FI125804B - Stripping Column - Google Patents

Description

Stripping column Field of the application

The present application relates to columns containing packing material. More particularly the present application relates to stripping columns wherein the packing material is arranged into movable racks. The present application also relates to methods for maintenance of a column and to methods for removing substances from a solution.

Background

Stripping is a physical separation process where one or more components are removed from a liquid stream by a vapour stream. In industrial applications the liquid and vapour streams can have co-current or counter-current flows. Stripping is usually carried out either in a packed or in a trayed column. Examples of stripping agents include steam, air, inert gases, and hydrocarbon gases.

Air stripping is a process where volatile components of a liquid are transferred into an air stream. It is an environmental engineering technology used for the purification of groundwater and wastewaters containing volatile compounds, such as volatile organic compounds (VOC) and/or other volatile components.

In air stripping a liquid, usually water or wastewater, is brought into intimate contact with a gas, usually air, so that some undesirable volatile substances present in the liquid phase can be released and carried away by the gas. Examples of air stripping technology include mechanical surface aeration, diffused aeration, spray fountains, spray or tray towers, and counter-current packed towers. These procedures and devices produce a condition in which a large surface area of the water to be treated is exposed to air, which promotes the transfer of the contaminant from the liquid phase to the gaseous phase.

Volatile compounds have relatively high vapour pressure and low aqueous solubility characterized by the compound’s Henry's law coefficient, which is the ratio of the concentration in air that is in equilibrium with its concentration in water. Pollutants with relatively high Henry’s Law coefficients can be economically stripped from water. These include BTEX compounds (benzene, toluene, ethylbenzene, and xylene) found in gasoline, and solvents including trichloroethylene and tetrachloroethylene. Ammonia can also be stripped from wastewaters and liquid digestates (often requiring pH adjustment prior to stripping). Since Henry’s law coefficient increases with temperature, stripping is easier at warmer temperatures.

Traditional packed columns consist of a vertical column with liquid flowing in the top and out the bottom. The vapour phase enters in the bottom of the column and exits out of the top. Packing is used to increase the contact area between the liquid and vapour phases. There are many different types of packing used.

Packed (bed) columns usually use engineered or random plastic packings or packing material. Design criteria for packed towers include surface area provided by the packing, column height and diameter, and air to water flow rates.

The process comprises counter-current flow of water and air through a packing material. In general, the packed tower contains a cylindrical drum equipped with a gas inlet and a distributing space at the bottom; a liquid inlet and a distributor at the top; gas and liquid outlets at top and bottom, respectively; and a supported mass of inert solid shapes, called tower packing. In the traditional system, water is pumped to the top of the tower, and is allowed to flow down over the inert packing, while air is pumped counter-current from the bottom of the tower. The contaminants of interest such as ammonia or volatile organic compounds (VOC) are stripped out of the water into the air stream. In practice, two methods are used to achieve contact between phases so that mass transfer can occur: continuous contact and staged contact. Different flow patterns used in practice include counter-current, co-current and cross-flow. The most common flow pattern is counter-current mode.

A problem in such systems is the contamination of the packing material, especially when wastewaters are treated. The high mass transfer efficiency provided by the packing in a stripper promotes the deposition of scale, such as insoluble metal oxides, and bacterial growth. Packings with high surfaces will be more efficient but will promote fouling as well. If the contaminated water contains free iron or other minerals, the action of the stripping air could cause some of these compounds to precipitate out and foul the packing media. Organic contaminants promote biological growth that accentuates the fouling problem. The bacteria present in the water will cause for example slime deposits in the packing material inside the stripping columns.

As slime, scale or other contamination builds up, it causes a pressure drop through the column and decreases the air flow. This decreases the stripping efficiency of the column considerably. Cleaning of the packing material is very laborious. Attempts have been made to wash the packing material in the column by using hot water and/or chemicals to avoid time-consuming manual dismantling of the column packing.

Summary

One embodiment provides a column, such as a stripping column, containing packing material, wherein the column contains -at least one rack for the packing material, the rack being movable from the column for maintenance, and -at least one opening hatch for said maintenance.

One embodiment provides a method for maintenance of column, such as a stripping column, the method comprising -providing said column, -opening a hatch of the column, -moving a rack out from the column, and -carrying out the maintenance.

