. ,
\
\.
.,' ; .... _ 'L J.._;
/-w
.
t ! " ~ l ,1 1FOR TEE DIRECT SYNTHESIS OF DIHETHYLDICHLOROSILAlrE
Laboratory of Chemical Technology, T. H. Delft,
January,
1969.
c.
Coulaloglou,232B, Rotterdamsedijk, Schiedam.
-~ -I INTRODUCTION
-
---. / The purpose of this work is the study of a fixed bed reactor for the direct synthesis of dimethyldichlorosilane.Although the fixed bed reactor has been replaced in the recent years by the fluidized bed in industrial applications, the low
installation-and operation costs of a fixed bed reactor make it worthwhile to reconsider the problem and detect its feasability.
By using rate data based on a) The Prague School investigations, and b) R. Voorhoeve's thesis at the T.H. Delft and carrying out design
calculations by a method which neglects the radial temperature gradients we reach at the follOldng results :
For a yearly production of 5000 t. (CH
3) 2SiC12 we need two reactors vii th 200 tubes, 2.06 inches diameter.
The operating conditions are
a. Feed rate
=
12 lb mOl/Sq ft, hr b. Feed temp.= 300°c
c. Conversion= 35% C~Cl and 60% Si d. Pressure =
7
atm.From the discussion which follOviS v1e conclude that because of the poor heat transfer properties and particularly because of the frequent shut-downs the synthesis of (C~)2SiCl in a fixed bed reactor is not economical-ly attractive.
CONTENTS
--
---1. The synthesis of Dimethyldichlorosilane1 .1 • The direct synthesis
1 .2. The catalyst and the contact mixture 1.3. The composition of the feed
1.4. The temperature
1 .5. The silicon conversion
Page 1 1 1 2 2
3
2. Kinetics 42.1. The catalyst particles
4
2.2. Effect of the copper content on the reaction rate 5 2.3. The effect of temperature and pressure on the rate equation 5
3.
General considerations9
3.1. General considerations of the problem 9
3.2. Proposed method of manufacturing 11
3.3. Reactor tubes 14 3.4. Cooling medium 14
3.5.
Contact mass-catalyst 143.6.
Heat of reaction 163.7.
Physical properties 164.
Process design 17 4.1. Statement of assumptions 174.2. Hall heat transfer coefficient 17
4.3. Effective thenlal conductivity 19
4.5.
The design procedure4.6.
Pro duction4.7.
Pressure drop in tubesI I I
4.8. The minimum fluidizing flovl rate
5.
Remarks and conclusions5.1.
The heat transfer coefficient502.
Isothermal conditions5.3.
Activity of the catalyst5.4.
The contact mixture6. Literature Page
23
29
3132
33
33
34 3535
36
TEE SYNTHESIS OF DIJlTETHYLDICHLOROSlLANE
1.1. The direct synthesis
The most widely used method for the preparation of Dimethyldichlorosi-lane is the direct synthesis. The' principle reaction can be represented as follows (Li t.1)
Si + 2CELCl Si - Cu (CH) S·Cl
5
3
2 1 2One of the most characteristic of the direct synthesis is the great number of by products resulting from the reaction of Si with CH
3
Cl,whichcan be described by a number of hypothetical equations Si - Cu ( ) 2Si + 4CH 3Cl CH3 3SiCl + CH3SiC13 Si - Cu Si + 3C~Cl CH 3SiC13 + C~C~ Si
+
HCl +C~Cl
SiCuC~HSiC12
Si + 2C1 2Naturally a still greater number of products is to be expected with higher organic halides. The reaction rate and the nature of the products depend on a large number of factors some of which have not yet been inden-tified. Among these are the starting material, the catalyst and the
technique of preparing the contact mixture, the temperature, the pressure, the type of the reactor and the degree of conversion of the silicon and the organic halide.
1.2. The cat alyst and the contact mixture
The best catalyst for the synthesis of dimethyldichlorosilane is
copper. The composition of the products depends to a great extent on the amount of copper in the contact mixture. The greater the amount of
copper present, the higher is the chlorine content of the resulting pro-ducts. When the copper content is increased the reactivity of the contact mixture increases, but the conversion of silicon is decreased (Lit.2).
- 2 '
-In industrial applications a copper content of 5 - 10% is normally used. The contact mixture consists of silicon, catalyst and other sub-stances (co-catalysts, promoters) and is prepared in a mechanical, phys-ical or chemphys-ical process. It has been suggested that the reactivity of the silicon copper contact mixture is connected with the formation of an intermetallic n-phase (Cu
3Si), t~e- presence of which is of great impor-tance for the selective synthesis of dimethyldichlorosilane.
1.3. The composition of the feed
The presence of hydrogen chloride can cause significant changes in the reaction rate and the composition of the product mixture, bccause hydrogen chloride reacts with silicon or silicon copper (Lit.2) to form trichlorosilanes and tetrachlorosilanes faster than that between silicon-copper and methyl chloride. Oxygen and oxygen compounds are undesirable. Small amounts of R2S (0,1 - 0,2 mol/100 molC~Cl) can increase the sil-icon conversion up to 90% and the reaction ráte sometimes by 507~.
The heat distribution in the contact mixture can be improved by dilu-ting the CH
3Cl in N2, but because of the difficulties caused in thc
distil-lat ion of the product the use of N
2 is recommended only to moderate a reaction that is going out of control.
1.4p The temperature
One of the most important parameters in the direct synthesis is the reaction temperature, which must be maintained at the required'level on
the surface of the contact mixture, where the reaction proceeds. It must
also be made sure that no hot spots develop in the agglornerates of the solidphase. In fixed bed reactors, in which local overheating may occur, the reactinn rate and the composition of the product mixture are highly temperature dependent. Another factor which has an indirect but strong effect on these two parameters is the flow pattern of the feed (Lit.1). The reaction between methyl chloride and silicon copper in a fixed bed reactor is generally characterized by moderate selectivity (70% at 3000 and only 50% at 3500 (IJit.1)). At higher temperatures the deposition of carbon and the absorption of chlorine on the surface of the contact mix-ture slow do"m the reaction and may even stop it. Other disadvantages of the fixed bed reactor are the uneven consumption of silicon, and the presence of the spent contact mixture throught out the process,
which strongly catalyses the decomposition of the CH)Cl. On the other hand the reaction of Si with CH)Cl in a fluidizedbed is more efficient-ly controlled and this is the strongest argument for replacing the fixed bed reactors first by stirred bed and then by fluidized bed reactors. Nevertheless many experiments are still carried out on the laboratory scale in fixed bed reactors, af ter special precautions have been taken for the effective temperature control. Other special reactors of rare application in the laboratory and the industry include the riser tube
reactor (Lit.4), the horizontal reactor and the slurry reactor (Lit.1).
1.5. The silicon conversion
The direct synthesis of dimethyldichlorsilane is an interesting process
from the chemical engineering viewpoint, because the abrasion of the
catalyst surface, which is highly undesirable in comparable process, has
here a favourable effect by constantly cleaning the surface of the con-tact mixture so that the deactivation of the catalyst from the deposition
of carbon and inorganic halides is slower. The direct synthesis is also
remarkable as a catalytic process in that a gas-solid reaction is
cata-lysed by a
secon
~
solido
It is expected that the conversion of siliconduring the process will influence the reaction rate and the selectivity. The most obvious consequence of the silicon consumption is the increase in the copper content of the contact mixture resulting mostly in an
increase in the halogen content of the product mixture.
