Cryogenic Method and System for Remediating Contaminataed Earth

The present invention is in the field of hazardous waste control and more particularly relates to remediation of contaminated earth.

Toxic substance migration in the Earth poses an increasing threat to the environment, and particularly to ground water supplies. Such toxic substance migration may originate from a number of sources, such as surface spills (e.g., oil, gasoline, pesticides, and the like), discarded chemicals (e.g., PCB's, heavy metals), nuclear accident and nuclear waste (e.g., PCB'S, solvents, methane gas). The entry of such hazardous materials into the ecosystem, and particularly the aquifer system, is well known to result in serious health problems for the general populace.

In recognition of such problems, there have been increasing efforts by both private environmental protection groups and governmental agencies, which taken together with increasing governmentally imposed restrictions on the disposal and use of toxic materials, to address the problem of long term, or permanent, safe storage of hazardous wastes, and to clean up existing hazardous waste sites.

There are many prior art techniques used to contain hazardous wastes, or other contaminants, in the ground. The most effective techniques appear to be those disclosed in U.S. Pat. Nos. 4,860,544 and 4,974,425, incorporated by reference herein, assigned to the assignee of the present invention. Generally, in accordance with the latter techniques, a frozen earth barrier, or two or more of such barriers, are established about the region containing the hazardous waste in situ. Those barriers are effective to contain, and thus prevent the migration of the enclosed hazardous waste, or other contaminants.

Once the contaminants have been contained, and even in cases where there is no containment, it is important to be able to remediate the contaminated earth. In the prior art, remediation is achieved by removing the contaminated earth to a remote site where the contaminants may be separated from the earth by conventional separation techniques. The most effective and safe approach for removal of contaminated earth for subsequent remediation is disclosed in U.S. Pat. No. 5,050,386, incorporated by reference herein, assigned to the assignee of the present invention. Generally, in accordance with an approach disclosed in that patent, the contaminated earth is frozen and then the periphery of sub-regions is thawed so that the generally frozen sub-regions may be easily removed from the site, like individual ice cubes from a tray of cubes. That approach is particularly effective where the contaminants are hazardous to humans, such as earth contaminated by radioactive waste. Even as the "cubes" are removed from the earth, a mist may be applied to the frozen cubes, which on contact freezes to establish a particulate-containing ice shell about the cube. This approach is effective for safely removing contaminated earth for remotely performed remediation.

However, it is desirable to minimize the need to transport the contaminated earth from both safety standpoints and economic standpoints. The best solution would be to remediate contaminated earth in situ, either contained as by the above noted frozen earth barrier techniques, or uncontained. In that case there would be less attendant cost, and improved safety risk factors.

Accordingly, it is an object of the present invention to provide an improved method and apparatus for remediating contaminated earth.

Another object is to provide an in situ method and apparatus for remediating contaminated earth.

Yet another object is to provide an improved contaminated earth remediation method and system that is economic and efficient to install and operate.

The present invention is a method and system for remediating a region containing contaminated earth. The method and system of the invention utilize repeated sweeps of a freeze-front (i.e., a liquid-solid phase interface) through a moist, porous transfer medium (which may be adjacent to the region-to-be-remediated, or which may be the region-to-be-remediated itself) in a manner drawing the contaminants from that region to a location where they may be removed. Generally, a central heat transfer assembly is disposed within the medium. That heat transfer assembly cools the medium disposed about it in a manner establishing an increasing-volume frozen sheath about the heat transfer assembly. As the sheath grows in size, a freeze-front is established that advances, or sweeps, across the medium, from the locus of the heat transfer assembly and eventually to a peripheral collection zone. In accordance with known principles, as the freeze-front advances across the medium, contaminant-bearing water in the unfrozen portion of the region-to-be-remediated is transported to the unfrozen side at the freeze-front (i.e., the liquid-solid phase interface).

When the water freezes, excluding the contaminants, it causes the freeze-front to advance across the region. After the freeze-front has advanced to the collection zone, with the result that water bearing a high concentration of contaminants is located at that zone, the sheath is melted, for example, by selectively operating heat sources located at or near that zone or by warming the heat transfer assembly. The resultant water, bearing a high concentration of contaminants, is then collected and removed. Following removal of the contaminated water, the contaminants may be separated from the water, and the relatively pure water may be injected back into the earth, preferably in the transfer medium and/or the region-to-be-remediated and a freeze-front may again be swept across the medium to reduce even further the contaminant level in the region.

Alternatively, the contaminant bearing water can be absorbed in a prepared receptacle bed. The bed can contain a component which can neutralize the contaminants or otherwise facilitate the safe containment or removal of the contaminants.

In the event there is only minimal water in the medium and/or region prior to freeze-front sweep, water may be injected into the earth. The entire cyclical freeze-front sweep/defrost/extract process may be performed on contaminated regions that are unconfined, or such regions that are confined, such as by frozen earth barriers of the type disclosed in the incorporated references.

