Field of the Invention

The present invention relates to methods and apparatus for removing liquid from a permeable material, and, more particularly, to such methods and apparatus which are especiall adapted to deliquify sludges and slurries which contain particulate matter suspended in a liquid. Background of the Invention

Over the years, many different methods have been developed for removing liquids from solids. One common method involves a step by which the liquid is evaporated, leaving dry solids. In practice, such an evaporation step is usually preceded by at least one other dewatering step designed to remove as much liquid as possible before evaporation. The use of a pre- evaporation step or steps is preferred primarily for two reasons. First, the large amount of heat required to vaporize the liquid makes the evaporation step exceedingly expensive. Second, the time required to boil the liquid can make the evaporation step very time consuming. Thus, in order to reduce the cost and/or length of the evaporation step, a multi-step liquid removal process might be employed in which an evaporation step is preceded by any one or all of the following pre-evaporation steps: gravity settling; low-pressure filtration; and high- pressure filtration. Known techniques for accomplishing these individual steps are discussed in detail in "The Chemical Engineers Handbook".

When separating solids from water, as in the case of a primary water treatment system, gravity settling typically results in a slurry or sludge that is l to 10 percent solids by weight. The sludge can then be subjected to high-pressure filtration producing a filter cake that is approximately 30 to 40 percent solids by weight. The maximum percent solids that can be achieved by pressure filtration is limited by the point at which "solids hold-up" occurs (i.e., the point in the process at which the solid particles are compressed together to such an extent that they behave as a rigid block and will not compact further) .

However, when such a point in the process is reached, a significant amount of liquid still exists within the intersticies between the solid particles, the ratio of solids to liquid at this point being referred to as the "equilibrium solids concentration". In order to remove the remaining liquid, the filter cake is typically heated above the boiling temperature o the liquid and the liquid is evaporated off. The amount of energ required to remove the remaining liquid is still extremely large, even when such an evaporation step is preceded by a filtratio step.

Most commercial filtration equipment, such as vacuu filters, leaf filters, centrifuges, belt filter presses and plat and frame filter presses, are incapable of producing filter cake having an "equilibrium solids concentration", and, therefore, i practice, they usually produce _-= ,what at best would be about a 4 percent solids filter cake. To completely dry a pound of such filter cake, 0.6 pounds of liquid would have to be removed. I the liquid were water, at ideal efficiency almost 1000 BTU pe pound would be required to evaporate the water, assuming suc evaporation occurred at atmospheric pressure. Additional energ would also be required to raise the temperature of th solid/liquid mixture from ambient to vaporization temperature. Because the conventional drying processes are not 100 percen efficient, values of 1500-2000 BTtT per pound of water removed ar typical for such processes.

In view of the foregoing discussion, there is a rea need for a process which is more effective than the known method for removing liquids from solids. Because energy conservation i such an important concern nowadays, there is a further need for deliquification process which, in addition to being mor effective than the known methods, is also economic. Summary of the Invention

In accordance with the present invention, a method an apparatus for removing liquid from a permeable material involve constraining the material such that it interfaces with at leas one surface which is permeable to the liquid contained in th material. The constrained material is then heated at one or mor

locations remote from the permeable surface or surfaces to temperature sufficiently high to cause in situ vaporization o the liquid in the vicinity of the remote location or locations As the vaporized liquid expands, it forces at least some of th remaining unvaporized liquid through the permeable surface o surfaces. The heating step is repeated until substantially all o the liquid has been removed from the material, whereby th material is substantially deliquified.

Several different types of heating techniques ar suitable for use in conjunction with the present invention. The include: direct resistance heating produced by passing electri current through the material; dielectric heating produced b exposing the material to electromagnetic wave energy; and direc conduction heating. The type of heating technique to be employe is dependent upon the desired efficiency and upon the type o liquid being removed.

