Chemical Engineering Design of an Operation Unit - Essay Example

Table of Contents Introduction 3 Material for the Operation Unit 4 Adsorbents Requirements 4 Capacity and Purity of CO2 5 Choice of Technology (Adsorption) 5 Operation Mode 6 Adsorbent Specification 6 Designing the Operation unit and the Processes 6 Pressure Aspects 8 Steps in the Adsorption Process 9 Conclusion 11 Bibliography 12 APPENDICES 17 Table of Figures Table 1: Isothermal data 17 Table 2: Technical Data about the Operation Unit 19 Figure 1: The Temperature Swing Adsorption Unit 8 Figure 2: Schematic of the CO2 Adsorption Operation Unit 10 Figure 3: Pressure Equalization using Two Modes 10 Chemical Engineering design of an operation unit Introduction Over the past, adsorption processes have increasingly gained commercial acceptance as effective and energy-saving separation techniques. The process begins with designing and setting up an operation unit for the separation process. After a stipulated start-up time, the unit or system attains a cyclic and steady state in which all conditions, both at the beginning and at the end of each cycle end up being identical. Adsorption processes work under the application of a determination process, which is direct, using a method that is Newton-based. This happens with accurate sensitivity in order to achieve quick as well as robust cyclic study state coverage. In chemical engineering, unit operation is the basic step in any separation process. Unit operation involves initiation of physical changes such as evaporation, crystallization, filtration, and separation among other physical processes. This paper involves a design of one operation unit for CO2 adsorption from flue gas of power plant. The adsorption process would make use of activated carbon, a source mainly from fuel substances in the mining industry, and it will be based on physical adsorption mechanism. Physical adsorption involves electrostatic attractive interaction of opposite charges and the idea of weak Van der Waals forces. It is chosen over chemisorption process because it uses lower enthalpy of adsorption, which is between 8-20 kJ mol-1 against 40-800 kJ mol-1 for chemisorptions. While activation energy in chemisorptions is small, it is zero for the physical process. Another factor considered is the low temperature of occurrence in physical adsorption, which depends on boiling point. Physical adsorption also allows the application of more than one layer that is adsorbed while chemisorption allows the chance of only one layer. Flue gas can be obtained from the combustion of a petroleum fuel such kerosene. The capture of carbon dioxide from power stations has increasingly become a key issue in research. CO2 separation is a benefit to the chemical industry, where CO2 is used for many purposes such as the production of methanol, urea, metal bicarbonates and carbonates. The concern on the production of CO2 is enhanced by the issue of global warming. Various capture approaches are typically applicable (Hicks et al. 2008). These approaches include membranes, cryogenic, absorption, and adsorption among other approaches. In this paper, adsorption process is used as the preferred approach in the gas separation process. Adsorption process of carbon dioxide gas from flue gas of power plant is critical for various reasons. Carbon dioxide typically affects power consumption. To establish an effective design for the operation unit suitable for this separation, experimental work is important in building a three-bed adsorption plant for carbon dioxide extraction from flue gas of a power plant (Arenillas et al. 2005). Aspects such as temperature, feed concentration, and evacuation pressure are considered. A step-by-step cycle is considered to depict the way each aspect influences performance of the adsorption process (Arenillas et al. 2005). In the adsorption process of separating CO2 from flue gas of a power plant, CO2 capture is developed and fully employed. Through both experiments and effective analysis, it can be established that feed gas temperatures, feed concentration, and evacuation pressure influence the capture cost and power consumption. The best results could be obtained mainly with 40°C of feed gas under a relatively deep vacuum condition. Feeding higher concentration of feed gas to the plant is known to enhance performance according to Chang et al. (2003). The choice of power plant as the carbon dioxide is based on the fact that fossil fuel plants contribute to over 30 per cent of all human practices contributing to global warming (Rinker, Ashour & Sandall 2000). It is thus important to come up with an effective way of capturing and separating carbon dioxide flue gas, which is a post-combustion affluent. This separation could be an effective way of preventing carbon dioxide release to the atmosphere (Himeno et al. 2007). Material for the Operation Unit Given that a physical mechanism is used for the adsorption process, activated carbon would be the main requirement for the project. Activated carbon is preferred due to its usage over the past. Besides, it has gained significant commercial applications. Another key requirement would be power supply to run the plant by supporting all chemical and physical processes. Flue gas is another requirement in this case. It can be obtained from sources like the combustion of a flue such as kerosene. Some packing material for the process include rasching ring, pro-pack, and intalox saddle, and they are packed in the absorber. Most of the required materials are obtained from the mining industry where the CO2 constituents can be established. They