Vapor-Phase Carbon Adsorption - Term Paper Example

Student Name: Registration No.: Title: Adsorption Course: Instructor: Department: Institution: Date Due: Vapor-phase adsorption unit is a remediation technology in which pollutants are removed from air by physical adsorption onto activated carbon grains. Carbon is "activated" for this purpose by processing the carbon to create porous particles with a large internal surface area (300 to 2,500 square meters or 3,200 to 27,000 square feet per gram of carbon) that attracts and adsorbs organic molecules as well as certain metal and inorganic molecules(Barry and John).Carbon can be used in conjunction with steam reforming. Steam reforming is a technology designed to destroy halogenated solvents (such as carbon tetrachloride, CCl4, and chloroform, CHCl3) adsorbed on activated carbon by reaction with superheated steam (steam reforming). The ability of some solids to remove colour from solutions containing dyes has been known for over a century(Barry and John). Similarly, air contaminated with unpleasant odours could be rendered odourless by passage of the air through a vessel containing charcoal. Adsorption is as a result of interactive forces of physical attraction between the surface of porous solids and component molecules being removed from the bulk phase(Barry and John). Thus adsorption is the accumulation of concentration at a surface (as opposed to absorption which is the accumulation of concentration within the bulk of a solid or liquid). According to the kinetic theory of gases the rate of adsorption of nitrogen at ambient temperature and 6 bar pressure is 2 × 104 Kg m-2 s-1. At atmospheric pressure this would translate to 0.33 ×104 Kg m-2 s-1. Ostensibly then, rates of adsorption are extremely rapid (Barry and John). In a vapour phase adsorption system the pollutants remain adsorbed onto the solid inside the column. Contrary, in an absorption system the gas is continuously removed from the column(Kennes and Viega). The adsorbent will thus finally get exhausted. Wherever possible adsorption processes are combined with a desorption process which aims at recovering the volatile solvent, thus reducing or at least compensating overall costs. The adsorbed solvent may be recovered in a vapour phase after steaming. The latter is afterwards liquefied in a condenser(Kennes and Viega). Since the organic solvent and the water phase are usually characterized by different densities, both phases can easily be separated. If at least two adsorption columns are connected in series, one can be regenerated while the other is still operating. Regenerated wet carbon is not suitable for adsorption and a drying step is required before reuse (Kennes and Viega). Activated carbon and certain resins will adsorb organic compounds in an aqueous phase, such as groundwater, or in a vapor phase, such as air. Stripping of groundwater that contains VOCs produces vapor-phase organic compounds(Marve and Ryan ). Ventilation of soil that contains VOCs produces vapor-phase carbon adsorption. In most cases activated carbon is used which is charcoal that can be produced from wood, coconut shells, fruit pits and other natural cellulose materials, or it can be made from lignite, peat, or bituminous coal. The most common activated carbon used for remediation is derived from bituminous coal. Lignite and coconut shells are also widely used for producing activated carbon. The material is activated by heating in a multiple-hearth furnace in a steam atmosphere where there is insufficient oxygen (air) to burn the material. The temperatures are high enough to drive off virtually all of the organic compounds, leaving almost pure carbon. Carbon that has been used to adsorb organic compounds can be reactivated in a multiple-hearth furnace, with the same process used to prepare virgin activated carbon, or in a kiln(Marve and Ryan ). Vapor Phase Carbon Adsorption Equipment Design The design calculations used are for sizing a vapor phase activated carbon adsorption unit. The treatment train consists of a blower, air stripper, heat exchanger, and carbon vessels to clean air from the air stripper containing the following volatile organic chemicals (VOC): perchloroethylene (PCE), trichloroethylene (TCE), benzene, and toluene (see Figure B-1). The water containing the VOCs enters the top of the air stripper column and flows generally downward through the packing material. At the same time, air flows upward through the column (countercurrent flow). Due to water and air contact, the volatile organic chemicals are transferred from the water phase to the air phase. The water gets out of the bottom of the column depleted in VOCs. The VOCs transferred by the air are removed at the top of the column. The air phase flows through a heat exchanger and it is heated from 289 to 300 K to reduce the relative humidity from 100 to 50%. From this point, the air phase flows through vessels filled with activated carbon which adsorbs the VOCs. The air phase, depleted of VOCs, is released to the atmosphere. Parameters Flow rate of the air phase entering the air blower: 1m3/s. 305 K is the Temperature of the stream vapor into the blower Run time between carbon changes: 3 months/vessel. There are two carbon vessels arranged in series. 