Separation of gas mixtures has been achieved using a variety of techniques. Examples of such techniques include distillation, membrane separation, gas-liquid absorption, and gas-solid adsorption. These techniques can be based on one or more of chemical and physical differences between the components being separated.
In distillation, the components of a gas mixture are separated by exploiting differences in the bubble- and dew-point temperatures of the components. Distillation columns are designed to provide intimate contacting of vapor and condensed liquid streams over a series of plates or packed sections, so that a temperature difference between the bottom and top of the column yields streams with different compositions. Distillation equipment is capital-intensive, and the operation of distillation columns is notoriously complex and difficult to automate, so that skilled operators are generally required.
Membranes have been used to separate gases from gases and gases from liquids. Gas separations can use either porous or nonporous polymeric membranes. In porous membrane separation, pore diameter is typically smaller than the mean free path of the gas molecules. In nonporous membranes, separation is generally based on differences in solubility and diffusivity in the polymer. Gas-liquid separation membranes are generally based on gas permeability or diffusion or on gas-permeable pores providing separation by capillary force.
U.S. Pat. No. 4,995,888 shows one example of gas separation from a liquid using a membrane in which the separation is based on absorption. Acid and base gases can be separated.
Soluble vapors can be separated from mixtures with inert gases by absorption into an appropriate liquid solvent. Gas-liquid absorption processes can employ packed beds of inert solid particles, with the liquid solvent gravity-flowing downward over the inert solid particle surfaces. The mixture of inert gas with soluble vapor flows upward through the bed, contacting the liquid in counter-current flow. Absorption of the soluble vapor into the liquid can provide a pure stream of inert gas exiting the top of the bed, but the soluble component is recovered as a solution in the liquid. This liquid solution must be subsequently separated if the soluble vapor and liquid solvents are to be re-used as pure components. In operation of a gas-liquid absorption bed, the velocities of both the liquid and gas streams are controlled to prevent either dry or flooded conditions in the bed.
U.S. Pat. No. 3,920,419 provides and example of a method of removing ammonia from an ammonia containing liquor using a packed column. The ammonia-containing liquor is fed to the top of the packed column, while monitoring the pH of the feed liquor. Caustic is added in controlled amounts to the feed liquor prior to introduction into the column to maintain a minimum pH of 10.5. A gaseous stripping medium is flowed in a countercurrent manner through the column at the minimum temperature of about 140° F., while regulating the flow of the feed liquor to the column. The flow of the stripping medium is controlled so that at least 99% of the ammonia is removed from the liquor.
Gas components of a gas mixture can also be separated using solid adsorbent materials. Since adsorption is a surface phenomenon, adsorption capacity increases with the specific surface area of the solid adsorbent, so materials with very high specific surface areas are commonly used. Some examples include adsorption of oxygen from air using activated carbon, adsorption of ammonia from nitrogen using metal salts (Christensen et al 2005, WO 2005/091418; Johannesen et al 2009, U.S. patent Pub. Nos. 2009/0123361 and 2009/0313976; Fulks et al 2009, SAE International Report 2009-01-0907), ammonia adsorption on dried coffee grounds and activated carbon for air deodorization (Kawasaki et al 2006, J. Oleo Sci. vol 55., no. 1, pp 31-35), and hydrogen purification by adsorption of impurities from reformate gas using zeolite molecular sieves.
Commercial gas-solid adsorption processes often employ packed beds of solid adsorbent particles operated first at high pressure to selectively remove one component of a gas mixture, followed by desorption of the bed at low pressure to recover the adsorbed species and regenerate the adsorbent. These cyclic operations are known as Pressure-Swing Adsorption (PSA) processes. Solid adsorbents may also be desorbed and regenerated by heating, known as Temperature-Swing Adsorption (TSA). The adsorbents used in gas-solid separations are often expensive, and their performance can be degraded by irreversible sorption of moisture, oils, or other contaminants. Adsorption of ammonia from inert gases using metal salts is limited by the rate of diffusion of ammonia into the bulk salt particles, and therefore does not allow for rapid adsorption and separation of ammonia from the inert gas.
Ammonia has been used to treat biomass material. In the treatment process, after contacting with biomass, the ammonia is separated, recovered and recycled, typically by what has been referred to as an Ammonia Fiber Explosion (AFEX) process.
Holtzapple et al., U.S. Pat. No. 5,171,592, discloses an AFEX process in which liquid ammonia is contacted with biomass material in a reactor. The ammonia vaporizes in the reactor causing the biomass to cool. The ammonia also causes the biomass to swell and decrystallize. When decrystallization is complete, the reactor is opened, causing the ammonia to flash off of the biomass. The flashed ammonia is recovered and recycled to contact fresh biomass.
Rajagopalan et al., WO 2006/055362, also discloses an AFEX process in which liquid ammonia is contacted with biomass material in a reactor. The reactor does not allow vaporization of the ammonia. However, when the biomass has been in contact with the liquid ammonia for a time sufficient to treat the biomass, the reactor is opened, causing the ammonia to flash off of the biomass. The flashed ammonia is recovered and recycled to contact fresh biomass.
Dale et al., U.S. patent Pub. No. 2009/0221042, discloses a process for treating biomass to render structural carbohydrates more accessible and/or digestible using concentrated ammonium hydroxide. The process uses steam to strip ammonia from biomass for recycling. The process yields of monosaccharides from the structural carbohydrates are considered to be good as indicated by enzymatic hydrolysis under standardized conditions (“Enzymatic saccharification of lignocellulosic biomass,” National Renewable Energy Laboratory Technical Report TP-510-42629, March 2008).
More efficient methods of separating gas components are desired. Particularly desired are methods that are less energy- and capital-intensive relative to known processes.
Also of interest would be methods that allow for use of biological or biomass materials in the separation process. Such materials are considered renewable and, therefore, of environmental benefit. The ability to use these materials for other downstream uses would also be of benefit.
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