|Title of Invention||
PROCESS FOR PREPARING ACETIC ACID
|Abstract||A PROCESS FOR PREPARING ACETIC ACID BY OXIDATION OF ETHANE BY MEANS OF MOLECULAR OXYGEN IN A FLUIDIZED BED OF CATALYST PARTICLES, AT5 A TEMPERATURE IN THE RANGE FROM 100 TO 500o C AND AT A REACTOR PRESSURE IN THE RANGE FROM 1 TO 50 BAR WHEREIN THE PARTICLE DIAMETER OF AT LEAST 70% OF THE CATALYST PARTICLES USED IS LESS THAN OR EQUAL TO 80 UM, THE DENSITY OF THE CATALYST PARTICLES USED IN THE FLUIDIZED-BED REACTOR IS IN THE RANGE FROM 500 KG/M3 TO 6000 KG/M3, THE FLUIDIZED BED INDEX IS IN THE RANGE FROM 1 TO 12 AND THE GAS VOLUME FLOW IS SELECTED SO THAT THE BUBBLE DIAMETER OF THE REACTION GAS MIXTURE FED IN THE FLUIDIZED-BED REACTOR IS LESS THAN 12 CM.|
The invention relates to a process for preparing acetic acid by oxidation of
ethane with high selectivity and in high yield in fluidized-bed reactors.
The catalytic gas-phase oxidation of ethane to ethylene and acetic acid has
been known for a long time. The reaction is influenced essentially by the
choice of reaction conditions; this is particularly true of the selectivity of the
reaction. In general, acetic acid is formed only as by-product, while the
main product is ethylene which may, under some circumstances, be
oxidized further to carbon dioxide.
DE 196 30 832 A1, DE 196 20 542 A1, DE 197 45 902 A1, WO 98/47850
and WO 98/47851 describe a process for preparing acetic acid by partial
oxidation of ethane using MoaPdbXcYd as catalysts in a fixed bed. The
catalyst has a high stability and a high selectivity in the oxidation of ethane to
acetic acid. However, difficulties are presented in the process by the fact that
the oxidation of ethane to acetic acid is strongly exothermic. Particularly in
relatively large fixed-bed reactors, the heat of reaction can be removed only
unsatisfactorily. This results in an increase in the temperature in the reactor
and thus a reduction in the selectivity of the reaction.
To circumvent this problem, attempts have been made to carry out the
reaction in a fluidized catalyst bed as described, for example, in
US 5,300,684, where cooling tubes are located directly in the fluidized-bed
reactor to remove heat. A further development of the process is described
in WO 00/14047. A disadvantage of the fluidized-bed process as described
there has hitherto been that the oxidation of ethane to acetic acid still
proceeds with a low selectivity and a lower space-time yield than in a fixed-
It is therefore an object of the present invention to provide a process for the
oxidation of ethane to acetic acid in a fluidized-bed reactor which can be
operated with higher selectivity and thus with a higher yield than the
processes described hitherto.
In the present invention, it has surprisingly been able to be shown that the
selectivity of the oxidation of ethane to acetic acid depends on the size of
the gas bubbles in the fluidized catalyst bed. In particular, bubble sizes of
a diameter of At a given average particle size of the catalyst used, the size of the bubbles
can be set by controlling the volume flow of gas. Bubble sizes of can only be achieved by use of catalyst particles having a mean diameter
of less than or equal to 80 µm.
The present invention accordingly provides a process for the direct catalytic
oxidation of ethane to acetic acid in a fluidized bed comprising up to 70% of
catalyst particles having a diameter of less than or equal to 80 |xm. The
fluidized bed preferably additionally contains smaller particles having a
particle size of from 10 to 60 urn, and the distribution of the particle sizes
should be such that 10-60% of the particles have a diameter of less than
60 µm. Particular preference is given to a distribution of the particle sizes
such that catalyst particles are present in a diameter range of 10-40 yon. In
a particularly preferred embodiment, the diameter of 20-50% of the catalyst
particles is less than 40 µm.
The fluidized bed index to be set (the fluidized bed index is the ratio of gas
velocity set to minimum fluidization velocity) has to be above 1. This means
that the fluidized bed has to be in the fluid state. At a pressure of 16 bar
and a temperature of 280°C, the minimum fluidization velocity at a particle
diameter of 80 µm is 5.85 mm/s. The fluidized bed index is preferably in the
range from 1 to 12.
