MAY 2010 Bornova-İZMİR REPORT TO DEPARTMENT OF CHEMICAL ENGINEERING EGE UNIVERSITY FOR COURSE: CHE386 CONCEPTUAL DESIGN II DESIGN OF A CONTINUOUS DISTILLATION COLUMN FOR A MULTICOMPONENT MIXTURE DESIGN REPORT IV Y.Doc.Dr. Serap CESUR SUBMITTED TO 25/05/2010 SUBMISSION DATE 05070008901 Ürün ARDA GROUP 3 05070008103 Berna KAYA 05070008849 Demet ACARGİL 05060008091 M. Serkan ACARSER 05060008017 Tayfun EVCİL
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Design of a Continuous Distillation Column for a Multi Component Mixture
Separation of multicomponent mixtures is very hard and expensive method in industry. For high purity products and high percentage recovery, continuous distillation columns are used. In this study, a saturated vapor mixture which has 5 different components is tried to be separated and with the help of assumptions, simplifications the plate type distillation column design methods is tried to be explored. In this report you can find the information about shortcut design in ideal and real calculations, using the SRK model. The necessary assumptions were done for both real and ideal calculations, and with the help of Fenske, Underwood and Kirkbridge equations, the number of plates were found.
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MAY 2010 Bornova-İZMİR
REPORT TO DEPARTMENT OF CHEMICAL ENGINEERING EGE UNIVERSITY
FOR
COURSE: CHE386 CONCEPTUAL DESIGN II
DESIGN OF A CONTINUOUS DISTILLATION COLUMN FOR A MULTICOMPONENT MIXTURE
DESIGN REPORT IV
Y.Doc.Dr. Serap CESUR
SUBMITTED TO
25/05/2010
SUBMISSION DATE
05070008901 Ürün ARDA
GROUP 3
05070008103 Berna KAYA 05070008849 Demet ACARGİL 05060008091 M. Serkan ACARSER 05060008017 Tayfun EVCİL
i
SUMMARY
Separation of multicomponent mixtures is very hard and expensive method in
industry. For high purity products and high percentage recovery, continuous distillation
columns are used.
In this study, a saturated vapor mixture which has 5 different components is tried to be
separated and with the help of assumptions, simplifications the plate type distillation column
design methods is tried to be explored.
In this report you can find the information about shortcut design in ideal and real calculations, using the SRK model. The necessary assumptions were done for both real and ideal calculations, and with the help of Fenske, Underwood and Kirkbridge equations, the number of plates were found.
TABLE OF CONTENTS
Summary ...................................................................................................................... i
6.1 Ideal System ................................................................................................... 20
6.2 Real System (Non-Ideal) ................................................................................ 27
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1.0 INTRODUCTION
Distillation is probably the most widely used separation process in the chemical and allied industries; its applications ranging from the rectification of alcohol, which has been practiced since antiquity, to the fractionation of crude oil. A good understanding of methods used for correlating vapour-liquid equilibrium data is essential to the understanding of distillation and other equilibrium-staged processes.
Distillation column design The design of a distillation column can be divided into the following steps:
1. Specify the degree of separation required: set product specifications.
2. Select the operating conditions: batch or continuous; operating pressure.
3. Select the type of contacting device: plates or packing.
4. Determine the stage and reflux requirements: the number of equilibrium stages.
5. Size the column: diameter, number of real stages.
6. Design the column internals: plates, distributors, packing supports.
7. Mechanical design: vessel and internal fittings.
The principal step will be to determine the stage and reflux requirements. This is a relatively simple procedure when the feed is a binary mixture, but a complex and difficult task when the feed contains more than two components (multicomponent systems).
Process Description The separation of liquid mixtures by distillation depends on differences in volatility between the components. The greater the relative volatilities, the easier the separation. The basic equipment required for continuous distillation is shown in Figure1. Vapor flows up the column and liquid counter-currently down the column. The vapor and liquid are brought into contact on plates, or packing. Part of the condensate from the condenser is returned to the top of the column to provide liquid flow above the feed point (reflux), and part of the liquid from the base of the
column is vaporized in the reboiler and returned to provide the vapor flow.
Figure 1. Distillation column (a) Basic column (b) Multiple feeds and side streams
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In the section below the feed, the more volatile components are stripped from the liquid and this is known as the stripping section. Above the feed, the concentration of the more volatile components is increased and this is called the enrichment, or more commonly, the rectifying section. Figure1a shows a column producing two product streams, referred to as tops and bottoms, from a single feed. Columns are occasionally used with more than one feed, and with side streams withdrawn at points up the column, Figure1b. This does not alter the basic operation, but complicates the analysis of the process, to some extent.
