Production of combustible gas

Aug. 17, 1965

J. S. NEGRA ETAL

PRODUCTION OP COMBUSTIBLE GAS Filed June 7, 1963

3,201,215

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JOHN S. NEGRA

ARNOLD R. BERNAS

INVENTORS.

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AGENT

United States Patent Office _F, _[aA³^Y’⁵,

Patented Aug. 17, 1965 ¹ · · ' ' ' ' ' 2 ' '' ·

3,201,215 PRODUCTION OF COMBUSTIBLE GAS

John S. Negra, South Plainfield, and Arnold R. Bernas, ^: Nixon, N.J., assignor to Chemical Construction Corporation, New York, N.Y., a corporation of Delaware Filed June 7, 1963, Ser. No. 286,296

Claims. (Cl. 48—215)

This invention relates to the gasification of naphtha with steam and. air, to produce a. stable combustible gas steam which may be further processed to a finished town gas with low content of illuminates such as ethylene. Naphtha is gasified according to the process of the present invention without the concomitant formation of free carbon, by the suitable selection of several· inter-related process variables. The reaction of naphtha with steam and . asr is controlled so as, to produce a crude gas steam containing a substantial proportion of lower hydrocarbons as well as carbon monoxide and hydrogen. Unsaturated low . hydrocarbons are hydrogenated in a preferred embodiment, so as to produce a stable town gas.

Naphtha is a relatively volatile petroleum refining product or intermediate, which is generally defined in terms of boiling range. Thus, according to a crude oil survey - in Industrial and Engineering Chemistry, 44, #11 (Nov.

195.:). p. 2578. naphtha is defined as follows: “Naphtha content (of crude oil) is the total distillate recovered in the U.S. Bureau of Mines routine analysis at a vapor temperature- of o92 F.” A more detailed definition of naphtha appears in “Petroleum Refining with Chemicals by Kalichevsky and Kobe (1956). A discussion of naphtha on p. 21-23 of this text indicates that different naphthas may have boiling ranges from a low point of 122° F., to a maximum of 400’ F. Thus “naphtha” is defined as a general term which is applied to fractions boiling in the .gasoline of low kerosene range. In general, then, naphtha is a low-boiling and readily volatilized liquid hydrocarbon cut, derived from crude oil distillation in .petroleum refining. This material consists mostly of straight-chain paraffinics in C-5 to C-9 range, however, up to.about 30% naphthenics together with up to 10% aromatics and unsaturates may also be present. In addition, naphtha also generally contains a significant proportion of sulfur in. the form of COS and mercaptans.

Naphtha may be utilized in a variety of ways. Thus, crude naphtha may be further refined and upgraded to yield a. variety of finished petroleum solvents. In many refineries, naphtha is “reformed” in the petroleum sense of the term. In this case, the crude naphtha is cracked, and hydrocarbon: molecules are re-assembled-in the presence of platinum or other suitable catalyst, so as to yield a substantial proportion of branched chain or aromatics molecules. This material is then blended with other refinery cuts for gasoline usage.

Naphtha has also been utilized in the prior art as a hydrocarbon raw material for the manufacture of a combustible gas, such as town gas. An improved process tor the gasification of naphtha to produce a combustible . town gas in provided in the present invention. Town gas is generally denned as a stable gas stream free of gums lorrtars and containing saturated lower hydrocarbons i hydrogen,, carbon monoxide and inert ballast gas com.ppnent, with a heating valve in the range of 400 to 1000 B.t.u./ft.³ and a specific gravity (air=l) of 05±0 05 A typical prior art procedure for producing a combustible gas such as town gas from naphtha is described in British Patent No. 923,385. In this process, naphtha or “light distillate” is reformed with steam, in the presence of a specific catalyst composition. It is claimed that rihe formation of free carbon, tars and gums is prevented. :A cyclic process for the production of fuel gas from high- sulfur naphtha is described in U.S. Patent No. 2,711,419. Preheated naphtha vapor, air and steam are reacted in the presence ot a nickel-type catalyst. Accumulated carbon and sulfur deposited on the catalyst are removed by a pe5 riodic burning-off period. Numerous other prior art procedures for the catalytic or non-catalytic gasification of naphtha have been proposed in the prior art. Generally speaking, the formation of free carbon, tars and gums remains as a problem, principally because of the diffi10 culty of cracking-the naphtha to form lower hydrocarbons without the concomitant formation of these undesirable side-products.

