WO2015060725A1

WO2015060725A1 - Apparatus and method for absorbing co2 from natural gas

Info

Publication number: WO2015060725A1
Application number: PCT/NO2013/050180
Authority: WO
Inventors: Torbjørn FIVELAND; Henrik Solgaard Andersen; Dag Eimer; Asbjørn STRAND; Eirik Manger
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2013-10-22
Filing date: 2013-10-22
Publication date: 2015-04-30
Anticipated expiration: 2016-04-22

Abstract

The invention relates to an apparatus 1 for absorbing C0₂ and/or H₂S from a gas stream of natural gas 2 comprising a stationary casing 4 having a longitudinal axis, a gas stream inlet 5 and a gas stream outlet 6, said casing comprising at least one drum 7 being rotatable about the longitudinal axis, said drum having at least one absorption section 8 provided with spray means 18 for spraying absorption liquid into the stream of natural gas containing C0₂ and/or H₂S to form a cross-flow of droplets of absorption liquid to the flow direction of the gas stream by aid of centrifugal force, and having means for disintegration 20 of the droplets of absorption liquid, said drum further having at least one droplet removal section 9 arranged after the absorption section in the flow direction of the gas stream, said apparatus having means for rotating 13 the drum such that the absorption section and the droplet removal section rotate about the longitudinal axis, and said apparatus having means for energy recovery 29 from the liquid. The invention also relates to a process for absorbing C0₂ and/or H₂S from a natural gas stream.

Description

Apparatus and method for absorbing C0₂from natural gas. Field of invention

The present invention relates to an apparatus and a method for absorbing C0₂ and/ or H₂S from natural gas.

Background of invention

A conventional design of a plant used for carbon dioxide removal from natural gas comprises a separate boiler, followed by a cooler for the gas before the carbon dioxide is removed in an absorption column where the natural gas is contacted counter-currently to an absorbent flowing downwards. The process units for such a plant are bulky and require instrumentation and control, in addition to a complex piping system to connect the various units.

The document US2003/0089232 describes a method and device for removal of C0₂from natural gas. The method comprises that the gas stream is brought in contact with one or more functional elements attached to, and rotating with, a hollow, rotating axle arranged in a gas duct and where fluids effecting the gas treatment flow through the functional element. The device comprises means for recovery of thermal energy from the hot exhaust gas by a rotating heat transfer element mounted in the duct and means for recovery of kinetic energy in the pressurized gas if there is a sufficient pressure in the gas.

The device described in US2003/0089232 provides improvements in the field of technology by reducing the complexity of the gas processing intended by enabling installation of all relevant processing and/or contacting devices in the gas duct itself, thus reducing the need for piping. The apparatus is compact and it reduces the need for piping by building the device into the exhaust gas duct.

However it is important to further improve the removal process of C0₂from natural gas, and it is particularly important to remove H₂S from natural gas produced in natural gas production sites since H₂S is a hazardous gas. Since the absorption process is very energy demanding it is also important to improve the energy efficiency of the absorption apparatus. Furthermore it is also important to reduce the C0₂ footprint from natural gas production sites.

Summary of invention

The object of the present invention is to provide an improved compact apparatus and process for absorbing C0₂ and/or H₂S from a gas stream of natural gas. This object is achieved by an apparatus according to the invention having the features as defined in independent claim 1. This object is also achieved by a process according to invention having the features as defined in independent claim 29. Further advantageous features and embodiments are defined in the dependent claims.

The apparatus according to the invention, also referred to as a compact rotating absorber (CRA), is advantageous in that it comprises a rotating drum which is provided in the gas duct. The drum is at least divided into an absorption section and a droplet removal section which rotate together with the drum.

Conventional technology has restrictions regarding maximum viscosity of the absorbent. This limits the range of chemicals that may be used and the concentration. The present invention allows for operation with highly viscous liquids as absorption liquids due to that the absorption process is carried out by forcing droplets of absorption liquid to move a in a cross- flow direction to the gas flow by aid of centrifugal force. The droplets of viscous absorption liquid are thereby efficiently distributed in the gas flow.

The apparatus according to the invention is also advantageous in that it comprises droplet disintegration means attached to the drum and arranged to rotate about an longitudinally arranged axle. The droplet disintegration means are constructed are constructed to disintegrate and finely atomize droplets of absorption liquid thereby improving the mass transfer of C0₂ and/or H₂S to the absorbent from the passing natural gas. The droplet disintegration means comprises a plurality of rotatable cylinders of permeable material such as densely perforated plates. The plurality of perforated cylinders have different radius and are coaxially arranged inside the main cylinder and rotate together with the drum , the main cylinder. The droplets of absorption liquid pass through the perforations of the cylinders when the droplets move in the cross flow direction and the gas flow passes axially between the perforated cylinders.

