Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
The present invention relates to the oxidation o~ a butane ~eed to
maleic anhydride.
Oxidation o~ hydrocarbons to maleic anhydride is well known. Feeds
which have been disclosed include benzene, butene, and n-butane. A series of
patents to Kerr, including United States Patents 3,156,705, '706, '707,
3,238,254, 3,255,211, '212, '213, 3,288,721, 3,351,565 and 3,385,796, dis-
closes vanadium-phosphorus oxide catalysts ~or oxidation of butene to maleic
anhydride.
Friedrichsen et al United States Patent 3,478,063 discloses oxida-
tion o~ olefinically unsaturated hydrocarbons with a catalyst containing
vanadium and phosphorus oxides and wherein the amount o~ phosphorus oxide is
at least equal to twice that Or the vanadium oxide and wherein the catalyst
contains at least one other oxide of chromium, iron, cobalt or nickel, and ~;
the catalyst is preferably on a carrier. The patent discloses at Col. 4
that the catalyst may have a sur~ace area ~rom 1 to lOO m /g.
Bergman United States Patent 3,293,268 discloses a vanadium-phos-
phorus oxide catalyst ~or oxidation of butane to maleic anhydride. Sur~ace
area is not disclosed ~or the catalyst in the Bergman reference. Also, as in
the Frledrichsen et al re~erence, the Bergman catalyst is prepared by an
aqueous solution method.
Schneider United States Patent 3,864,280 discloses a vanadium-
phosphorus mixed oxide catalyst having an intrinsic surface area o~ 7 to 5O
m /g. The Schneider catalyst can be prepared using an organic medium as op-
posed to an aqueous medium.
The use o~ recycle o~ unreacted constituents to a reactor is, o
course, well known and ~requently is employed in various processes. For
example, in the oxidation o~ ethylene to ethylene oxide, ethylene and air are
mixed with recycle gas containing mainly nitrogen and unconverted ethylene, ~ -
and -the mixture is passed over the catalyst. The catalyst is typically ~
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contained in tubes in a heat exchanger-type reactor with a boiling cooling
fluid on the outside of the tubes to absorb the heat of reaction and control
the temperature. Typically the catalyst for the ethylene oxidation is sil-
ver oxide on a refractory support, and typical operating conditions include
a tempera-ture in the range 392F to 572 F and a pressure of 10 to 30 atmo-
spheres.
Bissot et al, in "Oxidation of Butane to Maleic Anhydride", IEC
Vol. 2, ~o. 1, March 1963, pp. 57-60, disclose that, in a process for con-
version of butane to maleic anhydride unreacted butane may be recycled to
the reactor. However, Bissot et al prefer to use sequential reaction, with
maleic anhydride separation between the reactors, and with unreacted butane ~ -
from the first reactor being fed to the second reactor, etc.
