Xavier University – Ateneo de Cagayan ACTIVITY #2 Binary Batch Distillation in a Bubble Cap Column January 30, 2023 Submitted as a Partial Fulfillment of the Requirements for the ChE 47 Chemical Engineering Laboratory 2 Course AY 2022-2023 Czarine N. Maderal – 20190016915 Laboratory Report Writer Catherine Kate E. Villarin – 20130003578 Angel Louisse T. Rabuyo - 20180013733 Daniel David V. Harris – 20190017419 Don Kim S. Lapeña – 20170011651 Laboratory Report co-writers Dr. Shierlyn S. Paclijan ChE 47 Instructor February 6, 2023 ABSTRACT Distillation is one of the most commonly known unit operations in which a liquid mixture is heated to cause phase separation of components due to their differing volatilities as represented by boiling point. In this experiment, the researchers set out to separate a binary 40% ethanolwater solution by means of a batch distillation process in a bubble cap distillation column. This was done in order to analyze the samples of the distillate and bottoms products at intervals using a refractometer to get their respective refractive index, and subsequently, molar composition. Operating the distillation at total reflux conditions and constant reflux conditions were then done to see their effects in the variation of the composition of ethanol over time. The experiment was carried out successfully of which the results showed that for total reflux conditions, the refractive index at first collection was 1.3529, translating to a mole composition of ethanol of 14.01%. At 30 minutes of distillation, the refractive index returned was 1.3579, or 23.16 mol% ethanol, meaning a 65.3% increase of ethanol in the distillate. Meanwhile, at constant reflux conditions, initial refractive index was 1.3612 (33.41 mol% ethanol), while after 20 minutes of distillation, the refractive index was 1.3615 ( 34.99 mol% ethanol), giving a 4.73% increase. It was seen ethanol composition would fluctuate over time but with an increasing trend for longer distillation time for total reflux, while at constant reflux, ethanol composition has decreased over time. With that, there would be a need to gradually increase the reflux ratio to counteract the effect of decreasing product purity in the distillate for better ethanol recovery. 2 INTRODUCTION Distillation is a well-known unit operation in which 2 or more components in a liquid solution are separated. The unit operation makes use of the principle that the component’s involved have differing volatiles, described by their known boiling points or vapor pressure, to yield separation by obtaining a volatile vapor phase and a liquid phase that vaporizes. The main requirement for separation to occur via distillation then is that the composition of the vapor be different from the composition of the liquid when at equilibrium at the boiling point of the liquid solution (Geankoplis, 2003). For this experiment, a binary distillation of an ethanol-water solution is being investigated. Here, ethanol is the desired product to be collected in the distillate. With that, when energy is supplied to the solution in the form of heat, the more volatile component (having a higher vapor pressure/lower boiling point) will vaporize into a gas. In a distillation column set-up, this rising vapor is allowed to escape from the column, thereby allowing for separation. The vapor phase which can then be condensed back into a liquid, known as the distillate (top product), contains a higher concentration of the volatile component in the original solution. This leaves behind a bottoms product of the less volatile component(s). For separation by distillation, all components are actually present in both the vapor and liquid phase, of which the effectiveness of separation is dependent upon how these components are distributed amongst the 2 phases when boiling occurs (Geankoplis, 2003). Figure 1. Basic Distillation Column Operation S3t-Up (Source: https://neutrium.net/unit-operations/distillation-fundamentals/) 3 As seen in the figure above, the rising vapor is cooled by a condensed to convert the gas back into a liquid. The condensed liquid is then held in what is known as a reflux drum. Some of the liquid is actually recycled back into the distillation column, known as reflux of the liquid. The reflux ratio, denoted as R, is defined as the molar ratio of the reflux returned to the distillation column to the amount of distillate product collected. In this experiment, batch distillation is to be conducted using a bubble cap distillation column. This is done so that the trickling down liquid of a certain composition of the different components, comes into contact with the rising vapors within the column also of different compositions, allowing