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Discussion Mass Exp 2

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  DISCUSSION The Bubble Cap Distillation Process designed for the separation of the components in a liquid mixture of volatile constituents. The liquid in a stage is flow to the stage below, while the vapor will flow upward to the stage above. Distillation process is a s is based on the fact that the vapor of a boiling mixture will be richer in the components that have lower boiling points. Ethanol is the more volatile component and therefore has a lower boiling point than water. When this vapor is cooled and condensed, the condensate will contain more volatile components. At the same time, the srcinal mixture will contain more of the less volatile material. Distillation columns are  been designed to achieve this separation efficiently. An important process variable is the reflux ratio at the top of the column. This plays an essential role in controlling the product purity and in determining the energy which is required in the reboiler. Also of importance is the condition of the feed stream entering the column. Batch distillations are also used commercially and are easier to analyse and control. In these processes, the feed is placed in the still at the beginning of the run and product is taken off the condenser as a function of time. Before start the experiment, a calibration curve 5% of etahanol-water solution were prepared in a 21 test tube. By using a refractometer ,the refractive index were obtained. As from result from table 1, the refractive index is increasing when the volume of ethanol increasing and the volume of water decreasing. Then, using the refractive index versus % concentration, the graph were  plotted. After that, 27 liters of distilled water and 3 liters of ethanol were prepared by mixed the appropiate quantities of the industries grade of ethanol in distilled water. Then, the heater controller were set about 230V and allow a period of 15 minutes. Next, under the operation under reflux ratio conditon, adjust the reflux ratio of 1.0 and the reboiler temperature was set up at 909 ºC which is the heater power adjust to 200V to obtain a sample of ethanol  –  water mixture from the reboiler through the sampling port and the composition was tested by using refractometer. The mole fraction were determined by the graph of refractive index versus mole fraction of ethanol in liquid. In this experiment, the samples were collected every 5 minutes from the reboiler and the overhead from the sampling port. Before that, make sure the flow rate and the temperature readings at the top and bottom of the column are steady. In this experiment, it carried out two different reflux ratio which are 1.0 and 2.0 as shown on the table 2(a) and 2(b) respectively.   For this experiment,the ethanol is the top product which has low boiling point and high volatility while water is the bottom product which has high boiling point and low volatility. From table 2(a), the result for reflux ratio 1.0 the first values from the refrective index at the top  products is 1.358 and the bottom product is 1.340. After that wait for 5 minutes and the time required to achieve the constant values is 20 minutes. The constant value after 20 minute is 1.354 at the top product and the bottom product is 1.333. The mole fraction get from the graph of composition vs refractive index and do the mass balance for the top product is 0.350mol and the  bottom product is 0.05mol. The initial feed fraction, X f   was maintained at 0.1mol and the waste fraction, X w  of 0.007mol. By using the fraction obtained, the rectification was given by the equation of   Y   n   =0.5X   n  +0.175 with the distillate mol fraction. From the graph plotted by using Mc-Cabe Method the sketch the number of stages is 1 with the plate is 0. The second experiment was conducted at 2.0 of reflux ratio. This experiment only required 15 minutes to stabilize which the top product maintained at the RI of 1.334 and dragged to the ethanol composition of 0.35mol. The bottom product was stabilized for the first 10 minutes but the final RI recorded at 1.330 which directly translated from the graph of ethanol mole composition at around 0mol. This experiment is best given by  Y   n   =0.5X   n  +0.116   for the rectification section. This experiment also indicates only 1 stage required to make the system stabilize for the top and bottom products with the plate is 0. There is a significant relationship of the rectification line equation with the reflux ratio adjustment. According to the theory, the increment of the reflux ratio will decreasing the y-intercept of the rectification section and moving away from the line from the equilibrium curve. Therefore, the number of stages required to achieve the separation also decreases. From this experiment, there is clearly observed that the y-intercept is decreasing with the increment of reflux ratio and also the stages required as per graph plotted based on Mc-Cabe Method. Usually, the reflux ratio adjustment made to control the purity of the distillate. In this case, the RI can be the reference of the distillate purity where the reflux ratio adjustment recorded the mole fraction of 0.350, and 0.350 respectively. In this experiment, there is a indication that the lower the reflux ratio, the better the distillate purity which actually against the theory that stated the higher the reflux ratio, the purer the distillate can be.   This shows the importance of the reflux ratio and its influence on the number of plates required. As being mention by Coulson & Richardson, Chemical Engineering book volume 2; it stated that any change in the reflux ratio will therefore modify the slope of the operating line. This will make a difference when drawing the stages or plates line. Based on theory also, higher reflux ratio, will lower the stages and it will increase the slope. When the slope is increase, the diameter is also increase and it will increase the vapor loading greatly. The Coulson & Richardson, Chemical Engineering book volume 2; stated that by increasing the reflux ratio, the operating cost will rise and are approximately proportional to (R + 1). But it also decrease the capital cost since the number of plates falls off. So this experiment follow the theory. There also some errors and problems while the experiment being conducting. First errors or  problems related with Refractometer handling for the refractive index reading., the result might  be inaccurate since the sample may be contaminated with others sample from different experiment that run simultaneously. Besides that, the sample plate on the Refractometer also needs to be properly clean to avoid parallax eror on the RI reading which can be higher or lower that the reading supposed to be. However, the technique of using this Refractometer also can influence on the accuracy of the result. Then another problems is when the preparation of mixtures. When the water and ethanol was placed in feed tank the amount of mixture maybe is more or less than 30 liters but it should followed the procedure 27 liters of distilled water and 3 liters of ethanol must be prepared. So it may lead to a different molarity thus impacted on the distillate fraction determined from the ethanol composition versus refractive index graph. Thus the results and number of stages also were effect.    CONCLUSION  From the experiment, the basic principle of distillation process had been known and been understood by operating the vapour -liquid separation experiment using a bubble cap column distillation process unit. Next, the respective composition for the sample top and bottom were analysed by refractometer in order to obtain the refractive index . Beside that, the number of stages was determined by using the Mc Cabe Thiele method. Then, when the number of stages was increased the value of distillate (Xd) is higher because the amount of top product is higher than the bottom product. It means when the reflux ratio is decreased the value of distillate is higher so the cost is low. So, it resulting the top product was produced more than bottom. Reflux Ratio 1:2 < 1:1,  , it shows that more lower reflux ratio, more applicable in reducing the cost in industries and when the reflux ratio was increased the number of stages was decreased. There are some recommendations in this experiment, make sure check the volume of the mixture or feed before transfer to feed tank. It might be affect the process which is the feed composition or the molarity of the mixture. So,it will influence the result of the operating lines and hence the number of stages required for separation. Add stage of bubble cap at distillation column because when the stage is added it will increase the efficiency of distillation (contact area between bubble cap and the solution are increased). Therefore the RI value obtain is almost to the pure ethanol. Make sure, the refractometer is cleaned well and on good condition along the experiment because it is very sensitive and may affect refractive index reading. Thus, the reboiler must be appropriately sized to ensure that enough vapour can be generated during cold and windy spells and that it can be turned down.
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