Thermal Cracking of Isobutane Kinetics and Product Distributions Alfons G. Buekens and Gilbert F. Froment' Laboratorium voor Petrochemische Techniek, Rijksuniversiteit Gent, Krijgslaan, 271, 9000 Gent. Belgium The thermal cracking of isobutane was studied in a flow apparatus between 620" and 820" C and at atmospheric pressure. Product distributions were established and a reaction scheme was deduced. The order of the isobutane decomposition was determined by comparing experiments with different degrees of feed dilution and was close to one. Owing to inhibition the first-order rate coefficient decreases with increasing conversion. Therefore, the rate coefficient i s considered a hyperbolic function of conversion. The activation energy of the first-order rate coefficient increases slightly with conversion. Finally, the radical nature of the process is discussed.
T h e rapidly growing demand for propylene leads to a need for processes producing propylene independently from ethylene. Isobutane is an attractive feedstock for this purpose. The thermal cracking of isobutane yields four main products formed by two parallel decomposition reactions: isobutene and hydrogen on one hand; propylene and methane on the other. The main by-products are ethylene, ethane, propane, 1-butene, 1,3-butadiene, 2methyl-1-butene, and aromatics. Most studies of isobutane pyrolysis were performed in static systems at temperatures below 620°C. Pease (1928) and Pease and Durgan (1930) showed the reaction was first order and homogeneous, Paul and Marek (1934) found an activation energy of 66 kcal per mole a t low conversions. Steacie and Puddington (1938) investigated the kinetics of this reaction thoroughly in a pressure range from 5 to 60 cm of Hg and a t temperatures from 522" to 582'C. Packing the reaction vessel with silica tubing did not affect the pyrolysis rate, proving the reaction homogeneous. The authors reported the reaction times required to reach a conversion of 12.5 and of 25% ( 0 1 2 5 , 0 2 5 ) . Owing t o inhibition the rates fall off rapidly with increasing conversion. At different temperatures a constant 0 1 2 S / 015ratio was found, hence the inhibition was considered to be temperature independent. Although the reaction is approximately first order, er5increased with decreasing initial pressure. For this reason the authors extrapolated their rate coefficients to infinite pressure and obtained the following expression:
hqj = 4.17 x exp (-63500/RT) sec-' (1) Fusy et al. (1966) showed that the isobutane pyrolysis can be described by a radical chain mechanism and that a t low temperature the chains may be partially terminated by wall removal of H atoms. Konar e t al. (1967) determined propane yields a t 551'C and a t various pressures. These yields are a measure of the rate of initiation
i-C,Hio
+
CH;
+ CHiCHCH, + i-C,Hio
to account for early formation of ethylene.
' T o whom correspondence should be addressed.
H ' + (CH3)qC=CH2 -+ (CH3)sC' (4) (CHs)?C' + i-C,HIo + (CH3)2CH'CHq + i-C,Hlo (5) These reactions effectively describe the trends in the product distribution, but fail t o account for inhibition, since no active radicals are eliminated. Illes et al. (1969) reported ethylene, propylene, and isobutene yields a t a constant residence time of 0.8 sec for temperatures ranging from 580"to 820" C. Conversions or methane yields are not given. Although direct comparison is difficult, the ethylene yields reported are improbably high. The literature data are summarized in Tables I and 11. This literature survey illustrates the need for reliable experimental data above 620°C. The aim of this work was to investigate i-butane pyrolysis under conditions as representative as possible for industrial operation. Experimental
Apparatus and Analysis. The equipment has been described (Buekens and Froment, 1968). The reactor Table I. Kinetic Data
Order
