FR2545080A1 - PROCESS FOR PRODUCING ALCOHOLS FROM LIGHT HYDROCARBONS - Google Patents

Abstract

THE INVENTION CONCERNS THE COUPLING OF A STEAMING REACTION CARRIED OUT IN ZONE 14 (STEAM TREATMENT OF METHANE IN PARTICULAR) AND OF AN ALCOHOLS SYNTHESIS REACTION (CARRIED OUT IN ZONE 20) FROM THE SYNTHETIC GAS PRODUCED IN THE VAPOREFORMING ZONE. THE INVENTION IS CHARACTERIZED BY A RECYCLING TO THE VAPOREFORMING ZONE OF AT LEAST PART OF THE CARBON GAS AND LIGHT HYDROCARBONS (BY LINE 3) PRESENT IN THE GASEOUS PHASE SOUL OF THE ALCOHOL SYNTHESIS ZONE. THE INVENTION APPLIES TO THE PRODUCTION OF METHANOL AND HIGHER APPROVED ALCOHOLS.

Description

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  The invention relates to a process for the production of at least one alcohol chosen from methanol and the higher homologous alcohols of linear and saturated methanol, comprising, for example, 2 to 10 carbon atoms per molecule and in particular ethanol and propanol, butanols and pentanols One or other of these alcohols, and generally some of these alcohols used in a mixture, have the particularity of being able to be added directly to a gasoline composed essentially of hydrocarbons (that is to say to a motor gasoline type normal or super type) to be able to obtain, by

  example, a moderately priced and excellent quality automotive gasoline.

  The methods for obtaining such mixtures of alcohols, from synthesis gas in one or more steps, have been widely described in the prior art, for example in French patent applications 82/16 345 of 23 September 1982. , 82/08 997 of May 21, 1982, 82/05 368 of March 26, 1982, 82/11 892 of July 5, 1982, and in the French patents N 2 369 234 (or USP 4 122 110), 2 444 654 (or USP

  4,291,126) Nos. 2,453,125 and 2,441,420.

  The method according to the invention comprises two steps; the first step is a steam reforming (steam reforming or reforming or steam reforming etc) that is to say the reaction of water with methane or light hydrocarbon gases (LPG) of formula Cn H 2 n + 2 with n = 1 to 4 This reaction produces carbon monoxide, carbon dioxide and hydrogen, thus a synthesis gas from which

  in a second step, one or more alcohols are produced.

  Such a process does not seem a priori very reasonable for thermodynamic considerations Indeed, the reaction (1) of methane water reforming:

CH 4 + H 20 CO + 3H 2 (1)

comes with balance (2) A

CO + H 20 C 2 + H 2 (2)

  B This equilibrium moves essentially in direction A because of the excess of water used in this type of reaction (1), excess necessary to avoid the coking (by exaggerated heating) of the reactor and the catalyst, which leads to a molar ratio H 2 / CO very often between 4.5 and 6 The displacement of the equilibrium (2) in the direction A is thus reflected in an excessively high content of hydrogen which subsequently, in the second stage, results in training too

  significant amount of methane, to the detriment of sought-after alcohols.

  This displacement in direction A is all the more clear, as at the temperatures generally used, even in the absence of an excess of water, the value Kp (equilibrium direction Aj is in favor of the displacement in the sense A, with consequent surplus production as well

and harmful to CG 2.

  The various values of Kp for various temperatures are recalled below: K = p CO 2 p H 2 p (A direction) p CG p H 20

  But precisely, the invention is based, however, on the injection

  In the first CO 2 reactor, C 02 can be used as the C 02 source, for this injection, C 02 originating at least in part from a compressed C 02 storage or from a neighboring unit (ammonia synthesis). example) or a deposit, or else the C 02 recovered on the fumes of steam reforming furnace etc but according to the invention, at least part of the CO 2 advantageously comes from the C02 withdrawn from the methanol synthesis reactor and / or its homologous alcohols And thus using the CO 2 from the alcohol synthesis reactor makes it possible to remedy the many problems generally posed by the presence of CO 2 in the gases drawn off from the synthesis reactor: it is indeed recommended to recycle these gases in the synthesis reactor but the presence of CO 2 in these gases has drawbacks, for the following reasons: To carry out the synthesis of alcohols from carbon monoxide and hydrogen, it is theoretically necessary to hydrogen lecules for a CO molecule, according to reaction (1): (1) n CO + 2 NH 2 C n H 2 n + 1 OH + (n-1) H 20 TK TC Log O l Kp K -P p

298 25 4.992 98175

300 27 4.947 88512

400 127 3,166 1466

500 227 2.114 130.0

600,327 1,429 26.85

700,427 0,952 8,95

800,527 0.603 4.01

900 627 0.341 2.19

1000 727 0.136 1.37

, ,,, '^ L,

  Thus, for this type of reaction, it is generally desirable

  table having to work with a molar ratio H 2 / CO close to 2, for example between 1.6 and 2.4 and, more particularly, between 1.8 and

  2.2, which will be made possible by the method of the invention.

