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Prediction of performance of acid hydrolysis in a cyclone reactor
Biomass 23 (1990) 319-326
~ _
Prediction of Performance of Acid Hydrolysis in a Cyclone Reactor H. C. C h e n Department of Chemical Engineering,MichiganState University,East Lansing, Michigan 48824, USA
& H. E. Grethlein* Michigan BiotechnologyInstitute, PO Box 27609, Lansing, Michigan48909, USA (Received 5 May 1989; revised version received 18 December 1989; accepted 22 December 1989) ABSTRACT A parametric study is presented of a kinetic model in which the relative residence time of the solid phase to liquid phase, the absolute residence time of the liquid phase, and temperature are varied. It is possible to obtain 80-87% glucose yield in hardwood under various values of these parameters which exceeds to 50-60% yield of the current plug flow reactor. Key words: acid hydrolysis, cyclone reactor, kinetic model of wood hydrolysis.
INTRODUCTION The utilization of the carbohydrate of lignocellulose such as wood or agricultural residue requires some type of hydrolysis of the cellulose and hemicellulose. The hydrolysis can be promoted thermochemically with dilute acid or enzymatically. For the cellulose, dilute acid hydrolysis is *To whom correspondence should be addressed. 319 Biomass 0144-4565/90/S03.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
320
H. C Chen, H. E. Grethlein
limited to a glucose yield based on the glucan content in the lignocellulose of about 50-60% by a continuous plug flow reactor 1 or 65% based on a perculation reactor (Madison Process). 2 On the other hand, high yield of glucose, approaching 95% or more of theory, is possible with enzymatic hydrolysis, l While the rates of hydrolysis can be increased considerably by pretreatment, the rate is still slow compared to acid hydrolysis so that the reaction time for enzymatic hydrolysis is about 24 h compared to 10 s for continuous acid hydrolysis. As a result, the capital cost for an acid hydrolysis plant is less than an enzymatic hydrolysis plant because of the small reactor and no plant is needed to produce the enzyme, which involves as much capital as the hydrolysis plant. 3 The major disadvantages of dilute acid hydrolysis is the relatively low yield of glucose from the glucan. Since the substrate is the major item in the cost of manufacture of glucose, a key objective is to consider reactor configurations which improve the glucose yield and not become too capital intensive. Several types of reactors have been considered, such as the progressive moving bed, 4 the multiple pass reactor and the recycle reactor. 5 The first type is an improvement on the Madison process but has a cycle time of 3 h. The latter two are modifications of the plug flow reactor but require extensive investment in solid-liquid separation equipment in order to recover the glucose from the partially-reacted cellulose which is reacted again. Greenwald et aL 6 analyzed four flow configurations for acid hydrolysis. They noted that increasing the solid residence time relative to the liquid, improves the yield but gives no way to realize this effect. When considering any of these improvements, the key point is to increase the reaction time of the lignocellulose while decreasing the reaction time of the soluble glucose, which reduces the secondary reaction of glucose to hydroxymethyl furfural (HMF). The simplest reactor configuration that has these properties is a liquid cyclone. Since the solids phase has a higher density than the liquid it is possible to increase the solids residence time relative to the liquid phase because of the centrifugal force developed in the cyclone. The liquid cyclone is used as a physical device for separation of starch in a corn wet-milling plant. The yield potential of the cyclone as a simultaneous continuous reactor and a solid retaining device is explored in this paper.
KINETIC MODEL IN A CYCLONE REACTOR The cyclone reactor will be modeled as a continuous, one-dimensional, plug flow reactor in which the solid phase moves at a slower velocity
Prediction of performance of acid hydrolysis in a cyclone reactor
321
than the liquid phase. This will give the maximum yield performance since it does not account for liquid back-mixing which will extend the liquid residence time. Using the kinetic equations for dilute acid hydrolysis of wood developed by Saeman, 7 we account for the residence for the solid as ts and the liquid as t~. The rate equations for cellulose consumption and for net glucose formation are given below.
