High Pressure Chemical Engineering, Proceedings of the 3rd International Symposium on High Pressure Chemical Engineering

576 Pages·1996·24.131 MB·3-710\576

by  Ph. Rudolf von Rohr and Ch. Trepp (Eds.)

Preface
During the annual Meeting of the EFCE-Working Party "High pressure technology"
held October ,8 1993, at Kaiseraugst, Switzerland, under the chairmanship of the
late Prof. Helmut Tiltscher, ti was decided to hold the 3rd International Symposium
no High Pressure Chemical Engineering no October 7 - ,9 1996 at HTE ni Zurich.
Here, the Institute of Process Engineering and Cryogenics (Proffs Rudolf von Rohr
and Trepp) have taken no the task of organizing the meeting.
The two previous conferences were held ni 1984 and 1990 ni Erlangen, Germany,
at a University with considerable impact no the promotion of this emerging
technology ni industry. Following the Erlangen tradition the Zurich symposium is
also subdivided ni three major sections, namely
Chemical reaction engineering
Separation processes and phase equilibria
Plant, apparatus, machinery, measurements, control
From the contributions offered, the papers section of the scientific committee has
chosen a number of more than 60 as oral and about na equal number for poster
presentations. This volume contains the text fo all contributions (oral and posters)
which have reached us ni time, except the four invited papers.
.R.R Emst: NMR, a powerful tool for the investigation of molecules and
materials at high and ambient pressure
.P .G Debenedetti: Materials processing with sUpercritical fluids
.C Suter: Competing through technology
.G Vetter: High pressure machinery- a permanent challenge for
engineers
For these na other form and way of publication will eb used.
eW are grateful to the lead scientists of this conference, experts ni the area, for their
contributions ni coordinating the evaluation of contributed papers and maintaining
the high quality of research papers ni these proceedings. eW especially acknow-
ledge the professional help of reviewers from lla over the world for selection of
papers and suggestions for improvement of the content of accepted papers
published ni the proceedings.
Finally, we greatly appreciate the cooperation provided by Drs Huub Manten-
Werker of Elsevier Science Publishers B.V. for her preparation of these fine
proceedings ni a very timely manner.
The Editors
vi
Scientific Committee
Prof. Ch. Trepp (CH), Chairman
Prof. A. Baiker (CH)
Prof. A. Bertucco )I(
Prof. G. .H Brunner (D)
Prof. .P G. Debenedetti (USA)
Mr. M. Gehrig (D)
Dr. K. W. Hutchenson (USA)
Prof. .I Kikic )I(
Dr. L. A. Kleintjens (NL)
Prof. Z. Knez (SLO)
Dr. J. Krekel (D)
Dr. .E Lack (A)
Prof. A. Laurent (F)
Prof. A. Merbach (CH)
Prof. S. Peter (D)
Dr. K. Steiner (CH)
Prof. G. Vetter (D)
Prof. W. .K Yuan (PRC)
Organizing Committee
Members of the Organizing Committee
(Institute of Process Engineering and Cryogenics ETHZ):
- Prof. Dr. Ph. Rudolf yon Rohr, Chairman
- Prof. Dr. Ch. Trepp
- Dr. W. D6rfler
- M. Weber
- H. Brandenberger
- .E Phasuk
- .R Zuercher
- S. Haferl
- M. Lustenberger
High Pressure Chemical gnireenignE
.hP Rudolf nov Rohr dna .hC Trepp )srotidE(
(cid:14)9 6991 Elsevier Science B.V. llA rights .devreser
Enhancing the Activity of Solid Acid Catalysts with Supercritical
Reaction Media: Experiments and Theory@
Bala Subramaniam + and Daniel M. Ginosar*
Department of Chemical and Petroleum Engineering
University of Kansas, Lawrence, KS 66045-2223, U. S. A.
A detailed mathematical model is presented for describing the coking and isomerization
activity of a Pt/y-AI203 catalyst in a tubular reactor. The main reaction is the acid-catalyzed
isomerization of 1-hexene (Pc = 31.7 bar; cT = 23 I~ Consistent with our experimental
observations, the model predicts that (i) near-critical reaction mixtures provide an optimum
combination of transport and solvent properties for extracting coke precursors from the
catalyst, thereby maximizing isomerization rates and minimizing catalyst deactivation rates; and
(ii) the mitigation of feed peroxides and the addition of inert co-solvents significantly reduce
oligomer formation in the fluid phase, thereby curtailing the coke formation (i.e., catalyst
deactivation) rates.
