Guidelines for CO2 Strippers
The use of CO2 strippers is becoming more common as RAS (recirculating aquaculture systems) become more intensive. Much has been written about the theory of design and sizing but little has been written about the practical aspects of stripper design and construction. This paper will cover the following topics
1. Cross flow versus counter flow operation
2. Forced draft versus induced draft air flow
3. Pressure distribution systems versus gravity flow distribution
4. Solutions for drift elimination
5. Design considerations to minimize heat loss
6. Media selection criterion
Carbon dioxide strippers can be grouped with aeration towers and deaeration towers. These devices are designed to move gas into or out of water. Moving O2 into the water and CO2 out of the water can be accomplished simultaneously in the same piece of equipment.
CO2 strippers are necessary water treatment devices for some types of intensive recirculating aquaculture systems. In general, there are two criteria that indicate the need for a CO2 stripper. If the biofilter is not a trickling filter and pure oxygen is the primary source of oxygen then it is likely that a CO2 stripper is necessary. Ultimately it is the sensitivity of the species of fish that will determine the need for a CO2 stripper.
Unlike a biofilter, the entire recirculating water stream does not have to flow through the stripper. Typically, a small side stream is all that is required. Fortunately, CO2 strippers can also assist with the other water quality maintenance chores. The two extra benefits of a CO2 stripper are the addition of oxygen and the extra biofiltration surface area provided by the packing.
Mode of Operation
CO2 strippers are similar to devices used by other industries. Water cooling towers, air pollution control scrubbers and adiabatic coolers are just a few of the many liquid - gas contactors being used by various industries. There are three ways to operate a liquid – gas contactor regardless of which liquid or gas are being contacted.
Cocurrent – The liquid and gas both flow in the same direction. For practical reasons, this means that both the gas and liquid flow down through the vessel. This is the least efficient method of achieving mass transfer. There is no advantage to this mode of operation so we will not discuss it further.
Counter Flow – The liquid and gas flow in parallel but opposite directions. In this case the liquid flows down while the gas flows up. This is the most efficient method of mass transfer. Counter flow is the traditional method and most common mode of operation for CO2 strippers in aquaculture.
Cross Flow – The liquid flows down while the gas flows horizontally. This method is slightly less efficient than counter flow but not significantly different for CO2 strippers. Cross flow CO2 strippers have the advantage of more flexibility with regards to configuration. Cross flow towers can be built shorter than counter flow towers. This can save on pump head and be an advantage in buildings with low ceilings.
Air flow through the stripper can be induced (pulled) through the tower or it can be forced (pushed) through the tower. Forced draft systems have the fan located in the dry air upstream of the packed section. Theoretically, this makes the fan less susceptible to corrosion. However, this is a dubious benefit if the tower is located inside a warm, humid greenhouse or other enclosure.
Induced draft fans tend to provide a more even air flow through the packing. If structured packing is used in the tower, low pressure, high efficiency axial (prop type) fans can be used. If high pressure drop random packing is used, lower efficiency centrifugal fans may be necessary.
Drift (Mist) Elimination
The purpose of a drift eliminator is to remove any water droplets from the air stream before it exits the tower. Most aquaculture CO2 strippers should not need a drift eliminator. If one is required, it is important to locate it in an area of the tower were the air velocity is 300 FPM or higher. Most modern drift eliminators have an upper velocity limit of about 700 – 800 FPM. In general, drift eliminators work better at higher velocities until the break though velocity is reached. However, higher velocities also mean higher pressure drops. A good target design velocity is around 500 FPM.
Drift eliminators should be located where the water collected can be returned to the tower. Drift eliminators can be used for air streams that are moving up or horizontally. They cannot be used for air streams moving downward.
Example of a Cellular Style Drift (Mist) Eliminator
Water distribution is the single most important operating variable that ensures full performance of the stripper. Uneven water distribution can have the following effects.
Loss of Performance – If all the surface area of the packing is not wetted, then less surface area will be available for mass transfer.
Plugging – If some areas of the packing are intermittently wetted, scaling and solids deposition will occur. In areas not flushed with a sufficient flow of water, thick biofilms may form.