In the present invention, naphtha is gasified with steam and air at elevated temperature, to produce a crude com15 bustible gas. After further treatment such as hydrogenation of unsaturates, a finished town gas is produced. A limited amount of process air is employed in the gasification stage of the present invention, to assist in the gasification of the naphtha and to provide a nitrogen gas com20 ponent for ballast in the finished town gas. The process of the present invention depends on a unique balance of reaction.conditions to achieve the gasification of naphtha, since this is accomplished without accumulated deposition of free carbon. In addition, no significant amount 25 of unreacted hydrocarbon is present in the final syn thesis gas.. . The basic process is carried out in a gasification-conditioning stage followed by a quench step. It has been found that the gasification stage must be of short duration in order to prevent the reactants from reaching 30 equilibrium with resultant deposition of free carbon in _: the catalyst bed. It has also been found that the reactants must be preheated in order -to provide a minimum temperature level in the gasification stage. Thus, in the present invention, process streams of naphtha, steam 35 and air are preheated and mixed. A partial reaction en sues, and the mixed process stream, now containing a variety of intermediate components but not in final reaction equilibrium, is. passed to a quench'step. The resulting ciude. combustiole gas stream consists of a gas mixture 40 containing methane, ethylene, hydrogen, nitrogen, carbon monoxide and carbon dioxide. The stream is essentially fiee of uni eacted hydrocarbons or solid particulate carbon.

. The process of the present invention possesses several 4a significant advantages. A primary advantage is that naplitha is completely converted to a stable crude combustible gas without the concomitant accumulation of free carbon or tars. No recycle or said stream disposal is required. . The process is continuous rather than cyclic 50 or intermittent. The process is non-catalytic in the gasi; ; fication stage, however as will appear infra a nickel cat· a.yst may be provided if desired to increase the B.t.u. value of the final town gas.

it is an object of the present invention to produce a 55 combustible gas by the gasification of naphtha.

. Another object is. to completely gasify naphtha in a continuous process, without accumulated deposition of tars or free carbon.

_A .4· fr^lri’^1,er object is to react naphtha with steam and air 60 without reaching final reaction equilibrium, under conditions ^SL,ch that a stable intermediate stream containing lower hydrocarbons, carbon monoxide and hydrogen is produced.

An additional object is to produce a stable combustible os town gas by the. non-catalytic gasification of naphtha, another object is to simultaneously crack, partially oxidize, and non-catalytically steam reform naphtha by reaction with steam and air under process conditions such _₍₎ that free carbon is not formed.

These and other objects and advantages of the present invention will become evident from the description which :,201,215 follows. Referring to the figure, stream 1 is a liquid naphtha, derived from petroleum refining or other types of crude oil processing. Thus, as described supra, stream 1 consists principally of paraffinic hydrocarbons in the C-5 to C-9 range, together with naphthenics as well as minor amounts of aromatics and sulfur compounds. The liquid stream 1 is vaporized and preheated in heater 2, to form naphtha vapor stream 3. Vapor stream 3 may be produced at any suitable temperature, ranging from the boiling point of naphtha up to about 1000° F. Above this temperature level the naphtha vapor may become unstable, and certain portions or components will readily crack to smaller molecules with concomitant carbon deposition. Stream 3 thus is preferably produced at a temperature ranging from 400° F. to 800° F.

Stream 4 consists of highly superheated steam, preheated usually to a temperature above 1500° F. and preferably to the range of 1500° F. to 1700° F. Although lower ratios are feasible, it has been found that a range of molar steam/carbon ratios between 3 and 6 is desirable in proportioning the relative flow rates of streams 4 and 3, in order to assure the avoidance of formation or accumulated deposition of carbon under normal operating conditions.

