CHE 415 - Rectification Formal Report 2006

Ryerson University Department of Chemical Engineering CHE 415 Unit Operations II Lab Report Experiment #1: Rectification Experiment Performed on: October 19, 2006 Report Submitted to Dr. By Group # 2 Section # 021 Group Members: 1. (Leader) 2. (Inspector) 3. (Data Reporter) Date Report Submitted: October 26, 2006. Marking Scheme Formatting Answer to all 6 questions in each Report Section / 10 General Appearance; Grammar and Spelling / 5 Complete and Informative Tables and Graphs / 15 Contents Accuracy and Precision of Results / 20 Comparison with Literature Data / 10 Influence of Procedural Design on Results / 10 Logic of Arguments / 20 Sample Calculations / 10 _____ Total: / 100 Table of Contents 11 22 55 66 77 99 1010 1111 1212 1212 1212 12Table of Figures 66 88 8 Introduction The objective of this experiment is to compare the theoretical equilibrium stages to the number of actual stages in a distillation unit. Also, the experiment will investigate the following aspects of distillation: Boilup rate, Maximum Overall Separation Efficiency, and Thermal Efficiency. Rectification is the process of purifying a substance by distillation. Distillation separates chemicals by the difference in their vapour phases. The two major types of distillation are continuous distillation and batch distillation. For this experiment, a continuous multistage distillation (fractionation) will be performed using a methanol-water binary system. The column will be operated at 1 atmosphere and total reflux. Total reflux allows all the distillate to return to the column after condensing. In this experiment, a methanol-water solution was used as feed in the reboiler. The pilot-scale glass distillation column consisted of six separation bubble cap trays. When the equilibrium was reached in the system, the distillate was retrieved at total reflux and alternative reflux ratios. It was determined based on the results that the overall separation efficiency of the distillation column was about 116.7%. The thermal efficiency was found to be about 75% as well, and the boilup rate was 1.56 g/s when the dial of the boiler was turned to 50% using all 4 heating coils. Based on the McCabe-Thiele method, the experimental number of equilibrium stages was determined to be 7 compared to 6 theoretical stages in the distillation unit. This gave an experimental error of 16.7%. Theoretical Background In this experiment, a large distillation unit was used. Distillation is a method of separating a feed mixture of two or more components into two or more products. For this experiment, a liquid-liquid mixture was used, but also a liquid-vapor mixture could be used also. Distillation, a mass transfer operation, is a form of separation and requires a second phase to be formed so that both the liquid and vapor phases of the components are present and in contact with each other on each stage of a distillation column. The components of the binary mixture also have different volatilities so that separately concentrate between the two phases individually. In general, distillation is based on the differences in boiling points of the components of a solution. For continuous distillation or fractionation, a multistage, countercurrent distillation column is used. In this process, the components in a binary mixture can be separated individually and recovered. The feed itself is composed of a more volatile component, light key (LK), and a less volatile component, heavy key (HK) (Benitez, 2002). For distillation, the number of equilibrium stages and amount of reflux required for a desired degree of separation of the feed can be determined with a useful technique called the McCabe and Thiele Method. It is a graphical method that combines the equilibrium curve for a binary system with the operating line curves for the rectifying and stripping sections of a distillation column. This method is extremely useful in its ability to show the many important aspects of a multistage distillation operation. For the McCabe and Thiele method the liquid-vapour equilibrium curve is needed. This curve is a plot of vapour mole fraction, y, versus liquid mole fraction, x. Reflux is the vapours from the top of the distillation column that are condensed in the condenser by cool water to provide contacting liquid. The equation is as follows: (1) Where R is the reflux ratio, Lo is the external reflux rate and D is the distillate rate. If the reflux ratio is increased, this means the slope of the rectifying section operating line would increase from L/V <1 to a limiting value of L/V =1 (Seader, 2002). In addition to being able to determine the equilibrium stages and reflux of a distillation column, other aspects of the operation can also be investigated: boilup ratio, thermal efficiency, and the maximum overall separation efficiency. The boilup ratio is defined as the ratio of vapour rate (boilup) to the bottoms product rate. The vapour rate (boilup) leaving the reboiler can be calculated using the following equation, (2) Where V is the volume of distillate collected at time t, and rdistillate is the density of the distillate. The thermal efficiency of a distillation column can be determined by the following equation: (3) Where, ?TH is the thermal efficiency coefficient, Qdistillate is heat