WO1992005877A1

WO1992005877A1 - Countercurrent washing of solids in a decanter centrifuge

Info

Publication number: WO1992005877A1
Application number: PCT/US1991/007073
Authority: WO
Inventors: David A. Flanigan; John D. Boadway
Original assignee: Conoco Specialty Products Inc.
Priority date: 1990-09-27
Filing date: 1991-09-25
Publication date: 1992-04-16

Abstract

This invention is directed to a method and apparatus for washing particulate solids contaminated with a solvent soluble contaminate in a decanter centrifuge. The decanter centrifuge (20) includes an inlet (60) for delivering particulate solids into a medial position (65) of the rotating drum (30) and a plurality of passages (69) through the helical blade (44) in the helical conveyor (40) for directing solvent into the drum (30) between the medial position (65) and the solids outlet (35). Thus, as the solids move toward the solids outlet (35) and the liquids move toward the liquid outlet (37), at the opposite end of the drum (30), the solids and liquids flow countercurrent to one another. The invention further relates to two-stage cleaning of particulate solids wherein the first stage of cleaning is performed by mixing the solids with a solvent and then delivering the mixture to the same decanter centrifuge. The second stage is performed fully within the decanter centrifuge and comprises the countercurrent flow cleaning discussed above.

Description

Description Countercurrent Washing of Solids In a Decanter Centrifuge Field of the Invention This invention relates to washing solids contaminated with a solvent soluble contaminant. The invention more particularly relates to a method and apparatus for removing an oil-based contaminant from particulate drill cuttings produced in a well drilling operation.

Background of the Invention

A variety of techniques have been used in the past to remove contaminants from particulate solids. For example, U.S. Patent No. 2,965,522 describes a technique for regenerating fouled filter aids by water washing to remove the foulants. The technique involves passing the fouled filter aid through a series of hydrocyclones with separate water streams being introduced to each hydrocyclone to wash the filter aid. U.S. Patent No. 4,288,329 and U.S. Patent No. 4,406,796 describe a technique for cleaning mill scale with solvents to remove oil and water. With increasing emphasis on environmental concerns, techniques have been developed to remove contaminants from soil as exemplified in U.S. Patent No. 4,606,774 and U.S. Patent No. 4,662,948.

Particular concerns with removal of contaminants from particulate solids have arisen in the drilling of oil and gas wells wherein drill cuttings generated during drilling operations are flushed from the wellbore by drilling fluids (sometimes referred to as "drilling muds") . A serious problem exists with disposal of these drill cuttings and other residual solids after separation from the drilling mud because of oil contamination. After being separated from the drilling mud, these solids have oil on their surfaces and sometimes within their porous structure. This is particularly true when an oil-based drilling mud is employed although formation oil may also be present regardless of the type of drilling mud used. Because of the increasing environmental concerns, it is essential to have economic means to clean these oil-contaminated drill cutting solids and enable disposal in an environmentally acceptable manner. This is very significant in offshore operations where it is desirable to be able to discharge the solids overboard from the drilling rigs and avoid having to transport them to shore for disposal. With respect to drill cuttings, a variety of techniques have been suggested for dealing with this problem. For example, the use of surfactant solutions to wash drill cuttings to remove oil and other contamination has been suggested in U.S. Patent No. 3,688,781, U.S. Patent No. 3,693,733, U.S. Patent No. 3,716,480, U.S. Patent No. 3,860,019, U.S. Patent No. 4,175,039, U.S. Patent No. 4,546,783, U.S. Patent No. 4,595,422 and U.S. Patent No. 4,645,608. These approaches attempt to wash absorbed oil-based mud from the surface of the drill cuttings and have been largely unsuccessful because oil-based muds are specially formulated with powerful oil wetting agents that resist the detergent action of aqueous wash solutions. Additionally, detergent-laden water is continuously discharged into the marine environment and may be even more toxic to marine organisms than the oil on the particulate drilling solids. Steam stripping of the cuttings to remove oil contamination has been suggested in U.S. Patent No. 4,209,381 and U.S. Patent No. 4,395,338. In these techniques, steam is used to strip the more volatile oils from oily drill cuttings followed in some cases by distillation of the remaining solids to remove the higher boiling oil fractions. The methods are particularly impractical offshore because of the excessively high energy requirements to generate the quantity of steam needed and the high temperatures needed to distill the oil.