In one embodiment, the material to be deliquified i constrained in such a manner that it interfaces with a pair o surfaces located on opposite sides of the material, at least on of the surfaces being permeable to the liquid contained in th material. By adapting the surfaces for relative movement towar and away from each other, they can cooperate to compress th constrained material before and/or during its heating. Thus, prior to the performance of each and every heating step, at leas some of the liquid can be expressed from the material through th permeable surface or surfaces. Such expression of the liquid ca be facilitated by creating a vacuum on the side of the permeabl surface or surfaces opposite from the constrained material. Brief Description of the Drawings

For a better understanding of the present invention, reference is made to the following detailed description o various exemplary embodiments thereof considered in conjunctio with the accompanying drawings, in which:

Figure 1 is a schematic illustration of an overal system, including conditioning apparatus and deliquificatio apparatus, designed to condition a liquid slurry and the deliquify it in accordance with the present invention;

Figure 2 is a schematic illustration of th deliquification apparatus used in the system of Figure 1, suc apparatus being shown in a slurry-filling phase of th deliquification process;

Figure 3 ^" is a schematic illustration of th deliquification apparatus of Figure 2, the apparatus being show in a liquid-expressing phase of the deliquification process;

Figure 4 is a schematic illustration of th deliquification apparatus of Figures 2 and 3, the apparatus bein shown in an in situ vaporization phase of the deliquificatio process;

Figure 5 is a graph showing an idealized profile of th temperature existing within the material being deliquified durin the performance of the in situ vaporization phase of th deliquification process;

Figure 6 is a schematic illustration of th deliquification apparatus of Figures 2-4, the apparatus bein shown in a solids discharge phase of the deliquification process

Figure 7 is a schematic illustration of a modifie embodiment of the deliquification apparatus illustrated i Figures 2-4 and 6, the modified embodiment of Figure 7 bein shown in an in situ vaporization phase of the deliquificatio process;

Figure 8 is a cross-sectional view, taken along lin VIII-VIII and looking in the direction of the arrows, of th deliquification apparatus illustrated in Figure 7;

Figure 9 is a schematic illustration of anothe modified embodiment of the deliquification apparatus illustrate in Figures 2-4 and 6, the modified embodiment of Figure 9 bein shown in a filter cake delivery phase of the deliquificatio process; and

Figure 10 is a schematic illustration of th deliquification apparatus of Figure 9, the apparatus being show in an in _. situ vaporization phase of the deliquification process.

Detailed Description of the Exemplary Embodiments

While the present invention is especially suited fo removing liquid from slurries of solid particles, it can also b used to remove liquid from any permeable, porous solid, provide that the solid is properly constrained. Even deformable solids, such as foams, could be deliquified in accordance with th present invention. Because the present invention has such specia utility for deliquifying slurries, the invention will b described below with particular reference to a slurr deliquification system.

Referring initially to Figure 1, a liquid slurr containing approximately 1 to 10 percent solids by weight i conveyed by a pump 10 from a source 12, such as a holding pond, settling tank, clarifier, etc., through a conduit 14 and into tank 16 where a bar screen 18 or similar device removes larg foreign objects (e.g., sticks, rocks, etc.) and deposits them i a receptacle 20. Another pump 22 conveys the slurry from the tan 16 to a macerator 24, which is driven by a motor 26. Th macerator 24 grinds up any large particles that have passe through the bar screen 18 to thereby homogenize the slurry. metering pump 28 delivers a chemical flocculate from a storage tank 30 to the slurry at a venturi port 32, which is downstrea from the macerator 24. The metering pump 28 can be slaved to the pump 22 so that the flocculate will be continuously added to the slurry flowing from the macerator 24.

The flocculate is typically a chemical that causes the solid particles in the slurry to agglomerate and hence separate from the liquid. The addition of the flocculate greatly improves the speed and efficiency of filtration. However, the amount of the flocculate added to the slurry is critical. A general purpose anionic flocculate, such as that supplied by Stockhausen Chemical AG, can be mixed with water to achieve a .5 to .1 percent solution by weight of the flocculate. There is an optimum mass ratio of slurry to flocculate solution that needs to be achieved. Typically, a 50 to 1 ratio of a .25 percent solution is added, but the amount is highly dependent upon the slurry composition. If too much flocculate is added, the slurry becomes gelatinous

and is more difficult to deliquify than if no flocculate wer added. If too little flocculate is added, filtration efficienc is relatively unaffected. The exact amount of flocculate to b added to a particular slurry is best determined by trial an error.