1. INTRODUCTION
The deactivation of solid acid catalysts, such as those used in reforming and alkylation
practice, by coking occurs because the coke precursors that are formed either in the fluid phase
or on the catalyst have relatively low volatilities at the operating pressure and temperature.
Supercritical media have been shown to offer a unique combination of solvent and transport
properties for the in situ extraction of coke-forming compounds from porous catalysts.
Reported investigations of the supercritical decoking concept are summarized elsewhere 1 .
Employing 1-hexene isomerization on a Pt/)'-AI203 reforming catalyst as a model reaction
system, we showed that isomerization rates are maximized and deactivation rates are minimized
when operating with near-critical reaction mixtures 2. The isomerization was carried out at
281 ~ which is about 1.1 times the critical temperature of 1-hexene. Since hexene isomers are
the main reaction products, the critical temperature and pressure of the reaction mixture remain
virtually unaffected by conversion. Thus, an optimum combination of gas-like transport
properties and liquid-like densities can be achieved with relatively small changes in reactor
pressure around the critical pressure (31.7 bars). Such an optimum combination of fluid
properties was found to be better than either gas-phase or dense supercritical (i.e., liquid-like)
reaction media for the ni situ extraction of coke-forming compounds.
The catalyst activity is determined by the coke extraction rate relative to the coke formation
rate. The coke extraction rate is determined by the solubility of the coke compounds in the
reaction mixture and the diffusivity of the extracted compounds through the porous catalyst.
The coke formation on the catalyst occurs from the hexenes, and more significantly, from
hexene oligomers formed in the fluid phase catalyzed by traces of peroxide impurities 3. A
detailed mathematical model is presented here to interpret the results presented in our earlier
paper 2 and to develop a better understanding of the underlying physicochemical processes.
yllaitraP@ detroppus yb eht .U .S National Science noitadnuoF )3225529-RSO(
+ oT mohw ecnednopserroc gninrecnoc this paper dluohs eb ;desserdda ude.snaku@mainamarbusb
yltneserP* ta EG&G Idaho Inc., .P .O xoB ,5261 Idaho Falls, DI 51438
2. MODEL
A single pore (radius ro, length 2L) model is employed to describe simultaneous transport,
isomerization, coke formation and coke extraction in the Pt/~-AI203 catalyst. The model is a
significant improvement over previous efforts summarized in a recent paper 4. Coke
formation on the catalyst is assumed to occur from oligomers formed both in the bulk phase
(catalyzed by peroxide impurities) and on the acid sites of the catalyst. The equilibrium
distribution of these oligomers between the bulk fluid phase and the catalyst surface depends
upon temperature and pressure. At supercritical conditions, more of the oligomers tend to be in
the fluid phase while at subcritical conditions the oligomers are more strongly adsorbed on the
catalyst. The oligomers adsorbed on the catalyst surface are termed extractable coke (E) and
undergo transformation on the acid sites to consolidated coke (C), which is not extractable by
the reaction mixture. Thus, the catalyst surface is populated by two species of coke.
The main assumptions relating to coke formation and extraction are:
(cid:12)9 Oligomer formation in the bulk fluid phase is instantaneous relative to oligomer
adsorption and reaction to form coke. The oligomer distribution (up to tetramers) is
approximated (i.e., lumped) by the trimer, 1-octadecene (O).
(cid:12)9 The surface oligomers (E) result from 1-hexene (A), hexene isomers (B) and the
fluid phase oligomers. The rate constants associated with the formation of the
surface oligomers from 1-hexene and the isomer products are identical.
(cid:12)9 All coke formation reactions (producing E or C) are first order with respect to the
coke precursor concentration.
(cid:12)9 The extraction of the surface oligomers from the catalyst is described by an effective
desorption rate constant (ke).
