Packed towers, tray towers, and even spray towers can be used for energy recovery through the direct contact of a heat-absorbing liquid (usually water) with a hot gas stream. These devices may also simultaneously remove soluble gaseous pollutants. In these systems, the rate of heat transfer is controlled by the difference in temperature between the hot gas and absorbing liquid. The amount of heat absorbed is controlled by the mass of liquid that passes through the device. Devices that are strictly counterflow, such as a packed or tray tower, exhibit higher heat transfer efficiency than devices such as a spray tower that may entrain liquid upward and therefore reduce the differential in temperature required for optimum heat transfer. With proper design, however, spray towers can be effectively used for heat recovery.
Typical Applications and Uses
Most wet scrubbers applied to hot sources inherently transfer heat. As mentioned in other chapters, the initial heat transfer may be used to adiabatically saturate the gas stream. Thereafter, more heat can be removed through the sensible cooling of the gas stream. As the gases mix with the liquid, both the latent heat (the heat removed as the water vapor in the gas stream changes phase to liquid water) and the sensible heat are recovered. This is quite different from what occurs with surface (nondirect contact) type heat exchangers such as economizers or tubular heat exchangers. Those devices recover only sensible heat.
Given the preceding, the best application sources for direct-contact heat recovery therefore are those devices that emit high humidity (lots of condensable water vapor from which the latent heat can be recovered) and hot gases. Such sources are boiler flue gas stacks wherein the hot gases can first be adiabatically saturated, sources that emit lots of water vapor or steam
(such as food processing or paper making), thermal oxidizers (particularly ones that oxidize wet fuels), steam vents, and similar hot wet sources.
The recovered heat may be used, for example, to preheat boiler feed water. It may be used to provide supplemental heat for a building or loading dock. In other applications, it can be used to thaw frozen products. More exotic uses harness the heat to drive electric generator systems, which in effect are like driving an air-conditioner backward. Sometimes the recovered heat is used to preheat the wastewater outfall from a plant so that further biological destruction can more favorably occur. In all of these, the economics must be investigated. As fossil fuel costs rise, the economics favor heat recovery. Conversely, low fossil fuel costs can make the use of heat recovery only marginally beneficial. Each application must be evaluated based upon its own merit (or lack thereof).
Within these applications are two basic types. In one, the goal is to recover the heat simply by direct contact with the liquid in the heat transfer tower. In another, the liquid circuit must be cleaner; thus, the recovered heat is subsequently transferred to a "clean" liquid stream. In the latter, a heat exchanger (liquid-liquid) is used to transfer the heat in the "dirty" heat recovery tower water to clean process water. The process water may itself be a heat transfer liquid (such as ethylene glycol) that will subsequently be used to indirectly heat a device (such as a jacketed process vessel or reactor).
Figure 22.1 shows the direct-contact heat recovery process wherein the tower water itself absorbs the heat and no further transfer is used—thus the
term direct. This type of application may be one that heats water used for area heating or other applications wherein there are limited concerns that the tower water may mix with other liquid streams. The feed water simply enters the liquid distributor at the top of the tower, descends through a gas/liquid contact zone (in this case a packed tower), and then is drained away. The tower is usually sized “water once through," since recirculating the heated water to the top of the tower would reduce the thermal efficiency by decreasing the temperature differential between the water and the gas.
Notice the use of a direct contact counterflow packed tower as the heat transfer device. The liquid circuit can be recycled and bled (as shown) or once through as mentioned earlier. The tower design is like any packed tower. The packing media may be metallic, nonmetallic (plastics), or, quite often, ceramic packing, since these devices are connected to hot sources and ceramic packing resists overheat better than, particularly, plastic packing. (See Section 22.5, “Design Basics.")
Figure 22.2 shows a similar system, but the dirty water heat is transferred to a clean liquid using a heat exchanger. With these systems, it is still best to operate water once through; however, sometimes the water is recirculated and only the make-up water is added to the top of the tower. In the latter design, the amount of heat being recovered is more important than the temperature to which the liquid is heated. With a recirculation system, the overall
Heat recovery indirect type diagram. (Bionomic Industries, Inc.) thermal efficiency is less, again given the reduced temperature differential in the tower, though in practice this lower efficiency is typically minor.
What if the hot gas source also contains acid gases and the like? Remember, the heat recovery tower is basically a packed, tray, or spray tower. These devices will also absorb soluble gases (as explained in other chapters). During the absorption, say, of acidic flue gas, heats of absorption are evolved. Add a reactive chemical such as caustic and heats of reaction are also evolved. These heats can also be recovered in the heat recovery tower.
In systems without heat recovery, the heats of absorption and reaction usually go overboard via heat carried away in the scrubber blowdown or in the water vapor leaving the stack or both. In systems with heat recovery, that heat is captured and perhaps can be used to offset the facilities use of expensive fossil fuels.
What about the water vapor plume? If the system did not use heat recovery, the scrubber may emit a dense water vapor plume. These plumes are quite visible, and the public often mistakes these plumes for pollution. With a heat recovery application, the plume is reduced if not eliminated. Often, only under rare atmospheric conditions (low temperature and high relative humidity) can a water vapor plume be seen.
