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Acknowledgement
I wish to express my sincere thanks and gratitude to DR. Ratna Datta,
Jadavpur University, for his continuous, encouragement and supervision
over my entire project work on “Design a styrene plant of capacity 30
tonnes per day”. His most valuable advices and the paths he showed to me
helped me in completion of my work.
I also express my thanks to all faculty members and my
fellow students of the Department of Chemical Engineering,
Jadavpur University, for their help and moral supports.
Thanking you,
Yours faithfully,
Bappa Saha
[Class: B.Ch.E. – IV
Sec: A-1
RollNo:000810301018]
Date: 14th Oct, 2012
Place: Jadavpur, Kolkata-32
JADAVPUR UNIVERSITY
Faculty of Engineering and Technology
Department of Chemical Engineering
Certificate of Approval
This is to certify that the project entitled “Determination of fuel characteristics
of blended and pure biodiesel”, submitted by Bappa Saha, a fourth year student,
bearing Roll No. 000810301018, and accepted for partial fulfillment of the
degree of Bachelor of Chemical Engineering from The Department of Chemical
Engineering, Faculty of Engineering And Technology, Jadavpur University,
Kolkata, is prepared entirely by himself under my supervision and guidance.
Head of the Department In-charge of Projet Work
(Prof. Chiranjib Bhattacharjee) (Prof. Ratna Datta)
Department of Chemical Engg. Department of Chemical Engg.
Jadavpur University, Jadavpur University,
Kolkata-32 Kolkata-32
INTRODUCTION:
The styrene process was developed in the 1930s by BASF
(Germany) and Dow Chemical (USA). Over 25×106 tons/year of
styrene monomer is produced worldwide.1 The annual
production of styrene in the U.S.A. exceeds 6×106 tons.2 The
major commercial process for the production of styrene is the
dehydrogenation of ethylbenzene, which accounts for 85% of
the commercial production.3 The potassium-promoted iron
oxide catalyst has been extensively used for styrene
production. 4 The average capacity of ethylbenzene
dehydrogenation plants is over 100,000 metric tons per year
and plants which have a capacity of 400,000 metric ton per year
is not uncommon.5 Obviously, a small improvement in the
plant operation will lead to a substantial increase of returns.
Nevertheless, the research towards the fundamental kinetic
modeling based upon the Hougen-Watson approach has not
been pursued by most styrene producers and researchers. They
rely on the empirical polynomial correlations for the unit
optimization.6-8 Furthermore, the reaction rates published in
the most of papers are not intrinsic but effective.9, 10 An
intrinsic kinetic model based upon the fundamental principles is
essentially required for the optimization of the various reactor
configurations with different operating conditions. The
objectives of this research are to develop the mathematical
kinetic model for the ethylbenzene dehydrogenation and to
investigate the effect of operating conditions on the fixed bed
industrial reactor formation of styrene, benzene, and toluene,
the understanding of the kinetic behavior of the minor by-
products, such as phenylacetylene, α-methylstyrene, β-
methylstyrene, cumene, n-propylbenzene, divinylbenzene, and
stilbene, is also important in terms of the styrene monomer
quality and separation cost of the final products. The formation
of these minor by-products is not taken into account for the
fundamental kinetic model. The general features of
ethylbenzene dehydrogenation are briefly discussed. The
theoretical and literature backgrounds are presented in each
chapter. Chapter III explains the experimental methods of
ethylbenzene dehydrogenation.
Problem statement :
It is proposed to design a styrene plant of capacity 30 tones par
day, by vapor phase catalytic dehydrogenation of
ethylbenzene, starting from ethylbenzene as raw material.
(1) Prepare an energy balance and mass balance of the
plant
(2) Design a catalytic fixed bed reactor involved in the
prosess with optimum conversion
Definition:
Styrene monomer is an aromatic hydrocarbon,under normal
condition it is clear,cocorless, flamableand toxic liquid.
