Chemistry: Distilation Essay, Research Paper

Abstract

This report outlines the steps taken to separate a 50:50 by volume ethanol and isopropanol side stream. The resulting separation must contain no more than 3% alcohol impurity in each product. A laboratory column, run at total reflux, was utilized to scale up to a forty foot high by one foot diameter column. The laboratory column allowed the team to determine vapor velocities and HETP values for the 0.24 inch Pro-Pakq packing. HETP is defined as the height of packing divided by the number of theoretical column stages. The column consisted of four main sections: packing, controls, a reboiler, and a condenser.

To complete the vapor velocity vs. HETP relationship, the vapor velocity must be found. The vapor velocity was found using a system energy balance. The design vapor velocity was determined to be 4.85 ft/hr. However, this vapor velocity did not result in the column flooding; therefore the scaled-up column is not designed to its full potential. Ideally, distillation columns should be designed at 70-80% of the flooding velocity. The column HETP was found by use of the Fenske equation and was determined to be an average of 4.55 inches.

As a result of the design parameters from the experimental column, the following design is proposed: the column will run at a vapor velocity of 4.85 ft/hr and will have a HETP of 4.30 inches. This will result in a packing height of 38.7 feet. The reboiler will have an area of 113.52 ft2 and the area of the condenser will have a value of 45.54 ft2 in which heat exchange will take place.

Table of Contents

Introduction 3

Theory and Methods 3

Apparatus 9

Procedure 11

Design of Experiments 12

Results and Discussion 13

Design Calculations 16

Final Design 20

List of Equipment 22

Operating Procedure for packed Column 22

Notation 24

References 25

Appendicies:

Laboratory Data Sheets A1

GC Analysis Data Sheets A6

Sample Calculations (R&D) B1

Intermediate Number Tables (R&D) B4

Intermediate Number Tables (Design) C1

Calibration Curves D1

Introduction

A chemical plant spends approximately 50 to 90% of capital investment on separation equipment (1,1) Therefore, the ability to utilize a small laboratory column and to scale-up a column is an important skill for a chemical engineer.

This report will outline the steps taken to design a packed distillation column. The column needs to separate a 50:50 mixture of ethanol and isopropanol into a distillate stream containing no more than 3 wt% isopropanol and a bottoms stream containing no more than 3 wt% ethanol. The design of the full-scale column was based on a laboratory simulation column. This column allowed the team to determine vapor velocities and HETP values for the 0.24 inch Pro-Pakq packing.

Once the simulation vapor velocities are determined, they can be translated to the column design and used in the design of the reboiler and condenser. Areas for the reboiler and condenser will be found and costs will be calculated. Finally, the actual packing height will be determined for the scaled-up column.

Theory and Methods

The primary goal of this project was to deterimine the design specifications for a one foot diameter by forty foot high packed distillation column. The laboratory was run at total reflux. Total reflux occurs when the entire overhead vapor flow is returned as reflux and all of the bottoms liquid is returned as boil up. Total reflux is useful for starting up columns, maintaining column operation when all or part of a plant is shut down, or for our purposes: determining column efficiency. However, since our column was a packed column and not a tray or sieve plate column, efficiency is measured not in terms of overall efficiency, but in terms of HETP. HETP is defined as follows:

HETP = height of column packing y number of theoretical stages (1)

There are two basic ways to determine the number of theoretical stages (Nmin): McCabe Thiele analysis and/or using the Fenske equation. Both of these methods will give Nmin, however, McCabe Thiele utilizes a graphical analysis and Fenske utilizes a numerical analysis.

To use McCabe Thiele graphical analysis, the condition of constant molal overflow (CMO) must be met. The assumptions behind CMO are as follows (1, 117):

1. The column must be adiabatic

2. The specific heat changes are negligible compared to latent heat changes

3. The heat of vaporization per mole, l, is constant

An additional criterion for utilizing McCabe Thiele analysis is the existence of good equilibrium data. Ideally, empirical equilibrium data can be found in several sources, but this data was not available for isopropanol and ethanol. To determine our equilibrium curve, we had to incorporate a different method. All of the equilibrium data was found by applying the below equation:

yA = (aABxA) y [1 + (aAB - 1)xA] (2)

where: yA = vapor fraction of most volatile component

xA = liquid fraction of most volatile component

aAB = relative volatility of species A and B

Once these conditions are met, the minimum number of stages can be determined. To determine the minimum number of stages, an equilibrium curve is needed. The x = y line on the equilibrium diagram is used as the operating line. Stages are then stepped off starting at the distillate composition and finishing at the bottoms composition. An important criteria in the equilibrium curve is that all compositions must be in molar units instead of mass units. All experimental compositions were found through gas chromatography.

