Special Applications: Venturi Scrubbers as Evaporators
Venturi type scrubbers have been used for decades for the direct contact evaporation of water from process liquids such as pulp mill black liquor. These devices were typically used in stages wherein the black liquor was gradually increased in concentration to avoid foaming issues and to provide adequate control over the solids. The Venturi design had advantages since the gas path is essentially open and thus reduces the chance of plugging.
The design has also been applied to waste incinerators wherein a zero or near-zero liquid discharge is required. Venturi scrubbers are used since they can recirculate at a higher solids content than most spray or tray type devices.
Of course, any wet scrubber that uses the direct contact of hot, dry gas with water will experience evaporation. As mentioned in Chapter 1, “Air Pollution Control 101," the proper application of a wet scrubber to a hot, dry gas source will result in that gas stream becoming saturated with water vapor. The source of that vapor is the evaporation of the water from the liquid being recycled. Fluidized bed type contactors can also be used as evaporators; however, the peak solids content that can be reached is typically less than that of a Venturi scrubber.
Direct contact concentrators have been used to increase the solids content of landfill leachate, to produce water from natural gas recovery operations (such as in "fracking"), and to a lesser extent for process liquid stream concentration. The device is used effectively wherein direct contact of the liquid with hot gases is acceptable or where surface type evaporators plug with solids on the heat exchange surfaces.
In recent years at the time of this writing, interest has increased in evaporating excess water from liquid streams, particularly from those liquid streams relating to gas recovery and/or leachate treatment. Evaporating excess water reduces the volume and cost of trucking or pumping the liquid streams away for treatment off-site. If the treatment is on-site, removing much of the excess water can reduce the size and cost of the on-site treatment equipment. Sometimes, the evaporated water can be recovered through condensation if the process to which the system is attached needs make-up water.
The direct contact concentrator works by mixing a hot gas source with a dispersion of droplets. The dispersion is typically produced using a Venturi scrubber that can operate at a high solids content rather than a spray nozzle type system. This technology was pioneered in the 1960s and 1970s in the direct contact evaporation of black liquor in the kraft paper industry. The key feature is evaporation using large droplets rather than small droplets. Why? Small droplets evaporate more quickly; however, the solids they contain can be spray dried into small particles that could result in a particulate air emission. Using large droplets is less efficient but reduces, if not eliminates, the fine particulate formation. The droplets are separated using chevron or cyclonic action as described in other chapters.
Primary Mechanism Used
The primary mechanism is evaporation. The evaporation is promoted and enhanced by increasing the surface area of the liquid and carrying away the evaporated water using a moving hot gas stream. When a liquid stream is mixed with a hot gas stream that is not saturated with water vapor, the "liquid" in the droplet heats up and changes phase to water vapor. In doing so, the solids in the liquid begin to concentrate. When the vapor surrounding the liquid becomes saturated with water vapor, the evaporation ceases. By constantly renewing the mix of unsaturated gases with the liquid, the evaporation can be caused to continue. At some point, however, the liquid solids content becomes so high that pumping and other practical conditions (such as erosion) limit the solids content to which the mixture can reach. The primary mechanism, however, is adiabatic saturation in the quencher and Venturi throat contact zones.
The direct contact concentrator is designed to evaporate water on a continuous basis from process wastewater streams. The typical system is designed to minimize the net energy input to accomplish the concentration and, if water recovery is needed, to reclaim to the greatest extent possible the evaporated water. In some instances, the recovered water can reduce the need for new infeed water in the process.
A photograph of a compact, transportable direct contact Venturi evaporator is shown in Figure 21.1. The major components consist of a hot gas inlet equipped with a quench section followed by a Venturi throat (often adjustable to vary the contact droplet size) and then a high-efficiency droplet separator. The hot gases are often pulled through the system using an induced draft fan.
We will look at a system that includes both evaporation and water recovery because it encompasses both stages. For evaporation only, the water vapor will exit directly to the atmosphere. The evaporation-only designs
Component diagram. (Bionomic Industries, Inc.)
are characterized by their visible water vapor plumes. With water recovery, because the plume is essentially condensed, the plume is essentially eliminated. The water recovery stage is discussed in greater detail in the next chapter since, in condensing the water, heat can also be recovered. Figure 21.2 shows the primary component parts of such a system.