Industrial alternate process for styrene production:
St is produced in industry mainly by two processes: I. dehydrogenation of ethyl benzene (EB) in presence of steam over iron oxide based catalysts. II. as a by-product in the oxidation of propene with ethyl benzene hydroperoxide and Mo complex-based catalysts. The former process (I), accounts for more than 90% of the worldwide capacity. The catalytic dehydrogenation route, in which the potassium promoted iron oxide catalyst is typically used since 1957, produces most of the Styrene .The process can be run industrially either adiabatically or isothermally over a fixed bed reactor in which the reactants are passed over the catalyst bed employing radial or axial flow [1,3]. Several catalysts, such as cobalt, copper, iron and zinc oxides, have been studied, both with and without promoters, but the potassium promoted iron oxide catalyst was found particularly efficient with respect to both selectivity and activity [2,3]
1.) C6H6+ C2H4↔C6 H5 C2 H5
2.) C6 H5 C2 H5↔C6 H5 C2 H3+ H2
However, styrene can be industrially produced by what is known as PO-SM Coproduction, where propylene oxide and styrene are made simultaneously. It proceeds as follows:
3.) C6H5CH2CH3 + O2↔C6H5CH(CH 3)OOH
4.)C6H5CH(CH 3)OOH + CH = CHCH3↔C 6H5 CH(CH3)OH +
H2COCHCH3
5.) C 6H5CH(CH3)OH ↔C 6H5 CH=CH2 +H2O
One downside to the PO-SM Coproduction is that the production capacity is dictated by the demand for propylene oxide. Since it is a more complex series of reactions makes it is less attractive to operate in industry.
Chemistry of Ethyl benzene Dehydrogenation
The main reaction produces styrene and hydrogen. Ethylbenzene ↔ styrene + H2, ΔHr (620oC) = 124.83 kJ/mol The dehydrogenation reaction is usually conducted at temperatures above 600oC with an excess of steam. The ethyl benzene dehydrogenation is an endothermic and reversible reaction with an increase in the number of mole due to
reaction. High equilibrium conversion can be achieved by a high temperature and a low ethyl benzene partial pressure. The main byproducts are benzene and toluene. Ethylbenzene ↔ benzene +C2H4, ΔHr (620oC) =101.50 kJ/mol Ethylbenzene + H2 ↔ toluene +CH4, ΔHr (620oC) = -65.06kJ/mol
Reaction thermodynamics
The dehydrogenation reaction of EB to Styrene is endothermic (DH=129.4kJ/mol) At room temperature, the reaction equilibrium is located far towards the educts side. It can be shifted towards the product side by increasing the temperature, which increases the equilibrium constant K due to the van´t Hoff relationship and by reducing the pressure, since two moles of product are formed from one mole of EB. Therefore the technical St synthesis is run at around 600°C with an excess of steam, the steam-EB mixtures has a molar ratios from 5:1 to 12:1. Styrene plants run their reactors under isothermal or adiabatic conditions with flow rates that ensure short contact times in order to prevent polymerization of Styrene. The equilibrium EB conversion at 600°C and 0.1 bar pressure is( ~ 83% ), and conversions between 50 and 60% are obtained in technical reactors. The
typical byproducts of the EB dehydrogenation are (~1%) benzene and (~2%) toluene formed by catalytic dealkylation and hydrodealkylation of Ethylebenzene,respectively, or they also can be formed by steam dealkylation.
Physical porperties of ethyl benzene Physical properties Styrene
Molar mass 104.15g/mol
Appearance Clear colorless to yellowish
Density at 20 oC 0.9059g/cm
3
Melting point -30.60 C
Boiling point 145.20 C
Solubility in water 280mg/L (150 C)
300mg/l (200 C)
400mg/L (400 C)
Solubility in organic solvent soluble in ethanol,ether,acetone
Air Solubility 0.73 mg/L 0.011 mg/L 1.36 mg/m3
Oder threshold 0.1 ppm
Specific gravity 0.906(water=1)(25 oC)
Conversion factor 1 ppm=4.33 mg/m3
Flash point 87 oF(closed cup)
Vapor pressure at 20 c 5 mmHg
Autoignition temperature 914 oF(490
