CSTR PDF
CSTR PDF
ABSTRACT
This paper presents development of design equations to evaluate the performance of Non-isothermal continuous stirred tank
adsorption tower (CSTAT) for sulphuric acid production from sulphur trioxide hydration using vanadium catalyst. The
performance parameters as a function of kinetics data considered in this work include reactor volume, height, space velocity,
space time and heat duty. Model performance equation were developed to determine the functional parameters of the reactor.
The developed performance models were simulated using Matlab R2007B within the operational limits of conversion degree
and other kinetic parameters. The results of simulation demonstrated reproducible behavior as adsorption tower functional
dimensions have prefect correlation to each other.
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      International Journal of Engineering and Technology (IJET)  Volume 4 No. 12, December, 2014
The general schematic presentation for the production of sulphuric acid is given below.
Air
Figure 1: Contact process for making sulfuric acid and Oleum from sulfur.
In the industrial chemical process, heterogeneous fluid-fluid             that the irreversible biomolecular nature of the reaction have Hr
reactions are made to take place for one of three reasons. First,         = -25kcal/mol at 250C.
the product of reaction may be a desired material. Such reactions
are numerous and can be found in practically all areas of the             Following the outcome of the work of Chenier [1987] as cited
chemical industry where organic and inorganic synthesis are               above, the rate expression for the formation and production of
employed. Fluid-fluid reactions may also be made to take place            sulphuric acid is summarized as in equation (3).
to facilitate the removal of an unwanted component from a fluid.
Thus the absorption of a solute gas by water may be accelerated           -RA     = K2     SO3  H 2O                              3
by adding a suitable material to the water which will react with
the solute being absorbed. The third reason for using fluid-fluid
systems is to obtain a vastly improved product distribution for           Hence from equation (2.33) the amount of SO3 and H2O that have
homogeneous multiple reactions than is possible by using the              reacted at any time t can be presented as;
single phase alon
SO3  H 2O 
            H 2 SO4                                  2                  XA        =          Fractional conversion of SO3           (%)
Liquid film
                                                                                              Inter face
        Concentration
                                                   Gas (SO3)                          CAi
                                                                                                                 ZL
                                                                               Gas Film                                                          CBL
                                                                                                r
                                                                       Distance normal to phase boundary
Sulphur trioxide (A) is absorbed into the steam (B) by diffusion. Therefore the effective rate of reaction by absorption is defined by
 RA 
             rDL
                       C Ai    C AL   rK L (C Ai  C AL )                                                   5
              ZL
Invoking the works of Krevelen and Hoftyzer, the factor r is related to C Ai, DL and KL to the concentration of steam B in the bulk liquid
CBL and to the second order reaction rate constant K2 for the absorption of SO3 in steam solution. Thus
                                                   K 2 DL C BL 
                                                                       1
                                                                           2
                                   r           =                               KL     .                    6
                           1               1        1
- RA        = (CA) CBL2 K 2 2                      DL2 ..                                                     7
Previous reports [ Octave levenspiel 1999] showed that the amount of SO 3 (CA) and steam (CBL) that have reacted in a bimolecular type
reaction
                K 2 2 D L2 C BO  C AO X A  2 C A0  C A0 X A                                                      
                       1               1                                          1
- RA =
                                                                                                                              8
                                               (m  X A )                  (1  X A ) .
        1          1           3                               1
=   K 2 2 DL2 C A0                 2                               2
Where
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        International Journal of Engineering and Technology (IJET)  Volume 4 No. 12, December, 2014
                          CB0
                   m=
                          C A0
m         =- The initial molar ratio of reactants
For non-isothermal operation of the continuous stirred tank reactor, the reactor volume model is obtained from the auto-thermal balance
principle (Conlson & Richardson, 1979), which is expressed mathematically as:
But,
Equation (10- 12), Which upon substitution into equation (9) gives
From which,
                                 GPCP T  T0   UAt T  Tc 
                   VR        =                                                                          14
                                          H R RA
          Recall that
                                                  C A20 m  X A            1  X A 
                                 1        1         3                1
                                 2            2
          - RA     =         K   2   DL                                  2                              15
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        International Journal of Engineering and Technology (IJET)  Volume 4 No. 12, December, 2014
                                   G p C p T  T0   UAt                              T  Tc 
          VR          =                                                                                                        16
                            H R                                                m  X A  2 1  X A 
                                           1               1               3                 1
                                               2           2                2
                                       K       2       D   L       C       A0
          Where,
          GP          =        Mass flow rate of product, (Kg/sec)
          CP          =        Specific heat of product, (KJ/Kg K)
          U           =        Overall heat transfer coefficient of material, (KJ/Sec m3K).
