METHOD AND DEVICE FOR REMOVING CONTAMINANTS FROM A CONTAMINATED GAS

STREAM

BACKGROUND OF THE INVENTION

The invention relates to a method for removing contaminants from a contaminated gas stream, such as a contaminated natural gas or air stream. Various processes are known to remove contaminating components, such as water, hydrates, carbon dioxide and/or hydrogen sulphide, from a natural gas stream.

The processes may be based on physical and/or chemical separation techniques. The physical separation techniques use differences in boiling, condensation and/or freezing points of the various contaminating components to selectively remove one or more of these components in a fractionating column, or differences in density to separate components with different densities in a centrifugal or cyclonic separator .

The chemical techniques may employ selective absorption or catalytic reactions to convert a contaminating component into a composition that can be easily separated.

The standard technique for removing hydrogen sulphide and carbon dioxide from natural gas is amine treatment, which is based on solvent absorption. In this process the contaminating components are bound on a molecule such as diethanol amine in an aqueous solution. The clean hydrocarbon gas is not absorbed and emerges in the product gas stream. The solution with the absorbed contaminant is recycled and heated by approximately

100 ⁰ C to drive off the gases, which then are collected in a waste stream. The major cost factors in this process are the energy requirement for waste gas regeneration, solvent losses and the fact that the waste gases are regenerated at near atmospheric pressure - any process such as reinjection requires compression.

The operating costs for any gas purification process need to be a relatively small fraction of the value of the clean gas produced. Amine plants with their extensive gas-liquid contacting schemes will be fairly large, expensive and uneconomic if the gas stream contains a large fraction of contaminants.

Known gas separation centrifuges rotate at about 50,000 revolutions per minute (RPM) to separate gaseous fractions with only minor differences in density. These fast rotating centrifuges are known as ultracentrifuges and have limited separation efficiency and can only handle a limited flux of gas . If a large gas stream containing a large fraction of contaminants is to be purified by means of centrifuges then a large amount of centrifuges or ultracentrifuges are required, which renders centrifugal separation uneconomical.

International patent application WO2006087332 discloses a method of separating contaminants from a contaminated gas stream, wherein the contaminated gas stream is cooled in a nozzle or expansion turbine to such a temperature that at least some contaminants condense and the cooled mixture is separated in a channelled centrifuge into a purified gas stream and a contaminants enriched side stream.

European patents 1017465, 1438540 and 1140363 disclose cyclonic separators for purifying a contaminated gas stream, wherein the gas stream is accelerated in a

nozzle and thereby cooled such that at least some contaminants condense and the cooled gas liquid mixture is induced to swirl in a fluid separation section such that the liquid components swirl along the outer surface of the fluid separation section into an annular liquid outlet and a purified gas stream flows into a central purified gas outlet conduit.

It is an object of the present invention to provide a method for removing solid, liquid and/or gaseous contaminants, such as water, hydrates, carbon dioxide and/or hydrogen sulphide, from a gas stream in an efficient and economical manner, even if the gas comprises a large fraction of contaminants. SUMMARY OF THE INVENTION In accordance with the invention there is provided a method for removing contaminants from a contaminated gas stream, the method comprising inducing the gas stream to flow through a conduit having a first and a second conduit section, which conduit sections each comprise the following components: a) a cyclonic fluid separation section in which the gas stream is induced to swirl such that a solid and/or liquid contaminants enriched fluid fraction flows to an outer region of the separation section and a contaminants depleted gas fraction flows into a central region of the separation section; b) a central purified gas outlet tube for discharging the contaminants depleted gas fraction from the central region of the centrifugal fluid separation section; c) an outer discharge tube for discharging the contaminants enriched fluid fraction from the outer region of the separation section; and

d) a venturi section, which is located upstream of the centrifugal fluid separation section and in which the gas stream is induced to flow at a higher axial velocity than in the centrifugal fluid separation section; wherein the method further comprises: connecting the central purified gas outlet tube of the second conduit section to the venturi section of the first conduit section such that a purified gas fraction is induced to flow from the second conduit section into the first conduit section; injecting a liquid contaminants absorbent into the venturi section of the first conduit section; and connecting the outer discharge tube of the first conduit section to the venturi section of the second conduit section, thereby inducing a contaminants and liquid contaminant absorbent enriched fluid fraction to flow from the outer discharge tube of the first conduit section into the venturi section of the second conduit section . It will be understood that the acceleration of the fluid velocity in the venturi section of the second conduit section will result in a reduction of the static fluid pressure in particular in the vicinity of a throat of this venturi section, so that a positive pressure difference is created between the separated liquid in the outer discharge tube of the first conduit section and the fluid flowing through the venturi in the second conduit section, thereby inducing the separated liquid to flow from the outer discharge tube of the first conduit section into the venturi of the second conduit section without requiring any pumping action.