In embodiments of the invention, the contaminant-concentrating feature of the invention may be combined with various methods of contaminated earth removal (with or without peripheral frozen earth contaminant barriers) to efficiently remove contaminants from the earth. Also, the invention may be used to merely cleanse a large region of the Earth, without collection of the displaced contaminants, but rather with the use of contaminant containing techniques such as those taught in U.S. Pat. Nos. 4,860,544 and 4,974,425. The contaminants may be contained indefinitely, while the cleansed region of the Earth is available for use.

In an alternative aspect of the invention, there is provided, in addition to the heat transfer assembly and the collection means, a means for establishing a flow of a liquid adjacent to the freeze-front and transverse to the direction of freeze-front advancement. The means for establishing this liquid flow can include at least one pair of substantially parallel porous tubes disposed in the earth adjacent to the heat transfer assembly. Also included is a means for establishing a hydrostatic pressure differential between the parallel tubes so that a liquid flow is established from a region of relatively high fluid pressure to a region of relatively low fluid pressure.

In one embodiment, the means for establishing a hydrostatic pressure differential is a pump, which can be located on the site, in the earth or on the earth's surface, or it can be located remote from the site. The pump is preferably disposed near the collection zone so that the freeze-front advances toward the pump.

In another embodiment, the liquid being pumped can be collected and recirculated in the earth.

The porous tubes can extend along an axis substantially parallel to the direction of freeze-front advancement. Alternatively, they can extend along an axis which is transverse to the direction of freeze-front advancement.

The foregoing and other objects of the invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings, in which:

FIG. 1 shows in a sectional diagrammatic form a remediation system that operates in accordance with the invention;

FIG. 2 shows a sectional view along lines 2, 3--2, 3 of the system of FIG. 1 and illustrates the refrigeration phase of the operation of that system;

FIG. 3 shows a sectional view along lines 2, 3--2, 3 of the system of FIG. 1 and illustrates the heating phase of the operation of that system;

FIG. 4 shows in a top plan diagrammatic form an alternative remediation system that operates in accordance with the invention;

FIG. 5 shows in perspective form a side view of a portion of the system as viewed along lines 5--5 of FIG. 4;

FIG. 6 shows in schematic form a perspective view of an alternative remediation system in accordance with the present invention;

FIG. 7 shows a perspective view of a heat exchanger panel of the system of FIG. 6;

FIG. 8 shows waveforms illustrative of the temperature of the respective heat exchanger panels of the system of FIG. 6;

FIG. 9 shows in schematic form a perspective view of alternative remediation system in accordance with the present invention;

FIG. 10 shows a sectional view along lines 10--10 of FIG. 9 of a heat exchanger element of the system of FIG. 9, with a drive tube for assisting in insertion into the ground;

FIG. 11 shows a diagrammatic view of an alternative remediation system in an rectangular configuration in accordance with the present invention;

FIG. 12 shows a diagrammatic sectional view along lines 12 of FIGS. 11 and 13 of alternative remediation systems in accordance with the present invention;

FIG. 13 shows a diagrammatic view of one of the remediation system in a conical configuration;

FIG. 14 shows a diagrammatic view of an alternative remediation system in accordance with the present invention;

FIG. 15 shows in schematic form a perspective view of a modified remediation system in accordance with the present invention;

FIG. 16 shows in schematic form a perspective view of an alternative modified remediation system in accordance with the present invention;

FIG. 17A shows in schematic form a perspective view of an alternative arrangement of the parallel porous tubes in the system of the present invention; and

FIG. 17B shows a top plan view of the remediation system of FIG. 17A.

In accordance with the present invention, a freeze-front is repetitively swept across a porous region in or adjacent to a region-to-be-remediated. As each freeze-front sweeps across, the contaminants on the liquid-side of the liquid-solid phase interface are drawn to and concentrated ahead of, and transported with, the advancing freeze-front. By controlling the motion of the freeze-fronts, the concentrated contaminants are steered to locations where they may be conveniently removed, preferably after the frozen region at that location is thawed.

The migratory physical effects associated with the melting and freezing of moisture in the ground have been known for many years. These effects take place over a range of temperatures lower than the freezing point in moist soils and other granular and porous media. In these media, water does not completely freeze at the normal 0.degree. C. transition temperature of pure water; instead, the water freezes progressively over a range of temperatures, and does not freeze completely until the temperature is lowered to less than about -35.degree. C. The persistent liquid is in the form of small regions of bulk liquid in crevices between soil grains, and as thin films at the interfaces between the ice and the soil grains and between the individual crystallites of ice. In situations where the temperature is not uniform, this liquid tends to migrate toward lower temperatures. When the ground moisture is impure, the impurities are carried along with the flowing liquid. The present invention establishes such conditions, so that a directed flow of contaminated liquid phase water migrates to a freeze-front. At the freeze-front, the water becomes frozen to cause the freeze-front to advance; however, the impurities tend to be excluded from the ice at the front, and under slow controlled growth conditions, the exclusion is nearly complete.