As catalysts, it is possible to use all catalysts known to those skilled in the
art. Catalysts which have been found to be advantageous are catalysts of
the formula (I):
where X is one or more elements selected from the group consisting of Cr,
Mn, Nb, B, Ta, Ti, V, Te, W and Re and
Y is one or more elements selected from the group consisting of B,
Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Au, Fe, Ru, Cs,
Mg, Ca, Sr, Ba, Li, K, Na, Rb, Be, Nb, Zr, Hf, Ni, P, Pb, Sb, Si, Sn,
TI and U.
The indices a, b, c and d are the gram atom ratios of the corresponding
elements, where, based on a = 1
b is in the range from 0.0001 to 0.01, preferably from 0.0001 to
c is in the range from 0.1 to 1 and
d is in the range from 0 to 1.
X and/or Y may also represent a plurality of elements, in which case the
indices c and d may assume different values for different elements.
The catalyst particularly preferably comprises at least one of the following
compositions in combination with oxygen:
Mo1 .00Pd0.0005Vo.45Nb0.03Sb0.01 Ca0.01
M01 .00Pd0.0005V0.45Nb0.03Sb0.01 Ca0.01 K0.015
M01 .00Pd0.00075Vo.55Nb0.03Sb0.01 Ca0.01
M01 .00Pd0.00075Vo.45Nb0.06Sb0.01 Ca0.01
M01 .00Pd0.0008Vo.55Nb0.06Sb0.01 Ca0.01
M01 .00Pd0.00085Vo.55Nb0.06Sb0.01 Ca0.01
M01 .00Pd0.00075V0.55Nb0.09Sb0.01 Ca0.01
M01 .00Pd0.00075V0.50Nb0.09W 0.01 Pd0.0003
Apart from the reaction gases ethane and oxygen, the reaction gas mixture
may further comprise water vapor and inert gases such as nitrogen or
carbon dioxide. It is also possible to employ a reaction gas composition
above the explosive limit in the fluidized-bed reactor. However, the reactor
is preferably operated using an excess of inert gas, water and/or ethane.
Water vapor accelerates the oxidation of ethane to acetic acid at the
expense of the oxidation of ethane to ethylene and thus results in an
increase in the selectivity to acetic acid.
The process is preferably carried out at low temperatures. Low
temperatures increase the selectivity of the oxidation reaction in favor of
acetic acid. A temperature range of from 100 to 500°C at a reactor
pressure of from 1 to 50 bar is preferred.
The density of the catalyst particles can be in the range from 500 to 6000
kg/m3. To carry out the process, it is possible to use, for example, a
fluidized-bed reactor as is described in US 5,300,684.
The process of the invention has a high selectivity, and increased space-
time yields can accordingly be achieved in the oxidation of ethane to acetic
acid at moderate temperatures in a fluidized-bed reactor. Furthermore,
reduction of the bubble size enables troublesome backmixing of the
reactants in the fluidized bed to be reduced. This likewise leads to an
increased selectivity of the reaction, resulting in an increase in the space-
time yield of acetic acid.
In a particular embodiment, especially when high ethane conversions are
desired, the reaction can be carried out in a multistage fluidized-bed
reactor with dedicated introduction of gas into each stage. Bubble growth
commences afresh in each stage, so that the bubble size is on average
smaller than in a single-stage fluidized bed. The small bubbles can also be
achieved by means of internals, e.g. meshes, in the fluidized bed. To
achieve high ethane conversions at the same time as a high selectivity to
acetic acid, an additional divided introduction of oxygen into individual
stages can be advantageous.
The advantageous bubble sizes in a fluidized bed and the corresponding
reaction parameters were determined from measured results obtained in
investigations of the catalytic gas-phase oxidation of ethane in a fixed bed.
The kinetic parameters determined therefrom for the reactions involved are
summarized in fig. 1a and 1b. The catalytic gas-phase oxidation of ethane
is catalyzed by two different catalytically active sites Z and X on the
catalyst. Z is a site where all oxidation steps occur, namely the oxidative
dehydrogenation of ethane, the partial and complete oxidations of ethene
and of acetic acid. X is a position on the catalyst which becomes active
only in the presence of water. The activated form of X (X-OHOH) leads to
conversion of ethene into acetic acid via a mechanism similar to the
Wacker reaction. The formation of acetic acid thus proceeds via two
different reaction paths - via the partial oxidation of ethene at the position Z
(step 2 in fig. 1a) and via a mechanism similar to the Wacker reaction at
position X (step 3 in fig. 1a). The necessary kinetic values to the
intermediates at the catalytically active surface in the steady state, viz. Z,
Z-OH2, Z-O, Z-CH2H4, X-OHOH and X-O, were determined analytically and
are given in table X.
The values determined in this way were used to simulate the fluidized-bed
reactor according to the bubble assemblage model (BAM) (K. Kato, C.H.