Stream 5 consists of air, preheated usually to a temperature above 800° F. and preferably to the range of 800° F. to 1200° F. The proportion of air employed in the process is quite small, thus only enough air is used to provide the desired thermal effects of temperature elevation due to naphtha combustion. The streams 4 and 5 are preferably combined, to form a mixed steam-air stream 6 at a temperature of at least 1100° F. Stream 6 is now combined with naphtha vapor stream 3, and the mixed stream 7 is immediately passed into residence or gasification chamber 8. It will be appreciated that streams 3, 4 and 5 may be separately passed into chamber 8, however premixing of the air and steam to form stream 6 is a preferable procedure since this results in better and more rapid mixing of the several streams. Thus, stream 3 is more rapidly dispersed and diluted due to the mixing with stream 6, prior to entry of the naphtha vapor into residence chamber 8. Consequently, the possibility of transient carbon formation or deposition due to cracking of the naphtha is reduced by the pre-mixing step.

In chamber 8, simultaneous reactions take place between and among the several reactants and intermediate components. The temperature of the process stream immediately rises, due to exothermic combustion of a portion of the naphtha with the oxygen content of the air. In addition, a portion of the naphtha is non-catalytically steam reformed due to the high temperature and high steam concentration in chamber 8. This endothermic reaction serves to produce free hydrogen, and also moderates the temperature rise due to combustion. A further portion of the naphtha is thermally cracked to lower hydrocarbons, due also to the high temperature level. However, simultaneous deposition and accumulation of free carbon does not instantaneously occur. Instead, the unstable lower hydrocarbons are selectively hydrogenated to a certain extent due to the in situ formation of hydrogen, which may possibly be formed in the nascent state. Thus, the resultant gaseous stream 9 contains significant proportions of steam, nitrogen, hydrogen, carbon dioxide, carbon monoxide unsaturated hydrocarbons (mostly ethylene), methane and possibly ethane. It should be understood, however, that some of these components are present on a transient or instantaneous basis. If stream 9 is allowed to reach stable equilibrium under these process conditions, significant formation and accumulated deposition of free carbon will take place.

Under some conditions, the temperature in unit 8 may be in the range of 1450° F. to 1500° F. However, with such low reaction temperatures, the formation of free carbon may readily occur, especially after all the residual free oxygen is consumed, unless the residence time is kept in the range of 0.05 to 0.33 second. In general, the residence time in chamber 8 must be kept below 1.0 second, and preferably in the range of 0.05 to 0.33 second, in order to achieve the desired reactions without carbon formation. In addition, the instantaneous mix temperature of stream 7 must be kept above 1000° F. since it has been found in practice that the various competing reactions will tend to form free carbon if the initial mix temperature is below 1000° F. This initial or instantaneous mixture temperature should preferably be in the range of 1400° F. to 1700° F. in order to preclude carbon formation due to process upsets. It will be evident that chamber S may actually, in terms of apparatus design, consist merely of an insulated pipe section extending between the point of mixing of the reactant streams and the entry of the gasified process stream into the following quench.

In summary, the competing reactions of naphtha combustion, cracking and non-catalytic thermal steam reform are carried out in the first stage of the process of the present invention. It has been determined that, by maintenance of reaction conditions within certain critical ranges, these competing reactions may be carried out without carbon accumulation. In addition, the resulting unstable process stream, when passed to the following quench step before further reaction ensues, is successfully stabilized to yield a crude combustible gas mixture without carbon accumulation. The present invention essentially accomplishes the gasification of naphtha by a process using preheated air and steam. It has been determined that the resulting mixed gas stream may be successfully maintained as a stable crude combustible gas, if the mixed gas stream is quenched before final process equilibrium is reached. Thus, the critical features of the present invention essentially involve the maintenance of several inter-related process variables within operating limits in which the new result of the present invention is achieved, namely the continuous gasification of naphtha.