The spent contact mixture can no longer be reacted with the organic halide in an economically feasible manner and is discharged from the reactor. The silicon of the spent contact mixture could be reacted with
hydrogen chloride to give trichlorosilane and silicon tetrachloride, but
the marke~-Ior these is limited (Lit.1). S. Nitzche and R. Rielde (Lit.5) have claimed that the spent contact mixture can be regenerated by adding
fresh copper to it and can then be used for the synthesis of phenyl-chlorosilanes, particularly phenyltrichlorosilane; a silicon conversion -of 98% is said to have been reached in this manner.
:;
4
-KINETICS
2.1. The catalyst partieles
Polarization-microscopic investigations of the polished surface of pieces of contact mixture have shown that the silicon particles are part-ly coated with a layer of the ~-phase (app. CU
3Si), which in turn is partly coated with a thin layer of free copper that has not yet been converted into the ~-phase (Lit.1). It appears that ~-phase is formed at the boundary surface between silicon and copper, whereas a relatively copper rieh outer surface remains. Thc latter disappears almost entirely during the first period of the synthesis converted into the rt-phase. It
LV
is significant that only the copper present i$ theform a Si-Cu alloy
,
participates in the conversion of silicon into methylchlorosilanes. In the process of heterogeneous catalysis we visualize five steps occuring in succession:
a. Diffusion of gaseous reactants to the active sites of the catalyst b. Adsorption on the solid surface
c. The actual reaction on the surface d. Desoption of the gaseous products
e. Diffusion of the gaseous products into the gaseous phase.
In the present case of the direct synthesis of dimethyldichlorosilane a sixth step is introduced which is the diffusion of silicon atoms, through the solid phase onto active sites of the catalyst (n-phase of Cu-Si), where they can react with the CB3Cl. This diffusion probably becomes the rate determining step towards the latter part of the process, aft er muoh siliconhas been consumed but not before (Lit.1). Because of this re-striction, in order to increase the silicon conversion, bydecreasing the proportion of the silicon surface that is coated by a layer of the ~-phase,
_we enlarge the silicon surface by dLTIinution of the particle size, a technique vlidely used in industry. This increases the fraction of the reactive Silicon/~-phase interphasial boundary on the surface of the sil-icon particles (Lit.6). p~ additional advantage of this is that the finer partieles ensure a faster ind_tiation of thereaction between the methyl
chloride and the silicon copper mixture. On the other hand the use of
fine silicon partieles is complicated by other factors such as the large
internal surface area which increase the danger of local overheating and
cracking of methyl chloride resul ting in a lovler selectivi ty.
2.2. Effect of the copper content on
the reaction rate
The relationship between the
cop-per content and the reaction rate is
non linear and beyond a certain
opti-mum quantity of copper the rate
be-Gins to fall~~(Fig.1)(Lit.1).
..
;:j o I 3 -,-j IJ} tu) ~2 r-l o (J) ~ r-l 1 o El ...2.3.
The effect of temperature andEressure on the rate eouation ~,?O I I
O~--~3--~6----9~--1~2~ Cu in contact mass,
%
VIAf ter systematic investigations the
Prague School arrived at the
follm-J-ing rate equation (1) (Lit.1).
Figs1. The effect of the copper
content, on the rate of the
reaction
r
=
---\l1here
r = The reaction rate, mol C}~Cl/Kg Cu.hr
k
=
The-rate constant, mol CrsC1/Kg Cu.hr PA=
'Partial pres3ure of CH3Cl, Atm.
PB ::: Partial pressure of (CIS)2SiC12' Atm.
K
.
=
~ d t - ... t f' CLT C - A ... -1rl sorp lon cons~an
0
L3~' Ö~~ •.t~
IC. -13::: A' asorp lon t - cons t 'an t OI n (CnrT ) 4,-(")1 ' t -1
3 2 "l\., 2' il. ffi.
For an infinitesimally small conversion of the methyl halide eq.(1)
-I
. I
\..1
t
where r o = The reaction rate at zero conversion of methyl halide. The kinetic data obtained by,the Prague School are surnmarized in Table (1).
TABLE 1.
KINETIC DATA :B'OR THE REACTIONS OF ALKYL HALIDES WITH Si-Cu. Temp. , k, TT I~ E DH A 1;' (c) hA .>J a 0
RX Si-Cu
°c
mole RX/kg ( atm -1 ( atm -1 (kcal/ (kcal/ (kcal/mixture CU/h x 105 ) x 103 ) mole) mole) mole) CISCI Pure Si+ 280 240 ; 760 530
11 .9% Cu 300 600 ; 680 400 25.3 4.7 20.6 320 1170 570 280 300 625 900 \
-
-Techn.Si-t 280 ! 1000 800> 570 r 10.0% Cu 300 2025 ·280 ) 520 20.1 21.0 - 0.9 ; 340 8000 100' , 460(c) Over-all activation energy of the initial reaction rate ro:
-. ---~/ tJ--'A ~/
~ V'~p- ?
jY ... ~ .
~ == ~ - ])I{
o a A
The experiments \-lere carried out wi th technical silicon and 101'1 feed
rate of the gas. The maximilm selectivity ''las giv8n by 65;; dimethylchloro
si--lane in t~ product mixture and the pressure was found to have a small effect an the selectivity.
R. Voorhoever points out that the system used in the investigations of Prague School did not permit accurate control of the temperature
.throughout the reactor. He used a differential fixed bed reactor (Lit.6) to determine the kinetics of the reaction betVleen methyl chloride and two kinds of contact mixture with an efficient temperature control. In one case the contact mixture was prepared by t he reaction of copper chloride with silicon. Table (2) shows bis results.
7
-TABLE 2
KINETIC DATA OBTAINED WITH A DIFFERENTIAL FIXED-BED REACTORa
Temp. , Hourly Crs Cl Amount
--,
W/F
~
Expt.
°c
yield, conversion of Si(W), rocNo. g/h (x),
%
g1 303 1.85 0.43 68.5 0.21 0.021
2 323 4.00 0.75 67.1 0.16 0.047
3 343 8.40 2.64 63.5 0.25 0.104
Expt. i Composition of the
pr
oduct
mixture
, w t 4 j
No. C C}~) 2SiCl2 C~SiCl3 CC~)3SiCl CH
3SiHCl2 CiCl4
1 76.0 13.1 . 4.1 4.1 2.6
2 74.2 15.2 4.3 4.0 2.3
.3 73.0 14.0 4.7 4.0 4.4
a
The contact mixture was prepared from Si and CuCl b
Fis the feed-rate of methyl chloride in g/h fY"".