An exemplary, a system 10 embodying the invention is shown in FIGS. 1-3. The system 10 is adapted to remove contaminants from a surface or sub-surface soil layer. In this example, a contaminated earth layer 12 overlies an uncontaminated layer 14. The system 10 includes a porous transfer layer 16 positioned on top of the contaminated layer 12. By way of example, the transfer layer 16 may be fine silt earth. A thermal insulator layer 18 overlies the transfer layer 16. By way of example, the thermal insulator layer 18 may be polystyrene foam, sawdust, sand, or other thermal insulating material.

The system 10 further includes a substantially planar array 20 of three elongated heat exchangers 22, 24, and 26. The heat exchangers 22, 24, 26 are generally parallel, with the array 20 having a slight tilt (the right end of the array 20, as illustrated, is higher than the left end of the array 20). That array 20 is disposed in the transfer layer 16. In other forms of the invention, different numbers of heat exchangers may be used. Also, other than strict parallel alignment, and strict planar configurations, may be used. All of such configurations are considered to be equivalent to, and within the definition of the term "planar array" as used herein.

The exchangers 22, 24 and 26 are generally parallel and lie in a plane which is sloped with respect to the local vertical V. Each heat exchanger of array 20 is a set of concentric pipes, including an outer pipe (denoted with suffix "a") and an inner pipe (denoted with suffix "b").

A corresponding planar array 30 of three elongated collector troughs 32, 34 and 36 is disposed generally parallel to and beneath the array 20 of heat exchangers, with each collector trough having a V-shaped cross-section transverse to its principal areas, and extending along and beneath an associated one of the heat exchangers 22, 24, and 26. Each collector trough has a perforated collection pipe (denoted by suffix "c") extending along its vertex. An array of two inverted elongated V-shaped troughs 38 and 39 are disposed to span the gaps between the edges of adjacent troughs in the array 30. Other collection trough configurations may also be used provided those troughs are positioned to capture down-flowing water in the medium 16.

A planar array 40 of elongated parallel heater elements is disposed between, and generally parallel to, the arrays 20 and 30. Each heater element of array 40 is a set of concentric pipes.

The heat exchange pipes of array 20 are coupled at their uppermost ends to a manifold 50 which is coupled to a supply of heat exchange fluid. The heat exchange fluid may be selectively controlled to be hot or cold, as described below. The heat exchange pipes are connected so the heat exchange fluid is directed up the toroidal regions between the inner and outer pipes (as indicated by arrow A) and down the cylindrical region within the inner pipe (as indicated by the arrow B).

The pipes of the heater elements of array 40 are similarly coupled at their uppermost ends to a manifold 60 which is coupled to a source of heat exchange fluid. The heat exchange fluid may be selectively controlled to be hot or relatively cold as described below. The concentric pipes of array 40 are coupled to direct the heat exchange fluid "up" the annular region between the pipes and "down" the cylindrical region within the inner pipe.

The lowermost ends of the collector troughs are coupled by a collector manifold 65 (exemplified by pipe 65A in FIG. 1) to a reservoir in collector 70. A pump assembly 72 is connected to collector 70, to permit pumping of water from the reservoir of collector 70 by way of an output pipe 75 to a separator apparatus 78, such as an external still or freeze purifier. The separator 78 separates the contaminants from the water and returns the pure water to the transfer layer 16 by way of an array 80 of vertically extending perforated pipes.

In operation, for cases where the soil in layer 12 is moist and the transfer layer 16 is moist, a controller 84 commences a refrigeration phase (illustrated in FIG. 2) in which cold heat exchange fluid is driven through manifold 50 and through the array 20, while the fluid in array 40 is maintained relatively cool. The temperature of the fluid and the flow rate in array 20 is controlled so that increasing volume ice sheaths are established outside each heat exchanger. The sheaths are indicated by the broken line circles in FIG. 2 and the growth of those sheaths is indicated by the arrows passing through the broken lines. In response to the cold freeze-front at the peripheral surface of the sheath, ground water, including contaminants, from soil layer 12 migrates upward from layer 12 into the transfer medium 16 and toward the freeze front. The flow of this water is indicated by the curved arrows in FIG. 2. In the illustrated embodiment, the refrigeration phase is terminated (by stopping the flow of cold heat exchange fluid) when the ice sheath extends to the limits, i.e., the collection zone, indicated by the broken line circles in FIG. 2.