dC/dt~ = - k, C
(1)
dG /dtl = akl C - k2 G
(2)
where a = t,/t~: the relative residence time. The rate constants have the usual Arrhenius expression modified for the catalytic effect of acid (A), namely
k, = kH,A m exp[ - E,/RT]
(3)
k2 = k2oA" exp[- E~/RT]
(4)
where Et and E2 are activation energies (cal/gmol), R is the universal gas constant (cal/gmol K), T is absolute temperature (K), kl0 and kz0 are preexponential factors (per min), A is acid concentration (%wt), and m and n are exponents. The concentration variables can be made dimensionless by dividing by the potential glucose concentration of the lignocellulose. The solution to eqns (1) and (2) gives the following equations for the fraction of remaining cellulose (potential glucose) and yield fraction of glucose. = Co exp( - ak 1tl) t~ = (~0 exp( - kzt~) + ( aCo/( a - kz/k~ )) (exp( - kzt ~)- exp( - ak~ t~))
(5) (6)
These equations become equal to those for the normal plug flow reactor when a = 1. Equation (6) corresponds to the concurrent case of Greenwald et aL 6 when Go = 0. In order to account for that fraction of the potential glucose that is lost by over reaction via HMF, we can calculate the loss by L=I -0-0 (7) For any given temperatures, optimum liquid residence time can be determined by analytical derivatives technique from eqn (6). Where
[ tl,op t = I n
(k2/akl) +
k2(3o(a-k2/k,)/ aZkl Co (k 2 -akl)
(8)
In order to see the possible improvements of the cyclone reactor, a parametric study of the above equations was completed. The simulated results are summarized in Figs 1, 2 and 3 for mixed hardwood with the kinetic parameters from Grethlein 8 given in Tables 1 and 2.
H. C. Chen, H. E. Grethlein
322 1.0
2 4 0 "C Solid Line - Glucose Yield Dashed line - Glucose Loss
0.8
0.05 0.02 0.10
0 J
,_0.6 0 t I=
0.01
rain.
0.4 0 (3
tl=
0.10
0.2
rain.
0.05
J J t
/
0.02
t .
0 . 0
- -
- -
.
- -
.
.
.
.
0.01
- -
0
5
10 a, Relative Residence Time
15
20
Fig. I. Glucose yield and loss at 240°C as a function of reaction conditions for acid hydrolysis of mixed hardwood in a cyclone reactor. For a plug flow reactor, a = 1. Glucose yield, ; glucose loss, - - 1.0
t I = 0 . 0 5 min Solid Line - Glucose Yield Dashed Line - Glucose Loss 240
0.8
f
O
0.6
e 0.4
~
° o
.
.
.
.
.
.
.
.
~
I
~
.
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.
_
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~
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~
,
~
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f
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10 a, Relative Residence Time
J
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i
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r
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Fig. 2. Glucose yield and loss at 0.05 re_in liquid residence time as a function of reaction conditions for acid hydrolysis of mixed hardwood in a cyclone reactor. See Fig. I for key.