for interaction of the vapor and condensed liquid. This interaction allows for better separation as it some of the less volatile component in the vapor to condense, while the more volatile component in the condensed liquid will rise again, allowing for better separation as desired. With this concept of reflux ratio having an impact on the distillation process, the different conditions of which the reflux ratio is utilized also affects how the distillation occurs. It is then important to know which reflux ratio condition is optimum for the distillation. This is because in the industry, the reflux ratio selected “affects the cost of the distillation column, both in the number of trays and the diameter, as well as the cost of operation which consists of costs of heat and cooling supply and power for the reflux pump” (Couper et al., 2010). For this experiment, the effect of varying reflux ratio conditions on the compositions of the distillate and bottoms product is studied. A distillation process is said to be in “total reflux” if all of the vapor produced during operation of the process, is then returned back to the column. The reflux ratio then is, R = ∞. Another method is in which the reflux is constant, meaning the amount of distillate returned is the same at a pre-determined value throughout the process, which gives changing composition of the distillate over time (Lopes & Song, 2010). At these varying reflux ratio conditions, analysis of how the composition of the distillate and bottoms product varies over time is done. For this experiment, analysis is done by using a refractometer to obtain the refractive index of the sample. A known relationship for the composition of an ethanol-water mixture given refractive index is then used to get the compositions of the distillate and bottoms product. In this experiment, the refractive index to ethanol-water composition relationship provided by Nowakowska (1939). The conversion from refractive index gives the composition of components 4 for a by-weight basis. The mole composition of components is of interest in this experiment so conversion of weight percent to mole percent for the binary solution is done by the simple formula: πππ π π΄ πππππ πππ π π΄ ∗ π΄π π π’ππππ 100 π π πππ’π‘πππ, %πππ (πΆππππππππ‘ π΄) = π₯ 100 πππ π π΄ πππ π π΅ + πππππ πππ π π΄ πππππ πππ π π΅ With this, the following objectives have been identified for the conducting of this experiment: 1. To prepare and separate a 40% ethanol solution by means of a batch distillation in a bubble cap column. 2. To determine and plot the composition of the distillate and bottoms products against time at total reflux ratio conditions. 3. To determine and plot the composition of the distillate and bottoms products against time at constant reflux ratio conditions 4. To determine the effects of constant reflux ratio conditions on the product composition 5 PROCEDURE This laboratory experiment investigates the principles of batch distillation using a bubble cap distillation unit to separate a mixture of ethanol and water at 40% v/v composition. Figure X below shows the equipment used for this laboratory. Figure 2. Distillation Column Setup A 40% ethanol solution sample was batch distilled at total reflux. A sample of the distillate and bottom product was analyzed for composition using a refractometer every 5 minutes. This went on for the entire 20 minutes. Following that, the same procedure was repeated for constant reflux configuration. To simplify, this experiment follows the schematic diagram as shown in figure X below. 6 Figure 3. Schematic Diagram of the Experiment 7 RESULTS AND DISCUSSION Schematic Diagram of the Distillation Unit Used The schematic diagram below depicts the distillation unit used in the experiment, as well as all the valves used to control the distillate, bottoms, and reflux streams. Figure 4. Schematic Diagram of the Distillation Unit For this setup, a feed containing 40%v ethanol and 60%v water is initially charged to a tank where heating is supplied, vapor is generated by boiling the liquid and withdrawn after entering the condenser. The batch distillation was run two times, one each for total and constant reflux. For total reflux, the reflux valve connecting the condenser and receiver is checked to be fully closed, allowing all the condensed stream to be fed back to the column as reflux. Meanwhile for constant reflux configuration, the real-time experimental runs manipulate the reflux stream, which affects the changes that occurred in it keeping the top composition product constant, the real-time experimental runs manipulate the reflux stream, which affects the changes that occurred in it. This experiment shows the effects of constant and total reflux on the top product composition. 