  In the processes of the prior art, generally the synthesis gas is obtained by gasification, that is to say coal, coke, petroleum heavy residues, bitumens, etc., and its composition does not correspond to the stoichiometric composition for obtaining of alcohols, this synthesis gas being deficient in hydrogen; its composition must therefore be adjusted by conversion of CO to water vapor and elimination of all or part of the C 02 formed But in this case, it is not beneficial to leave too much C 02 in synthesis gas because the synthesis of alcohols from CO 2 requires more hydrogen than in the case of synthesis from CO (reaction (2)): N CO 2 + 3 NH 2 C 2 n + H OH + (2 n-1) H (2) During the synthesis of the alcohols, a reaction effluent is recovered which contains unconverted gases (CO + H 2), methanol, water, hydrocarbons ( methane and heavier hydrocarbons thus having at least 2 carbon atoms per molecule) and alcohols homologous to methanol All this effluent is cooled so as to condense the major part of methanol and homologous alcohols and most of the water. not condensed contains essentially

  carbon monoxide, hydrogen, methane, other hydro-

  carbides and carbon dioxide This gaseous fraction, in the prior art, is recycled at least in part to the inlet of the reactor, as a make-up gas to raise the H 2 / CO molar ratio (at the inlet of the reactor). synthesis reactor) at a value of at least about 1.8 and

about 2, preferably.

  But the process just described has a disadvantage

  major event which is the presence of relatively large quantities

  It has been explained above that the production of alcohols from C 02 requires more hydrogen than from CO. A recycling of C 02 is therefore ultimately detrimental. of C 02 in the reaction zone adversely affects the equilibrium reaction (2) which also exists in this synthesis of alcohols A

CO + H 20 = -CO 2+ H 2 (2)

B

  and occurs in the reaction zone.

  It follows that in the prior art it was necessary to

  often time-consuming decarbonation of non-condensed gases

  sand, of course a significant loss of carbon.

  The idea of the present invention is not to perform any

  decarbonation of non-condensed gases As gases are at a

  In the case of a cage structure of the order of 100 to 20 ° C., the invention involves heating them at a temperature of the order of 650 to 950 ° C. (preferably between 730 and 890 ° C.) to recycle them at least partially towards the steam reforming reactor which will operate precisely between 650 and 950 ° C., preferably between 730 and 890 ° C., thus playing on the value of Kp in

  advantageously displacing the equilibrium (2) in the direction B -

  The idea of the invention is thus to couple (a) a steam reforming (steam reforming) carried out at high temperature and thus requiring, at the chosen temperatures, the beneficial contribution of CO 2 and (b) a synthesis reaction of alcohols supplying CO 2, which is detrimental to this reaction, and light hydrocarbons which are therefore advantageously recycled to the steam reforming reactor. The essential advantages of the present process are thus as follows: it is no longer necessary to proceed with the decarbonation of non-condensable gases from the alcohol synthesis reactor,

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  the equilibrium (2) of the steam-reforming reaction is displaced in the B direction since the C 02 recycled and injected into the steam-reforming zone reacts with the hydrogen produced in the steam-reforming reaction. sense B will decrease the molar ratio H 2 / CO (generally too high, from a value of 4.5 to 6) to a value of the order of 2 to 3 (preferably 2.2 to 2.8) of so that

  During this first-stage reaction (steam-reforming),

  a little C 02 but essentially a mixture of CO and hydrogen in a molar ratio of 2 to 3 (preferably 2.2 to 2.8) ideal to be

  used as synthesis gas of alcohols during the second stage.