supplied to the distillate, and Qreboiler is the heat supplied to the reboiler. To determine the heat supplied to the distillate, the following equation is used: (4) Where xA and xB are the liquid mole fractions within the distillate and, ?A and ?B are the heat of vaporization. The other aspect of distillation that can be investigated is the overall separation efficiency which is determined by the following equation: (5) Where, Ntheoretical is the number of theoretical plates, and Nactual is the number of the actually plates in the experiment. Experimental Method In this experiment, a large pilot-scale glass distillation column was used. Before operation, all valves and electrical equipment were closed and shutoff. For this experiment, a feed methanol-water solution was already made up and inside the pot above the reboiler. The density of this feed solution was taken and recorded. Before the solution is heated, the cooling water was turned on to be sent to through the condensers. To start up the apparatus, the main switch was turned on and all the corresponding heating coils (4 in total) of the boiler were switched on. In addition, the voltage to the boiler was also ramped up to the max (100%) to quicken boiling the solution. As the solution started to bubble, the heat supplied was slowly decreases either by reducing the amount of heating coils in operation or reducing the boiler voltage. For this experiment, all heating coils were in use throughout the experiment, but the boiler voltage was reduced to around 60%. The system was allowed to reach equilibrium which was also at total reflux. This point was determined to be when initial liquid that accumulated on the condensers started to return back to the column unto the trays. Once equilibrium was reached a volume of the distillate was collected for 30 seconds and its density was measured. After the experiment was completed, all electrical switches to the boiler were turned off, but the cooling water that supplied the condensers were left on for a short period after shutdown because to the unit was still very hot. Experimental Apparatus Figure 1: Pilot-scale Glass Distillation Column Results and Discussion In this rectification experiment, important parameters such as boilup ratio, overall separation efficiency and the thermal efficiency were investigated using a binary mixture of methanol and water. To heat the mixture, a boiler with 4 circuits having 2 kW power rating of each circuit was provided. This boiler has a dial from 0 – 100 that indicate the percentage of the boiler power rating that is in used, and a boiler power rating is used to determine the heat supplied from the boiler (Q1). Then, a sample of the distillate was taken on the top of the tower and its temperature was measured by thermometer as well as its composition by a Pycnometer (density). Before any distillate sample was taken, it was observed that a little entrainment (flooding) occurred, or in other words, the heat supply coming from the boiler was a little bit too high so that there was a little flooding in between the columns, especially in the column next to reboiler, but throughout the experiment, the heat supply was kept constant so temperature profile of the column was assumed to be in steady state. After the calculation was performed, the boilup rate was found to be 1.56 g/s when the dial of the boiler was turned to #50 using all 4 coils (4 kJ/s). This indicates that there was 1.56 g of methanol being evaporated and sent to the top of the distillation column every second. Also, the experimental separation efficiency of the column was found to be in 7 stages. To determine this number, firstly the plot of liquid-vapour equilibrium graph was made and a 45o degree line was also plotted across the diagram. The liquid mole fraction in the distillate, Xd, was found to be about 0.95 when the condenser temperature was found to be 66.68 oC and this is our starting point. Temperature-composition correlation graph can be found in Physical Sciences Data 37 Vapour-Liquid Equilibrium Data by Shuzo Ohe. After that, the operating line needed to be constructed. The starting point of the operating line was determined by extending a vertical line from the Xd until it touches the 45o line. The end point of the starting point was found by dividing the Xd by R+1. Therefore, the number of experimental stages was found by making as many right angle triangles as possible in cascade motion in the area bounded by the equilibrium curve and the operating line as was shown by Figure 2 below: Figure 2: VLE Curve for a Methanol-Water Binary System Meanwhile the theoretical stage determination was done by visually counting how many columns that exists in the tower itself. By comparing the theoretical and experimental separation stages, it was found that overall separation efficiency is about 116.7%. This number is reasonable considering the calculation yields 1 stage off; in other words, the separation efficiency number states that it would take 7 stages to separate the amount of methanol, which was found from the distillate composition, from the water within a specific reflux ratio. The reflux ratio that was chosen in this experiment is the total reflux (R=1). The last point that was under investigation was the thermal efficiency of the column. The thermal