          At          =        Effective area of heat transfer, (m2)
          XA          =        Conversion degree
          T           =        Operational temperature of reaction, (K)
          T0          =        Initial temperature of reaction, (K)
          Tc          =        Temperature of cooling fluid, (K)
          HR         =        Heat of reaction, (KJ/mol)
          CA0         =        Initial concentration not SO3, (mol/m3)
          K2          =        Absorption reaction rate constant, (1/sec)
          DL          =        Liquid phase diffusivity of SO 3,     (m2/sec)
          m           =        Initial molar ratio of reactants.
VR = r 2 h 17
                                        VR
                      h        =                                                                                               18
                                        r 2
          Putting equation (16) into equation (18) results in
                                     G p C p T  T0   UAt T  Tc 
          h       =                                                                                                            19
                          r 2  H R  K                                 C A20 m  X A                1  X A 
                                                   1           1                3                 1
                                                       2           2
                                                       2   D           L
                                                                                                      2
The space time Ts is mathematically defined (octave levenspiel, 1986 and coulson & Richardson, 1979) as
                            Volume of reactor                                        VR
          Ts          =                                                        =                                               20
                           Volumetric flow rate                                      V0
But
                                p
          Ts          =                VR                                                                                      22
                                Gp
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        International Journal of Engineering and Technology (IJET)  Volume 4 No. 12, December, 2014
                                        p G pC p T  T0   UAt T  Tc 
          Ts        =                                                                                                        23
                                 H R  GP K                               C A20 m  X A                  1  X A 
                                                         1           1         3                      1
                                                             2           2
                                                             2   D       L
                                                                                                          2
                                1   V
          Vs        =               0                                                                                       24
                                Ts  VR
                         H R G p K                       C A20 m  X A 
                                                                    1  X A 
                                         1           1           3                      1
                                             2       2
                                             2   D   L
                                                                                            2
                                  p G pC p T  T0   UAt T  Tc 
          Vs        =                                                                                                        25
The steady state heat generation model for reactor is given (Rase, 1977) as
Q = (-Hr) FA0 XA 26
The heat generation per reactor volume is obtained by dividing both sides of equation (26) by the reactor volume, i.e
          Rq        =   Q
                                  
                                       H R  FA0                                XA                                        27
                        VR                                   VR
                   H R 2    FA0 X A K 22 D 22 C A20 m  X A                                   1  X A 
                                                 1       1           3                      1
                                                                                                2
          Rq =                                                                                                               28
                                  G pCP T  T0   UAt T  Tc 
Figure 4 demostrates hypothetical non-isothermal continuous stirred tank adsorption tower(CSTAT) for sulphur trioxide hydration process.
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      International Journal of Engineering and Technology (IJET)  Volume 4 No. 12, December, 2014
The computation of the functional parameters of the reactor as         computational procedure is given in Fig 4 Performance
                                                                       dimensions such as reactor volume, length, space time, space
shown in figure                                                        velocity, heat generation per unit volume, and heat exchanger
                                                                       functional parameters capable of maintaining non-isothermal
                                                                       conditions were cleverly inculcated into the computer algorithm.
3.2     Computational Method                                           The equations of these performance measures were expressed as
                                                                       a function of fractional conversions and characteristic
                                                                       operational temperature.
The developed models as presented in section 3.1 were
programmed using MATLAB, and the flow chart describing the
START
                                                READ
                                    Gp, Cp, Tc, Vo, U, AT, T0, CAO,
                                         HR, K2, DL, M, D1
                                               INITIALIZE
                                               XA = 0.95
                                                 T = 313
                                             PRINT
                                       T; XA; VR, h; Ts; Vs;
                                             QG ; RQ
XA = XA + 0.01
                                                       Yes
                                             T = T + 10
                                              T > 363
                                                      Yes
                                               STOP
Figure:4 Flow chart Describing the computational procedure of non-Isothermal CSTAT performance dimension
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        International Journal of Engineering and Technology (IJET)  Volume 4 No. 12, December, 2014
The reactor performance models were evaluated with variables obtained from stoichiometric calculations from the reaction mechanism
presented in section 1 equation 2. Such functional variables inculcated into the computer algorithm for the purpose of simulation of the
performance dimensions include molar flow rate, concentration etc.
    4. RESULTS AND DISCUSSION                                            were designed with hastelloy because it has excellent corrosion
                                                                         and sulphuric acid resistance properties.
Industrial reactors for the production of sulphuric acid over a          The reactors performance models developed in chapter three
range of reaction time t = 60 to 1800 Sec, degree of conversion          were simulated with the aid of MATLAB R2007b. The results
XA = 0.95 to 0.99 and operating temperature T = 313 to 363K              provided information for the functional reactors parameters viz:
have been investigated and designed. The reactors have a                 The reactor volume and the rate of heat generation per unit
capacity of 1.389x103 Kg/hr of sulphuric acid. These reactors            volume of the continuous reactors and the semi-batch reactor.
