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The Effect of Reflux Ratio on Tray Distillation Efficiency
 

The Effect of Reflux Ratio on Tray Distillation Efficiency

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Tray distillation is a crucial chemical engineering technique used to separate compound mixtures in various industrial settings. Such as chemical plants, oil refineries, and natural gas processing. Distillation is performed in a column with many different levels called trays. A liquid feed stream travels through the column and after exposure to heat is either condensed or vaporized, enabling the mixture to be separated based on the difference in volatilities. In order to design a cost-productive distillation apparatus the separation efficiencies of the trays in the system are studied. Here we will investigate the separation efficiency of a Sieve tray column used to separate a mixture of methanol, isopropanol, and water.

In a distillation column, the liquid feed is introduced and it flows downward while simultaneously contacting a rising vapor stream. When the liquid reaches the bottom it enters a re-boiler and is either vaporized and re-enters the column or remains liquid and exits the system. This exiting stream, called the bottoms, contains the heavier components. The boil up ratio VB, is the ratio of liquid recycled into the column to the amount of liquid leaving in the bottoms. The vapor stream flows upward through the column and is condensed before entering a reflux drum. It is then split into two streams. The distillate containing the more volatile components which exit the system, and the reflux stream which is cycled to the column. The reflux ratio, which is the ratio of reflux rate to distillate rate can influence separation efficiency. In total reflux mode, 100% of the streams are recycled back into the column. However, practical distillations are operated in partial reflux mode to achieve economic separation. Now let's take a look at the McCabe-Thiele analysis to determine the number of stages needed for a separation using VLE data for the two components. Starting from the distillate composition, draw an operating line with slope equal to RD over RD plus one. Starting from the bottoms composition draw an operating line with slope equal to VB plus one over VB. Step down the plot between the BLE curve and operating lines until the bottom is reached. The number of steps minutes one for the reboiler is equal to the number of trays needed. To evaluate efficiency, the Murphree Method is used. The Murphree efficiency for a single tray, EML, describes the change of the liquid composition over the tray divided by the change off the exiting liquid composition, assuming it was in equilibrium with the exiting vapor. Low efficiencies on a tray are often associated with low interfacial area or low superficial velocities, enabling engineers to pinpoint problems in a column and improve design. Now that we've discussed how a distillation column works and how the reflux ratio can affect the separation, let's test and demonstrate the effects in a laboratory experiment.

Before you start, familiarize yourself with the tray distillation unit. The unit consists of a column containing six sieve trays. The feed reservoir contains the mixture of methanol 50 weight percent, isopropanol 30 weight percent, and water 20 weight percent. Which is directed via a pump to the feed preheater and then to the column. The distillate from the column is collected in the total condenser equipped with a valve to collect the samples. A reflux drum, reflux pump, and preheater are used to provide a continuous reflux of which the ratio is adjusted for a better efficiency. Lastly, the reboiler and bottoms pump provide heating of the mixture and the bottoms valve is used to obtain the samples for analysis. The majority of the trade distillation unit is operated using a graphical interface. To start the experiment for the total reflux mode, turn on the cooling water and check the level of the reboiler liquid. Adjust the level by either adding feed liquid or removing some liquid using the bottom pump. Turn on the main reboiler heater and the strip heater. Then, via the controls, set the reboiler temperature controller to manual and adjust the output to at least 60%. And wait for the overhead vapor to condense, filling the reflux drum. Once the reflux drum has reached a level of 50%, set the reflux flow controller I auto with a set point of 20% and turn on the reflux pump. As soon as you measure the reflux flow on the controller, gradually decrease the set point in 2% increments every 20 to 30 seconds until the reflux flow rate is 12 to 13% of span. When the reboiler heater was activated, the system also started the reflux preheater. Now set the controller in auto and give the reflux preheater a set point of approximately 65 degrees Celsius. Make sure that the reflux drum level is around 50%, and if necessary manually adjust the rate by changing the set point on the reflux flow controller to provide a constant reflux drum level of 25 to 75%. When all flows, levels, temperature, and compositions are close to their set points and aren't changing significantly for approximately two minutes, one can say that steady state of the total reflux mode has been achieved.

Now that the system has reached a steady state, let's transition to the finite reflux mode. Set the feed flow controller to auto with a set point of approximately 120 cubic centimeters per minute. Next turn on the feed pump and the feed preheater, set the controller in auto, and give it a set point of approximately 65 degrees Celsius. Once the feed settings are set, put the reflux flow rate controller in auto and set the starting point of the reflux flow to a set point of approximately 80%. Start withdrawing the distillate product to maintain the reflux drum level between 25 and 75%. Put the distillate flow controller in auto and adjust it's set point above zero flow. Start withdrawing the bottoms product to maintain a constant level of 60 to 80% in the reboiler. Put the bottoms flow controller in auto, turn on the bottom pump, and adjust the set point to a flow rate above zero. Repeat the finite reflux operation at approximately three different reflux ratios while keeping the boil up rate constant. This is carried out to adjust to different steady states in the finite reflux mode. When all flows, levels, temperature, and composition are close to their set points and are not changing significantly, then stead state is reached. Once a stead state is reached, open the sample valves and use sample bottles to collect one set of three to four milliliter samples of the bottoms and distillate products at their respective sample points. Using a pipette, insert it through the top of the feed tank to collect a feed sample. Next, using a curved needle syringe, insert it through the septum port of each tray to acquire tray samples. Secure the samples and analyze them using a gas chromatograph after the experiment is finished.

Now that you have finished the experiment, let's focus on analyzing the results. The McCabe-Thiele analysis for this system shows that 4.5 stages are needed for the separation. Though the system utilizes six stages plus the reboiler. Next, obtain the mass fraction of the samples using the GC data. Apply the Murphree tray efficiency equation and calculate the efficiency of each tray. Tray two was significantly more efficient than its counterparts and visual observation showed it to be very frothy. So high in interfacial area. Tray one was even more frothy, but some entrainment could be observed. This behavior is a consequence of a low surface tension for an alcoholic mixture. In the top two trays, almost all of the water had been removed, leaving behind mostly methanol with some isopropanol. Tray three had poor methanol efficiency, which is observed when a different compound, water in this case, undergoes a profound concentration change on the tray. Now repeat the calculations for each reflux ratio to determine the effect on the composition of the distillate and bottoms. In general, the lower reflux rate reduces the methanol purity of the distillate. A higher reflux rate improves the separation at high operating costs.

Lastly, let's take a look at a couple applications of trade distillation and measuring tray efficiencies in the chemical industry. Oil refineries separate crude oils into multiple products. The feed stream is heated crude oil at atmospheric pressure. Fuels, such as fuel oil for ships, diesel, kerosene, naphtha, and gasoline are separated based on their boiling points and thus chain length. Chemical engineers use tray efficiencies to optimize the separation processes of the desired products. To generate distilled spirits such as vodka or whiskey, a mixture of grain fermentation products known as wash, which is 10 to 12% alcohol by volume is boiled in a still and the resulted vapor is separated by simple or trade distillation. This allows the ethanol to be separated from other alcohols like propanol and water, which have higher boiling points.

You've just watched Jove's introduction to distillation. You should now understand the distillation process, how to operate a tray distillation unit, and how to evaluate its efficiency. You have also seen several applications of distillation in industrial settings. Thanks for watching.

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