One embodiment provides use of said column for removing substances from a solution.

The aspects of the invention are characterized in the independent claim. Various embodiments are disclosed in the dependent claims. The features recited in dependent claims and in the description are mutually freely combinable unless otherwise explicitly stated.

The feature that the packing material is arranged into one or more movable racks enables easy maintenance of the packing material. The racks may be moved outside the column for checkout, cleaning and/or replacing the packing material. The modular arrangement of the racks enables a targeted maintenance of a specific section of the packing material in a short time. The condition of the packing material may be checked briefly by simply drawing a rack outside the column for a visual estimate.

The packing material arranged into one or more movable racks also provides an effect that not all the packing material is arranged in the same room inside the column but there are spaces between the racks. This promotes even liquid distribution in the column, and the presence of specific water redistributors is not necessary.

Brief description of the figures

Figure 1 shows a stripping column filled with the packing material

Figure 2 shows the outer side of the column of Figure 1

Figures 3a-c show an example of a rack frame seen from left, behind and right

Figure 4 shows an arrangement of a frame with four stacked racks

Figure 5 shows an example of a column having two hatches

Figure 6 shows an example of a column with the hatches opened

Figures 7a-c show detailed views of racks

Figure 8 shows an example of a column containing six stacked racks installed on rails

Figure 9 shows an example of a packing body made of plastics

Figure 10a-c show examples of liquid guides in a rack arrangement seen from the front side

Detailed description

The present application relates in general to containers such as columns, towers, reactors, vessels and the like containing packing material. The containers may be used for example for treating liquid, such as for liquid purification. One example of a method suitable to be carried out in such a system or arrangement is stripping. In one example the stripping is air stripping.

One embodiment provides a column, such as a stripping column, containing packing material. In one embodiment the column is stripping column. The stripping column may be also called a stripping tower, or a stripping tower column or a vessel. In one embodiment said stripping column is an air stripping column.

The stripping column may be manufactured for example of fiber reinforced plastics (FRP), or metal, such as aluminium or stainless steel. Tower internals such as distributors and supports may be manufactured for example of polymeric materials, or metal, such as aluminium or stainless steel.

In one embodiment the column is cylindrical. Cylindrical form is a traditional form for such a column. In general the column is in upright position. The column may also be angular. In one embodiment the cross-section of the column is square or substantially square, meaning for example that the edges of the column may be rounded. The height of a column may be in the range of 5-15 meters, such as in the range of 8-12 meters for example about 10 or 11 meters. The diameter of a column may be in the range of 1.5-3 meters, such as in the range of 2-2.5 meters, for example about 2.4 meters.

The column contains at least one rack for the packing material. The packing material is organized into the one or more racks to obtain a modular arrangement. In one embodiment the rack is installed on rails and movable from the column for maintenance or service. In one embodiment the rack is pivoted and movable from the column for maintenance or service. The rack may be pivoted to the column or to a frame inside the column. The column also contains at least one opening hatch for said maintenance, for example two, three or four hatches. The rack may be moved on the rails or on the pivot in such way that it may be moved outside the column for inspection, cleaning or removing the packing material through said hatch. There may be one hatch for one rack, or more than one rack may be accessible through one hatch, for example two, three or four racks through one hatch. Said hatches are usually arranged to be operable without tools, for example they are not sealed by bolting. The feature that the hatches can be opened easily facilitates the ease of maintenance of the column.

In one example the rack stays outside the column during the maintenance or service, but is not removed from the column. The rack may be partly on rails or it may remain attached to a pivot. In one example the contaminated or aged packing material is removed from the rack when the rack is pulled out from the column, and preferably replaced with clean or new packing material. In one embodiment the rack is removable from the column. This means that the rack or a part of it may be completely removed or demounted, for example lifted away from the column, rails, rack frame and/or pivot and moved to another location for example by using a crane or a forklift. In one example a new rack containing clean or new packing material is installed to replace the removed rack. In one example the contaminated rack is cleaned, for example with a pressure washer. In such case the packing material may remain in the rack. In one example the packing material is removed from the rack, and preferably the rack is washed, for example with a pressure washer. In general the maintenance or service requires certain amount of working space. For example, in general a column of 2x2 m requires approximately 2x6 meters of working space. This may, however, depend on the need of any specific equipment, such as forklifts, cranes, pressure washers etc.

