Many processes contain dirty gas streams that need to be cooled and cleaned prior to the next step in the process. This article will look at directly cooling these streams and methods for effectively removing contaminants from dirty gas streams.
Direct-contact gas coolers allow the cooling medium — usually water — to contact a gas or air stream directly. In some respects, this is the opposite of a cooling tower, in which the air removes heat from the cooling water. In a direct-contact gas cooler, the cooling liquid is used to reduce the temperature of the gas stream.
A direct-contact cooler is preferable to indirect cooling in some process applications for many reasons. In most cases, direct cooling is the simplest approach, and the use of direct cooling provides economical, reliable cooling. In other cases, direct cooling provides benefits for a manufacturing facility’s water balance or allows cooling of a dirty gas steam that is problematic for indirect cooling.
The most common direct-contact gas-cooling method used is the quenching of hot gas from combustion, drying or other heated processes. This cooler design uses the latent heat of evaporation of water to cool the hot gas stream. Quenching a hot gas stream increases the water content in the gas stream and the volume of the stream. It also allows for the use of materials and processes downstream that may not be feasible if the feed gas remained at the higher temperature.
One example of this type of direct gas cooling is quenching the exhaust gas from waste combustion. Combustion exhaust often contains contaminants that are corrosive to metals but not to materials such as fiber-reinforced plastic (FRP) or polypropylene. Such materials typically are limited to temperatures below 194°F (90°C), however. Gas cooling reduces the temperature of the exhaust gas so that the corrosion-resistant materials can be used.
Different kinds of direct-contact coolers are available. Cooling towers and quench systems typically operate using the latent heat of evaporation of water only. Other types of direct coolers use sensible heat — or the heat capacity available by heating the liquid medium — to reduce the gas stream temperature. These include once-through water coolers, chilled-fluid coolers and cryogenic coolers such as those incorporating the injection of liquid nitrogen.
All of these cooler designs have benefits and drawbacks. Evaporating water to cool a gas stream provides highly effective heat transfer. However, it uses water that may not be recoverable. Also, the cooling available from evaporation is limited by the liquid capacity of the gas stream being cooled. By contrast, sensible heat coolers often require large quantities of liquid. Also consider that cryogenic cooling can be effective, but the use of liquefied gas for cooling is expensive. As a result, cryogenic cooling usually is limited to small, point-of-use applications.
The majority of direct-cooling solutions work well when the gas stream to be cooled is clean. Dirty gas streams, however, present their own challenges.
Meeting Dirty Gas Stream Challenges with Scrubber Coolers
In systems such as processes where air is recycled back to a drying system, the level of solids in the gas stream may make indirect cooling infeasible. Where significant levels of solids are carried in the gas stream to be cooled, it can be advantageous to remove the solids from the air stream when cooling. While it may be possible to clean the gas stream prior to cooling, doing so creates additional capital, operational and maintenance costs.
One approach for a unified cleaning and cooling operation is a crossflow scrubber cooler. The crossflow design can fit into many plant layouts without requiring the up-and-down pipe run needed for vertical designs. The crossflow design also allows for effective staging of cleaning and cooling within the scrubber.
Where significant levels of solids are carried in the gas stream to be cooled, it can be advantageous to remove the solids from the air stream when cooling. One approach for a unified cleaning and cooling operation is a crossflow scrubber cooler.
The crossflow cooler consists of three main sections: an evaporative-cooling section, a sensible-cooling section and a mist eliminator. While the crossflow scrubber can be operated without one of the first two sections, in most cases, it is beneficial to have all three.
Evaporative-Cooling Stage. In the first section, the cooling liquid — usually water — is sprayed into the incoming gas stream. Gas-and-air intermixing and contact can be enhanced by using a venturi inlet or packing. Using the venturi inlet, packing or both also assists in the initial stage of cleaning the dirty gas stream, which is intended to remove any larger particulate in the gas. At the same time, the initial stage also conditions the gas and brings it to the equilibrium gas temperature with the water or other liquid content in the gas stream. In most cases, half or more of the gas-stream cooling is achieved by evaporation of the liquid sprayed into the gas stream.
