Thursday 10 March 2011

ACETYLENE CONVERTER


THEORY OF REACTOR DESIGN

·        Heterogeneous catalytic reactors are the most important single class of reactors utilized by chemical industry. Whether their importance is measured by the whole sale value of goods produced, the processing capacity or the overall investment in the reactors and associated peripheral equipment, there is no doubt as to prime economic role that reactors of this type play in modern technology society.
·        Commercially significant types of Heterogeneous Catalytic Reactors.
The types of reactors used in industry for carrying out heterogeneous catalytic reactions may be classified in terms of relatively small numbers of categories. One simple means of classification is in terms of relative motion of the catalyst particles or lack there of we consider.

·        Reactors in which the solid catalyst particles remain in a fixed position relative to one another (fixed bed, trickle bed and moving bed reactors).
·        Reactors in which the particles are suspended in a fluid and are constantly moving about (fluidized bed and slurry reactors).

Fixed Bed Reactors.

In its most basic form a fixed bed reactor consists of a cylindrical tube filled with catalyst pellets.   Reactants How   through   the catalyst bed and are converted in to products. Fixed bed reactors are   often   revered to   as packed bed reactors.   They   may be regarded as the work horse of the chemical industry with respect
to number of reactors employed and the economic value of
materials produced. Ammonia synthesis, sulphuric acid production
(by oxidation of SO2 to SO3) and nitric acid production (by
ammonia oxidation) are only a few of the extremely high tonnage
process that make extensive use of various forms of packed bed
reactors.
The   catalyst   constituting   the   fixed   bed   will   generally   be employed in one of the following configurations
·        A single large bed
·        Multiple horizontal beds supported on trays arranged in a
          Vertical stack
·        Multiple parallel packed tubes in a single shell
·        Multiple beds each in their own shell
The use of multiple catalyst sections usually arises because of the need to maintain adequate temperature control with in the system. Other constraints leading to the use of multiple beds include those of pressure drop or adequate fluid distribution. In addition to the shell and tube configuration, some of possibilities for heat transfer to or from fixed bed reactors include the use of internal heat exchangers, annular cooling spaces or cooling thimbles are circulation of a portion of the reacting gases through an external heat exchanger.
The packing itself may consist of spherical, cylindrical or randomly shaped pellets, weir screens or gauzes, crushed particles or a variety of other physical configurations. The particles usually are 0.25 to 1.0 cm in diameter. The structure of the catalyst pellets is such that the internal surface area far exceeds the superficial (external) surface area so that the contact area is in principle, independent of pellet size. To make effective use of the internal surface area, one must use a pellet size that minimizes diffusional resistance with in the catalyst pellet but that also gives rise to an appropriate pressure drop across the catalyst bed. Some considerations which are important in the handling and use of catalysts for fixed bed operation in industrial situations are discussed in the catalyst hand books.
The most commonly used direction of reactant. Flow is down ward through the bed. This approach gives a stable bed that will not fluidize, dance or lift out of the reactor. This approach minimizes catalyst attrition and potential entrainment of catalyst fines. When processing
conditions are such that the reactor is subjected to wide variations in
feed flow rates or when the feed is a dense fluid, it is imperative that
the flow direction be downward. The attendant advantages of-downward flow is the tendency of bed to compress itself and the
gravitation of catalyst fines (resulting from attrition) down through the
bed. Both phenomena may lead to increased pressure drop and
channeling or mal distribution of the flow. Up flow has the advantage of lifting catalyst fines or fragmented particles from the bed there by avoiding channeling and blockage of bed. However, this mode of operation is disadvantageous because it may lead to unstable beds at high flow rates. It leads to dancing in a pulsating flow that caused catalyst abrasion and in unusual circumstances may lead to fluidization.
A fixed bed reactor has many unique and value able advantages relative to other reactor types. One of its prime attributes is its simplicity, with the attendant consequence of low costs for construction, operation and maintenance relative to moving bed or (fluidized bed operation. It requires a minimum of auxiliary equipment and is particularly appropriate for use in small commercial units when investments of large sums for control, catalyst handling and supporting facilities would be economically prohibitive-. Another major advantage of this mode of operation is implicit in the- u.-e of the term "fixed bed reactor" (i.e.) there are no problems in separating the catalyst from the reactor effluent stream. (In many fluidized bed systems catalyst recovery can be quite troublesome and require substantial equipment costs). An other important attribute of fixed bed reactors is the wide variation in space times at which they can be operated. This flexibility is extremely important in situations where one is likely lo encounter wide variations in the quantity or quality of the feed stock to be processed. For example high temperatures or high pressure reactions employing solid catalyst, economic considerations usually dictate that the process becomes commercially viable only when a fixed bed reactor is employed.