The controller 84 then commences a heating phase, illustrated in FIG. 3. In this phase, controller 84 establishes a flow of hot heat exchange fluid through the array 40. As a result of this flow, the peripheral regions of the ice sheaths (i.e., near to where the high concentration of contaminants resides) begin to melt (indicated by the inward arrows through the broken circles in FIG. 3). The liquid phase water, along with the concentrated contaminants, is then gravity-driven (indicated by the downward straight arrows in FIG. 3) to the collector troughs of array 30, where that concentrated contaminated water runs off through the troughs via collector manifold 65 to the reservoir of collector 70. That water is then pumped out via pipe 75 by pump 72 for separation at separator 78.

In a latter portion of the heater phase, controller 84 establishes a flow of relatively warm heat exchange fluid through manifold 50 and array 20. This flow melts the remainder of the ice sheaths, resulting in a run-off of relatively contaminant-free water to reservoir 70. The heating phase is now complete. The water in reservoir 70 or a portion of it may be returned to the soil in the region being processed.

Following completion of the heating phase, the controller 84 may repeat the two phase operation again to pass the freeze-front formed by the peripheral surfaces of the ice sheaths through the transfer layer 16 and then melt that freeze-front and collect the run-off.

The above embodiment illustrates the invention configured for removing contaminants from a surface layer of earth, where the system 10 is disposed within a transfer layer above the region-to-be-remediated. This approach is preferred when it is hazardous to perform excavating at the site. Alternatively, the system 10 could be positioned directly within the surface soil layer 12, but that would required trenching to establish the various elements of system 10 directly in the region-to-be-remediated.

An alternative form of the invention is shown in FIGS. 4 and 5 in which a freeze-front sweep is established directly in the region-to-be-remediated, but wherein substantially no excavation is required. A system 100, illustrated in FIGS. 4 and 5, includes an elongated heat exchanger assembly 102 which extends directly into a generally cylindrical region-to-be-remediated 104 (denoted by dash-dot lines in FIGS. 4 and 5). The heat exchanger 102 is a vertically-oriented set of coaxial pipes 102a and 102b which are similar to the coaxial pipes 22a and 22b of heat exchanger 22 of the embodiment of FIGS. 1-3.

A circular array of six parallel collector elements 111-116 is disposed vertically about the heat exchanger 102 at the lateral surface of the region 104. Each of elements 111-116 includes a perforated outer pipe disposed about a coaxial inner pipe. In alternative embodiments, different numbers of collector elements may be used, with differing inter-collector spacings.

A circular array of eight heater elements 121-126 is disposed vertically between the heat exchanger 102 and the array of collector elements 111-116. In alternative embodiments, different numbers of heating elements may be used, also with differing inter-element spacings. Each heater element is a substantially vertically oriented set of coaxial pipes which are similar to the coaxial pipes of the heater elements of array 40 of the embodiment of FIGS. 1-3.

Although not shown in FIGS. 4 and 5, the heat exchanger 102 is coupled via a manifold to a heat exchange fluid source in a manner similar to heat exchangers of the embodiment of FIGS. 1-3. Similarly, the heating elements 121-128 are coupled via a manifold to a heat exchange fluid source in a manner similar to the heating elements of array 40 of the embodiment of FIGS. 1-3.

In operation, a controller controls the flow rate and temperature of heat exchange fluid in the respective manifolds to accomplish operation of the system 100 which is similar to that of system 10 described above. That is, during a refrigeration cycle, an increasing volume ice sheath is established about heat exchanger 102 so that a freeze-front sweeps across the volume 104, concentrating contaminants ahead of (i.e., outward of) that freeze-front. Then, during a heating cycle, the controller first causes melting near the periphery of the sheath and the resultant contaminant-bearing water flows through the perforated pipes of collector elements 111-116. That water is pumped out through the central pipes of those elements. Then, hot heat exchange fluid is driven through heat exchanger 102 so that the remainder of the ice sheath melts, leaving relatively pure water in the region 104.

Then, the controller repeats the cycle to establish another sweep of a freeze-front across the region 104. This cycle is repeated until a desired level of remediation is attained. The system illustrated in FIGS. 4-5 operates to establish an outwardly advancing freeze-front (from element 102 to collectors 111-116). In other forms of the invention having a similar structure, the peripheral elements 111-116 may be used to establish an inwardly advancing freeze-front directed to element 102 which would operate as a collector of the "focused" flow of contaminants.

An alternative form of the invention is shown in FIG. 6. In that figure, an array of four heat exchanger (freeze/thaw) plate assemblies (or panels) 181-184 is shown extending into the Earth, downward from a surface region E. The respective panels are spaced apart in the horizontal direction and extend from an input end (panel 181) to an output end (panel 184). As shown in FIG. 7 for exemplary panel 181, the panel 181 includes a first set of heating transfer elements in the form of concentric pipes 186 extending vertically at one side of panel 181, a second set of heat transfer elements in the form of concentric pipes 188 extending vertically at the opposite side of panel 181, and a perforated, substantially planar plate 187 spanning the region between pipe sets 186 and 188. As illustrated, each of the pipe sets 186 and 188 is of the same form as the pipes of heat exchanger 102 of the configuration of FIGS. 4 and 5. While the heat transfer elements are shown vertical, the invention may be configured with such elements angularly offset from vertical.

