Sultan Qaboos University

College of Engineering PCE Department

Chemical Reaction Engineering

PBR Design for Phthalic Anhydride Production

Ahmed Hamed Al-Qasmi 88805 Abdullah Ghusen Al-Abri 89672 Abdullah Mohammed Al-Kharosi 90033

I

Abstract These papers illustrate a reactor design to produce Phthalic anhydride (PAN) in continuous plant through the oxidation of o-xylene. Maleic anhydride is also obtained as by-product in this process. Pressure drop, side reactions and maintaining a relatively constant driving force for heat transfer are considered in this design. The production of phthalic anhydride is approximately 20,000 tons/year as required. The process was simulated using the process simulator Aspen plus.

II

Table of Contents Abstract ................................................................................................................................. I Table of Contents .................................................................................................................. II List of Tables ....................................................................................................................... III List of figures ....................................................................................................................... IV Introduction .......................................................................................................................... 1

Uses .................................................................................................................................. 1 Manufacturing company .................................................................................................... 2 The feed stock ................................................................................................................... 2 Process description ............................................................................................................ 3 Reactions path way............................................................................................................ 3 Type of reactors ................................................................................................................ 4 Catalyst ............................................................................................................................. 5 Kinetics ............................................................................................................................. 6

First Design: Reactor Volume Using Simple Reaction Rate Expression .................................... 7 Mass balance: .................................................................................................................... 8 Calculating the volume ...................................................................................................... 8 The simulation .................................................................................................................. 9

Second Design: Pressure Drop and Reactor Configuration .................................................... 11 Splitting the Reactor into Tubes ....................................................................................... 13 Tube Length effect .......................................................................................................... 13 Particle Diameter effect ................................................................................................... 14 The simulation ................................................................................................................ 16

Third Design: Multiple Reactions ........................................................................................ 18 Plot Flow ........................................................................................................................ 19 Effect of Temperature in Selectivity ................................................................................ 20 Aspen simulation ............................................................................................................. 21

Final Design: Energy Balance............................................................................................. 23 Temperature profile ......................................................................................................... 23 Dynamic stability ............................................................................................................ 24

Aspen Simulation ................................................................................................................ 25 Economic analysis............................................................................................................... 30 Design summary.................................................................................................................. 31 Conclusion .......................................................................................................................... 31

III

References........................................................................................................................... 32 Appendix ............................................................................................................................. 33

List of Tables Table 1: Component Price ..................................................................................................... 2 Table 2: Fixed-Bed. vs. Fluidized-Bed Reactors for Selective Hydrocarbon Oxidation Reactions .............................................................................................................................. 4 Table 3:Catalyst Properties and Behavior in o-Xylene Oxidation .......................................... 5 Table 4: Mass Balance Results .............................................................................................. 8 Table 5:Polymath design results ............................................................................................ 8 Table 6: Heat and material balance ...................................................................................... 9 Table 7: Design Results without Sipliting to Tube ................................................................ 11 Table 8: Heat and material balance .................................................................................... 16 Table 9: Heat and material balance .................................................................................... 17 Table 10: Third Design Results ........................................................................................... 19 Table 11: Heat and Material Results in Aspen Plus ............................................................. 21 Table 12: Components Properties ....................................................................................... 23 Table 13: Gain Calculation ................................................................................................. 24 Table 14: Gain Calculation after Modification .................................................................... 25 Table 15 :Aspen Results ....................................................................................................... 29 Table 16: Total Cost Results ................................................................................................ 30 Table 17: Reactor Configurations ....................................................................................... 31 Table 18: Feed Rates........................................................................................................... 31 Table 19: Products Rates .................................................................................................... 31 Table 20 : Coolant Specification ......................................................................................... 31

IV

List of figures Figure 1: Major uses of phthalic anhydride[1] ...................................................................... 1 Figure 2: Major product chain of phthalic anhydride ............................................................ 2 Figure 3: Reactions Pathway ................................................................................................ 4 Figure 4:Effect of temperature in the conversion ................................................................... 9 Figure 5 : Aspen Reactor Model ............................................................................................. 9 Figure 6 : conversion versus heat duty ................................................................................. 10 Figure 7:Effect of temperature in the conversion ................................................................. 12 Figure 8:Effect of Pressure Drop in the conversion at Different Temprature ....................... 12 Figure 9:The effect of number of tubes in the pressure drop ................................................ 13 Figure 10:Effect of Tube Length in Pressure Drop. ............................................................. 14 Figure 11: Comparing two paticle diameters effect in pressure drop. .................................. 14 Figure 12: Final design results............................................................................................ 15 Figure 13: Shows the Catalyst Weight Effects in Pressure Drop and Conversion ................. 15 Figure 14: Molar flow rate of each component versus the catalyst weight ............................ 19 Figure 15: Selectivity of Phthalic Anhydride at Different Temperature ................................ 20 Figure 16: Selectivity of Phthalic Anhydride at Different Temperature ................................ 20 Figure 17: Molar Flow of each Component with Reactor Length obtained from Aspen ........ 22 Figure 18: Temperature Profile ........................................................................................... 23 Figure 19:Coolant Temperature Profile .............................................................................. 24 Figure 20: Reactor and Coolant Temperature Profiles ........................................................ 24 Figure 21:Aspen Flowsheet ................................................................................................. 25 Figure 22 : Model Type ....................................................................................................... 26 Figure 23 : Design Dimensions ........................................................................................... 26 Figure 24: Pressure Drop Calculation ................................................................................ 27 Figure 25: Catalyst specification ......................................................................................... 27 Figure 26 : Reactions in Aspen ............................................................................................. 28 Figure 27: Kinetics Specification ........................................................................................ 28 Figure 28 : Temperature Profile ........................................................................................... 29

