phase made up of nearly uniform diameter drops. While it is possible to design the dispersed phase inlet nozzle to achieve the desired characteristics, providing a uniform flow in the continuous phase is more difficult. Great care must be taken or maldistribution of the continuous phase may lead to diminished heat transfer. Thus, the design of continuous phase inlet nozzles is sometimes proprietary, or patented.
The bubble column or sieve tray column (see Figure 3) enhances the internal heat transfer coefficient by repeatedly reforming the drops at each tray. Proper tray or baffle design can lead to shorter columns, and potentially small heat exchanger volume for the same service. Their major disadvantage is fouling, corrosion or blockage of some of the holes in the sieve tray. Details of design methods and references to recent improvements are given by Jacobs (1988) and Jacobs (1995a, 1995b).
Hausbrand, E. (1933) Condensing and Cooling Apparatus , 5th ed, Van Nostrand, New York.
Jacobs, H. R. (1988) Direct Contact Heat Transfer for Process Technologies. ASME Journal of Heat Transfer , Vol. 110, pp. 1259-1270.
Jacobs, H. R. (1995a) Direct Contact Heat Exchangers, Heat Exchanger Design Handbook.
Jacobs, H. R. (1995b) Direct Contact Heat Transfer, Heat Exchanger Design Handbook.