Design of  Acetylene Converter


Compound
Kgmol/hr
Kg/hr
CH4
C2H2
C2H4
C2H6
C3H4

C3H6

H2
1.214
3.423
132.439
15.745
0.07
0.426
6.846
19.36
88.998
3708.29
472.35
2.8
17.892
13.692
TOTAL
160.163
4323.384

Average molecular weight                                              = 4323.384/160.163       
                                                                                           = 26.99
Assuming gases behave like ideal gas vapour density    =  PM/RT

P                                                                                        = 30atm
M                                                                                       = 27 
R                                                                                       = 82.05                      T                                                                                        = 353K
Vapour density                                                             = 30*27/82.05*353
                                                                                          = 0.0279 g/ml
                                                                                          = 27.996 Kg/m3
                                                                                          = 1.75 lbm/ft3   

For the conversion of acetylene, space velocity for palladium catalyst is
Space velocity                                                                  = 1000/hr
Vol. of reactor                       = Flow rate of Feed/vol.of reactor
Volumetric flow rate           = Mass flow rate per unit density
Mass flow rate                                                                 = 4323.384 Kg/hr
                                                                                         = 1.64 lbm/sec
 Density                                                                            = 27.966 Kg/ m3

Volumetric flow rate                                                = 4323.384/27.966

                                                                                         = 154.59 m3/hr
Voidage volume of reator                                     = Vol. flow rate per
                                                                                             unit space velocity
                                                                                         =154.59/1000
                                                                                         = 0.15459 m3

Volume of catalyst                                                          = 0.15459*100/40

                                                                                        = 0.386m3
Volume of reactor                                                           = 0.386/0.7
                                                                                        = 0.551m3

Length of reactor                                                            = 1.845 m

                                                                                        = 6.04 ft
Diameter of reactor                                                        = 0.615 m
                                                                                        = 2.0 ft
Volume of reactor occupied by catalyst                         = 0.386m3
                                                                                        = 386Litres
Weight of catalyst                                                          = 386 Litres* 500 g/lit
                                                                                        = 19300 gm
                                                                                        = 19.3 Kg

Size of particle                                                               = 5 mm

                                                                                        = 0.016 ft
Size of screen                                                                 = 0.417 mm
Mesh No                                                                        =35
m                                                                                                                                                                    =0.0266 lbm/sec.ft
Cross sectional area                                                        =Pi *D2/ 4
                                                                                  =Pi*22/4=3.4ft2
Mass flow rate/cross sectional area                               =2.645/3.14    
                                                                                       =0.843 lb/sec.ft2
 Î                                                                              =0.4
DP/L                        =  (G/rgDp)(1-Î/Î)[150(1-Î)m/Dp +1.75G]
                                                                                  =260.67 lb/ft2
                                                                                  =1.8 Psi

ENARGY BALANCE

 

Heat of reaction
                           C2H2 + H2                            C2H4

DHT =  å[(HT-HO)+Hf,o]products - å[(HT - HO) + DHf,0]Reactants
             = [3.12 +14.522] – [(2.96 + 54.329) + (2.374 + 0)]
        = - 42.02 cal / gmole
        = - 75 BTU/lbmol
        = - 166.75 BTU/Kgmol





HT                                                                                                        = - 513.71 BTU/hr
Enthalpy of entering stream at 176F             = 250 BTU / lb
                                                                       =20.39 BTU/Kgmole
                                                                       =20.39BTU/Kgmole*163.63
                                                                          Kgmole/hr
                                                                       =3275.24 BTU /hr
Enthalpy of leaving stream                            = 3275.24 + 513.71
                                                                       =3779.41 BTU/hr
Temperature of outlet stream                                                      
     Cp ( T2 - T1 )        =    DH
     Cp ( T2 – 176 )     =     75
     T2                          =   181.4 F

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