A collector element 189 in the form of a trough (having a V-shaped cross-section) is disposed beneath the principal plane of the plate of panel 184. The principal axis of element 189 is angularly offset from the horizontal so that end 189a is lower than end 189b. A reservoir assembly 190 underlies the end 189a to collect any liquid phase water that runs down trough 189. The reservoir assembly 190 includes a pump to drive such collected water (and dissolved contaminants) via pipe 191 from the region -to-be-remediated V.

In operation, a controller 192 controls the flow and temperature of heat exchange fluid through the pipe sets 186 and 188 to effectively control the temperature of panels 181-184. By controlling these temperatures to first freeze and then thaw earth adjacent to the panels on a successively delayed basis, a freeze front is established in the volume V, initially adjacent to panel 181, which is then advanced across the volume V toward the panel 184. By way of example, the temperature of the respective panels is shown as a function of time in FIG. 8. As the freeze-front advances, contaminants in the earth in volume V are drawn toward, and concentrated at, the liquid side of, and driven ahead of, the liquid-solid phase freeze-front as it advances across the volume V. The repetitive nature of the temperature control illustrated in FIG. 8 results in a succession of freeze fronts sweeping across the volume V. Upon heating of the panel 184, the freeze-front melts and the resultant liquid phase water (and dissolved contaminants) flow downward to collector element 189 and in turn to reservoir 190, where that water is pumped from volume V.

To illustrate the effectiveness of the invention, the following calculations give the theoretical performance for a design of the invention consisting of an array of vertical freeze/thaw panels measuring 40 m wide.times.3 m high, place 5 m apart. The calculations set forth approximations for the freezing time and power demand as a function of the advance of the freezing front for various conditions of soil moisture and initial soil temperature.

The calculations are based on a one-dimensional approximation of heat transfer with a phase change, known as "Neumann's solution". The formula relates the position of the interface X(t) as a function of time t, to the thermal conductivity K of the frozen ground, the temperature difference .DELTA.T between the melting temperature T.sub.m and the T.sub.s of the refrigerated plane at X=0, and the latent heat/unit volume of the soil L.sub.p V.sub.w, where V.sub.w is the volume fraction of water in the soil. The exact solution for the case of soil initially unfrozen and at the melting point (i.e., where the refrigeration causes freezing only, and does not have to cool the soil to T.sub.m) is especially simple; see Eq. (1) below. The exact solution is modified for an initial soil temperature T.sub.0 >T.sub.m, by a correction factor, as shown in Eq. (2) below. Quantitative illustrations are set forth under simplifying assumptions: energy cost is for refrigeration only, with no allowance for pump loss or heat input from surroundings.

The Neumann solution which assumes that the soil is initially unfrozen, at a temperature T.sub.0 =T.sub.m, the fusion temperature is:

where X(t)=position of interface as function of time

K=thermal conductivity in frozen state

.DELTA.T=(T.sub.m -T.sub.s), difference between refrigeration and fusion temperatures

L.sub.p =latent heat of fusion/unit volume

V.sub.w =volume fraction of water

A correction for T.sub.0 >T.sub.m replaces the latent heat L by an effective value, which includes the sensible heat to cool the soil to the freezing point.

where c.sub.21 =c.sub.1 /c.sub.2

subscripts 1, 2=frozen, unfrozen

.PHI.=(T.sub.0 -T.sub.m)/T.sub.m -T.sub.s)

S.sub.T =c.sub.1 (T.sub.m -T.sub.s)/L.sub.p V.sub.w

The instantaneous power demand is given by

The following design and field conditions are used in this exemplary computation:

freeze/thaw panels 40 m.times.3 m, 5 m apart

soil and temperature parameters: T.sub.s =-35.degree. C.; V.sub.w =0.15;

K.sub.1 =6.4.times.10.sup.-3 J/cm deg; L.sub.p =305j/cm3

Table 1 sets forth the freezing time and power versus interface distance; T.sub.0 =T.sub.m

Table 2 sets forth the effect of soil moisture on freezing X(t) (cms) for three values of V.sub.w.

Table 3 sets forth the effect of initial soil temperature on X/X.sub.0, where X.sub.0 is calculated for T.sub.0 =T.sub.m.

The total energy to freeze over the full five meter distance between panels is 2.75.times.10.sup.10 J=7,640 KWH. At $0.06/KWH, the cost is $459., a unit cost of $0.77/m.sup.3 =$0.59/cubic yard. This assumes T.sub.0 =T.sub.M ; for higher initial soil temperature, the energy cost will be somewhat greater; the factor of increase depends on V.sub.w ; for T.sub.0 =40.degree. and V.sub.w =0.15, the cost is increased by 10%.

