GB2098644A - Method of cleaning polluted subsoil and apparatus for carrying out the method - Google Patents

Abstract

A method of cleaning polluted subsoil includes the step of placing a plurality of pipes (4) in the earth, the interior of the pipes (4) communicating near or at the lower ends thereof with the pore space of the subsoil (3) and the upper ends of the pipes being above the surface of the earth, followed by the step of passing steam downwards through the pipes (4) into the subsoil (3) and displacing fluids present in the pore space in upward direction to the surface of the earth, and continuing the steam injection at least until vapour is emerging from the earth. The apparatus for cleaning polluted subsoil, includes a steam generator (11) whose steam outlet (10) communicates with the upper ends of the pipes (4). <IMAGE>

Description

SPECIFICATION Method of cleaning polluted subsoil and apparatus for carrying out the method Lately, problems are arising at increased frequency in those industrial areas where spillages of liquids occur, which liquids pollute the earth.

Fortunately, the majority of such liquid spillages only pollute the surface soil, that is the upper layer of the earth wherein the small plants (that do not root extremely deep) grow. These spillages are often of relatively small volume. They should, however, be removed as soon as possible, since otherwise the liquids may seep to lower levels thereby entering the subsoil lying below the surface soil. Also, most of such liquid spills give off a nasty smell or even noxious vapours, which is another reason for quickly removing such liquids from the soil.

Small-sized spillages that in the initial phase of the spillage only moisten the surface soil, can be removed in a simple manner by lifting the polluted volume from the surface soil and subjecting the polluted material to a heat treatment in a furnace.

Hereby the polluting liquid will either be burned or vapourized and the cleaned soil can - if desired be eplaced to the original location.

When relatively large volumes of polluting liquid have been spilled and/or when the spillage of liquid has occurred in the past or from buried storage tanks, the polluted area will be at a relatively great depth below the surface of the earth. Such pollution of the subsoil - that is that part of the soil lying below the upper layer of the earth in which layer the plants grow-can by the above-described cleaning method only be removed at astronomically high cost as extremely large volumes of soil have to be dug outfortreatment in furnaces.

Since this latter type of pollution is lately met rather frequently, a better and cheaper method for removing this type of pollution should be designed as otherwise in future large areas will become inhabitable and/or unsuitable for the recovery of potable water therefrom.

Object of the invention is a method of cleaning polluted subsoil in a relatively cheap and simple manner whereby the areas overlying the polluted zone will not be disturbed or be disturbed to a small extent only.

The method according to the invention includes the step of placing a plurality of pipes in the earth, the interior of the pipes communicating near or at the lower ends thereof with the pore space of the subsoil and the upper ends of the pipes being above the surface of the earth, followed by the step of passing steam downwards through the pipes into the pore space of the subsoil and displacing fluids present in the pore space in upwards direction to the surface of the earth, and continuing the steam injection at least until vapour is emerging from the surface of the earth.

The apparatus according to the invention includes a steam generator and a plurality of pipes arranged in the earth, the pipes at or near the lower ends thereof communicating with the pore space of the subsoil and with the upper ends thereof communicating with the steam outlet of the generator.

The invention will be described by way of example in more detail with reference to the drawing which shows a vertical section over a polluted area of the subsoil and the earth surface thereabove.

In the drawing, an area 1 polluted by a spillage of hydrocarbon liquid (such as kerosine) is shown. This area 1 is situated partly in the surface soil 2 and for the larger part thereof in the subsoil 3. Pipes 4 have been placed vertically in the soil 2 and subsoil 3 in a manner such that the lower open ends 5 of the pipes are below the lowest level of the polluted area 1. The pipes have been placed in any suitable manner, such as by drilling or ramming. The outer surface of the pipes should fit closely in the hole or bore wherein the pipe is located, this in order to prevent shortcircuiting of the steam that is to be injected in a later stage of the method. This short-circuiting would then take place via the space left between the outer wall of a pipe and the wall of the hole or bore surrounding the pipe.