To make the system transportable, these evaporators are often skid mounted. All the components mentioned above are fitted onto a support skid. Figure 21.3 shows such a skid being moved in the fabrication shop just prior to shipment. The view angle is from the pump end, and the support skid can be clearly seen. If the components are too large to facilitate truck shipment, multiple units can be set side by side or subassemblies can be produced that house the larger components. The droplet separators, for example, may be too large for easy transport and thus could be shipped separately. Both horizontal separators and vertical cyclonic separators can be used. These systems are basically classic quencher/Venturi scrubbers applied to the specific task of evaporation.
The components for the system with condensate recovery, forced draft, include the following:
1. A heat source. This may be a burner combusting natural gas, syngas, producer gas, oil, biofuel, or other heat-producing liquid fuel.
It could also be a solid-fuel burner. The heat input is typically in the range of 300 to 1800°F.
Skid-mounted Venturi evaporator. (Bionomic Industries, Inc.)
- 2. A gas inlet with safety bypass to atmosphere. The bypass allows the heat to be rejected safely in case of the heat source system malfunction. It is also used during burner purge for gas-fired burner startup.
- 3. A blower capable of overcoming the system gas-flow resistance. The blower is equipped with an inlet filter and, if necessary, a sound reduction device.
- 4. A Venturi scrubber (co-current flow) or counterflow tray or fluidized two-fluid contact device. The device brings the hot, dry gas stream into intimate contact with the liquid stream that requires concentration, resulting in the evaporation of moisture from that liquid. Since solids in the liquid are concentrated during the evaporation, the gas- liquid contact device must have a superior ability to operate at a high suspended and dissolved solids content.
- 5. A pump to draw the concentrated liquid from the sump of the contact device and recirculate the liquid so that the solids concentration will increase to a desired level.
- 6. An optional heat exchanger to preheat the process liquid to enhance the evaporation. The hot concentrate extracted from the bottom of the contact device exchanges part of its heat to preheat the infeed liquid.
- 7. A droplet control stage to separate the droplets created in the device from the gas stream. The droplet control helps keep the concentrated liquid in the device for the maximum evaporation cycles. If condensate recovery is not required, the gases would proceed to an induced draft fan and typically a stack. If condensate recovery is required, the following additional equipment is used.
- 8. A direct-contact condenser tower (such as a packed tower) that places cooled water into direct contact with the water-saturated gas stream, thus condensing and recovering water.
- 9. A heat exchanger to extract the heat from the liquid recycled in the direct-contact condenser.
- 10. A pump to recycle the recycled water and condensate through the heat exchanger and back to the direct-contact condenser.
- 11. A pump to recycle the cooling loop water to an air-cooled heat exchanger or heat sink (lake, pond, cold plant process stream exchanger, etc.).
- 12. A bleed line to bleed away the condensate.
- 13. An exhaust stack to vent the residual gases.
There are some disadvantages to the technique. Venturi type concentrators send the hot gases co-current with the liquid stream. This means the heated liquid passes in the same direction as the hot air and in doing so minimizes the difference in temperature between the liquid and the gas, which is known to reduce the evaporation rate. The infeed liquid is typically fed at the hottest point in the device (the inlet), wherein it dilutes the liquid that is being recycled and proceeds co-current with the gas stream. Adding the dilute liquid to the hottest zone can cause spray drying of solids from the liquid and a visible emission of particulate. Diluting with the infeed and allowing the infeed to move co-current with the concentrate limits the maximum solids concentration that the liquid can reach.
If the process liquid contains odorous compounds, those compounds could possibly strip from the liquid stream, thus requiring further treatment. The direct-contact technology works best in concentrating liquid streams that contain inert or dissolved solids but are devoid of low-solubility liquid or gases that can be stripped from the liquid.
To achieve the best results, the direct-contact concentrator should be applied only to sources wherein concern for stripping of odors or volatile organic compounds is at a minimum; otherwise, these compounds could strip from the system. If such a liquid stream must be treated, the gases from the concentrator could be subsequently scrubbed (for example, using a neutralizing or oxidizing chemistry) with the blowdown from that stage being returned for concentration in the Venturi. As an alternative, if the vent gases do contain stripped compounds, it may be possible to send those gases to a thermal oxidizer and then use the resulting hot gases in the concentrator.
A density control is suggested to limit the recycle solids before their concentration can cause mechanical problems. It is not uncommon to require decanters or centrifugal separation devices to further increase the solids content.
If the water vapor is to be recovered, careful control of the cooling water to the condensing stage is suggested. An efficient and more precise way of doing this is to use a temperature-regulating control valve in the cooling water bypass around the heat exchanger (rather than varying pump speed).