Next, the slurry and the flocculate must be adequatel mixed to achieve the desired results. Mixing times of at least 3 seconds are normally required. Such mixing can be achieved b first passing the slurry/flocculate stream through a static mixe 34, which includes a sealed conduit having internal vanes adapte to induce turbulence in the flowing stream, and then into dynamic mixing chamber 36, which includes mixers 38.

Having been properly conditioned, the slurry is no ready for filtration in deliquification apparatus 40 includin cylinder 42, which is impermeable to the liquid contained in th slurry and which is made from an electrically non-conductiv material. An inlet port 44 for the slurry is located at one en of the cylinder 42, while an outlet port 46 for the dry solids i located at an opposite end of the cylinder 42. A valve 4 controls the flow of the slurry from the mixing chamber 36 to th deliquification apparatus 40.

Pistons 50, 52 are mounted for reciprocating movemen toward and away from each other within the cylinder 42, th pistons 50, 52 being moved by any suitable mechanism such a hydraulic actuators. The pistons 50, 52 are hollow wit electrically conductive filter faces 54, 56, respectively, eac of the filter faces 54, 56 having an air flow permeability in range of from 5 to about 15 scfm per square inch at 15 ps pressure differential. The pistons 50, 52 are also electricall conductive so that an electrical flow path exists from the filte faces 54, 56 to the exterior of the cylinder 42. The electrica flow path also includes a power supply 58, such as a 110 volt 6 cycle power supply or a source of radio frequency current in range of from about 8 X 10 ² to about 3 X 10 ⁵ cycles per second and a source 60 of line current at constant voltage. The piston 50, 52 are connected to a two-way pump 62 by flexible conduit 64, 66, each of which is made out of an electrically no

conductive material. The two-way pump 62 allows the pressure t be lowered inside the pistons 50, 52 and permits cooling gases t be circulated to the filter faces 54, 56 for reasons which wil be explained hereinafter.

The operation of the deliquification apparatus 40 will now be described with reference to Figures 2-6. Referring to Figure 2, after the piston 52 is moved away from the piston 50 to the position shown, the valve 48 is opened to allow th conditioned slurry to flow from the mixing chamber 36 to the cylinder 42 and then fill the void between the pistons 50, 52. By operating the pump 62 to lower the pressure within the pistons 50, 52, some of the liquid contained in the slurry entering the cylinder 42 is drawn through, the filter faces 54, 56 and conveyed to the pump 62 through the conduits 64, 66. When the void between the pistons 50, 52 is filled with a slurry mass 68 (see Figure 2) , the valve 48 is closed and the pistons 50, 52 are moved toward each other (see Figure 3) while the pump 62 continues to maintain a low pressure condition inside the pistons 50, 52, resulting in additional liquid being expressed through the filter faces 54, 56 and conveyed to the pump 62 through the conduits 64, 66.

In a typical slurry of fine particulate matter, such as a primary drinking water sediment sludge, pressure filtration at 200 psi confining pressure results in a filter cake 70 (see Figure 4) of 30% to 40% solids concentration by weight. To remove the remaining liquid from the filter cake 70, the pressure is maintained on the filter cake 70 by continuing to urge the pistons 50, 52 toward each other and by continuing to maintain a low-pressure condition inside the pistons 50, 52. The filter cake 70 is then heated so as to cause the in situ vaporization of the liquid located in a central region 72 of the filter cake 70. As the filter cake 70 is being heated, the filter faces 54, 56 are preferably maintained at a temperature lower than the vaporization temperature of the liquid, thereby ensuring that the greatest temperature rise and hence vaporization will occur only

in the region 72. The graph shown in Figure 5 depicts a idealized profile of the temperature that exists within th filter cake 70 along a path extending between the filter face 54, 56.

As boiling occurs and as the vapor pressure increase in the region 72, the vapor expands and moves in the form of front through the filter cake 70 towards the filter faces 54, 56 the vapor front sweeping non-vaporized liquid from the pores o the filter cake 70 and forcing the non-vaporized liquid out o the filter cake 70 through the filter faces 54, 56. Because th filter cake 70 is constrained between the cylinder 42 and th pistons 50, 52 during such in situ vaporization of the liquid the cylinder 42 and the pistons 50, 52 must be capable o withstanding the vapor pressure of the liquid so that the volum of enclosure 74 defined thereby does not substantially increas as the vaporized liquid expands.