1

Introduction Phthalic anhydride (PAN) is an organic compound with the formula C6H4(CO)2O. It is the anhydride of phthalic acid. This colorless solid is an important industrial chemical. It is one of the most important products of modem large-scale organic synthesis, and it has a wide application in various branches of chemical industry. Uses PAN is an intermediate petrochemical used for the production of plasticizers especially for polyvinylchloride (PVC), unsaturated Polyesters, dyes and alkyd resins which are used in the manufacture of reinforced plastics and in specialized applications such as surface coatings. It is trade in either molten form or as a white powder. About 50% of all phthalic anhydride produced is used in the manufacture of plasticizers for PVC processing. Unsaturated polyester resins are uses (15-20) % of all PAN produced and (10-15) % used in alkyd resins.[1]

The figure below shows the major product chain of phthalic anhydride in chemical industry.

Figure 1: Major uses of phthalic anhydride[1]

2

Figure 2: Major product chain of phthalic anhydride Manufacturing company PAN is currently produced in many region of the world. In Australia one company has two plants were built in 1960s and use naphthalene as the feed. Several large producers of PAN operate in Japan, Korea, and Taiwan. The major company that produce PAN are NCP coatings inc , Arkema inc, Esco company and Durcon inc . In 2006, the worldwide production of PAN has slowly increased with production approaching the estimated capacity of 3,600,000 tones/year.[2] The selling price has varied due fluctuating demands and production rates but is currently around $(850-950)/ton with purity of 99.5%.[3] The feed stock PAN is produced from o-xylene feedstock or naphthalene. In 1896 the feed stock for the liquid phase oxidation was naphthalene. In the United State and Germany they try to come up with process to oxide naphthalene using vapor phase and they success in 1930.The shortage of naphthalene with availability of o-xylene the company forced to use it. Since the 1960s o-xylene has replaced naphthalene as feedstock more and more. The selling price of pure o-xylene around $5.05/kg while the price for naphthalene around $5.96/kg[1].Currently 90% of phthalic anhydride is produced from o-xylene because of the yield since it is the main factor affecting the choice of the feed stock and the price. Table 1: Component Price

Component Price($) O-xylene 5.05/kg Naphthalene 5.96/kg

3

Process description The manufacturing of Phthalic Anhydride (PAN) is done in several steps. As simplicity, mixture of O-xylene and air is preheated and converting o-xylene to vapor knowing that the boiling point of the o-xylene is 144.4 C. Then, the mixture is fed to a catalytic reactor. The reaction is highly exothermic, so to maintain the temperature, the reactor is designed as packed bed reactor with heat exchanger. Water is used to cool the reactor. The products then purified using other units like distillation. There are five reactions happen beside the production of phthalic anhydride by oxidation of o-xylene. These reactions are listed below.[6] + + + . + + . + + . + + + + The first reaction is the main reaction and it is assumed 70% selectivity. The second Reaction refers to the formation of the by-product maleic anhydride (MA) and a 10% selectivity is considered. The third and the fourth reactions represent the complete and incomplete combustions of o-xylene with 15% and 5% selectivity, respectively. Reactions path way In the reactor, the o-xylene is subject to a variety of oxidation reaction to the desired product of Phthalic anhydride; by product Maleic anhydride and products of combustion. The oxidation of o-xylene feed with the air produce Phthalic anhydride. The reactor effluent containing pure Phthalic anhydride reacts with excess oxygen to produce water and carbon dioxide. Also, o-xylene reacts with the oxygen to produce the Maleic anhydride as by product that also react with excess oxygen to produce water and carbon dioxide.

4

Type of reactors In the past, phthalic anhydride (PAN) was produced in a slurry. After o-xylene has replaced naphthalene as feedstock, the reactors were developed and fluidized bed and fixed bed reactors were used. Naphthalene-based feedstock is made up of vaporized naphthalene and compressed air. It is transferred to the fluidized bed reactor and oxidized in the presence of a catalyst. In PAN production using o-xylene as the basic feedstock, filtered air is preheated, compressed, and mixed with vaporized o-xylene and fed into the fixed-bed tubular reactors. Nowadays, fixed bed catalytic reactor is commonly used. The table below shows comparison between fluidized bed and fixed bed for many parameters.[5] Table 2: Fixed-Bed. vs. Fluidized-Bed Reactors for Selective Hydrocarbon Oxidation Reactions

Fixed-bed reactor

Fluidized-bed reactor Hydrocarbon concentration

Below flammability limit Flammable region possible

Oxygen concentration Large excess Near stoichiometric Temperature control Hot spot Nearly isothermal Catalyst effectiveness Poor to average Good

