US3479298A - Production of hydrogen - Google Patents

Description

Nov. 18, 1969 M. c. SZE ET AL PRODUCTION OF HYDROGEN Filed Aug. 4., 1964 United States Patent 3,479,298 PRODUCTION OF HYDROGEN Morgan C. Sze, Garden City, N.Y., and Donald B. Stewart, Towaco, N.J., assignors to The Lummus Company, New York, N.Y., a corporation of Delaware Filed Aug. 4, 1964, Ser. No. 387,292 Int. Cl. C01]: N16

US. ,Cl. 252373 8 Claims ABSTRACT OF THE DISCLOSURE Process for producing a hydrogen-containing gas from a light hydrocarbon, such as natural gas, wherein a natural gas-steam mixture (mole ratio of steam to natural gas of 3.03.8:1) is reacted in a primary reformer, maintained at a ressure above 400 p.s.i.g. and a temperature above 1450 F to partially convert the natural gas. The effiuent from the primary reformer is mixed with a mixture of steam and a free oxygen-containing gas, preheated to a temperature of 1000-l500 F., in a secondary reformer to produce a hydrogen rich efiiuent, containing less than 0.5% natural gas.

This invention relates to the reforming of hydrocarbons, and more particularly to the steam reforming of a hydrocarbon to produce a gas rich in hydrogen, which is suitable for further treatment to form ammonia, methanol, etc.

Hydrocarbon feeds generally utilized include methane, ethane, propane, butanes and/or pentanes. In most prior art processes reforming was conducted at low pressures, i.e., 15 to 50 p.s.i.g., and at temperatures of fro-m 1225 to 1450 F. Under such conditions steam was added to the feed to provide a steam to carbon ratio of from about 2:1 to 4:1 expressed as moles of steam to atoms of carbon. If the synthesis gas was to be used for the production of ammonia, air is introduced at some point in the process to provide a gas, which, after various treatments, contain approximately three moles of hydrogen for every mole of nitrogen.

Recent developments, particularly in the metallurgical and catalyst arts have permitted the operation of catalytic steam reformer units at increasingly higher pressures. At such higher pressures, in order to obtain the necessary methane decomposition at reasonable tubewall temperatures, it is necessary to operate the reformers at higher steam to carbon ratio, as for instance set forth in US. Patent No. 3,081,268 wherein ratios of from 4:1 to 8:1 are described. However, operating at such higher steam to carbon ratios increases the duty of the radiant heating section of the reformer, thus requiring more fuel to be fired. Additionally, at such higher pressures, methane decomposition, for instance, in the primary reformer is reduced thereby requiring the secondary reformer to perform an increasing amount of methane decomposition.

It is a primary object of our invention to provide an improved process for the reforming of hydrocarbons to form gaseous streams rich in hydrogen and carbon monoxide.

A further object of our invention is to provide an improved process for the steam reforming of hydrocarbons utilizing a process having lower fuel requirements.

Another object of our invention is to provide an improved process for the steam reforming of hydrocarbons at high pressures at steam and carbon ratios of from 30:10 to 3.8:10.

Still another object of our invention is to provide an improved process for the steam reforming of hydrocarbons at high pressures whereby the duty of the radiant heating section of the primary reformer is actually reduced by the lower steam to carbon ratios.

3,479,298 Patented Nov. 18, 1969 ICC A still further object of our invention is to provide an improved process for the steam reforming of hydrocarbons at high pressures and lower steam to carbon ratios whereby the methane decomposition in the secondary decomposer is easily improved.

Yet another object of our invention is to provide an improved process for the steam reforming of hydrocarbons whereby wider flexibility of operation is achieved with greater range of feedstocks.

A further object of our invention is to provide an improved process for the steam reforming of hydrocarbons whereby deposition of carbon on the catalyst in the primary reformer is substantially eliminated when the hydrocarbon feed contains appreciable amounts of olefins.

Further objects of our invention will become apparent from the following description when taken in conjunction with the accompanying drawing illustrating a schematic flow diagram of our improved process.