The conduit may further comprise a third conduit section, which also comprises the components a),b),c)and

d), wherein the method further comprises: connecting the central purified gas outlet tube of the third conduit section to the venturi section of the second conduit section such that a purified gas fraction is induced to flow from the third conduit section into the second conduit section; and connecting the outer discharge tube of the second conduit section to the venturi section of the third conduit section, thereby inducing a contaminants and liquid contaminant absorbent enriched fluid fraction to flow from the outer discharge tube of the second conduit section into the venturi section of the third conduit section .

Optionally, the conduit may comprise a series of n conduit sections, wherein n is in the range from 4 to 40, which conduit sections each comprise the components a),b),c)and d) as described in claim 1 and wherein the method further comprises: connecting the central purified gas outlet tube of the n-th conduit section to the venturi section of the

(n-l)th conduit section such that a purified gas fraction is induced to flow from the nth conduit section into the (n-l)th conduit section; and connecting the outer discharge tube of the (n-l)th conduit section to the venturi section of the n-th conduit section, thereby inducing a contaminants and liquid contaminant absorbent enriched fluid fraction to flow from the outer discharge tube of the (n-l)th conduit section into the venturi section of the n-th conduit section.

The liquid contaminants absorbent may comprise Mono Ethylene Glycol (MEG) , an amine compound or aqueous amine compound solution, and/or any other absorbent fluid and

be configured to absorb water, hydrogen sulphide, carbon dioxide and/or other contaminants from the contaminated gas stream.

The conduit may be located horizontally or vertically at the bottom of a body of water in the vicinity of the wellhead of an underwater gas production well and may form part of an underwater gas processing and conditioning facility, which receives a contaminated gas stream from at least one wellhead of at least one underwater gas production well and which discharges an at least partially purified gas stream into a downstream processing facility or into a subsea gas transportation conduit which may have a length of more than hundred kilometres Alternatively, the conduit may be located on an offshore gas production platform or form part of an onshore gas production and processing facility.

In accordance with the invention there is furthermore provided a system for removing contaminants from a contaminated gas stream and a purified gas stream from which contaminants have been removed by means of the method according to the invention.

These and other features, embodiments and advantages of method and system according to the invention are described in the accompanying claims, abstract and the following detailed description of a preferred embodiment in which reference is made to the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWINGS

FIG.l is a schematic longitudinal sectional view of a conduit in which a contaminated gas stream is purified in accordance with the method according to the invention; and

FIG.2 is a schematic longitudinal sectional view of an alternative embodiment of the conduit shown in FIG.l,

wherein the conduit sections are not directly connected to each other, but are interconnected by pipes. DETAILED DESCRIPTION OF THE SHOWN EMBODIMENTS OF THE INVENTION FIG.l shows a conduit 10, which comprises first, second third, fourth and fifth conduit sections 11,12,13,14 and 15, respectively, and which is connected between a contaminated gas inlet conduit 16 and purified dry gas outlet conduit 17. Each of the conduit sections 11-15 comprises the following components: a) a cyclonic fluid separation section 11A-15A in which the gas stream is induced to swirl by swirl imparting vanes 11G-15G such that a solid and/or liquid contaminants enriched fluid fraction 11B-15B flows to an outer region of the separation section 11A-15A and a contaminants depleted gas fraction 11C-15C flows into a central region of the separation section 11A-15A; b) a central purified gas outlet tube 11D-15D for discharging the contaminants depleted gas fraction from the central region of the centrifugal fluid separation section 1 IA-I 5A; c) an outer discharge tube 11E-15E for discharging the contaminants enriched fluid fraction from the outer region of the separation section 1 IA-I 5A; and d) a venturi section 11F-15F, which is located upstream of the centrifugal fluid separation section 11A-15A and in which the gas stream is induced to flow at a higher axial velocity than in the centrifugal fluid separation section 11A-15A such that the static pressure of the accelerated gas stream is reduced in particular in the vicinity of the throat of the venturi section 11F-15F.

In the embodiment shown each of the conduit sections 11-15 furthermore comprises a optional static fluid mixer 11H-15H, which is arranged between the venturi section 11F-15F and the swirl imparting vanes 11G-15G and a series of anti-swirl flow straightening vanes 11K-15K arranged in the central purified gas outlet tube 11D-15D. In accordance with the invention: the central purified gas outlet tube 12D of the second conduit section 12 is directly, as shown in FIG.l, or indirectly, for example by means of a U-shaped connection tubular 32 as shown in FIG.2, connected to the venturi section HF of the first conduit section such that a purified gas fraction 12C is induced to flow from the second conduit section 12 into the first conduit section 11; a liquid contaminants absorbent (e.g. lean MEG) is injected via a lean absorbent supply conduit 18 into the venturi section HF of the first conduit section 11; and the outer discharge tube HE of the first conduit section H is connected to the venturi section 12F of the second conduit section 12, thereby inducing a contaminants and contaminant absorbent enriched fluid fraction 19 to flow from the outer discharge tube HE of the first conduit section 11 into the venturi section 12F of the second conduit section 12. Furthermore : the outer discharge tube 12E of the second conduit section 12 is connected to the venturi section 13F of the third conduit section 13, thereby inducing a contaminants and liquid contaminant absorbent enriched fluid fraction 20 to flow from the outer discharge tube 12E of the second conduit section 12 into the venturi section 13F of the third conduit section 13;

