In order to be useful, many substances must be available in pure form.

Transcription

1 DISTILLATION COLUMN INTRODUCTION In order to be useful, many substances must be available in pure form. In the real world substances are seldom found or produced pure, they are frequently found mixed together in solution. In a true solution the components are mixed together at the molecular level. They will not separate out spontaneously. Usual filtration or centrifugation methods will not separate them. Distillation is the most common industrial separation technique. It is a process in which a solution of two or more substances is separated into its component fractions by the application and removal of heat. Distillation is based on the fact that the vapour of a boiling mixture will be richer in the components that have lower boiling points. When this vapour is cooled and condensed, the condensate will contain more of the more volatile components. At the same time, the original mixture will contain more of the less volatile material. Distillation consumes large amounts of energy - both for heating and cooling. This makes it expensive to operate. The challenge for chemical engineers is to achieve separation of the components of desired purity with high efficiency. Distillation columns are specifically designed to achieve these goals. HISTORY Distillation in one form or another has been known for thousands of years. Today distillation is still used to produce spirits for consumption and is also applied widely in the chemical industry - one notable example being the production of petroleum products from crude oil. DISTILLATION AT MCMASTER In the Department of Chemical Engineering at McMaster University, the distillation apparatus you will use is pilot scale - this is fairly large for a university setting. Operating this column involves manipulation of many components, especially valves. Each component has an identifier beginning with a letter or two followed by a three-digit number. For example, the valves are labeled with yellow tags and start with the letter V. The first digit identifies the general location of the component. For example, if the first digit is a 3, the component is associated with one of the trays. The second digit tells what specific tray the component is attached to. The top tray is labeled number 1 and the bottom tray is number 9. The third digit tells whether the component is on the vapour side

2 or the liquid side or is part of the sampling manifold. For example, the yellow tag labeled V 362 is a valve attached to tray number six on the liquid side. COLUMN COMPONENTS The individual parts of a distillation column function to either transfer material or transfer heat energy. The column itself is called the shell. Within the shell are trays, also called plates. There are many tray designs; our column uses bubble-cap trays. The reboiler is where heat energy is put into the process. This provides the energy needed to convert liquid to vapour. The condenser is where heat energy is removed from the process. This is where vapour is converted to liquid. A reflux drum receives and holds the liquid condensed from vapour. In our case the drum is a glass column. This reflux liquid can be fed back into the column. DISTILLATION PROCESS In a commercial continuous operation the liquid feed solution, is usually supplied to a tray at the middle of the column called the feed tray. The part of the column above the feed tray is called the rectification or enriching section. The bottom part of the column is called the stripping section. The feed flows down the column to the reboiler. In the reboiler heat energy is used to generate vapour. This vapour rises to the column and works its way to the top. The vapour leaving the top of the column is condensed. Some of this distillate is removed as top product; some is recycled back to the top of the column as reflux. Liquid removed from the reboiler is called the bottom product. Our column operates in a batch mode. Once it has been charged with the binary solution, water and methanol, we neither add nor remove material, except for the small samples taken for composition analysis. So, we do not make use of a feed tray. And all of the collected distillate is returned to the top of the column - total reflux. Within the trays there are flows of liquid and vapour. Liquid flows from each tray down to the next. Vapour flows from each tray up to the next. The two phases are flowing counter to one another. The design of the bubble cap causes the vapour to bubble through the liquid producing both heat and mass transfer. In a given tray, the vapour is richer in the more volatile component than the liquid. The trays that are physically higher in the column have vapour and liquid compositions richer in the more volatile components than the lower trays.

3 In addition to direct measurement, the composition of the liquids and vapours in the trays can be inferred from their temperatures. Higher temperatures closer to the boiling point of water indicate a solution rich in water. Lower temperatures closer to the boiling point of methanol indicate a solution richer in methanol. OPERATING AND SAFETY PROCEDURES You will be working in a small group with other engineers-in-training. The distillation column is located in the pit area of JHE-A106. When you arrive in the lab you will evaluate the potential hazards associated with the experiment and prepare a safety report. What possible hazards do the use of steam as a heat source present? The solution being distilled is methanol/water. What should you be aware of when you work with methanol? How will you protect yourself against the potential hazards you ve identified? What can go wrong with the experiment? What would you do in such an emergency situation? In the lab: 1) Eye protection must be worn at all times. 2) Heat-resistant gloves must be used when handling steam components. 3) Immediately notify the TA/technician if the pressure gauges on the column show a pressure greater than atmospheric. 4) You should become familiar with the MSDS (the manufacturer s safety data sheet) for methanol. 5) Students handling the distillate must wear nitrile gloves and close the sampling flask provided immediately after sampling to prevent methanol loss before composition measurement. During your three sessions you will be taking samples from various locations in the apparatus. You will take these samples out of the pit area to the fume hood and use instruments to analyze the relative amounts of methanol and water. You will take the data and analyze it using the computers in the cluster adjacent to the lab. Your group members will have to decide how to apportion the workload. Don t forget that there must always be at least one group member physically standing at the column - never leave it unattended. START-UP PROCEDURE These next few instructions are for your Lab Assistant- they correspond to steps 1 through 7 in the written instructions and will be done by the Lab Assistant before you arrive.