In accordance with our invention, the hydrocarbon feed such as methane, ethane, propane or other light hydrocarbons are mixed with steam at a pressure of from 400 p.s.i.g. or greater, and preheated in the convection section of a primary reformer to a temperature of above about 800 F. The molal ratio of steam to atoms of carbons in the hydrocarbon feed stream is in the range of from 3.0 to 3.8. The preheated gas stream is then passed through the catalyst tubes in the reformer wherein the tubes are heated by the combustion of a fuel gas. The primary re former eflluent is withdrawn at an outlet temperature of about 1450 F. or higher. At this pressure and temperature, the primary reformer effluent is passed to a secondary reformer.

A mixture of steam and air, also preheated in the convection section of the primary reformer is introduced into the secondary reformer at a temperature in the range of 1000 to 1500 F. and mixed with the primary reformer effluent to complete the decomposition of the methane. The process gas leaving the secondary reformer is a raw ammonia synthesis gas containing hydrogen and carbon monoxide at approximately three times the nitrogen content, and with a methane content of 0.5 mole percent or lower. In the event that the hydrocarbon feed contains some olefins, some air is added to the steam admixed with the hydrocarbon feed to the primary reformer so that the oxygen content of the feed to the catalyst tubes is less than about 1.0 mole percent. By introducing air into the steam admixed with the hydrocarbon feed, deposition of carbon on the catalyst is essentially eliminated thereby permitting wider flexibility of operation with a greater range of feedstocks.

If a mixture of steam and oxygen is introduced into the secondary reformer, then the process gas leaving the secondary reformer is a gas suitable for further treatment to yield methanol or high purity hydrogen.

Referring now to the drawing, a hydrocarbon feed, in line 10, such as methane, ethane, propane or other lighter hydrocarbons, is admixed with steam in line 11 and passed to a primary reformer, generally indicated as 13 including a convection section 14 and a radiant heating section 15. Solely for the following description of the flow diagram, natural gas will be considered to be the feed to the process, however, this is in no way intended to limit the scope of the appended claims. The natural gas and steam at a pressure of about 400 p.s.i.g. to about 800 p.s.i.g. is preheated in the coil 16 positioned within the convection section 14 of reformer 13 and heated to a temperature of about 950 F. to about 1000 F. The molal ratio of steam to carbon atoms in the feed in line 12 is controlled so that such ratio is in the range of from 3.0 to 3.8.

As hereinabove mentioned, in the even that the hydrocarbon feed gas contains minor quantities of olefins, air

in line 17 is passed via line 18 under the control of valve 19, and admixed with the steam in line 11. Olefinic hydrocarbons exhibit a strong tendency to deposit carbonaceous matters on the catalyst contained in the tubes of the radiant section of the primary reformer 13. By using small amounts of air or oxygen in the feed to the radiant heating section deposition of carbon in the catalyst is substantially eliminated.

The preheated natural gas and steam mixture is withdrawn from the coil 16 and passed through line 20 to the catalyst tubes in parallel, generally indicated as 21, vertically disposed within the radiant heat section 15 of the reformer 13. The catalyst tubes are heated by the combustion of a fuel in line 22 which is passed to burners, schematically illustrated as 23. 'In the radiant heating section 15 of the reformer 13, the preheated mixture of natural gas and steam is heated to a temperature whereby the mixture is withdrawn in line 24 from the radiant heating section 15 at a temperature of at least 1450 F. The upper temperature to which the gas mixture can be heated in the radiant heating section 21 is set by present metallurgical limitations. The gaseous effiuent in line 24 is thereafter introduced into a secondary reformer, generally indicated as 25.

Air in line 17 is admixed with steam in line 26 and passed through line 27 to a coil 28 also positioned in the convection section 14 of the primary reformer 13. The mixture of steam and air is preheated in the convection section 15 to a temperature in the range of from 1000 to 1400 F. The preheated mixture is withdrawn through line 29 and admixed with the gaseous mixture in line 24 in the upper portion of the secondary reformer 25. At the temperature and pressure of the gaseous streams in lines 24 and 29, further decomposition of residual quantities of methane in line 24 is effected during passage through the catalyst bed 30 of the secondary reformer 25. A gaseous effluent is withdrawn from the secondary reformer through line 31 and passed to subsequent processing units (not shown) for further treatment. The gaseous effluent in line 31 contains hydrogen and carbon monoxide at approximately three times the molar content of nitrogen in such gas stream. The gaseous stream in line 31 also contains less than about 0.5% methane.