The aging of the packing material also affects the performance thereof. The used high temperature may also promote the aging of the material. It may be necessary to change aged packings to new ones after a certain time period. In such case the main reason for the maintenance is not necessarily the cleaning of the material. In general, the maintenance, or service, as used herein refers to any operation wherein the handling of the packing material, such as washing, removal or refilling, is required. The maintenance may also refer to service or maintenance of the racks, rails or any parts of the racks, rails, frames or column. The racks may stay outside the column completely or partially or they may be completely removed during the service or maintenance.

One embodiment provides a method for maintenance of a column, the method comprising providing the column described herein, opening the hatch in the column, moving a rack out from the column, and carrying out the maintenance. After the maintenance the rack is moved back, or a new rack is provided and moved into the column and the hatch is closed. The maintenance refers to any maintenance or service actions described herein, such as inspection, cleaning or removing the packing material, the racks, the rails, the frame or the column.

A rack as used herein refers to a frame structure arranged to hold a plurality of packing bodies. Also the term “module” may be used. In general, the rack is a boxlike structure having water- and air-permeable parts, for example the bottom part. The upper part of the rack may be open, i.e. without a cover, or it may also have a water- and air-permeable part, which is capable of retaining the packing bodies. In one example a rack contains a bottom permeable to liquids and gases, which may be implemented by using a net, a grille, a grid or a perforated sheet. In general, a net, a grille or a grid with aperture, hole or slot size smaller than the diameter of a single packing body is useful. The racks may be made of metal, plastics or composite materials, such as fiber reinforced plastics (FRP), aluminium, or stainless steel. The nets, sheets or other parts of the racks may be also made of the metal, plastics or composite materials, such as fiber reinforced plastics (FRP), aluminium, or stainless steel.

In one embodiment the rack is hexahedral. Such a rack has a square cross-section, or the rack is hexahedron in shape, such as (rectangular) cuboid (three pairs of rectangles) or a cube. In a rectangular cuboid, all angles are right angles, and opposite faces of a cuboid are equal. Flexahedral racks may be installed on rails for easy movement. Even though the column was cylindrical and therefore the packing material in hexahedral racks would not fill the interior completely, the benefits provided by the easy accessibility of the packing material are greater than the minor loss in efficiency caused by a smaller volume of the packing material in the column. Also the width of the corresponding hatch can be kept relatively small. Examples of the dimensions of the hexahedral racks include a width in the range of 100-300 cm, such as in the range of 150-250 cm, for example about 150 cm or 200 cm, a height in the range of 50-300 cm, such as in the range of 100-250 cm, for example about 200 cm, and/or a depth in the range of 200-400 cm, such as in the range of 150-300 cm, for example about 200 or 250 cm. The volume of a rack may be in the range of 1-10 m3, such in the range of 4-8 m3 In one example the rack has a width and a depth of about 200 cm. In one example the rack has a width, depth and height of about 200 cm. In one example hexahedral racks are used in a column having a square or substantially square cross-section. In such case the racks fill the column efficiently. The hatches have substantially the same width as the column, as shown in Figure 5.

In one embodiment the rack has a depth or length greater than the depth of the column, for example twice the depth of the column, or the width of the column in the case of cylindrical column. In such case a part of the rack, for example half of the rack, is outside the column during the usage. There are openings at the opposite sides of the column enabling the movement of the rack on rails through the column. When the maintenance or service is required, the rack is pushed through the column in such way that the used packing material comes outside the column and a part of the rack containing clean material is inserted to the column. In this way the interruption in the use is very short, as the maintenance can be carried out during the use of the column. The openings must be sealed during the use. In this embodiment there is a cover plate in the opening of the column (in the middle of the rack) and at the ends of the rack, which are generally sealed with gaskets to prevent leakage. This principle is shown in Figure 7c, wherein there are seals or gaskets 78 between the rack 76 and the cover plate 79. In one example the rack has a width of 2 meters and length (depth) of 4 meters.