Catalyst cost Least expensive More expensive Capital investment Expensive Less expensive Raw material Cost Lower Higher

Raw material availability More available Less available CO2 emission Less More

PAN Yield Higher Lower

Figure 3: Reactions Pathway

5

Catalyst There are different types of catalysts which are used for the oxidation of o-xylane to PAN. Types that contain V2O5 supported on TiO2 are common and affective catalysts for this process. The amount of V2O5 is used affects the selectivity of PAN and carbon dioxide and the temperature for total conversion. Table (3) shows some comparison. It seems that the catalyst contain 5.6% is more active. It has selectivity of 50% PAN when 100% conversion of o-xylene and the best temperature is 553K.[8] Table 3:Catalyst Properties and Behavior in o-Xylene Oxidation

Catalyst Vanadium (wt%)

Surface area (m2/g)

Yield of PA (%)

Temp. (K)

VPTGA(1) 0.68 111 26 558 VPTGA(5) 2.85 96 17 513 VPTGA(10) 5.6 104 50 553 VPTGB 0.58 152 25 568 VP25 0.13 56 18 593

6

Kinetics The kinetic of the five reactions are:[7]

7

First Design: Reactor Volume Using Simple Reaction Rate Expression In this design we develop an isothermal reactor model to estimate the reactor volume for the production of phthalic anhydride by oxidation of o-xylene based on the reaction below CH + 3O CHO + 3HO A catalyst weight of a packed bed reactor model is obtained and then converts the weight to a reactor volume using catalyst bulk density. In this design we will explore a reactor with a single reaction. However, reactions to byproducts will be added in the third design. The designing of fixed bed reactor depend on the following rate of the reaction that obtained from literature: r = kPP ln = + 19.837 k =4.1219e8 kmole /(kg catalyst).atm2 hr. The activation energy is 27000 cal/mol[1]. The design differential mole balance equation is d(x)d(w) = rFa With the following specifications: T=673K P=3atm W (f) =21071kg Polymath solved the differential equation and gave the relation between the weight of catalyst and the conversion. The typical conversion is 85%. The calculations were repeated at different temperatures to visualize the effect of temperature in the conversion and the weight of the catalyst required to achieve the specified conversion. The calculations can be obtained in matlab which is easier and the code is in the appendix. Table (4) shows the results at 400 C and 3 atm. Figure (4) shows the effect of temperature in the conversion. From figure (4), the weight of the catalyst required to reach 85% conversion decrease with increasing in temperature. At 633 K, 75 000 kg of catalysts is needed however; 21000 kg is only required to achieve the specified conversion.

8

Mass balance: Table 4: Mass Balance Results

species Initial (Kmol/s)

Change (kmol/s)

Final (Kmol/s)

0.0523 -0.04453 7.858 10 0.7259 -0.1333 0.5925

0 0.0444 0.0444 0 0.1333 0.1333

2.7308 0 2.7308 Total 3.509 0 3.509

Table 5:Polymath design results

Variable Final value 1 w (kg) 2.107E+04 2 X 0.8500067 3 T (K) 673 4 P (atm) 3

Calculating the volume To calculate the volume of the packed bed reactor, we use the weight catalyst, the density of the particle and the porous media coefficient by the following equation: V = w(1 ) = 210713357(1 0.5) = 12.55m Where the (w) is the weight of catalyst in kg The is the density of the particle in kg/m3 [2] is the porose media of the catalyst (V2O5/TiO2)[2]

9

Hea t and Material Ba la nce TableStrea m ID FEED P RODUCTTem perature C 399. 9 399. 9P ressure ba r 3.040 3.000Vapor Frac 1.000 1.000Mole Flow km ol/hr 12686.360 12686.360Ma ss Flow kg/hr 380541.602 380541.602Volum e Flow cum/hr 233529.366 236623.630Enthalpy Gca l/hr 37 .683 -4.826Mole Flow km ol/hr C8H10-01 188. 597 30 .143 O2 2612.819 2137.456 C8H4O-01 158. 454 H2O 475. 363 N2 9884.945 9884.945

The simulation We simulate the packed bed reactor in Aspen Plus using the RPLUG reactor. The specifications of the feed are 673K and 3 atm. The table below shows the result obtained of heat and material balance.

Figure 4:Effect of temperature in the conversion

Table 6: Heat and material balance

Figure 5 : Aspen Reactor Model

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The conversion can be calculating using the flow rate of the o-xylene obtained from aspen plus by the equation below, and we get very close answer to the calculated result. x = FFF = 188.597 30.143188.597 = 0.84 To construct the plot of conversion versus heat duty we use the data composition obtained and multiplied by the total flow of the outlet and use it to find the conversion.

The figure above shows the relationship between the conversions versus heat duty. As can be seen from the figure, as we increase the conversion the heat duty increase. Actually there is a proportional linear relationship between them.