An alternative to McCabe Thiele graphical analysis is the Fenske numerical analysis. The major requirement for using the Fenske equation is an accurate value for the relative volatility of the binary system. In this case, aAB is known and thus the Fenske equation can be used. The Fenske equation is listed below (1, 276):

Nmin = {ln[(xA/xB)Dist] / [(xA/xB)Bot]} / (ln aAB) (3)

where: xA,Dist = distillate mole fraction of more volatile component

xB,Dist = distillate mole fraction of less volatile component

xA,Bot = bottoms mole fraction of more volatile component

xB,Bot = bottoms mole fraction of less volatile component

To complete the vapor velocity vs. HETP relationship, vapor velocity must be found. With the assumption of an adiabatic column, the vapor velocity can be determine by using an overall system balance or through a condenser balance. The overall and condenser balances give a heat duty. This heat duty can then be used to determine vapor velocity. The system contained three electrical heaters, therefore, the heat duty was determined through and electrical power equation:

P = V2 / R (4)

where: P = power input or heat duty (J/s)

V = voltage input (volts)

R = resistance of the heater (W)

To accomplish the condenser balance the cooling water flow rate and temperature drop across the condenser must be known. Once these values are found, the following energy balance can be applied:

Q = mCpDT (5)

where: Q = heat duty (J/s)

m = cooling water flow rate

Cp = heat capacity of cooling water

DT = temperature increase on water side of condenser

Either of these heat duties can be used to find the vapor velocity. The vapor velocity is found using a system energy balance. The result of this balance is shown in the following equation:

Q = S V(xi)dist(DHvap,i) (6)

where: Q = heat duty (J/s)

V = vapor velocity (m/s)

xi,dist = mole fraction of component I in the distillate

DHvap,i = heat of vaporization of component I

After the vapor velocity vs. HETP graph is plotted, the flooding point can be determined. The flooding point is the point at which vapor entrainment occurs. On the vapor velocity vs. HETP graph, flooding is shown at the point where the plot changes from a linear relationship to an exponential plot. Once the design vapor velocity is determined, the column scale up may take place. The appropriate velocity is used to design the condenser and reboiler for the system. The vapor velocity is converted to mass flow rate by employing the column diameter according the Equation (7):

m = VAr (7)

where: m = mass flow rate (kg/s)

V = vapor velocity (m/s)

A = cross sectional area of the column (m2)

r = density of the fluid (kg/m3)

Equation (6) can be used to find the heat duty for the condenser and then Equation (5) can be used to determine the cooling water flow rate. To calculate the condenser area, the following equation is used:

Qc = UADTlm (8)

where: Qc = heat duty of the condenser (J/s)

U = overall heat transfer coefficient (BTU/ft2*hr*F)

A = area of heat exchange (m2)

DTlm = logarithmic mean temperature difference

The final step in this design process is to determine the height of packing in the column. In order to accomplish this, the actual number of stages must be determined using McCabe Thiele analysis. To use McCabe Thiele to determine actual number of stages, the following conditions must be identified.

1. Distillate and bottoms compositions

2. Minimum and actual reflux ratio (L/D)

3. Feed temperature and slope of the feed line

The slope of the feed line is dependent on the feed temperature. To find the slope of the feed line, the following equations were used (1, 138):

q = (L’ – L) / F (9)

L’ = L + F + c (10)

c = (Cp * DT * F) / l (11)

slope = q / (q-1) (12)

where: F = feed flow rate

L = liquid flow rate above feed stage

L’ = liquid flow rate below feed stage

DT = degrees of superheat or subcooling (K)

l = latent heat

To find the minimum reflux ratio, a pinch point is used. The slope of the line from the destillate composition on the x = y line to the point where the feed line touches the equilibrium curve is the minimum (L/V) value. Using this value, the minimum reflux ratio can be determined using Equation (13).