The upper ends of the pipes 4 are in communication with a manifold 6 via suitable conduits 7. The manifold 6 communicates via a conduit 8 and a valve 9 with the steam-outlet 10 of a steam generator 11 that is placed at some distance from the surface area 12 overlying the polluted area 1.

A hood 13 is placed over part of this area 12 and possibly, if the circumstances allow such, with the lower rim thereof in the soil 2, in order to close off the exterior of the hood from the atmosphere.

A condenser 14 is placed inside the hood 13.

Below this condenser, a collecting tray 15 is arranged that communicates via a conduit 16, a pump 17, and a hose 18 with the interior of a tank car 19.

Cooling fluid is circulated through the condenser 14 by means of a pump 20, the conduits 21, 22 and a heat exchanger 23.

The polluting liquid is present in the space between the grains of the surface soil 2 and the subsoil 3. The volume of this space (also indicated as pore space) that is polluted by the liquid is situated within the area 1. The polluting liquid is to be driven out of the pore space of area 1 to the surface. To this end, steam is generated in the steam generator 11, and passed on via the steam outlet 10, the valve 9 and the conduit 8 to the steam manifold 6, and subsequently from this manifold via the conduits 7 to the pipes 4. Initially, the steam will condense in the pipes 4 but after these have been heated up, the steam will flow out of the lower ends 5 thereof into the pore space of the subsoil 3 and heat up the grains of the subsoil as well as the hydrocarbon liquid that is present in the pore space of the subsoil.The hydrocarbon liquid is vapourized at the increase of temperature in the lower levels of the contaminated area and the vapourized hydrocarbon liquid together with the injected steam will flow upwards (see arrows 24).

Initially, the vapours will condense at higher levels in the polluted area 1, but since the injection of steam is continued, the temperature of this area will gradually be increased to a value at which no condesation of steam or hydrocarbon vapour will take place.

The vapours will then flow upwards through the pore space of the subsoil 3 and the soil 2 and pass out of the surface of that part of the area 12 that is located below the hood 13. Inside the hood 13 the vapours condense on the surface of the condenser pipes 14 that are continuously being cooled by the cooling liquid circulating therethrough as well as through the heat exchanger 23 by the action of the pump 20.

The condensed vapours form a mixture of water and hydrocarbon liquid that is collected on the tray 15 and subsequently pumped by means of the pump 17 into the tank car 19. The mixture of liquids is then transported to a refinery or other suitable location where the components ofthe mixture can be separated from one another.

It will be understood that the rate at which the steam is flowing into the subsoil 3 is chosen such that the fluid pressure that is being built up in the polluted area 1 is below the pressure at which cratering or fracturing of the soil would occur. When cratering occurs, a conically-shaped part of the soil is lifted by the fluid pressure in the pore space of the soil, as a result whereof steam will escape between the lifted part and the remaining part of the soil.

When fracturing occurs, a horizontal fracture is formed by the increase of fluid pressure at the level of the lower ends 5 of the pipes 4. The steam flows into this fracture, as a result whereof a large area of the soil is lifted as a whole. The steam continues to flow into the fracture without flowing upwards to the surface.

The above-mentioned phenomena will not occur if the steam is injected at a sufficiently low rate to keep the pressure at the level of the lower ends 5 of the pipes 4 below fracturing or cratering pressure, which ever is the lowest. No exact figures can be given for the magnitude of this rate since this is highly dependent on the characteristics of the subsoil such as the permeability and injectivity of the pore space of the subsoil. In general, however, it can be said that the steam injection rate should be chosen such that the steam flow rate in upward direction in the pore space of the subsoil at a level about midway between the lower ends of the pipes 4 and the earth surface is between .02 ton/m2/day and 2 ton/m2/day.

