There are two heat exchangers above the distillation column. Then the reflux tank will be filled with the condensate that flow from condenser. The electromagnetic controlled in the column by switching of on and off controlled the reflux ratio.
The remaining from the reflux tank then turned back to the top of the column. Therefore, the refractive index of every sample analysed by refractor meter. The rectification is the mass transfer between the two phases in equilibrium. Figure 2 - Process Flow Diagram for Common Distillation Column Process From the distillation column system, there are two principle method that need to be followed in order to carry out the experiment which is batch distillation method and continuous steady state distillation process.
Batch distillation process is the process when there is no reflux and the vapor that boiling the liquid mixture to be separated and condensing vapor not allowing any liquid to return into the top column.
Therefore, the continuous distillation process is vice versa. Some example of continuous distillation is flash distillation single stage partial vaporization and rectification distillation. In the industries, there are many categories of products that use distillation for separation such as petroleum refining, petrochemical, process of natural gas and beverage.
In generally, the typical aim to remove the light component from mixture of heavy component. Other example of distillation process is within the food industries which concentrating essential oils and flavour, deodorization of fats and oil.
Distillation is a physical process used to separate chemicals from a mixture depending on the differences in volatilities of the components that make up the mixture. In the distillation process, a volatile vapor phase and a liquid phase that vaporizes are involved Geankoplis, Their operation is based on the difference in boiling temperatures of the liquid mixture components.
The binary mixture of methylcyclohexane-toluene is used in the experiment to understand the concept of distillation. The vapor raised in the boiler flow into the unit at the bottom of the column. The velocity of the vapor passing through the column depends on the boil-up rate.
As the vapor passes straight upward through the liquid on the sieve trays Tham M. The pressure drop is expected to increase as the boil — up rate is increases.
As the vapor exits the top of the unit, it is cooled by a condenser Tham M. The condensate is stored in the reflux drum. The refractive indexes can be measured for mixtures of known concentration made up for the binary system. The refractometer works using the principle of light refraction through liquids ProSciTech, It measure the critical angle of the liquid or mixture under test and different concentration will yield to different reading of the critical angle.
The lamp in the reboiler which is LOW LEVEL lamp section of console illuminated along with the reboiler power, reflux timer, column temperature and process temperature displays. The leaks were checked. The V6 and V7 closed. The water in the reboiler begin to heat up and observed by the selecting T9 on the process temperature digital display.
Then V6 and V7 closed. The overall pressure drop was recorded. The degree of forming was observed and recorded. The refractometer was being calibrated by zeroing the instrument using distilled water before RI testing. I of pure methyl cyclohexane and pure toluene was measured.
VolMCH Voltol. The degree of separation of the liquid mixture depends greatly on the volatility of one component to the others. The most volatile component will be boiled as vapor thus separating it with the least volatile component.
The liquid product will be collected at the bottom and the vapor product will be collected at the top part of the distillation column. This experiment was divided into 2 parts, experiment A and experiment B respectively. The main objective of experiment A is to determine the pressure drop over the distillation column for various boil-up rates. A mixture of toluene and Methylcyclohexane were fed into the distillation column and were boiled up by using different power input.
From the result obtained from conducting experiment A, graph of pressure drop versus boil up rare as in Figure 5 is plotted. From that, we can see that the graph undergoes slight drop at the beginning and then increases sharply before it goes for slight drop again.
The drop pattern of the graph is probably caused by error in which the value of the pressure drop were taken. Supposedly, the value shown on the manometer were let to stabilize first before the value could be taken to ensure accuracy of data.
Besides that, the varying power input will also affect the degree of foaming, and the refractive index of the system. The degree of foaming varies as the power input were set differently. The different degree of foaming is dependent on the upward vapor flow.
Apart from that, the refractive index of the mixture was observed to increases when the power input increases.
The refractive index value of the condensate from Experiment A is taken to compare with the calibration curve from Experiment B. As for experiment B, the main objective is to determine the refractive index of MCH for the calibration curve. Samples with different MCH mole fraction were prepared and each of the samples refractive index were measured by using refractometer.
Based on the graph, we can see that the graph undergoes steady drop and sudden increases before it drop steadily again. The sudden increases is probably caused by error in which the reading were taken from the refractometer. Supposedly, the sample stage were clean properly before the next reading can be taken. This to ensure that the current sample was not mixed with the previous sample as it can affect the refractive property of the sample.
Comparing with the calibration curve from Experiment B, the refractive index of the condensates from Experiment A, shows that the major constituent of the top product of the distillation column is methylcyclohexane since the refractive index value is closer to that of high concentration of methylcyclohexane.
However based on the results, as the power input is increases, the composition of the major constituent is also changing. At feed-stage pressure, the feed of LK mole fraction zF may be liquid, vapor, or a mixture of the two. The mole fraction of LK is xD in the distillate and xB in the bottoms product.