8 Variation of Top Product (Ethanol) against Time at Total Reflux Ratio For total reflux, the amount of liquid returned to the top of the distillation column is constantly increased, stating that the distillate is collected at the distillation stages which for this setup, ethanol composition is collected from the 2nd stage. Figure X below shows the instantaneous ethanol mole fraction in the distillate over time, where the overhead composition (ethanol) increase, decrease, and further increase reaching the 30-minute mark, in which the observable time is extended for further investigation. In contrast, the bottoms composition (water) decreased with time. This phenomenon can be explained by the adjustment of the solution and the process for the first minutes of the process as well as the fluctuations in the observed temperature of the column, which could be due to the inconsistency of heat supplied to the equipment. The maximum composition of 24%mol ethanol of the overhead product is attained in the 30-minute mark as shown below. This means that more time is required to distill, resulting in higher ethanol concentrations in the distillate for a longer period because the vapor current formed inside the column is enriched in the more volatile component. Figure 5. Product Composition against time for Total Reflux The ethanol vapor condensed and returned to the column, allowing the vaporized mixture and liquid reflux to achieve equilibrium. Because the composition of the liquid in the still changes, so does the instantaneous composition of the distillate. The concentration of the ethanol product 9 increases with time as the distillation progresses at constant total reflux, while the concentration of the water product decreases with time as the solution is distilled from the bottom still. This demonstrates that ethanol and water separate as their curves move away from each other over time. Variation of Top Product (Ethanol) against Time at Constant Reflux Ratio At constant reflux, reflux is set to a predetermined value that is kept constant throughout the run. Because the composition of the pot liquid changes, the instantaneous composition of the distillate also changes. Following total reflux configuration, the column is adjusted to constant reflux to allow distillate collection in the receiver this time. The graph below shows that the overhead composition decreases and increases over time, whereas the bottom composition decreases. As the batch distillation system operates under unsteady-state conditions, this can contribute to the changes in the concentration of distillate in the receiver. Figure 6. Product Composition against time for Constant Reflux 10 Effect of Constant Reflux Ratio on Product Composition With the constant reflux operation in batch distillation, the product composition of ethanol has decreased over time. As a result, there is a need to gradually increase the reflux ratio to counteract the effect of decreasing product purity in the distillate. Higher reflux conditions allow for better ethanol recovery in this process. Because the mass transfer between the vapor and the reflux occurs when the reflux comes into contact with the vapor coming towards the upper portion of the column, the concentration of the vapor to be condensed increases as the reflux ratio increases. However, because the introduction of reflux increases the distillation time, it is necessary to evaluate the energy of the best operating condition. Error Analysis Potential sources of error that may have occurred during the experiment were caused by instrumental and human errors during the experiment. The experiment is heavily reliant on the instrumental readings of the refractometer which could produce inaccuracies specially that the instrument was not calibrated before use. Also, the refractometer was used without first cleaning the surface on which the sample was placed which could leave traces of previous sample would affecting the accuracy of the subsequent samples. In addition, there seems to be lack of notice about the regulation of the heat supplied to the distillation column since the recorded temperature from the provided thermal gun have inconsistencies over time. This could also be from the frequent opening of the boiler still where some of the heat escaped, hence decreasing the temperature in the still. 