oC)
Henry’s law constant 2.61x10-3 atm-m3/mol Odor If pure,sweet and pleasent
Physical porperties of styrene
Physical properties Ethylbenzene
Molar mass 106.17g/mol
Appearance Clear colorless
Density 0.8670g/cm3(20
oC)
o.8669 g/cm3(200
oC)
.86262 g/cm3(250
oC)
Melting point -94.9760 C
Boiling point 136.190 C
Solubility in water 140mg/L (150 C)
152mg/l (200 C)
160mg/L (250 C)
Solubility in organic solvent soluble in ethanol,ether,acetone
Oder threshold Water Air
0.029 mg/L o.140 mg/l 2.3 ppm
Conversion factor(25 oC,1atm) 1mg/m
3=0.230ppm
Specific gravity 0.867 at 20 oC (water = 1)
Flammability limits 0.8(lower)vol%-6.7(upper)vol%
Flash point 15oC
Vapor pressure at 20 c 7 mmHg
Autoignition temperature 810 oF(432
oC)
Henry’s law constant 6.6*10-3
atm-m3/mol
Physical state Liquid
Refractive Index(at 200 oC) 1.49320
Latent Heat(veporizatin) 335 j/kg
Heating value(gross) 42999 j/kg
Heating value(net) 40928 j/kg
Kinematic viscosity(at 98.90 oC) 0.390*10
-6 m
2/s
Surface tension 28.48 mN/m
Specific heat capacity(idea gas, 250
oC)
1169 -1 k-1
Physical porperties of ethyl benzene
Physical properties Methane(CH4)
Molar mass 16.04g/mol
Appearance Clear colorless to yellowish
Gas Density at 70 oF 0.0416lb/ft
3
Melting point -296.50 F
Boiling point -258.70 F
Solubility in water 280mg/L (150 C)
300mg/l (200 C)
400mg/L (400 C)
Solubility in organic solvent soluble in ethanol,ether,acetone
Specific heat 8.53 Btu/lbmol-oF
Specific gravity 0.466gm/cu.m (-164oC )
Conversion factor
Flash point 914 oF(490
oC)
Vapor pressure at 20 c
Autoignition temperature
Henry’s law constant
Odor
Physical porperties of toluene
Physical properties Toluene
Molar mass 92.14g/mol
Appearance Clear colorless to yellowish
Density at 25 oC 0.867g/cm
3
Melting point -93.990 C
Boiling point 110-1110 C
Solubility in water 0.052% at 25 0 C
Specific heat 1.72 kj/kg k
Flash point 4.40 C
Vapor pressure at 20 c 28.5 torr
Hazard class Flamable liquid
Styrene from Ethyl benzene
• Ethyl benzene is mainly used to produce styrene.
• Over 90% of the 12.7 billion pounds of EB produced in
the U.S. during 1998 was dehydrogenated to styrene.
• Styrene (vinyl benzene)
– A liquid (b.p. 145.2C)
– Polymerizes easily when initiated by a free radical or when
exposed to light.
• Dehydrogenation of ethyl benzene to styrene
– Catalysts:-
• Metal oxide catalysts. Oxides of Fe, Cr, Si, Co, Zn, or their
mixtures
– Reaction conditions:
• Temperature : 600-700o C
• Pressure : 1 atm or or lower
• 90% styrene yield
• 60-70% conversion:
Process Description
See Figure 1. The raw material is ethyl benzene, and steam is
fed as an inert. In the suggested process, ethyl benzene is
preheated in E-501 to a saturated vapor. This is then mixed with
steam produced from the fired heater H-501. The steam provides
the heat of reaction and serves as an inert diluent to help shift
the reaction to the right. Steam also tends to limit side reactions
and helps to extend catalyst life by reducing coke formation on
the catalyst. The ratio of steam to ethyl benzene entering reactor
R-501 in Stream 6 ranges between 6 and 12. The main reaction:
C6H5CH2CH3 → C6H5CHCH2 + H2 …………….. (1)
Ethyl benzene styrene
is endothermic, reversible, and limited by equilibrium. The
reaction occurs at high temperatures (600 – 700 0c) and low
pressures (0.4-1.4 bar) in order to shift the equilibrium to the
right to favor styrene production. In R-501, the process uses a
proprietary iron catalyst that minimizes (but does not eliminate)
side reactions at higher temperatures. For simplicity, assume that
the only side reaction that occurs in R-501 is the hydrogenation
of ethyl benzene to form toluene and methane:
C6H5CH2CH3 + H2 → C6H5CH3 + CH4 ……….(2)
Ethyl benzene toluene
The primary reaction is limited by equilibrium, and is assumed
to approach 80% of equilibrium. The selectivity of the toluene
side reaction is a function of reactor temperature.
