HYDROCARBONS

The present invention relates to the removal of volatile sulphur containing compounds from liquid hydrocarbons.

Liquid hydrocarbons often contain undesired volatile sulphur containing compounds. Examples of such liquid hydrocarbons are crude oil and liquefied natural gas. For instance, the value of crude oil is a function at least in part of the volatile sulphur containing compound content of that oil. The value of a barrel of crude oil drops by about one US Dollar for eve one hundred parts per million by weight of volatile sulphur containing compounds. It would thus be of very great commercial significance if "sour" oil could be converted to "sweet" oil by removing volatile sulphur containing compounds. The loss in value arises partly from costs associated with the handling of sour oil given that volatile sulphur containing compounds, especially compounds such as hydrogen sulphide, are both toxic and corrosive, and the cost involved in desouring oil in a refinery.

Sour crude oil is very common. Marginal oil fields can often be uneconomic to develop when found to be geologically sour. The economics of such fields could be transformed if the sour oil could be readily sweetened before delivery to the refinery but the known methods for sweetening sour oil cannot be applied economically other than in the refinery.

It is well known to remove hydrogen sulphide (a volatile sulphur containing compound) from for example gaseous waste streams resulting from various industrial chemical processes. US Patent No. 4278646 describes various methods for removing hydrogen sulphide and stresses that extremely efficient processes are required to effectively capture relatively small amounts of hydrogen sulphide which if released into the environment would infringe the increasingly stringent environmental controls that are being applied by local authorities and the like. These processes cannot of course be applied directly to

the problem of removing volatile sulphur containing compounds from liquid hydrocarbons (such as crude oil) and are not therefore applied for that purpose in oil refineries. However the processes may be applied in oil refineries to clean tail gases.

It is known that hydrogen sulphide may be removed from liquid hydrocarbons by mixing the hydrocarbons with steam and passing the vapour phase over a catalyst containing uranoso-uranic oxide and/or uranium trioxide supported on an inert carrier at 350 to 500 ^υ C. Such a process is clearly not applicable to bulk crude oil.

It is an object of the present invention to provide a method and apparatus for removing volatile sulphur containing compounds from liquid hydrocarbons in an economically viable manner.

According to the present invention, there is provided a method for removing sulphur containing compounds from a body of liquid hydrocarbons stored in a sealed container, wherein a gas is passed through the liquid hydrocarbon such that at least some of the sulphur containing compound is stripped from the liquid hydrocarbon by the gas, the gas which has passed though the liquid hydrocarbon is processed to remove at leas! some of the sulphur containing compounds and the processed gas is recirculated through the liquid hydrocarbon.

The invention is most suited for removing sulphur containing compounds which are volatile and/or soluble.

The method of the invention is particularly useful for the removal of volatile sulphur containing compounds from liquid hydrocarbons such as oil (e.g. crude oil) or liquefied natural gas.

Various volatile sulphur containing compounds may be removed according to the method of the invention however the method is particularly useful for the removal of hydrogen sulphide.

A preferred use of the method of the invention is for the removal of hydrogen sulphide from oil.

The present invention may be applied during transit of oil from an oil field to a refinery. Typically a tanker takes several days at least to transport a cargo of crude to a refinery and throughout that period the volatile sulphur containing compounds content of the cargo may be gradually reduced by for example recirculating tanker exhaust gases through the cargo. Gas bubbled through the oil will collect in the ullage above the cargo and can be extracted from that space, passed through a volatile sulphur containing compound removing process, and then rcbubbled through the cargo. It will of course be appreciated that any gas which can be passed through the cargo without causing a risk of explosion can be used and that the invention could be practised in a land-based installation by for example passing an inert gas through a body of oil stored in a storage tank.

Various gas processing methods may be applied to remove the volatile sulphur containing compounds but it is preferred to pass the gas through an aqueous solution of ferric ions, said solution will preferably contain a chclating agent such as nitriloacetic acid (NTA). Such a process can be applied in the case of an embodiment of the invention in which the gas passed through the oil is exhaust gas from an oil tanker engine. Alternatively volatile sulphur containing compounds could be absorbed by contacting it with oxides of iron in a suitable sponge matrix. Wood shavings are an example of such a sponge matrix.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which;

Figure 1 is a schematic illustration of a first embodiment of the invention on an oil transport tanker; and

Figure 2 illustrates a modification of the embodiment of Figure 1.

Referring to Figure 1, this illustrates an embodiment of the invention in which a body of oil 1 is retained within the cargo holder of an oil transport tanker only the deck 2 of which is shown. An ullage space 3 is defined between the upper surface of the oil 4 and the deck.

