METHOD OF PROCESSING GAS ASSOCIATED WITH OIL

Abstract

The invention relates to a method for producing combustible gas for gas engines from associated gas obtained during oil production, the associated gas containing methane, ethane, propane, hydrocarbons having more than three carbon atoms, and optionally propene, wherein a gaseous fraction and a liquid fraction are obtained by partially condensing the associated gas, wherein the condensation process is performed under such pressure and temperature conditions that the liquid phase is substantially free from methane, ethane, propane, and optionally propene, and that substantially the entire methane, ethane, propane, and optionally propene are contained in the gaseous phase.

Description

The invention concerns a method of producing combustible gas for gas engines from associated gas which is produced in crude oil production and which contains methane, ethane, propane, hydrocarbons having more than three carbon atoms and optionally propene, wherein a gaseous fraction and a liquid fraction are obtained by partially condensing the associated gas.

In crude oil production from crude oil deposits by means of crude oil production stations an associated gas is produced, which almost exclusively consists of hydrocarbons. This involves a mixture of different gaseous hydrocarbons, primarily alkanes and alkenes. That associated gas is frequently referred to as “associated petroleum gas” or APG, and earlier also as “flare gas”.

It will be noted however that this associated gas is unsuitable for feeding into a gas pipeline as, in comparison with natural gas which contains methane as its main component, it has a complex mixture of different hydrocarbons. Consumers which are optimized for given gas compositions and gas qualities cannot be optimally operated with such mixtures. Therefore that associated gas has long been of low significance for energy utilization. For many years it was burnt without being put to use, and the earlier term of “flare gas” is also derived from that consideration. That method is not only environmentally harmful but it also represents a waste of valuable resources.

A possible way of using the associated gas involves combustion in boilers operated with gas burners. It will be noted however that the thermal energy requirement at the oil production stations which are situated in the most exposed locations is generally not as great as there is associated gas. Direct utilization of the gas, by it being burnt in gas engines operated on the basis of the Otto cycle in order to produce power by means of a generator as a further consequence falls foul of the excessively low methane number of the associated gas. The methane number is a measurement in respect of anti-knock quality and an excessively low methane number means that the anti-knock quality of the associated gas is excessively low to convert it into power and heat with co-generating heat and power installations.

To make the associated gas useable for combustion in a gas engine using the Otto cycle a gas processing installation generally has to be disposed upstream of the gas engine. Different technologies are known for that purpose, inter alia the membrane technology or cooling the gas to extremely low temperatures. In that respect it is usual to produce almost natural gas quality, that is to say to increase it to a methane content of over 85%. Deposited higher-grade hydrocarbons can further be used as a valuable substance by cracking or by direct thermal utilization.

The membrane technology for processing associated gases represents a relatively high technical and financial involvement. The alternative, cooling the gas with the aim of providing that the higher-grade hydrocarbons condense out, is also very complicated and expensive, nonetheless requiring very low temperatures of the order of magnitude. Thus for example propane (C₃H₈) becomes liquid at −42 degrees Celsius, and ethane (C₂H₆) even becomes liquid only at −89 degrees Celsius. Such temperatures can be produced for example with turbo-expanders. That also requires a very high level of technical and financial implementation.

WO 2007/070198 A2 presents a method of processing associated gas in crude oil production. That method primarily involves producing a methane-rich gaseous phase and a liquid phase with a low methane content. In that respect different pressure and temperature conditions are referred to, wherein attention is primarily directed to very low temperatures and very high pressures so that this affords a gaseous phase containing almost exclusively methane and small traces of ethane. It will be noted however that the liquid phase produced in that case still has a high methane and ethane content, which can be deduced from the described method conditions and examples. That however is also understandable as the production of a liquid phase with a high useful energy value is in the forefront in WO 2007/070198 A2.

The methods applied hitherto, in which the associated gas was cooled to such low temperatures and pressures that almost pure gaseous methane and a condensate with the other hydrocarbons is produced, is extremely complicated and expensive in energy terms as cooling to the boiling point of ethane or ethene is required to achieve enrichment of methane in the gaseous phase. WO 2007/070198 A2 allows certain residues of hydrocarbon compounds with two carbon atoms in the gas. It will be noted however that the choice of the method conditions provides that the liquid phase has a very high methane content which is naturally at the expense of the quality of the gaseous phase obtained. The gaseous phase obtained is therefore suitable for implementation in a gas engine in view of the methane number but the complicated and expensive method conditions make operation of the gas engine uneconomical. The high-grade liquid phase obtained however still has to be transported away.

The object of the present invention is to improve the method of the kind set forth in the opening part of this specification such that the products obtained can be better employed in further utilization. In particular the invention seeks to provide that the gaseous fraction obtained can be burnt in a gas engine operated on the basis of the Otto cycle in order to produce power and heat by way of a co-generating heat and power installation. In that respect production of the gaseous fraction is to be as inexpensive as possible from the economic point of view by making use of the prevailing temperature and pressure conditions of the associated gas obtained in oil production.