By operating the primary reformer at pressures of above 400 p.s.i.g.; and at a steam to carbon ratio of from 3.0 to 3.8; and by using an air and steam mixture preheated in the primary reformer for introduction into the secondary reformer, the duty of the radiant heating section 15 of the primary reformer 13 is considerably reduced. As hereinbefore mentioned, at increasing pressures, methane decomposition is reduced in the primary reformer. Consequently, it is necessary to provide for a higher enthalpy of the feed streams to the secondary reformer to provide for the same overall methane decomposition as obtained in the reforming processes heretofore operated at lower pressures.

In known prior processes, all of the steam requirements were admixed with the hydrocarbon feed prior to passage through the catalyst tubes of the radiant heating section. With such processes, the total steam preheat duty was limited by the cracking temperature of the heaviest hydrocarbon constituent in the gas feed prior to contact of the hydrocarbon feed with the catalyst in the tubes of the radiant heating section. By preheating an air and steam mixture in the convection section of the primary reformer, the steam can be preheated to a much higher temperature. A more eflicient use is made of the fuel fired in the radiant section of the primary reformer.

Further advantages of our process may be had by reference to the following examples illustrating inter alia, the net fuel savings obtained by reforming a hydrocarbon in accordance with our process. In all of the following examples, 100 moles of natural gas (100% CH is introduce-d into the process. The temperature of the feed to the radiant heating section is set at 950 F. while the outlet temperature from the secondary reformer is maintained at 1700 F. The quantity of air which is passed through line 17 and admixed with the steam in line 26 to the subsequently introduced into the secondary reformer is maintained as to provide a ratio of hydrogen and carbon monoxide to nitrogen in the gaseous effiuent withdrawn from the secondary reformer of about 3.0.

EXAMPLE I Case No 1 2 3 Pressure (p.s.i.g.) 400 400 400 Steam to primary reformer (moles) 630 370 370 Steam to secondary reformer (moles) 0 262 262 Total steam added (moles) 630 632 632 Air to secondary reformer (moles) 142. 6 142. 6 142. 6 Temp. of air+steam to secondary F.) 1,000 1,000 1, 200 Gas at outlet secondary reformer:

0. S3 0. 83 0. 83 43. 9 43. 9 43. 9 292. 8 292. 9 292. 9 55. 2 55. 3 55. 3 111. 3 111. 3 111. 3 H O (moles) 536.9 537.9 537.9 Fired process duty in primary reformer (MM Btu/hr.) (radiant section) 9. 8. 580 8. 355 Net fuel saving, percent 6. 5 9. 0

There was a net fuel saving of 9% with the operation according to case 3 as compared to the operation of case 1 where all of the steam is passed together with the feed through the primary reformer prior to introduction to the secondary reformer.

EXAMPLE 11 Case No 1 2 3 Pressure (p.s.i.g.) 400 4.00 400 Steam to primary reformer (moles) 1,090 370 370 Steam to secondary reformer (moles) 0 722 722 Total steam added (moles) 1,092 1, 092 Air to secondary reformer (moles) 143. 5 143. 5 Temp. of air+steam to secondary F. 1,000 1, 000 1, 200 Gas at outlet secondary reformer:

0H4 (moles) 0. 18 0. 18 0.18 00 (moles) 31.3 31.3 31. 3 H2 (moles) 307. 7 307. 7 307. 7 C02 (moles) 68.6 68.6 68. 6 N2 (moles) 112.0 112.0 112.0 H1O (molest 983.4 983.9 983.9 Fired process duty in primary reformer (MM Btu/hr.) (radiant section) 12. 398 10.712 10. 493 Net fuel saving, percent 13. 6 15. 4

The above example illustrates the additional net fuel savings when introducing 60% more steam to the process as compared with Example I.