If the injectivity of the subsoil is rather low and steam does not start to flow upwards through the pore space, the steam pressure at the lower ends 5 of the pipes 4 should be gradually increased. How ever, care should then be taken that the pressure does not rise above the pressure at which cratering or fracturing occurs. If this would threaten to occur, a plurality of additional pipes (not shown) should be placed parallel to the pipes 4 and distributed over the area 12 below the hood 13. The open lower ends of these additional pipes should be at a higher level than the level of the open lower ends 5 of the pipes 4.

Further, the upper ends of the additional pipes should be open to the interior of the hood 13. By means of these additional pipes, the polluting liquid present between the levels of the open lower ends of the pipes 4 and the additional pipes will be vapourized by a steam supply at relatively low pressure, since the vapourized liquid and steam do not have to pass through the entire height of the pore space of the polluted area 1, but will find a short-circuit to the interior of the hood via the additional pipes.When pure steam starts to leave the additional pipes, these pipes should be closed off and further additional pipes should be placed that end at increasingly higher levels in the polluted area 1, until finally the total volume ofthis area is being steamed out and the vapours pass directly out of the soil 2 into the interiorofthe hood 13.

It will be appreciated that at large horizontal extensians of the polluted area 1, the cleaning of this area should take place section-wise. In the embodiment of the invention shown in the drawing, pipes 14 are then withdrawn from the cleaned section of area 72 and replaced in an adjoining section of this area. The hood 13 is also removed to this adjoining section and the cleaning operation is then repeated in the manner as described already hereinabove. If necessary, further sections are also cleaned in the same way.

In the embodiment of the invention as described hereinabove with reference to the drawing, the pore space of the area 1 was not fully saturated with liquid(s). As a result thereof, the steam and the condensed water could flow upwards through the pore space and removal ofthevapourizable polluting liquid(s) could start after the temperature of the subsoil was sufficiently raised to vapourize the polluting liquid(s).

However, in other locations, polluted areas will be present having the pore space thereof fully liquid saturated. The liquid in the pore space may be the polluting liquid or consist of ground water and polluting liquid. When removing this type of pollution with the method of the present invention, the steam and the condensed water will displace the liquid(s) present in the pore space to the surface and provisions are to be made to prevent excessive flooding of the surface area 12. This can be done by casting a dam or a small dike around this area (or around parts of this area) and pumping the liquid(s) emerging at the surface to a suitable storage, or to a separation plant where the polluting liquid is separated from the ground water, which latter is then returned to a location outside the area 12. Also, holes may be dug in the soil for collecting the displaced; liquids, from where they are transported to a site where they can be further treated. When the polluted area 1 has been sufficiently heated, a mixture of steam and vapourized polluting liquid starts to emerge from the surface 12, which vapour-mixture is condensed in the hood 13 as has been described hereinbefore with reference to the drawing.

If after the above described treatments of the pol luted area 1, any liquids or any solids that were dis solved in the polluting liquid are found to have remained in the pore space of the contaminated area, these solids or liquids can be removed from the area by dismounting all the equipment shown in the drawing but for all or some of the pipes 4. Subsequently clean water is supplied to the surface overlying the area 1, and suitable pumping means are mounted to cooperate with the pipes 4 in a manner such that the water that flows down through the pore space of the area 1 is being recovered via these pipes. The water thus recovered is mixed with the liquids and solids that were left in the area 1 after the steam treatment. After separating these liquids and solids from the recovered water, the water may be re-used.The use of small dikes is recommended when flooding those parts of the earth surface overlying the area 1.

In an area with an originally high ground water level, the above described flushing step carried out after the steam treatment may be carried out in an alternative manner by supplying the flushing water via the pipes 4 (which have been disconnected from the steam generator 11) to the lower levels of the area 1. The water will fill up the pore space of this area, and displace to the surface any liquids or soluble solids that have been left in the pore space after the steam treatment. At the surface, flushing water and any other liquids (together with any solids dissolved therein) are collected and subsequently removed forfurthertreatment.