Air bypass – The air will follow the path of least resistance. The parts of the tower with the greatest water flow will receive the least air flow and the part with the least water will receive the most air. This contributes to the loss of performance.
Even water distribution across the top of the packing is absolutely essential to full performance and trouble free operation. There are several good ways to distribute water across the top of the packing depending on the configuration of the tower. The choice is between single nozzle coverage with a solid cone nozzle and multiple nozzles that provide overlapping patterns. A few guidelines are useful to help the selection process.
1. Avoid small holes. Small holes tend to plug up and/or biofoul. The minimum orifice in any systems should be 12 mm.
2. Avoid high pressure nozzles. High pressure systems waste energy, create small droplets, require more maintenance and are generally more expensive.
3. Avoid moving parts. Moving parts such as spinners, rotating arms and oscillating bars tend to have shorter useful lives and require more maintenance than fixed systems.
4. Avoid spraying water on the wall. The edge of the spray should hit at the corner where the packing meets the wall. Water that hits the wall tends to stay on the wall and does not travel through the packing.
For counter flow systems, the simplest and best system is a pressurized nozzle arrangement. Nozzles mounted in a piping system can provide even distribution, access to the packing and unrestricted air flow through the tower. They are simple to build and support. Their only drawback is the pressure required to operate the nozzle. Operating pressure should be between 2 – 5 psi.
Small round counter flow towers up to 3-4 ft. in diameter should use a single, solid cone, round pattern nozzle in the center of the tower. It is more difficult to achieve an even pattern of droplets with multiple nozzles in a round tower.
Small square towers up to 3-4 ft. on a side should use a single, solid cone, square pattern nozzle in the center of the tower. However, it is easier to get even coverage with multiple nozzles in a square or rectangular tower than in a round tower.
The alternative to pressurized piping systems is a pan distribution system. Pan distribution systems operate with very low head requirements and space requirements. There are two main drawbacks to these systems. The weight of the water in the pan must be considered when designing the structure. The other drawback is the obstruction of the pan to the air flow in counter flow systems. Provisions must be made for the air to flow around or through the pan.
Pan distribution systems must use target nozzles to achieve a sufficiently even water pattern. Drip type pans with small holes cannot provide an even distribution. The small holes in drip pans are also very easily plugged. Here is an example of a target nozzle.
For cross flow systems, pan distribution systems are best for all but the smallest system. For very small systems, a header pipe with cover plate distribution system is best. Pressure nozzles do not work well for cross flow systems.
Although many different materials have been used for packings in CO2 strippers, most modern strippers are designed with one of two types of media. The older types of media are known as random or dumped packings and come in a variety of shapes and sizes. Here are a few examples.
The newer types of packings are known as structured media and have been used in aquaculture for the past 30 years. They are also known as cellular or film fills and are available in numerous shapes and configurations. Structured packings have a number of advantages that make them the packing of choice for CO2 strippers. This is a typical example of cross corrugated structured packings
Structured packings are typically constructed of vacuum formed sheets of PVC (polyvinyl chloride). Continuous vacuum forming is a high speed automated process that can efficiently produce large amounts of material. This method of construction allows structured packings to be produced for a much lower cost per unit surface area than injection molded, random dumped packings. PVC is a relatively low cost resin with much better mechanical properties than PP (polypropylene) or HDPE (high density polyethylene).
In order for the media to maximize mass transfer, water must be able to wet the surface and form a film. PVC is initially hydrophobic but normally becomes fully wetable within 1 to 2 weeks. Most random medias are made from HDPE or PP that take several months to become fully wetable.
The vacuum formed sheets of PVC are welded or glued together to form rectangular blocks. Some packings have internal "tubes" that only allow flow along one axis of the block. Other types of structured packings known as cross corrugated packings allow flow along 2 axes of the block. Most structured packings used in CO2 strippers are of the cross corrugated type.
important feature of structured medias is their very high void fractions. Void
fraction is the percentage of open space or volume in the packing. To phrase it
another way, void fraction is the space not occupied by the packing itself.