Prediction of performance of acid hydrolysis in a cyclone reactor 1.0
323
--
t l = 0 . 0 2 min Solid Line - G l u c o s e Yield D a s h e d Line - G l u c o s e Loss /
0.8
0 J
,_0.6 0
>-
~0.4 0 o
221 °
0.2 t__
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,
,
,
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220
,
,
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,
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,
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'
e, Nelat,ve
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i
I
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,
,
10 Residence
,
'
'
,
,
Time
,
I
15
'
'
'
'
'
'
'
'
'2/0
Fig. 3. Glucose yield and loss at 0.02 min liquid residence time as a function of reaction conditions for acid hydrolysis of mixed hardwood in a cyclone reactor, t~-- 0-02 min. See Fig. 1 for key. TABLE 1 Kinetic Parameters for Acid Hydrolysis of Mixed Hardwood (90% Birch, 10% Maple)
Kinetic parameters
Values
~o fraction potential glucose, instantly available C0 fraction potential glucose, initially as cellulose kl0 (min-l) k2o (min- l) m n EL (cal gmol -l) E2 (cal gmol- l) R (cal gmol-l K-1)
0"006 0.994 1.45 x 1015 3"96 x 109 1'161 0"569 33 720 21 000 1-987
RESULTS AND DISCUSSION The yield potential of the cyclone reactor strongly depends on the relative residence time 'a', as shown in eqns (5) and (6). Since the product ak~ occurs together, this has the effect of increasing k~ relative to k 2 which improves the selectivity when a > 1.0. Simulated results are
H. C. Chen, H. E. Grethlein
324
TABLE 2 Rate Constant for Various Temperatures for Acid Concentration, A = 1"0%
Temperature(°C) 200 220 240 260
k2
kl (min -I)
(min -t)
0"380 1.629 6'233 21"57
0"783 1"939 4"472 9"687
presented in Figs 1-3. At constant temperature (Fig. 1) or constant liquid residence time (Figs 2 and 3), loss increases continuously as relative residence time increases. For higher temperature or longer liquid residence time, loss becomes larger and can be up to 35% of potential glucose at a = 20. On the other hand, at constant temperature and liquid residence time, glucose yield increases as relative residence time increases until a maximum glucose yield is achieved. After this point, the glucose yield reduces slightly due to the cellulose consumption becoming less significant than the loss from over reaction via HME The longest liquid residence time (0"1 min in this study) produces the local maximum glucose yield for small values of 'a' (0 < a_< 5.2) at constant temperature 240°C (Fig. 1 ). However, at larger relative residence times, the smaller liquid residence time becomes dominant and produces larger local maxima. This trend is also true for operation at constant liquid residence times (Figs 2 and 3). For low values of 'a', the largest glucose yield comes at the highest temperature. For higher values of 'a', the lower temperature gives higher yields. The relative residence time 'a' could potentially be controlled by the innovative cyclone reactor design (Petty et al.9). However, for the feasible operating range of relative residence times 1 _
Prediction of performance of acid hydrolysis in a cyclone reactor
E
"d aj~
%
0
0
~
0 0
t"q
t"q
:= ~<
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0
.=_ 0 0 ©
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325
326
H. C. Chen, H. E. Grethlein REFERENCES
1. Grethlein, H. E. & Converse, A. O., Flow reactor for acid hydrolysis on pretreatment of cellulosic bimoass. In: Liquid Fuel Developments, ed. D. L. Wise. CRC Press, Boca Raton, FL, 1983, pp. 97-128. 2. Hajney, G. J., Biological utilization of wood for production of chemicals and foodstuffs. Research Paper FPL 385, March 1981, For. Prod. Lab, Madison, WI, 1981. 3. Grethlein, H. E., Comparison of the economics of acid and enzymatic hydrolysis of newsprint. Biotechnol. Bioeng., 20 (1978) 503-25. 4. Wright, J. D., Evaluation of acid hydrolysis. Report, SERI/TR-231-207, Sept., SERI, Golden, CO, 1983. 5. Currier, P. M., Investigation of acid hydrolysis of delignified cellulose by multiple pass through a plug flow reactor. Bachelor of Eng. thesis, Thayer School of Eng., Hanover, NH, 1982. 6. Greenwald, G. C., Nystrom, J. M. & Lee, L. S., Yield predictions for various types of acid hydrolysis reactors. Biotechnol. Bioeng. Symp., 13 (1987) 27-33. 7. Saeman, J. E, Kinetics of wood saccharification. Ind. Eng. Chem., 37 (1945) 45. 8. Grethlein, H. E., Acid hydrolysis review in anaerobic digestion and carbohydrate hydrolysis of waste: Comm. EUR. Communities Report No.: EUR 9347, ed. G. L. Ferrero, M. P. Ferranti & H. Naveau. Elsevier Applied Science Publishers, London, 1984, pp. 14-31. 9. Petty, C. A., Chen, H. C. & Dvorak, R. G., Improved Hydrocyclones. US Patent 4 855 066, 8 August 1989.