Wen, Chem. Eng. Sci. 24, 1351-1368 (1969)).
The model describes the fluidized bed as a two-phase system which consists
of a bubble phase which is low in catalyst and a catalyst-rich emulsion phase;
the two-phase structure of such a reaction mixture comprising particles in a
fluidized bed with gas being passed through it can be verified with the naked
eye. Since the emulsion phase contains a high proportion of solid, the
catalytic reaction takes place there, while virtually no reaction takes place in
the bubble phase owing to the low concentration of the catalytic material. The
model of the fluidized-bed reactor therefore has to describe the transfer of
reactants and products between the bubble phase and the emulsion phase
and at the same time take account of the growth of the bubbles with
increasing distance from the gas distributor, an effect which is well known
from fluidized beds. For this reason, the fluidized bed was divided into
segments with a height corresponding to the local bubble diameter. The
bubble diameter, the velocity of the bubbles, the expansion of the bed and
the local volume fractions of the phases were calculated by the methods of
Werther (J. Werther, Chem. Eng. Sci. 47 (9-11), 2457-2462 (1992)) and
Murray (J.D. Murray, J. Fluid. Mech. 21, 465 (1965)). For both bubble phase
and emulsion phase, each segment was described as an ideal stirred tank.
Owing to the good heat removal from fluidized-bed reactors, approximately
isothermal reaction conditions can be achieved; for this reason, the reactor
was regarded as isothermal for the purposes of modeling.
The mass equation for the two phases is given by the following equations:
The equations can be solved for each reaction component i and each
segment k, for example by means of the Wegstein method. The mass
transfer between the bubble phase and the emulsion phase, which is
expressed by the coefficient Kbeik, can be determined quantitatively by
means of a correlation of Sit and Grace (S.P., Sit, J.R. Grace, Chem. Eng.
Sci. 36, 327-335 (1981)). The minimum fluidization velocity umf is derived
as described in Wen et al (C. Y. Wen, Y. H. Yu, AlChE J. 12, 610 (1966)).
The direct catalytic oxidation of ethane to acetic acid in a fluidized bed can
be described to a good degree of accuracy by means of the model
The results are shown in figs. 2 to 7. xm in the figures and tables is the
modified space-time given by the quotient of catalyst mass and volume
flow under the respective reaction conditions.
Fig. 2 shows the bubble diameter as a function of the height above the gas
feed point for different particle diameters of 60 µm and 80 µm.
Fig. 3 shows the conversion selectivity curves and fig. 4 shows the yield of
acetic acid as a function of the residence time for various particle and
bubble diameters under the following reaction conditions: T = 260°C, Ptot
=16 bar, feed composition C2H6 : O2 : H2O : N2 = 40 : 8 : 20 : 32.
Fig. 5 shows the conversion selectivity curves and fig. 6 the yield of acetic
acid as a function of the residence time for multistage fluidized beds (see
above); the points indicate the composition at the outlet of each stage
(reaction conditions: T = 260°C, Ptot =16 bar, dp = 60 µm, feed composition
C2H6 : O2 : H2O : N2 = 40 : 8 : 20 : 32 (in the case of divided introduction of
oxygen in the first stage: 40 :1.6 : 20 : 32)).
Fig. 7 shows the conversion selectivity curves and fig. 8 the yield of acetic
acid as a function of the residence time for a fixed bed, single-stage
fluidized bed and 5-stage fluidized bed with divided introduction of oxygen;
the points indicate the composition at the outlet of each stage (reaction
conditions: T = 260°C, Ptot =16 bar, dp = 60µm, feed composition
C2H6 : O2 : H2O : N2 = 40 : 16 : 20 : 32 (in the case of divided introduction
of oxygen in the first stage: 40 : 3.2 : 20 : 32)).
The present calculations were carried out under the assumption of a
reactor diameter Di of 4 m, a particle size of 60 yum or 80 µm (according to
the Geldart classification A), a density of the particles of 3100 kg/m3 and a
porosity of the fluidized reactor bed of 0.5 at the minimum fluidization
velocity. The corresponding minimum fluidization velocity was 3.3 m/s
(60 urn) or 5.85 m/s (at 80 urn). To achieve comparable fluid-dynamic
conditions in the reactor in the simulation, the fluidized bed index u/umf in
the reactor was kept constant at 8.0 for all simulation conditions (different
temperature, different composition of the gas stream fed in) by adjusting
the gas velocity at the reactor inlet accordingly in the range from 4.67 m3/s
to 5.40 m3/s. As gas distributor, a nozzle plate distributor having 800
orifices per m2 was assumed.