--~
c Fx/W in g of C}~Cl per g of Si per h. r't" ... ·~'V"" ro =
From table (2) we see that the composition of the product mixture is independent of the temperature in the range 300 - 340oC. The variation
of~e reaction rate with temperature can be calculated by substituting
the appropriate value for the temperature and the value 26~9 ~ 1 kcal/mol
for the activation energy in the Arrhenius rate equation. In a second series of experiments R. Voorhoeve used contact mixture consisted of a
-comminuted silicon-copper alloy exploiting the advantage that copper
is present from thc very beginning in the catalytically active form i.e.
as the n-phase. Some of the results are reproduced in Fig.(2) and Fig. (3). From Fig.(2) we see the agreement of the rate equation with the Arrhenius
law and Fig.(3) shows that in the temperature range investigated the
temperature has hardly any effect on the comllosition of the product mixture and this must be the result of efficient heat transfer of the
reactor. When the reactor has good heat transfer properties, local over-heating is avoided and the composition
o
Temperature C 350 325 300 275
o
r - - r . . . - - - r - - - r - - - - . - - .
fr-,.;:' _ of the product mixture is basically
-0.4
,J'
independent of the reaction mixture. By applying eq.(1) at tempera~ure
o
300 C R. Voorhoeve shows that at high conversions the reaction rate reaches a maxL~um at low pressures (Lit.1). Thus for methyl chloride conversions of 60% and 50% at 3000C the maximum reaction rates are reached at pres -sure of about 6 and 8 atm. res
pecti-~-0.8
~
-1.2
1.60 1.70 1.80 1000T-1
Fig.2. Arrhenius curves for the reaction between methyl chlorid.e and Si-Cu.
vely. This is because the products are absorbed on the surface of the contact mixture, and thus the number of active sites on it decreases
-at higher pressures and conversions (Lit.1). The adsorption is still stronger at lower temperatures, and the maximum reaction rates coincide wi th stilllovier pressures. Such maxima are responsible for the fact that the methyl chloride conversion in industrial reactors is lirnited to about 50% and the optlinum pressure is about 6 atm. At higher con-version or higher pressure the selectivity of the industrial process is decreased.
---Product composi tion, %1-1
100'r-~--'--'-ïï--.---r--'.-~~-r--~ 80
"
0 lil 8 0 60 ~.le2SiC12 40 20 NeSiC1 3,
0•
~ ~.,
280 290 300 310 320 330 340 350 360 370 380 'l'empcraiure°
c
Fig.3. T1..,ü ' -o effect of temperature on the compo-si tion of th8 metr.ylchlorosilane prOduct m ix-ture obtained at 4.6 at:l.
9
-GENERAL CONSIDERATIONS
3.1. General considerations of the problem
Before carrying out the reactor calculations it is necessary to have enough rate data in the form we can use in the design equations. The rate equation according to Baza~t et al is
~~---~--- (1 )
The molal flow rate of CE)Cl entering the reactor is F. At a point \-lhere the conversion is X i t will be NA
=
}1 (1 - x). The molal rate of(Crs)2 Si(CI)2 is NB = ~ and the total floltl rate Nt = F(1-x) + F ~ =
F (1 - ~).
Assuming that the perfect gas law holds and calling P
t the total pressure, we have NA 1 - x PA - Nt Ft = Ft 1
-
x 2 x PB 2 x Ft = 2-x x Ft 1--2Hence the rate equation (1) becomes
From table (1) we At 2800C k=1000 At 3000C k=2025 At 3400C k=8000 kK 1 - x A 1
-
-
x r -A- 2(1
+J
K 1-x A -1-2S. 2have for contact K =: 800.10-5 A -5 K A= 280.10 _t:; K A
=
100.10 .-I Ftptf
Ft +K:s
2 -x x mixture consisting-3
KE=
570.10-3
K...,= 520.10 15K:s=
460.10-3
(2 ) of Techn.Si+10% CuBy plotting the reaction rate constant versus temperature in Fig. (4) we get a straight line \'Jhich is expressed by the usual Arrhenius rate
6 ~ ~ 0 r-I 5
,
,
4 3 2,
,
,
1.6
1.8
2.0
2.2
1 T· 1000Fig.
4.
Plot of the reaction rate constant, versus temperatureat
7
atm.
equation.
9 12·16~
k
=
5570.10 .e-
T.
(3)k
=
rate constant in mol CIS Cl/Kg Cu., h.T = absolute temperature oK.
The change of the adsorption equilibrium constants with the temperature is _shown in fig.(5). We see that
KB
changes slightly with temperature at the range of 280 - 340oC: Rence we can assume that it is constant, -1
and equal to about 0.500 Atm. •
For P
t
=
7 atm~k = given by eq.(3) or fig. (4) KA = by fig. (5) •
We calculate the reaction rate for different temperatures and conversions (see tables 3,
4, 5,
6 and 7). The results are plctted in fig. (6).---'-~---- . ___ ---.l___ _
I
909
\ ~ \ I • S op <Xl r<'I 0 ~ • ~..
-
,,_
_
- - - - ---
K:s
--
-... I • El op <Xl 300 lî'I 0 ... • ~<Xl 100 280 290
Fig.
5.
Effect of temperature on the adsorption constants KA'KB
at P =
7
atm.o 0
We observe three regions of the rate curves in the range 280 - 340 C
(fig. 6). In the first region of the curve the reaction rate drops till 300oC. This is owed to the decrease of the adsorption constant KA and
i
the small increase of the rate constant k. At the _second region!
~~~~i:n~3~:~)
t:::
::::~:nb::::S~n::e:::: :::::l:/::::~:~:r::
~:Wh:::-a steep increase of tne rate constant and only a small decrease of the
adsorption constant (see fig. 5,6).Further the reaction rate drops once
more because the adsorption constants become very small and cannot be
balanced ,by the increase of the rate constant. In addition to the drop
of the reaction rate above 3300C the selectivity of the reaction is
smaller too and above 3500C CH
3CI is decomposed extensivily. This is
obviously the reason 'n'hy the optimum temperature conditions lie bet1>leen
, 0
280 and 340 C.
3p2. Proposed method of m~rillfacturing
o
Methylchloride \'lill enter the reactor at a temperature of 300 C, a pressure of 7 atm. anci a molal rate of :E' = ~ lb mol/ ft 2 ohr.' ,.;hich will
" .. ' .~ ,.. '.
"(
12 -TABLE
3
TOe KA k KA·Pt IVt
~
-Atm.-1 mol
e~el/kg.Cu,h
-1Atm. 280 80.10-4 1000 560.10-4 0,236 50,0-10-3 290 50.10-4 1370 350.10-4 0,187 500.10-3 300 28.10-4 2025 196.10-4 0,140 500.10-3 310 23,5.10 -4 2950 164,5.10 -4 0,128 50,0-10-3 320 18,9.10 -4 4210 132,3.10 -4 0,115 500.10-3 330 14,4.10 -4 5970 100,8.10 -4 0,100 500.10-3 340 10,0.10 -4 8000 70.10 -4 0,084 500.10-3 TABLE 4 1-x 1-~ 1-x 2-x x
Fr
x 2 1--x 2-x 1-~ 2 2°
1 1 1 2°
1 Q,1 0,9000 0,9500 0,9473 ,1,9000 0,0521 0,9738 0,2 0,8000 0,9000 0,8888 1 ,8000 0,1111 0,9419 0,3 0,7000 0,8500 0,8235 1 ,7000 0,1764 0,9074 0,4 0,6000 0,8000 0,7500 1,6000 0,2500 0,8658 0,5 0,5000 0,7500 0,6666 1,5000 0,3333 0,8164 0,6 0,4000 0,7000 0,5714 1,4000 0,4285 , 0,7565 ! Iprovide the reactant gas vii th the appropriate residence ... time for a con-version of about 30 - 35%. The Si concon-version is chosen to be about
60 - 65~, although a more realistic one would be a silicon conversion of about 50%. The temperature of the reaction must not exceed the level of 3400e because of the low selectivity and the decomposition of e~el above
this temperature. We assume that the selectivity is of the order of 80% in the range280 - 3400e and this is in agreement with the experimental results of R. VoorhoevG (Lit.1), altbough it was claimed that the selec-tivity drops at the e:ld of the process af ter a high silicon conversion. (Bazant et al)
i-x k.KAeP t x x 12" 0 1 0,1 0,947~ 0,2 0,888E 0,3 0,8235 0,4 0,7500 0,5 0,0666 0,6 0,5714
!
