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Flue gases consist of nitrogen, carbon (IV) oxide, water vapor, carbon (II) oxide, oxygen and oxides of both nitrogen and sulphur. Adsorption separates CO2. This method occurs in several ways that include; Electrical swing adsorption (ESA), pressure swing adsorption (PSA), single chamber adsorption (SCA) and temperature swing adsorption (TSA).
The rates of adsorption depend on CO2 partial pressure, temperature, surface area of the sorbent and surface forces between the sorbent and CO2. PSA is the commonest method as it uses less energy compared to the rate of regeneration. In ESA method, CO2 goes through adsorption onto a sieve and then electricity passes via the sieve to increase the sieve temperature, making it release CO2.
In SCA, the temperature of the gases cools to 300C. This process increases the adsorption of CO2 as other gases exit. Scientists apply a vacuum to liberate CO2 via a different outlet from the sorbent for sequestration. The PSA has two chambers in which gases as passed back and forth continuously as pressure is increased forcing the adsorption of CO2. As the residual flue gases move to the other chambers, the pressure of CO2 falls, and thus, collects in one site.
Cryogenic distillation relies on separating flue gases from CO2 and nitrogen gas. Nitrogen gas and CO2 separate when the temperature falls to -56.60C and the pressure increases to around -7.4 atmospheres in the cryogenic chamber. CO2 liquefies while nitrogen escapes in gaseous form through the top of the chamber. A similar method, refrigeration under pressure, can separate N2 and CO2, but the method is more complex and requires high pressure and temperature.
Another cryogenic distillation process has CO2 separated from flue gases. The gases then cool in the presence of a stream of nitrogen gas from the refrigerant. Water vapor condenses during this process to prevent chemical corrosion. Traps remove solid particles and heat. This step helps in the conservation of heat. O2, CO2 and N2 flow directly to an adjacent chamber against the entering gases. This step helps cool the gases for separation.
The refrigeration under pressure has two condensers in which raw flue gases pass through after the removal of water vapor. The gases are compressed and further cooled in different chambers. The experimenters then separate them into two flows. One flows through the bottom coolers while the other passes through a heat exchanger, transferring heat to the existing flow.
The gases cool down, and CO2 separates out as a liquid or dry ice depending on conditions of temperature and pressure. The CO2 gas then passes through a heat exchanger to cool entering gases before decompressing it. The liquid CO2 obtained is more than 99.95% pure and is ready for transportation. However, this process is energy intensive. Water vapor can form ice that may clog the pipes if not completely removed.
Experimenters can separate CO2 from light hydrocarbons by a membrane that traditionally, separated hydrogen gas. Membranes used include inorganic, polymeric, metallic, solid and liquid. Their selectivity depends on their ability to interact with target molecule. Experimenters allow the gas to diffuse across the membrane by absorption or solution diffusion mechanism. The membranes are porous and so allow CO2 to penetrate through them.
The membrane stops large gas molecules while CO2 collects in a chamber at low pressure. Some membranes use absorbent materials that selectively dissolve CO2. Thick membranes are less permeable as compared to thin ones due to the varying distances across a membrane. The flue gases go into a separation tank where diffusion takes place. The permeable side has 10% pressure difference to the feed side. Therefore, a vacuum constantly pulls CO2 across the membrane. CO2 flows to a collecting tank. Scientists allow other flue gases to escape.
Further separation of CO2 collected improves its purity. Construction of equipment in membrane separation is quite simple, although the major setback is that membranes are not very permeable and selective to only CO2. Other gases penetrate through the membrane making secondary separation essential in the purification of CO2. Many organic membranes perform poorly at high temperatures, making them inefficient.
Hydrate formation and dissociation
Hydrate formation and dissociation can also help in the separation of CO2 from flue gases. It consists of cage cavities with ice-like structures that can trap CO2 and other gases. CO2 is easily false into the trap as compared to other gases because it easily forms hydrates with water. Hydrates form best at temperatures below 100C and high pressure. The hydrates then melt, making them emit a stream of CO2.
This method is more efficient than traditional means since it does not use lots of energies and makes transportation easier. However, the hydrates formed can plug the lines, and this needs a mechanized and functional plant to prevent plugging. The addition of Tetrahydrofuran (THF) reduces pressure. Hydrates form best, when the temperature reduces to 20C, and THF used that reduced the pressure to 3.95 atmospheres. This experiment produces 70% CO2 mixed with N2.
Electrical desorption and Redox technology
Electrical desorption is another way of CO2 separation from flue gases when used with electrically conductive sorbent (Quinone carrier) that requires a minimum amount of energy. The National Renewable Energy Foundation recommends the use of the redox technology that binds CO2 to itself at high pressure. Reduction allows CO2 to be bond with the carrier while oxidation of the carrier makes it release CO2 at low pressure. This method too requires low energy.
On the other hand, experimenters can separate CO2 from flue gases by passing the gases through ammonia solution forming solid ammonium carbonate. Other gases escape as they are insoluble in ammonia except SO2 that forms ammonium sulphate. These products work as soil fertilizers. The process requires no application of energy or alteration in the pressure, making it more efficient and fast. The process is cost effective as sales of fertilizers can offset the costs. Use of chemical solvents is under testing for use in future.
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If all the above methods were could be in ranks, separation by membrane diffusion could be more efficient as the processes involved are easily attainable. It requires less energy to move gases through the system, atmospheric pressure and temperature of up to 3500C that does not destroy the membrane. On the other hand, absorption is another promising way of CO2 separation. The energy required is relatively low. Experimenters carefully chose the reagents. They are non-corrosive to equipment used and are resistant to degradation, making them reused for some time.
The only problem is making the permeable membranes that are currently under research and testing. Adsorption comes third in this ranking. It uses less energy, though it requires selection of sorbents that can selectively separate CO2 from flue gases. Cryogenic distillation uses expensive equipment and is less efficient as conditions are hard to achieve and maintain. The CO2 collected by this method is not pure. Hydrate formation and absorption of ammonia are undergoing testing though they are promisingly efficient.