The goal of distillation is to produce a distillate rich in the LK i. Whether the separation is achievable depends on relative volatility of the two components A and B.
For components with close boiling points, the temperature change over the column is small and relative volatility is almost constant. An equilibrium curve for the benzene—toluene system is shown in Figure-9, where the fixed pressure is 1 atm, at which pure benzene and pure toluene boil at o F and oF, respectively. In , McCabe and Thiele published a graphical method for combining the equilibrium curve with mass balance operating lines to obtain, for a binary-feed mixture and selected column pressure, the number of equilibrium stages and reflux required for a desired separation of feed components.
Although computer-aided methods are more accurate and easier to apply, the graphical McCabe—Thiele method greatly facilitates visualization of the fundamentals of multistage distillation, and therefore the effort required to learn the method is well justified.
The distillate can be a liquid from a total condenser, or a vapor from a partial condenser. The feed-phase condition must be known at column pressure, assumed to be uniform throughout the column. The type of condenser and reboiler must be specified, as well as the ratio of reflux to minimum reflux.
Lastly, condenser and reboiler heat duties are obtained from energy balances. Besides the equilibrium curve, the McCabe—Thiele method includes a 45o reference line, operating lines for the upper rectifying section and the lower stripping section of the column, and a fifth line the q-line or feed line for the phase or thermal condition of the feed.
Rectifying Section Operating Line Figure-8, shows that the rectifying section of equilibrium stages extends from the top stage, 1, to just above the feed stage, f. Consider a top portion of the rectifying stages, including the total condenser, as shown by the envelope in Figure A material balance for the LK over the envelope for the total condenser and stages 1 to n is as follows, where y and x refer, respectively, to LK vapor and liquid mole fractions.
This is the case if: 1. The two components have equal and constant molar enthalpies of vaporization latent heats. Component sensible-enthalpy changes and heat of mixing are negligible compared to latent heat changes. The column is insulated, so heat loss is negligible. Column pressure is uniform thus, no pressure drop. These are the McCabe—Thiele assumptions leading to the condition of constant molar overflow in the rectifying section, where the molar flow rates remain constant as the liquid overflows each weir from one stage to the next.
Since a total mole balance for the rectifying-section envelope in Figure gives: If L is constant, then V is also constant for a fixed D. Energy balances are needed only to determine condenser and reboiler duties. A vertical line is dropped until it intersects the operating line at y2, x1 , the compositions of the passing streams between stages 1 and 2.
Horizontal- and vertical- line constructions are continued down the rectifying section to give a staircase construction, which is arbitrarily terminated at stage 5. The optimal termination stage is considered in later section of this article. Stripping Section Operating Line The stripping section extends from the feed to the bottom stage. This is the inverse of the flow conditions in the rectifying section. FIGURE Vapor leaving the partial reboiler is assumed to be in equilibrium with the liquid bottoms product, B, making the partial reboiler an equilibrium stage.
The vapor rate leaving it is the boil up, , and its ratio to the bottoms product rate: is the boil up ratio. With the constant-molar overflow assumption, VB is constant in the stripping section.
From that point, the staircase is constructed by drawing horizontal and then vertical lines between the operating line and equilibrium curve, as in Figure, where the staircase is arbitrarily terminated at stage m. Next, the termination of the two operating lines at the feed stage is considered. Feed Stage Considerations — The Equation of q-Line In determining the operating lines for the rectifying and stripping sections, it is noted that although xD and xB can be selected independently, R and VB are not independent of each other, but related by the feed-phase condition.
For cases a and e in Figure, VB and R cannot be related by simple material balances. An energy balance is necessary to convert sensible enthalpy of subcooling or superheating into heat of vaporization. Instead of using the stripping-section operating-line equation to locate the stripping operating line on the McCabe—Thiele diagram, it is common to use an alternative method that involves the q-line, shown in Figure Hence in order to plot the equation of q-Line on the equilibrium diagram the following information is needed: 1.
Point: zF, zF 2. The point of intersection must lie somewhere between the equilibrium curve and the 45o line. For a saturated-liquid feed, the q-line is vertical; for a saturated vapor, the q- line is horizontal. This figure shows the Effect of thermal condition of feed on slope of the q-line. Consider a binary mixture of components A and B, to be separated into two product streams using conventional distillation.
The mixture is fed in the column as a saturated liquid i. The overhead vapor stream is cooled and completely condensed, and then it flows into the reflux drum. The cooling of the overhead vapor is accomplished with cooling water. The liquid from the reflux drum is partly pumped back in the column top tray, N with a molar flow rate FR reflux stream and is partly removed as the distillate product with a molar flow rate FD.
Let us call MRD the liquid holdup in the reflux drum and xD the molar fraction of component A in the liquid of the reflux drum. It is clear that XD is the composition for both the reflux and distillate streams. The composition of the recirculating back to column stream is xR. Let MB be the liquid holdup at the base of the column. The column contains N trays numbered from the bottom of the column to the top. Let M, be the liquid holdup on the ith tray. The vapor holdup on each tray will be assumed to be negligible.