In the calculation for the multistage fluidized-bed reactor without divided
introduction of oxygen, each stage was modeled as an individual fluidized-
bed reactor with its own gas distributor. In the case of a multistage fluidized
bed with divided introduction of oxygen, the same flow of oxygen is added
to the gas stream before each stage. The volume flow thus increases from
stage to stage, so that the fluidized bed index increases slightly from 7.5 to
8.5. The volume flow was chosen so that the fluidized bed index u/umf in
the middle stage of the fluidized bed is 8.
In the figures, the simulation results for an ideal isothermal fixed-bed
reactor under the same conditions are given for comparison (abbreviation:
PLF). The modified residence time tmod specified in the figures and in the
appendix is defined as the mass of catalyst divided by the volume flow at
the reactor inlet under the given reaction conditions.
Example 1: Influence of the particle size
Simulation calculations were carried out for two different particle diameters,
viz. 60µm and 80µm. In the case of the 60µm particles, smaller bubbles
having a maximum diameter of 4 cm were formed, while in the case of the
80 µm particles, bubbles having a diameter of up to 12 cm are formed.
The effect of better mass transfer and reduced backmixing in the case of small
bubbles on the selectivities and the yield of acetic acid are shown in figures 3
and 4. In addition, figures 3 and 4 show that at a bubble diameter of dB = 1
cm, the achievable yields become the same as those in the fixed-bed reactor.
Furthermore, the results show that at constant conversion, the selectivity to
acetic acid increases with decreasing bubble sizes. In addition, when the
particle diameter becomes smaller, the yield of acetic acid YHOac and also
the space-time yield increase (fig. 4).
The example shows that at a maximum bubble size dB of 1 cm, virtually
identical results in respect of the selectivity to acetic acid and the space-time
yield are obtained in the fluidized-bed reactor and the fixed-bed reactor,
since at this bubble size virtually no inhibition of mass transfer between
bubbles and emulsion phase occurs and, in addition, backmixing is reduced.
It can thus be shown that the low selectivity and space-time yield is at least
partly attributable to undesirable backmixing in the fluidized bed and to slow
mass transfer between the bubble phase and emulsion phase which leads to
oxygen from the bubbles only slowly getting into the emulsion phase. In
addition, a high oxygen concentration in the bubbles leads to increased
carbon dioxide formation as a result of unselective further oxidation of the
acetic acid and thus to a reduction in the acetic acid selectivity.
Example 2: Multistage fluidized-bed reactor
An additional possible way of reducing the undesirable backmixing in the
fluidized bed is, apart from a reduction in the particle size, the use of a
multistage fluidized-bed reactor. As a result of the gas distributors at the
beginning of each stage, the average bubble size over the fluidized-bed
reactor is smaller than in a single-stage fluidized bed, since bubble growth
starts afresh in each stage.
Figures 5 and 6 show the results for various multistage fluidized-bed reactors:
• "3St" - 3-stage fluidized-bed reactor with 201 of catalyst/stage, height of
the fluidized bed per stage = 1.04 m
• "5St" - 5-stage fluidized-bed reactor with 10 t of catalyst/stage, height of
the fluidized bed per stage = 0.53 m
• "5St,vert" - 5-stage fluidized-bed reactor with divided introduction of
• 15 t of catalyst/stage, height of the fluidized bed per stage = 0.78 m
For comparison, the results for the single-stage fluidized-bed reactor (BAM)
and the ideal fixed-bed reactor (PLF) are also shown in figures 5 and 6.
At low ethane conversions, ethylene is formed with significantly higher
selectivity in the 5-stage fluidized bed with divided introduction of oxygen
than in the other fluidized bed reactors. This is attributable to the low
oxygen concentration. In contrast, at higher ethane conversions, the
differences in the selectivities between the various fluidized-bed reactors
are low, ( reactor of SHOac = 72.2 % is obtained in the 5-stage fluidized bed with
divided introduction of oxygen (at XC2H6= 8.6 %). At the same ethane
conversion, the selectivity in the fixed-bed reactor is SHOac = 77.5 %.
The results in figure 5 show that use of a multistage fluidized bed enables
the space-time yield to be increased significantly compared to a singe-
stage fluidized bed. On the other hand, the divided introduction of oxygen
leads to a reduction in the space-time yield, since the lower oxygen partial
pressure reduces the reaction rate compared to the other reactors.
The simulation of the 5-stage fluidized-bed reactor with divided introduction of
oxygen was repeated for a higher oxygen concentration which allows a higher
ethane conversion and thus a higher acetic acid yield. Owing to the higher
oxygen partial pressure, the reaction rate is increased. As before, the catalyst
mass is 15 t/stage. The results of the simulation are shown in figures 7 and 8.