The particular method of heating is critical, becaus the filter cake 70 should be heated without heating th deliquification apparatus 40. Direct resistance heating is on acceptable heating method. In accordance with such a method electric current at 110 volts and 60 Hz (cycles per second) i passed through the filter cake 70 by attaching the power suppl 58 to the pistons 50, 52 such that as the voltage is applie across the filter faces 54, 56 of the pistons 50, 52 respectively, the current flows through the filter cake 7 causing resistance heating to occur. Since the filter faces 54 56 are maintained at a temperature below the boiling (i.e. vaporization) temperature of the liquid, vaporization occurs at location (i.e., in the region 72) which is equidistance from th filter faces 54, 56.

When direct resistance heating is employed, the curren flow selected at the commencement of the heating step i important to the overall process efficiency. More particularly the character of the temperature profile through the filter cak 70 resulting from a given current flow has been found to b determinative of displacement efficiency (i.e., the amount o non-vaporized liquid removed per liquid vaporized) . In general,

current must be selected for a given filter cake 70 such that a optimum balance is achieved between the ideal temperature profil as shown in Figure 5 and the heat loss through the filter face 54, 56.

The current flow at any set voltage during th subsequent stages of the process will be automatically regulate by the resulting conductive properties of the pore network withi the filter cake 70. That is, as the conductive liquid is remove from the pore network, conductivity and hence current flow ar reduced. Thus, the voltage may be increased as conductivit decreases in order to reduce total processing time. The proces may be terminated when a predetermined minimum current flow i achieved or when a uniform temperature profile occurs (i.e., whe the temperature of those portions of the filter cake 70 whic interface with the filter faces 54, 56 is equal to th vaporization temperature of the liquid) .

The direct resistance heating method works well fo permeable solid/liquid materials" whose liquid component is polar liquid, such as water or ammonia.. To heat material containing liquids that are non-polar, such as alcohols and othe hydrocarbons, it is desirable that the power supply 58 be a radi frequency current source, instead of a 110 volt, 60 Hz source, whereby the filter faces 54, 56 would act as sending an receiving antennas. The exact frequency of the RF source, whic typically would be in a range of from abou 3 X 10 ⁵ to about 8 X 10 ⁸ cycles per. second, must be tuned in t the optimum frequency of that which can be absorbed by th liquid.

Once substantially all of the liquid has been remove from the filter cake 70, the pistons 50, 52 are moved to the positions indicated in Figure 6 (i.e., positions which woul align the filter cake 70 with the outlet port 46 in the cylinde 42 of the deliquification apparatus 40) . The outlet port 46 ca then be opened to permit the discharge of the substantiall deliquified filter cake 70.

Two alternate embodiments of the deliquificatio apparatus 40 are illustrated in Figures 7 and 8 and in Figures and 10, respectively. The various elements illustrated in Figure

7 and 8 and in Figures 9 and 10 which correspond to element described above with respect to the deliquification apparatus 4 are designated by corresponding reference numerals increased b one hundred and two hundred, respectively. Unless otherwis indicated, the alternate embodiments illustrated in Figures 7 an

8 and in Figures 9 and 10 operate in the same manner as th deliquification apparatus 40.

With reference to Figures 7 and 8, a deliquificatio apparatus 140 includes a cylinder 142, which is permeable to th liquid contained in a slurry being processed in th deliquification apparatus 140 and which is made from a electrically non-conductive material. Pistons 150, 152, which ar made from an electrically non-conductive material, include filte faces 154, 156, which are impermeable to the liquid contained i the slurry and which are made from an electrically non-conductiv material. The cylinder 142 is contained within a housing 176 which cooperates with the cylinder 142 to form an annular chambe 178. Conduits 164, 166 connect the chamber 178 to a pump 162. A inlet port 144, which extends through the chamber 178, i provided with a valve 148. An outlet port 146 extends through th chamber 178 and communicates with the cylinder 142. Microwav horns 180 are spaced at 120 degree intervals around the perimete of the cylinder 178 (see Figure 8) . Although three of the horn 180 are shown in Figure 8, only one would suffice. The horns 18 are positioned such that they are in direct alignment with filter cake 170 constrained between the pistons 150, 152 and th cylinder 142.