Remediation can vary greatly, depending on the type of soil, V.sub.w, the nature of the contaminants and the speed of cooling. For an indication of the remediation ratio, there exist reports of segregation coefficients measured in different systems: residues of explosives chemicals in soil, simple salts in bulk H.sub.2 O, and mineral salts in soil. The segregation (or cleanup) factors vary from less than 10.sup.-3 to 0.16. For a factor 0.1 (near the maximum of the range). the cleanup ratio after four passes is 10.sup.-4. This would correspond to a reduction of a contamination level from 100 ppm to 10 ppb. At the calculated freezing rate given in Table 1, this could be achieved in 6.5 years.

In an alternate form of the invention, an array of elongated heat exchangers may be established in the Earth, wherein the heat exchangers of the array extend downward from a surface region E and into a sub-surface volume-to-be-remediated V. In one such system, illustrated in FIG. 9, an array 150 of heat exchangers includes three rows of four exchangers. Each of the heat exchangers may be of the same type as heat exchanger 102 of the embodiment of FIG. 5. Alternatively, each heat exchanger may be of the type shown in section in FIG. 10, denoted by reference designation 102'. In that figure, a cylindrical drive tube 154 is coaxially disposed about an elongated cylindrical central element 156. A pointed tip, or auger, element 158 is disposed at the distal tip of exchanger 102, and drive element 160 is disposed at the proximal end. In the preferred form, the tube 154 is hardened stainless steel, central element 156 is copper or aluminum (or other high thermal conductivity metal), tip element 158 is hardened stainless steel, and drive element 160 is hardened stainless steel. Other materials may be used in alternative embodiments. With this configuration, the heat exchangers 102' are adapted to implantation in the Earth using conventional driving techniques, permitting rapid inexpensive implementation.

Following implantation, the drive element 160 and tube 154 is withdrawn, leaving the highly conductive central element 156 in place. A controller 170 is adapted to selectively control the temperature of the various central elements of the heat exchangers, as in a phased array, to establish controlled and directed freeze fronts passing through the region of the Earth underlying surface region E and disposed about the array 150. As with conventional phased arrays, the resolution of the array is a function of the inter-element spacing of the array of heat exchangers. In one form of operation, the rows of the array are alternatively heated and cooled to establish a substantially planar freeze-front which initially extends across the first row through the volume V and then advances to the second row, and finally advances to the last row. Behind the freeze-front, the heat exchangers are heated to cause melting in that portion of volume V. This alternate heating and cooling of the heat exchangers establishes a succession of freeze-fronts passing, or sweeping, across the sub-surface volume V disposed about the array 150. Contaminants in that volume are driven ahead of each freeze-front and concentrated at the distal end of the volume, i.e., adjacent to Row 3 at array 150, thereby establishing repetitive remediating action in the Earth. Upon heating of the elements of Row 3 in a manner thawing the Earth adjacent thereto, the concentrated contaminants may be collected and extracted from the Earth, for example, using collection pipes in the same manner as in the pipes 111-116 of the configuration of FIGS. 4 and 5.

Further modifications of this alternative form of the invention are shown in FIGS. 11-14 which permit remediation of the Earth by transporting the contaminants to a collection zone which may be at or near the surface of the Earth. In one modification, opposed pairs of planar arrays of elongated heat exchangers are implanted into the Earth at angles with respect to the surface such that they collectively form a set of nested, elongated V-shaped cross-section arrays which extend underneath a relatively large area of contaminated earth to be remediated. In another form, non-planar, i.e. curved arrays maybe used. For example, conical arrays of elongated heat exchangers are implanted to collectively form a set of nested conical arrays which extend underneath an area of contaminated earth to be remediated. Other forms, such as horizontal and vertical cylindrical arrays may also be used.

FIG. 11 shows a top (surface) view of a system 200, in which five pairs (235a-235j, 235b-235i, 235c-235h, 234d-235g, 235e-235f) of six element planar arrays of heat exchangers are embedded in the Earth under a rectangular collection zone.

As shown in FIGS. 11 and 12, the system 200 includes ten planar arrays of heat exchangers 235a-235j, each consisting of six elongated coaxial pipes similar to those described in FIGS. 9 and 10 oriented in a common plane. In this embodiment, five planar arrays 235a-235e, in spaced parallel relationship, are inserted into or adjacent to the left side (as shown) of the area containing the contaminated earth 210 at an angle with respect to the surface 215 such that at least a portion of each planar array underlies the area to be remediated. The right side (as shown), containing planar arrays 235f-2345j, is a mirror image of the left. Together, the ten arrays form a set of nested V-shaped "channels" beneath the area to be remediated.