~ _
Prediction of Performance of Acid Hydrolysis in a Cyclone Reactor H. C. C h e n Department of Chemical Engineering,MichiganState University,East Lansing, Michigan 48824, USA
& H. E. Grethlein* Michigan BiotechnologyInstitute, PO Box 27609, Lansing, Michigan48909, USA (Received 5 May 1989; revised version received 18 December 1989; accepted 22 December 1989) ABSTRACT A parametric study is presented of a kinetic model in which the relative residence time of the solid phase to liquid phase, the absolute residence time of the liquid phase, and temperature are varied. It is possible to obtain 80-87% glucose yield in hardwood under various values of these parameters which exceeds to 50-60% yield of the current plug flow reactor. Key words: acid hydrolysis, cyclone reactor, kinetic model of wood hydrolysis.
INTRODUCTION The utilization of the carbohydrate of lignocellulose such as wood or agricultural residue requires some type of hydrolysis of the cellulose and hemicellulose. The hydrolysis can be promoted thermochemically with dilute acid or enzymatically. For the cellulose, dilute acid hydrolysis is *To whom correspondence should be addressed. 319 Biomass 0144-4565/90/S03.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
320
H. C Chen, H. E. Grethlein
limited to a glucose yield based on the glucan content in the lignocellulose of about 50-60% by a continuous plug flow reactor 1 or 65% based on a perculation reactor (Madison Process). 2 On the other hand, high yield of glucose, approaching 95% or more of theory, is possible with enzymatic hydrolysis, l While the rates of hydrolysis can be increased considerably by pretreatment, the rate is still slow compared to acid hydrolysis so that the reaction time for enzymatic hydrolysis is about 24 h compared to 10 s for continuous acid hydrolysis. As a result, the capital cost for an acid hydrolysis plant is less than an enzymatic hydrolysis plant because of the small reactor and no plant is needed to produce the enzyme, which involves as much capital as the hydrolysis plant. 3 The major disadvantages of dilute acid hydrolysis is the relatively low yield of glucose from the glucan. Since the substrate is the major item in the cost of manufacture of glucose, a key objective is to consider reactor configurations which improve the glucose yield and not become too capital intensive. Several types of reactors have been considered, such as the progressive moving bed, 4 the multiple pass reactor and the recycle reactor. 5 The first type is an improvement on the Madison process but has a cycle time of 3 h. The latter two are modifications of the plug flow reactor but require extensive investment in solid-liquid separation equipment in order to recover the glucose from the partially-reacted cellulose which is reacted again. Greenwald et aL 6 analyzed four flow configurations for acid hydrolysis. They noted that increasing the solid residence time relative to the liquid, improves the yield but gives no way to realize this effect. When considering any of these improvements, the key point is to increase the reaction time of the lignocellulose while decreasing the reaction time of the soluble glucose, which reduces the secondary reaction of glucose to hydroxymethyl furfural (HMF). The simplest reactor configuration that has these properties is a liquid cyclone. Since the solids phase has a higher density than the liquid it is possible to increase the solids residence time relative to the liquid phase because of the centrifugal force developed in the cyclone. The liquid cyclone is used as a physical device for separation of starch in a corn wet-milling plant. The yield potential of the cyclone as a simultaneous continuous reactor and a solid retaining device is explored in this paper.
KINETIC MODEL IN A CYCLONE REACTOR The cyclone reactor will be modeled as a continuous, one-dimensional, plug flow reactor in which the solid phase moves at a slower velocity
Prediction of performance of acid hydrolysis in a cyclone reactor
321
than the liquid phase. This will give the maximum yield performance since it does not account for liquid back-mixing which will extend the liquid residence time. Using the kinetic equations for dilute acid hydrolysis of wood developed by Saeman, 7 we account for the residence for the solid as ts and the liquid as t~. The rate equations for cellulose consumption and for net glucose formation are given below.