Stream 9 now passes into the quench unit TO, in which the gas stream is quenched to a temperature below 800° F., so as to terminate the reaction before final equilibrium is attained, and thereby prevent carbon formation. Quench liquid stream 11 is any suitable coolant liquid and may consist of a stable heavy hydrocarbon oil, however stream 11 will preferably consist of water. Warmed quench liquid is removed via 12. The cooled process gas stream is removed via 13 at a temperature below 800° F., and consists of a crude combustible gas typically containing hydrogen, carbon monoxide, ethylene, methane, carbon dioxide and nitrogen. Stream 13 is directly usable as a gaseous fuel in certain applications, however stream 13 is preferably further processed to saturate any unsaturated hydrocarbons such as. ethylene, which act as illuminants and thus are undesirable in town gas. In addition, it is generally desirable to subject the gas stream to the water gas shift reaction in which carbon monoxide is converted to carbon dioxide by reaction with steam, in order to increase the hydrogen content of the gas stream. Carbon dioxide is also usually removed from the gas stream, in order to increase the B.t.u. content by decreasing the ballast or inerts content.

Stream 13 is first passed into water gas shift converter 14, together with further steam which is supplied via 15 when required, to react with the carbon monoxide i content of stream 13. In some cases stream 15 may be¹omitted, if sufficient steam is already present in stream' 13. Converter. 14 is provided with at least one bed 16° of catalyst, usually consisting of active iron oxide plus alkali oxide promoter, deposited on a suitable carrier. Unit 14 may consist of the apparatus described in U.S. Patent No. 3,010,807, or other suitable apparatus. When the gas stream passes through converter 14, contact with catalyst bed 16 results in the water gas shift reaction between carbon monoxide and steam, yielding carbon dioxide and hydrogen. The resulting gas stream removed

3,201,215 s

via 17 has low or negligible carbon monoxide content, and is relatively high in content of free hydrogen.

Stream 17 is now passed through catalytic hydrogenation unit 18, which is provided with gauze layers 19 consisting. of a platinum group metal such as platinum or _gpalladium. As an alternative, unit 18 may be provided with, a catalyst bed consisting of platinum deposited on a suitable carrier. In any case, catalytic hydrogenation of unsaturates in the gas stream is carried out in unit 18. Thus in the case of ethylene, addition of hydrogen ,θ to the unsaturated double bond produces ethane.

The resulting process stream 20 withdrawn from unit 18 has a negligible content of unsaturates. Stream 20 is now passed through scrubber 21, where the gas stream is contacted with a liquid scrubbing agent such as hot aqueous potassium carbonate solution or aqueous monoethanolamine solution, for removal of carbon dioxide. The scrubbing unit 21 is provided with a packed section 22 for gas-liquid contact, alternatively bubble cap trays or grid trays may be provided for this purpose. The ₂₀scrubbing liquid solution is admitted via 23, and liquid solution containing absorbed carbon dioxide is removed via 24 for external regeneration.

The final gas stream of reduced carbon dioxide content is removed from unit 21 via 25, and may be subsequently 25 cooled or otherwise treated by means not shown for the removal of water vapor. In any case, a finished town gas is produced, containing principally ethane, methane, hydrogen and nitrogen. _; Nitrogen is a desirable component in finished town gas, since it serves as ballast in diluting _gothe gas stream to provide the proper B.t.u. value. Due to the hydrogenation in unit 18, the finished town gas is low in unsaturates, which are undesirable since these compounds act as illuminants during combustion and also have gum-forming tendencies. ₃As an alternative embodiment of the present invention, a suitable catalyst such as metallic nickel rings may be provided in unit 8, in order to promote the formation of hydrogen by catalytic steam reform; thus yielding a final gas product of higher B.t.u. value. 40

It has been found that operating pressure does not appear to be a significant variable in the process of the present invention. Although pressure is not critical, an operating pressure in the range of 1 to 22 atmospheres is preferable'since reform plant equipment size is re- 45 duced, arid also become subsequent compression costs are reduced. In addition, gasification at elevated pressure yields a high pressure process gas which thus may •range of 1450° F. to 1500° F. However, of a longer residence interval up to 1.0 second is required, then the initial streams 3, 4 and 5 must be preheated to higher levels so as to provide a temperature range of 1650° F. to 1690° F. in chamber 8, in order to prevent accumulated deposition of free carbon in actual operation of the process.