Other thermal methods involving heating of the cuttings to volatilize or incinerate the oil contamination have been suggested in U.S. Patent No. 3,693,951, U.S. Patent No. 4,139,462, U.S. Patent No. 4,304,609, U.S. Patent No. 4,411,074, U.S. Patent No. 4,606,283, U.S. Patent No. 4,683,963, U.S. Patent No. 4,726,301 and EP Publication Application No. 6005273. Typically, the high temperatures required for these processes is supplied by electrical resistance heating, electrical induction heating, infrared heaters, or high temperature heat transfer fluids. The methods have been at least partly unsuccessful for reasons already cited. The total amount of energy to heat all of the solids and boil all of the liquids off the cuttings is excessively high. Also, it is very dangerous to operate any equipment offshore in which hydrocarbon vapors are generated at temperatures well above their flash point.

Solvent washing or extraction techniques to remove oil contamination from cuttings have been suggested in U.S. Patent No. 4,040,866, U.S. Patent No. 4,434,028, U.S. Patent No. 4,836,302 and PCT Published Application No. WO82/01737.

In particular, U.S. Patent No. 4,040,866 teaches the use of a mutual solvent to clean oily drill cuttings. A mutual solvent is one that is soluble in both oil and water. In this process, oily liquid is removed from the solids with a mutual solvent like ethylene glycol monobutyl ether; however, the mutual solvent and oil mixture remains on the cuttings and must be washed away with water followed by centrifuging to recover the cuttings. This method has proven impractical because two undesirable process streams are created. Large quantities of solvent (approximately equal to the original volume of oily liquid on the solids) are washed from the solids with water and discharged with the water into the environment. It is probable that the solvent is even more toxic to marine organisms than the oil which was removed from the cuttings. Additionally, large volumes of mutual solvent become contaminated with dissolved oil and must be either discarded or purified and recycled. The cost of mutual solvents prohibits simple disposal. Further, the high boiling point and high latent heat of vaporization of mutual solvents make their separation from oil by distillation difficult, expensive and hazardous. U.S. Patent No. 4,434,028 teaches a high pressure process for the use of a solvent which is miscible with oil but essentially immiscible with water to clean oily drill cuttings. In this process, a substance that is typically a gas at ambient temperature and pressure is compressed sufficiently to convert the gas to a liquid which then becomes a suitable solvent for the oil associated with drill cuttings. The liquified gas is then flowed, batchwise, through a vessel packed with oily solids. When the solids have been washed sufficiently clean, the chamber is depressurized allowing the solvent to flash into a vapor, leaving the solids free of oil and solvent. The oil-contaminated solvent can also be flashed to a vapor to separate it from the oil and allow it to be recycled. This process has not been successful on offshore drill sites for several possible reasons. U.S. Patent No. 4,836,302 teaches a system for cleaning drill cuttings using a cleaning column. The system is stated to overcome many of the problems associated with the techniques described above and is contained so as to minimize solvent escaping to the atmosphere.

PCT Published Application No. WO82/01737 describes a technique for reducing oil contamination on drill cuttings which involves washing the contaminated drill cuttings, preferably after screening to remove fines and supernatant drilling mud, with a single halogenated solvent. The resulting slurry is then macerated and processed through a single continuously running decanter centrifuge to separate the cleaned solids. While the system will reduce oil contamination on drill cuttings, a substantial volume of solvent is required to achieve adequate cleaning.

Copending U.S. Application Serial No. 07/487,350 filed February 28, 1990 relates to cleaning oil contaminated drill cuttings in a two stage process. Each of the stages of the process includes a solvent mixing step and a centrifuge separation step. Thus, the adhering contaminant is dissolved in solution and separated from the particulate drill cuttings with the solvent. The second stage repeats the steps of the first stage thus cleaning the residual contaminants and providing more completely cleaned particulate drill cuttings. In related copending U.S. Application Serial No. 07/487,351 filed on February 28, 1990, the two stage process of cleaning oil contaminated drill cuttings is modified. The first stage comprises a solvent mixing step and a cyclone separation step to separate solids from the dissolved contaminant. The second stage comprises a countercurrent extraction step where the solids are fed to a vessel while solvent is fed countercurrent to the solids therein to dissolve and strip contaminant. The solvent is thereafter separated from the solids in a decanter centrifuge. Each of these systems requires valuable platform space in an offshore installation and any weight or space savings would be a substantial improvement.