are dissolved in an HCl solution with a concentration of 37 per cent at room temperature. HCl solution and Tetraethyl orthosilicate are also part of the requirements (Zhao, Chen & Zhao 2009). Besides, adequate and steady power supply is required to maintain the required levels of pressure and temperature. Adsorbents Requirements In order to obtain good results, the adsorbents should be having high selectivity. High selectivity would allow sharp separations in terms of surface and pore shape or size. High capacity of the adsorbents is required for minimizing the amount of sorbent used. Again, there should be favorable kinetic and transport properties to allow rapid adsorption (Lecture14 2013). In order to preserve the sorbent amount and its properties, there need to be thermal and chemical stabilities. This should include low solubility within the contacting fluid. Mechanical strength and hardness are required to avoid crushing and erosion aspects. Resistance to both poison and fouling should be ensured to achieve long life. Again, a free-flowing tendency to ease the emptying or filling of vessels should be ensured (Lecture 14 2013). There should be hardly any tendency for promoting undesirable reactions. Besides, all these requirements should be facilitated with cost considerations. Capacity and Purity of CO2 The adsorption technology is initiated to yield the purest CO2 product possible. The adsorption technology aims to achieve a CO2 purity of above 99%, with the target of the final product being 99.5%. This purity level is achievable only with application of the correct procedures. The use of a modified thermo-gravimetric analysis is initiated in order to determine the adsorption process as well as the adsorption properties of the used adsorbents. This determination could be done with the use of H2O saturator. For the adsorbent, 10 mg is place within a sample cell and then heated to a temperature of 373 K (Hiyoshi, Yogo & Yashima 2005). The heating is done under the flow of N2 at the rate of 50 ml per min. The flow is maintained at a temperature of 373 K for 30 minutes. This is done until there is hardly any weight loss observed. A subsequent cooling of the sample is then done until the cooling reaches the 348 K. At least 15 per cent of dry CO2 is introduced at the rate of 25 ml per minute (1000). The gas is then switched to nitrogen flow at the rate of 50 ml per minute (Hiyoshi, Yogo & Yashima, 2005). This is done to advance the adsorption process at a constant temperature. Each adsorption cycle requires at least two hours, equivalent to 120 minutes. In this technology, maximizing on the purity of the product is enhanced by determining the accuracy and sensitivity of the thermo-gravimetric analysis. The accuracy and sensitivity are 0.1 per cent and 10 μg respectively. The influence of CO2 adsorption capacity by moisture is likely to occur, which implies that investigation has to be initiated with cyclic measurements in order to access the adsorbents’ stability (Hiyoshi, Yogo & Yashima 2005). The experiment requirements and technical data for the operation unit are tabulated in table 1 and table 2 below respectively. Choice of Technology (Adsorption) The technology chosen in this case is adsorption. The choice of this technology is based on resource availability and ease of application as compared to the material membrane. In the case of adsorption process, a special vessel is chosen for the purpose of loading air with water in order to get wet air (Reynhardt et al. 2005). Wet air is then combined with the prevailing dry air as well as the start concentration. The results could be measures with the use of hygrometer particularly at the adsorber inlet (Hook 1997). Adsorption process occurs within the adsorption tower. In this case, absorption of the moisture occurs at the adsorbans. Measurements such as concentration against time are determined mainly for various loadings. The extension operation makes it possible to carry out the adsorption process with flue gas (Siriwardance, Shen & Fisher 2005). With the use of this technology, flue gas is tested among other adsorbans. Again, the determination of isothermal adsorption lines breakdown curves is done at a constant temperature. This curve determination is done for various temperature loadings (Hook 1997). Operation Mode For this operation unit, a relatively low power plant is used which require a maximum power supply of between 208 and 250 volts. The capacity of carbon dioxide from the power plant is estimated at 1500 liter per hour as per the provided gas flow rate of 25 liters per minute. This occurs under a number of assumptions. One of the assumptions is that the adsorbent has 90 per cent of CO2 working capacity (Siriwardance, Shen & Fisher 2005). Again, 8 percent is considered to be significantly low. The adsorption cycle time for the temperature swing adsorption is two hours of Bed utilization, which results to 90 per cent of the product (Zhao et al. 1998). These assumptions bring about the calculation of the required sorbent. Adsorbent Specifications Activated carbon is chosen for this adsorption process. Adsorption vessel and the carbon storage vessels is pressurized, heated, or put under vacuum. The weight of contaminant to be adsorbed per weight can be absorbed (Erskine & Schuliger 1971). This is based on estimated isothermal data that is supplied by literature or carbon manufacturer. The choice of the activated carbon is based on isotherm aspects, run time between carbon changes, and steady and cheaper carbon source among other aspects. The amount of carbon to be used for the runtime