If the process requirement is to strip a volatile component from a relatively non-
volatile solvent, the rectifying section may be omitted, and the column would then be called a stripping column. In some operations, where the top product is required as a vapor, only sufficient liquid is condensed to provide the reflux flow to the column, and the condenser is referred to as a partial condenser. When the liquid is totally condensed, the liquid returned to the column will have the same composition as the top product. In a partial condenser the reflux will be in equilibrium with the vapor leaving the condenser. Virtually pure top and bottom products can be obtained in a single column from a binary feed, but where the feed contains more than two components; only a single “pure” product can be produced, either from the top or bottom of the column. Several columns will be needed to separate a multicomponent feed into its constituent parts.
The problem of determining the stage and reflux requirements for multicomponent
distillations is much more complex than for binary mixtures. With a multicomponent mixture, fixing one component composition does not uniquely determine the other component compositions and the stage temperature.
Also when the feed contains more than two components it is not possible to specify the complete composition of the top and bottom products independently. The separation between the top and bottom products is specified by setting limits on two “key” components, between which it is desired to make the separation.
SHORT-CUT METHODS FOR STAGE AND REFLUX REQUIREMENTS Most of the short-cut methods were developed for the design of separation columns
for hydrocarbon systems in the petroleum and petrochemical systems industries, and caution must be exercised when applying them to other systems. They usually depend on the assumption of constant relative volatility, and should not be used for severely non-ideal systems.
In this project, during the shortcut calculations, Fenske, Underwood, Gilliand and Kirkbridge Equations were used. Trial and error procedures were all made in excel.
Qr 1.0981*10 [kJ/h] 1.5182*107 2.0782*107 1.3445*107 1.6457*107 2.2480*107 7 R 3.5214 D,min 3.5214 3.5214 3.7715 3.7715 3.7715 N 35.0220 min 35.0220 35.0220 37.6120 37.6120 37.6120
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3.0 DISCUSSION AND CONCLUSION
The object of this report is to design a multi-component distillation column for ideal and non-ideal situations. Consider the case of the distillation of a multi-component mixture in a multi-stage distillation column. As before, the number of degrees of freedom is determined by the description rule (i.e., D.O.F. = number of variables set during construction or controlled during operation by independent means). Generally, all the variables associated with the feed, such as its composition, flow rate and enthalpy are set, as is the column pressure, which leaves four degrees of freedom and two basic types of problems (design and simulation), as was the case for a binary distillation.
For a design problem, the goal is to determine the number of plates needed and the location of the feed plate, and the following is generally specified
• Separation variable #1, the recovery of the light key component in the top (distillate) product.
• Separation variable #2, the recovery of the heavy key component in the bottom product.
• Condenser and reboiler loads • Diameter and height of the column • The fact that the optimal feed plate is used that minimizes the total number of plates. • The reflux ratio.
Generally, in a multi-component distillation column, only two components will exist in significant quantities in both the bottom and top products. These are the two key components. The heavy non-key components will essentially all end up in the bottom product while the light non-key components will essentially all end up in the top product. This is why the separation variables described above are given in terms of the key components.
In our process, the components were methanol, ethanol, neopentanol, n-butanol and 1-pentanol. Our feed rate was 100 kmol/h; the light-key component was neopentanol and the heavy-key component was n-butanol. Our mole fractions in the feed were 15%, 25%, 25%,15% and 20% respectively (See Table 2). The separation targets for top and bottom products were 95%. In order to find temperature and equilibrium constants for the feed, a temperature was assumed. The Psat values were calculated at this temperature, by using Antoine equation (See Table 5). The equilibrium constants were calculated with this assumed temperature (See Table 2). By using Goal-seek, we equalized . The dew-point
temperature was calculated as 113.66°C. According to the calculated temperature, the equilibrium constants were found as 5.14, 3.5, 1.02, 0.86, and 0.42. To calculate relative volatilities of the components, the ratio of K value of each component to the K value of the heavy-key product was taken. After this, the relative volatilities were found as, 5.95, 4.05, 1.18, 1 and 0.48 (See Table 6).