High void fractions allow free and unrestricted flow of water or air and
water. A modern packing like structured
packing for CO2 stripping applications should have a void fraction of 95% or
Structured medias are resistance to plugging or clogging. This parameter is very important but difficult to quantify. Plugging or clogging of a stripper can happen through mechanical trapping of particles in the same way a screen or other particulate filter works. Plugging can also result from the growth of the biofilms and bridging across the spaces within the packing. Plugging tendency for various packings can be predicted or compared by looking at the void fraction and free passage diameter. The free passage diameter is the more important variable. The best way to understand free passage diameter is to imagine a marble or ball bearing being dropped through the packing. The size of the largest ball that will pass through the packing is the free passage diameter.
Another feature of structured media is excellent mechanical strength. In a large stripper, it is very desirable that the media be able to safely support the weight of one or more workers. Aside from supporting maintenance traffic, good mechanical strength means better dimensional stability, reduced vessel support requirements and longer life. Unlike any other type of packing, structured packings can span distances of up to 10 ft. between supports. It is more common however to support them on beams that are 2 to 3 ft on center.
CO2 strippers come in all shapes and sizes and structured media can be cut to fit any shape vessel. If maintenance is required due to plugging or the need to sterilize the system between crops, structured packings can be easily moved with a minimum of labor and specialized equipment. The large blocks are easy to handle and move around. Random packings must be moved with shovels or buckets and are cumbersome to handle.
CO2 strippers are sometimes built with out any fans. These systems depend on diffusion, thermal convection or water flow induced convection to carry away the CO2 laden air. These systems are typically much less efficient than ones designed with fans to move air through the stripper. Here are a couple performance curves showing the removal efficiency versus air flow for two temperature levels. The basis for the design is counter flow operation, 30 ppm CO2 entering and 10 ppm leaving. The packing is a cross corrugated structured media with a 19 mm sheet spacing. The system is set for isothermal conditions. (i.e. no heat transfer between air and water) These calculations do not take into account any removal or addition of CO2 due to chemical equilibrium reactions.
The flows are given in terms of air velocity and specific water loading. In this way the towers can be scaled to fit any size application.
There are a number of common mistakes made by people trying to build CO2 strippers.
1. Using waterfall type systems
Water cascading over a weir or pouring in a solid stream from a pipe does not generate enough surface area to achieve much mass transfer. It is a waste of pump head energy to let water fall in an unbroken stream into a pool.
2. Totally enclosed vessels with no air movement.
If a CO2 stripper does not have any air movement through the vessel, no CO2 will be removed from the water. There will be a small amount of CO2 removed when the unit is first used but as the CO2 builds up in the vessel, the CO2 transfer out of water slowly decreases as the CO2 concentration in the air increases. When the CO2 in the air reaches equilibrium with the CO2 in the water no further removal of CO2 from the water is possible.
3. Uneven or inadequate water distribution.
This is one of the most common problems in packed towers of all kinds. Continuous and even distribution of water over all of the packing surfaces is essential to reach full performance. If the packing surface is not wetted, it will not do any work. A good water distribution system is a small part of the cost of any tower but it is a major contributor to overall system performance.
4. Using inefficient or unsuitable packings.
Many people try to reinvent the wheel by utilizing some common or cheap objects for tower packings. The truth is that almost anything will work as a tower packing. Old car bodies will work if you use enough of them. The problem is that old car bodies are not the most economical choice when you consider both the capital and operating costs of the system. Ping pong balls, lava rock, milk crates, window screens and other materials will also do the job but again, they are not the most economical choice. Structured packings are used in a wide variety of industries for heat and mass transfer applications. Up to this point in time, no one has developed a better or more cost effective media for gas-liquid mass transfer operations.
There are many different ways to remove CO2 from the water in recirculating aquaculture systems. From the standpoint of minimizing capital, operating and maintenance costs, the best solution to the problem is an induced draft, cross flow stripper with structured packing. They are simple to build and operate and have several auxiliary benefits including biofiltration and reoxygenation of the water. If designed properly, they can also be used for cooling purposes.
The calculations in this paper were performed with the use of VOC design software developed by MRL Corporation. This software is available for sale from:
15590 Triple Crown Court
Ft. Myers, FL 33912 USA
Contact: Marcel LeFevre
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Author: Matt Smith