Continuous Binary Distillation University of Illinois
Continuous Binary Distillation
Lab Prep Report
Unit Operations Lab 2
January 24, 2011
Group 6
Sana Buch
Priya Chetty
Liliana Gutierrez
Linda Quan
Vijeta Patel
Lipi Vahanwala
1. Introduction
Distillation is the most widely used separation process that separates a mixture based on differences in the conditions required to change the phase of components of the mixture. Distillation contributes to more than 50% of plant operating costs (Modeling & Simulation of Binary Distillation Column). Distillation is used mainly for commercial purposes, such as production of gasoline, distilled water, xylene, alcohol, paraffin, and kerosene (Modelling & Simulation of Binary Distillation Column). In large chemical complexes, the distillations are continuous with the feed entering at some point near the middle of the column and the product being taken off the top and the bottom. Distillation is very flexible and there are schemes which include multiple feeds to the column and the take-off of multiple products. However, a distillation column can produce only two products (the top and the bottom) which are of high purity. Sequences of distillation columns are commonly used to produce numerous high-purity products.
The process of distillation can be categorized according to the following (Guzman 4).
1) Method of separation (Guzman 5)
a. Simple distillation is distillation without reflux, where no condensate is allowed to return to the single stage still pot to contact the rising vapors.
b. Fractional distillation is distillation with reflux in which a portion of the condensate is allowed to return to the distilling column to contact the rising vapors.
2) Nature of the process feed (Guzman 5)
a. Binary component
b. Multi-component
3) Method of operation (Guzman 5)
a. Batch Column is the design of a batch column, which is more complex than a continuous distillation column as it requires consideration of unsteady-state behavior (Diweker, Madhvan 713). Batch distillation is most often used with smaller volume products (“The Distillation Group, Inc”). The batch column distillation column can be designed by keeping two modes of operation (Diweker, Madhvan 713).
i. Variable reflux and constant product composition of all or one component
ii. Constant reflux and variable product composition
b. Continuous Column, on the other side, is an ongoing distillation in which a liquid mixture is continuously fed into the process and separated fractions are removed continuously (M&S). The simulation of continuous distillation for multi-component mixture is well developed in chemical engineering due to its commercial importance. Continuous distillation is most often used with big volume products like jet fuel, benzene, and plastic monomers (“The Distillation Group, Inc”).
4) Types of columns used (Guzman 5)
a. Plate/staged column type provides each plate contact between vapor and liquid in continuous countercurrent flow. Each plate constitutes a single stage where there’s a simultaneous partial condensation of vapor and partial vaporization of liquid.
b. Packed column type are plate columns containing packing columns that provides high interfacial area for the exchange of the components between the vapor and liquid phases.
This lab is mainly designed to separate binary components including methanol and water using the fractional distillation method in the six stages batch column. The 32 liter mixture is created using 5vol% of 99.8% anhydrous methanol and water. Then sufficient amount of steam is supplied to begin the process of distillation. After it starts boiling, the feed reaches equilibrium and samples of liquid and vapor from each stage are collected. The collected sample is observed under a refractometer, a device that measures the index of refraction. The data obtained from the refractometer is compared with the calibration data obtain at room temperature in order to determine the composition.
2. Literature Review/Theory
Distillation is the separation or partial separation of a liquid feed mixture into components or fractions by selective boiling (or evaporation) and condensation (Wankat 86). In a distillation column, the separation occurs because different components have different volatilities. The component with more volatility is easy to vaporize therefore the output vapor from distillation column will be enriched with more volatile component while the output liquid phase will be more enriched with less volatile component (Wankat 91). When a mixture reaches a specific temperature and pressure a certain amount of the mixture moves into the vapor phase until the vapor reaches the mixture’s vapor pressure (Wankat 98). This point is known as the vapor-liquid equilibrium. Volatility is a measure of a pure component’s vapor pressure at a set pressure and temperature in a specific mixture. It is incorrectly assumed that the components of a mixture will separate based on their boiling points when pure. Rather, the boiling point of a mixture is based on the total vapor pressure of a mixture, which is a sum of the vapor pressures of each individual component in the mixture. This is known as the Dalton’s law and can be expressed as:
(1)
Vapor pressure of mixture (kPa)
Vapor pressure of component a (kPa)
Vapor pressure of component b (kPa)
This means that a component will not boil off “cleanly” meaning it is impossible through distillation to obtain a pure substance. The vapor created above a mixture is also a mixture of components. The composition of the vapor is based on the volatility of each of the substances. Raoult’s law helps us to determine what the volatility, or “K value” of a substance. This in turn allows us to find the mole fraction of a component in the vapor phase.