In one embodiment the rack is cylindrical. Such a rack has a round cross-section. When cylindrical racks are used, the interior of the column may be filled more efficiently. However, the movement of the rack may be more challenging than with hexahedral racks. In one example a cylindrical rack is installed on one or more rails, which are below or above the rack. In one example a cylindrical rack is installed on a vertical pivot or axle on one side in such way that the rack may be turned on the pivot or axle outside the column in the horizontal direction. In one example the rack may be lifted from a pivot for replacement and/or maintenance. In such case the pivot may comprise a simple neck and the rack has a corresponding hole arranged to fit to the neck. The pivot is load-bearing so it carries the rack so that the rack is movable around the pivot, i.e. it may be said that the rack is pivotally movable or pivoted. Also, a rack of other shape may be pivoted, such as a hexahedral rack. When a rack is pivoted, no lifting tools, such as cranes or forklifts, are required in the maintenance for accessing the packing material.

In one example a rack comprises a frame for receiving a container for the packing material. The container may be a box or the like arranged to hold a plurality of packing bodies. Such a box may have water- and air-permeable parts as described above for the rack. Such a rack structure may be called as a two-part rack, wherein the frame part may on the rails or pivoted, while the box part is removable from the frame for maintenance. The container part may be lifted into the rack frame and out from the rack frame. The rack frame may contain a framework for the container, which may be inserted into the rack frame from the top.

In one embodiment a rack has at least two separable parts, i.e. the rack may be split into two. In one example the rack has two separable parts. This feature facilitates the maintenance of the rack, such as the washing of the rack. In one example the rack is split along the horizontal line, i.e. the two parts are stacked. In one example the height of such a two-part rack is in the range of 1.5-2.5 meters, for example about 2 meters. The height of one part may be half the height of the whole rack, for example about 1 meter. When using a pressure washer to wash the rack from the upper side and bottom side, only half of the height of the packing material needs to be reached with the spray, for example about 0.5 meters. The at least two separable parts may be attached to each other with attaching means which allow easy dismantling of the rack. In one example the rack includes parts, such as pegs, bars or the like, going through corresponding holes in the rack holding the at least two parts of the rack together, for example two or four of such parts, for example at the corners of a rack. Also other means for locking the separable parts together may be used, for example clamps or the like, which may be unlocked easily, especially manually without tools.

The rails as used herein refer to parts onto which the racks may be installed movably. The rack contains a corresponding part arranged to fit to the rail, or arranged to receive the rail. In general a part of the rack lays on a part of a rail. A simple form of a rail is a plate rail, for example made of metal and welded to a frame or to the interior of the column. The rack is placed on the rails. The rails installed in the frame or in the column may be called fixed rails. In one example a rail has an L-profile. In one example a rail has a U-profile. In one example a rail has a square profile. A rail may have a width and/or height in the range of 3-15 cm, for example in the range of 5-10 cm.

In one embodiment the rail or the rack contains a liquid guide for distributing liquid. In general the guide directs liquid towards the inner part of the packing material in the next rack below as the liquid is coming down through the column packing. The guide is located below the rack, and it is generally a longitudinal part near the edge of the bottom of a rack. In one example the guide is located in the rail 80 attached to a frame 30. Such a guide 84, 85, 86 may be for example a longitudinal ridge 84 or protruding longitudinal strip in the middle of the vertical side of a rail 80 (Figure 10a), or it may be a shaped part 85 of an L-profile rail 80 (figure 10b), for example an arched part of a horizontal part of the L-profile. In general, the guide is directed downwards, for example having an angle of about 10-80°, for example about 30-60° with a horizontal line. The guide 84, 85, 86 may have a width in the range of 1-10 cm, for example in the range of 2-5 cm. The guide 84, 85, 86 promotes even liquid distribution in the system and no separate planar water redistributors or the like platforms are required. In one example the guide 86 is in the bottom of the rack 41 near the edge of the rack (Figure 10c). A hexahedral rack may have two, three or four guides 86.

In one example the rail is a rod. In one example the rods are arranged to fit into a corresponding pipe or tube or U-profile attached to a rack and arranged to receive said rod. The rack may also have a hole in the frame arranged to receive said rod.

In one example there are at least two rails arranged to be positioned on both sides of the rack. In one example the rails are positioned below a rack or at the lower or bottom part of the rack. In one example the rails are positioned on the sides of a rack, for example at the lower part, at the middle part or at the upper part of the rack. In one example U-profile rails are used, wherein there is a corresponding rail on the sides the rack arranged to fit to the U-profile rail. In one example there are grooves on the sides of a rack for receiving the rail. In one example there are U-profile rails on the sides of a rack for receiving the rail. In one example the rails are positioned above a rack. A rack being installed “on rails” refers to all embodiments and examples disclosed herein regardless of whether the rails are below the rack, at the sides of the rack, or above the rack, or to any other suitable embodiments.