  in the overall balance of the two reactions (steam-reforming and synthesis of alcohols), the C 02 present not being eliminated but returned to the first stage, does not cause loss of carbon In the coupling of the two reactions, all the carbon involved is converted into alcohols Finally, the light hydrocarbons, reaction by-products which can be formed simultaneously with the alcohols in the synthesis reactor are thus advantageously recycled together with the C 02, to the reactor of These light hydrocarbons also comprise methane not steam reformed in section (14) of Scheme 1. Scheme 1 illustrates the process according to the invention. The feedstock (natural gas, for example methane or at least one other light hydrocarbon or a mixture such gases) is sent through lines 1 and 5 in the steam reforming zone 14. The steam introduction pipe and the condensate recovery pipe are not indicated. Supplemental carbon dioxide, derived at least in part from the natural gas pool, is introduced through lines 2 and 26 of carbon dioxide

  recycling and hydrocarbons reaction by-products are introduced.

  by the pipe 3 Possibly, by the pipe 4, arrives

  carbon dioxide recovered at the same time from the vaporefor-

  mage The steam reforming zone operates between / 3 and 890 ° C, under a pressure of between 1 and 4 Megapascals and with a flow rate of 2000 to 10,000 volumes of gas (normal temperature and pressure) per volume

of catalyst and per hour.

2 L 545 O O

  The effluent from the steam reforming zone, drawn off by the conduction

  6, is sent by lines 7 and 8 in a compressor 19 Even-

  part of the effluent runs through the pipes 15 and 16 through an area 18 in which any part of the carbon dioxide is recovered by any suitable means from the steam reforming effluent, which part is at least partially returned to the zone The effluent from the steam reforming zone, compressed to the required pressure used in the alcohol synthesis zone (pressure generally between 5 and 20 Megapascals), enters the pipe 17 and 9 in the catalytic synthesis zone 20 of alcohols operating between 200 and 4000 C, and preferably between 250 and 3500 C. The effluent of the zone 20, withdrawn via the pipe 10 is separated in the zone 21 in a part of a liquid phase containing water and alcohols, this liquid phase being withdrawn through line 12, and secondly a gaseous phase containing unconverted gases, light hydrocarbons and C 02, this gaseous phase being drawn off

  22 At least part of this gaseous phase, possibly

  the temperature prevailing in zone 14 of vapo-

  Reforming is recycled in this zone 14 by the pipe 3 Generally, 2% to 15% by volume and preferably 2.5% to 10% by volume are recycled.

  total volume of the gas phase of the pipe 22.

  Preferably it may also be wise to recycle at least a portion of the gaseous phase of the pipe 22, to the zone 20 for synthesizing alcohols, via the pipe 11, the compressor 24 and the pipe Finally, part of the gaseous phase can also be eliminated (purge)

by driving 13.

  The synthesis of alcohols is carried out at a pressure of between 5 and 25 Megapascals, (M Pa) and preferentially between 6 and 20 M Pa;

  the reaction temperature is between 200 and 4000 C and preferably

  usually between 250 and 3500 C; the hourly volumetric velocity (expressed in volumes T P N of gaseous mixture per volume of catalyst and per hour) is usually between 1500 and 60 000 h 1 and

  preferably between 2000 and 20 000 h -1.

  Many types of catalysts have been described for this reaction, in particular in the patents mentioned above they are suitable

  perfectly for the implementation of the present invention, in particular

  The higher homologous alcohols obtained in the present process are, for example, ethanol, propanol, butanol, pentanol, hexanol, heptanol, and the like.

Example.

  The steam-reforming of methane (zone 14) is carried out at a temperature of

  temperature of 880 C, operating pressure of 2 Megapascals, a water / methane ratio of 2.72 (by volume), an hourly volumetric velocity of 5000 h-1 (normal temperature and pressure) The steam reforming reactor

  contains 50 tonnes of nickel-alumina-calcium-potassium catalyst (approx.

  by weight: 20% Ni O, 5% potassium hydroxide, 60% alumina and 15% lime

Ca (OH) 2).

  There is no purge (absence of the pipe 13) Part of the gas phase of the pipe 22 is recycled to the first stage and another part to the second stage A recycle of C 02 is

performed by driving 4.

  The alcohol synthesis reaction section (zone 20) comprises two reactors arranged in series, each of them comprising several catalyst beds, with intermediate cooling. The catalyst prepared according to the teaching of the French patent application of the applicant NE 82 / 05 368, corresponds to the formula (in atoms) Cuo, 8 Co 0.35 To 11.1 Zno 5 Lao 1 Nao 2 There are 6 tons of catalyst per reactor;

  The operating pressure is 13 Megapascals and the temperature 300 C.

