US3456032A

US3456032A - Utilization of propane recovered from liquefied natural gas

Info

Publication number: US3456032A
Authority: US
Inventors: Ludwig Kniel
Original assignee: Lummus Co
Current assignee: CB&I Technology Inc
Priority date: 1963-10-14
Filing date: 1966-03-14
Publication date: 1969-07-15
Anticipated expiration: 1986-07-15

Description

United States Patent 3,456,032 UTILIZATION OF PROPANE RECOVERED FROM LIQUEFIED NATURAL GAS Ludwig Kniel, Scarsdale, N.Y., assignor to The Lummus Company, New York, N.Y., a corporation of Delaware Continuation-impart of application Ser. No. 316,048, Oct. 14, 1963. This application Mar. 14, 1966, Ser. No. 534,013

Int. Cl. 'C07c 11/04 US. Cl. 260683 Claims ABSTRACT OF THE DISCLOSURE A process for utilization of propane recovered from liquefied natural gas in which the cold potential of the liquefied natural gas is used to provide the refrigeration requirements of an ethylene plant. The liquefied natural gas is separated in a first fractionation zone into a methane stream and a bottom fraction of heavier hydrocarbons. The heavier hydrocarbon stream is separated into an ethane stream that is used in a cracking plant to produce ethylene and a propane stream that is blended with the OE gas stream of the ethane cracking step to provide a fuel gas of similar heating value to methane.

This application is a continuation-in-part of US. appli cation Ser. No. 316,048, filed Oct. 14, 1963, now abandoned, and entitled, Process.

The present invention relates generally to the utilization of propane recovered from liquefied natural gas and, more particularly, it relates to use of propane so recovered as a fuel in lieu of natural gas or methane. Ordinarily, propane can not be so utilized due to its high heat value. More particularly, the present invention involves a process wherein the cold potential of liquefied natural gas is used to satisfy the refrigeration needs of an ethylene plant, and the mixing of off-gases from the ethylene plant with propane, whereby the off-gas is upgraded and the resulting gas has the approximate heating value of methane. Ethane in the liquefied natural gas is used as the charge stock to the ethylene plant.

In the operation of transporting LNG by special tankers over long distances to supply foreign markets with gas, it is important to be able to effect economies wherever possible which will lower the price of the gas to the eventual consumer. In many instances, natural gas being transhipped is not pure methane, but also contains ethane, propane and heavier hydrocarbons as well. The heavier than methane components may be present in such large amounts as to raise the heating value of the gas to well above that of pure methane. In such cases, it is necessary to reform such heavier fractions in such a way as to make their heating value compatible with that of methane, if methane is to be the send-out gas. In some instances, it is even necessary to reform the methane for a lower heating value to make it compatible with the gas which is being distributed in an existing system.

In the process described in the above-identified copending application, substantial economies are effected by making use of the cold potential of the LNG. According to that application, ethane and propane are recovered from the LNG and converted to ethylene, while making use of the LNG cold potential to satisfy the requirements of the ethylene plant for refrigeration. Depending on the available cold potential and the amount of ethane and heavier fractions contained in the LNG, it may be profitable to introduce an extraneous charge stock such as naphtha into the ethylene plant, for the purpose of producing additional ethylene and simultaneously using up completely the available cold potential. The present in- 3,456,032 Patented July 15, 1969 ice vention is concerned with a modification of the process of said application such that it may be still more economic.

In considering the relative economies of ethane and propane derived from LNG as charge stock to an ethylene plant, relative costs should be noted. Due to the different yields of ethylene that can be obtained from ethane or propane as charge stocks, propane is about twice as expensive as a charge stock than is ethane, and the transport of propane specifically for this purpose in special tankers over long distances may not be a profitable venture. It is known that in many geographical areas, light naphtha may be had at a very low cost per pound which, after taking credit for byproudcts produced from naphtha while subjecting it to a pyrolytic treatment for the production of ethylene, will make the cost of naphtha charge stock per pound of ethylene very attractive. The availability of naphtha as a competitive charge stock puts a limit on the price that may be set for ethane and pro pane as charge stock for such purposes. Ethane may assure the operator of an LNG ocean transport route a reasonable profit while propane would not, when either stock were used as charge for an ethylene plant. Obviously in such instance it would be necessary to dispose of the propane as bottled gas or reform it and any heavier constituents if present to a heating value close to that of methane.

It is thus a general object of the present to obviate the need for the above-mentioned reforming step and provide an improved and even more economic regasification process.

Another object of the present invention is to dispose of propane in an economic manner.

It is another object of the present invention to recover ethane from LNG during regasification thereof and to utilize this gas as feedstock for an ethylene plant.

Yet another object of the present invention is to utilize the caloric potential of regasification of LNG to effect a separation of the ethylene-producing constituents from the LNG.