M
V
KAPt x I ; - x ;1--l
-
2 I I 0 II
i 1 0,1 : 0,9738 I : I°
,
2 I 0,9419 1 1 0,3 i 0,9074 0,4 1 0,8658 0,5 0,8164 0,6 0,7565 TABLE 5 280°C 290°C 300°C 310°C 320°C 330°C 340n '"'C 56,00 47",95 39,69 48~52 55,69 60,17 56,02 : I : 53,00 45,42 37,59 45,96 52,75 56,99 53,12 ; 49,77 42,61 35,27 43,12 49,49 53,47S 49,84 46;11 39,48 32,68 39,95 45,86 49,54 46,13 42,00 35,96 29,76 36,39 41,76 45,12 4290Ó 37,33 31,96 26,45 32,343 37,12 40,10 37,38 I 31,99 27,39 22,67 27,72 31,82 34,38I
32, C~-280°C 0,236 0,229 0,222 0,214 0,204 0,192 0,178 1\
I i \ l TABLE 6 K P1.::2s
A t 1_2S. 2 290°C 300°CI
310°CI
0,187 0,140 0,128 0,182 0,136 0,124 0,176 0,131 0,120 0,170 0,127 0,116 0,162 0~121 0,108 0,152 0,114 0,104 0,141 0,105 0,096 I Ii
j 1 --320°C 330°C 340°C. , 0,115i
0,100 0,084 0, 111 I 0,097 0,081 0,108 0,094 0,079 0,104 0,090 0,076 0,099 1 0,086 0,072 0,093I
0,081 O,Oó8 0,086 1 0,075 0,063 ' 1I
- 14
-TABLE 7
r. mol CE)CI/kg Cu, hr or lb mol CILCI.1 0
3/
lb C hrj u, TOC
280
290
300
310
320
330
340
x0
3
6,67
34,05 30,5
38,1
4
44,85 49,72
47,78
0,1
33,15
31,13 27,84 34,76
40,83 45,24
43,36
0,2
30,49
28,11
25,03 31,20
36,55 40,48
38,78
0,3
27,20
24,95 22,14 27,57
32,23 35,66
34,10
0,4
2
3
,76
21,70 19,16 23,
9
4
27,8
30,77
2
9
,37
0,5
20,21
18,36 16,12 20,02
23,39 25,76
24,51
/0,6
16,50
14,80 13,01
16,13
18,8
I
20,7
19,65
3.3.
Reactor tubesThe fact that it is necessary not to exceed the mentioned temperature
l~vel in the catalyst bed and that .the reaction is considerably exothermic, leads us to the choice of small diameter tubes with better heat-transfer properties. It also means that the radial temperature variation will be less.
2.06
inches internal diameter40/
21
NichelChromium alloy steeltubes were employed for the reaction duty because this material has superior
strength properties at elevated temperatures and good corrosion resistance to organic halides.
3.40
Cooling medium'I:Y' 0
.~)'"' .çI'" Stearn of about
8.5
atm at a temperature of175
C wi11 be used as cooling,
~
medium.3.5. Contact mass-catalyst
The contact mass consists of
0.50
mm diameter particles containing10
%
copper. \'Ie assume that the catalyst is present from the beginning in·the form of Si-Cu alloy (C~Si). Af ter about
60 - 65
%
Si conversion the contact..
(5 ~ "-r<'I o ..-c r-l o r<'I ... >-'-< o rl o S ~ , f 501
/
/
~
.
x=o 46//
/
~
x=0,1 , 42 I(
Ij
/
/
~
x=0,2 / 38'~
L_- •.-
/
,//
/
/
-34/t
~
~
~
11/
1/
/
X7
30 ~, I / """'"-.
~
~
l/
/
/ '
;5
/x=O,4r""
~
/
/ " .;'/ ,,/ 26/
~""
>/~
/
~/
/ " / ...~
. /p~
VP ""'" ,// -22-
-l><
V
~
, t> 18 , 14 280 290 300 310 320 :530 340 350 360 Temperatureoe
Fig.; 6. Plot of reaction . rate against temperature at 7 Atm.
I
I!
.. j --
--- ---
- -
-16
mass will be discharged and replaced by a new one. The porosity of catalyst was estimated to be equal to 0.5.
3.6.
Heat of reaction ,The complexity of the reaction and the side reactions occuring during the process make it difficult to ~stimate with great accuracy the heat of reaction. For the present conditions the heat of reaction is estimated to be about -92.000 cal/mol Si' which is equal to -46.000 cal/mol CH
3 Cl (Lit.1).
3.70
Physical propertiesThe physical properties of the reactant and products were estimated at
o
\
- - - -
17
-PROCESS DESIGN
4.1. Stat ement of assumptions
The following assumptions are made
a. Smooth variation of the properties b. Eddy diffusion dominant
c. Axial diffusive process neglected
d. Insignificant temperature varations in the radial direction across the diameter of the reactor.
e. Heat transfer coefficient at the wall and the bed.
4.2.
Wall heat transfer eoiffieientI
Leva's correlation may be used for this purpose (Lit.7)
cooling
, -
--
'
where h .d t .W k g 0.7 d.e-4,6.~
h =w wall transfer coefficient, Btu/ sq ft.hr. 0 F
d t k g d P G
tL
For -= = = -= tube diameter, ft. CH 3Cl thermal conduetivity, part iele diameter, ftsuperficial mass velocity, CH 3Cl viseosity, lb/ft• hr d t
=
0,172 ft d=
0,0016 ft P ~=
0,050 lb/ft •hr The Reynolds Re=
~
=
12 x 50,5 • 0,0016=
P
0,050 19,4I
I
I
- 1$ -and d
p/
d=
0,0093 t / Hence eq (4) becomes 3,5. k (19,4) 0,7 " 3,5.902.8 hw=
d t .e-f.
6•O'0093=
0,172.1,043=
3,15 hw=
3,15 Btu/s ft.hr.oF---g---I
The wall transfer coefficient is a critical quantity in the design method
and small changes in it can cause large difference in the conversion.
The use of Leva's correlation for this small Reynolds number is a
question-able procedure and only experimental resul ts ,dIl provide us wi th an
accurate wall transfer coefficient. However it is expected that the wall
transfer coefficient will have a low value not only because of the small Reynold number, but also because of the very smal 1 partiele size which decreases the size of the eddies ànd the distance over which each mixing
process occurs. Also, there is a large number of more or less stagnant
films between the fluid and the solid partieles, vlhich the heat must cross
in reaching the wall. Colburn~ (Lit.8) claimed that.the ratio of heat
transfer coefficients varied with the ratio d
p/d
value of about d
I
=
0,15 (TabIe 8). tp d t TABLE 8 I·~ :, i reaching a miximum
J
~
:>
Ratio of heat transfer coefficient in packed- (h )and empty (hi)tube (8).