United States Patent 3,904,652 discloses the oxidation of n-butane
to maleic anhydride using enriched oxygen and with a recycle stream of re-
actor effluent which lowers the oxygen concentration in the total feed to
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the reactor. It is known that explosive mixtures of butane and oxygen exist -~
and that some oxygen concentra-tions can cause an oxygen-butane-nitrogen mix- --
ture to go into the explosive range, see, for example, Bureau of Mines Bul- -
letin 503 (1952), Figure 35, page 62, and Bureau of Mines Bulletin 627 ;;
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~ (1965), Figure 21, page 23. In United States Patent 3,904,652, the reactor
~eed mixture is kept below explosive (flammable) limits by the addition of ;-~
an inert gas, e.g., nitrogen, to the enriched oxygen fresh feed. -
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Butane conversion levels in United States Patent 3,904,652 are 30 ;
to 70% per pass. The unconverted butane passes out of the oxidizer reactor
as par-t of the reactor effluent. The effluent is processed to remove maleic
anhydride. The maleic anhydride-free effluent is then divided into two
parts, a recycle stream which is recycled back to the reactor, and a purge
stream which is removed from the system. According to this patent, the
oxygen level in the ~resh feed is not desired to be below 50%; lower levels
are not desired, according to the patent, as the lower levels would require
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increasing the amount of purge gas to be withdrawn from the recycle stream
and hence would increase the unreacted butane loss in the process of said
Patent. ~;
SUMMARY OF THE INVENTION
According to the present invention a process is provided for pro-
ducing maleic anhydride ~rom n-butane which comprises:
(a) feeding n-butane and air to a reactor;
(b) contacting the butane and air with a catalyst comprising van-
adium and phosphorus oxides at reaction conditions including a temperature
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between 550 and 1000 F so as to convert 15 to 28% of the butane to maleic
anhydride and obtain a reactor effluent comprising unreacted butane, nitrogen
and maleic anhydride;
(c) removing maleic anhydride from -the reactor effluent to obtain
maleic anhydride-lean effluent;
(d) recycling a portion of the maleic anhydride lean effluent to
the reactor;
(e) removing butane from the other portion of the maleic anhydride-
lean effluent; and
(f) recycling the removed butane to the reactor.
Preferably the temperature in, and/or space velocity through, the --
reactor is adJusted to obtain a weight percent selectivity o~ at least 75%,
more preferably at least 90% of the butane converted. ~
Among other factors, the present invention is based on our findings ~;
that the use of the specified conversion levels (percent of the butane feed
converted per pass) of the process of the present invention resul-ts in un-
expectedly long run lengths of high selectivity and activity for the cat-
alyst. We have found that the process is especially attractive by virtue of
the integration of the specified conversion level, and the use of a recycle
wherein a purge stream is treated for butane recovery and recycle of the
butane to the process, and wherein air is used as the source of oxygen for
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the oxidation reaction.
Related to the above factors, particularly the finding of improved
run length of high selectivity, is the supportive finding that there is a
substantial lowering and flattening of the temperature profile curve for op-
eration in accordance with the present invention at conversions between 15
and 28%, for example at about 25%, versus conversions at higher levels such
as 35% This lowering and flattening of the temperature profile is ex-
plained in more detail with respect to Figure 2. The temperature profile -'
represents the temperatures along the length of the reactor tubes starting
from the inlet of the feed gas to the tubes and proceeding to the outlet por-
tion of the tubes.
Thus, compared to prior art processes, we have found that using the
conversion levels of the present invention enables the butane recycle process -
to be opera:ted at lower temperature and for longer catalyst run life for a
given productivity (productivity is the pounds of maleic anhydride produced
per unit time per volume of catalyst) in a recycle process using air as the
oxygen source, particularly when the oxygen in the reaction zone feed is
maintained at about 6-12 volume percent as wel:l as the conversion being main-
tained within the stipulated levels. In the process of the present invention
the oxygen concentration should be kept below the explosive level.
It is to be appreciated that as the conversion per pass is dropped,
more butane needs to be fed -to the reactor per unit time in order to maintain
a given productivity, and that the increase in the butane concentration is a
factor which tends to raise the catalyst bed hot spot temperature because ~`
the oxidation of butane is an exothermic reaction. However, through our
experimental datag we have found that for a given productivity the process
of the present invention generally operates at a lower temperature -than
would be the case for the same productivity on a once-through operation or
for recycle operation at higher conversion.
Particularly preferred butane conversion levels for the process of
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the present invention have been found to be below 30%, and above 1 or 2%,
for example in the range of 20 to 28% ~ and especially preferred levels have
been found to be 22 to 27% of n-butane oxidized to maleic anhydride and by-
products per pass through the reactor.