Frequency factor, -1 sec
Activation energy, cal/mole
Temp range, "C
1
1.7 x l0l4
65,000
600
1
7.8 x 10"
66,000
550-610
1
8.3 x 10''
63,500
522-582
(2)
Konar et al. (1968a) postulated the radical isomerization reaction
i-CsH++ i-C4H1o + n-CsHi
Konar et al. (1968b) investigated the self-inhibition of the pyrolysis reaction and the changes in the distribution of the major products, both of which occur a t an unusually early stage of reaction. Up to 3% conversion, the ratios of (HZ)/(C,Hg) and (CH4)/(C3Hs) are strictly equal to one, but the ratio ( C H 4 ) / ( H 2 )changes markedly with conversion. At 497°C the rate coefficient drops t o half its initial value a t a conversion of only 1%. The instantaneous reaction rate a t fixed conditions is a ;unction of the amount of isobutene present, whether generated or added. The authors account qualitatively for both phenomena by adding two reactions to the Rice-Herzfeld reaction scheme:
(3)
Reference
Pease and Durgan (1930) Paul and Marek 119341
Steacie-and Puddington (1938)
Ind. Eng. Chem. Process Des. Develop.,Vol. 10, No. 3, 1971 309
Table II. Product Distribution Data Selectivity, mole
Conversion, Reference
Temp.,
Marek and Neuhaus (1933) Frey and Hepp (1933) Frey and Hepp (1933) Hurd and Spence (1929) Steacie and Puddington (1938) Steacie and Puddington (1938) Steacie and Puddington (1938)
O
C
600 575 575 600 522-582 522-582 522-582
%
H2
CHI
0.0 6.7 17.4 22.0 12.0 25.0 51.0
64.0 58.7 46.5 46.5 57.3 56.1 47.8
35.6 41.0 41.7 77.4 38.6 61.4 85.8
effluent stream was analyzed on a Wilkens Moduline 202 gas chromatograph with hot wire detector. An eight-meter squalane column was used for the general analysis, a 10-meter dimethylsulfolane column for the separation of 1-butene from isobutene. The hydrogen-to-methane ratio was determined off line on a Perkin-Elmer F-11 gas chromatograph with hot wire detector. A three-meter Porapak Q column was used. The peak surfaces were measured by means of a Varian electronic integrator 477. Conditions. The experiments covered the following range of variables: temperatures, t, 620" to 820" C; pressure, p , atmospheric; isobutane flow range, Fa, 0.5 to 10 moles per hr; space time, V I F O ,3 t o 150 liters sec per moles total feed (residence times from 0.03 t o 2.5 sec); dilution I
C?H4
2.1 7.6 2.4 5.0 7.4
Yo
C2H6
C7H6
2.4 2.2 2.3
34.5 31.2 30.8 37.7 31.6 39.6 41.5
3.0 6.1 10.0
CIH,
3.8 9.5
Figure 3. Product distribution at
C.Hs
64.0 61.9 57.1 49.1 54.3 49.4 38.7
740°C
I
loO{
l
o
20
40
60
80
{ )*I
loo
120
iio
160
0
ibo
5
Figure
Figure 1 . lsobutane conversion vs. V/Fo
I
//--=-I \
I V - 11t set) c-'*
120
140
160
180
Figure 2 . lsobutane conversion (moles cracked/lOO moles
fed) and conversion into the main products at 780"C (moles formed/lOO moles fed)
310
Ind. Eng. Chem.Process Des. Develop., Vol. 10, No. 3, 1971
4. Product distribution
at
780"C
factor, 6 , 0 to 10 moles nitrogen per mole isobutane fed. The isobutane used contained 0.5 mole "c ethane, 2.5 mole 7% propane, and 0.5 mole c7c n-butane. The product distributions were checked with a set of runs with pure isobutane (Air Liquide CH 25, >99.5% pure. only detected impurity, n-butane). The reactor wall was deactivated with CS, a t 450" to 550" C. Results. The kinetic analysis is based upon conversion x vs. space time V / F o diagrams. Figure 1 represents the experiments without dilution of the feed; x stands for the isobutane conversion, as derived from the exit stream composition on a no loss basis, V is the equivalent reactor volume defined below. Figure 2 shows the conversion of isobutane and the yields for the main products as a function of V I F o a t 780" C. Product distributions a t 740" and 780" C are represented in Figures 3 and 4. The material balance was checked by carbon/ hydrogen balances. The hydrogen surplus may be considered a measure of coke formation and is shown in Figure 5.