(2)
Mole fraction of component a in vapor phase (dimensionless)
vpa = Vapor pressure of component a (kPa)
xa = Mole fraction of component a in liquid phase (dimensionless)
Psat = Vapor pressure of mixture (kPa)
Equation 2 (Raoult’s law) holds true only for those components that do not form azeotrope (J.M. Smith, H.C. Van Ness, M.M Abbott 350. Not all components obey Raoult’s law because some components have high solubility with each other which leads to the formation of azeotrope. An azeotrope is a mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture (Azeotrope). The simplest of all distillation techniques is called flash distillation. Flash distillation occurs when a mixture at a specific temperature and pressure is allowed to drop in pressure. This changes the vapor-liquid equilibrium of the mixture and creates a vapor rich in the most volatile component(s). This is also the crudest form of distillation and does not allow for refinement of the distillates.
A more complicated form of distillation is batch distillation. Batch distillation uses both a boiler and a condenser, but only allows one separation, or cut, to be taken from the mixture.
Continuous distillation is the most complicated and most common form. It has a boiler, condenser and multiple trays or packing which allows the vapor to condense as it moves up the column and cools. The trays or packing allows for a better separation of the components in the mixture which in the end gives purer products. It also adds a lot of complexity to the system.
Figure 1
The McCabe-Thiele method greatly simplified the process of determining the size of the tower and the number of trays. The method uses a graphical representation of the material balance equations as operating lines on a graph of the liquid composition (x-axis) and the vapor composition (y-axis). The bottom line in Figure 1 is the x-y line. This starts at the origin and ends where x and y both equal 1.This line would represent a distillation column that operated at total reflux and total boil-up, meaning that all of the vapor and all of the liquid is recycled back into the system. The next line added is the vapor –liquid equilibrium line for a binary system which is found experimentally. By moving step-wise between the two lines we can find the number of theoretical plates needed for a specific separation of a binary mixture and the liquid and vapor composition at any point in the distillation column. These stage lines can be seen in Figure 1.
Figure 2: McCabe-Thiele diagram with operating lines and feed line added.
In practice, we want to draw a purified product out of the column in the form of either a distillate (top of the column) or the bottoms product (bottom of the column) or both. This requires a column that operates at a partial reflux and/or a partial boil-up ratio. This means that we cannot use the x-y line for such a column. In Figure 2 we see the addition of a line for reflux ratio (slope L/V) and a line for the boil-up (slope L’/V’). The boil-up line’s slope increases as we increase the amount of bottoms product that we remove from the system. Subsequently, as we remove more distillate as product we decrease the slope of the top operating line, which is the line for the reflux ratio. By changing the amount of liquid re-boiled, which is liquid returned to the column as a vapor, or by changing the amount of vapor refluxed, returned to the column as a liquid, we change the number of theoretical plates necessary for a given separation. The q (quality) line in Figure 4 is the feed line which is the composition of the stream entering the distillation column. We can see from the diagram in Figure 4 that where that line intersects with the two operating lines is the feed stage, or the tray at where the incoming stream enters.
The quality q is defined as:
(3)
Where:
· [=] quality of the feed (dimensionless)
· [=] liquid flow rate below the feed (mole/hr)
· L [=] liquid flow rate above the feed (kJ/kg)
· F [=] feed flow rate (mole/hr)
· H [=] saturated vapor enthalpy of feed (kJ/kg)
· hf [=] enthalpy of feed (kJ/kg)
· h [=] saturated liquid enthalpy of feed (kJ/kg)
The feed line can then be defined as:
(4)
Where:
· y [=] vapor mole fraction of methanol (dimensionless)
· q [=] quality of the feed (dimensionless)
· x [= ] liquid mole fraction of methanol (dimensionless)
· ZF [=] mole fraction of methanol in feed (dimensionless)
The top operating line is defined as
(5)
Where:
· y [=] vapor mole fraction of methanol (dimensionless)
· L0 [=] liquid reflux rate into column (mole/hr)
· D [=] distillate flow rate (mole/hr)
· x [=] liquid mole fraction of methanol (dimensionless)
· xD [=] mole fraction of methanol in distillate (dimensionless)
The McCabe-Thiele method is widely used for binary mixtures. Multi-component distillation is much more complicated and the calculations involved are trial-and-error, so it is convenient to do them on computers (Wankat 176).