The overall reaction network with multiple pathways for coke (C) formation is as follows:
ak a akw ak c
A ~--- B ~--- E---~ C
akw ak c
A --rE--+ C
ko a k c
+--~E >--- C
ek
The mass balances for the main reactant, the oligomer, the extractable coke and the
consolidated coke in the catalyst pore are as follows:
~ De-~z - aGPc) O=aC (1)
o( OCo
Z~" De z~o j - a (Cop c) Co+ (ke Sa Pc /Mw) W e =0 (2)
OWe
)3(
~o = a (kw Mw /Sa) (Ca + Cb) + a (k~ Mw /Sa) C~ - ke We - a (kc Pc ) We
a%
(4)
= a (k c Pc) We
The initial and boundary conditions are:
Ca -t-( L, t) - ;a~C C O (+ L, t) - ~C (5a)
aCqo C~c o
c)z (0, t) - z-~--- (O, t) - W e (z, O)- W c (z, O)- 0 (5b)
Note that in the hexene balance (eq. ,)1 the yield of coke is neglected as being insignificant
when compared to the yield of the isomers. A linear deactivation function (a) is employed for
all surface-catalyzed reactions as follows:
( / tw
W e -t- W c = 1-~ (6)
1-
a - Ws Ws
The reduction in the effective pore radius (r), as coke accumulates, is related to the empty
pore radius (ro), the coke density (pc), and the total coke (W t ) as follows:
2 2
r -r o-(2r oW t/pw) (7)
Lee et al. ,5 6 and Tsai et al. 7 employed the following modified version of the Spry-
Sawyer correlation to describe restricted diffusion of solutes in 3021A-'7 catalysts:
I 1 -
rm/r
IDe -(1 rm/ro; (-~o) 2 (8)
~D 2 -
where p was found to range from 4.9 to 5.1 at liquid-like conditions, and decreased to 4.4 as
the solvent temperature approached its critical value. Since there are no reported correlations of
diffusion as a function of density ranging from subcritical to supercritical values, an
approximation is used in this work. Eq. (8) was employed altering the value of the exponent p
with reaction mixture density (Or) as follows:
p = 0 for Pr _< 1.0; p = 5.1 (Pr )1 for 0.1 < Pr < 2.0; and p = 1.5 for Pr 2.0 (9)
- >
Thus, at subcritical, gas-like conditions, there is no restricted diffusion. As the density
increases beyond the critical point, the extent of pore-diffusion restriction increases.
Erkey and Akgerman 8 reported an adsorption equilibrium constant for naphthalene in
alumina pores filled with supercritical CO2. Analysis shows that this desorption equilibrium
constant is simply proportional to the solubility of naphthalene in CO2 as measured by
Tsekhanskaya et al. 9. Hence, the desorption rate constant was estimated from the following
type of correlation reported for the naphthalene-ethylene system by Tsekhanskaya et al. 9:
In (k )e = yek C( 1 + C 2 )rP (10)
where the parameters Cl and c2 were fit to the measured solubility data and kef was estimated
by a fit with our experimental data.
For fixed or chosen values of the parameters, the model equations (eqs. 1-4) along with the
initial and boundary conditions (eqs. 5) are solved iteratively by a centered-in-space, forward-
in-time, finite difference scheme to obtain (i) the hexene and hexene oligomer concentration
profiles in the pore fluid phase, and (ii) the coke (extractable + consolidated) accumulation
profile. The effectiveness factor (7/) is estimated from the hexene concentration profile as
follows:
D e (L, t) -~z (L, t)
r/(t) - (11)
LPckaC~a
The foregoing effectiveness factor was used to predict temporal hexene isomerization rates
)sbor( at the exit of an isothermal tubular reactor as follows:
bor s - ~ 1-exp - Ft (12)
The reaction rate and adsorption equilibrium constants (kw, ,ok ck and kef) were obtained
from a fit of the model simulation results employing a limited number of experimental data.
The data used in the model simulations are presented in Table .1 Details of the simulation may
be found elsewhere 10. In the following section, the predicted isomerization rates are
compared with experimentally observed isomerization rates reported earlier 2.