peratures and conversions; its selectivity remains below 0.5%. The Ci-selectivity increases with temperature (Table 111). Because of secondary formation the selectivity decreases only slightly for conversions up to 7 0 7 ; beyond that conversion it drops sharply, however. Propane formation is favored by low temperatures (Figure 7 ) . Up to 70% conversion the propadiene selectivity exceeds that for methylacetylene, although the latter is the more stable compound (Figure 8). The C4-selectivities decrease with increasing temperature and also with increasing conversion (Figures 3, 4,
Table 111. Initial Product Distribution Temp, ' C
CI-selectivityQ
Ci-selectivity'
lsobutene selectivityn
640 660 680 '700 720 740 '760 '780 800
46 41 44 46 49 49 51 54 49
41.5 42 43 45 47 49 51 51.5 50
57 56.5 55 53.5 52.5 51 49 48 49
Moles formed 100 moles decomposed. :.THANE
!
SELECTIVITY
Product Distribution
Initial Product Distribution. Table I11 shows the initial selectivities for the primary products as a function of temperature. At low temperature the initial selectivity for hydrogen is 55 i 5 . This value decreases with increasing temperature. The initial selectivity for C2hydrocarbons is almost zero, for Ci- and C6-hydrocarbons, zero. Influence of Conversion and Temperature. Product distributions at 740" and 780°C are shown in Figures 3 and 4 , hydrogen selectivity in Figure 6. The methane selectivity reveals two opposite trends: an increase in temperature favors C1 + Cs formation while a decrease in temperature favors methane formation by secondary reactions. Above 30% conversion the methane selectivity is almost independent of temperature. C?-hydrocarbons are typical secondary products (Figures 3, 4, and 7). Acetylene is only detected a t high tem-
4-
4/
;,f?OfANE
SELECTIVITY
3
I
!-BUTENE S E L k T I V I T Y
'
7,
3-
3-
21-
.-n -
CARBON-HYDROGEN BALANCE
I.3-8UTADIENE SELECTIVITY
'
7
2.5
$4K
5'0
75
7,CONVERSION
100
Figure 7. Selectivities for C2H6, C3H8, 1-C4Hs, 2-C4Hs, and 1,3-C4H6 as a function of conversion
2
13::
21-
3-
8009
21-
I 0
20
40
% CONVERSION
60
80
I
100
0. 3-
78OoC
Figure 5 . Carbon/hydrogen balance as a function of conversion and temperature
-
760 + 78OOC 720 + 740'C 680+ 7 0 0 T + (660'C
\*.
I
.
:I
700'c
C.
I
1
I
0
20
40
%CONVERSION
60
80
I
100
Figure 6. Hydrogen selectivity as a function of conversion
Figure 8. Selectivities for propadiene and Cs as a function of conversion a t different temperatures (800", 780°, 740°, and 700°C) Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
31 1
Table 111). At low conversion the C,-fraction consists only of isobutene, at higher conversions 1-butene, 2-butene, and 1,3-butadiene are also produced (Figure 7). The C5-selectivity (Figure 8) exhibits a maximum, the value of which increases slightly with temperature. At low conversion 2-methyl-1-butene is the only Ci-component detected. At higher conversions isoprene, pentadiehe, and 2-methyl-2-butene become important. The Cs-fraction consists of benzene; it is detected from a conversion of 7 0 5 upward. Up to a conversion of 60% essentially two moles of product are formed per mole isobutane decomposed. The expansion coefficient c (Figure 9) is approximated by the following equation c
= 1.96
+ 0 . 8 ~ ~(0
5 x < 1)
(6)
The temperature dependence of the product distribution is confirmed by comparison of our data with these from Table I1 and from Konar et al. (1968b).