The rails or the racks may further include any means facilitating the movement of the racks on the rails, such as wheels or sliding planes or parts, for example plastic parts. A rack may have further rails on sides, or below or above, arranged to be fitted into a U-profile rail in the frame, or onto an L-profile rail in the frame. In one example there are two rails arranged to support the bottom of a rack. In one example there are further two rails arranged to support the top of a rack. There may be a further moving rail arranged to support the rack in the pulled-out position and/or to enable prolonged movement of the rack. The moving rail may be installed in the rack or in the fixed rail.

The column may include a frame for the at least one rack. The frame includes rails attached at positions enabling installation of the rack(s), for example the stacking of the racks. The frame may be made of metal, such as steel. An example of a frame is shown in Figures 3a-c, wherein the frame is shown from left, behind and right.

There is usually more than one rack in the column. In one embodiment the column contains at least two stacked racks. The column may contain for example two, three, four, five, six, seven, eight, nine or ten stacked racks, or even more. When more than one rack is used, an empty space without packing material will be left between two racks. This may enhance the liquid distribution in the column and in the packing material, and no planar water redistributor generally used in the prior art is required. Therefore, in one embodiment the column contains no liquid redistributor(s). The use of more than one rack enables a targeted maintenance or inspection of the packing material. It also enables the distribution of the workload as only a part of the packing material needs to be handled at once, and the racks may be drawn out even manually. Also the height of a single rack is low enough enabling washing of the packing material on site.

In one embodiment the packing material comprises a plurality of separate packing bodies. Plurality may refer to tens or hundreds or packing bodies, or even more, per one rack. A rack may be filled completely or partially with the packing bodies, for example about 100%, about 95%, about 90%, about 80% or about 70% of the rack volume. The packing bodies or pieces are usually bodies having a specific form, in general open structure, and providing a large surface area. The packing may have varying material, surface area, flow area, and associated pressure drop. Uniform packing material, such as structured packing, is excluded from this embodiment. Structured packing is usually formed from corrugated sheets of perforated embossed metal or wire gauze. The result is a very open honeycomb structure with inclined flow channels giving a relatively high surface area but with very low resistance to gas flow.

Examples of suitable packing material bodies include tile, intalox saddle, raschig ring, pall ring, Zbigniew Bialecki ring, berl saddle and tri-pak. The packing material may be made of for example metal, plastics or composite materials. For example usually pall ring and tri-pak may be made of plastics. Generally used plastics include polypropylene or polyethylene. Figure 9 is a photograph of a single Bialecki ring type of packing body made of plastics used in the embodiments. The diameter of a packing body is generally in the range of 15-200 mm. The height of a packing body is in the same range, and in many cases the height is equal to the diameter. The specific area provided by the packing material may be for example in the range of 20-250 m2/m3. In one example the packing material is made of light material, such as plastics and/or composite materials. Light packing material facilitates manual handling of the material and/or racks.

The packing material is designed to provide high surface area, flow area, and/or to prevent pressure drop across the packing. Also important is the ability of the packing material to not stack on top of itself. If such stacking occurs, it drastically reduces the surface area of the material.

In some examples the packing material is inert material, i.e. it does not react in the process. In another example the packing material is not inert and it may play a role in the process, for example it may take part in the reactions as a catalyst or as a reactant.

The columns, such as stripping columns or air stripping columns as described herein may be used for removing a variety of different substances from liquids. Examples of such substances include acetone, ammonia, amyl acetate, amyl alcohol, aniline, benzene, butyl acetate, carbon tetrachloride, chloroform, dimethyl formamide, fermentation broths, iso-octanol, isopropyl alcohol, kerosene, methanol, methyl iso-butyl ketone, methylene dichloride, para-nitrochlorobenzene, phenol, saturated salt solutions, tert butanol, tributyl phosphate, trichloroethylene, and xylene.