P I d I P/d 0,05 0,10 0,15 0,20 0,25
!
0,30 t h P/h. . 5,5 7,0 7,8 7,5 7,0 6,6 l4.3 Effective thermal conductivity K
e
The effective thermal conductivity is estimated by the analytical method proposed by Vl. Argo and
J.
Smith (Lit.9).a. Radiation contribution K'
r "
e Ta3
K'
= 4·---·d .0,173.r 2-e p 1004
T
=
mean temperature of the bed ae = emissivity
d = part iele dia~eter
P K'
=
4-ih2.
.0,0016.0,173· ,10708 3 r 1 ,5 10 b. Point eonductivity K' p=
=
= = K logK'=
-1,76 + 0,0129·-ä p e (5) 1070 oR 0,5 0,0016 ft 0,004 (6)Ks
=
particle conductivity == 48,4 Btu/sg.ft.hr.oF/fte
=
void fraction=0,31. For d1
=P d 0,0093 e = 0, 31 ( Li t . 1 0 ) t ~ r 6 logK;= -1,76+0,0129. 0
,
3
1
=
-1,70+0,0129.15 ,1=
0,254 K' = 1,8-12 _____ _
c. Coefficient h between pellet and gas
c For Re <350
.
2/
3.
(d . G) -0. 51
(~)
(?;)
= 1.95 ..lL- (7)~
I '120
-i
=
0,33.Btu/lb~F.c == mean specific heat of reaction mixture
p
i
(c ; == specific heat of C~Cl at constant pressure== 0,38 Btu/1boF
P1 )
c = specific heat of (CEJ)2 SiC12 at constant pressure =
f2
0,20 Btu/1b0:E').
-..
G =:superticial mass velocity
=:CEJC1 viscosity
== 606.1b/ sq. ft • hr
= 0,050. 1b/ft • hr
kg
=
CH)Cl conductivity == 0,020 BtU/Sq ft.hr OF/ftBy replacing in eq.(7) we get
I
~
)
2/
3
"
~~h~c~~ _ 0,33.0,05 == 1 ,95 (19,4) -0,51 0,33 x 606 0,02 and d. Coefficient hand h r p 0,33.606.1,95 h = =94 c 0,86.19,4.0,51 h c == 94The calculation of the radiation and point-to-point contact coefficient
from eq (8) and (9) is a trial- and error procedure, since both
expres-sions involve the summation coefficient h.
h == K' (2K' T hd ) r r s p d .K P s d .K P s
KS = particle conductivity
=
48,4 Btu/sq ft.:b..2.~. oP/ft(8)
!'
We assume that h = 2445, then eq (8) becomes:
0,004(2.48,4 + 2445.0,0016)
and from eq.(9)
h r = ---
=
5,2 h = P 0,0016.48,4 h r = 5,2 1,8(96,8 + 0,0016.2445) 0,0016.48,4 = 2.346,4Since the sum of h c + h r + h p = _ 94 + 5,2 ~ 2346,4 = 2445,6 , the original assumption of 2445 for h is satisfactory.
e. Knovling the coefficient h at the surface of the partiele, the series contribution to K can be evaluated from eq.(10).
e
h.K .d K'series=
s p=
2K + hd s l? 2445.48,4.0,0016 188,3 =2.48,4+2445.0,0016=
100,7=
1,87 (10)f. Xurbulent d1ffusion contribution Ktd
The turbulent diffusion contribution is evaluated from eq (11)
-
.~ ~. d .c .GK.
td
=
(~e
m)
e.
d .u
n
Pem
=
Average Peclet Number = ~De
=
Effective diffusivity;
- ,
~- - - _ .
-I
- '
22-d .G
From R.A. Bernard and R.H. Wilhelm (Lit.11) for ~ = 19,7
and d / P d = 0,0093/ t / Pem
- - - =
9
d 2 1 + 19,4 (p/
dt ) Pem=
9(1+
19,4.0,00932) = 9.1,0016=
9,014Rence eq.(11) becomes
d.c G 0,0016.606.0,33
Kt~
=
~em:d
=
9,014.0,31=
0,107g. Molecular conductivity contribution K '
c
Xhe thermal conductivity of CE3C1.at 315
°c
is abouth. Effective thermal conductivity k~.
K = eek' + Kt'd + K' ) + (1 - e) K'series --e g r =.0,31 (0,020 + 0,107 + 0,004) + 0,69.1,87
=
0,041 + 1,3=
1,34 Etu/ ft h 0F/ sq • r. Ft K = 1,34 Btu/ f t h 0F/ e sq • r Ft~'
T
.,--
---j,...~-I
I ,\ , , \ j ' LI
I/
23
-404. The over all heat transfer coefficient
The heat transfer capability of a fixed bed ofregularly shaped small size solids may be expressed in the form of an over all transfer coefficient from eq.(12) (Lit.12).
I
I
(12 ),
I
i
U = over all coefficient Btul 0
sq ft.hr, F
h = heat transfer coefficient for cooling fluid outside of wall
o
r
=
radius of bed solid, ft .Btul sq f+-IJ. h r, oF
?
We assume that the outside of the wall Rence from /éq. (12)
heat
~~e
is negligible..:::.
!
t
0,086U 3,15 + 4 x 1,34
=
0,326U = 3,1 Btu/c oq . ft • hr , OF
4.5.
Thc design procedureAt this point we have enough data to proceed to the evaluation oI' the conversion, the temperature profile along the reactor and the size of the_. reactor.
The rela.tionships required are
a. The basic design equation r.dVI = F.dx
or (13)
S r () Q' Z = :E'. dx
CO ·lBo
r
=
rate.
Jj,
J
(-
24
-I
equation,lb Cu, per hour W = mass of the solid catalyst, lb F = feed rate, lb mol/
hr x = conversion
z
=
longitudinal distance in reactor, ft Sc= cross sectional area oft~be,
ft2eB= Bulk density of solid catalyst, lb/ cu. ft. b. The rate equation
In fig. (6) the reaction rate is shown as a function of temperature and conversion.
c. The energy balance taking into account the heat transfer to the reactor wall
F·dx· (-DI-I) - ·U.S - (tD-tv,)-dz = Sm.c .dtm (14)
R l Pi
DH
= heat of reaction, Btu/lb mol oF2
S = internal surface of tube per ft, ft /ft R
hl = wall temperature, oF
tm = bulk mean temperature of reaction mixture, oF mi - weight rate of flOiv of component i, lb/ hr
c = specific heat of component i at constant pressure, Btu/1b*OF
Pi
The bulk densi ty of the catalyst
e
B
is found as follOi-Js Thc.density of the contact mixture particlese~
= Ö.(0,1 eCu + 0,9 eSi)ó
= porosity of particles=
0,5
~....;--eCu= density of copper = 555,61b/cu ft
.eSi= density of silicon
=
151,71b/ cu fte~ =0,5-(0,1°555,6+151,7-0,9)= 96 lb/cu ft I 1 J I j f ,\ " I,
________________
---_______________
~I---The void fraction of the bed e = 0,31
Rence the bulk density of the contact mixture partieles ec is
ec = e~·(1-e) = 96.0,69 = 66,3 lb/cu ft
and the bulk density of the catalyst (10% copper)
The heat of reaction is DH=-46.000 cal/
g
mol
CB)CI 0C'
which in British units is equal to-46.000 Btu/1b mo. 1 CH-C "5 1, oF
The wall transfer area/per unit length of tube SR = 0,541 sq ft/ft ';: il.I ...