The rotation of the droplet disintegration means increases the turbulence in the gas and thereby increase the mass transfer of C0₂ and/ or H₂S from natural gas to the absorbent, absorption liquid. Another advantage is that the speed difference between the droplets and the gas stream, the relative speed, is high. This has the benefit that turbulence within the droplets increases, thus absorption is improved.

The apparatus according to the present invention is provided with means for recovering kinetic energy from the supplied liquid, the absorption liquid. The absorption liquid is removed, drained, from the drum by use of diffusers such that kinetic energy in the liquid is recovered and converted into hydrodynamic energy, a pressure head, which is used as hydraulic driving force of the drum. In particular it is the kinetic energy in a liquid layer formed on an inside mantle surface of the drum, the main cylinder which is recovered. This saves energy and may replace pumps downstream.

Furthermore, the apparatus according to the invention allows that absorption liquid is recovered and separated into rich absorption liquid and semi lean absorption liquid, such that the semi lean absorption liquid can be recirculated to the apparatus for further use. This is advantageous in that the absorption liquid can be used in a very efficient manner.

Known processes for removing C0₂ and/ or H₂S from natural gas consists of very large units. This new invention reduces the size of the absorber by more than 90% in addition to a cost reduction of at least 50%. The apparatus according to the invention provides a compact technology which can be constructed at a factory and transported on site. This will greatly reduce the construction costs.

In one embodiment of the apparatus according to the invention the process for C0₂ and/ or H₂S removal is improved by efficiently washing the cleaned natural gas before venting to the atmosphere. This is achieved by arranging a particular washing section in the drum.

In this specification the expressions "absorbent" and "absorption liquid" are equal and used for the same matter.

The present invention will now be discussed in more detail with the aid of preferred illustrative embodiments shown in the drawings, in which:

Figure 1 schematically shows a cross sectional view along the length of an apparatus according to one embodiment of the invention. Figure 2 schematically shows an enlargement of a part of the apparatus shown in figure 1.

Figure 3 schematically shows an enlargement a part of another embodiment of the apparatus according to the invention.

Figure 4 schematically shows a cross sectional view along the length of another embodiment of the apparatus according to the invention. Figure 5 schematically shows a cross sectional view along the length of another embodiment of the apparatus according to the invention.

Figure 6 schematically shows a cross sectional view along the length of another embodiment of the apparatus according to the invention.

Figure 7 schematically shows a cross sectional view along the line a-a in the enlargement shown in figure 2. Figure 8 schematically shows a cross sectional view along the length of another embodiment of the apparatus according to the invention.

Figure 9 schematically shows an enlargement a part of another embodiment of the apparatus according to the invention.

Detailed description of embodiments of the invention

With reference to the figures, it should be noted that the figures only show details which are important for the understanding of the invention. Moreover, same features are indicated by the same reference number throughout the specification.

It should also be noted that the illustrated apparatus and the features thereof are out of proportion in relation to each other and the indicated features and dimensions thereof are very exaggerated in size in comparison to the size of the drum and casing. The features are represented in this manner for explanatory purposes only.

Figure 1 schematically illustrates an apparatus 1 , a compact rotary absorber, for absorbing C0₂ and/ or H₂S from a natural gas stream 2 according to the invention. The apparatus 1 is connected to a gas duct 3 wherein a natural containing C0₂ and/ or H₂S to be treated flows. The natural gas from a gas production site such as a gas well enters the apparatus 1 via the gas duct 3 at a velocity in the range of 15-30 m/s.

The apparatus 1 comprises a compact rotating absorber 1 which is provided with a stationary casing 4 having a longitudinal axis, a gas stream inlet 5 and a gas stream outlet 6.

The casing 4 encloses at least one drum 7, also referred to as main cylinder, which is rotatable about the longitudinal axis A inside the casing. The drum 7 is open in both ends, such that gas 2 flows through the casing and the drum from the gas stream inlet 5 to the gas stream outlet 6.