Table 1
Parameter values used in the simulations
Variable Definition Value Source
tF flow rate of hexene feed to reactor 4.46 (10 -7) kg mol/s (a)
ak rate constant for A ~B 0.85 (10 )3- m3/kg cat/s (a)
ko rate constant for O ~We 0.11 (10 )3- m3/kg cat/s (b)
wk rate constant for A, B ~ eW 9.9 (10 )7- m3/kg cat/s (b)
kc rate constant for eW eW+-- 1.4 (10 )7- m3/kg cat/s (b)
kef adsorption equilibrium constant (eq. )21 0.11 (b)
L pore length 3.04 (10 )4- m (a)
Mw molecular weight of coke 252.5 kg/kg mol (a)
or mean radius of empty pore 32 (10 ~176 m (a)
cP bulk density of catalyst 2.1 ( 01 )3 kg/m 3 (a)
wP density of coke 5.1 (103) kg/m 3 (a)
aS catalyst surface area 174.9 (103) m2/kg cat (a)
cOC catalyst loading in reactor 0.1 ( 01 )3- kg (a)
Ws coke laydown at total deactivation 2.37 (10 )6- kg/m 2 (a)
ameasured; bfitted
3. RESULTS AND DISCUSSION
3.1 Reaction Mixture Density Effects
The simulated and experimental variations of the end-of-run (i.e., 8 hr.) isomerization rates
with density are compared in Figure .1 Details of the experiments are provided elsewhere 2,
3. At subcritical densities, the extraction of coke precursors is insignificant. Hence, an
increase in the concentration of the hexene and coke precursors (i.e., oligomers) leads to lower
isomerization rates. At near-critical densities, the extraction of coke precursors becomes
significant. Hence, the isomerization rate increases. Both the experimental and simulated rates
show a decreasing trend when the density is increased from near-critical to supercritical values.
This is attributed to pore-diffusion limitations as the fluid changes from gas-like to liquid-like.
Above 2.0 ,cP the isomerization rate increases with density as the ability of the reaction mixture
to extract the coke precursors increases.
t
1.0 16
T =281 ~ C T =281 ~ C
o Experiment ~o 14 o Experiment
<~ 0.8 t Simulation Ju Simulation
-~< 12-
z~ rr
._o_
z
<~-~ 6.0 O 10
U --F
<
Wv > 8
4.0 --F
o
~x Ju< 6
s
,, I I I I I 4
0 1 2 0 1 2
REDUCED DENSITY REDUCED DENSITY
Figure .1 Comparison of experimental and Figure 2. Comparison of experimental and
simulated variations of end-of-run (8 hr) simulated deactivation rates at various
isomerization rates with density reaction mixture densities
A complementary trend is seen in the simulated and experimental end-of-run deactivation
rates (defined as the percent decrease in the isomerization rate between six and eight hours into
the run). As seen in Figure 2, both the simulated and experimental deactivation rates show an
increasing trend with density in the subcritical range, a decreasing trend as the density
approaches its critical value, and an increase with density up to 2.0 ,cP above which the
deactivation rates are lower.
3 4
(-
10 min. -{ o Experiment 10min. f o Experiment
tu~, .,t Simulation L Simulation
t-
<~3 -~...~ 8 .rh ( (cid:12)9 Experiment
rr"~ h .r ( _ etneomitairlepXis
zo.~ 02 Zcn O -- --Simulation
0..~ -7 T=281~ O ~
--.I
"~~7 g/cc
N
rr~l m T=281oC
!.IL ~.
,..- P = 277 bar, 2.09 cP
g5 o1
xm_ ........
0 I i I I I I ,, I I I I 0 I 1 I I
0 20 40 60 80 100 0 1 2
SOLVENT MOLE% OLIGOMER PRODUCTION (%)
Figure 3. Effect of solvent addition on Figure 4. Effect of oligomer production
experimental and simulated initial and end- on experimental and simulated initial and
of-run isomerization rates end-of-run isomerization rates
3.2 Solvent Effects
By adding an inert co-solvent with the hexene feed, and adjusting the operating pressure
such that the reaction mixture density is maintained constant, the coke precursor concentrations
can be reduced while maintaining the in situ extraction of the extractable coke compounds 3.
However, dilution of the feed with the co-solvent also reduces the isomerization rate. Figure 3
compares simulated initial (i.e., 01 min.) and end-of-run (8 hr.) isomerization reaction rates
with experimental results obtained with n-pentane as the co-solvent. In all the experiments, the
reaction mixture density is maintained constant at 0.47(103) kg/m .3 The simulated and
experimental initial reaction rates show excellent agreement, decreasing linearly with co-solvent
addition, as would be expected for a first-order reaction.