87.6 kPa is the Atmospheric pressure (the elevation of the site is approximately 1600 m above sea level). 289 K is the temperature of air phase leaving the air stripper Contaminants and their concentrations going out of the air stripper are as shown below: a) 15 ppmv of Perchloroethylene (PCE) b) 14 ppmv of Trichloroethylene (TCE) c) 9 ppmv of Benzene: d) 5 ppmv Toluene b. Design Steps. (1) Determination of the amount of carbon needed for 3 months. (2) Determination of the size of the carbon adsorption vessels. (3) Determination of the total pressure drop through the treatment train: (a) Air stripper and coupled piping, valves, and instrumentation. (b) Air stripper and heat exchanger piping, valves, and instrumentation. (c) Heat exchanger. (d) Heat exchanger and carbon vessels piping, valves, and instruments. (e) Carbon vessels. (f) Inter Carbon vessels piping. (g) Carbon vessel and ambient air discharge point. (4) Determination of the type and size of the blower. (5) Determination of the type and size of the heat exchanger. c. Comprehensive calculations. 1. Determination of the amount of carbon needed for 3 months. It is assumed that two carbon vessels in series will be used. The air from the air stripper will flow through the first vessel (the lead vessel) that adsorbs most of the organic vapors. The air phase will then flow through the second vessel (lag vessel) and be discharged to the atmosphere. When the effluent and influent concentrations are equal, the flow will be redirected first through the lag vessel. It is assumed that the second vessel contains equal amount of carbon as the first vessel. The lag vessel now functions as the lead vessel. The other vessel will contain the exhausted carbon regenerated on-line as the lag vessel. The amount of time that a carbon vessel should remain on-line is very site-specific. This design system is for 3 months between carbon changes of the lead vessel. The exact amount of carbon needed is hard to determine. In estimating the amount of activated carbon needed the method shown below was used. (a) Calculation of the Partial Pressure of Each Contaminant in the Vapor Stream. Making an assumption that the pressure of (87.6 kPa) in the carbon vessel is equal to the discharge pressure, if pressure being used differs from the actual pressure, there will be need for recalculation of partial pressure for the correct pressure in the carbon vessels (Dutta, 2002: 70). The contaminant weight adsorbed per weight of activated carbon can be projected from isothermal data supplied by the carbon manufacturer. From the given data, the contaminant adsorbed per carbon weight was derived from the Fruendlich isotherm relationship: The K and (1/n) values were obtained for one carbon type at 298 K. The values must be obtained for each carbon type being evaluated and for each temperature (see Table B-1). Table B-1 Fruendlich isothermal data b) Determination of the Weight of Each Contaminant to be adsorbed Per Unit Time. This calculation is applicable on an estimated quantity of carbon. It is always recommended by Manufacturers that calculation of the carbon needed for the three or four most prevalent constituents with an addition of a safety factor. Safety factors may be range from 20% more carbon than the mount calculated for non-regenerable systems to 100% for a very conservative design. In this design we can see that the estimated carbon total M is: (6870 kg)(2)= (13740) kg for 3 months 2. Determination of the Size of the Carbon Adsorption Vessels. (a) Estimation of the Diameter of the Carbon Vessel. The designer can use a short vessel with a larger diameter or tall vessel with a smaller diameter. The two vessels will hold the same amount of carbon. The diameter for a reasonable superficial velocity is calculated. Superficial velocity (V) is the velocity that the vapor would attain through the carbon bed that if this vessel were empty (V = Q/A, where Q is the vapor flow rate and A is the cross-sectional area of the vessel). Several carbons may be used for various superficial velocities. Manufacturers’ specifications for superficial velocities range from 2.5 cm/s to several hundred. Typical superficial velocities are 5 to 50 cm/s. superficial velocities are directly proportional to pressure drop through the vessel. This results in an increase in the cost of energy. In this design, a superficial air velocity through the carbon vessels is initially assumed to be 25 cm/s. The resulting diameter D of the vessel is calculated as follows: Combining the equation it gives: (b) Estimation of the depth of the carbon in the vessels. Where Vol = is the volume of the carbon in the vessel M = is the weight of the carbon H = is the depth of the carbon in the vessel. Combining, rearranging and estimating the carbon density to be 489 kg/m3 yields In the above calculation the carbon vessel is too deep. By lessening the superficial velocity through the carbon bed from 25 cm/s to 12.5 cm/s the diameter of the vessel becomes 3.2 m, the depth of carbon becomes 3.6 m, which is acceptable, alternatively one can use four vessels 2 ×2). 