00.10.20.30.40.50.60.70.80.9

0 10000 20000 30000 40000 50000 60000

X

Q (KW)

Figure 6 : conversion versus heat duty

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Second Design: Pressure Drop and Reactor Configuration Based on the previous design (isothermal reactor), the pressure drop consideration is taken into account in this design. The minimum pressure ratio (P/P0) is 0.9 that give us the outlet pressure of 2.7atm.The consideration of pressure drop is important to get the suitable outlet pressure that needed for downstream operation. Moreover, if we have very large pressure drop, we have to increase the pressure before it fed to the next operation hence increase the capital and operating cost. The optimization will be done in this design to find the typical conversion which is 0.85 and reasonable reactor configuration using multiple tubes, changing weight of catalyst and the temperature. The differential mole balance equation is d(x)d(w) = rFa And the momentum balance Ergun equation is ()() = (1+x) With the following specifications: T=673K P=3atm W (f) =21071kg = y(0) =1 Polymath solved the differential equations and gave the relation between the weight of catalyst and the conversion. The calculations were repeated at different temperatures to visualize the effect of temperature in the conversion with consideration of pressure drop and compare it with the result of previous design. Table 7: Design Results without Sipliting to Tube

Variable Final value 1 w (kg) 2.107E+04 2 x 0.7044096 3 y 0.4936886 4 T (K) 673 5 N 1 6 P0 (atm) 3 7 D (m) 3.5 8 V (m3) 12.55347 9 L (m) 1.304257

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Figure (7) above shows the effect of catalyst weight on the conversion at different range of temperature with consideration of pressure drop. As can be seen, the conversion increases as the catalyst weight increase. When the pressure drop increased the concentration decreased resulting in a decreased rate of reaction. As a result of this smaller reaction rate, the conversion will be less with pressure drop than without pressure drop as we observed that on figure (8).

00.10.20.30.40.50.60.70.8

0 5000 10000 15000 20000 25000

X

W(kg)

Weight VS conversion T=673KT=650KT=630KT=600KT=573K

00.10.20.30.40.50.60.70.80.9

0 5000 10000 15000 20000 25000

X

W(kg)

Weight VS conversion

T=673K without (y)T=673K with (y)T=630K without(y)T=630K with(y)

Figure 7:Effect of temperature in the conversion

Figure 8:Effect of Pressure Drop in the conversion at Different Temprature

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Splitting the Reactor into Tubes After the calculation of conversion and pressure drop, it can be seen that the pressure drop is less than 10%. The proper solution is to split the flow in tubes to decrease the pressure drop since it function of mass flow rate. From figure (9), it is clear that dividing the flow rate into tubes affect the pressure drop. Pressure drop is decreasing with increasing the number of tubes. So, using banks of tubes will solve the pressure drop problem and the design will meet the specifications.

Tube Length effect After optimizing the design to achieve the required conversion 85% and the pressure drop constrain 10% maximum, the number of tubes was chosen to be 25140 tubes. The weight of catalyst is 5.8 kg in each tube. The tube diameter was selected as 1.5 inches. The length of the tube is changing to discuss the effect of tube length in the pressure drop. Figure (10) shows the results of different tube length in pressure drop. As can be seen from the figure, increasing the tube length will increase the pressure drop and that due to the friction. Also, since the volume of the reactor is fixed, increasing the length will decrease the diameter and as a result the friction will be greater and pressure drop will increase. From the calculation, a length of three meters gives a pressure drop of 0.2885 atm which is a proximately 10%.

Figure 9:The effect of number of tubes in the pressure drop

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Particle Diameter effect As well known theoretically that particles with smaller size will minimize the spacing between them and as a result the fluid needs more force to pass through them. A large pressure drop will gain if smaller particle is used. For studying the particle size effect, the particles diameter that was used is doubled and the calculation is repeated. Figure (11) gives the result of both diameters used to calculate the pressure drop. As expected, the higher diameter has a less pressure drop of 0.1365 atm which is 5% approximately.

Figure 10:Effect of Tube Length in Pressure Drop.

Figure 11: Comparing two paticle diameters effect in pressure drop.

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After optimizing all parameters, the result shows below

Variable Final value 1 w (kg) 2.152E+04 2 x 0.8507969 3 y 0.9009362 4 T (k) 617 5 N 2.514E+04 6 P0 (atm) 3 7 D (in) 1.5 8 L (m) 0.653 9 Vt (m3) 86.87042

Figure 12: Final design results The number of tubes is considered to be 25140 tubes and contains 5.8 kg of catalyst in each one. The conversion is calculated and gives 85% and the pressure ratio is 0.9. Figure (13) shows the catalyst weight effects in pressure drop and conversion. The tube diameter of each tube is 1.5 inches and a length of 0.653 meter.