Similarly, it will be recognized that a minimum steam/carbon ratio of 1.5 is generally required, in order to satisfy material balance considerations by providing sufficient steam for complete reaction with the naphtha. However, a steam/carbon ratio in the range of 3 to 6 has been found to be optimum in providing complete reaction, satisfactory reaction rate, and minimum tendency for carbon formation due to process upsets. Higher proportions of process air will generally be required if minimum steam/carbon ratios are adopted, in order to prevent carbon deposition. In general, the molar air/carbon ratio will range between 0.1 and 1.0.

Following are examples of industrial application of the process of the present invention in the production of a stable combustible gas from naphtha.

. TABLE I

Production of crude combustible gas

Run No___________________________:_____ 1 2 3

Steam/Carbon Ratio—__________

Air/Carbon Ratio....——_________

Reaction Time, sec..... Pressure, p.s.i.g............. ....... .

Gasification Temp., ⁰ F.:

Inlet_..________

Outlet._______________

Exit Gas Analysis, percent Vol. (Dry .

Basis): ,: .:

Carbon Dioxide...____________....

Carbon Monoxide..

Ethylene_____.....................-.

Methane.— ______.................

Hydrogen. ___________——_____ Nitrogen. — — ________________

3.6 0.58 0.48 . 240

1, 700 1,500

9.9

8.5

5.8

20.5.

27.8.

27.5

4.4 ,0.59 ; 0« 37 .150

1,700 1,485

3.6 : 0.59

0.30 .100

1,700

1,500

9.6

12.5 . 7.2

16.0

31.1

23.6

Runs 1 and 2 were non-catalytic, while in run 3 a nickel type catalyst was provided in the gasification step. It is evident that a higher, content of free hydrogen was present in the exit gas from run 3, thus the B.t.u. value /was somewhat greater due to the catalytic effect. _:In the following data table, the conversion of the crude .combustible gas from run .3 to finished town gas is illustrated. This conversion is carried out in accordance with the various process steps described supra. .

TABLE II

Production of finished town gas

Gas Component

<td> Crude</td><td> After</td><td> After</td><td> After</td><td> Final</td>

<td> Combus-</td><td> Carbon</td><td> Hydro-</td><td> Carbon</td><td> Town</td>

<td> tible Gas</td><td> Monoxide</td><td> genator</td><td> Dioxide</td><td> Gas, Vol.</td>

<td></td><td> Converter</td><td></td><td> Scrubber</td><td> Percent</td>

Carbon Dioxide. _____——

Carbon Monoxide.—

Ethylene.. ___

Ethane/....——_ Methane.———,———— Hydrogen________________...

Nitrogen.————, be directly treated for carbori dioxide removal by hot ¹ potassium carbonate scrubbing.

It will be evident to those skilled in the art that the /significant process variables in the gasification of naphtha according to the present invention are closely interrelated. Thus, the required minimum preheat temperature of the reactant streams piror to chamber 8 vzill depend principally on the residence time in 8 prior to entry of the mixed stream via 9 into quench unit 10. With lower residence times in the range of 0.05 to 0.10 second, it has been found that the process may be successfully carried out with a residence chamber temperature in the:

The final town gas product had,a net heating value of 437 B.t.u./ft.³ and a specific gravity of 0.502.

Following is an analysis of the commercial naphtha employed in the above runs. It is evident that naphthas of varying compositions and .analyses,may be successfully reformed by suitable selection of process variables within the scope of the present invention.

TABLE III

Specification of tested naphtha

Initial boiling point (° F.)___________________ 120

End point (° F.).____________________________ 250

3,201,815

Specific gravity @ 60° F. 0.70

Composition (wt. percent):

Paraffins 57.4

Naphthenes 32.5

Aromatics 9.4

Olefins 0.7

Sulfur content (p.p.m.) 5-10

From the above analyses of the gas stream in Table I, certain conclusions may be reached with respect to probable reaction mechanism. Thus, since no oxygen is present, combustion of naphtha has already taken place to completion. Since some unsaturates as well as methane are present, it is evident that some thermal cracking of naphtha also took place, probably together with some hydrogenation of unstable free radicals and unsaturated carbon linkages. This thermal cracking thus was accomplished without carbon accumulation. Finally some free hydrogen is also present hence non-catalytic (thermal) steam reform of naphtha also took place.