dC/dt~ = - k, C
(1)
dG /dtl = akl C - k2 G
(2)
where a = t,/t~: the relative residence time. The rate constants have the usual Arrhenius expression modified for the catalytic effect of acid (A), namely
k, = kH,A m exp[ - E,/RT]
(3)
k2 = k2oA" exp[- E~/RT]
(4)
where Et and E2 are activation energies (cal/gmol), R is the universal gas constant (cal/gmol K), T is absolute temperature (K), kl0 and kz0 are preexponential factors (per min), A is acid concentration (%wt), and m and n are exponents. The concentration variables can be made dimensionless by dividing by the potential glucose concentration of the lignocellulose. The solution to eqns (1) and (2) gives the following equations for the fraction of remaining cellulose (potential glucose) and yield fraction of glucose. = Co exp( - ak 1tl) t~ = (~0 exp( - kzt~) + ( aCo/( a - kz/k~ )) (exp( - kzt ~)- exp( - ak~ t~))
(5) (6)
These equations become equal to those for the normal plug flow reactor when a = 1. Equation (6) corresponds to the concurrent case of Greenwald et aL 6 when Go = 0. In order to account for that fraction of the potential glucose that is lost by over reaction via HMF, we can calculate the loss by L=I -0-0 (7) For any given temperatures, optimum liquid residence time can be determined by analytical derivatives technique from eqn (6). Where
[ tl,op t = I n
(k2/akl) +
k2(3o(a-k2/k,)/ aZkl Co (k 2 -akl)
(8)
In order to see the possible improvements of the cyclone reactor, a parametric study of the above equations was completed. The simulated results are summarized in Figs 1, 2 and 3 for mixed hardwood with the kinetic parameters from Grethlein 8 given in Tables 1 and 2.
H. C. Chen, H. E. Grethlein
322 1.0
2 4 0 "C Solid Line - Glucose Yield Dashed line - Glucose Loss
0.8
0.05 0.02 0.10
0 J
,_0.6 0 t I=
0.01
rain.
0.4 0 (3
tl=
0.10
0.2
rain.
0.05
J J t
/
0.02
t .
0 . 0
- -
- -
.
- -
.
.
.
.
0.01
- -
0
5
10 a, Relative Residence Time
15
20
Fig. I. Glucose yield and loss at 240°C as a function of reaction conditions for acid hydrolysis of mixed hardwood in a cyclone reactor. For a plug flow reactor, a = 1. Glucose yield, ; glucose loss, - - 1.0
t I = 0 . 0 5 min Solid Line - Glucose Yield Dashed Line - Glucose Loss 240
0.8
f
O
0.6
e 0.4
~
° o
.
.
.
.
.
.
.
.
~
I
~
.
28o
.
_
T=26o
-c_
0.2
0.0
Li
0
-
~
~
~
~
q
I
5
E
~
,
~
~
J
~
~
f
~
10 a, Relative Residence Time
J
~
]
15
~
J
~
i
J
r
~
~
~
]
2O
Fig. 2. Glucose yield and loss at 0.05 re_in liquid residence time as a function of reaction conditions for acid hydrolysis of mixed hardwood in a cyclone reactor. See Fig. I for key.