The drum 7 comprises at least one absorption section 8 and at least one droplet removal section 9. The sections are arranged directly next to each other within the drum. The drum 7 is provided with a mantle surface 11 which functions as the outer perimeter in the radial direction of each section. The drum is further provided with a hollow centre axle 12 arranged coaxial with the rotation axis A of the drum 7. The hollow centre axle 12 is arranged to extend through the absorption section 8 and the droplet removal section 9, the hollow axle may also only be provided in the absorption section. Preferably the hollow centre axle 12 is stationary, however it may also be arranged to rotate about the longitudinal axis.

The design of the absorption section 8 and the droplet removal section 9 will be adjusted according to their intended purpose, however the drum 7 and the sections 8,9 therein are arranged and shaped such that the axial movement of the natural gas 2 is not blocked or hindered to avoid pressure loss.

The apparatus 1 is provided with mechanical driving means 13 of conventional type for causing rotation of the drum whereby the absorption section and the droplet removal section rotate about the longitudinal axis A with the same rotational speed. The drum 7 rests in bearing arrangements that serve as necessary support and to facilitate rotation at significant speed without undue friction. The number of support points, and thus bearings, will be determined by good mechanical engineering practice. Bearings are provided in houses that are cooled as necessary according to good practice. Absorption liquid 14, preferably lean or semi lean absorption liquid, for the gas treatment enters the drum 7 via an inlet conduit 16. The drum is provided with at least one inlet 16 conduit for absorption liquid being arranged along the rotation axis A of the drum, preferably the inlet conduit 16 is arranged axially inside the hollow centre axle 12. Figure 1 further show that the absorption section 9 is arranged next to the gas stream inlet 5 of the drum. In the absorption section, C0₂ and/ or H₂S in the natural gas 2 is exposed to and treated by the absorption liquid whereby C0₂ and/ or H₂S is absorbed.

The absorption section 8 is provided with spray means 18 arranged along the axis of rotation A of the drum 7, which are connected to the inlet conduit 16 for the absorption liquid. The spray means 18 can be formed like a plurality of nozzles attached to the hollow axle 12. However, other means for spraying liquids may also be envisaged, such as perforated pipes or similar. The plurality of spray means 18 are evenly distributed about circumference of the hollow axle. In figure 1 , the spray means 18, the nozzles, are distributed along the whole length of the part of the hollow axle which is located in the absorption section 8.

Preferably the spray means 18 are arranged to spray absorption liquid 14 in the radial direction, a cross flow direction, to the flow direction of the passing stream of natural gas and towards droplet disintegration means. In operation of the apparatus 1 the drum 7 rotates and a centrifugal force arise that influences the droplets of absorption liquid and causes the droplets to move from the centre in a direction towards the circumference of the drum 7, thereby forming a cross-flow 19 of droplets in relation to the direction of the gas stream.

The absorption section 8 is further provided with means for disintegration of droplets 20. The purpose of this is to disintegrate, finely atomize, the droplets of absorption liquid, thereby enhancing the absorption process and the mass transfer of C0₂ and/ or H₂S from natural gas 2 to the absorption liquid 14. The means for disintegration 20 of droplets also increase the turbulence inside the droplets which also improves the mass transfer of C0₂ and/ or H₂S and the absorption process.

Preferably the means for disintegration of droplets comprises a plurality of cylinders 20 which are coaxial and are arranged to rotate about the longitudinal axis A. Each cylinder 20 of the disintegration means is open in both ends, and has a length which corresponds to the length of the absorption section 8.

Figure 2 shows an enlargement of a part of the apparatus shown in figure 1. In figure 2 it can be seen that the cylinders 20 are provided with perforated mantle surface 21 , in order to let the droplets move from the centre towards the perimeter of the drum 7 and thus passing through the perforated mantle surfaces 21 of the plurality of coaxial arranged rotating cylinders 20. The mantle surface 21 of each cylinder is perforated all around the

circumference and along the whole length of the cylinder.

Preferably the perforations 22 of the mantle surface are adapted such that the droplets pass through the perforations 22 under influence of centrifugal forces without forming a liquid layer on the inside surface of the cylinder mantle 21. Since the perforated cylinders rotate together with the drum 7, the droplets are disintegrated and finely divided when passing through the perforations 22, which improves the absorption. Trials have shown that it takes about 0.01 - 0.02 seconds for a droplet to move from one cylinder to the next, thus very good mixing inside the droplets is achieved. The cylinders 20 further provide an increased surface for mass transfer purposes in that the absorption liquid is distributed over the perforated mantle surfaces of the cylinders achieving high exposure to the passing natural gas. The average droplet size decreases with distance from the centre and the volume of gas between the cylinders increases further out from the hollow axle which allows for high level of absorption. The absorption liquid and droplets thereof absorbs C0₂ and/or H₂S and becomes rich when moving in the radial direction towards the drum passing the gas stream. The means for disintegration of droplets comprises 3-50 or more coaxial arranged cylinders. As can be seen in figure 2, the perforated cylinders 20 are consecutive arranged between the hollow centre axle 12 supporting the spray means 18 and the mantle surface 11 of the drum 7. The cylinders 20 are arranged such that every cylinder is larger than the cylinders arranged closer to the hollow centre axle. A radial distance d, a separating space, is arranged between two consecutive arranged cylinders 20 which provides for passage of gas in the axial direction between the cylinders 20 and through the section. The size of the space d is adapted such that the droplets of absorption liquid are exposed to the passing natural gas stream 2 in order to achieve good absorption without blocking or hindering the stream of gas. Preferably the separating space is approximately 0,5-5 cm, more preferably 0,5-3 cm. The co-axial perforated cylinders 20 of the droplet disintegration means are fixed in relation to the drum by conventional fastening means not shown in the figure but this may easily be done with use of appropriate stays.

Figure 2 further shows that the mantle surface 1 1 of the drum 7 is provided with a plurality of liquid outlet passages 23, the liquid outlet passages being provided as holes through the mantle surface 1 1 of the drum 7. In the absorption section, rich absorption liquid which has absorbed C0₂ and/or H₂S is drained from the drum via the holes 23 and is collected by the enclosing casing 4. The apparatus 1 according to the invention is arranged to recover kinetic energy from the absorption liquid. In figure 2 it is further shown that the mantle surface 1 1 of the drum 7 is therefore provided with means for recovering energy, diffusers 29 connected to the liquid outlet passages 23. The diffusers are not indicated on the apparatus shown in figure 1 , however in figure 2 the diffusers 29 are indicated as longitudinally extended tube like passages. The energy recovery means will be described in detail below. Figure 1 further shows that the casing 4 is provided with at least one rich absorption liquid outlet conduit 24 and at least one semi lean absorption liquid outlet conduit 25, such that the absorption liquid removed from the drum is directed to different outlets 24, 25 at the lower side of the casing 4. Preferably the rich absorption liquid outlet conduit 24 is arranged near the gas stream inlet of the casing, and the semi lean absorption liquid outlet conduit 25 is arranged near the droplet removal section 9. The casing may be provided with guiding means for guiding the collected absorption liquid towards the outlet conduits (not shown in the figures). Typically the rich absorption liquid is removed from the apparatus and supplied to a regenerator for absorption liquid.

Figure 3 schematically shows a part of another embodiment of the apparatus according to the invention_where the apparatus is provided with means 30 for recirculation of semi lean absorption liquid, also referred to as SLA, to the inlet conduit for absorption liquid of the apparatus. The semi lean absorption liquid removed from the apparatus via the semi lean absorption liquid outlet conduit 25 is thereby cycled to the absorption liquid inlet 16 of the apparatus and sprayed into the absorption section to absorb C0₂ and/ or H₂S from the gas stream and exit the apparatus as rich absorption liquid, also referred to as RA, to improve the efficiency of the absorption process. In another embodiment of the invention, the apparatus 1 is adapted such that semi lean absorption liquid is removed from the apparatus via the semi lean absorption liquid outlet conduit 25 and is added by the means for recirculation 30 to the lean absorption liquid LA shown as a dashed line in figure 3 supplied from a regenerator. The mixture is supplied to the absorption liquid inlet of the apparatus and sprayed into the absorption section to absorb C0₂ and/ or H₂S from the gas stream and exits the apparatus as rich absorption liquid RA.

The present invention allows that the absorption liquid used in the absorption process has an amine concentration up to 95% by weight. The absorption liquid comprises one of the following MEA, MDEA, DEA, or a mixture of different amines or other absorbents such as ionic liquid. Furthermore, the centrifugal effect used in absorption process and apparatus according to the invention allows that the absorption liquid has a viscosity of up to 1500 nPa-s.

Natural gas recovered from gas wells is typically highly pressurized. The apparatus and process according to the invention are adapted to operate above atmospheric pressure, typically the natural gas is pressurized up to 80 Bar, preferably in the range of 40-80 Bar. During the absorption process some droplets from the absorption section may be carried forwards with the stream of gas 2. In order to avoid that droplets of absorption liquid are released to the atmosphere, a droplet removal section 9 is provided directly after the absorption section in the flow direction of the gas stream to remove droplets of absorption liquid from the gas stream. As can be seen in figure 1 the droplet removal section 9 is provided with a packed bed 28 for collecting and removing droplets of absorption liquid from the passing gas stream. The packed bed 28 comprises a material with very high surface area per cubic meter (up to 3000-4000 m²/m³) such as metal foam or the like. The packed bed is arranged to be connected to the drum 7, by conventional attaching means not shown in the figure. Absorption liquid droplets carried over to this section will come in contact with the material and be forced to the outer perimeter by the centrifugal force. The droplets are drained from the droplet removal section 9 via liquid outlet passages 23 arranged on the drum surface 11 The absorption liquid drained from the droplet removal section is typically semi lean, having absorbed some of C0₂ and/ or H₂S present in the natural gas. The liquid outlet passages 23 are provided as holes through the mantle surface of the drum, as previously described.

With reference to figure 4 another embodiment of the apparatus 1 according to the invention is schematically shown as a cross sectional view along the length of the apparatus wherein a washing section 10 is arranged directly after the droplet removal section 9 in the flow direction of the gas stream. When the stream of natural gas flows through absorption section 8 and the droplet removal section 9 some absorption liquid, absorbents, may be dissolved in the gas and it is advantageous to remove these droplets of absorption liquid before the gas is vented to the atmosphere This is particularly necessary when very low level of C0₂ and/ or H₂S is to be achieved.

Washing liquid, preferably water, is sprayed into the gas stream in order to remove absorption liquid, absorbents, dissolved in the gas stream 2. A washing liquid inlet conduit 17 is provided to supply washing liquid to the washing section 10. The hollow centre axle 12 previously described may also extends through the washing section 10 to be provided with spray means 18 to spray wash liquid 15 into the stream of natural gas. Alternatively, the washing section 10 is provided with other types of spray means (not shown in the figure) arranged in the centre of the washing section and connected to a washing liquid inlet conduit 17. The spray means are used for spraying wash liquid, water, into the passing cleaned natural gas stream. In one embodiment of the invention the pH of the wash liquid is adjusted to further improve the washing process. Preferably the washing section 10 is provided with means for disintegration of droplets 20 of the same kind as used in the absorption section as showed in figure 1. The sprayed wash liquid 15 forms a cross-flow of droplets by effect of centrifugal force causing the wash liquid droplets to move in a direction towards the perimeter of the drum thereby coming into contact with the passing gas stream. The plurality of rotating perforated cylinders 20 in the means for disintegration of droplets finely atomizes the droplets of wash liquid which improves the mass transfer of dissolved absorption liquid from the passing natural gas stream. Any absorption liquid dissolved in the passing gas stream will be absorbed by the wash liquid such that natural gas free of absorption liquid can be vented. Preferably the wash liquid is drained from the washing section via a plurality of liquid outlet passages, holes 23 provided in the mantle surface 11 of the drum 7, as previously described. The wash liquid drained from the drum is collected by the casing 4 and removed via a wash liquid outlet conduit 26.

Figure 5 schematically shows a cross sectional view along the length of another embodiment of the apparatus according to the invention where the washing section 10 is directly followed by at least one additional droplet removal section 9 of the same kind as previously described in order to remove any droplets of wash liquid from the cleaned natural gas in advance of venting the natural gas to the atmosphere.

Even if the apparatus shown in figure 1 only illustrates one absorption section 8 and one droplet removal section 9 it should be understood that there may be more such sections arranged in series within the drum. In particular if the natural gas contains both C0₂ and H₂S, it is possible to arrange several gas treating sections after each other and to use different absorption liquids for the absorption of H₂S and C0₂ Figure 6 schematically shows another embodiment of the apparatus according to the invention where drum 7 is provided with a first and a second absorption section 8, followed by a first and a second droplet removal section 9 respectively. This is advantageous in case the C0₂ and/ or H₂S content in the cleaned natural gas has to fulfil very strict requirements. As previously mentioned the apparatus 1 according to the invention also comprise means for recovering of energy from the absorption liquid after having treated the gas. In particular it is the kinetic energy in the liquid layer formed on the inside of the mantle surface of the drum which is recovered. This is achieved by draining the absorption liquid from the drum via diffusers 29 whereby kinetic energy in the liquid layer is converted into a pressure head. The pressure head is then used as a hydraulic driving force for rotating of the drum. Figure 2 and 7 shows the means for energy recovery which comprises a plurality of diffusers 29 which are connected to the liquid outlet passages 23 on the outside surface of drum mantle. The liquid outlet passages 23 are distributed over the entire mantle surface, preferably symmetrically such that unbalance of the drum is avoided. The droplets of liquid move towards the perimeter of the drum, and form a layer of liquid 32 on the inside of the mantle surface 11 of the drum. The drum is provided with liquid outlet passages 23 draining the liquid from the drum 7, but the size and number of outlet passages are adapted such that there is a liquid layer formed on the inside surface 1 1 of the drum .This has the effect that gas is blocked from passing through liquid outlet passages 23. Preferably every outlet passage 23 is provided with a diffuser 29 arranged on the outside surface of the drum mantle 1 1. Preferably the diffusers 29 are symmetrically arranged around the circumference of the drum and are arranged along the whole length of the drum.

In figure 2 and 7 the diffusers 29 are indicated as longitudinally extended tube like passages. In figure 7 it is showed that the diffusers are provided with tapered cross section such that the liquid drained from the inside of the drum 7 is accelerated when passing through the diffusers. The diffuser outlets 29a, are formed such that kinetic energy in the liquid drained from the liquid layer 32 is converted into a pressure head by forcing the absorption liquid through the plurality of diffusers 29. The pressure head in the liquid is converted into a hydraulic driving force contributing to driving the drum. The diffusers are directed towards the casing and have a diffuser outlet 29a which is directed such that the ejected liquid stream hits the inside of the casing wall, and preferable in the opposite direction to the rotation of the drum. The diffuser outlet 29a is directed such that the angle a between the tangent of drum surface at the outlet passage23 and the diffuser outlet 29a is less than perpendicular (a< 90 degrees). This implies that the liquid stream leaving the diffuser outlet 29 is directed to hit the casing 4 such that hydraulic drive of the drum 7 is achieved. In figure 7 the liquid is ejected from the diffusers in a direction towards the casing wall. This saves energy and may replace pumps downstream. As previously mentioned typically natural gas from a gas well is pressurized up to 80 Bar, preferably in the range of 40-80 Bar, and the cleaned gas leaving the absorption apparatus is provided with a full body rotation. It is thus advantageous to recover the kinetic energy from the gas and in another embodiment of the invention, shown in figure 8, the apparatus 1 is additionally provided with a turbine 31 arranged in the gas stream outlet 6 of the apparatus for energy recovery from the pressurized the gas. Alternatively the turbine may be arranged in the gas stream inlet 5, this is advantageous in that the gas stream is forced to start rotating about the longitudinal direction before entering the absorption apparatus, which improves the absorption process and decreases the pressure drop in the apparatus.

It is advantageous to prevent liquid from moving axially along the inside of the mantle surface 1 1 of the drum, to avoid undesirable mixing of rich absorption liquid and semi lean absorption liquid. Figure 9 schematically shows an enlargement a part of another embodiment of the apparatus according to the invention. As shown in figure 9 the surface 1 1 of drum 7 can be provided with at least one ring shaped flange 27 and/ or seal arranged to separate the absorption section and the droplet removal section. The ring shaped flange is adapted to hinder axial flow of liquid. Also the inside wall of the casing 4 may be provided with a ring shaped flange and/or a seal to separate the liquids, not shown in the figures. The liquid may drip from the casing "ceiling" back on to the outside surface of the drum 7 from where it will be flung back on to the casing wall, and eventually it will reach the casings lowest point from where it may be drained.

In another embodiment of the apparatus according to the invention the drum may provided with at least one compensating balance duct extending radially through the mantle surface of the drum (not shown in the figures). The balance duct is formed like a short tube which extends radially through the mantle surface of the drum and has a length which exceeds the liquid layer formed on the inside of the drum. This has the effect that in case gas has passed through the diffusers to the outside of the drum, the gas can return into the drum via the balancing ducts, which functions as balancing compensation of the drum.

By placing all primary gas processing equipment inside a gas duct, there is by definition no piping between processing equipment. Hence the piping costs are reduced. Since the usual contractions and expansions of gas flow cross-sectional area due to flow from vessel to pipe and from pipe to vessel are removed, pressure drop in the process is also reduced.

The transfer of mass is intensified through the increased turbulence around the droplet disintegrating means, the perforated cylinders, by the rotation of the assembly. The units of the processing equipment have been shrunk through this intensification of the process thereby achieving a compact rotary absorption apparatus.

The above-mentioned embodiments, variants and examples of the present invention may be freely combined within the limits of the following claims without departing from the invention.

Claims

1. Apparatus (1) for absorbing C0₂ and/or H₂S from a gas stream of natural gas (2) comprising a stationary casing (4) having a longitudinal axis, a gas stream inlet (5) and a gas stream outlet (6), said casing comprising at least one drum (7) being rotatable about the longitudinal axis (A), said drum having

-at least one absorption section (8) arranged within the drum provided with spray means (18) for spraying absorption liquid (14) into the natural gas stream containing C0₂ and/or H₂S to form a cross-flow of absorption liquid droplets (19) to the flow direction of the gas stream (2) by aid of centrifugal force, and having means for disintegration (20) of droplets,

-at least one droplet removal section (9) arranged after the absorption section (8) in the flow direction of the gas stream,

- said apparatus (1) having means for rotating the drum (13) such that the absorption section

(8) and the droplet removal section (9) rotate about the longitudinal axis (A) together, and said apparatus having means for energy recovery (29) from the liquid.

2. Apparatus according to claim 1 wherein said means for disintegration (20) of droplets comprises a plurality of coaxially arranged cylinders (20) provided to rotate about the longitudinal axis (A), said cylinders being provided with perforated mantle surfaces (21).

3. Apparatus according to claim 1 or 2 wherein perforations (22) of the mantle surface (21) are adapted such that the droplets pass through the perforations without forming a liquid layer on an inside surface of the cylinder mantle (21).

4. Apparatus according to any of the preceding claims wherein the droplet removal section

(9) is provided with a packed bed (28) for collecting and removing droplets of absorption liquid from the passing gas stream (2).

5. Apparatus according to any of the preceding claims wherein the drum is provided with at least one inlet conduit (16) for absorption liquid being arranged along the rotation axis (A) of the drum (7).

6. Apparatus according to any of the preceding claims wherein the spray means (18) are arranged along the axis of rotation (A) of the drum (7), and said spray means (18) being arranged to spray absorbent liquid (19) in a cross-flow direction to the passing gas stream.

7. Apparatus according to any of the preceding claims wherein the absorption section (8) is provided with a plurality of liquid outlet passages (23) for rich absorbent, the liquid outlet passages being provided as holes (23) through the mantle surface (1 1) of the drum.

8. Apparatus according to any of the preceding claims wherein droplet removal section (9) is provided with a plurality of liquid outlet passages (23) for semi lean absorbent, the liquid outlet passages being provided as holes (23) through the mantle surface (11) of the drum.

9. Apparatus according to any of the preceding claims wherein the means for energy recovery (29) comprises a plurality of diffusers (29) which are connected to the liquid outlet passages (23) on the outside surface of drum mantle (11).

10. Apparatus according to claim 9 wherein each diffuser (29) is provided with a tapered cross section such that the liquid drained from the drum (4) is accelerated when passing through the diffuser.

1 1. Apparatus according to any of claims 9-10 wherein each diffuser (29) is directed towards the casing (4) and having a diffuser outlet (29a) which is directed such that the liquid is ejected in a direction towards the casing wall.

12. Apparatus according to any of claims 9-11 wherein the diffusers (29) are symmetrically arranged around the circumference of the drum (7).

13. Apparatus according to any of the preceding claims wherein the diffusers (29) are arranged along the whole length of the drum (7).

14. Apparatus according to any of the preceding claims comprising a gas turbine (31) to recover kinetic energy in the gas stream (2).

15. Apparatus according to any of the preceding claims having at least one washing section (10) arranged after droplet removal section (9) in the direction of the gas stream (2).

16. Apparatus according to any of the preceding claims wherein the drum (7) is provided with at least one inlet conduit for wash liquid (17) being arranged along the rotation axis of the drum.

17. Apparatus according to any of the preceding claims wherein the washing section (10) is provided with spray means (18) for spraying wash liquid into the gas stream (2) to form a cross-flow of droplets of wash liquid (15a).

18. Apparatus according to any of the preceding claims wherein the washing section (10) is provided with means for disintegration (20) of droplets.

19. Apparatus according to claim wherein the washing section (10) is directly followed by at least one droplet removal section (9) in the direction of flow of the gas stream.

20. Apparatus according to any of the preceding claims wherein washing section (10) is provided with a plurality of liquid outlet passages (23) for wash liquid, the liquid outlets being provided as holes (23) through the mantle surface (1 1) of the drum.

21. Apparatus according to any of the preceding claims comprising diffusers (29) connected to liquid outlet passages (23) for wash liquid on the outside surface of drum mantle (11) to recover kinetic energy from the wash liquid.

22. Apparatus according to any of the preceding claims wherein the drum(7) is provided with at least one additional absorption section (8), said additional absorption section being followed by at least one additional droplet removal section (9).

23. Apparatus according to any of the preceding claims wherein the drum is provided with at least one compensating balance duct extending radial through the mantle surface (11) of the drum.

24. Apparatus according to any of the preceding claims provided with means for recirculation (30) of semi lean absorbent to the inlet conduit (16) for absorbent liquid of the drum.

25. Apparatus according to any of the preceding claims wherein the surface (11) of the drum is provided with at least one flange (27) and/ or seal arranged to separate the sections (8,9, 10).

26. Apparatus according to any of the preceding claims wherein the apparatus (1) is adapted to operate above atmospheric pressure.

27. Apparatus according to any of the preceding claims wherein the natural gas is pressurized up to 80 Bar, preferably in the range of 40-80 Bar.

28. Process for absorbing C0₂ and/or H₂S from a gas stream of natural gas (2)

comprising the steps of:

- supplying natural gas containing C0₂ and/or H₂S and supplying absorption liquid (14) to a drum (7) being rotatable about a longitudinal axis (A), said drum having at least one absorption section (8) and at least one droplet removal section (9),

- rotating the drum with the absorption section and the droplet removal section,

- spraying absorption liquid (14) into the natural gas stream in the absorption section, -forming a cross-flow of droplets (19) of absorption liquid to the flow direction of the gas stream and causing the droplets to move by aid of centrifugal force towards the perimeter of the drum (7),

-absorbing C0₂ and/or H₂S from the natural gas stream,

-disintegrating the droplets of absorption liquid (19),

- removing the droplets of absorbent liquid from the gas stream, and

-recovering energy from the liquid.

29. Process according to claims 28 wherein the disintegration of the droplets comprises forcing droplets to pass through perforations (22) arranged on a plurality of coaxially arranged cylinders (20) having perforated mantle surfaces (21).

30. Process according to claims 28 or 29 wherein the absorption liquid (14) is sprayed in a cross-flow direction (19) to the flow direction of the gas stream.

31. Process according to any of claims 28-30 comprising forming a layer of liquid (32) on the inside of the mantle surface (11) of the drum (7).

32. Process according to any of claims 28-31 wherein kinetic energy in the liquid drained from the liquid layer (32) is recovered as hydraulic drive to rotate the drum (7).

33. Process according to any of claims 28-32 comprising draining the liquid from the drum by forcing the liquid through the diffusers (29) by centrifugal force, and ejecting the liquid from the diffusers in a direction towards the casing wall (4).

34. Process according to any of claims 28-33 comprising rotating the drum by hydraulic drive from the liquid drained by the diffusers (29) from the drum (7).

35. Process according to any of claims 28-34 comprising recovering kinetic energy from the gas stream by a gas turbine (31) arranged in the gas stream outlet.

36. Process according to any of claims 28-35 comprising spraying a wash liquid (15) into the gas stream, and forming a cross-flow of droplets (15d) of wash liquid causing the droplets to move in a direction towards the circumference of the drum.

37. Process according to any of claims 28-36 comprising disintegrating the droplets (15a) of wash liquid during the movement towards the circumference of the drum.

38. Process according to any of claims 28-37 wherein the wash liquid is sprayed in a cross- flow direction (18) to the flow direction of the gas stream.

39. Process according to any of claims 28-38 wherein pH of the wash liquid is adjustable.

40. Process according to any of claims 28-39 wherein the wash liquid is water.

41. Process according to any of claims 28-40 wherein the absorption liquid has an amine concentration up to 95% by weight.

42. Process according to any of claims 28-41 wherein the absorption liquid comprises one of the following MEA, MDEA, DEA, or a mixture of different amines or other absorbents such as ionic liquid or brine.

43. Process according to any of claims 28-42 where the absorption liquid has a viscosity of up to 1500 nPa-s.

44. Process according to any of claims 28-43 comprising balancing compensation of the drum in order to recover gas from the outside of the drum.

45. Process according to any of claims 28-44 claim wherein the process is carried out at a pressure of up to 80 Bar, preferably in the range of 40-80 Bar.