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In the ideal calculation, since we only had specific target separation for the light-key and heavy- key products were known (which was 95% for light-key at top and 95% for heavy at the bottom), the other separations were assumed 98% for methanol, 96% for ethanol, 95% for neopentanol, 5% for n-butanol and 2% 1-pentanol (See Table 3). The flow rate of top product was found as 63.6 kmol/h. As a result, the mole fractions for the top product were calculated as 0.231, 0.377, 0.373, 0.012, and 0.006 (See Table 3). In order to find temperature and equilibrium constants, a temperature was assumed. The Psat
In the bottom product, the separations were assumed 2% for methanol, 4% for ethanol, 5% for neopentanol, 95% for n-butanol and 98% 1-pentanol (See Table 4). The flow rate of bottom product was found as 36.4 kmol/h. As a result, the mole fractions for the top product were calculated as 0.008, 0.027, 0.034, 0.391 and 0.538 (See Table 4). In order to find temperature and equilibrium constants, a temperature was assumed. The P
values were calculated at this temperature, by using Antoine equation (See Table 5).The equilibrium constants were calculated with this assumed temperature. By using Goal-seek, we equalized . The
dew-point temperature was calculated as 96.84°C.According to the calculated temperature, the equilibrium constants were found as 3.10, 2, 0.55, 0.45, and 0.21 (See Table 3). To calculate relative volatilities of the components, the ratio of K value of each component to the K value of the heavy-key product was taken. After this, the relative volatilities were found as, 6.87, 4.43, 1.22, 1 and 0.46 (See Table 6).
sat
The average relative volatilities of the components were determined by taking the geometric mean of the components’ volatilities at the feed, top product and bottom product. The values were 6.11, 4.12, 1.19, 1 and 0.48, respectively (See Table 6).
values were calculated at this temperature, by using Antoine equation (See Table 5). The equilibrium constants were calculated with this assumed temperature. By using Goal-seek, we equalized . The dew-point temperature was calculated as 122.39°C. According to the calculated temperature, the equilibrium constants were found as 6.57, 4.59, 1.37, 1.18 and 0.58 (See Table 4). To calculate relative volatilities of the components, the ratio of K value of each component to the K value of the heavy-key product was taken. After this, the relative volatilities were found as, 5.58, 3.90, 1.16, 1 and 0.49 (See Table 6).
Fenske equation was used to calculate the minimum number of trays. As a result, the minimum plate number was found as 34.33 (See Table 7).
To find the minimum reflux ratio, the θ parameter was calculated at saturated vapor condition (q=0), and it must be between the relative volatilities of the light-key and heavy-key component, and was calculated using Goal-seek. As a result, the θ value was found as 1.075. Using Underwood equation, the minimum reflux ratio was determined as 3.57 (See Table 7).
The ratio of was assumed as 1.5, 1.8 and 2.4 respectively and the actual reflux values were calculated and tabulated (See Table 8).
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The Gilliland correlation was used to calculate actual plate number. For this, X and Y parameters were calculated and Nactual was found as 56, 51 and 46, using the Y function and Nmin
To determine the feed location, we used Kirkbridge equation. For N
value (See Table 8).
actual being 56, the entry location was found as the 26th tray. The NR and NS
To calculate reboiler and condenser duty, the average latent heat of vaporization was calculated. Our reboiler and condenser were selected as total reboiler and condenser. As a result, the reboiler duty was found as 3712.1kW for R
values were tabulated (See Table 8).
min
The height and diameter of the column were calculated lastly. The diameter was found as 0.698m and the height was 28.5 m. Other values were tabulated (See Table 8 and 9).
/R at 1.5. The condenser duty was found as 4376.1kW for the same ratio. Other values were tabulated (See Table 8).
In the case of the non-ideal system, the critical temperature, critical pressure, density, molecular weight and acentric factor were looked up from Ref.4 (See Table 10). The constants for the SRK Model were taken, a temperature was assumed and the reduced temperatures were calculated accordingly. The parameters αSRK, ai, bi, βi and qi were determined. Zv
i was calculated using Goal-seek and Ivi and Φv
i were determined by using the related equations in Ref.2.Same procedure was followed for ZL
i, ILi and ΦL
i. The equilibrium
constants were determined using . The liquid mole fractions were calculated to be used in
Modified Raoult’s Law. The Pisat values were determined by Antoine equation. The vapor-
liquid equilibrium was proven by the equality of fugacity coefficients of vapor and liquid phases. The activity coefficient was calculated from Modified Raoult’s Law. The assumed dew-point temperature was checked using the γi and Psat
The Fenske, Underwood, Gilliland and Kirkbridge equations were used like in the ideal system calculations. From these equations, N
values, and from Goal-seek, the actual dew-point temperatures was calculated as 106.6°C for feed, 96.98°C for top product. Same procedure was applied to the bottom product to find bubble-point temperature, which was found as 128.5°C. The average relative volatilities were calculated and found as 7.47, 4.01, 1.16, 1 and 0.59 respectively (See Table 32).