Accordingly, it is the object of the present invention to provide a method and apparatus for cleaning solids which overcomes the above noted drawbacks and disadvantages of the prior art.

It is a further object of the present invention to provide a method and apparatus for countercurrent cleaning of solids in a decanter centrifuge.

It is a further particular object of the present invention to provide a method and apparatus for two stage countercurrent cleaning of particulate solids in a decanter centrifuge.

The above and other objects are achieved by the present invention which includes a method of cleaning solids contaminated with solvent-soluble contaminant in a decanter centrifuge. The method particularly comprises delivering solids contaminated with solvent-soluble contaminant into the decanter centrifuge and directing a liquid solvent via a selected portion of the helical blade to the internal surface of the drum to mix with the solids and dissolve contaminant therefrom. The drum is rotated at a sufficient rotational speed so as to form a layer of solids along the internal surface of the drum, separate the liquid solvent from the solids within the rotating drum, and move the liquids toward the liquid outlet. The helical conveyor is rotated at a rotational speed which is slightly different than the rotational speed of the drum so that the helical blade pushes the layer of solids toward the solids outlet. The solids are discharged from the drum at the solids outlet of the decanter centrifuge and the liquid solvent is discharged from the drum at the liquid outlet of the decanter centrifuge.

The invention also relates to an apparatus for performing the above method and comprises a generally elongate hollow drum having a frustoconical peripheral wall portion, a solids outlet at the small end of said frustoconical portion, a liquid outlet at the opposite end of said drum and an internal surface extending from said solids outlet to said liquid outlet. A helical conveyor is coaxially positioned in the drum and includes a generally continuous helical blade with its peripheral edge extending toward and proximately spaced from the internal surface of said drum and extending substantially the length of said drum. The apparatus further includes means for delivering solids contaminated with solvent-soluble contaminant to the drum and means for directing liquid solvent via a selected portion of the helical blade to the internal surface of the drum. Means for rotating the drum at a sufficient rotational speed is provided so as to form a layer of solids along the internal surface of the drum and a mechanism for rotating the conveyor at a rotational speed slightly different than the rotational speed of said drum so that the helical blade moves the solids toward solids outlet. Brief Description of the Drawings

Some of the objects of the invention have been stated and others will appear as the description proceeds when taken in connection with the accompanying drawings, in which—

Figure 1 is a cross sectional view of the preferred embodiment of a decanter centrifuge particularly illustrating the features of the present invention;

Figure 2 is an enlarged fragmentary cross sectional view of a portion of the decanter centrifuge illustrated in Figure 1; and

Figure 3 is schematic view of a system for cleaning particulate solids contaminated with a solvent soluble contaminate incorporating the decanter centrifuge illustrated in Figure 1. Detailed Description of the Preferred Fmhoriiτm«*»n-

Referring now more particularly to the drawings, Figure 1 illustrates a decanter centrifuge generally indicated by the reference number 10. The decanter centrifuge 10 comprises an elongate casing 20 for housing internal moving elements as will be described below. The casing 20 should have a robust construction to contain the moving elements in the event of a catastrophic failure and may be formed of any suitable material such as cast iron, steel, aluminum, or the like. The casing 20 is preferably comprised of upper and lower portions 20A and 20B, respectively.

A drum 30 is mounted within the casing 20 by suitable bearings 21 for rotation about an elongate axis λ. The drum 30 is preferably formed of an elongate shell comprised of a frustoconical peripheral wall portion 31 and a cylindrical peripheral wall portion 32 coaxially extending from the large end of the frustoconical portion 31. A relatively smooth internal surface 33 extends around the inner circumference of the drum 30 along the length of the frustoconical and cylindrical portions 31 and 32. The drum 30 has a first end 34 at the small end of the frustoconical portion 31 which includes a number of ports 35 defining a solids outlet for discharging solids separated in the drum 30. The ports 35 are illustrated as extending radially through the shell of the frustoconical peripheral wall portion 31, however, the ports 35 may alternatively extend parallel to the axis λ through the end 34 of the drum 30. Other suitable arrangements of the ports 35 may be designed to allow particulate solid material to exit the drum 30 adjacent the end 34 and be received by the casing 20. To collect the particulate solids exiting the ports 35, the casing 20 includes a solids receiving chamber 22 adjacent the first end 34 of the drum 30. The solids receiving chamber 22 is defined by suitable baffles or dividers so as to guide the solids out of the casing 20 through an exit 23.