I
O
I,
V = V’ exp[- E
(T l - %1) ] d V ’
(7)
The use of the equivalent reactor volume concept as a basis for kinetic analysis implies an a priori knowledge of the activation energy. The calculation of V was based on a value of 55 kcal per mole for E , in close agreement with the experimental activation energy. Kinetic Analysis. The kinetic analysis is carried out along the lines discussed in detail in a previous paper on propane pyrolysis (Buekens and Froment, 1968). The reaction order is determined by comparison of experiments at the same conversion level, obtained with a different dilution factor. A distinction is made between the instantaneous rate coefficient, k , as determined by the differential method and the mean rate coefficient, %, as calculated by the integral method. Typical values for the reaction order are presented in Table IV. The reaction order decreases from a value between 1.0 and 1.5 at low conversion to a value of 1 a t higher conversions. The same behavior was observed in the pyrolysis 312
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
I
20
40
9. Expansion
Figure
% CONVERSION
ab
60
1 0
factor as a function of conversion
Table IV. Determination of the Reaction Order Temp,
Conversion, Yo
C
620 640 640 ti40 660
Kinetic Study
Equivalent Reactor Volume. An important problem encountered in kinetic studies of pyrolysis reactions is the proper measurement of the gas temperatures. At high cracking temperatures a radial temperature profile is inevitable owing to the endothermic nature of cracking reactions. When wall temperatures are measured and heat transfer is not accounted for, the difference between the wall and the true gas temperature results in a strongly curved Arrhenius diagram. In the present study the thermocouples were inserted in the tube axis so that gas temperatures are measured. Owing to heat radiation and conduction, the measured temperature is closer to an average cross sectional than to the true axial value. In kinetic studies the longitudinal temperature profile may be accounted for by the equivalent reactor volume concept. The effectiveness of an elementary reaction volume is proportional to the factor exp ( - E / R T ) which describes the temperature dependence of the rate coefficient. In this work the Hougen and Watson (1947) equivalent reactor concept is applied (Froment e t al., 1961). The equivalent reactor volume a t the reference temperature T R(“ K ) is calculated by the formula
1
EXPANSION FACTOR
3,0/
Order
1.25 1.15 1.05 1.05 0.95 t o 1.0
1.8 2.7 6.0 11.0 > 15
of propane (Buekens and Froment, 1968). Halstead et al. (1969) noticed a similar decrease of the order in the pyrolysis of neopentane and also concluded that inhibition lowers the apparent reaction order. In the following, the kinetic analysis will be based on a first-order rate law. First-Order Kinetics. Integral Rate Coeficients, These are calculated numerically by integration of the material balance (Equation 8),after substitution of r in function of the rate coefficient, h, the total concentration, et, and the dilution and expansion factors 6 and C .
x.
Fodx = rdV -
(8)
1-x - _ _ 1 + 6 + (6 - l ) x
r = kc = %et ~
_
(9)
The calculated K values are grouped into narrow conversion intervals. I n each of these intervals the best Arrhenius parameters are determined by linear regression. The activation energy, E , of the integral rate coefficient, h, increases slightly with conversion. This trend is represented by
E, = Eo + A E ~ A X - X where
Eo = 52,600 =t2300 cal/mole a E / h = 9500 j= 6000 calimole Eo and A E / A x are determined by weighted least squares. The limits cited are 95% confidence intervals. Figure 10 represents an Arrhenius diagram for fir = A , exp ( - E r / RT). Point Rate Coefficient, k. The influence of inhibition on k can be described by the semiempirical hyperbolic law k = k o j ( 1 + ax) (10) Combination of Equations 8, 9, and 10 leads to:
koct
rv S,x ( 1 + a x ) { 1 + 6 + [ t ( x ) - l ] . x ) dx =
-
1-x
(11)
k o and a are determined from Equation 11 by linear regression, the results are represented in an Arrhenius diagram (Figure 11). At 620" and 640" C the conversion range was too limited so that no significant values of a could be determined. In the Arrhenius diagram two values of a are included, estimated from results of Steacie and Puddington (1938) ( a = 23 a t 562°C) and from Konar et al. (1968b) ( a N 100 a t 497°C). These values are in good agreement with ours. The ko and a values were correlated by an Arrhenius law; to each value a weight inversely proportional to the standard deviation on ko or a was assigned. The Arrhenius parameters are given in Table V.