period of 150 days is determined. For this operation unit, two carbon vessels are used in series as indicated in figure 1 above. The flue gas flows from the first vessel to the second vessel and be discharged (Valenzuela & Myers1989). The flow is directed to the lag vessel after the concentration of the effluent equals that of the influent. The two vessels have equal amounts of carbon. Exhausted carbon is regenerated and put back in the lag vessel. Because it is difficult to know the contaminants in carbon, the exact amount of carbon required may be difficult to establish (Valenzuela & Myers1989). Carbon contaminants are Perchloroethylene (PCE), Trichloroethylene (TCE), Benzene, and Toluene. The amount of carbon required can be calculated from the following equation with respect to the isothermal data in table 1(Appendix I), whose values are predicted from Fruendlich isotherm relationship (U.S. Army Corps of Engineers 2001). From the isothermal data, the weight of each contaminant can be established in order to establish the amount of carbon required for the runtime period (see Appendix I). Designing the Operation unit and the Processes In the course of designing the operation unit, aspects like the diameter and depth are considered. The diameter of the tower depends on aspects like liquid flow and vapor. The diameter determination is driven by vapor loads, but flow rate is mostly critical in the case of acid absorption (Himeno, Komatsu & Fujita 2005). Given that CO2 is an acidic gas, flow rate is a consideration for the unit dimensions. This aspect is also considered given a case where the product specification is required. Designing the Operation Unit starts from the vessel choice, in which aspects related to its dimensions are considered as well as adsorbent specifications. In a typical operation unit, since the technology makes use of temperature swing adsorption, CO2 is adsorbed at a temperature range of 10°C to 60°C. Regeneration is generally conducted at a temperature greater than 100°C (Khatri et al. 2006). Since with large beds much longer time is taken to heat up due to the regeneration process that uses steam and similarly long time in cooling down, the size of the apparatus has to be considered (Pulido et al. 2009). The rate of carbon usage is evaluated against aspects like initial costs for the larger units as well as the high maintenance and operation cost associated with smaller units. When determining the size of the vessel a bed expansion of between 20% to 50% should be considered to allow backwashing (Reynhardt et al. 2005). The design can have a short vessel with large diameter or it could have a small diameter, but vast in length. The two types would hold the same amount. Air is used in the process of cooling down (adsorption process) due to possible heat transfer limitations. The steam, which is used for the heating process, is also used to attack some sorbents. This is allowed mainly given that the steam could condense and collect on the surface of the attacked sorbent. Mass transfer as well as diffusion entail a relatively rate limiting step, which require large beds for CO2 adsorption (Zheng et al. 2005). This occurs due to the high volume of CO2 requiring adsorption as well as large granules for the two beds. Another key concern in the operation unit is pressure drop across the two beds. Designing the operation unit would require an estimation of its dimensions such as the volume, diameter, and depth. The calculations are possibly formulated from some given information (see Appendix III). From table 2 in Appendix IV, which gives a summary of the calculations, it is possible to design the TSA unit as shown in figure 1 below. Figure 1: The Temperature Swing Adsorption Unit Pressure Aspects Pressure drop across the adsorption beds is highly affected by the activated carbon’s particle size. The particle size also affect the diffusion mechanism. These aspects implies that the particle size of the activated carbon affect the entire adsorption process. Typically, pressure drop is inversely proportional to the particle size. Pressure drop is also affected by piping configuration, contact time, as well as surface loading rate (Valenzuela & Myers1989). Pressure safeguard like rupture disks are used with corrosion allowance being used because wet carbon is very corrosive. The partial pressure for the operation unit is critical in this case. In calculating the partial pressure, it is assumed that the carbon vessel pressure is equal to the discharge pressure. Taking discharge pressure to be 12.7 psia, the partial pressure can be calculated as follows (Valenzuela & Myers1989). For the partial pressure calculations, see Appendix V. Steps in the Adsorption Process Adsorption of CO2 takes place in a determined process. The operation mode for the adsorption process involve four steps in which various processes occur. In designing the operation unit, the application of two beds is considered. The operation unit in this case operates mainly in a four step cycle. The Steps The four steps of the cycle are explained below: Step 1 In this step, Bed-1 is set at a high pressure. Bed-2 is conversely at a relatively low pressure. The feed to be used is first brought to Bed-1 like in the case of distillation (Xu, 2005). This is done at some point along the bed. A portion of the gas obtained from bed-1 is directed to Bed-2 for recycling. The rest of this gas is considered to be the raffinate product. Some of the gas from Bed-2 is directed to Bed-1 for recycling as well. The rest is considered to be the extracted CO2 product (Zelenák et al. 2008). Step 2 In