(L/V) = (L/D) y [(L/D) +1)]

where: (L/V) = slope of top operating line

(L/D) = reflux ratio

The actual reflux ratio can then be determined, and the actual slope of the top operating line can be found by using Equation (13). Once the feed line and the top operating line have been plotted, the bottom operating line can be found. The bottom operating line runs from the bottoms composition on the x = y line to the point where the top operating line intersects the feed line. After all three lines are known, stages can be stepped off starting with the bottoms composition.

When the total number of stages is known, the height of packing can be determined using the HETP from the experimental column. The height of packing is then found by using Equation (1).

Apparatus

The column consists of four main sections: the condenser, packing, reboiler, and controls. The condenser system consists of a glass pipe connected to the top of the packing section. The heat transfer medium for the condenser is cooling water, which flow through a spiral glass tube running vertically down the body of the condenser shell. The cooling water inlet enters the condenser shell at the base where the condenser and the packing section meet. The condenser is essentially and extension of the shell of the packing section. Therefore the vapor is free to flow from the packing section up into the condenser section where the vapor is condensed.

There is a small are between the condenser and the packing section where the top sample is collected. After the vapor is condensed, it flows down the sides of the condenser onto a lip at the bottom of the condenser section. This lip overflows into a magnetically actuated funnel that will distribute the liquid either down into the packing section or into a side draw-off pipe, which leads to the top sampler port. During normal operation the actuated funnel is turned off and the liquid is solely allowed to flow down in the packing section. However, during the sample period, the magnetic is initiated and the funnel pulses between the packing section and the sample port. The packing section is 3.31 ft in height and is filled with 0.24 inch diameter by 0.24 inch length protruded Pro-Pak? packing. This packing has a stainless steel material of construction with a surface area of 372 square feet per cubic-feet. The material of construction of the entire body of the column is vacuum-jacketed 3″ X 60″ glass. There are internal liquid distributors at the halfway point and at the top of the column. There is also a packing support screen on the bottom of this section.

The reboiler has a round still pot character constructed of glass. The reboiler is surrounded by an electric-heating mantle, which contains four sections. Three of the four heaters were active during our experimentation. Each heater is rated at 1000 Watts. The voltage supply is at 120 volts. During our design of experiments, our team controlled the percentage of the voltage that was allowed to flow through the system. The reboiler also contains a bottom sample port. This piece of equipment consists of a glass tube that extends from the center of the reboiler up through the top surface of the reboiler and connects with a two-way glass valve. This valve allows either a nitrogen purge to be placed on the reboiler or bottom material to be siphoned out. During normal operation, the valve is positioned to allow the nitrogen to blanket the reboiler in order to maintain steady boiling. Only during sampling is the valve position changed to that a sample may be siphoned out.

The control system consists of fiver thermocouples, four heaters, a rotameter, and a magnetic actuator. A thermocouple is located at both the cooling water inlet and outlet. Also, thermocouples are located at the column reflux, reboiler, and center of the column. The temperature reading is transmitted to a digital monitor for display at the base of the column. The heaters are controlled by a dial setting on the control panel, which ranges from 10% to 100% of the voltage supply. The water flow rate can also be controlled from this control panel. The rotameter has a discharge valve that can be manually manipulated to adjust flow. The rotameter reading is in percent of scale. The last control unit is the magnetic actuator, which can be initiated from the control panel. This unit creates the pulsating action of the sample funnel just below the condenser.

Procedure

To ensure that our team would be ready to collect composition data for each trial condition, we produced a 50:50 Ethanol:Isopropanol standard mixture using an analytical balance. Our team injected pure samples of ethanol, isopropanol, and water independently to determine the peak location of each component. Also, set mixtures of 60:40, and 40:60 ethanol:isopropanol standards were injected. A 1 ml sample of each of the standards was injected into the GC and resulting peak areas were recorded. Utilizing the mass of the components, the area of the GC results for each sample, and the GC calibration charts for the above standards, we were able to calculate the mole % of ethanol and isopropanol in each sample. These values determined the composition of ethanol and isopropanol in the top e and bottom samples later in the procedure. The next preparatory step was to calibrate the rotameter. Water was collected in a plastic bucket for 2 minutes then measured in a graduated cylinder to determine the volumetric flow rate. We varied the percent of scale reading on the rotameter in intervals of 20% from 20% to 100%. Two random replicates were accomplished to provide data for our calibration curve.