efficiency is the ratio of heat that is needed to produce distillate and the heat supplied from the boiler. By using the parameters of the process of the experiment such as turning the boiler heat rate to 50, it was found that heat of the process was 2.985 kJ/s where the heat supplied from the boiler was 4 kJ/s and from these two numbers, the thermal efficiency was found to be about 75%. It was suspected that the remaining 25% of heat was lost due to mechanical limitation of the boiler, heat lost to surrounding and the small entrainment that occurred during separation process. Error Analysis One notable source of error in this experiment was the occurrence of the entrainment. This entrainment possibly was due to the fact that the boiler was slow in heating the mixture to boiling; therefore, in the beginning of the experiment, the boiler rating was raised to the maximum to encourage quicker boiling of the mixture. After some mist was observed on the boiler column, the heat supply was toned down to 50% setting where apparently, excessive boiling did occur and hence, a little entrainment occurred. However, in order to maintain good temperature profile and to avoid sharp temperature fluctuation, the boiler rate was not changed. Conclusion and Recommendations This experiment was performed in order to determine the three important variables of a distillation column. These aspects are the boilup rate, the maximum overall separation efficiency and the thermal efficiency. And based on the results, it can be concluded that the experiment was quite successful, the rectification column performance was quite dependable and also the temperature and heat supply setting was closed to where it should be to produce 100% separation efficiency. A recommendation for this experiment would be to inspect and conduct a maintenance check on the boiler power rating system to enhance the thermal efficiency of the distillation column, and would also reduce experimental time needed to for the solution to reach a boiling. The method to determine the reflux ratio could also be modified so that the effect of changing the reflux ratio on the composition of the distillate can be performed. References Benitez, Jaime. Principles and Modern Applications of Mass Transfer Operations, John Wiley & Sons Inc., New York, 2002. p. 300 Seader, J.D. and Henley, Ernest J. Separation Process Principles, 2nd Edition, John Wiley & Sons, New York, 2006. p. 252, 255-259 Appendix Raw Data from Experiment Mass of starting solution + pycnometer (g) = 39.4 Mass of Pycnometer (g) = 15.2 Heat rating (dial position of the boiler) = 50 Temperature Data (thermocouple, in oC) minutes #1 (Condenser) #2 (sixth column) #3 (third column) #4 (reboiler) 5 153.7 151.7 171.6 190 10 151.9 152.5 163.9 191.4 15 150.5 153.6 170.1 191 20 153.5 151.5 170.3 190.5 25 152.6 150.7 167.4 190.2 30 151.5 150.7 169.5 190.4 Mass of distillate obtained (gr) = 64.62, 131, 33.5 (1st, 2nd,3rd trial) Volume of distillate obtained (ml) = 80, 56, 42 Time needed to obtain distillate (s) = 45, 25, 35 Mass of pycnometer (gr) = 15.2 Mass of residual (gr) = 35.395, 34.872, 34,956 Sample Calculations For Boil up ratio: Total reflux ratio was employed for all the calculations due to the setup of the system, where the entire overhead vapor is being condensed and is then returned to the top stage. Therefore, Assuming total reflux for the rectification process (L/V =1): Thus, B = 0.878 gr/ml * 80 ml / 45sec = 1.56 gr/s Where d is the density of solution, V is the volume of the solution and t is the amount of time taken to collect the volume of the solution, B is the boil up ratio, L is the moles of liquid in the still, D is the distillate flow rate from still, and R is the reflux ratio In this experiment, 3 data of boilup rate was taken by varying the heat rate supplied by the reboiler. For Maximum overall separation efficiency: From the vapor-liquid equilibrium curve for methanol and water in the preceding page, the mole fraction of methanol in the distillate xdistillate, can be found from the temperature in the condenser. The temperature in the condenser of this experiment was 66.68 0C, so xdistillate = 0.95 From this data we will be able to determine the y intercept of the rectifying line using: xdistallate / (R + 1) = 0.95 / (1 + 1) =0.475 The number of theoretical stages can be determined by plotting these 2 data points on an x-y diagram along with the 450 line. # Stages = 8 – 1 (from reboiler) = 7 stages Efficiency = (number theoretical plates / actual number plates) * 100 = (7/6) * 100 = 116.67% Thermal Efficiency, ?, Calculations Again, to inquire the vapor-liquid mole fraction of the methanol in the reboiler, the above graph was used. This time, the temperature reading used is 88.056 oC, which points at xm,4 = 0.1 and ym,4 = 0.417. If xm,4 = 0.1, then liquid mole fraction of water, xw,4 at that temperature is 0.9. xM,4 = 0.1 and xW,4 = 0.9 w = 40.66 Kj/mol = 2.257 Kj/gr Similarly for methanol: m = 35.27 Kj/mol = 1.101 Kj/gr Therefore, Q4 = 1.394 gr/s * (0.1*1.101+0.9*2.257) Kj/gr = 2.985 Kj/s For Q1 = Heat supplied for the reboiler = heat rate from the heater The heat boiler dial was placed on 50 and all 4 circuits were used, thus: Q1 = 0.5*4*2 kW = 4 kW = 4 kJ/s = Q4/Q1 * 100% = 2.985/ 4 *100% = 74.625 % (i) (i) (i) (i) (i) (i) (i) (i)