In a method of the general kind set forth in the opening part of this specification that object is attained in that the condensation process is performed under such pressure and temperature conditions that the liquid phase is substantially free from methane, ethane, propane and optionally propene, and substantially the entire methane, ethane, propane and optionally propene is contained in the gaseous phase.

The basic idea of the invention, in the production of associated gas, is to transfer hydrocarbons with a high methane number into the gas phase and to condense out hydrocarbons with a low methane number. That provides a gaseous mixture which contains substantially all the methane, ethane, propane and possibly propene. In that way the methane number in the gaseous phase and thus also the anti-knock quality of the gas can be increased. The particular realization is that hydrocarbons with more than three carbon atoms enormously reduce the anti-knock quality while hydrocarbons with up to three carbon atoms can be excellently well used in a gas engine. In particular n-butane and isobutane are responsible for the anti-knock rating being reduced while a gas with the main constituents methane, ethane and propane (optionally also propene) has a methane number which is excellently well suited for implementation in gas engines, in particular also those with upstream-connected compressor devices. The aim therefore is to separate off n-butane and isobutane.

In contrast to the state of the art therefore a particularly high-grade gaseous phase can be obtained in that way, with the valuable constituents methane, ethane and propane, while the liquid phase contains the higher-grade hydrocarbons, wherein that phase is still excellently well suited for possible further utilization. From that point of view the liquid phase with a high methane content, as is obtained in accordance with WO 2007/070198 A2, is a waste when considered in energy terms as gas engines are markedly more sensitive than installations for combustion of the liquid phase produced. The gas obtained in accordance with WO 2007/070198 A2 is also not optimum in terms of energy yield as a large part of the methane is lost.

It is preferably provided in accordance with the invention that the condensation process is performed at a temperature of between −5 degrees Celsius and −14 degrees Celsius. Particularly preferably the temperature range is between −7 degrees Celsius and −14 degrees Celsius. As the boiling point of isobutane is −11.7 degrees Celsius, it is therefore desirable if the temperature is below −11.7 degrees Celsius. In many geographical areas of use of the method the prevailing ambient temperatures are in the region of those values so that expensive cooling can thereby either be entirely avoided or can be effected in correspondingly inexpensive fashion, thereby permitting economically particularly advantageous production of the gaseous fraction.

It is preferably provided that the condensation process is effected in a plurality of stages, wherein cooling is effected to a temperature of between −5 and −8 degrees Celsius in one stage and to a temperature of between −8 and −14 or −12 degrees Celsius in a further stage. In that way low-boiling hydrocarbons can be collected in a first fraction and higher-boiling hydrocarbons can be collected in a second liquid fraction. In that way however it is also possible for any water contained in the associated gas to be condensed in a first step.

It is advantageous under the above-selected temperature conditions if at the same time certain pressure conditions are set as temperature and pressure are responsible for the condensation characteristics of gases. It is preferably provided in that respect that the pressure in the condensation process is between 1 bar and 16 bars, preferably between 10 bars and 16 bars, particularly preferably between 14 bars and 16 bars. In many cases the associated gas produced in crude oil production issues at a pressure in the region of those values so that the choice of that range permits economically particularly advantageous production of the gaseous fraction. In addition the costs of pressure equipment for pressures to be generated of up to 16 bars are relatively low in comparison with pressure equipment in a higher pressure equipment category.

It is further preferably provided that the gaseous fraction has a methane number of at least 40, preferably at least 45. The methane number as a measurement of the anti-knock quality of the engine is important for the further purpose of use. In the present case the gaseous fraction obtained is in fact to be used in a gas engine so that there should be a methane number of at least 40, preferably at least 45.

In addition in a variant it can be provided that the gaseous phase is substantially free from n-butane and isobutane.

In a further variant it can be provided that water vapor which is possibly present in the associated gas is removed. In that respect it is possible to use for example absorption agents and molecular sieves like zeolites or known drying agents like inorganic salts. A composition of a possible associated gas from an oil production station will now be described by reference to an example in following Table 1:

TABLE 1
Composition of an associated gas (Example):
Compound                 Mol. %
CO2                      0.05
N2                       3.78
Methane CH4              36.67
Ethane                   15.29
Propane                  23.17
Isobutane                6.16
n-Butane                 9.73
Isopentane               2.08
n-Pentane                1.88
Hexane                   0.76
Cyclohexane              0.06
Heptanes                 0.18
Octanes                  0.14
Nonanes                  0.04
Decanes and higher-grade 0.01
hydrocarbons
Total                    100

The mixture described in the Table has a methane number of 32.7 and is not suitable for combustion in an Otto-cycle gas engine. After cooling to about −14 degrees Celsius there was a gaseous mixture which almost exclusively consisted of methane, ethane and propane and has a methane number of 45. In addition the gas contains traces of carbon dioxide and nitrogen. The other constituents are almost completely contained in the condensate. Following Tables 2 and 3 also show once again the difference between the subject-matter of the invention and the state of the art.

