Combustion processes are one the major generators of greenhouse gases, such as C
ID: 106553 • Letter: C
Question
Combustion processes are one the major generators of greenhouse gases, such as CO_2. Carbon capture and storage (CCS) is therefore necessary to control CO_2 levels from emissions. Separation of CO_2 from flue gases is important from an environmental point of view, and also because pure CO_2 has many applications in food/beverage and different chemical industries. Assume that a combustion process generates 500 tons/day of flue gases containing 15%volume of CO_2 in a dry basis, and that 90% of the CO_2 needs to be captured. Your goal for this exercise is to design two systems for separation of CO_2: 1) gas absorption (using multiple equilibrium stages), and 2) membrane processes (multiple membranes). Calculate all the relevant parameters for each case (including, but not limited to, the flux for membranes, membrane configuration and system volume, as well as the number of ideal stages for absorption). As much as possible, try to compare the two processes, providing a recommendation as to which system you believe would be more appropriate, based on your calculations. Addition information: Gas Absorption of CO_2 can be performed with a variety of solvents, such as monoethanolamine (MEA), 2 - amino - 2 - methyl - l - propanol (AMP), diethanolamine (DEA), or methyl diethanolamine (MDEA). Several types of membranes can be used for CO_2 separation. This includes inorganic membranes, such as zeolite membranes (porous), as well as dense inorganic membranes, such as palladium and its alloys, or solid electrolytes, like zirconia. Sometimes, polymeric membranes (non - porous), such as polyacetylenes, polyaniline, and polyarene ethers.Explanation / Answer
CO2 removal by membrane and gas:
Carbon dioxide membranes operate on the principle of selective permeation. Each gas component has a specific permeation rate. The rate of permeation is determined by the rate which a component dissolves into the membrane surface and the rate at which it diffuses through the membrane.
The components with higher permeation rates (such as CO2, H2, and H2S) will permeate faster through the membrane module than components with lower permeation rates (such as N2, C1, C2 and heavier hydrocarbons). For example, carbon dioxide is a “fast,” more permeable, gas than methane. When a stream consisting of these two gases contacts the membrane, the carbon dioxide will permeate through the fiber at a faster rate than the methane. Thus, the feed stream is separated into a methane-rich (residual) stream on the exterior of the membrane fiber and a carbon dioxide-rich (permeate) stream on the interior of the membrane fiber.
The primary driving force of the separation is the differential partial pressure of the permeating component. Therefore, the pressure difference between the feed gas and permeate gas and the concentration of the permeating component determine the product purity and the amount of carbon dioxide membrane surface required.
Gas treating membrane systems provide a safe and efficient option for water vapor and carbon dioxide removal from natural gas, especially in remote locations. Membrane systems are extremely adaptable to various gas volumes, CO2 concentrations, and/or product-gas specifications. A spiral wound cellulose acetate membrane unit offers the greatest efficiency per Mcf of product removed compared to any other competing CO2 removal system.
The sequestration of CO2 as a greenhouse mitigation option is becoming an increasingly important priority for industry. Theoretically membrane based CO2 removal systems have the potential to provide a cost effective, low maintenance approach for removing CO2 from gas streams. . Lower costs for CO2 avoided can be achieved using an MEA amine based absorption separation system. Gas separation membranes would require significant improvements in CO2 permeability and selectivity, together with reductions in the cost of membranes and changes to the process configurations and operating pressures to be competitive against MEA systems for the purposes of geo-sequestration.
Gas separation membranes allow one component in a gas stream to pass through faster than the others. There are many different types of gas separation membrane, including porous inorganic membranes, palladium membranes, polymeric membranes and zeolites. Membranes cannot usually achieve high degrees of separation, so multiple stages and/or recycle of one of the streams is necessary. This leads to increased complexity, energy consumption and costs. Several membranes with different characteristics may be required to separate high-purity CO2. Solvent assisted membranes are being developed to combine the best features of membranes and solvent scrubbing. Much development is required before membranes could be used on a large scale for capture in power stations.
CO2 can be separated from other gases by cooling and condensation. Cryogenic separation is widely used commercially for streams that already have high CO2 concentrations (typically >90%) but it is not used for more dilute CO2 streams. A major disadvantage of cryogenic separation of CO2 is the amount of energy required to provide the refrigeration necessary for the process, particularly for dilute gas streams. Another disadvantage is that some components, such as water, have to be removed before the gas stream is cooled, to avoid blockages. Cryogenic separation has the advantage that it enables direct production of liquid CO2, which is needed for certain transport options, such as transport by ship. Cryogenics would normally only be applied to high concentration, high pressure gases, such as in pre-combustion capture processes or oxygen fired combustion.
The conditions for CO2 separation in pre-combustion capture processes will be quite different from those in post-combustion capture. For example, in a coal IGCC process, modified for capture, the CO2 concentration would be about 35-40% at a pressure of 20 bar or more. In that case, physical solvents, such as Selexol, could be used for pre-combustion capture of CO2, with the advantage that the CO2 can be released mainly by de-pressurization, thereby avoiding the high heat consumption of amine scrubbing processes. However, de-pressurization of the solvent still results in a significant energy penalty. Gas separation by selective transport through polymeric membranes is one of the fastest growing branches of membrane technology. However, the existing polymeric membrane materials are inadequate to fully exploit the application opportunities on industrial scale; the improvement in permeability is at the expense of selectivity, and vice versa. A new type of membrane material emerging with the potential for future applications is mixed matrix materials composed of homogeneously interpenetrating polymeric and inorganic particle matrices. Compared to original polymeric membranes, significant improvement in separation properties with trivial loss in membrane flexibility is expected for the resultant mixed matrix membranes (MMMs). This review first gives an outline of the concept and the key advances of MMMs. Subsequently, recent developments are presented, including two immediate challenges: achieving an optimized interface structure, and forming asymmetric or composite membrane with an ultrathin and defect-free mixed matrix skin. Attractive avenues to overcome these challenges are emphasized. The review of the Maxwell model demonstrates how the transport properties of MMMs are related to the polymer matrix, molecular sieves, as well as membrane morphology. Finally, future directions of MMMs’ fabrication and application are suggested.