Preferred operating conditions are as follows:
More Most
Preferred Preferred Preferred
Temperature, F 650-950 700-850 70o-800
Pressure, psig 10-1000 20-50 25-40
Space rate, VHS~ 1000-10,000 2000-5000 3500-4500
Feed n-butane content, com-
bined fresh + recycle
feed, vol.% 1~5 1.5-4 2-3.5
% effluent recycled directly 20-95 65-go 80
(remainder of effluent n-butane
recycled after separation from
other effluent gases)
Total volume of gas at 70 F and 1 atm. per hour per cubic foot of catalyst
volume.
The purge portion of -the effluen-t recycle is treated for n-butane
recovery for example by cryogenic cooling to selectively condense out the
butane or by absorption of the butane in a solvent selective for butane
absorption, but preferably by adsorption of n-butane onto a solid adsorbent
selective for n-butane adsorption followed by a separate desorption step to
recover a relatively pure stream of n-butane from the adsorbent. Preferred
adsorbents are those wherein the adsorbent is an activated carbon adsorbent
having a low a~finity for polar compounds such as water and carbon dioxide.
Particularly preferred adsorbents are activated carbon produced from coalg
especially from bituminous coal, and having a high surface area between ;~ `about 400 and 1500 m /g, preferably with a large percent (about 25% or more)
of the pore volume coming from pores of 15-30 Angstroms in radius, and also
preferably of small Tyler mesh size (about 4-60 mesh size).
A preferred cyclic adsorption-desorption system for the recovery
of n-butane from the purge stream consists of 3 stages: adsorption, desorp-
tion, and cooling (with drying when steam i3 used for desorption). To
provide a cycle for repetitive operation using multiple beds, the absorption
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time desirably is equal to -the total time for desorption and cooling.
Adsorption is performed at as low a temperature as practical, usually in the
range of 60-200 F, and a-t superatmospheric pressure. The time required for
saturation of the adsorbent aepends on these conditions in addition to the
concentration of butane in the gas and the size of the carbon bed relative
to the amount of gas to be processed. The time may be as short as 1/4 hour
or as long as 12 hours. Desorption of -the n-butane can be accomplished in a
variety of ways, depending on the form in which it is desired to be re- ~
covered. ~-
Thus, it can be desorbed by use of a heated inert gas, e.g., N2 or
C02, or mixtures of these gases with air to provide a gaseous mixture suit-
able for recycle to the catalytic reactor. Alternatively, desorption can be
carried out with low-pressure steam (15-100 psig), followed by condensation
and separation of liquid C4 and water layers in the usual manner. The liquid
C4 can then be returned to the plant feed inventory. ~`-
After desorption with heated gas (either fixed or condensable), the
adsorption bed can be dried with inert hot gas, such as hot N2 or C02, and
cooled before starting the cycle again. Cooling can be accomplished by any -;
of several means, e.g., by circulating a cold fluia through cooling coils
imbedded in the carbon, by passing a cooled gas through the bed, or by a ~
combination of these. The method chosen is dependent on the rate of cooling -
desired. We have found it advantageous to use the spent gas from another
bed, operating on the adsorption cycle, to do the cooling. Alternatively,
drying and cooling can be achieved in a combined step.
Spent carbon may require periodic regeneration to regain its ad-
sorption capacity. Preferred regeneration operation is an in-situ treatment
of the carbon by hot flue gases at 400-1000 F.
According to a particularly preferred embodiment of the present -
invention, the butane is recycled to "extinction," i.e., all the butane fed
to the process is ultimatel~ converted by way of recycle of unconverted
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butane in the off-gas and recycle of recovered butane from the adsorption-
desorption system.
Preferred catalysts for use in the present invention are high-
activity, high-surface-area vanadium-phosphorus oxide catalysts, preferably
having an intrinsic surface area between about 7 and 50 m /g.
Particularly preferred catalysts for use in the process of the
present invention are those disclosed in Schneider United States
Patent 3,864,280, specifically crystalline phosphorus-vanadium
mixed oxide catalysts containing pentavalent phosphorus, vanadium `~
and oxygen, said vanadium having an average valence in the range
from about +3.9 to +4.6, said oxide having a phosphorus-to-
vanadium atomic ratio in the range from about 0.9-1.8 to 1, and
an intrinsic surface area in the range from about 7 to 50 m /g.