TABLE 2
Gas processing by cooling to −12 degrees Celsius
	Compound	Boiling point (° Celsius)
	H2	Hydrogen	−253
	N2	Nitrogen	−196
	CO	Carbon monoxide	−192
	O2	Oxygen	−183
	CH4	Methane	−162
	C2H4	Ethene	−104
	C2H6	Ethane	−89
	CO2	Carbon dioxide	−79
	C3H6	Propene	−48
	C3H8	Propane	−42
	C4H10	Isobutane	−12
	C4H10	n-Butane	−1
	C5H12	Isopentane	28
	C5H12	Pentane	36
	C5H12	n-Pentane	36
	C6H14	Hexane	69
	C6H6	Benzene	80
	C7H16	Heptane	98

TABLE 3
Gas processing by cooling to −48 degrees Celsius
	Compound	Boiling point (° Celsius)
	H2	Hydrogen	−253
	N2	Nitrogen	−196
	CO	Carbon monoxide	−192
	O2	Oxygen	−183
	CH4	Methane	−162
	C2H4	Ethene	−104
	C2H6	Ethane	−89
	CO2	Carbon dioxide	−79
	C3H6	Propene	−48
	C3H8	Propane	−42
	C4H10	Isobutane	−12
	C4H10	n-Butane	−1
	C5H12	Isopentane	28
	C5H12	Pentane	36
	C5H12	n-Pentane	36
	C6H14	Hexane	69
	C6H6	Benzene	80
	C7H16	Heptane	98

As can be seen from Tables 2 and 3 the subject-matter of the invention (Table 2) provides that all gaseous constituents with boiling points below isobutane are collected in the gaseous phase and those with a boiling point thereabove are collected in the condensate. In the state of the art (Table 3) which involves cooling to −48 degrees Celsius enrichment in the gaseous phase is effected exclusively in respect of those compounds which have a boiling point of CO₂ and below, concentration of the other constituents occurs in the condensate. In accordance with WO 2007/070198 A2 only enrichment in respect of CH₄ in the gaseous phase would be observed at all, clean separation is not effected.

The applicant's calculations showed that, with typical associated gas which occurs in oil production stations, only few higher-grade hydrocarbons have to be separated off to be able to operate modern internal combustion engines which are designed or adapted specifically for gases with a low resistance to knocking. These include heptane (C₇H₁₆), benzene (C₆H₆), n-pentane and isopentane (C₅H₁₂) as well as n-butane and isobutane (C₄H₁₀). All those components already condense at −12 degrees Celsius so that it is sufficient for the gas to be cooled down to that temperature. That can be implemented inexpensively and with a low level of complication with commercial refrigerating machines such as for example water chillers and with commercially available heat exchangers and condensate locks.

The advantage of the method is that the gas does not have to be cooled down to very low temperatures as was otherwise usual but only to between about −5 and −14 degrees Celsius, preferably −12 degrees Celsius, to make the gas suitable specifically for internal combustion engines in regard to anti-knock rating. That achieves a marked reduction in costs.

Claims

1. A method of producing combustible gas for gas engines from associated gas which is produced in crude oil production and which contains methane, ethane, propane, hydrocarbons having more than three carbon atoms and optionally propene, wherein a gaseous fraction and a liquid fraction are obtained by partially condensing the associated gas, characterized in that the condensation process is performed under such pressure and temperature conditions that the liquid phase is substantially free from methane, ethane, propane and optionally propene, and substantially the entire methane, ethane, propane and optionally propene is contained in the gaseous phase.

2. A method as set forth in claim 1 characterized in that the condensation process is performed at a temperature of between −5 degrees Celsius and −14 degrees Celsius.

3. A method as set forth in claim 2 characterized in that the condensation process is performed at a temperature of between −7 degrees Celsius and −12 degrees Celsius.

4. A method as set forth in claim 1 characterized in that the condensation process is effected in a plurality of stages, wherein cooling is effected to a temperature of between −5 and −8 degrees Celsius in one stage and to a temperature of between −8 and −12 degrees Celsius in a further stage.

5. A method as set forth in claim 1 characterized in that the pressure in the condensation process is between 1 bar and 16 bars, preferably between 10 bars and 16 bars, particularly preferably between 14 bars and 16 bars.

6. A method as set forth in claim 1 characterized in that the gaseous fraction has a methane number of at least 40, preferably at least 45.

7. A method as set forth in claim 1 characterized in that the gaseous phase is substantially free from n-butane and isobutane.

8. A method as set forth in claim 1 characterized in that water vapor which is possibly present in the associated gas is removed.

9. A method as set forth in claim 8 characterized in that the water vapor is removed by condensation, absorption or combinations thereof.