Figure 13: Shows the Catalyst Weight Effects in Pressure Drop and Conversion

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Heat and Materi al Balance TableStream ID FEED PRODUCTTemperature C 399 .9 399 .9Pressure bar 3.0 40 1.4 95Vapor Frac 1.0 00 1.0 00Mole Flow kmol/hr 126 86.3 60 126 86.3 60Mass Flow kg/hr 380 541.602 380 541.602Volume Flow cum/hr 233 529.366 474 674.606Enthalpy Gcal/hr 37.683 2.3 01Mole Flow kmol/hr C8H10-0 1 188 .597 56.708 O2 261 2.81 9 221 7.15 3 C8H4O-01 131 .889 H2O 395 .666 N2 988 4.94 5 988 4.94 5

The simulation Based on the simulation done in memo 2 of packed bed reactor in Aspen Plus using the RPLUG reactor, the pressure drop was including in this memo. We specify Ergun friction correlation to calculate process steam pressure drop. The specifications of the feed in the first run are 673K and 3 atm with the following configurations and catalyst weight. N (number of tube) =1 D=3.5m L=1.304257m W=21071kg The table below shows the result obtained of heat and material balance. The conversion can be calculating using the flow rate of the o-xylene obtained from aspen plus by the equation below, and we get very close answer to those obtained in POLYMATH. x = FFF = 188.597 56.708188.597 = 0.699 The conversion obtained using POLMATH was 0.704. As we observed the difference between them is 0.7%.

Table 8: Heat and material balance

17

Heat and Material Balance Tab leStream ID FEE D PRODUCTTemperature C 343. 9 343. 9Pressure bar 3. 040 2. 738Vapor Frac 1. 000 1. 000Mole Flow kmol/hr 12686. 360 12686. 360Mass Flow kg/hr 380541.602 380541. 602Volume Flow cum/hr 214097.502 237669. 115Enthalpy Gcal /hr 31. 909 -10.926Mole Flow kmol/hr C8H10-01 188. 597 28.761 O2 2612. 819 2133. 313 C8H4O-01 159. 835 H2O 479. 506 N2 9884. 945 9884. 945

In the second run, the specifications of the feed are 617K and 3 atm with the following configurations and catalyst weight that give us the desired conversion which is 0.85. N (number of tube) =25140 D=0.0381m for each tube L=3.03m W=21520 kg The table below shows the result obtained of heat and material balance.

The calculated conversion using flow rate of the o-xylene obtained from Aspen is 0.848 with the difference of 0.2% from POLYMATH result. In both trails the ratio of the pressure (P/Po) is exactly the same we obtained in POLYMATH.

Table 9: Heat and material balance

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Third Design: Multiple Reactions Based on the previous design (isothermal reactor), the side reactions are included in this design. There are four reactions happen beside the production of phthalic anhydride by oxidation of o-xylene. The reactions and their kinetics [7] are below.

The differential mole balance equations are ()() = r ()() = r ()() = r ()() = r ()() = r ()() = r And the momentum balance Ergun equation is d(y)d(w) = 2y FF TT

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With the following specifications: Table 10: Third Design Results

Variable Final value 1 w (kg) 23055 2 y 0.9005 3 T (k) 673 4 N 2.25E+04 5 P0 (atm) 3 6 D (in) 1.5 7 L (m) 0.653 8 Vt (m3) 86.87042

Plot Flow

Figure (14) shows the molar flow of each component with the weight of catalyst. The molar flow rate of all products is always increasing. Water is produced with high amount. Pressure drop was fixed to be 10% only.

Figure 14: Molar flow rate of each component versus the catalyst weight

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Effect of Temperature in Selectivity As can be seen from figure (15), the selectivity of phthalic anhydride is increasing with the decrease of temperature. It is as expected since the activation energy of a reaction that produce phthalic anhydride has the lowest value than the others. The selectivity of maleic anhydride is decreasing with increase of temperature. The relation is shown in figure (16).

Figure 15: Selectivity of Phthalic Anhydride at Different Temperature

Figure 16: Selectivity of Phthalic Anhydride at Different Temperature

0

0.5

1

1.5

2

2.5

3

0 5000 10000 15000 20000 25000

sd_pa

W(kg)

T=673KT=655KT=640K

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5000 10000 15000 20000 25000

sd_ma

W(kg)

sd_ma Vs W

T=673KT=655KT=640K

21

Heat and Material Balance TableStream ID FEED PRODUCTTemp erature C 399.9 399.9Pressure bar 3.040 2.737Vap or Frac 1.000 1.000Mole Flow kmol/hr 22547.160 22626.729Mass Flow kg/hr 676363.053 676363.053Volume Flow cum/hr 415046.066 462533.781Enthalp y Gcal/hr 66.980 -47.109Mole Flow kmol/hr C8H10-01 335.520 92.947 O2 4646.160 3432.062 C8H4O-01 160.015 H2O 848.566 N2 17565.480 17565.480 CO2 483.391 C4H2O-01 44.268

Aspen simulation Based on the simulation done in memo 3 of packed bed reactor in Aspen Plus using the RPLUG reactor, the four side reactions are added in the configuration with their kinetics. The specifications of the feed are 673K and 3 atm with the following configurations and catalyst weight. N (number of tube) =1 D=5.35m L=0.558m W=21071kg The table below shows the result obtained of heat and material balance.