As regards the end-of-run isomerization rates, whereas the experimental results show a
maximum at roughly 50 mol% co-solvent, the simulated results show a continuous decrease in
isomerization rate with co-solvent addition. This might suggest a higher extent of coke
extraction with the n-pentane co-solvent relative to that predicted for hexene. Such solvent
effects have been reported previously [ 11 .] The model does not account for solvent type and is
hence unable to predict co-solvent effect on the solubility of coke precursors.
3.30ligomer Effects
The oligomers are formed in the fluid phase, aided by organic peroxide impurities in the
feed. Oligomer production increases linearly with reaction mixture density. Experimentally,
coke formation was observed to increase when oligomers were added to the reactor feed,
decrease when oligomer concentrations were reduced by dilution with an inert co-solvent, and
increase with higher oligomer production due to an increase in feed peroxide concentration [3].
The initial and end-of-run isomerization reaction rates obtained from a set of model simulations
are compared to experimental results in Figure 4 for runs at ~ 281 277 bar, yielding a reduced
density of 2.09. The oligomer production was varied from nearly zero to 2% by increasing the
peroxide content in the feed by aeration. The simulated and experimental isomerization rates
show excellent agreement, increasing with a decrease in oligomer production.
Figure 5. Effect of oligomer reduction on Figure 6. Effect of oligomer reduction on
simulated end-of-run isomerization rates simulated end-of-run deactivation rates
Figure 5 shows the effect of reducing the oligomer production to 50%, 90% and 99.99%
of the value obtained with an untreated hexene feed (i.e., without oligomer reduction). The
reduction may be achieved by adsorbing the organic peroxides in the feed with activated
alumina [ 12]. With 50% oligomer reduction, the simulated isomerization rates show a similar
qualitative trend as the rates with no oligomer reduction. In both these simulations, a rate
maximum is observed around the critical density. Despite the similarity in trends, the
isomerization rates predicted with 50% oligomer reduction are roughly 30% higher than those
with no oligomer reduction. With 90% reduction in oligomer production and beyond, the
isomerization rate trends change. Although there is still a minimum in the subcritical region
(0.48 pc)t,h ere is no maximum around the critical density. In all cases, however, an increase
in the isomerization rate is accompanied by a decrease in the deactivation rate (Figure 6).
4. CONCLUSIONS
A detailed mathematical model is presented for describing the coking and isomerization
activity of a Pt/7-A1203 catalyst in a tubular reactor. The main reaction is the acid-catalyzed
isomerization of 1-hexene. Consistent with our earlier experimental results ,2 3, the model
predicts that (i) near-critical reaction mixtures provide an optimum combination of transport and
solvent properties for extracting coke precursors from the porous catalyst, thereby maximizing
isomerization rates and minimizing catalyst deactivation rates; and (ii) the mitigation of feed
peroxides and the addition of inert co-solvents significantly reduce oligomer formation in the
fluid phase, thereby curtailing the coke formation (i.e., catalyst deactivation) rates. Indeed, as
we had reported recently 12, reduction of organic peroxides in the olefinic feed in conjunction
with supercritical operation leads to dramatic improvements in catalyst activity. Nearly steady
catalyst activity at high conversions (roughly 70%) is observed at 281~ 70 bars and a space
velocity of 531 g hexene/h/g cat. We are currently investigating the feasibility of applying the
supercritical decoking concept to enhance the activity of solid-acid catalysts for the alkylation of
1-butene with isobutane. If successful, such a process would clearly be an environmentally
superior alternative to conventional alkylation technology that employs hazardous liquids such
as sulfuric and hydrofluoric acids as catalysts.
NOTATION (partial list; others defined in the main text and in Table )1
Ci ~ concentration of species i at the pore mouth or in the reactor fluid phase, kmol m 3-
iC concentration of species i in the fluid phase inside the catalyst pore, kmol m 3-
Di e effective diffusivity of species i in the pore at time t, m 2 s 1-
Di ~ effective diffusivity of species i in the empty pore, m 2 s 1-
ek effective desorption rate constant, s 1-
mr molecular radius of diffusing solute, m
cW consolidated coke laydown on catalyst, kg/m 2
eW extractable coke laydown on catalyst, kg/m 2
REFERENCES
.1 P.E. Savage, .S Gopalan, T. .I Mizan, C. J. Martinio and E. E. Brock, AIChE J., 14
(1995) 1723.