Once the preparatory work was complete, we were ready to run design trials. The column was already in total reflux and heated to a certain temperature when our team arrived in the laboratory. To begin our experimentation, our team determined what trial conditions the column could operate at. The manipulated variables were heater percent of voltage and cooling water flow rate. The manipulated variables were set to the trial conditions and the column was allowed to reach equilibrium before any data was taken. It was evident that the column had reached steady state when the cooling water outlet temperature was constant. During this time frame, the condenser was observed to ensure that the capacity was not exceeded. We watched the point in the condenser where the condensation was complete. When this point reached the top of the column, we had reached a maximum boil. A constraint that was placed on the design was not to exceed 110 volts on the variac control. During this period there is a nitrogen blanket on the reboiler to help avoid eruption of the liquid up the column.

Once the system reached steady state, we sample both the reflux and reboiler compositions. During our reflux sample collection procedure, we used a metal bucket consisting of a metal tube spiraling through the apparatus and exiting out the bottom of the bucket. The entry point of the metal tube was connected to plastic tubing that could be attached to the sample port. The bucket was filled with ice to promote condensation and/or cooling. This device served as a pseudo condenser to cool the liquid sample we were collecting. Two individuals would attach the condenser device to the top sample port and allow any residual material in the sample line to drain into a waste glass beaker by opening the sample port valves. Once the line was empty, the sample port valve was closed and the waste beaker was replaced with a sample flask and the magnetic actuator was initialed. At that point the sample point valve was re-opened to allow process material to flow out of the system. Once enough material was collected to do a GC analysis the magnetic actuator was terminated and the sample port valve was closed. The sample flask was covered and set aside for GC analysis.

The next step was to obtain a bottom sample from the reboiler. A clean flask was connected to the plastic tubing existing the bottom sample two-way valve. The valve position was then changed from the nitrogen purge to the sample flask. The rubber ball placed on the flask side outlet was used to pull material from the reboiler through the tubing into the flask. Once enough material was collected for GC analysis, the two-way sample valve was re-positioned back to allow nitrogen flow into the reboiler. The sample flask was disconnected and covered for later GC analysis. The reflux and reboiler samples were both analyzed separately. The procedure, however, was the same. A 1 ml sample was injected into the GC and the resulting GC peak areas were recorded to determine composition. The procedure was completed for each trial condition.

Design of Experiments

There were two overall variables that were manipulated in the design of experiments. The first was the percentage of the supply voltage (120v) provided to each of four reboiler heaters. We decided to run the entire range of the variac control for the heater. We emphasized on the higher percentage of the heater voltage control in an effort to cause the column to flood. The project team needed the column to flood in order to determine from the HETP vs. Vapor velocity graph what the parameters were for scaling up to the existing column. The second manipulated variable was the cooling water flow rate. The cooling water flow rate was manipulated in order to ensure a sizeable temperature change across the condenser. Wankat presents a reasonable value for the change in cooling water temperature between the inlet and outlet as 30-to-40o F. (1, p.443). Therefore, we used this rule of thumb as a parameter for deciding the values we were going to operate the flow rate of cooling water during each trial. In our design of experiments we replicated all of our individual trail conditions multiple times. A complete summary of the entire design of experiments is located in Appendix B, p. 5-6.

Table 1: Design of Experiments (Sample of each trial condition)

Results & Discussion

The goal of the packed column distillation project was to use the results from a lab simulation column to design the specification on the packing height and the size of a reboiler and condenser for a column, which already exists. An HETP vs. vapor velocity curve was generated in order to identify the point at which the lab column was flooding. The point before the curve slope increased exponentially into the flooding region is the operating point from which the vapor velocity and HETP are extracted, to be used in designing the specifications for the existing column.