The reactor effluent, Stream 7, is cooled in E-502 to produce
steam and then enters a three-phase separator (V-501). The
bottom phase of V-501 is waste water (Stream 11). This must be
decanted and sent for further processing before discharge. This
treatment is not shown in the PFD, but it is an expense which
must be included in the economic analysis. Stream 9 leaves the
top of the separator and contains all the light gases (methane and
hydrogen) and can be used as a fuel gas. Stream 10 contains
most of the toluene, ethyl benzene and styrene.
Stream 10 flows through a pressure-reducing valve and then
enters a distillation train (T-501 and T-502). The distillation
columns operate at (different) constant pressures, the values of
which are governed by the properties of the heating steam and
cooling water used, and the composition of the top and bottom
products, as described later. Most of the toluene is removed at
the top of first column (T-501) in Stream 16. The remaining
toluene, ethyl benzene and styrene leaving the bottom of this
column in Stream 15 enter the second column (T-502). From T-
502, Stream 24 (containing ethyl benzene, toluene and styrene)
is recycled and mixed with fresh ethyl benzene before the
reactor. The bottom product of T-502 leaving in Stream 28
constitutes the styrene (with small amounts of ethyl benzene
and toluene) leaving Unit 500.
Tolue
-ne
Purifi-
cation
Colu-
mn
Adiabatic Fixed Bed
Reactor Pre-Heater
Separator
Benzene
tolune column
Final
Purif
-ier
Colu
-mn
Pure Ethyl
Benzene
Hydrogen and
Methane Gas
Ethyl Benzene
Recycle
99% Styrene
Ethyl Benzene
Recycle
Process flow diagram
Styrene plant
MASS BALANCE AND ENERGY BALANCE:
C6H5CH2CH3 → C6H5CHCH2 + H2…………. (1)
Ethyl benzene styrene
C6H5CH2CH3 + H2 → C6H5CH3 + CH4 ….…(2)
Ethyl benzene toluene methane
Molecular weight of Ethyl benzene = 106
Molecular weight of Styrene = 104
Molecular weight of Benzene = 78
Molecular weight of Toluene= 80
The plant capacity for ethyl benzene plant =30 tones per day
For which we are suppose to produce the product as
follows: The product ethyl benzene which we will produce will contain:-
Styrene produced =
(30*1000) / 24 (kg/hr) *(104/106) (1/kg) *(1/104) (kmol)
=11.8 kmol/hr
Since we are using gas phase dehydrogenation process for styrene
production, for which yield and conversion are
as follows:
Yield=90%
Conversion=65% (w.r.t ethyl benzene)
Now consider FBR for Styrene plant
Styrene produced =11.8 kmol/hr
Now we know that:-
Yield = [{(moles of product produced)*(stochiomertric coefficient)}
/(moles of reactant converted)]/100
90/100=11.8/moles of reactant converted
Moles of reactant converted(ethyl benzene)= 13.4 kmol/hr
Now according to the reaction(1) and (2) in the reactor, we can say
that :-
For 1 mole of Styrene = 1 mole of ethyl benzene needed
Now after reactor 11.80 kmol of Styrene produced
So ethylene consume is = 11.8kmol
Thus rest ethyl benzene is = 13.4 – 11.8=1.6 kmol of ethyl benzene
Since conversion is 60%
It means that some of ethyl benzene had converted in to toluene and
methane(side reaction will occurred <10%) rest ethyl benzene are
unconverted and it is recycled.
Material balance of reactor
Input:
Ethylbenzene : 13.4 kmol/hr
Output
Ethyl benzene: 1.3 kmol/hr
Styrene : 11.8 kmol/hr
Hydrogen : 11.5 kmol/hr
Toluene : 0.27 kmol/hr
Methane : 0.27 kmol/hr
Material balance over Toluene column
Input
Ethyl benzene: 1.3 kmol/hr
Styrene :11.8 kmol/hr
Toluene :0.27 kmol/hr
Output
Toluene :0.27 kmol/hr
Output
Styrene :11.8 kmol/hr
Ethyl Benzene:1.3 kmol/hr
Material balance over final purification column
Input
Ethyl Benzene:1.3 kmol/hr
Styrene :11.8 kmol/hr
Output
Ethyl Benzene :1.3 kmol/hr
Output
Styrene:11.8 kmol/hr
ENERGY BALANCE
Estimated heat capacities at different temperature:
In the reactor the inlet feed temp is 6600C where the outlet
temp is 6000C. Hence for the energy balance about the reactor
it is assumed that the physical properties are constant, Ie
values at average temp (600+660)/2 = 6300C.