The conversion can be calculating using the flow rate of the o-xylene obtained from aspen plus by the equation below, and we get very close answer to those obtained in POLYMATH. x = FFF = 335.520 92.947335.520 = 0.722 The conversion obtained using POLMATH was 0.726. As we observed the difference between them is 0.5%.However in this memo we care more about the exit flow rate of

Table 11: Heat and Material Results in Aspen Plus

22

our product (phthalic anhydride) to get the desired amount specified (20,000 tons/year). The outlet flow rate obtained using Aspen is 160.0 kmol/hr which is very close answer calculated by POLYMATH(160.5 kmol/hr). Figure (17) shows the flow rate of all species except the flow rate of O2 in the primary axis with reactor length. As can be seen, the reactant flow rate (O-xylene and Oxygen) are decreasing with the reactor length while the product species increasing with the reactor length. These results are expected from the reactions since all it are producing water and four reactions producing CO2 and only one reaction are producing phthalic anhydride.

0500100015002000250030003500400045005000

0100200300400500600700800900

0 0.1 0.2 0.3 0.4 0.5 0.6

flow r

ate (k

mol/h

)

flow r

ate (k

mol/h

)

L(m)

flow rate VS Length

f_O-xylenef_phthalicf_waterf_CO2f_o2

Figure 17: Molar Flow of each Component with Reactor Length obtained from Aspen

23

Final Design: Energy Balance Energy balance equation was added to the previous design. All reactions in phthalic anhydride production are exothermic reactions. The multi tubular reactor acts as a shell and tube heat exchanger with co-current coolant. The energy balance of the coolant was considered and steam was used as a coolant. The calculated heat capacity (Cp) of each component at the inlet temperature ( 673 oC ) are shown in table(12). Table (13) contains the Heat of formation for each component that was used to calculate the heat of reactions. Table 12: Components Properties

Component o-xylene Oxygen Phthalic anhydride

Maleic anhydride

Carbon dioxide

Water Nitrogen Cp (KJ/kmol.K)

458.6

32.73 723.5 269.9 48.96 37.23 30.6 H (KJ/mol) 19.1 0 -371.4 -398.2 -393.5 -241.80 0

The overall heat transfer coefficient was found in literature to be 58 (W/m2.K). the coolant inlet temperature is 655 K. the pressure drop was maintained not to exceed 10%. Temperature profile Figure (18) shows the temperature gradient with the weight of catalyst. A hot spot appear in the reactor. The hot spot temperature which is the highest temperature is calculated to be 672.8 oC. Then the temperature decreases and goes to constant. The outlet temperature of the reactor is 656.1 oC which is almost acceptable. Coolant temperature increases along the reactor and Figure (19) shows the relation.

Figure 18: Temperature Profile

24

Dynamic stability The reactor was tested dynamically by considering the gain. The inlet temperature of the coolant was changed to study the effect of changing in the hot spot temperature. The following results were obtained: Table 13: Gain Calculation

Ta ( K ) Th ( K ) Gain 638 715.2 17 636 689.1 9.1

Figure 19:Coolant Temperature Profile

Figure 20: Reactor and Coolant Temperature Profiles

25

From the results, the design is not stable dynamically since the gain is more than 2. As a solution the amount of inert component should be increased and in this case the ratio of the air to o-xylene was increased from 20 to 25 ( kg.air/kg.xylene). After the air amount was changed and other parameters were optimized, the dynamic results were shown in table (14). The coolant inlet temperature was set to be 643 K and hot spot temperature was found to be 688.1. Table 14: Gain Calculation after Modification

Ta ( K ) Th ( K ) Gain 642 681.2 1.9 644 684.8 1.7

Aspen Simulation After the reactor was designed to obtain the target production of phtalic anhydride, the reactor was simulated in Aspen Plus. The flow sheet of the model was shown in figure (21). PBR model was used with co-current coolant and specified heat transfer coefficient U of 3.5 (kw/m2.K). Figure (22) shows the model specification. The reactor configuration used in Aspen is the calculated dimensions. A length of 0.6 m, 1.5 in diameter and 36000 tubes was considered.

Figure 21:Aspen Flowsheet

26

Figure 22 : Model Type

Figure 23 : Design Dimensions The inlet reactor pressure is specified to be 3 atm. The Ergun equation was used to calculate the pressure drop in the reactor. Catalyst weight, porosity and particles diameter were added in the model specification. Figures (24) and (25) show the progress.

27

Figure 24: Pressure Drop Calculation

Figure 25: Catalyst specification

The five reactions were added to the design. POWERLAW type of kinetics was considered and the all kinetics were added. Rate phase, basis and activation energies were specified for each reaction. Figure (27) explains the kinetics specification way.

28

Figure 26 : Reactions in Aspen

Figure 27: Kinetics Specification

After running the simulation, the results were tabulated and then compared with the calculated values. The production of phthalic anhydride is consistent. The calculated phthalic anhydride is 163 kmol/hr and Aspen calculated to be 160 kmol/hr. The pressure drop was 0.3 atm as same as the calculated value. The temperature profile through the reactor was plotted and a hot spot can be determined. The hot spot temperature is 391.5 C, 665 K and calculated is 670 K. The results were shown below.