In the operation of the deliquification apparatus 140 the liquid expressed by the pistons 150, 152 passes through th cylinder 142, rather than through the filter faces 154, 156 After the completion of such a pressure filtration step, th pressure is maintained on the filter cake 170 and microwav radiation emitted by the horns 180, which are connected to suitable power supply (not shown) , is absorbed by the filter ca

170, resulting in its heating in a central region 172. Th temperature of the cylinder 142 is, at least initially, below th vaporization temperature of the liquid in order to ensure tha vaporization occurs in the region 172. As the resulting vapo front sweeps through the filter cake 170, the non-vaporize liquid is swept from the pores of the filter cake 170 and force out of the filter cake 170 through the cylinder 142.

The heating step can be controlled by monitoring th temperature of the filter cake 170 at its periphery (i.e., a that portion which interfaces with the cylinder 142) . When th temperature being monitored reaches the boiling temperature o the liquid, the microwave energy is switched off and the cylinde 142 is cooled by using the pump 162 and the conduits 164, 166 t circulate cooling gas through the chamber 178. The heating ste can then be repeated, with or without the maintenance of an mechanical or other force on the filter cake 170, to remove an residual liquid from the filter cake 170.

With reference to Figures 9 and 10, a deliquificatio apparatus 240 includes a cylinder 242, which is impermeable t the liquid contained in a filter cake 270 being processed in th deliquification apparatus 240. Pistons 250, 252 are mounted fo reciprocating movement within the cylinder 242. The piston 250 i solid (i.e., impermeable to the liquid contained in the filte cake 270) and is adapted to be heated to a temperature which i greater than the vaporization temperature of the liquid containe in the filter cake 270. The piston 252 has a filter face 25 which is permeable to the liquid contained in the filter cak 270. A conduit 266 connects the piston 252 to a pump 262. A inlet port 244 and an outlet port 246, each of which is larg enough to receive the filter cake 270, are provided in th cylinder 242.

In the operation of the deliquification apparatus 240 the filter face 256 is at a temperature below the boilin temperature of the liquid to be removed. The piston 250 i maintained at a temperature above the boiling temperature of th liquid and is either massive enough to avoid a significan temperature drop or is provided with heating means capable o

supplying enough energy to vaporize a portion of the liqui without having the piston 250 experience a significan temperature drop. The best results have been obtained by havin the piston 250 at an elevated temperature which is approximatel 1001F above the boiling temperature of the liquid. The filte cake 270 is then placed in the cylinder 242 through the inle port 244 (see Figure 9) and not allowed to contact the pisto 250. The piston 252 is moved rapidly to bring the filter cake 27 into contact with the piston 250. Sufficient force is exerted b both of the pistons 250, 252 so as to constrain the filter cak 270 at a pressure greater than the vapor pressure of the liqui contained therein. A partial vacuum can also be maintained insid the piston 252 by the pump 262 to evacuate the liquid expelle from the filter cake 270. Vaporization of the liquid whic interfaces with the piston 25Q occurs almost instantaneously. A the resulting vapor front sweeps through the filter cake 270, th non-vaporized liquid is swept from the pores of the filter cak 270 and forced out of the filter cake 270 through the filter fac 256.

The heating step can be controlled by monitoring th temperature of the filter cake 270 at that portion thereof whic interfaces with the filter face 256. When the temperature bein monitored reaches the boiling temperature of the liquid, th pistons 250, 252 can be moved away from the filter cake 270 an then cooled by using the pump 262 to draw air through the inle port 244 and into the piston 252 through the filter face 256 Alternatively, only the piston 252, including the filter fac 256, can be cooled, whereby the piston 250 is maintained at o near the elevated temperature referred to above so as to reduc the additional energy required to reheat the piston 250 i preparation for the performance of another heating step for th purpose of removing any residual liquid from the filter cake 270

The deliquification apparatus 240 is primarily intende to deliquify the filter cake 270 or a permeable, porous soli which contains a liquid. However, it could also be utilized i connection with the deliquification of sludges and slurries.

It will be understood that the embodiments describe herein are merely exemplary and that a person skilled in the ar may make many variations and modifications without departing fro the spirit and scope of the invention. All such variations an modifications are intended to be included within the scope of th invention as defined in the appended claims.