2. D.M. Ginosar and B. Subramaniam, in Catalyst Deactivation: Stud. in Surf. Sci. and
Catal., 88, Delmon, B. and G. F. Froment (eds.), Elsevier, Amsterdam, (1994) 327.
3. D.M. Ginosar and B. Subramaniam, J. Catal., 152 (1995) 31.
4. B.J. McCoy and B. Subramaniam, AIChE J., 41 (1995) 317.
5. S.Y. Lee, J. D. Seader, C. H. Tsai, and F. E. Massoth, Ind. Eng. Chem. Res., 30
(1991) 29.
6. S.Y. Lee, J. D. Seader, C. H. Tsai, and F. E. Massoth, Ind. Eng. Chem. Res., 30
( 1991) 607.
7. C.H. Tsai, F. E. Massoth, .S Y. Lee and J. D. Seader, Ind. Eng. Chem. Res., 30 (1991)
22.
8. C. Erkey and A. Akgerman, AIChE J., 36 (1990) 1715.
9. Y.V. Tsekhanskaya, M. B. Imotev and E. V. Mushkina, Russ. J. Phys. Chem., 39
(1964) 1173.
10. D. M. Ginosar, Ph.D. Dissertation, University of Kansas, 1994.
11. J. C. Fetzer, J. A. Graham, R. F. Arrendale, M. S. Klee and L. B. Rogers, Sep. Sci.
Tech., 61 (1981) 97.
.21 M. C. Clark and B. Subramaniam, Chem. Engng. Sci., 15 (1996) 2369.
hgiH erusserP lacimehC gnireenignE
.hP floduR nov rhoR dna .hC pperT )srotidE( ll
(cid:14)9 6991 reiveslE ecneicS .V.B llA sthgir .devreser
Influence of Chain Architecture on High-Pressure Copolymer
Solution Behavior: Experiments and Modeling
Sang-Ho Lee and Mark .A McHugh
Department of Chemical Engineering, Johns Hopkins University, Baltimore,
Maryland 21218, USA
Phase behavior studies with poly(ethylene-co-methyl acrylate), poly
(ethylene-co-butyl acrylate), poly(ethylene-co-acrylic acid), and poly(ethylene-co-
methacrylic acid) were performed in the normal alkanes, their olefinic analogs,
dimethyl ether, chlorodifluoromethane, and carbon dioxide up to 250 ~ and
2,700 bar. The backbone architecture of the copolymers as well as the solvent
quality greatly influences the solution behavior in supercritical fluids. The effect
of cosolvent was also studied using dimethyl ether and ethanol as cosolvent in
butane at varying concentrations of cosolvent, exhibiting that the cosolvent
effect diminishes with increasing cosolvent concentrations.
1. INTRODUCTION
In the 1960's several extensive investigations were performed on the high-
pressure solution behavior of the polyethylene (PE) 1,2. The high-pressure
process is also currently being used to produce polar/nonpolar, ethylene-based
copolymers since many catalytic processes fail as the polar comonomer is a
poison for the catalyst. Once again, high-pressure phase behavior becomes an
important consideration when processing ethylene-based copolymers using
techniques formerly developed to produce PE.
Over the past five years we have been investigating the role of backbone
architecture on the phase behavior of ethylene-based copolymers in a variety of
supercritical fluids 3. We have studied the phase behavior of poly(ethylene-co-
methyl acrylate), poly(ethylene-co-butyl acrylate), poly(ethylene-co-acrylic acid),
and poly(ethylene-co-methacrylic acid) in many solvents. These copolymers can
be distinguished from one another based on their ability to form hydrogen bonds.
We have performed systematic phase behavior studies with these random
copolymers since the properties of a given copolymer varies with the percent of
each constituent in the backbone of the copolymer. Likewise, the properties of
the solvents needed to solubilize a given type of copolymer change as a function
of copolymer architecture. The solvents that have been investigated include the
alkanes, their olefinic analogs, dimethyl ether, chlorodifluoromethane, and carbon
dioxide. This wide range of solvents allows us to systematically vary solvent