Prediction of performance of acid hydrolysis in a cyclone reactor 1.0
323
--
t l = 0 . 0 2 min Solid Line - G l u c o s e Yield D a s h e d Line - G l u c o s e Loss /
0.8
0 J
,_0.6 0
>-
~0.4 0 o
221 °
0.2 t__
0.0 0
,
~
/
,
,
,
,
,
220
,
,
I
5
,
,
'
,
'
'
'
e, Nelat,ve
'
i
I
'
,
,
10 Residence
,
'
'
,
,
Time
,
I
15
'
'
'
'
'
'
'
'
'2/0
Fig. 3. Glucose yield and loss at 0.02 min liquid residence time as a function of reaction conditions for acid hydrolysis of mixed hardwood in a cyclone reactor, t~-- 0-02 min. See Fig. 1 for key. TABLE 1 Kinetic Parameters for Acid Hydrolysis of Mixed Hardwood (90% Birch, 10% Maple)
Kinetic parameters
Values
~o fraction potential glucose, instantly available C0 fraction potential glucose, initially as cellulose kl0 (min-l) k2o (min- l) m n EL (cal gmol -l) E2 (cal gmol- l) R (cal gmol-l K-1)
0"006 0.994 1.45 x 1015 3"96 x 109 1'161 0"569 33 720 21 000 1-987
RESULTS AND DISCUSSION The yield potential of the cyclone reactor strongly depends on the relative residence time 'a', as shown in eqns (5) and (6). Since the product ak~ occurs together, this has the effect of increasing k~ relative to k 2 which improves the selectivity when a > 1.0. Simulated results are
H. C. Chen, H. E. Grethlein
324
TABLE 2 Rate Constant for Various Temperatures for Acid Concentration, A = 1"0%
Temperature(°C) 200 220 240 260
k2
kl (min -I)
(min -t)
0"380 1.629 6'233 21"57
0"783 1"939 4"472 9"687
presented in Figs 1-3. At constant temperature (Fig. 1) or constant liquid residence time (Figs 2 and 3), loss increases continuously as relative residence time increases. For higher temperature or longer liquid residence time, loss becomes larger and can be up to 35% of potential glucose at a = 20. On the other hand, at constant temperature and liquid residence time, glucose yield increases as relative residence time increases until a maximum glucose yield is achieved. After this point, the glucose yield reduces slightly due to the cellulose consumption becoming less significant than the loss from over reaction via HME The longest liquid residence time (0"1 min in this study) produces the local maximum glucose yield for small values of 'a' (0 < a_< 5.2) at constant temperature 240°C (Fig. 1 ). However, at larger relative residence times, the smaller liquid residence time becomes dominant and produces larger local maxima. This trend is also true for operation at constant liquid residence times (Figs 2 and 3). For low values of 'a', the largest glucose yield comes at the highest temperature. For higher values of 'a', the lower temperature gives higher yields. The relative residence time 'a' could potentially be controlled by the innovative cyclone reactor design (Petty et al.9). However, for the feasible operating range of relative residence times 1 _
Prediction of performance of acid hydrolysis in a cyclone reactor
E
"d aj~
%
0
0
~
0 0
t"q
t"q
:= ~<
¢.,a
0
.=_ 0 0 ©
'a
0
0 66666666
.ca
325
326
H. C. Chen, H. E. Grethlein REFERENCES
1. Grethlein, H. E. & Converse, A. O., Flow reactor for acid hydrolysis on pretreatment of cellulosic bimoass. In: Liquid Fuel Developments, ed. D. L. Wise. CRC Press, Boca Raton, FL, 1983, pp. 97-128. 2. Hajney, G. J., Biological utilization of wood for production of chemicals and foodstuffs. Research Paper FPL 385, March 1981, For. Prod. Lab, Madison, WI, 1981. 3. Grethlein, H. E., Comparison of the economics of acid and enzymatic hydrolysis of newsprint. Biotechnol. Bioeng., 20 (1978) 503-25. 4. Wright, J. D., Evaluation of acid hydrolysis. Report, SERI/TR-231-207, Sept., SERI, Golden, CO, 1983. 5. Currier, P. M., Investigation of acid hydrolysis of delignified cellulose by multiple pass through a plug flow reactor. Bachelor of Eng. thesis, Thayer School of Eng., Hanover, NH, 1982. 6. Greenwald, G. C., Nystrom, J. M. & Lee, L. S., Yield predictions for various types of acid hydrolysis reactors. Biotechnol. Bioeng. Symp., 13 (1987) 27-33. 7. Saeman, J. E, Kinetics of wood saccharification. Ind. Eng. Chem., 37 (1945) 45. 8. Grethlein, H. E., Acid hydrolysis review in anaerobic digestion and carbohydrate hydrolysis of waste: Comm. EUR. Communities Report No.: EUR 9347, ed. G. L. Ferrero, M. P. Ferranti & H. Naveau. Elsevier Applied Science Publishers, London, 1984, pp. 14-31. 9. Petty, C. A., Chen, H. C. & Dvorak, R. G., Improved Hydrocyclones. US Patent 4 855 066, 8 August 1989.