As defined in the '280 patent, the term "intrinsic
surface area" is used herein to mean the surface area of the
mixed vanadium-phosphorus oxide material itself, i.e., per se, in
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the absence of a support or ca~raer.
The catalysts of U.S. Patent 3,864~280 are particula~ly suited for
use in the process of the present inventlon.
The proGe~s of the present invention is espe~ially advantageous in
that the catalyst of the '280 patent is not overox-idized and the "B-phaæ" of
the catalyst is substantially retalned durang operation.
In the process of the present invention we have found that it is
advantageous, espæ ially in tenms of obtalmng lcng catal~vst life of high
selectlvit~, to maantain the oxygen concentration m the total feed gas to
the reactor at a~out 6 to 12 volume percent, p~eferably 7 to 11 volume percent,
and most prefe~ably abc~tt 8 to 10 volume Eercent. To achieve th~s o~ygen
level, ~he air feed rate an~/or the butane o~nversion level ~ thin the
stipulated ranges) and/or the resh-~eed butane content can be varied. me
above oxygen concent~ations have been found especially advantageous when
used in conjunctlon with the pLe~erred catalyst.s, i.e., the hi~h-surface-area
catalysts disclosed Ln the 1280 patent.
Th~ tenm "fresh butane"is i~lust~a~ by reerence to Figure 1,
which shows the fresh or net butane eed to the pro~ess in line 1. The total
butane fed to the react3r mcludes the fresh butane, the butane recovered
frcn th~ reactor off-gas by adso~ption ~see line 16), and ~utane an the off-
gas which is recycled directly to the reactor via line 3 without separating
nitrogen, oxygen arld aarb~n oxldes from the of-gas.
T}le te~m"Select-av~ty" as used hereln to mean the weight percent OL
maleic ar~}ydrade cbtainRd per pa~d of butane ccnverted.
me selectiv:Lties herean are on a welght pe~cent basis;
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the theoretical maximum selec-tivity is 169 weight percent if 1 mol of butane
is converted entirely to maleic anhydride with no by-produc-t formation.
In the present invention, whether air or a high oxygen content
feed, such as purified oxygenor enriched oxygen, is used as the oxygen
source, the oxygen content in the reactor total feed mixture can be adjusted
by the amount of recycle to the reactor. In addition to use of the recycle
to adjust oxygen concentration, an ou-tside source of inert gas, such as nit-
rogen, argon or helium, can also be used to adjust the oxygen concentration.
According to the most preferred embodiments of the present inven-
tion, air is used as the sole oxygen source.
Where one simply wanted to take advantage of long catalyst life - -
and high selectivity found by using -the conversion levels as specified here-
in, e.g., in an embodiment where there is an alternate use for the recovered ~ ;~
n-butane or where recovery of n-butane is omitted altogether, such as when a
portion of the off-gas is burned as fuel, an al-ternate embodiment of the
invention may be defined in terms of the following improvement: In a pro-
cess for producing maleic anhydride from n-butane wherein (a) n-butane is
contacted with an oxygen-containing gas and a catalys-t comprising vanadium-
phosphorus oxides to thereby convert the butaneto maleic-anhydride, ;
(b) maleic anhydride is removed from the reactor effluent to obtain a
maleic anhydride-lean off-gas, and (c) at least a portion of the off-gas
is recycled to the reactor, the improvement is made which comprises main-
taining the conversion level within the ranges previously stipulated herein,
preferably between about 22 and 27% per pass.
Preferred selectivities and catalysts and operating
temperatures and pressures for this alternate embodiment are as
aforesaid for the preferred embodiment wherein air is used as the ---
3356'~
oxygen source and n-butane is rec~vered for recycle to ex~inction.