Still another object of the present invention is to provide a process wherein during regasification of LNG, ethane is recovered and converted to ethylene which may be used elsewhere; heavier than propane fractions may be used as feedstock or fuel for the ethylene plant, and also wherein the methane and hydrogen residue gas from the ethylene plant may be blended with the recovered propane to provide a gas which closely approximates the heating value of methane.

Various other objects and advantages of the invention will become clear from the following description of an embodiment thereof, and the novel features will be particularly pointed out in connection with the appended claims.

In accordance with the invention, liquefied natural gas is separated into a methane fraction, which goes directly into the gas distribution system, and ethane and heavier fractions over a demethanizer. In this process, the methane is vaporized and the cold potential available at the requisite temperature level is recovered in the manner set forth in the above identified copending application. Subsequently, the ethane is fractionated from the propane and sent to an ethylene plant. The ethane undergoes pyrolytic treatment and the resulting ethylene can be used, for example, for synthesis. Heavier fractions from the ethylene plant are used for fuel. Methane and hydrogen from the ethylene plant are blended with the propane to give a gas having a heating value close to that of methane.

Assuming, for example, that the liquefied natural gas landed at the distribution terminal from a foreign source contains 84.3 volume percent methane, 11.0 volume percent ethane and 4.7 volume percent propane, the following products will emerge from the ethylene plant per one hundred cubic feet of LNG:

Refrigeration for the ethylene plant is supplied by the cold potential recovered in the regasification of the LNG and superheating of the methane fraction.

If the available cold potential from the vaporizing and superheating functions noted above is in excess of the refrigeration needs of the ethylene plant, the charge stock to the ethylene plant would be supplemented by extraneous naphtha, rather than propane, to the extent of balancing out the available cold potential. If the available cold potential is insufiicient, refrigeration must of course be supplemented with other means.

The heavier fraction from the propane separation may also be used as a starting material for further synthesis operations.

The total fuel gas from the ethylene plant in the above example (i.e. hydrogen, carbon monoxide and methane), amounting to 11.78 volumes, is mixed with 4.7 volumes of propane, to produce 16.48 volumes of gas having about the same heating value as pure methane and a slightly lower density. If the ratio of propane to ethane in the LNG were higher than in the above example, not all of the propane could be blended off with hydrogen from the ethylene plant in this manner. Propane not so blended oif would have to be disposed of elsewhere, for example as bottled gas, except that in cases where extraneous naphtha is fed to the ethylene plant, a part of such excess propane may be blended off, with hydrogen and methane produced during the pyrolysis of such extraneous naphtha.

Fuel for the ethylene plant is provided in part by the 0.42 volume of hydrocarbons heavier than ethane, which predominately consist of propylene, butylenes and butadiene which may be supplemented by extraneous naphtha or, in locations where naphtha is not available, by methane from the send-out line or by methane boil-off from the LNG storage tank.

In summary, the process of the invention provides for LNG treatment by partial regasification, recovery of a C fraction for direct use after superheating, recovery of a C fraction for use as ethylene plant feedstock, where the cold potential of the regasification and superheating steps is used to supply refrigeration, and the use of a C fraction plus ethylene plant off-gas (CH H in a blend of about the same heating value as the C fraction. In this manner, the uneconomic pyrolytic treatment of propane is avoided completely. Excess ethylene plant off-gas is utilized for fuel, and excess propane can be either sold as bottle gas or compensated for by feeding naphtha to the ethylene plant.

A better understanding of the invention will be gained by referring to the accompanying drawing, which is a simplified, schematic, flow sheet or flow diagram illustrating an embodiment of the invention.

With reference to the drawing, liquefied natural gas from a storage tank (not shown) is passed in line 2 and at least partially vaporized in element 4 which furnishes refrigeration to the ethylene plant, for example by a suitable heat exchange medium in

lines

6 and 8. Preferably, however, the cold potential of the LNG is employed in the ethylene plant in the manner described in the aboveidentified copending application, wherein heat exchange with an ethylene plant process stream is used rather than an indirect heat exchange medium.

The partially vaporized LNG is then passed through line 10 into demethanizer 12. A substantially pure methane fraction is removed as overhead in line 14, is passed through unit 4' Where it is superheated to distribution temperature, supplying additional cold potential to the ethylene plant, via a heat exchange medium in lines 6 and 8'. The methane is then passed into the distribution system.

Ethane and heavier hydrocarbons are removed from demethanizer 12 as a bottoms product in line 16 and passed into the deethanizer 18, wherein an ethane fraction is removed in line 22 and passed directly to ethylene plant 24. Ethylene is removed from ethylene plant 24 in line 26, for separate usage. Hydrogen, carbon monoxide and any methane produced in ethylene plant 24 are removed via line 28. Under the appropriate conditions, gases in line 28 are combined with the propane and heavier bottoms product from the deethanizer 18 in line 20, and the combined stream, having approximately the same heating value as methane but of slightly less density, is blended with the methane and passed to the distribution system.