3 Calculation of the Total Pressure Drops through the Units in the Process Train. The real pressure drops must be calculated for each application thus each will give different results. a) 13 cm H2O for blower through Air Stripper, Valves, and Instruments (estimate). b) 2.5 cm of H2O for Air Stripper for Heat Exchanger Piping, Valves, and Instruments (estimate). c) 2.5 cm of H2O for Heat Exchanger (estimate). d) 2.5 cm of H2O for Heat Exchanger to Carbon Vessels Piping, Valves, and Instruments (estimate). e) Carbon Vessels. The drop in pressure in the carbon bed depends on; the type and amount of carbon, vapor velocity through the carbon bed, and the depth of the bed. For one specific carbon in manufacturers’ recommended drop in pressure through the carbon is 6 cm H2O per meter of carbon. For the 3.6-m bed of carbon which is being used, the pressure drop is 21 cm of H2O for the lead and lag vessels, for a total of 42 cm of H2O (see Figure B-1). f) 2.5 cm of H2O between Carbon Vessels (estimate). g) 2.5 cm of H2O Carbon Vessels to Ambient Air Discharge Point (estimate). 4 Determination of the Size and Type of Blower. a) Size of Blower. Design the blower to handle 1 m3/s for the above total system pressure drop. Pressure exiting from the blower is the pressure going out of the carbon units (87.6 kPa) plus the drop in pressure through the treatment train. The exit pressure from the blower is determined as shown: Blower performance curves may be obtained from the manufacturer or the design engineer may obtain the power from thermodynamic relationships as follows (Nag, 2006: 234). Substituting in the equation it gives: In the absence of manufacturers’ data, the efficiency of a combination of the blower and motor is estimated to be 40%. Thus the actual size of the motor is: b) Type of Blower. In this design high-pressure centrifugal blowers are used. 5 Determination of the Size and Type of Heat Exchanger. Relative humidity (RH) of the vapor stream entering the carbon vessels should not exceed 40 to 70%. A heat exchanger is used to raise the temperature (lower the RH) or lower the temperature (raise the RH) as needed. High RH reduces the adsorption capacity of the carbon. High temperature reduces the capacity of the carbon. A good compromise between temperature and humidity is to raise or lower the RH to about 50%. The type of heat exchanger depends on the amount of heating or cooling needed. Assume that in the air stripper the vapor stream is cooled to 289 K (the temperature of the water in the air stripper). Assume the vapor leaving the air stripper is saturated with moisture (100% RH). The temperature must be increased in the heat exchanger from 289 to 299.8 K so as to reduce the RH to 50%. The Fruendlich isothermal data of 298 K in this design is close to the 300 K which is the temperature of the vapor entering the carbon units. For large temperature difference, the contaminant adsorbed per carbon (x/m) weight must be determined. An increase in pressure causes the blower to raise the temperature of stream vapor . If the vapor is directly taken from the blower to the carbon units, there is need to reduce the temperature of the vapor that was not heated, in a heat exchanger. The estimation of the rise in temperature is hard, thus this must be obtained from manufacturer's blower literature. Figure B-1. Vapor phase activated carbon treatment train (SI units). Costs (Installation and Capital) Item cost Building, piping and valves $710,000 Equipment $250,000 Polyphosphate storage and feed system (Recommended, but optional) $25,000 Vertical Diffusion Vane $125,000 Installation cost $150,000 Administrative,Engineering, and legal $150,000 Contingency (Approx. 25%) $290,000 TOTAL COST $1,700,000 Other associated cost include Operation and Maintenance Cost Item cost Maintenance $ 11,000 Electrical $12,500 Chemicals $ 2,000 Total $25,000 Cited Work (s) Barry, Crittenden and Thomas John. Adsorption Technology and Design. Melbourne: Butterworth-Heinemann, 1998. Dutta, S. Environmental Treatment Technologies for Hazardous and Medical Wastes ... New Delhi: Tata Mcraw-Hill, 2002. Kennes, C and M C Viega. Bioreactors for Waste Gas Treatment. Netherlands: Kluwer Academic publishers, 2001. Marve, Hyman and Dupont Ryan . Groundwater and soil remediation: process design and cost estimating of proven technologies. Danver MA: ASCE Press, 2001. Nag, P.K. Engineering thermodynamics. New Delhi: Tata McGraw-Hill , 2006. Read More