In the above described treatments it may sometimes be advantageous to prevent horizontal displacements of liquids between the pore space of the polluted area 1 and the pore space of the area surrounding the polluted area. Such displacement may be prevented by forming a barrier in the soil such as a cofferdam or a frozen area, or by water wells that locally reduce the watertable. Also, a combination of these measures may be taken.

Also, the polluted area 1 may at least partly be fenced in to prevent horizontal liquid displacements between the fenced-in area and the rest of the soil, whereafterthe ground water level in the fenced-in area is lowered by wells that are distributed over the fenced-in area. If necessary, the ground water that is pumped to the surface is treated for removal of any pollutant present therein. After lowering ofthe water table, the steam treatment as described with reference to the drawing is carried out. If necessary, the level of the water table is being controlled and measures are taken to keep it at a desired depth.

When a hood is applied for catching the vapours that emerge from the surface when the underlying soil and subsoil is being steam-treated, any type of hood (other than the one shown in the drawing), that is suitable for the purpose may be applied. The same applies for the condenser. If condensation of vapours takes place against the walls of the hood, suitable gutters should be mounted to prevent the condensation products from re-entering the surface soil 2.

Although the pipes 4 shown in the drawing are vertically positioned in the soil, the invention is not restricted thereto. If necessary, such as when cleaning a polluted area that is fully or for part thereof situated below an inaccessible surface area (such as a housing area), the pipes may be driven into the soil in a slanted position whereby the upper ends of the pipes are outside the inaccessible surface area, but the lower ends of the pipes are situated below this area. If possible, a hood should be applied, and suih hood should cover the surface area where the vapours emerge from the earth.

An attractive manner of placing the pipes is the following. The lower end of each pipe is closed off by a streamlined nose portion, whereafterthe pipe is driven or hammered into the soil. The nose portion reduces the friction that is met during the driving operation, and also prevents grains of the soil from entering the pipe. Moreover, this way of setting the pipe ensures a close fit of the pipe in the hole, which is advantageous during the subsequent steaming period as no steam can then escape through the hole to the surface. After the pipe has been placed, some openings (also known as perforations) are shot in the side wall of the pipe at a level that is below the lowest level of the polluted area. Such openings are shot by perforating guns that are well known in the oil-recovery industry and therefore do not require a detailed description.In an alternative manner, the openings may be drilled in the pipe prior to setting the pipe in the soil. Although some grains may enter these latter holes when the pipe is being driven into the soil, these grains will not block the holes and steam flow through the holes will not be hampered.

Although wet steam may be applied in the present process, the use of dry steam or even superheated steam is preferred in view of the large heat content thereof.

It will be appreciated that the interior of the hood 13 should be sealed off from the atmosphere to prevent the escape of the vapours to the atmosphere.

When the lower rim of the hood cannot be sealed off by the soil, other suitable sealing means may be used for this purpose.

When cleaning polluted areas that extend to very great depths in the subsoil, the vertical flow resistance to the steam in the subsoil is often very high, which requires high steam pressures at the levels where steam is being passed into the pore space of the subsoil. However, this may give rise to cratering or fracturing of the soil. This problem can be solved by initially placing the plurality of steam pipes such that the steam is injected at a relatively shallow level in the polluted area. After the surface soil and that part of the subsoil situated between the surface soil and the steam injection level has been cleaned, the steam pipes are driven to a greater depth in the polluted area, whereafter that part of the subsoil between the new steam injection level and the already clean area is being cleaned by a steam treatment.

This procedure is repeated until the lowermost level ofthe polluted area has been reached. Subsequently, the steam pipes are placed in an adjoining section, which section is then cleaned in the same manner.

Although cratering of the soil should be obviated during the steaming-process to prevent shortcircuiting of steam between the lower ends of the steam pipes and the surface of the earth, such cratering may be advantageous prior to starting the steaming-process if one or more relatively impermeable intermediate layers are present in the subsoil at a level above the steam injection level. In such case, steam or another fluid (gas or liquid) is passed at high pressure into the pore space of the subsoil at a level below the intermediate layer(s) that would hamper the passage of steam in upward direction through the pore space of the subsoil. The high fluid pressure will break up the subsoil thereby possibly forming a crater to the surface.By subsequently injecting steam via the pipes at a reduced pressure, the crater will collapse but the steam will be allowed to pass through the broken-up intermediate layer(s) at relatively low pressure losses, and flow in upward direction for displacing the polluting liquid thereform.