In Figurea we see the material flows in and out of the feed tray. Similarly, Figureb and 18c show the material flows for the top Nth and bottom first trays, while Figured refers to any other tray. Vapor holdup on each tray will be neglected. The molar heats of vaporization of both components A and B are approximately equal. This means that 1 mole of condensing vapor releases enough heat to vaporize 1 mole of liquid. The heat losses from the column to the surroundings are assumed to be negligible.
The relative volatility of the two components remains constant throughout the column. The first three assumptions imply that: and there is no need for energy balance around each tray. The last two assumptions imply that a simple vapor-liquid equilibrium relationship can be used to relate the molar fraction of A in the vapor leaving the ith tray yi with the molar fraction of A in the liquid leaving the same tray xi : The final assumptions that we will make are the following: 1.
Neglect the dynamics of the condenser and the reboiler. It is clear that these two units heat exchangers constitute processing systems on their own right and as such they have a dynamic behavior. Therefore, accurate modeling should include the state equations, which describe the dynamic behavior of condenser and reboiler. Neglect the momentum balance for each tray and assume that the molar flow rate of the liquid leaving each tray is related to the liquid holdup of the tray through the Francis weir formula: Let us now develop the state equations that will describe the dynamic behavior of a distillation column.
The fundamental quantities are total mass and mass of component A. But the question is: What is the system around which we will make the balances? From a practical point of view the boundary of the system of interest is outlined by dashed lines in Figure Such a boundary clearly identifies the inputs and outputs of practical significance for the overall system. It is also evident that unless we can describe how the concentrations and liquid holdups on each tray change with time, we cannot find how the variables of practical significance, such as xD and xB, change with time.
Therefore, we are forced to consider the balances around each tray. Thus, we have see also Figure All the equations above are state equations and describe the dynamic behavior of the distillation column.
The modeling steps outlined above indicate that the overall procedure may be tedious and full of simplifying assumptions. At times the resulting model is overwhelming in size and the solution of the corresponding equations may be cumbersome.
An exact integer number of stages is rare; usually fractions of stages arise. In that figure, point P is the intersection of the q-line with the two operating lines. The feed- stage location is the transfer point for stepping off stages between the rectifying-section operating line and the equilibrium curve to stepping off stages between the stripping- section operating line and the equilibrium curve. The smallest optimal number of total equilibrium stages occurs when the transfer is made at the first opportunity after a horizontal line of the staircase passes over point P, as in Figure, where the feed stage is stage 3 from the top and a fortuitous total of exactly five stages is required four plus a partial reboiler.
FIGURE An example of distillation design calculations is given on the next page for demonstration of concepts presented before. Condenser Type Types of condensers are shown in Figure A total condenser is suitable for reflux-drum pressures to psia 1. A partial condenser is appropriate from psia to psia 2. A mixed condenser can provide both vapor and liquid distillates. A refrigerant is often used as coolant above psia when components are difficult to condense.
A partial condenser provides an additional stage, based on the assumption that liquid reflux leaving the reflux drum is in equilibrium with the vapor distillate. Either can provide the large heat-transfer surface required. In the former case, liquid leaving the sump reservoir at the bottom of the column enters the kettle, where it is partially vaporized by transfer of heat from tubes carrying condensing steam or some other heat-transfer fluid.
The bottoms product liquid leaving the reboiler is assumed to be in equilibrium with the vapor returning to the bottom tray.
Thus, a kettle reboilers, which is sometimes located in the bottom of a column, is a partial reboiler equivalent to one equilibrium stage. Vertical thermosyphon reboilers are shown in Figuresb and 21c. In the former, bottoms product and reboiler feed are both withdrawn from the column bottom sump. Circulation through the reboiler tubes occurs because of a difference in static heads of the supply liquid and the partially vaporized fluid in the reboiler tubes. The partial vaporization provides enrichment in the exiting vapor.
But the exiting liquid is then mixed with liquid leaving the bottom tray, which contains a higher percentage of volatiles. This type of reboiler thus provides only a fraction of a stage and it is best to take no credit for it. In the more complex and less-common vertical thermosyphon reboiler of Figure- 21c, the reboiler liquid is withdrawn from the bottom-tray downcomer.
Partially vaporized liquid is returned to the column, where the bottoms product from the bottom sump is withdrawn.
This type of reboiler functions as an equilibrium stage. Thermosyphon reboilers are favored when 1 the bottoms product contains thermally sensitive compounds, 2 bottoms pressure is high, 3 only a small deltaT is available for heat transfer, and 4 heavy fouling occurs.
A pump may be added to a thermosyphon reboiler to improve circulation. Liquid residence time in the column bottom sump should be at least 1 minute and perhaps as much as 5 minutes or more.
Large columns may have a foot-high sump.
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