CP of steam at 6300C= 2.223 kJ/kg k
CP of EB at 6300C = 2.827 kJ/kg k
CP of styrene at 6300C = 2.610 kJ/kg k
Heat of raction = HR|25 =129.4 kJ/mol
Heat of formation (HR)= HR|25 + ∑( Ƴi* CPi)|PORDUCT -∑(Ƴi*CPi)|REACTANT
of styrene at 6300 = 86.99 kJ/mol
STEAM REQUIREMENT:
Let m is the mass of steam added to the feed per hour, in order
to supply the heat of reaction in the reactor .The steam and the
reactant feed entered the reactor at a temperature 6600C and
leave at 6000C temperature. Hence, it can be assumed that the
temperature drop solely due to the reaction the approx. energy
balance is made;
m*CPw*∆T +mST* CPST*∆T + mEB*CPEB *∆T + QR = 0
From the above balance the steam flow rate been calculated
as; m*2.223*(660-600) +122.97*2.613*(660-600)
+1422.68*2.827*(660-600)+86.99*11.8
m= 10948.07 kg/ hr
Henceforth the mol ratio of steam to hydrocarbon feed is
approximately calculated = 20:1
TEMPERATURE OF THE PREHEATED STREAM:
Let T be the temp of the preheated coming out of the
preheater. Assume the mixing of
the steam to hydrocarbon stream is adiabatic.
It is known that the temperature of the preheated steam is
8000C and after mixing with
the hydrocarbon stream, the temperature is dropped to 6600C.
Therefore, the heat balance
can be given as:
10948.07*2.293*(800-660)=(1450.5*2.133+4638.5*2.267)(660-
T)
i.e., T= 401.730 C
TEMPERATURE DETERMINATION OF THE REACTION
PRODUCT LEAVING THE PREHEATER:
Again, from mass balance about the reactor the wt fraction or
composition of the stream leaving the reactor can be given as:
Steam= 47.64%
EB = 13.95%
Styrene=36.63%
Toluene= 0.67%
Benzene= 0.38%
Hydrogen=0.73%
Therefore the average Cp value been determined=2.322 kJ/kg k
& the Cp value for the feed stream to the reactor= 2.235 kJ/kg k
The boiling point temp of the feed mixture at atmospheric
pressure is calculated by using
Roult’s law is=1380C
And the latent heat of vaporization of the mixture= 341.03
kJ/kg.
The Cp value for the liquid mixture = 1.939 kJ/kg k
Let the outlet temp of the product stream from the preheater=t
Therefore, the approx energy balance can be given as:
17036.97*2.322*(630-t)= 6088.9*(1.939*(138-
30)+341.03+2.235*(401-38))
i.e., t=472.90C
WASTE HEAT RECOVERY
The excess of superheat from the stream leaving the reactor
and through steam preheater,
is been utilized in generating steam in boiler, which further fed
back to the reactor
system.
The temp of the product stream leaving the preheater=472.90C
& Cp value of the stream is =2.322 kJ/kg k
Let the boiler operated at a temp of 1000C
The water feed to the condenser is at temp= 400C
The latent heat of vaporization of water at 1000C is= 2260 kJ/kg
Therefore the amt. of steam generated is:
17036.97*2.322*(472.9-100) = M*(4.184*(100-40)+2260)
ie, M=5874.72 kg/hr
hence the amount of steam been generated= 5874.72kg/hr
ENERGY REQUIRED FOR SUPER HEATING OF STEAM:
The steam is been heated to a temperature of 8000C in the
super heater.
The specific heat of steam at 4000C = 2.065 kJ/kg k
Therefore the heat requirement in the super heater=
Q= (10948.07/3600)*2.065*(800-100)
= 4395.95 KW
Fixed Bed Catalytic Reactor Process Design:
Feed composition component
lb mole/hr
Ethylbenzene 28.5
The catalytic dehydrogenation of ethyl benzene and found that
with a certain catalyst the rate could be represented by certain
catalyst.