I
="
Discussion of the Radical Mechanism
I t is now generally accepted that the pyrolysis of paraffins proceeds by a free radical chain mechanism. Such a mechanism mainly consists of four types of reactions: Initiations, which occur by carbon-carbon bond rupture
C HB 1
CH,-CH-CH,
--*
CHi
+ CHiCHCH,
Terminations or radical recombination reactions 0.01
2CHI CHi
+ i-C4Hi
2 -+
3 +
i-ChHio + H ' i-C,Hio + CH,'
+
5 +
/' '
10
,1 13
12
4 'l +(OK-')
CZHG
Figure 1 1 . Arrhenius diagram fork0 a n d o
.
L-CsHiz
0 Konar
et 01. (1968b)
A Steacie
Hydrogen abstraction from the parent hydrocarbon, which forms a large unstable R p radical: 4
~
9
and Puddington (1938)
Table V. Arrhenius Parameters of ko and a
E
IogioA zt AlogioA"
+ Hz i-CdH9 + CH, i-C4Hi
ko a
11.62 zt 0.43 1.32 -4.89 zt 0.57
52.1 -20.6 -24.0
-4.16 zc
a'
i l E , kcal/rnole'
* 2.0 T I
6.1 2.5
"95% confidence limits. 'Including values for a , calculated from literature data. 10-
7 1-
KX
X
1 2 3 4 5 6 7
0.25 0.35 0.45 0.55 0.65 0.75 0.85
-Y :_
R Kdecomposition, yielding an olefin and a n active, chainpropagating Rp radical
CH3 CH,-C-CH,
C Hs H'
+ CH,-k
= CH2
CHs
x .
CH,-CH-CHZ
7 +
CHI
+ CH,--CH=CH?
The radical mechanism, formed by reactions 1 to 7 , describes isobutane pyrolysis a t initial conditions. The dynamic equilibrium between initiation and termination steps determines the radical concentration level. The chain process is formed by reactions 4 to 7 . Addition of 4 + 6 and 5 + 7 leads to the two parallel over-all reactions:
0,l-
9
6
--+
10
11 T'oK-l' 1o4
112
Figure 10. Arrhenius diagram for k , Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
313
This type of mechanism gives rise to an over-all reaction order of 3/L when termination 2 is predominant and to first-order kinetics when termination 3 is more important. A computation of radical concentrations a t steady state shows that (CH;) >> (i-C,H,) a t the temperatures and pressures investigated. Thus an initial order of % and an activation energy of E N E: + '4 ( E 1 - E?) N 50 kcal/moleaare predicted. These values are compatible with the experimental results from Tables I V and V. At noninitial conditions the pyrolysis reaction is strongly inhibited by isobutene and propylene. Part of the inhibition may be explained by addition reactions of the type:
taneous appearance of inhibition (reactions 10, 11) and of changes in product distribution (reactions 8, 9), as reported by Konar et al. (196813). Ethane is formed by recombination 2 and by hydrogen abstraction by ethyl; propane is formed by the corresponding reaction of propyl. At high temperatures acetylene and propadiene are formed by decomposition of vinyl and allyl radicals. Propadiene isomerizes further into methylacetylene:
CHz=CH'
12 +
13
CH;-CH'-CH;
+
CH=CH
+ H'
CHz=C=CH,
+ H'
14
CH?=C=CHZ 5 CH,--C=CH 1-Olefins and diolefins are formed by termination reactions.
CHI-CH'-CH;
+ CHj
CH;-C(CHs)'-CH;
15 -+
CH2=CH--CH*-CHs
+ CHj
16 +
CH2=
CHs
I
C-CHz-CH,
These reactions are responsible for the formation of C2H, by decomposition of the n-C3Hi' radical and for changes in the distribution of the main products, according to the over-all reaction:
CH;-CH'-CH;
17 + CHsCHClHi +
CHs
I
CH2=CH-CHz-CH-CH3 Yet, it is believed that inhibition is mainly owing to the formation of allylic radicals:
2 CH;-CH'-CH;
18
+
CHz=CH-CH,-CH,-CH=
I
CH,--C=CHz
+ H'
10 +
+ H'
CH,-CH=CH,
I
CH;-C-CH; 11 +
CH;-CH'-CH;
+ H?
CH;-CH*-CH;
19 4
CHz= CH-CHZ-CH
+ H~
These radicals are less active in chain propagation than Rp and decompose much slower than Rk. Reactions 10 and 11 result in the replacement of the active Rp radicals by relatively stable allylic radicals. At moderate t o high conversion levels, recombination of an allylic radical with Rp and recombination of two allylic radicals becomes the major termination step. At fixed temperature the rate of addition reactions 8 or 9 is proportional to the rate of the corresponding H-abstraction reaction 10 or 11. This explains the simul-
+ CHz=CH'
9
10
12
13
Our data lead to log1,A = 2.50 =t 0.24 and E = 5.2 0.9 kcalimole, in fair agreement with log,,A = 2.35 0.47 and E = 5.0 + 1.5 kcal/mole, as can be derived from the literature data. Both kinetic and selectivity data of the isobutane pyrolysis show a striking similarity with corresponding data of the propane pyrolysis. This indicates similar cracking mechanisms. Nomenclature
Figure 12. Arrhenius diagram for (C~Hs),/(i-C4Hs), ABuekens and Froment 0 Konar et al. (1 968b) OFusy et al. (1966)
314
V M a r e k and Neuhaus (1933) I , Steacie and Puddington (1938)
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971
= CH2
Further secondary reactions of these compounds either result in the formation of lower molecular weight compounds (H2,CH,, C 2 H 4 . ,, .), or in the formation of cyclic and aromatic hydrocarbons. When conversion increases, the number of possible radical reactions with secondary products augments rapidly. Initial values for the ratio (C3Hs)/(i-C,Ha) are compared with literature values in the Arrhenius diagram of Figure 12. This ratio is mainly determined by the relative rates of formation of secondary and tertiary isobutyl radicals:
+
-0.5
CHz
A = frequency factor, sec-' C C ' " - mole" a = inhibition coefficient, dimensionless c = concentration, molesicc ct = total concentration, molesicc
~
E = activation energy. cal/mole Fo = molar feed rate of isobutane, molesisec k = rate coefficient, sec ’, C C ” ’ mole ko = initial rate coefficient, same dimension K = mean rate coefficient, same dimension n = order of reaction, dimensionless P = pressure, a t m R = gas constant, calimole ( “ C ) r = rate of reaction, mole/cc ’sec T = temperature, K TK = reference temperature, K t = temperature, C x = conversion. fractional, unless otherwise specified equivalent reactor volume, cc V‘ = actual reactor volume, cc c = expansion factor, moles formed mole decomposed 6 = dilution factor, mole /mole e = reaction time, sec ‘I
v =
literature Cited
Buekens, A. G., Froment, G. F., Ind. Eng. Chem. Process Des. Develop., 7, 435 (1968). Frey, F. E., Hepp, H. J., Ind. Eng. Chem., 25, 441 (1933). Froment, G. F., Pycke, H., Goethals, G., Chem. Eng. Sci., 13, 180 (1961). Fusy, J., Martin R., Dzierzynski, M., Niclause M., Bull. Soc. Chim., 1966, p 3783.