The cross sectional area of the tube
2
Sc
n~d
= 3à
14 = 0,1782=0,023~
sq.ftThe molal rate/per tube
F = 12·S
c
=
12-0,0232=
0,278 lb mol/hrThe heat capacity of the reaction mixture Sm. cdenends on the tempera-1. p. _ .. ture and conversion. Por an average temperature l of 31SoC and conversion x
+ F • liill • ~. c
2 2 P2
rill
1, lffi2 = Holecular weight of C~Cl and (CI~)2()S1 C12 respectively
By substi tuting these values in the desibJTI equation (13) we get
,
or
dz = 1 80·d.x , r
26
-(A)
The energy balance equation (14) may be written
,t
0,278-46.000-dx-1 ,8-3, 1-0,541 ~ (tm-tw) ·dz·1 ,8=0,278- (19,2-7,7') ·à.tm-1,8 (The
ture
conversion factor 1,80F/ OC have been introduced so that the tempera-may be expressed in degrees centigrade)
or
For conversion x = 0 the energy balance eq~ation becomes
.12,788·d.x-1,68-(tm-tw)-dz = ·5,34 (-lh .. , for x = 0,05 12.7SS·d.x-1,6S-(tm-tw)·dz = 5,23 for x = 0,10 12.78S·dx-1 ,68· (tm-bl) -dz = 5,13-dtm for x = 0,15 12.788.d.x-1,6S-(tm-tw)-dz = 5,02-dtm
Proceeding in this way we can get the form of the energy balance equa.tion for every conversion.
Equations (A) (B) and fig (6) or eq (2) are the design energy and rate relationships necessary to solve the problems. A numeri cal approach is indicated ivhich is carried out in the following llay
1.
An
incremental value of conversion dx = 0,05 is chosen 2. A temperature at the end of the increment is assu~ed3. The rates are obtained at the beginning and end of the increment from fig_(6)
4. The increment dz is computed fr om eq (A) 5. The assumed temperature is checked in eq (B)
,.I f .1 /. ..
f
.
-a...J~)
·
I -.:5
':.,.J ... 01I
27
-The design equation (A) may be written in a simplified form considering the average reaction rate in the increment dx = O,OS
dz
=
1,s-0,oS·{l +1) ·
-21 =\r
1r
2dz = 0, 04S -(
1
r +1 )
(A1 )1 r 2
r
1, r2
=
the rates at the beginning and end of an increment dx=O,OS Illustrating these calculations we start with an increment dx = O,OSFirst increment It is assumed that ;tm 1 = 303 oe Hence dtm = tm l - 300° = 3 0 e
From fig_ (6) the rate at the beginning of the increment, zero conversion and t = 300 oe is 30,4-10-3 and at the end of it, t = 303 oe is 31,3-10-3 , then in eq (Al) dz = 0,045- 1 + 30,410-3 1 - - ' - - - = 2,91 ft 31,3-10-3 In eq.(B
1) we check the assumed value of tm1
/
"
-
;.'"
.:
'1
,
,-
.
0"",,.,.,... :J, 'ol 12.788-0,05 - 1,6S-(300-17So)-2,91=5,34e dtm= 119,7 - 0,32-12S-2,91 = tm 1 - 300 - -tm1 - 300 = 3,1 tI!l1 z: 303,1(versus the assumed value of 303°C)
Second increment
I
,
t'
~,34-(tm1-300)
At' the end of the second increment we assume Tm~
=
306,S
-Then from fig.(6)
r
1 = 31,3-10-3
r
2 = 32,2-10-3 and eq.
A,
gi yes+ __ 1.:-_ _
=
2,83 32,2-10-3Checking the assumed value of tm
2 in eq, (B2), 5,23.dtm = 12_788 - 0,05 - 1,68-(303-11'5)-2,83 dtm = 122,2 - 0,322·128-2,83
=
3,50Ctm2 = 303 + 3,5
= 306,5
(Versus the assumed value of 306,5)
Continuing the computation give the results summarized in Table (9) and shovln in fig. (7)
TABLE 9
Convers ion x TeI:1perature
°c
Catalyst bed depth_ :ft°
300°
~ '\ 0,05 303 - 2,91-
0,10 306,5 2,83 ~ 0,15 316,5 2,62 Ó,20 ~, 331,5 2,'35 '. I 0,25 336,5 2,34)i
i I 0,30 , 328,5 2,48 /~ 0,35 290 3,20 • '7 ~\
0,37 (265 ) (4,8) \ " ).
- . _ - 29 -. / 340~ ________ ~ __________________________________________ -, 330 320 310 300 290 2 4 6 8 10 12 14 16 18Catalyst bed depth-ft.
Fig_ 7. Longitudinal ternperature profile of the bed. 4.6 Production
i } ' It is obvious from fig_ (7) and' table (9) that the reaction slows dovm
/' for CH
3Cl conversions greater than 0,35. The length of the tubes for this
~ c:> conversion is equal to 18,73 ft _ A reactor consis ting of 2,06 inches tubes,
.--ft high operating 260 days/year (about 1 day in operation and 9
r-
18,73
~'Á-hours shut-down) the yearly production is estiluated as follo\vs
The mass of contact mixture per tube is
Mc= Vte c
=
0,434-66,3=28,78 lb/tube andMSi=the mass of silicon in a tube=28,7S-0,9=25,9 lb/+ b
For 60% silicon conversion the mass of silicon converteN- rs 1'1si = 25,9. 0,6 . ) = 15,54 lb
or
Msi
=
1,11 lb mol30
-The amount of C~Cl to convert 15,54 lb·Si is
"IK M .-2 hCLLC};: Slo j 0,35 · 1,11·2 = 0,35
=
6,34 lb mol ~.{,Z ;r {L~ ,~v 'IFor a feed rate 0,278-lb mol CB)Cl/ hr
b2i
'
it will take about h
=
0,278=
23 hr to convert 60% of Si.For a selectivity of 80% the production per tube per hour will be
p' = 0,278 -0,35-0,8 = r 2 or 0,0389-115 = 4.473 lb/ 1 lr, tube or P ' r = 4,473-1000 453,5 = 2,03 kg / hr tube .
,
And the yearly production per tube
P r
=
260-24·2,03 = 12.670 . kgf year, tube'"
.A
reactor with 100 tubes will produce P =·100·12.670=
1.267.000 kgf .r year
and with 200 tubes
P
=
200·12.670=
2.534.000 kgfr year
In a reactor of L
=
15,53 ft the C~Cl conversion will be 30% Thenh ·= 22 hr
p'-
r - '
1 74 k / g hr tube,
A reactor with 100 tubes will produce P r
=
1.086.000 kgf year31 -and with 200 tubes
P r = 2.172.000 kg! year
Because the synthesis of dimethyldichlorosilane is a semibatch process it is necessary to use at least ~wo reactors, even it is expected that the use of three reactors will secure smoother operation of the whole
unit_
/
4.7- Pressure drop in the tubes
Using Kozeny's equation for laminar flow we evaluate the pressure drop in the tubes (Lit.13)
2 -1 DP _ ~G .a •
L
L - 2 a·u g"e·e~",) l ~'~~J..~tr forSL
.4
00a.~
'\~)
(15)
r--e
---
<
v> \)
P <e. v., D~ . . - - - " ' : .1.