Figure 1 is a schematic process flow diagram illustrating a pre-
ferred embodiment of the present invention.
Figure 2 shows a temperatu~e profile for the reactor at various
conversicn levels.
Example 1
Feed butane Ln line la, made up from 97 lbs/hr o~ fresh-~eHd
butane in line 1 and 36 lbs/hr of recycled butane ln line 16 is mixed with
about 1426 lbsjhr of fresh make-up air m trcduced via line 2, and about 5995
lbs/hr of rec~cled off-gas, and the moxture is introduced into oxid~zer
reacto~ 4 thro~gh l.ine 3~a). ~he oxidizer reacto~ consists of conventional
heat exchanger-type design with catalyst packed in tubes su~rounded b~v a
heat-transfer liqu~d la "salt bath"). The reaction mLxture .is oxidized
in the presence o a catalyst effective for accelerating the reaction of
n-butane with air to form malew anh~d~ide. Preferred catalysts comprise
mixed oxides of ~anad~um and phQspho~us, especlally those described Ln the
previously cited United States Patent 3,864,28(), and p~eerred reaction
temperatures are Ln the range 700-800CF.
Following oxidaticn~ the gaseous eff~usnt 1a~s th~ough line 5
lnto absorber 6. Abcut 104 lbs/h~ ~f male~c anhyd~ide is absorbed ~n the
organic solvent flcwmg into the absorker from line 7~ The gaæous st~eam,
7450 Ibs/hr~ leaves the abscrber through line 9 at a t~mperature o abcut 160
and is given a wa~er wash in vessel 10~ ~ash water is intrcduced through
line 11 and the used wash water leaves th~ough lane 12. The solvent-maleic
anhydride stream leaves the absorber through line 8 ~ox m~leic anhydri~e
stripping in stripper 17 and then th~ough line 18 for further F~ification
of the crude product.
Washed off-gas leaving the water wash through lme 13 at a
tempexature
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of about 100 to 120F is split into streams 3 and 14. About 5995 lbs/hr of
this washed off-gas is compressed and recycled back to the oxidizer through
line 3 and the remaining 1336 lbs/hr of off-gas is passed through line 14
for butane recovery in adsorber 15.
Butane in line 14 is adsorbed in the adsorber by a cyclic opera-
-tion using multiple beds filled with adsorbents such as activated carbon.
The denuded off-gas, 1300 lbs/hr, mainly containing oxygen, nitrogen and
oxides of carbon, is vented out and -the butane adsorbed on the adsorbent is
recovered by suitable desorp-tion such as steaming at about 250 F followed
by condensation and phase separation. The recovered bu-tane, 36 lbs/hr, is
then recycled back through line 16 to the reactor.
Example_2
The oxidizer, absorber and the recycle operations for off-gas and
butane were carried out as in Example 1 in a pilot plant, but withou-t re-
cycle of butane recovered by adsorption. The temperature of the catalyst
bed was measured by use of a set of thermocouples placed in an axial thermo-
well located at the center of one of the oxidizer -tubes. A typical temper-
ature profile along the catalyst bed normally showed an initial increase in
the ca-talyst temperature in the first part of the catalyst bed, followed by
a decrease in the temperature in the latter part o~ the bed. Typical pro-
files obtained at various operating conditions are shown in Figure 2. In
Figure 2, the abscissa is -the percent distance a]ong the tube from the feed
inlet, and the ordinate is the temperature recorded at that level. The three
flat portions of the curves in the first 10~ of the tube are the salt-bath
temperatures (SBT).
The temperature of` the heat-transfer liquid bath (S~T) was dropped
by about 14 F to ascertain, among other factors, the effect of lower opera-t-
ing temperatures on the oxidizer operation. The decrease in the maleic an-
hydride production rate (productivity) due to such a temperature decrease
was compensated for by increasing the butane feed rate. The -temperature
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profiles of Figure 2 thus represent the cumulative result of the two changes,
i.e., salt bath temperature decrease and butane feed increase.