The drum 30 further comprises a second end 36 at the opposite end of the drum 30 which includes weirs 37 to provide a liquid outlet from the drum 30. The weirs 37 are adjustable ports so that the centrifuge may be operated with a variable thickness fluid layer on the inside of drum

30. However, it should be understood that other arrangements for defining the liquid outlet are known and the weirs 37 are simply a representative design for a liquid outlet. The casing 20 further includes a liquid receiving chamber 24 similar to the solids receiving chamber 22 for receiving the liquid discharged from the drum 30. The liquid receiving chamber 24 is defined by the cylindrical space between the end of the casing 20 and the end of the drum 30. The liquid receiving chamber 24 further includes a liquid exit 25 through which the liquids may be discharged from the decanter centrifuge 10. The liquid receiving chamber 24 may further comprise dividers or baffles 38 to guide the liquids to the liquid exit 25 and further prevent the liquids from contaminating the solids or otherwise being retained in the casing 20.

A helical conveyor 40 is disposed inside the drum 30 and mounted by suitable bearings 41 to the drum 30 for rotation about the axis λ. The conveyor 40 is supported by the bearings 41 at opposite ends thereof and comprises a cylindrical or generally cylindrical scroll tube 42 which is generally uniformly spaced from the internal surface 33 along the length of the drum 30. A generally continuous helical blade 44 is attached to the outer surface of the scroll tube 42 and extends radially outwardly therefrom. The helical blade is preferably comprised of a continuous helical ribbon mounted to the scroll tube 42. However, the helical blade may alternatively be comprised of a succession of blade sections fixed to and aligned to form a helical blade. As is indicated in the drawing, the peripheral edge of the helical blade 44 extends radially toward and is proximately spaced from the internal surface 33 of the drum 30. The space between the peripheral edge and the internal surface 33 is typically about 3 to about 8 millimeters, however, the smallest distance is preferred. The peripheral edge of the helical blade 44 may be tapered or angled as indicated in the Figure such that the side of the blade which pushes the material along the drum extends closer to the internal surface 33 than the back side of the blade 44. The tapered peripheral edge reduces drag on the blade and thus the power required to rotate the conveyor 40. The tapered peripheral edge also tends to prevent the solids from blocking the exit of the passages 69 which carry solvent to the internal surface 33. In a further alternative embodiment (not shown) , the peripheral edge may be provided with abrasion resistant ceramic plates at the leading edge of the blade 44 so as to extend closer to the internal surface 33 of the drum 30. The advantages of this embodiment are similar to the tapered edge embodiment with the additional advantage of wear resistance.

The helical blade 44 is further disposed at an angle with respect to the axis A so that any rotation of the drum 30 relative to the helical conveyor 40 causes the blade 44 to move any materials along the inside of the drum 30 rather than around inside the drum. The aspect of moving materials along the drum 30 rather than around the drum will be used to separate the liquids and solids as will be more clearly explained later.

The drum 30 and helical conveyor 40 are mounted in the casing 20 for rotation about the axis λ. A drive motor 50 is secured to one end of the casing 20 for rotating the drum 30 and the conveyor 40. The drive motor 50 includes a rotating drive shaft 51 extending parallel to the axis λ which carries a pair of spaced apart drive pulleys 52 and 53. The helical conveyor 40 includes a driven pulley 54 adjacent the second end 36 of the drum 30 which is also in a common plane with the first drive pulley 52. A drive belt 55 overlies the first drive pulley 52 and the conveyor driven pulley 54 in a conventional manner to connect the drive motor 50 to the helical conveyor 40 for rotating the same. In a similar manner, the drum 30 includes a driven pulley 56 adjacent the second end 36 thereof and which lies in a common plane with the second drive pulley 53. A drive belt 57 overlies the pulleys 53 and 56 to connect the drive motor 50 to the drum 30 for rotating the drum 30. Suitable means (not shown) may be used for adjusting the tension on the drive belts 55 and 57 to prevent slippage thereof on one of the pulleys 52, 53, 54, and 56.