G (1_ ~ ) b'1. !:( \ -.J
.::. ~ G - 606 lb! sq fot' .nrp
= oO,05 -lb!ft.hr _~e'=
densi ty of C~CI =.
=
0,31 g = 4,18.108a
=
specific surface of the bed .sq ft! cu ft=
§JJ-eJ
D a=
Hence6
{1-0 221 }=
2587 0,0016 606 G a~ 2587-0,05 P sq ft/cu ft. =4,7
L - 8 2 •=
175 psf! ft~p
5-6062.2.587(606-1
4,18-10 .0,45·0,31 2587·0,05DP
178 ~=
144'= 1,2 psi/ft DE=
1,2-18,73 = 23,4 psirfJ~
rwJ
vr~lr
T:'AS
çJpc>
P "/
re,
cU-
,L.cJ,2';'/~
~),
(>_1)1-"l,,\;e)
y',~
)"'7'
I
4_8 The minimum fluidizing flow rate
-The CB)Cl will pass upward through the bed. In order to check that the gas velocity is less than the minimum fluidizing velocity C we must
m
estimate it for the conditions under which the reactor
operaf~s.
Using equations (16) proposed by Hiller and Logwinuk (Lit.13) we have= 5,23°105·e1,1·(ep-e)0,9.dn2 ~
e
= density of CH3Cl=0,45 lb/cu ft
e
p=
density of particles ~~lb/cu ftD = 0,0016 ft p ~ = 0,050 lb/ ft.hr Rence 5,23.105.0,451,1.(95,55)0,9.256.10-5 0,050 (16 )
=
5,23-2,56.0,415-60,5 0,05 = 336,18 0,05 = 6.723 lb/ sq ft h , r.33
-'
-REMARKS ANTI CONCLUSIONS
From the previous discussion we dravl the following conclusions:
5.1. The heat transfer coefficient
This critical parameter for the design of a fixed bed reactor was
found to be low at the present operating conditions (hw=3,15 Btu/Sq ft,hro:. Although Leva's correlation for the wall transfer coefficient is a
questionable procedure for not provide us with enough
(d
I
= 0,0093)p d
low Reynolds numbers, the litterature data for so small d and d
I
ratiosP P d
t
does
t
In an attempt to improve the heat transfer properties of the reactor by using higher gas velocities the results we got were not satisÎactory,
(For F= 24 lb moll sq fi: _, h the h = 5,1 r w Btul s q , ft h) probably, because r
of the very small particle size of the contact mixture. On the other hand the conversion was low (about 10-15%) and the pressure drop very high. The length of the reactor necessary for 0,05 conversion vlas of the order dz = 6ft . .
Another mean to irnprove the heat transfer properties was the use of smaller tube diameter. Here follm"l the resul ts we got
For
F = 10 lb mol CELCI/ --5 sq ft, hr d
t = 1,61 inches Feed temp.= 2950C
Cooling me~ium temperature 2000C The total length of the reactor
L = 15,85 ft
The conversion = 30% CB)CI and 60% Si The production p'= 5.500 kgf tube
r year,
and for a yearly production of 1.5000000 kgf we needed a reactor
year
vli th 273 tubes, a considerably expensi ve construction.
I
/
./o
0,1 0,2 0,3 0,4 0,5 0,6Conversion x Fig. 80 Graphical integration of the design equation at
T = 3300C and P
=
7 atm.--It seems that we could improve the heat transfer properties of the reactor by introducing an inert gas (N
2) in the feed stream. We have
already seen (Par. 1 .3) that the use of N
2 \vould cause difficulties in
the distillation of the product and the acceptance of this method cannot be predicted without taking into account the separation problems.
5.2 Isother!1".al conditions
If we"could operate the reactor isothermally, at the temperature of higher reaction rate T = 330oC, something which is qui tOe unrealistic for
the direct synthesis of (CH
3)2 SiC12, the length of the reactor for
COI1-version x= 35% C~Cl eQual to that of the examined problem is found as
follows:
35
-7 W_jO,35
d.x \'.,/ F -- -
r S ~ ~/ (17) JvV 0 . À YJ'/ "
'it ~...r ('w,.JU-'\ S t ,25 '33,7 ;-~~2
hrl lb Cu lb -mol C~Cl and ~ . " \ W = P-8,42 = 0,278·8,42=
2,340 lb CuThe bulk density or Cu
eB
=6,63
lb/cu ft
...
\
Hence the volume
6tf
the tube is~ \
Vt = 6,63 = 0,35~U ft
and the length ~Of the' reactor _ ~ _ 0,353
L - S - 0 0232 = 15,2 ft ~
c '
503
Activity of the catalystIt is expected that the conversions will be lower than that predictod by eq. (2), because of the uneven consumption of· silicon and the decrease of the activity and selectivity of' the catalyst vlith time.V.S---Fikhtengol' ts and A .L. Klebanskii claimed that in the fixed bed process the relative
amount 'Of dimethyldichlorosilane drops from 60 to 30% in the course of
20 hours (Lit.14).
5.4. The contact Dixture
This which makes the fixed bed reactor quite unattractive for the direct synthesis of (C~) 2SiC12 is the frequent shut-dm'm for removal of the contact mixture. This becomes the overriding cons~deration for rejecti LG
LITERATURE
==========
1. R.J.H. Voorhoeve, "Organohalosilanes Precursors to Silicones", Elsevier Publishing Company, Amsterdam, 1967.
R.J.H. Voorhoeve, Thesis, Delft, ,1964
2. E.G. Rochow, J. Am. Chem. Soc. 67p 963 (1945)
3. P.G. Dudani and H.G. Plust, Nature 194, 85 (1962) 4. C.E. Reed and J.T. Coe, Chem.Abstr. 40, 1536 (1946)
5.
S. Nitzche and R. Rielde, Chem. Abstr. 57, 13.803 (1962)6. R.J.H. Voorhoeve, B.J.H. Geertsema and J.C. Vlugter, J. Catalysis 4, 43
7.
Leva,Che~. Eng. (Aug. 1957)8. J.M. Smith "Chemical Engineering Kinetics" IvIcGra'\'l - Hili Book Company, Inc., New York, 1956
9. W.B. Argo and J.H. Smith, Chemical Engineering Progress, 49 : 443 (1953)
10. J.H. Perry "Chemical Engineers' Handbook" 4th edition,
r.TcGraw-Hill Book Company, Inc., New York, 5-51, (1963)
11. R.A.< Bernard and R.Il. Wilhelm, Chem. Eng. Progr.,
46 : 233 (1950)
12. J. Beek and E. Singer, A procedure for sealing-up a catalytic-Reactor, Chem. Eng. Progr. 47 : 534 (1951)
13. S.H. Walas, "Reaction Kinetics for Che~ical Engineers",
HcGravl-Hill Book Company, Inc, Nevl York, (1959)
14. V.S. Fikhtengal'ts and A.L. Klebanskii, J. Gen. Chem. U.S.S.R. 27, 2535 (1957).
I.