Important assumptions in distillation are constant molar overflow and constant relative volatility. There are three requirements at the heart of assuming constant molar overflow that were met within our analysis. (1, p. 117-118) First the column was assumed to be adiabatic. The specific heat changes are negligible compared to latent heat changes. The heat of vaporization per mold does not change with composition. In assuming constant relative volatility alpha for ethanol:isopropanol equals 1.18, with ethanol as the light key component. (2, p. 13-2)

Two items of preparation needed to be accomplished before the experimentation began. The vapor liquid equilibrium diagram and the rotameter calibration chart needed to be developed. The rotameter calibration chart was used to determine the flow rate of cooling water through the condenser system in each trial, which was needed in the determination of the vapor velocity (Appendix B, p. 1) Since ethanol was the more volatile component, the VLE diagram represents ethanol. Given the assumption of constant relative volatility, the compositions were calculated for the VLE diagram through equation 2. This diagram was used in stepping off stages to determine the HETP of the lab column. An example of the VLE diagram can be observed in Appendix B p. 2.

The first step in this process was to design the experiments in order to extract useful results, which would facilitate the scale-up process to the 1-foot in diameter by 40-foot high existing tower. In the design of experiments the voltage supplied to the reboiler heaters and the flow rate of cooling water through the condenser were varied. (Appendix B, p. 5-6) At each trial condition the composition of the reflux and reboiler was analyzed. The distillate and bottoms compositions of ethanol were used to step off stages at total reflux, which was the condition that the lab column was operated. This operating condition is where all the overhead vapors are returned to the column as reflux, and all the underflow is returned as boil-up. Total reflux gives the minimum number of stages required for a given separations. (1, p. 177-78) At total reflux the x-y line is the operating line and the process of stepping off stages was started with the distillate composition of ethanol. The HETP for each lab simulation trial was calculated using the minimum number of stages from McCabe-Thiele analysis through equation 1. The average HETP determined through McCabe-Thiele analysis was 3.76 inches, with a standard deviation of 0.5427.

An alternate method of calculating the minimum number of stages was determined, Equation 2 was used to determine the HETP value. The Fenske results gave an average HETP of 4.55 inches with a standard deviation of 0.4939. The Fenske Method gives a comparable HETP result when compared to McCabe-Thiele results, but has a lower standard deviation. Hence, the Fenske analysis is more consistent that the McCabe Thiele analysis. A summary for the design of experiments with the corresponding HETP values for both methods is located in Appendix B, p. 5-6.

Once the HETP was determined, the vapor velocity needed to be analyzed to generate the HETP vs. vapor velocity graph. Again, the two methods were analyzed to determine the optimum analysis method. Method A involved an energy balance around the column condenser. In this analysis, Equation 5 is a cooling water energy balance and Equation 6 is an energy balance with respect to the process material, ethanol, and isopropanol.

The cooling water energy balance determined the heat duty of the condenser. That duty was then used in the process component energy balance over the condenser to determine the vapor velocity. The relationship exemplifies that as the vapor flow rate is increased through the column , the cooling water temperature change will increase due to the additional heat load that was created. A complete summary of the vapor velocity results for each trial can be found in Appendix B, p. 6.

Graphs of HETP vs. vapor velocity were created to represent each method for determining both number of equilibrium stages and vapor velocity.

From the HETP versus vapor velocity graphs, it is evident that there is a clear relationship between the vapor flow rate and the HETP. As the vapor flow rate increases, the HETP also increases in a linear fashion. The R2 value for the Fenske method produced much higher significance between the variables then the results from the McCabe-Thiele analysis. The R2 from the Fenske Method given a value of approximately 0.643, while that of the McCabe-Thiele valued at 0.547. The r2 value representing the significance between HETP and vapor velocity did not change appreciable between the two methods for calculating the vapor velocity. The condenser energy balance was chosen as the method for representing the HETP vs. vapor velocity data, which was used in the scale-up process. From the graph of HETP vs. vapor velocity, the flooding value was determined at 4.85 ft/hr.