                                                                         The reactor length, space time, and space velocity for the
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      International Journal of Engineering and Technology (IJET)  Volume 4 No. 12, December, 2014
continuous reactors, while the height of reactor were obtained for                     operating temperature T and degree of conversion within the
the continuous stirred tank reactors and the semi-batch reactor.                       limits t, T and XA as specified. A plot of heat generation RQ
Similarly, information for the pressure drop in the plug flow                          versus operating temperature T was curvilinear and found to be
reactor, whose diameter Di was varied from 0.02 to 0.1 m was                           increasing with increasing operating temperature T within the
also obtained. Suitable heat exchangers were also designed for                         range of XA = 0.95 to 0.99. Similar plots were made RQ versus
the isothermal reactors and the semi-batch reactor to remove the                       XA within the range of T = 313 to 363K. The graphs were also
heat of reaction occasioned during the process. It is the purpose                      curvilinear with negative gradient. At fairly above 99%
of this section to present and discuss the results of the reactor                      conversion of sulphur trioxide, there was a sharp drop tending to
types and the heat exchangers and to compare their performance.                        the abscissa of the graph. This behavior explains the infinity of
                                                                                       the rate of heat generation per unit reactor volume at 100%
The functional parameters of the reactors are tabulated in figures                     degree of conversion of sulphur trioxide. Finally the rate of heat
17, 18, 19, 20, 21, 22, 23, and 24. And appendix 1-2. The results                      generation per unit reactor volume decreases with increasing
showed that the reactor volume is dependent on operating                               reaction time and degree of conversion within the range of
temperature T and degree of conversion X A. The volume of the                          temperature as specified.
reactor would tend to infinity at 100% conversion. The variation
of the reactor volume, as a result of sulphur trioxide addition to                     Figures 5 to 10 illustrated the variation of space time with
water, with reaction time, operating temperature and degree of                         operating temperature and degree of conversion X A as specified
conversion is illustrated in figures 5, 6, 7, 8, 9, 10, 11, and 12.                    within the range of T = 313 to 363K and XA = 0.95 to 0.99. The
From the results it was observed that volume of the reactors                           plots were curvilinear as well within the range of T and XA
increases with increasing degree of conversion and decreases                           investigated. However, for the addition of sulphur trioxide to
with increasing operating temperature. This characteristic                             water, the highest conversion was observed for the highest space
behavior was observed to be in agreement with the usual reactor                        time with the lowest operating temperature.
prototypes dependable features of performance parameters vis-
avis the kinetic data (Abowei 1989).                                                   The space time TS, was observed to be increasing with
                                                                                       increasing degree of conversion and decreases with increasing
Figures 11 and 12 illustrated the variation of heat generation per                     operating temperature within the range specified.
unit volume of the reactors as a function of reaction time t,
                                                          -3
                                                       x 10
                                                 1.4
                                                 1.2
                           REACTOR VOLUME (m3)
                                                  1
                                                                                                                  xA=95
                                                                                                                  xA=96
                                                 0.8                                                              xA=97
                                                                                                                  xA=98
                                                                                                                  xA=99
                                                 0.6
0.4
0.2
                                                  0
                                                  310          320    330         340       350             360           370
                                                                            TEMPERATURE (K)
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                                                 -4
                                              x 10
                                      1.8
                                                                                                                              313
                                                                                                                              323
                                      1.6
                                                                                                                              333
                                                                                                                              343
                                      1.4
                REACTOR VOLUME (m3)                                                                                           353
                                                                                                                              363
                                      1.2
0.8
0.6
0.4
0.2
                                       0
                                       0.94              0.95          0.96          0.97       0.98              0.99              1
                                                                              CONVERSION DEGREE
Figure 6: plot of Reactor Volume against Conversion Degree for Non-Isothermal CSTAT
                                                                                                                         xA=95
                                      4
           SPACE TIME (sec)
                                                                                                                         xA=96
                                                                                                                         xA=97
                                                                                                                         xA=98
                                      3                                                                                  xA=99
                                      0
                                      310               320           330           340       350                 360               370
                                                                              TEMPERATURE (K)
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                              0.8
                                                                                                                                       313
                                                                                                                                       323
                              0.7
                                                                                                                                       333
                                                                                                                                       343
                              0.6                                                                                                      353
                                                                                                                                       363
           SPACE TIME (sec)
0.5
0.4
0.3
0.2
0.1
                                     0
                                     0.94                        0.95          0.96       0.97       0.98                  0.99              1
                                                                                   CONVERSION DEGREE
Figure 8: Plot of Space Time against Conversion Degree for non-isothermal CSTAT
35
                                                      30
                                                                                                                              xA=95
                                                                                                                              xA=96