C6H5CH2CH3↔C6H5CHCH2 + H2 (1)
Ethyl benzene styrene
The global rate was given as
rp= k(pe – 1/k*(psph))
where,
pe= partial pressure of ethylbenzene
ps= partial pressure of styrene
ph=partial pressure of hydrogen
the specific reaction rate and equilibrium constant
are
logk = - 4770/T + 4.10
where k is the pound mole of styrene produced per
hr/(atm)(lb catalyst) and T is degree kelvin
Temperature, T(°c) K
400 .0017
500 .0025
600 .0023
700 .0014
It is desired to estimate the volume of reactor necessary
to produce 30 tones of styrene par day, using vertical
tubes 4 ft in diameter, packed with catalyst pellets,
consider this problem by taking into account the side
reaction producing benzene and toluene. However to
simplify the calculation in this inductor example,
supposed that the sole reaction is the dehydrogenation
to styrene, and there is no heat exchange between the
reactor and the surrounding. Assumed that under normal
operation the exit conversion will be 65%. However also
prepare graphs conversion and temperature vs catalyst
bed depth, up to equilibrium conditions. The feed rate
per reactor tube is 13.5 lb moles/hr for ethylbenzene and
270lb moles/hr for stem. In addition
Temperature of mixed feed entering reactor =625 °c
Bulk density of catalyst as packed =90 lb/cu ft
Average pressure in rector tubes =1.2 atm
Heat of reaction = 60000 btu/lb mole
Surrounding temperature = 70 °F
The reaction is endothermic, so that heat must be
supplied to maintain the temperature. Energy must be
supplied by adding stream to the feed to provide a
reservoir of energy in its heat capacity.
In this problem the operation is adiabatic and the energy
balance is given by :
(F/M)(-∆H)dx =FtCpdT ……………………………….(01)
Since x refers to the conversion of ethylbenzene,
F/M =13.5 lb moles/hr. As there is large access of steam,
it will be satisfactory to take Cp=0.52 then the heat
capacity of the reaction mixture will be
FtCp=(270*18 + 13.5*106)(0.52) = 3270 Btu/°F
Substituting numerical value in equation (01), we obtain
13.5(-60000)dx=3270dT
-dT = 248x
T – 1616 = -248x ………………………………(02)
Where T is in degree and 625°c isthe entering
temperature of the feed.
The weight of catalyst expressed as :dW = pBAcdz
Ac= Cross sectional area
So we get, FdX= rppBAcdz …………………………(03)
dz = F/(rppBA) …………………………(04)
dz = 13.5dx/(90*0.78548*16)(rp) =(.0119/rp)dx
The partial pressure can be expressed in term of the
conversion as follows. At any conversion x the mole of
each component are
Steam= 20
Ethyl benzene =1-x
Hydrogen = x
Total moles = 21 + x
Then, pe= (1-x)*(1.2)/(21+x)
ps = ph= x*(1.2)/(21 +x)
then the total equation becomes
rp =1.2*k*[(1-x) – (1.2*x2)]/K*(21+x))/(21+x) ..…….(05)
or, with the expression for k determined by wanners and
Dybdal,
rp =1.2*(12600)*e-(19800/T)[(1-x) – (1.2*x2)/K*(21+x)]
……………………………………….(06)
Substituting this value of rpin equation (06) gives the
expression for the catalyst bed depth in terms of the
conversion and temperature,
dz=((21+x) *e(19800/T)/(1270000))*
[(1-x) – (1.2*x2)]/K*(21+x)]-1dx ………………………………(07)
Equation (02) and (07) can be solved numerically for the
bed depth for any conversion. If the coefficient of dx in
equation (07) is designated as α then we may wright eq.
(07) as: ∆z =α ∆x ………………………………..(07)’
At, z =0, x=0 and T=625°c
αo = (21/(1270000))*e12.25*(1/(1-0)) =3.30
If an increment ∆x of 0.1 is chosen, the temperature at
the end of the increment is, from equation (02),
T1 = 1616 – 248(0.1) =1591 °R =611°c
Then at the end of the first increment
α1 =((21+0.1) *e(12.43)/(1270000))*
[(1-0.1) – (1.2*0.12)]/0.28*(21+0.1)]-1=4.65
(Note: In the exponential term T has been converted to
degree Rankine.