The HETP vs. vapor velocity chart, which represents the values for HETP from the Fenske equation and the vapor velocity through the total column energy balance is the relationship used to scale-up to the existing column. At flow rates higher than achieved in this laboratory project the HETP will increase at a much faster rate and the packing becomes much less efficient. This region is characterized as the flooding region. The graphs of HETP vs. vapor velocity in our laboratory show a visible flooding region. The slopes of the curves are linear. The curves are linear for the operating range up to the flooding then increase exponentially as the curve enters the flooding region. In the design of experiments we attempted many different approaches to induce flooding into the system without success. Such approaches were to use a wide range of temperature drops across the condenser cooling water and to increase the heat load on the reboiler to a maximum level. The value for vapor velocity used in the scale-up was 4.85 ft/hr. The vapor velocity value was used in the design as a parameter when doing the energy balances around the condenser and reboiler for the existing column. Once the energy balance was complete and the heat duty was known, the heat exchangers were sized. The HETP value that corresponds to 4.85 ft/min is 4.3 inches. This value was used in the design of the packing height. Using a number of equilibrium stages for the existing column, which was obtained through McCabe-Thiele analysis, and the HETP value from the lab column, the height of packing for the existing column was determined.

Design Calculations

*Feed Composition

rETOH = 0.789 kg/L rISO = 0.785 kg/L

Mol. Wt.ETOH = 46.07 g/kmol Mol. Wt.ISO = 60.09 g/kmol

*Assumed basis of 1 Liter

0.5 L. * 0.789 (g/kmol) * (1 kmol/46.07 kg) = 0.0085631 kmol ETOH

0.5 L. * 0.785 (g/kmol) * (1 kmol/60.09 kg) = 0.0065319 kmol ISO

*Total moles

0.0085631 kmol ETOH + 0.0065319 kmol ISO = 0.015095 kmol

*Mole fraction of ETOH in feed

x ETOH = (0.0085631 kmol ETOH)/ 0.015095 kmol = 0.567

*Mole fraction ISO in feed

x ISO = 1 – 0.567 = 0.433

*Reflux Ratio

RMIN = (xD – y)/(y-x)

RMIN = (0.97- 0.61)/(0.61- 0.567) = 8.3721

RACT = 1.25 * RMIN

RACT = 1.25 * 8.3721 = 10.4651

*Design Column Vapor Flow Rate

vEXP = 0.970 (in/min) * (1 ft./12 in) * (60 min/1 hr) = 4.85 ft./hr

HETP = 4.30 in.

VEXP = VDESIGN = mEXP/(rEXPAEXP) = mDESIGN / (rDESIGNADESIGN)

rEXP = 0.0154 kmol/L rDESIGN = 0.0159 kmol/L

mEXP = (4.85 ft/hr)* (0.0154 kmol/L)* (p(.25)2ft2)) * (28.306 L/ft3) = 0.41512 kmol/hr

mDESIGN = (0.41512 kmol/hr)/*(1/0.0154 kmol/L)*(1/(p(.25)2ft2)*(0.0159 kmol/L)

(p(1)2(ft)) = 6.857567 kmol/hr

MWAVG,D = 46.493 kg/kmol

VD = (6.857567 kmol/hr) * (46.493 kg/kmol)

VD = 318.82886 kg/hr

*Equation of Top Operating Line

y = (L/V)x + (1-(L/V))xD = (RACT / RACT +1)x + (1/ RACT +1)(0.97)

= 0.912779x + 0.084605

*Distillate Rate

R = (V-D)/D = 10.4651

318.82886 (kg/hr) – D = 10.4651D

D = 27.808642 (kg/hr)

R = L/D = 10.4651 * 27.808642 (kg/hr) = L

L = 291.02022 (kg/hr)

*Bottoms Flow Rate

L/V = R (z -xB) + q (xD – xB)

R(z -xB) + q(xD-xB) -(xD-z)

z= Feed mole fraction of ethanol

q= 1 (feed assumes to be liquid)

L/V = 10.4651(0.567-0.03) + 1(0.97-0.03)

10.4651(0.567-0.03) + 1(0.97-0.03) -(0.97-0.567)

L/V = 1.05

L/V = (VB + 1)/ VB = 1.05

VB = 20

B = V/ VB = (318.82886 kg/hr)/20 = 15.941443 (kg/hr)

*Feed Flow Rate

F = D + B = 15.941443 (kg/hr) + 318.82886 (kg/hr) = 334.7703 kg/hr

*Bottom Operating Line

y = (L/V)x – ((L/V)-1) xB = 1.05x – 0.0015

* Condenser Heat Duty

QCOND = V * DHVAP

DHVAP = xETOH * DHVAP,ETOH + xISOP * DHVAP,ISOP

QCOND =

Bibliography

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