the outer discharge tube 13E of the third conduit section 13 is connected to the venturi section 14F of the fourth conduit section 14, thereby inducing a contaminants and liquid contaminant absorbent enriched fluid fraction 21 to flow from the outer discharge tube 13E of the third conduit section 13 into the venturi section 14F of the fourth conduit section 14; the outer discharge tube 14E of the fourth conduit section 14 is connected to the venturi section 15F of the fifth conduit section 15, thereby inducing a contaminants and liquid contaminant absorbent enriched fluid fraction 22 to flow from the outer discharge tube 14E of the fourth conduit section 14 into the venturi section 15F of the fifth conduit section 15; and - the outer discharge tube 15E of the fifth conduit section 15 is connected to a contaminants enriched MEG outlet conduit 23 through which a contaminants enriched MEG fluid stream is discharged from the conduit 10 and flows to a MEG purification unit 24 in which the contaminants enriched MEG fluid stream is separated into a contaminants enriched and MEG depleted fluid fraction 25 and a contaminants depleted lean MEG fraction 26, which is recycled into the lean MEG supply conduit 18.

An advantage of the method according to the invention is that MEG is supplied to the conduit 10 via a single lean MEG supply conduit 18 into the venturi section HF of the first, most downstream, conduit section 11 and is then automatically recycled into the more upstream conduit sections 12-15 as a result of the relatively low static pressure in the throat of each of the venturi sections 12F-15F of these more upstream sections 12-15 relative to the static pressure in the wider outer discharge tubes 11E-14E of the downstream sections.

FIG.2 shows an alternative embodiment of a conduit according to the invention, wherein the conduit comprises four conduit sections 11-14, which are similar to the conduit sections 11-14 shown in FIG.l and in which similar reference numerals are used to identify similar components, but where the conduit sections 11-14 are not aligned and are not directly connected to each other, but are interconnected by U-shaped or other shaped pipes 30,31 and 32, and where the contaminants enriched liquid absorbent discharged by the fourth conduit section 14 is fed into an absorbent purification unit (not shown) from which the lean absorbent is pumped into the venturi section HF of the first section.

The outer discharge tube HE of the first conduit section 11 is connected to the venturi section 12F of the second conduit section 12, thereby inducing a contaminants and contaminant absorbent enriched fluid fraction 19 to be sucked from the outer discharge tube HE of the first conduit section 11 into the venturi section 12F of the second conduit section 12.

The outer discharge tube 12E of the second conduit section 12 is connected to the venturi section 13F of the third conduit section 13, thereby inducing a contaminants and contaminant absorbent enriched fluid fraction 20 to be sucked from the outer discharge tube 12E of the second conduit section 12 into the venturi section 13F of the third conduit section 13.

The outer discharge tube 13E of the third conduit section 13 is connected to the venturi section 14F of the fourth conduit section 14, thereby inducing a contaminants and contaminant absorbent enriched fluid fraction 21 to be sucked from the outer discharge tube

13E of the third conduit section 13 into the venturi section 14F of the fourth conduit section 14.

It will be understood that the absorbent recirculation system according to the invention may operate without any circulation pumps between the conduit section, which enhances the reliability of the recirculation since circulation pumps are prone to wear and require regular maintenance, inspection and replacement . It will be understood that in the case that the conduit comprises n conduit sections, the contaminated gas stream will first enter the n-th conduit section, than the purified gas stream from the n-th conduit section will enter the (n-l)th conduit section, than the gas stream from the (n-l)th conduit section will enter the (n-2)th conduit section etc. Finally the purified gas stream from the first conduit section will leave the total conduit .

The cyclonic fluid separation sections in the present invention are especially in-line separation. The gas stream is flowing through a pipe-line shaped conduit, the conduit provided with internals as describe above. Thus, no tangential inflow of the gas stream, the tangential flow is induced by specific internals as vanes and swirls .

The contaminated gas stream of the invention is preferably a natural gas stream, suitably comprising at least 50 vol% of methane, contaminated with water, carbon dioxide, hydrogen sulphide and/or mercaptans.