In the instant embodiment, each planar array is angled at approximately 45 degrees with respect to vertical with the lowermost ends converging under the area to be remediated to form a set of nested V-shaped arrays. In a manner similar to FIGS. 9 and 10, a controller 270 and manifolds 260A and 260B regulate the temperature and flow rate of heat exchange fluid (not shown) to selectively control the temperature of each array and to establish controlled and directed freeze-fronts through the area to be remediated 210, generally starting from the channel formed by arrays 235b-235i and propagating toward and past the channel formed by arrays 235e-235f and toward collection element or zone 240. In this configuration, the combined effect of both sides permits transportation of the contaminants with a vertical component of motion (i.e. against the force of gravity) and with a horizontal component of motion which directs the contaminants toward a central axis of the V-shaped channels. By controlling the sides unequally, variations in the horizontal component of motion may be introduced.

Collection of the contaminants may be achieved by any means previously disclosed herein, such as FIGS. 1-3 or 4-5. Alternatively, an absorbing material 240 may be disposed at the surface to absorb the liquid phase water and contaminants. The absorbing material can contain a component which neutralizes the contaminants or otherwise facilitates safe removal.

As shown in FIGS. 11 and 12, the outermost planar array 235a and 235j on each side can be used to maintain a frozen earth barrier to contain the contaminants and prevent them from seeping deeper into the earth. In addition heat exchangers 280, similar to heat exchangers 102' of FIG. 10, can be used to form a frozen earth barrier at each end of the area to be remediated, thereby completely enclosing the region to be remediated.

FIG. 13 shows a top (surface) view of a system 200; in which five conical arrays of heat exchangers are embedded in the Earth under a circular collection zone. The heat exchangers may be configured in the form of nested, concentric cones or conical surfaces 235a'-235j' in and adjacent to the area to be remediated 210, as shown in FIG. 13. This embodiment is very similar to that of FIGS. 11 and 12, the difference being the geometric orientation of the heat exchangers. The sectional view along lines 12--12 would appear substantially as shown in FIG. 12. In operation, the configuration of FIGS. 12 and 13 is substantially the same as the configuration of FIGS. 11 and 12, except that the freeze front is direct with a radial horizontal component of motion toward the axis of symmetry of the nested conical arrays.

The embodiments of FIGS. 11-13 are advantageous for remediating existing contaminated areas and for preventing contaminants from seeping further into the earth where they could jeopardize the aquifer or other environmental system. The heat exchangers can be implanted by conventional driving or boring techniques with limited exposure to the people involved. In addition, the contaminants are driven to the surface, thus reducing the energy necessary to extract the liquid phase water containing contaminants for removal.

FIG. 14 shows system 300 which implements another alternative embodiment of the invention wherein planar arrays of elongated heat exchangers 335a-335d are oriented horizontally and configured in vertically spaced parallel relationship with one another in or adjacent to the area to be remediated 310. A collection medium or zone 340 is established over the area 310. The system is also provided with a controller, manifolds and source of heat exchange fluid (not shown) to establish controlled and directed freeze fronts through the area to be remediated 310. In this configuration, the system produces planar freeze fronts moving upward against gravity toward the surface or toward a collection medium or element 340 for removal as described herein above. In a manner similar to the embodiments of FIGS. 11-13, the lowermost planar array can be used to form a frozen earth containment barrier 380 and additional vertically oriented planar arrays can be provided about the perimeter of area 310 to form vertical frozen earth barriers for containment.

In the embodiment of FIG. 14, the heat exchangers can be implanted by utilizing conventional trenching or horizontal boring techniques. This configuration can also be used to remediate earth adjacent to the side of a cliff, or excavation, for example, where side access would be safely permitted. It is also contemplated that the heat exchangers can be implanted prior to contamination and used to remediate as well as to form a frozen earth barrier to protect against a subsequent spill of contaminant materials.

FIG. 15 illustrates another modification of the present invention. As described previously in connection with FIG. 9, an array 150 of elongated heat exchangers 102 is established in a region of contaminated earth and extends downward from a surface region E into a sub-surface volume to be remediated V. Controlled heating and cooling of a heat transfer medium within the heat exchangers establishes a freeze-front which sweeps across the rows of heat exchangers in the direction of arrows F.

As the contaminant concentration increases ahead of the freeze-front, the freezing point of the liquid in the earth is depressed and the liquid bearing the contaminants remains in liquid form until and unless the temperature is lowered sufficiently to cause it to freeze. Non-uniform concentrations of contaminants cause the solid/liquid interface to be highly unstable at a given temperature, as regions of higher contaminant concentrations remain liquid while regions of lower contaminant concentrations freeze. The instability is manifested as pockets of contaminant-rich liquid being surrounded by contaminant-poor ice.

To enhance the stable formation of the freeze-front region, a flow of a liquid can be established in the earth adjacent to the advancing freeze-front in a direction which is transverse to the direction of freeze-front advancement. The liquid cross-flow between the rows of heat exchangers significantly enhances the stability of the advancing freeze-front by making the concentration of contaminants in the earth just ahead of the freeze-front more homogeneous.