Halstead, M. P., Konar, R . S.,Leathard, D. A., Marshall, R . M., Purnell, J. M., Proc. Roy. Soc., A310, 525 (1969). Hougen, 0. A., Watson, K. M., “Chemical Process Principles,” Vol. 111, p 884, Wiley, New York, 1947. Hurd, D . C., Spence, L. U.,J . Amer. Chem. Soc., 51, 3353 (1929). Illes, V., Pleszkats, I., Szepeny, L., Erdoel Kohle, 22, 201 (1969). Konar, R. S.,Marshall, R. M., Purnell, J. H., Trans. Faraday Soc., 64, 405 (1968a). Konar, R . S., Purnell, J. H., Quinn, C. P., J . Chem. Soc., A , 1967, p 1543. Konar, R . S., Purnell, J. H., Quinn, C. P., Trans. Faraday Soc., 64, 1319 (196813). Marek, L. F., Neuhaus, M., Ind. Eng. Chem., 25, 516 (1933). Paul, R. E., Marek, L. F., ibid., 26, 454 (1934). Pease, R. N., J . Amer. Chem.Soc., 50, 1779 (1928). Pease, R. N., Durgan, E. S..ibid., 52, 1261 (1930). Steacie, E. W. R., Puddington, I. E., Can. J . Res., B16, 260 (1938).
RECEIVED for review May 8, 1970 ACCEPTED September 18, 1970
Use of Piston Bed for Trapping of Particles Newton C. M. Landis’ and Herbert F. Wiegandt’ Cornell University, Ithaca, N . Y . 14850 A moving packed-bed column was tested for clarification of water-containing colloidal organic solids. Feed suspension entered the bottom of the column; filtrate exited through ports in the walls of the column. The bed was propelled upward by the viscous drag of liquid flow through the bed. Filter media were continuously added to the bottom of the column and removed from the top. A mat of inpurities could not form on the constantly renewed surface of the filter bed. The suspension feed rate and/or bed movement rate could be adjusted so that impurities were uniformly distributed in the filter bed. Using sand as the media, filtration rates up to 9 gal per min ft2 were obtained.
T(Wiegandt, he hydraulically propelled piston bed was reported 1960) as a means of washing ice crystals in the freezing process for saline water conversion. I t also is used in a process for continuous crystallation and purification (Wiegandt and Lafay, 1967). I n this investigation, hydraulically propelled piston beds of polyethylene, sand, and carbon particles were tested to clarify water suspensions of corn starch and wheat starch. T h e purpose of the experimentation was: T o compare the moving bed with a stationary bed for different filter media; and to evaluate the filtration properties of a moving bed of these media. A sketch of a piston-bed column is shown in Figure 1. I n operation of the piston bed as a solid-liquid separating
’ Present address, Eastman Kodak Co., Kodak Park, Rochester, N. Y. 14650. ’ To whom correspondence should be addressed.
device or filter, the filter media, such as sand, fibers, etc., are mixed with a feed which contains suspended impurities (colloidal or nearly colloidal in size) to be removed. The resulting slurry enters the column a t A. The liquid moves upward through the bed and exits a t B. The media add t o the bottom of the moving bed; the impurities move upward with the liquid within the moving bed until they become entrapped within the media. The bed is propelled upward by the viscous drag of the liquid on the lower bed. The bed movement rate is a small fraction, perhaps l j l 0 0 , of the superficial liquid flow rate. The impurities move past the liquid exit port entrapped within the bed of media and are removed a t C in Figure 1. The filter bed is not washed. A principal feature of this clarification technique is that it is continuous-Le., flows into the filter do not have t o be stopped periodically for backwashing of the filter media. I t is impossible for a m a t to form on the surface Ind. Eng. Chern. Process Des. Develop., Vol. 10, No. 3, 1971
315