One example provides a method for removing substances from solutions using the column described herein, such as a stripping column, for example an air stripping column. The method for removing substance from solutions comprises -providing a solution containing said substance, -leading said solution into the top of the column described herein, -leading gas, such as air, to the bottom of the column for stripping the substance into the gas stream. One or more substances may be removed simultaneously. The substance(s) may be removed partially or completely.

In one example the solutions are aqueous solutions. The aqueous solution may refer to water, such as groundwater or waste water. One example provides a method for removing ammonia from waste water using the column described herein. Ammonia stripping is a desorption process for lowering the ammonia content of a wastewater stream. Some wastewaters contain large amounts of ammonia and/or nitrogen-containing compounds that may readily form ammonia. It is often easier and less expensive to remove nitrogen from wastewater in the form of ammonia than to convert it to nitrate-nitrogen before removing it. Ammonia is a weak base which reacts with water (a weak acid) to form ammonium hydroxide. In ammonia stripping, lime or caustic is added to the wastewater until the pH rises to 10.8-11.5 which converts ammonium hydroxide ions to ammonia gas. In one example the solutions are organic solutions.

In general, there are two variations of ammonia stripping towers, cross-flow and counter-current. In a cross-flow tower, the solvent gas (air) enters along the entire depth of fill and flows through the packing, as the alkaline wastewater flows downward. A counter-current tower draws air through openings at the bottom, as wastewater is pumped to the top of a packed tower with a pump. Free ammonia (NH3) is stripped from falling water droplets into the air stream. However, one disadvantage of the ammonia stripping is scale formation in the packing material. The scale is traditionally removed by using chemicals, such as hydrochloric acid. In some cases the high pH of the wastewater or the high temperature used in the process corrupts the packing material, which then needs to be replaced. Also in such cases the dismantling of the packing material is often required.

In one example the free ammonia is let to another reactor wherein it is reacted with sulphuric acid. In this reaction ammonium sulphate is formed and it may be recovered. If the air flow of the process is closed all the nitrogen will be used in the preparation of ammonium sulphate and the odour problems are avoided.

The method for removing ammonia from waste comprises -providing waste water containing ammonia, -adjusting the pH of the waste water to basic range, such as pH in the range of 10-12, -leading said basic water into the top of the stripping column described herein, -leading air to the bottom of the column for stripping the ammonia into the air stream. The treated water is led away from the column. The free ammonia in the air is lead away from the column, for example to another reactor to be treated, for example with sulphuric acid.

A general air stripping column wherein the packing material is not organized in racks is shown in Figure 1. The column 10 is in upright position and it has an inlet 14 and outlet 15 for air, the inlet 14 being at the bottom or at the lower part of the column 10 and the outlet 15 at the top or at the upper part of the column 10. For the water to be treated there is an inlet 16 at the upper part of the column 10 and an outlet 17 at the bottom or at the lower part of the column 10. The water inlet 16 is connected to a water distributor 18 at the upper part of the column 10, above the packing material 12. The column is filled with packing material 12 and has a water distributor or a support grid 11 at the middle part of the packing material dividing the packing material to an upper section 12a and a lower section 12b. In figure 2 there are round hatches 22, 24 in the column 10 wall for maintenance, such as hatches for inserting 22 and hatches for removing 24 of the packing material. In general such service hatches 22, 24 are manholes which are usually bolted and not designed to be opened without tools.

An example of the frame 30 wherein the racks according to embodiments are to be installed is shown in Figures 3a, b and c, the figures showing the left side (a), the back side (b) and the right side (c) of the frame. No rails are shown in Figures 3a-c.

Figure 4 shows an example of the racks 41-44 installed to a frame 30. In the Figure 4 four stacked racks 41-44 are shown from the front side of the installation. There is also an outlet 17 for the water and an inlet 14 for the air at the lower part of the frame, and an inlet 16 for the water and an outlet 15 for the air at the upper part of the frame. In Figure 4 there is also an optional service hatch 48 at the lower part of the arrangement for cleaning the floor of the column. The arrangement shown in Figure 4 may be installed inside a cylindrical column. The arrangement shown in Figure 4 may also be installed inside a quadrangular column (having a square cross-section), or the arrangement may be in the form of a quadrangular column. In such case there is less empty space inside the column, and the external dimensions of the column are smaller. Such a column requires less space and may be installed in a variety of locations.

Figure 5 shows the arrangement of Figure 4 installed in a column having a square cross-section. The column has two hatches 52, 54, which are closed in the figure 5.