11 CONCLUSIONS AND RECOMMENDATIONS Distillation is mostly used for separation of liquid components with varying volatility such as ethanol and water as per the experiment. The experiment showed the effects on the compositions of the distillate and bottoms product at total and constant reflux. From the results and discussion, we concluded the following: 1. For total reflux, the amount of liquid returned to the top of the distillation column is constantly increased. The overhead composition (ethanol) fluctuates over time, while the bottoms composition (water) decreased with time. 2. For constant reflux, the overhead composition (ethanol) decreases and increases over time, whereas the bottom composition (water) decreases. 3. Constant reflux is not a preferred method as more time is required to distill, resulting in higher ethanol concentrations in the distillate for a longer period because the vapor current formed inside the column is enriched in the more volatile component. For the recommendations for similar experiments, we recommend the following: 1. Calibrate the refractometer properly to ensure correct data gathering. 2. Always clean the refractometer with de-ionized or distilled water and wipe with tissue to avoid incorrect readings of the composition. 3. Instead of using a thermal gun to read the temperature, invest in a thermometer that can be placed inside the still to have an accurate and precise temperature reading. 4. For additional analysis, a batch distillation at varying reflux is recommended to understand more on its effect to the production composition of the liquid. 12 LITERATURE CITED Couper, J. R., Penney, W. R., Fair, J. R., & Walas, S. M. (2010). Chemical Process Equipment: Selection and Design. Geankoplis, Christie. (2003). Transport processes and separation process principles (includes unit operations) fourth edition. Mendes Lopes, Maira & Song, Tah. (2010). Batch distillation: Better at constant or variable reflux?. Chemical Engineering and Processing: Process Intensification. 49. 1298-1304. 10.1016/j.cep.2010.09.019. Neutrium (2017, April 16). Distillation Fundamentals. Retrieved February 5, 2023, from https://neutrium.net/unit-operations/distillation-fundamentals/ Nowakowska, J. (1939). The Refractive Indices of Ethyl Alcohol and Water Mixtures. 13 NOMENCLATURE Symbol Description π· π πΏ πΏ1 πΏ2 π₯1 Distillate, mol Vapor, mol Liquid, mol Initial moles of mixture inside the still, mol Left moles in the still, mol Initial composition of liquid π₯2 Final composition of liquid 14 APPENDICES A1. Raw Data A.1.1 Empirical Data at Total Reflux Time (Minutes) 5 10 15 20 25 30 Ethanol Water Ethanol Water Ethanol Water Ethanol Water Ethanol Water Ethanol Water Bottom Product Distillate Product Refractive Index 1.3601 1.3529 1.3499 1.3557 1.3462 1.3513 1.3443 1.3539 1.3458 1.3566 1.345 1.3579 A.1.2 Empirical Data at Constant Reflux Time (Minutes) 5 10 15 20 Ethanol Water Ethanol Water Ethanol Water Ethanol Water Bottom Product Distillate Product Refractive Index 1.3442 1.3612 1.3434 1.3612 1.3428 1.3615 1.3421 1.3615 15 A2. Analysis of Data and Sample Calculations A.2.1 (a) Calculation of %mole composition at Total Reflux Bottoms At 5 mins Refractive Index (n) = 1.3601 Based on The Refractive Indices of Ethyl Alcohol and Water Mixtures by Nowakowska J. At n = 1.35940 %w/w = 48.49 and n = 53.60 %w/w= 53.60 By interpolation, % π€ 53.60 − 48.49 (πΈπ‘βππππ) = 48.49 + (1.3601 − 1.35940) ( ) = 51.14% π€ 1.36075 − 1.35940 Then, % π€ (πππ‘ππ) = 100 − 51.14 = 48.86% π€ π€ Converting % π€ to %πππ 51.14 46.07 π/πππ %πππ (π΅ − πΈπ‘βππππ) = = 29.16% 51.14 48.86 + 46.07 π/πππ 18.02 π/πππ %πππ (π΅ − πππ‘ππ) = 100 − 29.16 = 70.84% (b) Calculation of %mole composition at Total Reflux Distillate At 5 mins Refractive Index (n) = 1.3529 Based on The Refractive Indices of Ethyl Alcohol and Water Mixtures by Nowakowska J. At n = 1.35188 %w/w = 29.41 and n = 1.35630 %w/w= 38.82 By interpolation, % π€ (πΈπ‘βππππ) = 29.41% π€ Then, % Converting % π€ π€ π€ (πππ‘ππ) = 100 − 29.41 = 70.59% π€ to %πππ 16 29.41 46.07 π/πππ %πππ (π· − πΈπ‘βππππ) = = 14.01% 29.41 70.59 + 46.07 π/πππ 18.02 π/πππ %πππ (π· − πππ‘ππ) = 100 − 14.01 = 85.99% A.2.2. Total Reflux: Variation of Product Composition with Time Time (Minutes) 5 10 15 20 25 30 Ethanol Water Ethanol Water Ethanol Water Ethanol Water Ethanol Water Ethanol Water Bottom Product % mole Refractive Index 29.16 1.3601 70.84 11.82 1.3499 88.18 8.77 1.3462 91.23 7.56 1.3443 92.44 8.51 1.3458 91.49 7.99 1.345 92 Distillate Product % mole Refractive Index 14.01 1.3529 85.99 19.03 1.3557 80.97 13.36 1.3513 86.64 16.59 1.3539 83.41 20.48 1.3566 79.52 23.16 1.3579 76.84 A.2.3 Constant Reflux: Variation of Product Composition with Time Time (Minutes) 5 10 15 20 Ethanol Water Ethanol Water Ethanol Water Ethanol Water Bottom Product % mole Refractive Index 7.5 1.3442 92.5 7.36 1.3434 92.64 6.55 1.3428 93.45 6.06 1.3421 93.94 Distillate Product % mole Refractive Index 33.41 1.3612 66.59 33.41 1.3612 66.59 34.99 1.3615 65.01 34.99 1.3615 65.01 17