APPENDIX I
1
1 ProceS8 design for zero partiele porosity
For
G
=
superficial maS8 velocity=
1212 lb/sq ft.hrtw
=
wall temperature=
145°Cand
S
= poroai ty of catalyst particles = 0We follow the same design procedure.
Wall transfer coefficient
__ G.d 12
The Reynolds number Re
r-
-
=From equation (4): = 3,5.0,02.(38,8)°,7
=
0,172.1,043 hw=
5,02 Btu/sq ft.hr.op 1212.0,0016 0,050 5,0213
Effeetive thermal conductivity Kea. Radiation contribution K' r
From
equation (5) K' lC 0,004 r..
-
...._--
..._--_
..
b. Point eonductivity K~ Prom equation (6) K' =..,E--... ... ___
1,8.
__
c. Coeffieient h . c between pellet and gas
From equation (7) he
=
0,33.1212.1,95=
146 0,86.6,23 hc == 146 -.-....
---.---=
38,8-""---" ~... ..'
r-
-
'
-
-II. d. Coefficient hr and hpWe
aesumeh
=
2.499.
From equation (8)and from equation (9)
h
_12
... __
=
... ____
2.347,5_
_
----
-.
-
~ . - ::.~,_". h'-':'~
Since the eum hc+ hr+ hp
=
146 + 5,2 + 2347,5 = 2.498,7 the original assumption of 2499 for h is satisfactory. e. Series contribution K'seriesFrom equation (10)
f.
Turbulent diffusion contribution Ktd For Re :: 38,8 dp/d t=
0,0093 = 9,2 Pem : 9,22 andfrom equation (11) Ktd = 0,225---
...
----g. Molecular conductivity contribution k~
" -'.' ... ~.
k~=O,020 BtU/ft2~hr.(OF/ft)
...---_
...---h. Effective thermal conductivity Ke
Ke
=
e (K~ +Kid
+ K;) + (1-e) K~eries~ 0,31 (0,020+0,225+0,004) + 0,69.1,9 f: 1,39 Ke= 1,39 BtU/eq.ft.hr.op/ft
l I l .
14
The overall
heat transfer coefficientFrom equation (12)
1 1 0,086
'U ~ ~ +
4
.
1,34
=
0,214u
=
4,67 Btu/sq ft.hr.oF ~---~--~15
The design procedureThe density of the contact mixture partieles f~ (See page 24).
For 10% Copper
r
~ = 0,1.555,6+151,7.0,9 = 192 lb/cu•ftThe void fraction of the bed e
=
0,31. Rence the bulk density of the contact mixture particlesFe
isre
=r~(1-e)=1920,69=132,5 lb/cu ftand the bulk density of the catalyst (10% copper)
The molal rate/per tube
, , ,
F ~ , 24.S c = 24.0,0232 = 0,556
.:.;/
~ ~.
The heat capacity of the reaction mixture Smi c Pi
Smic = F.MB1.{1-x).c +F.MB2·~·c
Pi P1 P2
By
substituting in equation (13) we get O,0232.13,25.r.dz = 0,556.dx oror
dz = 1 80 dx , r dz=
1,80.dX(~
+ ');~
1 r 2 (A)The energy balance equation (14) may be written
25.576 .dx-2-.53. (tm-tw) .dz=O, 556. (19,2-7, 7x) .d tm (B) For x=O and an increment of dx=0,025
1 1 )
dz
= 0,0225 (-- +
--r,
r
259,8-0,237.(tm-tw).dz=dtm
For a feed temperature of 270°C it is assumed that the temperature tm1 at the end of the first 1ncrement is 2900
c.
From fig. (6) the rate
at
the of the increment, zero conversion and t=
270° C is 34,3.10-3 , and at the end of it, t = 290° C is 33,2.10-3 •Then in equa.tion CA1)
1
,
dz=0,0225 + =1,3
.... :---
....
34,3.10 -3 33,2.'0 -3 In eq.(B,) we check the assumed value of tm,
59,8-0,23~(290~).1,3=dtm=tm1-270
59,8-39,7
=
tm,-270 20,1=
tm,-270tm,= 270+20,1=290,10C (versus the assumed value of 290°C)
ft.
Cont1nu1ng the computation give the results summarized in Table (10) and fig. (9).
560 ~20 SOO ~80 !60 (.) o E-4 4 Fig. (9) Conversion x
0
0,025 0,050 0,075 0,100 0,125 0,150 . 0,200 0,250 0,300 0,350 0,400 ' ; . 6 .8 . _.i,....
..
.
. . .. ,.~,.. .. ; 10 12 14 16 18 20 22Catalyst bed depth, f t
Longitudinal temperature profile of the bed.
TABLE 10.
Temperature
°c
f' . Catalyst bed depth, ft.270,0 0 290,0 1,30 300,0 1,45 308,0 1,43 322,5 1,19 338,0 1,06 346,5 1,12 348,0 2,47 343,0 2,58 339,5 2,60 334,0 2,71 291,5 3,50 ./
I --'
.
_
... -. ,', ~..
I . . -, i ,'? VI. 16 ProductionA reactor coneisting of 2,06 inches tubes, 15,20 ft high
..
----operating 260 days/year (about 1 day in operation and 9 hours shut down) the yearly production for eonversion x=0,30 ie estimated as follows:
V
t=
L.Se=
15,20.0,0232 = 0,353 eu ft The mass of contact mixture per tube isMc= 't·re= 0,353.132,5 = 46,77 lb/tube
M
Si = the mass of silicon in a tube=46,77.0,9=42,1 lb/tubeFor 60% silicon eonversion the mass of silicon converted is
M
Si = 42,1.0,6 = 25,26 lb = 1,8 lb.mol The amount of CH 3Cl to convert 1,8 lb.mol Si isM
Si ·2 1,8.2 MCH Cl=0,30
= 0,30=
12 lb mol CH3Cl 3For a feed rate 0,556 lb mol CH
3Cl/hr it will take 12
about h = 0,556 = 21,7
hr.
to convert 60% ofS1 •
Assuming a mean select1vity of 65% during the course of 21,7 hours, the production per tube will be
;;=O,~56.0,30.0,65=O,0542
lb mol(CH3)2SiC12! hr, tubeor
0,0542.115=
6,233 lb/hr , tubeor
P; = 2,82 kg/hr, tubeAnd the yearly production w111 be
I •
•
A reactor
with
100
tubes will produce
Fr= 100.17.597
=
1.759.700
kg/year
and
w1th
150
tubesP
r=
150.17.597
=
2.639.550 kg/year
17 lressure drop in the tubes
From equation
(15)
~
=
359
pSf/ft=
2,4
psi/ftand
dP =
2,4.L
=
2,4.15,20
=
36,5
psiIa
The minimum fluidizing flow rate\" , ) ,
~l
L.
l ,
---From equation
(16)
we see that the minimum fluidizing
flow
rate is
about5,5
times higher than the
ueed velocity of CH,
.
VIII.
REMARKS ANTI CONCLUSIONS
We see from the previous discussion that considering
the particles without porosity the production/year is
improved but the main disadvantagesof the fixed bed reactor i.e. the poor heat transfer properties and the
frequent shut-down remain.
For a production or 5000 tUbes/year we need:
Two reactors with 150 tubes, 2,06 inches diameter and
15,20 ft length.
The operating conditions are
a. Feed rate = 24 lb mOl/Bq ft, hr b. Feed temperature 2700 C c. Conversion