this step, pressure levels within the two beds are made equal. The equalization of the pressures is done through a connection of the top ends or even the bottom ends of the two beds. Bed-2 is then pressurized to a high pressure with the gas drawn from Bed-1 (Yue et al. 2008). Step 3 In this step, the feed is directed to Bed-2. The end streams are then recycles like in the case of Step 1. Step 4 In this step, the same setting as in step 2 is initiated. This involves the equalization of the two pressures. The entire process could be presented in figure 2 below: Figure 2: Schematic of the CO2 Adsorption Operation Unit Steps 1 to 4 are adequate to complete one cycle. Within the same process, the feed may be introduced to the low pressure bed. This could be initiated in place of the high pressure bed. From the same process, a mathematical model could be developed as a way of stimulating all the four steps of the operation unit. The model would provide a way of assessing the unit performance for the CO2 adsorption and recovery from flue gas (Xu et al. 2002). In such a case, flue gas in the power plant is considered to be an only a binary mixture of nitrogen and carbon dioxide. This happens because the amount of nitrogen adsorbed within the bed is relatively small as compared to the amount of carbon dioxide (Li &Tezel 2007). Nitrogen is thus considered to be a non-adsorbing component. Again, the process’s theoretical energy, which is required, is calculated. The calculation takes the theoretical energy to be the sum of all energies required to recycle the gas from the lower pressure bed to high pressure bed. It is also considered from the aspect of pressurizing any bed from low pressure to high pressure as well as evacuating one bed from high pressure to low pressure (Lee, Keener & Yang 2009). The simulation results are presented in figure 3 below. Figure 3: Pressure Equalization using Two Modes In figure 3 above, pressure equalization as well as pressure resetting is shown for the operation unit. The pressure equalization could be done from the carbon dioxide rich end or from the nitrogen rich end. The CO2 rich end is represented by mode-1 while the N2 rich end is represented by mode-2 (Walton, Abney & LeVan 2006). Conclusion In the process of conducting the adsorption process, material and requirement consideration is first done. The materials need to be suitable and economical depending on the aspired product quantity and purity. Aspects like power supply, the course and availability of the flue gas, activated carbon, HCl, and any other requirements need to be established first. This would not only ease the process, but would also enhance accuracy and the purity of the final product. Planning is also good for economical reasons. More importantly, suitable adsorbents for removing CO2 from flue gas should have a number of attributes. One of the properties is high capacity for CO2 adsorption. The high capacity is suitable for screening new adsorbents. In this case, considering high skills or adequate knowledge for the adsorption equilibrium is essential. The knowledge helps in the evaluation of potential adsorbents. Another key attribute is fast kinetics since adsorption kinetics influences the working adsorption within the dynamic processes like fix bed column adsorption. The third attribute to consider is high CO2 selectivity (Morris and Wheatley 2008). This attribute is critical given that it directly affects the purity degree of the product, which further affects the economy of the entire process. It is also important to consider the regeneration mild conditions. This would ensure the ease of the used adsorbent due to a meaningful selection of the right material for the CO2 separation. 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Res., 44, pp.3099-3105. APPENDICES Appendix I Table 1: Isothermal data (U.S. Army Corps of Engineers 2001) The Contaminants Temperature K C (kPa) 1/n x/m PCE 298 K 1.0 1.3×10–3 0.144 0.384 TCE 298 K 0.95 1.2×10–3 0.263 0.162 Benzene 298 K 0.388 0.79×10–3 0.131 0.152 Toluene 298 K 0.565 0.44×10–4 0.111 0.240 Table 2: Shows the isothermal data for activated carbon. The K and 1/n values are obtained at a temperature of 77. Appendix II Estimation of Carbon amount for the Runtime Period An application of safety factor of 100% to the total amount is two times the calculated amount (DEPARTMENT OF THE ARMY, U.S. Army Corps of Engineers 2001). Appendix III Estimating the Column Diameter, Depth, and Volume Useful Information Flow rate = 1 m3/s. Temperature of vapor stream = 305 K. Run time between carbon changes = 5 months/vessel. Number of carbon vessels = two (in series). Atmospheric pressure = 87.6 kPa Temperature of air phase = 289 K. Adsorbent contaminants and the relative concentrations are Perchloroethylene (PCE) =15 ppmv. Trichloroethylene (TCE) =14 ppmv Benzene = 9 ppmv. Toluene = 5 ppmv. Diameter Estimation A = Q/V Where, A = Vessel cross-sectional area Q = Vapor flow rate V = Superficial velocity (Assumed to be 25 cm/s) Thus,  D =  Depth Estimate H = depth of carbon in the vessel The Vessel volume (Valenzuela & Myers1989) Volume is given by: Thus, volume of each empty vessel is 28.385m3 Appendix IV Table 2: Technical Data about the Operation Unit Variable Description Carbon source Carbon Mining Company CO2 Purity required 99.5% Column Diameter 2.24m Column Bed Area 3.942 m2 Column Volume 28.385 m3 Flow Rate 60 m3 /min Superficial Velocity Rate 25cm/s Bed Depth 7.2 m EBCT (each column) 2 min Weight of Activated carbon per Column per day 76.33 kg Run Time 150 days Appendix V Partial Pressure Estimation Appendix VI Read More