What about water consumption? Some installations want to not only recover heat but also conserve water. The water that initially evaporates as the hot gases mix with the water in the heat recovery tower can often also be recovered. Indeed, the amount recovered typically must be balanced with the water pollution control system's capability at the specific site. In some applications wherein a high hydrocarbon-based fuel is oxidized, the combustion product water vapor is also recovered.
Energy recovery devices are typically adaptations of packed, tray, or spray towers. The operating principles are described in Chapters 9, 10, 12, 14, and 17.
The difference with the heat recovery designs is that they are often configured to operate based upon the liquid rate rather than the gas rate. For example, a customer may want to heat a given amount of wastewater to a not-to-exceed temperature. The heat recovery tower would therefore be sized based primarily on that liquid rate and temperature. The hot source may offer more heat than is necessary, therefore, the tower may need only a slip stream from that source. The hot source heat production may be marginal, however, and the tower may be designed to recirculate, and bleed based upon the final (controlled) liquid temperature specified.
Primary Mechanism Used
For heat recovery, the mechanism is basic. Heat tends to move from hot sources to colder surfaces. In these devices, the hot “surface" is the incoming gas mixture, and the cold surface is a constantly replenished liquid surface (usually water). These towers are sized much like the soluble gas absorbers described in other chapters.
As mentioned earlier, the difference in temperature between the hot gas source and the liquid determines the rate of heat transfer. The greater the temperature difference (delta T), the greater the heat transfer rate. The amount of heat finally transferred, however, is controlled by the mass of liquid passing through the tower. In English units, the basic equation is as follows:
In all these devices, the hot gases are brought into direct contact with the recirculated liquid in preferentially a counterflow mode.
Given that the source to which the device may be applied is hot, the towers often are designed for a thermal upset. If packed towers are used, the packing may be heat-resistant polypropylene or even ceramic saddles. A photo of the type of packing often used is shown in Figure 22.3. This general type of packing is often used as heat-transfer media in regenerative thermal oxidizers.
Given the extra weight per cubic foot of ceramic packing, the tower needs to be designed for this greater load, particularly at the packing support. Often, on larger diameter towers (more than 4-5 feet), an additional central beam or column is used to transfer the packing weight to the vessel wall or base.
Ceramic/porcelain saddles. (Lantec Products, Inc.)
Ceramic packing also has wetting characteristics different from plastic or metal packing. The surface is lower per unit volume, and the liquid tends to be retained on the rougher surface of the packing rather than slide off. These parameters, however, have been well defined by the packing makers. The packing supplier is therefore consulted regarding the specific packing depth and water rate. Typical packing depths are 6-10 ft.
Vertical gas velocities for towers using ceramic packing range from about 4 to 6 ft/s. Vertical velocities for towers packed with heat-resistant plastic packing range from about 5 to 7 ft/s. The liquid (irrigation) rates per horizontal square foot are dictated by the type packing but range from about 8 g/ft2 to about 12 g/ft2. The irrigation rate ensures that the packing is fully wetted. Dry locations or areas that are not constantly wetted decrease the heat transfer rate. For liquid distribution, weir type or low-pressure (under 15 psig) spray type liquid distributors are commonly used.
Though the vessels are often fiberglass-reinforced plastic construction, sometimes stainless steel is used given the possibility for overheat. If stainless steel is used in a tray type design, stainless steel trays are also used.
For spray towers for heat recovery, multiple spray zones are used with a bias toward locating them higher in the tower so that the descending droplets can better mix with the hot gases. In addition, the droplet eliminator often receives any make-up (assumed cold) water so that the droplet eliminator acts as an additional mass-transfer surface.
It is best to accumulate the maximum and operating conditions of the heat source both for its thermal (BTU) capacity and for the presence of any contaminant gases. Since these devices place the liquid in direct contact with the gas stream, both heat and contaminants can be passed to the liquid. If contaminant gases exist (for example, SO, from a fossil-fueled boiler source), the design can also include chemical neutralization of the absorbed contaminant. The system design would need to include chemical addition (i.e., pH control and conductivity control) equipment.
If the source does not include contaminants, basic temperature control can be used. A common method is to use a liquid outlet temperature sensor as a primary control point with sump level as a secondary control. Since liquid is typically evaporated from these systems and the make-up water (or liquid stream) is typically at a fixed rate, sometimes only a portion of the liquid flow is sent through the heat recovery unit. The flows can be recombined after the device, or that portion can be heated to a specific higher temperature.
Since the heat recovery demand may vary (say, seasonally), these systems often use a hot gas bypass (incorporating diverting dampers) so that the recovery unit is used only when needed.
If a fan is used to move gases through the unit, a bit more energy can sometimes be recovered if the fan is placed ahead of the heat recovery tower, thus recovering the heat of compression of the fan. In this case, the exhaust stack is often attached to the tower exhaust; thus, the tower becomes a stack base.