EXAMPLE III Case No.. 1 2 3 Pressure (p.s.i.g.) 600 600 600 Steam to primary reformer (moles) 630 370 370 Steam to secondary reformer (moles) 0 260 260 Total steam added (moles) 630 630 630 Air to secondary reformer (moles) 141. 3 141. 3 141. 3 Temp. 0f air+steam to secondary t F.) 1,000 1,000 1, 200 Gas at outlet secondary reformer:

CH4 (moles) 1. 77 1. 78 1. 78 00 (moles) 43. 3 43. 3 43. 3 H2 (moles). 290. 3 290. 2 290. 2 C 02 (moles). 55. 0 54. 9 54.9 N2 (moles) 110.2 110. 2 110.2 H2O (moles) 537. 6 536. 3 536. 3 Fired process duty in primary reformer (MM B.t.u./hr.) (Radiant section) 9.144 8. 528 8. 310 N et fuel saving, percent- 6. 7 9. 1

Example III illustrates improved net fuel savings when operating at higher pressures but at the same quantity of steam as set forth in Example I.

As hereinbefore set forth, should it be desirable to produce a gas suitable for the synthesis of methanol, oxygen instead of air is to be admixed with the steam in line 26. Consequently, the term free-oxygen containing gas as used in the claims is to be interpreted as air, oxygen, or oxygen in admixture with inert gases.

It will be understood that the embodiments of the invention set forth above are illustrative only and that certain changes may be made by those skilled in the art within the scope of the invention as defined in the claims appended hereto.

We claim:

1. A process for the steam reforming of light hydrocarbons which comprises:

(a) admixing said hydrocarbon feed and steam at a pressure above about 400 p.s.i.g. to provide a feed stream having a molal ratio of steam to carbon atoms of from 3.0:1 to 3.821;

(b) heating said feed stream to a temperature above about 1450 F. in the presence of a reforming catalyst in a primary reforming zone to form a gaseous effiuent including hydrogen;

(c) preheating a mixture of steam and a free oxyencontaining gas to a temperature of about 1000 to about 1500' F.;

(d) mixing the gaseous effluent of step (b) without further heating, with an amount of the mixture o step (c) to provide an efiluent in step (f) containing no greater than about 0.5% of light hydrocarbons;

(e) passing the resulting mixture of step ((1) through a secondary reforming zone; and

(f) withdrawing a gaseous efiluent from said secondary reforming zone which is rich in hydrogen and contains no greater than about 0.5% of light hydrocarbons.

2. The process defined in claim 1 wherein air is admixed with the steam of step (a) to provide an oxygen content of the stream passing through the catalyst of less than 1.0 mole percent.

3. The process defined in claim 1 wherein the free oxygen-containing gas employed in step (c) is air and the air is present in the mixture in an amount to provide an effluent in step (f) containing about three moles of hydrogen and carbon monoxide per mole of nitrogen.

4. A process for reforming a light hydrocarbon which comprises:

(a) forming a feed mixture of said light hydrocarbon and steam at a pressure above about 400 p.s.i.g. and having a molal ratio of steam to carbon atoms of from 3.011 to 3.821;

(b) preheating said feed mixture in the convection zone of a primary reformer to a temperature above about 800 F.;

(c) heating said preheated feed mixture in a radiant heating zone of said primary reformer and in the presence of a reforming catalyst;

(d) withdrawing a gaseous effluent including hydrogen from said radiant heating zone at a temperature above about 1450 F.

(e) preheating a mixture of a free-oxygen containing gas and steam in said convection section of said primary reformer to a temperature between about 1000 F. and about 1500 F.;

(f) mixing in a secondary reforming zone the effiuent of step (d), without further heating thereof, with an amount of the mixture of step (e) to provide a secondary reforming zone efiluent containing no greater than about 0.5% of light hydrocarbons; and

(g) withdrawing a hydrogen-enriched gas from said secondary reforming zone containing no greater than about 0.5% of light hydrocarbons.

' 5. The process defined in claim 4 wherein the freeoxygen containing gas is air.

References Cited UNITED STATES PATENTS 1,957,743 5/1934 Wietzel et al 48-l97 2,700,598 l/1955 Odell 48l96 3,264,066 8/1966 Iuartulli et a1 48197 X 3,278,452 10/1966 Vorum 48197 X JOSEPH SCOVRONEK, Primary Examiner US. Cl. X.R.

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