In contrast to the results obtained with the low oxygen concentration
(figures 5 and 6), the conversion-selectivity curves in figures 7 and 8 show
more significant differences between the single-stage fluidized-bed reactor
and the 5-stage fluidized-bed reactor. As a result of the divided introduction
of oxygen, a significantly higher selectivity to acetic acid compared to the
single-stage fluidized-bed reactor is obtained at the same ethane
conversion (at Xc2H6 = 16 %: SHoac,1-st = 65.4 % or SHOac,5-st = 69.7 %). The
loss of selectivity in the fluidized bed compared to the fixed bed is relatively
small (SHoac,PLF = 73.6 %). As before, the space-time yield obtained with
divided introduction of oxygen is reduced compared to the fixed-bed
reactor; however, it is at the level of the single-stage fluidized-bed reactor.
1. A process for preparing acetic acid by oxidation of ethane by
means of molecular oxygen in a fluidized bed of catalyst particles,
at a temperature in the range from 100 to 500° C and at a reactor
pressure In the range from 1 to 50 bar wherein the particle
diameter of at least 70 % of the catalyst particles used is less than
or equal to 80 urn, the density of the catalyst particles used in the
fluktaed-bed reactor is In the range from 500 kg/m9 to 6 000
kg/m3, the flukized bed index is in the range from 1 to 12 and the
gas volume flow is selected so that the bubble diameter of the
reaction gas mixture fed in the fluidized-bed reactor Is less than
2. A process as claimed In claim 1,-wherein the gas volume flow is
set so that the bubble diameter of the reaction gas mixture fed in
is less than 5 cm.
3. A process as claimed in claim 1 or 2, wherein the particle diameter
of from 10 % to 60 % of the catalyst particles Is less than 60 urn.
4. A process as claimed in any of claims 1 to 3, wherein from 20 to
50 % of the particles have a diameter of less than 40 urn.
5. A process as claimed In any of claims 1 to 4, wherein the catalyst y
particles are within a particle diameter range from 10 µm to
6. A process as claimed In any of the preceding claims, wherein the
fluidlzed bed Index Is in the range from 1 to 10.
7. A process as claimed in any of the preceding claims, wherein the
catalyst used has the formula (I),
X is one or more elements selected from the group consisting of
Cr, Mn, Nb,Ta, B,Tl,V,Te, Re and W,
Y is one or more elements selected from the group consisting of
B, Al, Ga, in, Pt, Zn, Cd, Bl, Ge, Go, Rh, Ir, Cu, Ag, Au, Fe, Ru, Cs,
Mg Ca, Sr, Ba, U, K, Na, Rb, Be, Nb, Zr, Hf, Ni, P, Pb, Sb, Si, Sn,
Tl and U and the indices a, b, c and d are the gram atom ratios of
the corresponding elements, where, based on a = 1, b Is in the
range from 0.0001 to 0.01, c is in the range from 0.1 to 1 aqd d
is in the range from 0 to 1, where X and/or Y may represent a
plurality of elements.
8. A process as claimed In any of the preceding claims, wherein the
9. A process as claimed in any of the preceding claims, wherein not
only ethane and motecular oxygen but also an inert gas, carbon
dioxide and/or water vapor are present in the reaction gas
10. A process as claimed in any of the preceding claims, wherein the
fluldized-bed reactor has a plurality of reaction stages or reaction
11. A process as claimed in any of the preceding claims, wherein
one or more reactants or water are fed into various zones or
stages of a single-stage or multistage reactor.
A process for preparing acetic add by oxidation of ethane by means
of molecular oxygen in a fluidized bed of catalyst paracles, at a
temperature in the range from 100 to 500° C and at a reactor
pressure in the range from 1 to 50 bar wherein the particle diameter
of at bast 70 % of the catalyst particles used is less than or equal to
80 µm, the density of the catalyst particles used in the fluldlzed-bed
reactor is in the range from 500 kg/m3 to 6 000 kg/m3, the fluidized
bed index is in the range from 1 to 12 and the gas volume flow is
selected so that the bubble diameter of the reaction gas mixture fad
in the fluidzed-bed reactor Is less than 12 cm.
|Indian Patent Application Number||449/KOLNP/2003|
|PG Journal Number||06/2008|
|Date of Filing||10-Apr-2003|
|Name of Patentee||CELANESE INTERNATIONAL CORPORATION,|
|Applicant Address||1601 WEST LBJ FREEWAY, DALLAS TEXAS 75381|
|PCT International Classification Number||C07C 51/215|
|PCT International Application Number||PCT/EP01/12001|
|PCT International Filing date||2001-10-17|