min was found as 39.56, θ was found as 1.051, Rmin was 3.516. For Rmin/R as 1.5, 1.8 and 2.4, our reflux ratios were found as 5.274, 6.328 and 8.438 respectively. For Rmin/R as 1.5, 1.8 and 2.4, Nact
The condenser and reboiler duties were calculated like in the ideal system. Q
were found as 64, 59, 53. The feed locations were found and tabulated (See Table 34).
c and Qr
Finally, the diameter and height were calculated and were found as 0.737m and 32.5 m respectively. Other values were tabulated (See Table 34 and 35).
were calculated and tabulated (See Table 34).
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According to these results, it can be seen that the ideal system and non-ideal system are slightly different from each other. For both systems, actual plate numbers decrease with increasing Rmin/R. Also, height of the column decreases with increasing Rmin/R. Both Qc and Qr increase with increasing Rmin
In real systems, the relative volatilities of components are greater than the relative volatilities of ideal system. Also, the number of plates used are greater than that of ideal systems. The condenser and reboiler duties of real systems are less than the ideal system’s duties. The diameter and height of the column in real systems are greater than the diameter and height of the column in ideal systems. Finally, the most suitable reflux ratio for the real system can be selected within the calculation of total and operation costs. However, the optimum reflux ratio can be chosen without any calculations of costs and it can be seen in Table 34 as 1.8 for this project.
/R, as expected.
In conclusion, the calculated values by using ChemCAD for the ideal and non ideal systems were tabulated in table 36.
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4.0 NOMENCLATURE
Symbol Name Unit
M Molecular Weight of i Component w,i [kg/kmol] ρ Density of i Component i [kg/m3
ρ]
Vapor Density of i Component vap [kg/m3
ρ]
Liquid Density of i Component liquid [kg/m3
F ]
Mol Number of Feed Stream [kmol/h] D Mol Number of Top Product [kmol/h] W Mol Number of Bottom Product [kmol/h] n Mole Number of i Component i [kmol/h] x Mole Fraction of i Component i P.R. Percentage Recovery of i Component i K Distribution Coefficient of i Component i P Saturated Vapor Pressure sat [bar] P Total Pressure t [bar] α Relative Volatilities of i Component i N Minimum Number of Plate min N Actual Number of Plate act N Number of Plate of Stripping Section S N Number of Plate of Enriching Section R R Minimum Reflux Ratio D,min R Actual Reflux Ratio D,act λ Latent Heat of Vaporization i
MW Molecular Weight of Mixing u Fluid Velocity v [m/s] D Diameter of Distillation Column c [m] T Reduce Temperature r f Fugacity Φ Fugacity Coefficient T Temperature [o
TC]
Critical Temperature c [K] P Critical Pressure c [bar] Q Reboiler Load r [kJ/s] Q Condenser Load c [kJ/s]
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5.0 REFERENCES
1. Coulson, J.M., Richardson, J.F., Chemical Engineering Series - Chemical Engineering Design, Vol. 6, 4th Ed., Great Britain Pergamon Press, 1977.
2. J.M. Smith, H.C. Van Ness, M.M. Abbott, Introduction to Chemical Engineering Thermodynamics, 2005, 7th
Ed., Mc Graw Hill Company, Singapore.
3. Felder, R.M., Rousseau, R.W., Elementary Principles of Chemical Process, 2nd
Ed., John Willey and Sons Inc, USA, 1986.
4. Yaws C.L., Yaws' Handbook of Thermodynamic and Physical Properties of Chemical Compounds, Lamar University, Beaumont, Texas,
Norwich, New York, 2004
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6.0 APPENDIX
6.1 Ideal System
(See Table 4).
The mixture of methanol, ethanol, neopentanol, n-butanol and 1-pentanol was fed to the column as 100 kmol/h. The given data for the feed, top and bottom product was tabulated in Table 2, 3, 4.
Calculation of Mole Numbers of Components in Feed Stream
1
2
3
4
5
100
0.15
0.25
0.25
0.15
0.20
F kmol hxxxxx
=
=
=
=
=
=
Assumption of Percentage Recoveries for Top Product