29

Table 15 :Aspen Results

Figure 28 : Temperature Profile

384385386387388389390391392

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Temp

eratur

e (c)

Tube Length (m)

30

Economic analysis The cost of production phthalic anhydride per year depends on purchase cost of the packed bed reactor with heat exchanger, annual cost of the raw material (air and o-xylene); cooling cost and catalyst cost (V2O5).The cost of production of 20000tons/year can be calculated using the following equation Cost=Purchase cost of the reactor + Annual cost of raw material + Annual cooling cost + catalyst cost- annual revenue of by product (malic anhydride)- Annual revenue of phthalic anhydride Purchase cost of the packed bed reactor with heat exchanger can be calculated using the following equation [1] ($) =1.25*1600(As). As = DLN Where N is number of tubes Cooling cost calculated by obtained first the heat duty then calculate mass flow rate using the following equation m = QCT Cooling cost ($) = 6.70 [1] Annual cost of the raw material and revenue of by product and the product depend on the flow rates multiply by it price. Table 16: Total Cost Results

Cost per year($) Purchase cost of the reactor 3244210

cost of raw material 106,143 cooling cost 365730 catalyst cost 670220

revenue of by product (malic anhydride) 3531407 revenue of phthalic anhydride 18,000,000

Total 17,145,104

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Design summary Table 17: Reactor Configurations

N (tubes) D (in) L (m) W ( kg ) T ( K ) P ( atm) 36000 1.5 0.6 40000 656 3

Table 18: Feed Rates

Component O-xylene Air Rate ( kg/s) 10 250

Table 19: Products Rates Component O-xylene Oxygen PAN MAN CO2 Water

Rate (kmol/s) 0.0256 1.4706 0.0454 0.0125 0.1352 0.2395

Table 20 : Coolant Specification Type of coolant Inlet temperature (K) Flow rate (kg/s)

Water 640 0.18

Conclusion The packed bed reactor design that used for phtalic anhydride production has been done. Pressure drop, side reactions, temperature variation were considered. After optimizing all parameters to achieve 20 ton/year of phthalic anhydride, reactor configurations listed in table (15). The reactor looks like a double pipe heat exchanger with 36000 tubes with 1.5 in. water steam at 640 K was used to maintain the temperature with a flow rate of 0.18 kg/s per tube. The feed rates to the reactor shown in table (16) and products rate tabulated in table (17).

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References

1. Chemical Engineering Design Project 2. Kirk-Othmer Encyclopedia of Chemical Technology Volume 18 3. Encyclopedia of Chemical Processing 4. Handbook of Industrial Catalysts By Lawrie Lloyd 5. CHEMSYSTEMS PERP PROGRAM Report Abstract Phthalic anhydride. 6. A novel route to produce phthalic anhydride by oxidation of o-xylene with air

over mesoporous V-Mo-MCM-41 molecular sieves. 7. Kinetics of the Selective Oxidation of o-Xylene to Phthalic Anhydride

Doctoral Thesis (Dissertation) to be awarded the degree of Doctor of Engineering (Dr.-Ing.) submitted by Dipl.-Ing. Robert Marx.

8. THE USE OF MATHEMATICAL MODELLING TO INVESTIGATE THE EFFECT OF CHEMISORPTION ON THE DYNAMIC BEHAVIOR OF CATALYTIC REACTORS. PARTIAL OXIDATION OF o-XYLENE IN FLUIDIZED BEDS By S. S. E. H. ELNASHAIE.

9. Simulation of a Reactor for the Partial Oxidation of o-Xylene to Phthalic Anhydride Packed with Ceramic Foam Monoliths By A. REITZMANN, A. BAREISS and B. KRAUSHAAR-CZARNETZKI.

10. Reactors and Separations Design Project, Phthalic Anhydride Production. 11. http://www.alibaba.com/showroom/price-of-phthalic-anhydride.html 12. http://www.metalprices.com/metal/vanadium/vanadium-pentoxide-v2o5-fob 13. Catalyst Deactivation 1994b edited by B. Delmon, G.F. Froment

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Appendix Final Design Matlab Code.

function main () clear all;close all;clc %----------------------------------------------- % OPERATING CONDITION %----------------------------------------------- To=656; % k R1=1.987; % cal/mol.k Po=3; % atm N=36000; % Tubes Ta0=640; % K m_c=.01; % kmol/s U=3.5; % Kw/m2.k Tr=298; % K D=.0254*1.5; % m w=40000; % kg %------------------------------------------------ % O-Xylene FEED %------------------------------------------------ m_xy=10/N; % kg/s %------------------------------------------------ %----------------------------------------------- %Reactions %----------------------------------------------- % (1) XY + 3 O2 ---> PA + 3 H2O % (2) PA + 7.5 O2 ---> 8 CO2 + 2 H2O % (3) XY + 10.5 O2 ---> 8 CO2 + 5 H2O % (4) XY + 7.5 O2 ---> MA + 4 CO2 + 4 H2O % (5) MA + 3 O2 ---> 4 CO2 + H2O % % Stoic % XY O2 PA MA CO2 W v=[ -1 -3 1 0 0 3 0 -7.5 -1 0 8 2 -1 -10.5 0 0 8 5 -1 -7.5 0 1 4 4 0 -3 0 -1 4 1]; %---------------------------------------------- % Cp % XY O2 PA MA CO2 W Cp=[ 458.588 32.7362 723.474 269.858 48.96 37.23 ]; % KJ/kmol.K Cpn2=30.5798; %------------------------------------------------ dCp=v*Cp'; %---------------------------------------------- % H % XY O2 PA MA CO2 W Ho=[19.1 0 -371.4 -398.2 -393.51 -241.83]*1e3; % KJ/kmol dHrx_o=v*Ho'; %----------------------------------------------- % K CALCULATION %----------------------------------------------- ko=[19.837 20.86 18.98 19.23 20.47]; E=[27000 31000 28600 27900 30400]; %------------------------------------------------ %================================================ % CALCULATION %------------------------------------------------ m_air=m_xy*25; % kg/s m_o2=0.232*m_air; % kg/s m_n2=(1-.232)*m_air; % kg/s % XY O2 N2 PA Mr=[106.16 32 28.02 148.1]; % kg/kmol n_xy=m_xy/Mr(1); % kmol/s n_o2=m_o2/Mr(2); % kmol/s n_n2=m_n2/Mr(3); % kmol/s