3. Experimental
3.1 Apparatus
No. / Manufacture / Component / Description/Usage/Safety1 / N/A / Condenser / Condenses the water.
2 / N/A / Water supply / A bulb valve used to fill the filling tank with water. Be cautious of splashing water when opening the valve.
3 / F&P Co / Liquid Rotameter / Measures the flowrate of the water flowing into the condenser.
4 / N/A / Stages 1-6 / There are 6 stages of the distillation column. At each stage, both liquid and vapor can be extracted. When extracting samples, be careful. Use a ladder or the staircase. Do not climb on the metal bars to extract samples.
5 / N/A / Filling tank / The filling tank holds the methanol and water until it can be drained into the round bottom flask of the distillation column.
6 / N/A / Ladder / Used to fill the holding tank and to extract samples at various stages of the distillation column.
7 / N/A / Ball valve for draining distillation column / Ball valve is used to drain the distillation column at the end of the experiment. Make sure the system is cooled to room temperature before draining; otherwise, it will break the glass.
8 / N/A / Fill/Drain valve / Needle valve used to fill the distillation column with the methanol and water solution from the filling tank. When the knob is pointing towards “fill” the filling tank can be filled with solution. When the knob is pointing towards drain, the filling tank will be drained of the solution and transported into round bottom flask of the distillation column.
9 / N/A / Pressure gauge / Reads the pressure of the water flowing into the condenser.
10 / N/A / Flow rate control valve / Needle valve used to control the flow rate of the water flowing into the condenser.
11 / N/A / Valve for pressure gauge / Needle valve used to control the pressure of the condenser.
12 / Powerstat / Heater box / Controls the boiler of the distillation column. Team must ask instructor to unlock and turn on the reboiler.
13 / N/A / Thermometer gauge / Reads temperature at the top of the distillation column.
14 / Newport / Digital Thermometer / Reads the temperature at each thermocouple location at each stage of the distillation column.
15 / N/A / Large round bottom flask / Holds the methanol and water solution while the boiler heats it up.
16 / N/A / Reboiler / Heats up the round bottom flask containing the methanol and water solution.
17 / N/A / Valve to release the vapor / For each stage there is a ball valve labeled “GAS” and the stage number. This releases the vapor into a tube in which a vapor sample at that stage can be collected.
18 / N/A / Tube for gas / A clear tube is connected to a valve in order to transfer the vapor from the distillation column to a small test tube.
19 / N/A / Valve to release the liquid / For each stage this is a ball valve labeled “LIQ” with the stage number. This releases the liquid into a tube in which the liquid sample at that stage can be collected.
20 / N/A / Tube for liquid / A clear tube connected to the liquid valve allows for an easy transfer of the liquid from the distillation column to a small test tube.
21 / N/A / Filling funnel / There if a funnel located at the mouth of the filling tank in which water and methanol can be easily poured into the tank.
22 / N/A / Filling tank valve / A green valve that drains the fluid out of the tank. When the green valve is facing the left side, the solution is contained in the tank. When the green valve is facing the right side, the solution is transported through the pipes and into the distillation column.
23 / Eyepiece / Used to see the sample.
24 / Thermometer / Reads the temperature of the water flowing into the refractometer from the ice bath.
25 / Light switch / Turns on the light source of the refractometer.
26 / Adjusting Knob / Used to align the horizontal refracting line to the center of the X.
27 / Refocusing knob / Used to focus the refracting line.
28 / Light source
29 / Scale illumination / This button makes the refractor index scale appear. Read the top numbers.
30 / Illuminating Prism / A small droplet of the sample is placed here. Be sure to clean the lens using distilled water and kimwipes in between samples.
31 / Tubing for cold water / The cold water from the ice bath is transported through the refractometer.
32 / Ice bath / Used to keep the water flowing through the refractometer cold.
33 / Capacity Controller / Controls the amount of fluid flowing through the refractometer.
34 / Temperature controller / Controls the heating coil that melts the ice to form water which flows through the refractometer.
35 / On/Off Switch / Turns the ice bath machine on and off.
36 / Digital thermometer / Reads the temperature of the ice bath.
37 / Heating coil / Melts the ice into water which flows through the refractometer.
3.2 Materials and Supplies