When treating a polluted area section-wise, steam may be found to short-circuit a section that is being treated, by flowing through the gas-filled pore space of a neighbouring section that has been cleaned previously. If such short-circuiting occurs, it can be prevented either by placing a cofferdam between the two sections, or by introducing liquid into the pore space of the cleaned section to obtain a liquid saturation thereof that is at least equal to the liquid saturation of the pore space of the section that is to be cleaned.

Summarizing the invention, it can be said that the method and apparatus according to the invention allow cleaning of earth layers at relatively large depths (say below 1 to 2 metres and several tens of metres), when these layers have been contaminated with liquids that can be vapourized by means of steam. There is no restriction as to the area over which the earth has been polluted, but it will be appreciated that very large areas have to be cleaned section-wise. This in-situ cleaning of the earth is particularly designed for cleaning polluted subsoil, that is the soil lying below the surface soil, which latter soil is that part of the earth wherein the small plants (otherthan trees and large-sized shrubs) are rooted and which can be considered to be about one metre deep. Cleaning of polluted surface soil can take place in a relatively cheap manner by replacing the volume of polluted soil by a fresh batch of soil and subjecting the polluted volume to a heat treatment in a furnace. This way of cleaning, however, is too costly for cleaning polluted subsoil. The present method, however, since it allows cleaning of the subsoil insitu is comparatively cheaper and less time consuming.

Claims (12)

1. Method of cleaning polluted subsoil including the step of placing a plurality of pipes in the earth, the interior of the pipes communicating near or at the lower ends thereof with the pore space of the subsoil and the upper ends of the pipes being above the surface of the earth, followed by the step of passing steam downwards through the pipes into the subsoil and displacing fluids present in the pore space in upward direction to the surface of the earth, and continuing the steam injection at least until vapour is emerging from the earth.

2. The method of claim 1, wherein the steam injection is stopped when pure steam is emerging from the earth.

3. The method of claim 1 or 2, wherein the level at which the plurality of pipes communicate with the subsoil is lowered at least once.

4. The method of claim 1, wherein vapours are allowed to emerge from the subsoil via additional pipes situated close to the plurality of pipes and communicating at or near the lower ends thereof with the subsoil at a level above the level at which the steam is passed into the subsoil, and having the open upper ends thereof located above the surface of the earth.

5. The method of claim 4, wherein the level at which the open lower ends of the additional pipes are situated is lifted at least once.

6. The method of any one of the claims 1-5, including the step of condensing the vapours that have emerged from the earth.

7. The method of cleaning polluted subsoil, substantially as described in the specification with refer ence to the drawing.

8. Apparatus for cleaning polluted subsoil, including a steam generator and a plurality of pipes arranged in the earth, the pipes at or near the lower ends thereof communicating with the pore space of the subsoil and with the upper ends thereof communicating with the steam outlet of the steam generator.

9. The apparatus of claim 8, including means at the surface of the earth for rendering harmless the fluids that have been driven out of the earth by the action of the steam.

10. The apparatus of claim 9, wherein the means include a hood and a condenser for condensing the fluids, the condenser being placed inside the hood, and being in communication with a vessel for storing the condensed fluids.

11. The apparatus according to any one of the claim 8-10, including additional pipes situated close to the plurality of pipes, said additional pipes communicating at or near the lower ends thereof with the pore space of the subsoil at a level above the level at which the plurality of pipes is in communication with the pore space, and having the open upper ends thereof above the surface of the earth.

12. Apparatus for cleaning polluted subsoil, substantially as described in the specification with refer ence to the drawing.