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A scavenge tank 5 is mounted on the deck and connected to an inlet pipe 6 which opens in the ullage space 3. The scavenge tank 5 is partially filled with a ferric chloride based solution 7 containing nitriloacetic acid. A gas pump 8 mounted on the deck is connected to the tank 5 by a pipe 9 and is connected to a sparger 10 immersed in the body of crude oil by a pipe 11.

As is conventional practice in oil transport tankers, the ullage space 3 will always be filled with an inert gas such as exhaust gas from the ships engines. In the present case however that inert gas is circulated through the crude oil by the gas pump 8 so as to be bubbled firstly through the crude oil and then through the solution 7. As the gas passes through the crude oil it strips volatile sulphur containing compounds from the oil and carries the volatile sulphur containing compounds into the tank 5. The volatile sulphur containing compounds are then absorbed in the solution 7. Given that oil will be retained within the tank for at least a matter of days it is not particularly important for the efficiency with which the inert gas strips volatile sulphur containing compounds from the oil to be very high nor for the solution 7 to remove ail of the volatile sulphur containing compounds from the gas which is pumped through it. The solution 7 will require periodic regeneration, but this can be simply achieved by sparging the solution 7 with air. Thus the entire system relies on very simple chemical processes, uses an inert gas such as exhaust gas from the ships engines, and chemical agents which can be handled without risk by relatively unqualified personnel. Such a system is therefore ideal for use in the relatively difficult environment associated with oil tanker transportation systems.

For instance, the process described above (with reference to the accompanying drawings) can be represented by the following formulae in which a means for the removal of hydrogen sulphide is given as way of example:

1. Stripping hydrogen sulphide from the crude oil:

HiS (crude) + Inert Gas → H ₂ S (in gas)

2. Scavenging hydrogen sulphide from the inert gas: 2Fe ⁺ + 2e — » 2Fe ⁺ (reduction reaction)

H ₂ S → S+2e + 2H ⁺ (oxidation reaction)

3. Regeneration of the scavenging solution: 2Fe -» 2Fe ⁺ + 2e (oxidation reaction) O ₂ + H ₂ O + 2e → 2OH (reduction reaction)

4. Neutralisation

20H + 2H ⁺ → 2H ₂ O

The overall reaction can be represented as follows: H ₂ S + ^ O ₂ → H ₂ O + SJ.

Fe ⁺ may be provided as an acid ferric chloride chelant solution. As an alternative to the use of the acidic ferric chloride chelant based solution (as represented above), an iron oxide or mixture of iron oxides could be used. The chemical process could be presented as follows:

2Fe ₂ O ₃ + 6H ₂ S → 2Fe ₂ S ₃ + 6H ₂ O

In either case the volatile sulphur containing compound stripping gas could be exhaust gas from the tanker engine or some other inert gas as described above.

The above embodiment may be modified for the removal of volatile sulphur containing compounds from liquid hydrocarbons in a land based facility.

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Referring to Figure 2, a body of oil 101 is retained within the facility only the top wall 102 of which is shown. An ullage space 103 is defined between the upper surface of the oil 104 and the top wall.

A scavenge tank 105 is mounted on the top wall and connected to an inlet pipe 106 which opens in the ullage space 103. The scavenge tank 105 is partially filled with a ferric chloride based solution 107 containing nitriloacetic acid. A gas pump 108 mounted on the deck is connected to the tank 105 by a pipe 109 and is connected to a sparger 110 immersed in the body of crude oil by a pipe 111. A pressurised container 112 filled with liquid nitrogen is connected to the system so as to maintain a predetermined nitrogen pressure in the ullage space 103.

Nitrogen is circulated through the crude oil by the gas pump 108 so as to be bubbled firstly through the crude oil and then through the solution 107. As the gas passes through the crude oil it strips volatile sulphur containing compounds from the oil and carries the volatile sulphur containing compounds into the tank 105. The volatile sulphur containing compounds are then absorbed in the solution 107. Given that oil will be retained within the facility for at least a matter of days it is not particularly important for the efficiency with which the nitrogen gas strips volatile sulphur containing compounds from the oil to be very high nor for the solution 107 to remove all of the volatile sulphur containing compounds from the gas which is pumped through it. The solution 107 will require periodic regeneration, but this can be simply achieved by sparging the solution 107 with air. Thus the entire system relies on very simple chemical processes, uses nitrogen and chemical agents which can be handled without risk by relatively unqualified personnel.

It will be appreciated that an alternative exhaust gases or some other inert gas may be used to strip volatile sulphur containing compounds from the oil contained within the facilit . It is also possible that nitrogen may be used instead of exhaust gases according to the embodiment of the invention for use on ships.

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It will be appreciated that many alternative chemical processes may be applied for the removal of volatile sulphur containing compounds from a gas which has been used to strip the volatile sulphur containing compounds from liquid hydrocarbons according to any embodiment of the invention. The possibilities outlined above are given by way of example only.

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