Figure 6 shows the column with the hatches 52, 54 opened. The racks 41-44 can be accessed through the opened hatches 52, 54, two racks through one hatch.

Figures 7a-c show detailed examples of the racks. Figure 7a shows a view from the top side of a rack 70. The rack has rails 72 on its sides arranged to be fitted onto a rail in the frame, for example an L-profile rail in the frame. Figure 7b shows a grill bottom 74 of a rack, which is permeable to liquid and gas. Figure 7c shows an example of the frame 76 of a rack as a cross-section along line A-A. There are seals 78 between the frame 76 and a plate 79 for sealing the rack with the column, such as the hatch or a cover plate. The seals may be made of elastic material, such as elastomers or silicone.

An example of a stripping column according to an embodiment is shown in figure 8. The column 10 is in upright position and it has an inlet 14 and outlet 15 for air, the inlet 14 being at the bottom or at the lower part of the column 10 and the outlet 15 at the top or at the upper part of the column 10. For the water to be treated there is an inlet 16 at the upper part of the column 10 and an outlet 17 at the bottom or at the lower part of the column 10. The bottom of the column may include a distributing space. The water inlet 16 is connected to a water distributor 18 at the upper part of the column 10, above the packing material 12. The column does not contain a separate water redistributor, support grids or any fixed platforms for the packing material.

The column 10 contains six stacked racks 41-46 installed on rails 80 in such way that the racks may slide on the rails and can be moved in the horizontal direction as a drawer. This enables pulling a rack outside the column through a hatch in the column wall. The racks are packed with the packing material 12. One rack 43 has been drawn out for maintenance through a hatch in the side of the column (column not shown). When the rack 43 is at the pulled-out position, the packing material 12 may be easily removed from the rack and the rack may be refilled with new packing material. Alternatively, it is possible to wash the packing material for example with a pressure washer.

From Figure 8 it can be seen that there is an empty space between the individual racks or between the packing materials in the individual racks. The distance d between the packing materials in adjacent racks may be at least 3 cm, for example at least 5 cm. In one example the distance d is in the range of 3-20 cm. In one example the distance d is in the range of 3-15 cm. In one example the distance d is in the range of 5-10 cm. A distance between two adjacent racks may also be defined, which may be at least 3 cm, for example at least 5 cm. In one example the distance between two adjacent racks is in the range of 1-20 cm. In one example the distance between two adjacent racks is in the range of 2-10 cm.

One embodiment provides a use of the column described herein for removing substances from a solution, such as an aqueous solution. In one embodiment the use is the use of stripping column described herein for removing ammonia from waste water, preferably by the air stripping method.

Claims (16)

A column, such as a stripping column, containing a filler, the column comprising: - at least one frame for the filler, which frame is removable for column maintenance, and - at least one opening hatch for said maintenance, characterized in that the frame is movable horizontally through the hatch . A column according to claim 1, wherein the frame is mounted on rails. The column of claim 1, wherein the frame is pivotable. A column according to any one of the preceding claims, wherein the frame is cubic. A column according to any one of claims 1 to 3, wherein the rack is cylindrical. A column according to any one of the preceding claims, wherein the frame is removable from the column. A column according to any one of the preceding claims, wherein the frame comprises at least two separable parts. A column according to any one of the preceding claims, wherein the column is cylindrical. A column according to any one of claims 1 to 7, wherein the column is angular, for example having a square or substantially square cross-section. A column according to any one of the preceding claims, wherein the column comprises at least two overlapping racks. A column according to any one of the preceding claims, wherein the filler comprises a plurality of discrete fillers. A column according to any one of the preceding claims, wherein the rail or rack comprises a fluid guide for fluid distribution. A column according to any one of the preceding claims, wherein the column does not comprise liquid redistributors or support nets. A column according to any one of the preceding claims, which is a Strip Stripping column, such as an air stripping column. A method of maintaining a column, such as a stripping column, comprising: - arranging a column according to any one of the preceding claims, - opening the column door, - moving the frame out of the column, and - performing maintenance. A method for removing material from solutions, the method comprising: - providing a solution containing said substance, - introducing said solution into the top of a column according to any one of claims 1-14, - introducing a gas such as air into the bottom of the column to strip the substance.