The axial temperature profile resulting from this lower per-pass
conversion level was measured using the aforementioned set of tbermocouples.
The above procedure was repeated at three different per-pass conversion
levels, at each time maintaining the maleic anhydride production rate at a
constant value.
It will be seen that the position of the "hot spot" remains the
same, but its temperature decreases with the changes. The response of the
"hot-spot" temperature to a lowering of the salt-bath temperature is a great-
er drop in temperature, as tabulated in Table I.
TABLE I
Salt-Bath Temperature~ F Hot-Spot Temperature 2 F
From To Change From To ~ e
721 707 14 802 777 25 ~;
707 700 7 777 763 ll~
In Figure 2, the numbers in each column of Ta`ble II represent,
se~uentially, the following conditions for each of the three temperature
runs~ total hours on the catalyst; (2) percent butane conversion "X";
(3) volume percent butane in the feed gas, "CI~"; (4) productivity (lbs
20 maleic anhydride/cu.ft. catalyst/hour), "Pr"; (5) weight percent yield of
maleic anhydride on a once-through basis, "Yot"; and (6) selectivity, which
is the weight percent yield of maleic anhydride on a total recycle basis,
"Z". ~' 'For convenience, Table II is also included at this point:
TABLE II
-in- Pr Yot Z
2200 34.5 1.93 3.86 76.6 105
2230 29.1 2.08 3.79 75.3 llO
2255 25.7 2.44 3.87 72 113.~
Referring to the values in Table II~ i-t will be seen that as the
3Q salt~bath temperature is lowered ~rom 721 to 707 to 700 F, the concentration
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of butane in the feed is increased from 1.93 to 2.08 to 2.44%. The increase
in butane concentration was made -to maintain productivity essentially con-
stant; the productivity was maintained a-t about 3.86 pounds of maleic an-
hydride per cubic foot of catalyst volume per hour. However, the once-
-through conversion dropped from 34.5 to 29.1 to 25.7%.
At the same time~ once-through yields drop from 76.6 to 75.3 to
72 weight percent.
However, selectivity increased from 105 to 110 to 113.6 weight
percent maleic anhydride, so that the ultimate yield of maleic anhydride per
pound of fresh butane feed is 1.136, using the 25.7% conversion level in ac-
cordance with the process of the present invention, vs. 1.05 pounds maleic
anhydride per pound of butane feed using the higher conversion level of
34.5% per pass, which is not in accordance wi-th the present invention.
Thus, a comparison of the data obtained at 25.7, 29.1 and 34.5%
per-pass conversion levels under these conditions shows our findings that:
(a) higher ultimate process yields and selec-tivities are obtained at the
indicated lower conversion levels of about 25% vs. 35%; and (b) substantial
lowering and flattening of the axial temperature profile was achieved by
operating the oxidizer at lower per-pass conversions, such as about 25%.
20 Since a flat profile is indicative of a uniform work rate of the entire cat-
alyst bed, such an operation is advantageous in terms of longer catalyst
life at a given productivity.
~ ower temperature aifferences between heat-exchanger bath temper-
ature (salt-bath temperature) and the ca-talyst temperature were also found,
and such, as well as the lower absolute catalyst bed temperature at the
given productivity also helps assure a more stable and longer operation of
the catalyst in the oxidizer.
A consequence of lower operating temperature and per-pass conver-
sion levels found was tha-t the useful life of the catalyst was extended a
30 surprising amount. Operating the oxidizer at higher per-pass conversion
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levels such as 35% was found to result in about 3.5 times faster loss of
catalyst selectivity -than at lower conversion levels such as 25~. -
For feed butane concentrations below about 5%, we have found that
operation below about 15% conversion is unattractive in terms of -the resul-
tant need to handle large recycle volumes and large butane recovery require-
ments for the adsorbers. :
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