Depending on the content of propane and heavier hydrocarbons in the LNG, a portion or substantially all of the propane and heavier fraction removed in line 20 may be diverted in line 34 and passed into depropanizer 36, wherein the propane is separated and passed via line 38 back to line 20, and the heavier fraction is passed in line 40, back to the ethylene plant. Again depending on the refrigeration requirements of the ethylene plant, extraneous naphtha in line 30 may be fed thereto; the object being of course, to ultize the entire cold potential of the LNG.

When extraneous naphtha is employed in the ethylene plant, byproducts are removed therefrom via line 32.

Excess propane (i.e. more than can be properly blended with the gases in line 28), may be bled off from line 38' for other usage.

It will be understood that various changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. In the claims, it is to be understood that the expression heat exchange medium refers to either heat exchange with a separate fluid or with an ethylene plant process stream.

What is claimed is:

1. A method for regasifying liquefied natural gas to utilize the caloric potential thereof that comprises:

(a) partially vaporizing said liquefied natural gas and recovering the cold potential thereof with a heat exchange medium;

(b) introducing said partially vaporized natural gas into a demethanizing zone and separating therein a gaseous overhead fraction containing predominantly methane and a liquid bottoms fraction containing ethane and heavier components;

(c) superheating said gaseous methane fraction to distribution temperature and recovering the cold potential thereof with a heat exchange medium and withdrawing said fraction as product;

(d) introducing said liquid bottoms fraction into a deethanizing zone and separating therein a gaseous ethane fraction as overhead and a bottoms fraction containing propane and any heavier components;

(e) introducing said ethane fraction as feedstock into an ethylene plan;

(f) supplying the refrigeration requirements of said ethylene plant with the cold potential recovered in steps (a) and (c);

(g) withdrawing as separate products from said ethylene plant an ethylene fraction and a lighter gas fraction containing substantially hydrogen, carbon monoxide, methane and the like; and

(h) blending said lighter gas fraction from step (g) with said propane fraction of step (d) to produce a gas having approximately the same heating value as the gas Withdrawn in step (c).

2. The method as claimed in claim 1, and additionally comprising introducing said bottoms fraction from step (d) into a depropanizing zone and separating therein a propane fraction as overhead and a heavier than propane fraction as bottoms product, and utilizing said propane overhead for blending in step (h).

3. The process as claimed in claim 2, and additionally comprising introducing said heavier than propane fraction to said ethylene plant as additional feedstock,

4. The process as claimed in claim 1, and additionally comprising cracking naphtha in said ethylene plant in addition to said ethane, whereby the cold potential of said vaporizing natural gas and said superheating methane is fully utilized, and additional ethylene is produced.

5. A method for regasifying liquefied natural gas containing a major proportion of methane and minor proportions of ethane and propane that comprises:

(b) separating a methane fraction from said vaporized gas in a first separation zone and superheating the said fraction with a heat exchange medium;

(c) separating an ethane fraction from the demethanized product of step (b);

(d) passing said ethane fraction to an ethylene plant;

(e) supplying the refrigeration requirements of said ethylene plant with the cold potential and superheat from steps (a) and (b);

(f) withdrawing said superheated methane as a product;

(g) withdrawing from said ethylene plant ethylene as one product and lighter gases including hydrogen, carbon monoxide and methane as a second product;

and (h) blending said lighter gas from step (g) with the deethanized gas remaining after step (c) to produce a gas having approximately the same heating value as methane.

6. The method as claimed in claim 6, and additionally comprising cracking an ethylene-producing hydrocarbon in said ethylene plant so as to produce additional ethylene and fully utilize the cold potential of said vaporizing natural gas and superheating methane.

7. The method as claimed in claim 6, and additionally comprising separating a propane fraction from the deethanized product of step (c), utilizing said propane fraction for blending in step (h), and utilizing the depro panized product as at least a portion of said ethyleneproducing hydrocarbon in said ethylene plant.

8. In a method for regasifying liquefied natural gas containing some ethane and propane, such method also including separating and utilizing said ethane as feedstock for a plant producing ethylene and as by-product, a hydrogen, carbon monoxide and methane-bearing off-gas, the improvements comprising separating propane from said liquefied natural gas and blending it with said oilgas, said blending being controlled to produce a mixture having the approximate heating value of methane.

9. The method as claimed in claim 8, and additionally comprising utilizing the cold potential of said liquefied natural gas to supply the refrigeration requirements of said ethylene plant.

10. The method as claimed in claim 8, and additionally comprising separating a heavier-than-propane fraction from said off-gas and utilizing it as additional feedstock to said ethylene plant.

References Cited UNITED STATES PATENTS 2,340,778 2/1944 Steward et al. 260683 2,498,806 2/1950 Hachm-uth 260-683 2,952,984 9/1960 Marshall 6227 2,672,489 3/ 1954 Holland 260--683 DELBERT E. GANTZ, Primary Examiner C. E. SPRESSER, Assistant Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,456,032 July 15, 1969 Ludwig Kniel It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 3, claim reference numeral "6" should read Signed and sealed this 28th day of April 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents