CONTACT AND SEPARATION COLUMN AND TRAY

Field of the Invention

The present invention relates to a contact and separation column, particularly for high capacity

gas/liquid mass transfer, with a stack of one or more contact and separation cells, where gas is intensively contacted with a liquid, which is first entrained with the gas flow and which is subsequently removed from the gas flow by centrifugal impact of a swirler. The

invention also pertains to a tray to be used in such a column and to a process for treating a gas with such a column .

The column can for instance be a heat transfer column, a fractionation column, a stripping column or a treating column, in particular a treating column, more particular a column for treating natural gas, to remove contaminants, such as carbon dioxide and/or hydrogen sulphide .

Background of the Invention

The term gas shall be used herein so as to also include vapour while the term liquid also includes liquid containing gas such as froth. The expression treating liquid is used to indicate compounds, preferably liquids, which selectively remove contaminants such as hydrogen sulphide and/or carbon dioxide by physical and/or

chemical phenomena.

Trays for gas/liquid mass transfer containing a cyclonic gas/liquid separation device differ from regular mass transfer trays in their applications. A tray in which gas and liquid are contacted and subsequently separated with a cyclonic gas/liquid device can be made significantly more compact than a regular - - tray where the separation between gas and liquid is by gravitational forces instead of centrifugal forces.

Critical issues for gas/liquid contact and separation columns are the mass transfer in the contact zone, the separation efficiency in the separation zone and the maximum capacity of gas and liquid which can be handled.

The mass transfer rate is related to the amount of liquid entrained per unit of gas in the contact zone. The required volumetric mass transfer rates increase with higher partial pressure of the impurities (such as CO ₂ ) to be absorbed. Mass transfer rates obtainable with prior art contact and separation columns were found to be limited. In offshore gas mining contact and separation columns are used with floating applications, e.g. on a floating device or boat. Sea motion induced sloshing or waving of the liquid poses further serious limitations on the obtainable mass transfer and thereby contacting effectiveness of the columns.

The separation efficiency is the weight percentage of liquid removed based on the total amount of liquid entering the gas/liquid separator.

The pressure drop per tray cannot exceed a certain value as otherwise so-called reverse flow will occur causing liquid to flow in a direction opposite to that intended which can be highly undesirable. The maximum pressure drop is a function of the vapour load, the liquid load and the geometry. It is advantageous to have a geometry allowing a large pressure drop, i.e. having a high maximum value, as this makes that the equipment can be used over a wide range of vapour and liquid loads.

US-A-3, 722, 839 describes a vapour-liquid contacting device having a contact zone and a separator downstream of the contact zone. The outlet of the separator is - - pinched. This flow restriction increases the pressure differential to facilitate liquid exit and aid

entrainment in the main vapour stream of any small amount of vapour escaping through the slots in the separator walls. Further pressure drop is caused by flow

straighteners and the liquid distribution pipes having small holes (1/16 inch) and the use of honeycomb packing in the contacting zone. Somebody skilled in the art will disregard this set-up for commercial use in view of the inherent large pressure-drop caused by this combination of features. This makes that the equipment is limited in the liquid and gas load which can be handled. Another disadvantage of this set-up is the piping between the trays which makes that all trays are mutually connected. This further limits the flexibility to change the gas and liquid load and makes it very complicated to manufacture and install.

WO-A-2013/087865 and WO-A-2013/087866 describe contact and separation columns which can be used for removing carbon dioxide and/or hydrogen sulphide from natural gas by allowing to intensely mix the amine absorbents with the gas and subsequently separate the cleaned gas in an efficient way. The exemplified

separation zones have closed walls. The columns have reduced pressure drop over the contact and separation zones thereby increasing the maximum gas load and liquid load which can be handled. They further have improved mass transfer, are robust and can be used offshore.

It is an object of the present invention to further increase the efficiency of separating gas from liquid in the separation zone of columns as described in WO-A- 2013/087865 and WO-A-2013/087866. - -

Summary of the Invention

A contact and separation column is disclosed

comprising a column wall encasing a stack of one or more contact and separation cells, wherein each cell

comprises:

- a tray with a number of gas flow openings opening into one or more contact and separation units, wherein each contact and separation unit comprises an upstream contact zone with liquid inlets, and one or more separation zones provided with a swirler and a top end with a gas outlet;

- a downcomer defining a liquid discharge for liquid overflowing the downcomer, which has a height exceeding the level of the liquid inlets; and

- a liquid supply for supplying liquid to the contact and separation cell,

wherein the wall of at least one of the separation zones has a plurality of discharge openings extending through the wall to allow fluid to pass through the wall from within the separation zone.

In a second aspect, a contact and separation tray is disclosed for use in the disclosed column.

In a third aspect a process for treating gas is disclosed making use of the disclosed column.

Detailed Description

Surprisingly, it has been found that a higher

separation efficiency is attained by having discharge openings through the wall of the separation zone.

Furthermore, it was found that this efficiency is

increased even further by having a relatively high percentage of the side wall consisting of such openings.

It was found to be beneficial if at least 1 % of the wall of the separation zone consisted of such openings. The wall of the separation zone is considered to be the wall - - of the zone containing the separation device including the wall surrounding this device which in our case is a swirler .

Preferably, of from 2 to 15 % of the wall surface of the separation zone (10) consists of discharge openings.

More preferably, at least 3 %wt, more specifically at least 4 %, most specifically at least 5 % of the wall surface consists of discharge openings. Preferably at most 14 %, more specifically at most 13 ~6 , more

specifically at most 12 %, more specifically at most 11 % and most specifically at most 10 % of the wall surface consists of discharge openings.

The discharge openings can have any shape such as slits, square holes, triangular holes and downward directing lips. These can be present over the full length of the separation zone or mainly in the downstream part. The overriding effect observed was the percentage of the wall of the separation zone which is open.

A further aspect of the present invention is that the swirler preferably is located at a distance from the gas inlet of from 50 to 90 % of the total length of the contact and separation zone, preferably from 60 to 90 %, more preferably from 65 to 85 %.

Preferably, the cross-sectional area of the contact zone is larger than the cross-sectional area at the separation zone as this causes a substantial increase of the mean residence time in the contact zone and,

consequently, of the volumetric gas/liquid mass transfer rate. This advantage can be further improved by spacing the swirler at a distance from the gas inlet.

The cross-sectional area is the surface area of the section made by a plane cutting the middle of the zone in question at a right angle to the longest axis of the - - contact and separation cell. The middle of a zone is taken in the direction of the longest axis of the contact and separation zone together.

Due to the larger cross-sectional area of the contact zone, the flow velocity of the gas in the contact zone will typically be lower than the flow velocity in the separation zone. While lower flow velocities result in improved mass transfer in the contact zone, faster flow velocities in the separation zone result in improved separation. The flow velocity can be represented by the

Souders-Brown density corrected gas load factor Cs = U _Gas * (Peas/ (pLiquid-pcas) ) ^1/2 where p _g is the gas density and pi is the liquid density and U _g is the linear gas velocity at the inlet of the swirler.

To obtain a good centrifugal separation at the swirler this factor can for instance be in the range of 0.3 m/s < Cs < 1.5 m/s . Such velocities are very high, particularly compared with regular prior art contacting devices (e.g., with trays or packing), which typically operate at a Cs factor below 0.1 m/s . In the contact zone such flow velocities would lead to very short gas/liquid contacting times and low mass transfer rates. Due to the larger cross-sectional area in the contact zone, the flow velocity can be substantially lower. The flow velocity of the gas in the contact zone can typically be at most

90 %, preferably at most 80 % or even more preferably at most 50% of the flow velocity in the separation zone.

In a specific embodiment the downcomers of the trays can be arranged offset relative to downcomers of a next tray, e.g., alternately at opposite sides in adjacent cells, wherein the liquid discharge of an upper contact and separation cell forms the liquid supply of a lower contact and separation cell. Liquid flowing into the - - downcomer of an upper cell flows into the underlying cell where it is circulated once again until it flows into the next downcomer.

The expression "upstream" and "downstream" is used with respect with the swirler.

The cross-sectional area of the contact zone can for instance be at least 30% larger than the cross-sectional area of a single separation zone, more specifically at least 50 %, more preferably at least 80 %, most

preferably at least 100 %.

Preferably, the cross-sectional area of a contact zone is at least 5% larger than the cross-sectional area of all separation zones belonging to the same unit as this contact zone, more specifically at least 8 ~6 , more preferably at least 10%. Most preferably, the cross- sectional area of a contact zone is at most 50% larger than the cross-sectional area of all separation zones belonging to the same unit as this contact zone.

The downcomers at an edge of the tray can comprise a weir which weir and the edge are spaced from the column wall to define the liquid discharge for liquid

overflowing the weir. The weir can be part of the tray or it can be a separate part. In an alternative embodiment, the downcomer can for instance be a pipe or any other suitable type of conduit. The tray may have one or more downcomers of the same or different types.

The contact zone is provided with one or more inlets for supplying liquid into the gas flow at a level below the top end of the downcomer. In use the liquid inlets are submerged in the liquid on the tray, i.e. the liquid inlets are below the liquid level of normal operation. Preferably, the liquid inlets are at or near the gas flow openings in the base plate of the tray, most - - preferably the liquid inlets are covered by the gas inlets. The liquid inlets can for example be slits or circular, square or triangular holes or of any desirable alternative shape.

Optionally, liquid distributing means can be used for distributing the liquid over the area of the gas flow openings in the tray. This way, liquid is not only introduced peripherally into the gas flow, but a more equal liquid distribution is obtained over the full gas flow area. It has been found that this substantially increases the mass transfer rate. Moreover, larger gas flow openings and contact zones with larger cross- sectional areas can be used, while maintaining high mass transfer rates. Examples of guiding means which can be applied are vanes or gutters, e.g., with a rectangular, semi-circular or V-shaped cross section.

The swirler can for example be a vane assembly imparting a rotary movement to the gas/liquid mixture. By this rotary movement the liquid droplets of the gas/liquid mixture are flung outwardly to impinge and coalesce on the inner surface of the conduit.

In a specific embodiment the contact zone is such that its cross section is a square or rectangle.

Preferably, the contact zone may have the shape of a box, carrying one or more tubular separation zones, such as cylindrical or conical separation zones. Due to the box shape the contact zones can be arranged close together, using a high proportion of the available space in the respective cell.

Preferably, the gas outlet of the top end of the separation zone is bordered by a liquid flow deflector.

Optionally, the liquid flow deflector comprises a return skirt having a U-shaped radial cross section with - - an open side arranged over a top edge of the separation zone. Due to the centrifugal impact of the swirler, liquid borne by the passing gas is collected on the cylindrical or conical inner wall of the separation zone, where it flows upwardly under the influence of the passing gas flow. At the upper edge of the contact zone, gas flows upwardly through the outlet, while the

separated liquid is deflected by the return skirt to flow down again outside the contact zone.

In a specific embodiment the cross-sectional area of the contact zones together take up the majority of tray surface, typically at least 60 %, more specifically at least 70 %, and preferably at least 80 %. In this respect the cross-sectional area of the contact zones refers to the joint cross section of all contact zones connected to the tray in question. The cross-sectional area of the separation zones is not taken into account.

When waves develop due to tilt or motion, the liquid level on the tray will vary, but the impact will remain very small as long as the wave height is relatively small in comparison with the average liquid level on the tray. The liquid level on the tray can be further increased by increasing the weir height, and optionally also tray spacing. Weir heights (up to 700 mm) which are like a factor 10 higher than on regular trays have been successfully tested for this purpose.

The one or more contact and separation trays may form a separate or integral part of the contact and

separation column. The tray comprises a base plate with a number of gas flow openings opening into one or more contact and separation units positioned on an upper side of the base plate. In a specific embodiment, each contact and separation unit comprises an upstream - - contact zone with liquid inlets which liquid inlets preferably are within the scope of the gas flow

openings, one or more downstream separation zones provided with a swirler and a top end with a gas outlet bordered by a liquid flow deflector;

wherein the contact zone has a larger cross-sectional area than the separation zone.

The tray can comprise a weir at one or more of the tray edges which weir can be part of the tray itself or it can be a separate part or part of the column wall.

In a specific exemplary embodiment of the tray, the contact and separation zones comprise a box shaped contact zone opening into one or more tubular separation zones, typically cylindrical and/or conical separation zones.

The contact zone can be empty, allowing free flow of the passing gas, or it may wholly or partly contain a packing material, such as corrugated plate material and/or wire mesh material. Such packing material increases the residence time of the liquid in the contact zone and, consequently, such packing supports further mass transfer.

The column of the present invention will further be provided with a liquid inlet, a liquid outlet, a gas inlet and a gas outlet.

The disclosed contact and separation column can be used for a process for treating gas with a liquid. To this end, gas is introduced into the contact and

separation zones via the gas flow openings, while treating liquid at the liquid inlet in a lower contact and separation cell of a contact and separation column.

The process can for example be used for treatment of gases comprising water, carbon dioxide and/or hydrogen - - sulphide, such as natural gas or shale gas. If mainly water is removed, the process generally is called dehydration. Most preferably, the process is used for removing carbon dioxide and/or hydrogen sulphide.

Natural gas is a hydrocarbon gas mixture containing a substantial amount, preferably consisting of at least 40 %wt, more specifically at least 50 %wt, most

specifically of from 60 to 95 %wt, of methane. It is customary to temporarily liquefy natural gas for ease of storage or transport. To prepare natural gas for

liquefaction it is treated to remove components that would freeze under liquefaction conditions or that would be destructive to liquefaction equipment, such as water, hydrogen sulphide and carbon dioxide. The liquids for treating the gas can be any liquid known to be suitable for this purpose. Typically, the liquids for removing the undesired compounds will contain water, glycols, methanol and/or amines, more specifically will be aqueous solutions of one or more amines, more

specifically alkylamines, more specifically one or more compounds chosen from the group consisting of

monoethanolamine, diethanolamine, methyldiethanolamine, diisopropanolamine and aminoethoxyethanol .

The mass transfer further can be improved by adding compounds known as accelerators which further increase the speed of chemical and/or physical interaction between the natural gas and the treating liquid. The mass transfer can be increased further by using a higher temperature for example by up to at most 30 °C above the normal operating temperature.

A further advantage of the intense mixing that takes place in the contact zone of the present invention is the increased selectivity for removing contaminants - - which have a higher absorption rate into the liquid.

Intense mixing for example tends to favour absorption of hydrogen sulphide over carbon dioxide which is highly advantageous when treating natural gas.

The disclosed process generally is designed to give almost pure methane.

The process can also be used for treatment of other gases or for stripping or fractionation, if so desired.

The invention will now be described by way of example in more detail, with reference to the drawings, wherein

Figure 1 shows schematically in plan view a contact and separation column of the invention;

Figure 2 shows the column of Figure 1 in a

longitudinal cross section along line A-A in Figure 1 ;

Figure 3 shows a section of the interior of the column of Figure 1 in perspective view;

Figure 4 shows a different perspective view of the section shown in Figure 3.

Figures 5A, 5B and 5C show various kinds of discharge openings.

Figure 1 - 4 show a contact and separation column 1 comprising a column wall 2 encasing a stack of two contact and separation cells 3. Each cell 3 comprises a tray 4 with a base plate 5 provided with a number of gas flow openings 6 opening into a respective contact and separation unit 7 positioned on the upper side of the tray 4. Each contact and separation unit 7 comprises a box-shaped upstream contact zone 8, 9 and cylindrical separation zones 10 on top of the contact zone 8, 9. The box shaped contact zones 8, 9 include a larger middle box

8 and two smaller boxes 9 flanking the larger middle one. The larger middle contact zone 8 carries a row of three separation zones 10, while the two smaller ones carry - - only two separation zones 10 as shown in Figure 1. This way, the cross-sectional area of each of the contact zones 8, 9 is larger than the joint cross-sectional areas of the separation zones 10 in fluid connection with a single contact zone. As the contact zone furthermore is much higher than the separation zones, the volume of the contact zone is much larger than the volume of the separation zones 10 in fluid connection with a single contact zone.

At their lower ends the contact and separation units

7 comprise a set of parallel vertical plates 11 bridging the gas flow openings 6 and spacing the contact zone 8, 9 from the base plate 5. Gas can flow through the gas flow openings 6 via the space between the vertical plates 11 into the contact zones 8, 9. The vertical plates 11 define inlets 12 for liquid present in the cell 3.

The cylindrical separation zones 10 are provided with a swirler 13 within their lower end and discharge

openings 18 in their walls. The top end 14 of the

separation zone forms a gas outlet which is bordered by a liquid flow deflector 15 formed by a ring with a U-shaped radial cross section with the open side arranged over the top edge of the separation zone 10. The walls of

separation zone 10 contain discharge openings 18.

The tray 4 further comprises an edge with a weir, which is spaced from the column wall 2 to define a liquid discharge 17 for discharging liquid overflowing the weir to a lower cell 3. The height of the weir determines the level of liquid present in the cell 3. At the lower side of the tray 4 the weir extends to a distance above the base plate 5 of a lower tray 4. This distance corresponds to the height of the vertical plates 11. This way, the weir and the adjacent section of the column wall 2 define - - a downcomer 16 serving as a liquid supply 17 for the next cell 3 below.

The trays 4 in the column 1 are arranged in a

mirrored position relative to adjacent upper or lower trays. As a result, the weir of each tray 4 is placed at a side of the column which is opposite to the side where the weir of a tray 4 above or below is arranged. With such an arrangement the weir of each tray 4 forms a downcomer 16 supplying liquid to the next lower cell 3. In alternative embodiments, other types of downcomers, such as pipes, can also be used.

Liquid flows down from a downcomer 16 into a cell 3, where the liquid is collected until it reaches the level of the upper edge of the weir. Meanwhile gas flows in the direction indicated by the arrow A in Figure 2. The gas flows via the gas flow openings 6 upwardly into the cell 3, via the contact zones 8, 9 and the separation zones 10 into the gas flow openings 6 of a next upper cell 3. At the liquid inlets 12 of the contact zones 8, 9 the liquid is forced to flow into the contact zone 8, 9 by a

hydrostatic force and a suction force exerted by the passing gas flow. These forces are balanced by a counter force resulting from the pressure drop in the gas flow due to the narrowing cross-sectional area. Liquid will gradually enter the contact zone 8, 9 and be dispersed into the passing gas flow. Due to the vertical plates 11 the liquid does not only enter the gas flow at the periphery of the contact zone, but over a larger area of the gas flow, resulting in an improved mass transfer of liquid into the gas flow. The contact zone 8, 9 is relatively high which contributes to a more intense contact between liquid and gas. When passing the swirler 13, the gas flow undergoes a centrifugal force. As a - - result liquid borne by the gas flow will be swept against the inner wall of the separation zone 10 and pushed upwardly by the passing gas flow. The gas flow exits the separation zone 10 via the outlet 14. The separated liquid reaching the top edge of the separation zone 8 is deflected by the return skirt 15 to the outer side of the cylindrical wall, where it flows downwardly back into the liquid of the cell 3.

Figure 5A shows discharge openings which are formed by making a U-shape slit and bending the separated square outwards to form downward directing lips. The side view is depicted as well.

Figure 5B shows discharge openings which are holes.

Figure 5C shows discharge openings which are open slits.

The present invention will now be illustrated with the following examples.

Example 1

A tube having a diameter of 8 cm contained a gas inlet at its bottom end and a liquid inlet at its side each feeding into a contacting zone having a height of 150 cm. The contacting zone feeds the mixture via a swirler to a separation zone having a height of 16.5 cm. The equipment was operated in once-through mode.

Air and water were added while differing the

percentage of the wall surface of the separation zone consisting of discharge openings. The comparison was made at a liquid addition of 6 m ³ /h and a tube gas load factor of 0.7 m/s . The gas load factor is as defined in the description.

If no openings were present, the separation

efficiency was found to be 94 %.

If downward directing lips as shown in Figure 5A were - - present constituting 3.7 % of the wall of the separation zone, the separation efficiency was found to be 95 %.

If holes as shown in Figure 5B were present

constituting 3.9 % of the wall of the separation zone, the separation efficiency was found to be 95.5 %.

If narrow slits were present constituting 5.9 % of the wall of the separation zone, the separation

efficiency was found to be 97.5 %.

If broad slits were present constituting 12.3 % of the wall of the separation zone, the separation

efficiency was found to be 98 %.

Example 2

The experiment of Example 1 was repeated except that the tube gas load factor was decreased to 0.5 m/s.

If no openings were present, the separation

efficiency was found to be 88 %.

If downward directing lips as shown in Figure 5A were present constituting 3.7 % of the wall of the separation zone, the separation efficiency was found to be 92 %.

If holes as shown in Figure 5B were present

constituting 3.9 % of the wall of the separation zone, the separation efficiency was found to be 94 %.

If narrow slits were present constituting 5.9 % of the wall of the separation zone, the separation

efficiency was found to be 96.5 %.

If broad slits were present constituting 12.3 % of the wall of the separation zone, the separation

efficiency was found to be 97.5 %.