The value of K is estimated to be 0.28 at 1591 °R from
the tabulation of data. )
The bed depth required for the first increment is given
by eq. (07)’ as
∆z = (3.30 + 4.65)*(0.1)/2 = 0.4 ft
Proceeding to the second increment, we find
T2 =1616 – 248*(0.2) = 1566 °R =597°c
α2 =((21+0.2) *e(12.66)/(1270000))*
[(1-0.2) – (1.2*0.22)]/0.22*(21+0.2)]-1= 6.60
z2-z1= α∆x = (α1 +α2)*(0.1)/2 =(4.65+6.60)*(0.1)/2 =0.56 ft
z2=0.4 +0.56= 0.96
The of further calculation are shown in table:
Conversion Temperature(°c) Catalyst bed depth(ft)
0 625 0
0.1 611 0.4
0.2 597 0.96
0.3 584 1.75
0.4 570 2.93
0.5 556 4.84
0.55 549 6.30
0.6 542 8.5
0.65 536 13.2
0.69 530 ∞
The rate of reaction becomes zero at conversion about
z=0.69 and a temperature of 530 °c, as determined from
equation (02) and (04) from the figure it is found that a
bed depth of 3.8 ft is required for a conversion of 65%.
The production of styrene from each reactor tube would
be,
Production/tube = 13.5*(104)(0.65)(24)=21,902 lb/day
Production/tube= 9.94 tons/day
Hence three 4 ft diameter reactor tubes packed with
catalyst to a depth of at least 3.8 ft would be required to
produced 30 tons/day of styrene.
CONCLUSION AND RECOMMENDATIONS
The catalytic dehydrogenation of ethylbenzene into styrene was
investigated in a tubular reactor over commercial potassium-
promoted iron oxide catalyst under atmospheric pressure. The
extensive kinetic experiments covered a wide range of operating
conditions and allowed the development of a fundamental kinetic
model. The kinetic study showed that the higher feed molar ratio of
H2O/EB give higher total ethylbenzene conversion and styrene
selectivity. The total ethylbenzene conversion and styrene selectivity
decreased as the addition of styrene or H2 to the feed mixture
increased. The addition of styrene or H2 leads to fast catalyst
deactivation. The intrinsic kinetics for the formation of styrene,
benzene, and toluene has been
modeled using the Hougen-Watson formula. The data analysis was
based on the integral method of kinetic analysis. The mathematical
model developed for the ethylbenzene dehydrogenation consists of
nonlinear simultaneous differential equations in multiple dependent
variables. The parameters were estimated from the minimization of
the multiresponse objective function which was performed by means
of the Marquardt algorithm. The significance of the individual model
parameters was tested by comparing the estimate bj with its standard
deviation. The estimate was significantly different from zero and
effectively contributes to the model. The kinetic model with set of
estimated198 parameters yielded an excellent fit of the experimental
data. The final estimated values of the adsorption enthalpies and
entropies was tested and validated using the physicochemical criteria
proposed by Boudart. The intrinsic kinetic parameters were used to
simulate 3-bed adiabatic industrial reactor with axial flow and radial
flow using the heterogeneous fixed bed reactor model.
Kinetic experiments for the formation of minor by-products, such
asphenylacetylene, α-methylstyrene, β-methylstyrene, cumene, n-
propylbenzene,divinylbenzene, and stilbene revealed that the
phenylacetylene selectivity did not depend on the total ethylbenzene
conversion. The selectivity of stilbene was highly increased with
increasing temperature. The selectivity of divinylbenzene was so low
(below0.01%) at all the reaction conditions that no correlation with
the ethylbenzene conversion was made. The selectivities of other
minor by-products decreased with increasing the total ethylbenzene
conversion. More research efforts can be contributed to the following
recommendations for
future work:
1. Experimental study for the coke formation and gasification using an
electrobalance to estimate the kinetic parameters for the coke
formation and
gasification, which leads to determine the dynamic equilibrium coke
content.
2. Process optimization of ethylbenzene dehydrogenation to
determine an optimal
reactor configuration and operating conditions, such as a molar ratio
of steam to
ethylbenzene, pressure, and temperature.
3. Empirical kinetic model for the production of minor by-products
which
correlates the selectivity with the total ethylbenzene conversion.
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McCabe-Smith-Harriott, 6th Edition, Published by McGraw . Hill International Edition, Chemical
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Chapter 8, Process Design of distillation Column, Introduction to Process Engineering & Design,
Second
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Engineering
Design, By R.K.Sinnot, Publisher by Butterworth-Heinemann Publications.
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