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Ft0=n_xy+n_o2+n_n2; % kmol/s % N2 XY O2 y0=[n_n2 n_xy n_o2]/Ft0; %------------------------------------------------------ % Alpha Calculation %------------------------------------------------ wf=w/N; P0=Po*101325; % Pa R=8.314; % kPa.m3/kmol.k por=0.5; Dp=.006; % m Vis=(-1e-08*To^2+5e-5*To+0.0039)/1000; % Pa.s rho_c=3357; % kg/m3 rho_b=rho_c*(1-por); % kg/m3 V=wf/(rho_c*(1-por)); % m3 L=4*V/(pi*D^2); % m M_avg=y0(3)*Mr(1)+y0(1)*Mr(3)+y0(3)*Mr(2); % kg/kmol Ac=pi/4*D^2; % m2 m=Ft0*M_avg; % kg/s G=m/Ac; % kg/m2.s rho_o=P0*M_avg/(R*To)/1000; % kg/m3 B0=G/(rho_o*Dp)*(1-por)/por^3*(150*(1-por)*Vis/Dp+1.75*G);% kg/m2.s2 a=2*B0/(rho_c*Ac*(1-por)*P0); % 1/kg %------------------------------------------------ %------------------------------------------------------ Fn=n_n2; Fi=[ Ft0*y0(2) Ft0*y0(3) 0 0 0 0 1 To Ta0]; ws=0:10/N:wf; n=length(ws); [w,z]=ode45(@(w,z)rate(w,z,Po,a,v,Ft0,dHrx_o,dCp,rho_b,D,Cp,Fn,Cpn2,To,ko,E,R1,m_c,U,Tr,Ta0),ws,Fi); %------------------------------------------------------ %====================================================== % RESULTS %------------------------------------------------------- disp(' Fxy Fo2 Fpa Fma Fco2 Fw y ') table=[z(:,1:6)*N z(:,7:9)] yf=z(n,7) Th=max(z(:,8)) D L x=(z(1,1)-z(n,1))/(z(1,1)) mp=z(n,3)*Mr(4)*(30326400/1000)*N % ton/year q=[z(:,1:6)*N]; plotyy(w,q,w,z(:,7)) plot(w,z(:,8:9)) Vt=V*N end %========================================================================== %========================================================================== %========================================================================== function dzdw=rate(w,z,Po,a,v,Ft0,dHrx_o,dCp,rho_b,D,Cp,Fn,Cpn2,To,ko,E,R1,m_c,U,Tr,Ta0) for i=1:6 F(i)=z(i); end y=z(7); T=z(8); Ta=z(9); Ft=sum(F)+Fn;

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% Heat dHrx=(dHrx_o+dCp*(T-Tr)); % KJ/kmol k=exp(ko - E/(R1*T))/3600; % kmol/(kgcat.atm2.s) Pxy=F(1)/Ft*Po*y*To/T; % atm Po2=F(2)/Ft*Po*y*To/T; % atm Ppa=F(3)/Ft*Po*y*To/T; % atm Pma=F(4)/Ft*Po*y*To/T; % atm % Reactions rs(1)=k(1)*Pxy*Po2; % kmol/kgcat.s rs(2)=k(2)*Ppa*Po2; % kmol/kgcat.s rs(3)=k(3)*Pxy*Po2; % kmol/kgcat.s rs(4)=k(4)*Pxy*Po2; % kmol/kgcat.s rs(5)=k(5)*Pma*Po2; % kmol/kgcat.s % Rates Evaluation for i=1:6 r(i)=0; for j=1:5 r(i)=r(i)+rs(j)*v(j,i); end end % differential for i=1:6 dzdw(i)=r(i); end dzdw(7)=-a/(2*y)*(Ft/Ft0)*(To/T); dzdw(8)=(((4*U)*(Ta-T)/(D*rho_b))+(sum(-rs.*dHrx')))/(sum(F.*Cp)+Fn*Cpn2); dzdw(9)=((4*U)*(T-Ta)/(D*rho_b))/(m_c*Cp(6)); dzdw=dzdw'; end