  Molar H 2 / CO ratio at the entrance of zone 14 (ie in line 5): 1.81 Molar H 2 / CO ratio at the entrance of zone 20 (ie say in driving 9): 1.97

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  9. Percentage of gaseous phase recycled in line 3 to total gas phase of line 22 3.25% Percentage of CO recycled in line 3 to CO 2 total of line 22: 3.25 % Percentage of CO 2 recycled in line 3 to CO 2 total of line 5 (steam reforming input): 14.42% The following table shows the mileage budget for the process; the water vapor consumed partly in Section 14 has not been indicated. N (El) W) (il) (01) (6) (8) W (9) (9) WWW (1) LAW qE z OIi soil qz elq 9 CD lx ') C> Ln% r 1.ni c 9.11 zz LIS Gold 6 Z 6 f, 19 OISEI

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  SIIE 9 191 you Viszi t E Zf'16 ú 1 OZ 61 Ozlo

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(DH) 3

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(OH) 30

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Claims (11)

  A process for producing at least one alcohol selected from the group consisting of methanol and higher homologous alcohols, the process of performing in two separate zones (14) and (20) a) a first steam reforming step performed in the zone (14) between 650 and 9500 C, during which water and at least one light hydrocarbon containing 1 to 4 carbon atoms are reacted together to produce a mixture of carbon monoxide and carbon monoxide; hydrogen according to reaction (1) (written below for the treatment methane). CH 4 + H 20 CO + 3H 2 (1)   This reaction (1) is accompanied by equilibrium (2) A CO + H CO + H (2) 2 2 2 (2   the reaction (1) and the equilibrium (2) thus together give a mixture of carbon monoxide with carbon dioxide and hydrogen, that is to say a synthesis gas, b) a second catalytic synthesis step at least one alcohol, produced in the zone 20, and during which, from the synthesis gas produced in the first step, running in the pipe (9), at least said alcohol is produced, the process being characterized in that at least a portion of the carbon dioxide and light hydrocarbons withdrawn (via the pipe (22 ") of the zone o is carried out in the second stage (b), is recycled at a temperature of between 650 and 9500 C by the line (3) in the zone o is carried out the first step, so as to move the balance (2) in the direction (B) to obtain in the zone o is carried out the first step, a report molar H 2 / CO between about 2 and 3, the use of this ratio H 2 / CO to work at the entrance of the zone in which the second   step with an optimum molar ratio H 2 / CO of between 1.6 and 2.4.   2. The method of claim 1 wherein the effluent of the zone o is carried out the second step is treated so as to collect firstly a liquid phase containing essentially the alcohol or alcohols produced and water, and on the other hand a gaseous phase containing generally carbon dioxide, the majority of unconverted gases, light and hydrocarbons, at least a part of this gaseous phase being   recycled to the area where the first step takes place.   3. Process according to claim 2, in which at least part of said gaseous phase is recycled in the o-zone. the second step.   4. Method according to one of claims 2 and 3 wherein furthermore   a portion of said gas phase is removed.   Process according to one of Claims 2 to 4, in which the volume   of said gaseous phase recycled in the first stage is between   2% and 15% relative to the total volume of said gas phase.   6. The method of claim 5 wherein said volume is between 2.5% and 10% relative to the total volume of said gas phase.   7. Method according to one of claims 1 to 6 wherein the first   step is carried out between 730 and 8900 C, the C 02 (or the CO 2 -containing gas) recycled to the first stage zone having been preheated between 730 and 8900 C.   The method according to one of claims 1 to 7 wherein the second   step is carried out between 250 and 3500 C, at a total pressure included between 6 and 20 Megapascals.   9. Method according to one of claims 1 to 8 wherein, between   two zones (14) and (20) in which the first and second steps are carried out, the line (15) is used to take a sample of carbon dioxide, which is recycled at least in part by the line (4) in the zone o takes place the said first step.   10. Method according to one of claims 1 to 9 wherein the said   H 2 / CO molar ratio obtained in the zone (14) where the first step is carried out is between approximately 2.0 and 3, the molar H 2 / CO ratio at the inlet of the zone (20) in which is carried out the second   step being between about 1.6 and 2.4.   11. The method of claim 10, wherein said molar ratio H 2 / CO obtained in zone (14) o is carried out the first step is between about 2.2 and 2.8 molar ratio H 2 / CO at the entrance to the zone (20), in which the second   step, ranging from about 1.8 to 2.2.