                                                      25
                              SPACE VELOCITY(sec-1)
                                                                                                                              xA=97
                                                                                                                              xA=98
                                                                                                                              xA=99
                                                      20
15
10
                                                       0
                                                       310          320         330         340       350               360           370
                                                                                      TEMPERATURE (K)
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                                  35
                                                                                                                                                           313
                                                                                                                                                           323
                                  30                                                                                                                       333
                                                                                                                                                           343
                                                                                                                                                           353
                                  25                                                                                                                       363
         SPACE VELOCITY (sec-1)
20
15
10
                                  0
                                  0.94                                                      0.95     0.96     0.97     0.98                     0.99              1
                                                                                                       CONVERSION DEGREE
Figure 10: plot of Space Velocity against Conversion Degree for non-Isothermal CSTAT
                                                                                        7
                                                                                     x 10
                                                                               4.5
                                   HEAT GENERATED PER UNIT VOLUME(kJ/sec.m3)
                                                                                4
                                                                                                                                               xA=95
                                                                                                                                               xA=96
                                                                               3.5
                                                                                                                                               xA=97
                                                                                                                                               xA=98
                                                                                3
                                                                                                                                               xA=99
2.5
1.5
0.5
                                                                                0
                                                                                310            320    330         340       350              360          370
                                                                                                            TEMPERATURE (K)
Figure 11: Plots of Heat Generated per unit Volume against Temperature for Non-Isothermal CSTAT
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                                                                       7
                                                                    x 10
                                                              4.5
2.5
1.5
0.5
                                                               0
                                                               0.94        0.95   0.96     0.97     0.98                 0.99            1
                                                                                    CONVERSION DEGREE
Figure 12 plot of Heat Generated per Unit Volume against Conversion Degree for non-Isothermal CSTAT
  The consideration of non-isothermity of the reactors is                                                    Critical examination of the results of the reactor types
  a reasonable assumption as long as the operation of the                                                    gives the following analysis:
  reactors is within the sonic limit. An observation
  deduced from this work is that the operating                                                                a.   At the same operating temperature, change in
  temperature tends to influence the reactor performance.                                                          degree of conversion, XA from 0.95 to.0.99
  Generally the operation is favoured by low temperature.                                                          curvilinearly increases the reactor volume and
  This confirms the reason why heat exchangers should                                                              space time of the non-isothermal CSTAT, while
  be incorporated in the design. The consideration of the                                                          the rate of heat generation per reactor volume and
  optimum limit of degree of conversion X A from 0.95 to                                                           space velocity decreases by the same proportion.
  0.99 is reasonable because at 100% conversion of                                                            b.   At the same degree of conversion, change in
  sulphur trioxide, the functional parameters of the                                                               operating temperature from 313 to 363K linearly
  reactors will all tends to infinity. In this case the                                                            increases the reactor volume and space time of the
  dimensions of the reactors have no limit.                                                                        non-isothermal CSTAT, while the rate of heat
                                                                                                                   generation per reactor volume and space velocity
  Work free days of 65 is allowed to produce the specified                                                         decreases curvilinear by the same proportion.
  quantity i.e. 1.389 x 103Kg/hr of sulphuric acid. Sulphur
  trioxide, SO3 can be produced by catalytic oxidation of                                               5. CONCLUSION                                           AND
  sulphur dioxide using vanadium pentoxide as catalyst.                                                    RECOMMENDATION
  From the results of the computation for the non-                                                           Model equation for the design of non-isothermal
  isothermal CSTAT it was found that; if the degree of                                                       CSTAT have been proposed for the production of
  conversion, XA was 0.95, the operational temperature,                                                      sulphuric acid via sulphur trioxide hydration process
  T was 313K, the reactor volume, VR were 2.5957E-                                                           using vanadium catalyst. Computer programs were
  05m3 and 7.8263E-06m3 when the reactant molar ratio,                                                       developed and utilized to simulate the performance
  m=1.0 and 1.5 respectively but increase of X A, and T                                                      parameters over a temperature interval of T=313 to
  resulted in increase of the reactor volume up to                                                           363K, and conversion degree, XA=0.95 to 0.99. The
  1.1432E-04 to 1.2781E-03m3 when m=1.0, T=363K                                                              result of the performance evaluation parameters shows
  and XA= 0.95 to 0.99 and 3.4469E-05 to 1.7897E-04m3                                                        the usual dependable characteristics of the kinetic data.
  when m=1.5.                                                                                                Further work need to be done to evaluate the
                                                                                                             performance of the various adsorption towers as a
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