Sensible-Cooling Stage. The second section — sensible cooling — cools the gas to the final design temperature and removes the contaminants to the level required for proper system performance. This is achieved by irrigating packing with a liquid that will cool the gas while, at the same time, clearing the packing of any accumulated solids removed from the gas stream. The heat balance and particulate-removal design of this section are critical to ensure proper operation of the cooler.
When direct cooling a clean gas stream, the design ends with the heat balance. By contrast, for a dirty gas stream, particulate collection must be considered and incorporated into the heat transfer design. This helps ensure the system will provide the proper gas exit temperature and the required reduction in contaminant loading. As a result, packing design must take into account the desired heat transfer and the required particulate removal rate to provide effective cleaning of the packing.
Generally, the gas can be cooled to within 5.4 to 9°F (3 to 5°C) of the cooling liquid temperature. During the design phase, the system is analyzed for the heat transfer achievable in the unit volume of the packing section. This helps determine the total volume of packing required. In a crossflow scrubber, it may be beneficial to use multiple contacting sections of sensible cooling in order to manage liquid usage, or to use multiple liquid streams to cool the gas.
Many styles of random and structured packing are available. Almost all can be designed to provide heat transfer contacting as well as particulate removal. However, many also will collect particulate solids in the packing and are difficult to keep clean and performing well. I have found the most effective packing for crossflow cooling of a dirty gas is structured monofilament media.
Structured monofilament media can be irrigated to help ensure optimal contact between the gas and the cooling liquid. Also, it is more resistant to clogging than random packing or monolithic structured packing. Further, the structured monofilament media can be cleaned for reuse as needed. Finally, structured monofilament media reduces operating costs by providing a lower pressure drop across the bed.
Mist-Elimination Stage. After the gas is cooled and cleaned, mist elimination is the final stage. This is a critical step to prevent any liquid droplets from reaching downstream process equipment.
A number of design options are available for the final mist eliminator. Vane-type mist eliminators can be used where only larger (above 10 microns) droplets need to be removed. For removal of smaller (3 to 5 microns) droplets, knitted-mesh mist eliminators provide effective service. In critical applications, structured monofilament mist eliminators can provide removal of particles down to 2 microns.
An effective packing for crossflow cooling of a dirty gas is structured monofilament media, which can be irrigated to help ensure optimal contact between the gas and the cooling liquid.
Cooling Recycled Nitrogen for a Specialty Chemicals Maker
A specialty chemicals manufacturer used a recycled nitrogen stream that was cooled from 140 to 68°F (60 to 20°C). The manufacturer had been using an indirect cooler for a number of years, but 1,000 mg/m3 of insoluble polymer in the 70,000-m3/hr nitrogen stream clogged the heat exchanger badly. Extensive annual maintenance was required clean the unit.
The chemical company considered other options and decided on a direct-cooled system. It consisted of an evaporative-cooling section, a two-stage sensible-cooling system and a mist eliminator. The evaporative-cooling stage sprays ~6,340 gal/hr (24,000 kg/hr) of water into the gas stream, evaporating ~115 gal/hr (435 kg/hr) to reduce the gas temperature from 140 to 104°F (60 to 40°C). This also reduces a small amount of the particulate. In the sensible-cooling section, two stages of structured monofilament media are sprayed with 50°F (10°C) chilled water to achieve the required 68°F (20°C) outlet temperature while removing more than 90 percent of the entrained solids. The sensible-cooling section is followed by a structured monofilament mist eliminator that effectively removes all of the entrained-water droplets and mist generated in the cooling operation.
The system was installed and operated successfully at a pressure drop of less than 150 mm of water. It has continued to run for more than two years and has not required maintenance.
In conclusion, the use of direct-contact coolers is an effective way to reduce the temperature of dirty gas streams. Unwanted contaminants can be removed, and the gas stream can be directed to downstream equipment or recycled into the process. PC