As shown in FIG. 15, one or more of pairs of substantially parallel porous tubes 400 can be placed in the earth to extend generally parallel to the direction of freeze-front advancement. The tubes 400 are placed adjacent to the array 150 of heat exchangers.

A liquid flow is established in the tubes 400 by means of a hydrostatic pressure differential which is created between the parallel tubes by a pump assembly 402 disposed in fluid connection with the parallel tubes 400 and between them at one end thereof. The liquid flows from a region of relatively high fluid pressure through the earth to a region of relatively low fluid pressure as shown by arrows L in FIGS. 15 and 16. The liquid is forced through the porous tubes 400 at a sufficient pressure to create a substantial liquid cross-flow component between the rows of heat exchangers from one porous parallel tube to another, as shown by arrows L. The porous tubes 400 are preferably plugged with caps 401 at the ends opposite from the pump assembly 402 so that cross-flow of liquid between the tubes is favored over axial flow through the tubes.

In FIG. 15 the porous tubes are placed substantially parallel to the earth's surface and thus must be placed in the earth by excavating the site to a depth sufficient to permit placement of the tubes at the desired location. This method of tube placement can be difficult and costly. FIG. 16 illustrates an alternative placement of the porous parallel tubes 400 of FIG. 15. In FIG. 16 the porous parallel tubes 400 are placed at a slight angle relative to the earth's surface so that they can be positioned in the contaminated earth by drilling a series of slant channels into the earth.

As shown in FIG. 15, an array 403 of pump assemblies and parallel tubes can be established in remediation sites having contaminated earth to substantial depths. The pump assemblies of the array 403 can be located within the earth or external to the earth's surface and can be controlled by an external controller 404 which coordinates the sequencing and operation of the individual pump assemblies of an array.

As shown in FIGS. 15 and 16, advancement of the freeze-front is achieved in the direction of arrows F toward the pump assembly 402 and away from the plugged ends of the parallel tubes. High pressure liquid is pumped into one of the parallel tubes and is forced out of the pores of the tubes along arrows L in a transverse direction across the contaminated earth ahead of the advancing freeze-front. Some liquid also flows axially through the parallel tubes, but only until it reaches the frozen region of earth at the solid/liquid interface of the freeze-front. Thus, as the freeze-front advances toward the pump assembly 402, liquid is forced along paths L across the earth between the parallel tubes ahead of the freeze-front with greater pressure, thereby further enhancing stable freeze-front formation and advancement. Behind the freeze-front the frozen earth is heated as previously discussed, and the liquid in the earth can flow into the pores of the relatively low fluid pressure tube to be recirculated by the pump assembly. In this way no additional liquid must be added to the contaminated earth site; the transverse liquid flow can be established with liquid already present in the earth. The environmental benefits of this arrangement include complete containment of the site during remediation.

An alternative arrangement of the parallel porous tubes 400 is shown in FIGS. 17A and 17B. In this embodiment, the tubes 400 establishing convection are placed vertically at the ends of the rows of elongated heat exchanges 102, between the planar arrays 150. A liquid is pumped through porous tubes 400 as previously described, and a convection flow established in the direction of arrows L, transverse to the direction of freeze-front advancement (arrows F).

As shown in FIG. 17B, the outermost heat exchanges 410 in each planar array 150 of heat exchanges can be used to maintain a frozen earth barrier, as previously described in connection with the embodiment of FIGS. 11 and 12, to contain the containments and prevent them from seeping into regions of the earth beyond the remediation region. The enclosures thus formed may be part of a full cryogenic containment barrier. This containment feature, although not shown with the embodiment of FIGS. 15 and 16, should be understood to be applicable to those and other embodiments.

The invention is described above as being effective for the in situ removal of contaminants from, i.e., remediating, regions of the Earth, where the contaminants are mobile, for example, soluble in water. However, the invention may also be used to remove immobile contaminants, such as insoluble precipitates or films adsorbed onto soil grains. In this form of the invention, prior to advancing the freeze-front across the region-to-be-remediated, a solubilizing agent is first injected into the region, whereupon that agent serves to make the previously immobile contaminants mobile. Then, the freeze-front sweeping process may be accomplished in the manner described above.

In addition, while the above description is in terms of "contaminants" and "remediation", it will be understood that such contaminants may include valuable materials, such as oil, which may be recovered using the invention, thereby "remediating" the "contaminated" region of the Earth. As used herein, the term "contaminants" refers to any material that may be removed from the Earth using the invention, and the term "remediation" refers to the removal of any such material. Thus, general soil "washing" is contemplated, as well as operations such as extraction of oil, minerals (e.g., from tailings or deposits), and bioremediation (at any desired temperature).

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.