4 Prior to the start of your experiment, 7 litres of a 20% methanol: water solution has been placed in the feed vessel. Valve V101 has been opened to allow hot water to flow through the pre-heater vessel and valve V120 has been opened to fill the pre-heater with the methanol: water solution. The feed pump is turned on to charge the reboiler, and then stopped. Valve V120 is re-opened and the feed pump is turned back on. This has been repeated until the liquid level in the reboiler is slightly below the feed point. Once this is achieved, valve V120 is closed and the hot water supply to the pre-heater has been turned off by closing valve V101. That ends the preparation by the Lab Assistant. The next instructions are for your student group. They begin at Step 8 in the written instructions. 1) Run cooling water through the condenser by opening V102. 2) To set the flow of cooling water, turn the needle valve V103 connected to a rotameter counterclockwise until the flow is approximately 1.5 on the scale. 3) Wearing thermal gloves you will apply steam to the system. Open the main steam valve V110 turning slowly 90 degrees to avoid a surge of steam. 4) Located at the base of the column is the steam trap. Open the by-pass valve V113 using thermal gloves. This will allow any dirty condensate to be flushed out of the system without clogging the steam trap. 5) To direct steam into the reboiler, slowly and carefully open valve V111 using thermal gloves. Next, slowly open valve V112. Proceeding slowly will prevent any surges of steam or excessive hammering in the system You may hear some minor knocking as the steam heats up the components and works its way through the system. 6) After a few minutes, the steam and condensate flowing through the reboiler should be clean. You can then close the bypass V113 which will direct the steam to the steam trap. 7) Wait 30 minutes to allow a sufficient amount of the condensed liquid, a mixture of methanol and water, to accumulate in the collector column located under the condenser. Then you can implement total reflux mode. Periodically check the pressure gauges PI 310, 350, and 390 on the distillation column. They should not register any pressure. REFLUX MODE

5 Under total reflux, fluid in the collector column is fed back to the top of the distillation column. 1) To do this you must start the reflux pump. It is easier to run the pump at a constant speed and adjust the flow rate using a needle valve than to vary the pump speed. 2) By manipulating needle valve V411 you can change the flow rate. V411 is integrated into a rotameter which allows you to measure the flow rate. The pump is taking liquid from the bottom of the collector column. You want to match this outflow rate to that of the condensate being delivered to the top of the collector column from the condenser. Your goal is to keep the level of liquid in the collector column constant. 3) Allow approximately 30 minutes to reach steady state conditions. SAMPLING PROCEDURE There are six liquid sampling points on the column. Whenever taking a sample, draw off a few milliliters of liquid to reject any old liquid sitting in the lines. Pour this liquid back into the hopper where it will eventually be returned to the system. Using a sampling flask, take a sample, then immediately close the flask with a rubber stopper and place it in a bucket of ice. Take samples as quickly as possible to get a snapshot of the conditions of the column. Takes samples of liquid in the trays from valves V323, V343, V363, and V383. Then sample condensate at V403 and use V404 to sample from the reboiler. The chilled samples can then be analyzed for methanol: water composition using either refractive index or density. SHUTDOWN PROCEDURE With the experiment over and all the data collected you can shut the system down. Close valve V411 on the reflux rotameter and turn off the reflux pump. Wearing thermal gloves close the main steam valve, V110.

6 Open the steam trap by-pass valve V113 using thermal gloves- this will quickly reduce the pressure in the steam system. Pressure gauges PI 101 to 104 should read zero within a few seconds. Wait until there is no residual pressure in the system then close V111 and V112 using thermal gloves. Run cooling water for a few minutes to cool the column, then close the cold water supply to the condenser by turning off V102 and then V103. SUMMARY In order to understand the processes happening during distillation you will make use of tools like: Raoult s Law; Dalton s Law; and the McCabe-Thiele method. This laboratory has introduced the key concepts and theories related to mass transfer. Once completed, you will have a good fundamental background of distillation and be familiar with the components of a distillation column. After the closing credits, please test your knowledge by completing the following crossword puzzle.