As is most clearly illustrated in Figure 1, the driven pulley 56 has a larger diameter than driven pulley 54. Accordingly, the helical conveyor 40 will rotate faster than the drum 30 under the driving force of the motor 50. Other mechanisms for obtaining a speed differential are known such as a drive mechanism with differential gears which permits controlled variation of the speed differential. The speed differential is also used to separate the liquid and solids as will be explained below. The decanter centrifuge 10 further includes a particulate solids inlet conduit 60 through which the particulate solids are received for cleaning. The solids inlet conduit 60 is disposed coaxially within the conveyor 40 and extends into the hollow interior of the scroll tube 42. The solids inlet conduit 60 extends from the end adjacent the first end 34 of the drum 30 about two thirds the length of the scroll tube 42 so as to reach about a central portion of the conveyor 40. The solids inlet conduit 60 is thereby disposed to deliver the particulate solids to a particulate feed chamber 65 within the scroll tube 42. The particulate feed chamber 65 is positioned at about a medial position of the scroll tube 42 and defined by suitable baffles or dividers or the like. The solids inlet chamber 65 further includes inlet ports 66 extending through the shell of the scroll tube 42. The inlet ports 66 are arranged between the blade 44 to permit the particulate solids to enter the drum 30 at a about a medial position of the drum 30. The medial position is preferably spaced from the ends of the cylindrical portion 32 of the drum 30 as will be explained below.

A liquid solvent inlet conduit 61 is disposed coaxially over the solids inlet conduit 60 so as to define an annular channel for the liquid solvent to enter the decanter centrifuge 10. The solvent inlet conduit 61 terminates closer to the small end of the scroll tube 42 thereby being disposed to deliver liquid solvent to a solvent feed chamber 67. A deflector 68 is mounted around the periphery of the solids inlet conduit 60 near the outlet end of the solvent inlet conduit 61 to deflect solvent traveling through the solvent inlet conduit 61 into the solvent feed chamber 67. As such, the solvent is prevented from traveling along the solids inlet conduit 60 into the solids inlet chamber 65 and is directed into the solvent feed chamber 67. The solvent feed chamber 67 is defined by suitable baffles or dividers or the like and includes a plurality of passages 69 for the liquid solvent to be admitted into the drum 30. The addition of solvent in this manner helps to keep the area between the bearings 41 and the solvent inlet conduit 61 free of particles which would be quite abrasive causing wear and seal failure. Other suitable arrangements for introducing the materials to the decanter centrifuge may be used. For example, one alternative (not shown) comprises opposed conduits entering the scroll tube from the opposite ends thereof.

Referring now also to Figure 2, a portion of the helical blade which is in the general proximity of the juncture of the frustoconical portion 31 and the cylindrical portion 32 is selected to carry the liquid solvent to the internal surface 33 of the drum 30. The passages 69 extend through the shell of the scroll tube 42 and preferably through the selected portion of the helical blade 44. However, the passages 69 may alternatively be formed of tubes which are positioned along either side of the blade 44. Preferably, if the passage is a tube along side the blade 44, the tube is positioned along the back side when considering the side pushing the materials as the front side. As such, the tube avoids the high stresses associated with turning the blade through the material. In either embodiment, the passages 69 are preferably arranged to extend to the peripheral edges of the selected portion of the blade 44 so that the solvent exits the passages at the internal surface 33 of the drum 30. Since the solvent is admitted into the drum 30 at the internal surface 33, which may be within or below other materials at the internal surface, the admission of the solvent is also referred to as injecting the solvent into the drum 30. While actual forced injection of the solvent is contemplated, such an injection system does not form a part of the preferred embodiment.

Turning now to the operation of the decanter centrifuge 10, the drive motor 50 rotates the drive shaft 51 in the clockwise direction when viewed in the direction of arrow 12. Thus, both the drum 30 and the conveyor 40 also rotate in the clockwise direction by rotation of the drive belts 55 and 57. However, as noted above the driven pulleys 54 and 56 are of slightly different diameters which provides a slightly different gear ratio. Thus, the drum 30 rotates at a slightly different rotational speed than the helical conveyor 40 in spite of the fact the they are both driven by the same shaft 51 and by drive pulleys 52 and 53 that are the same diameter. It should be noted that other suitable means for rotating the drum 30 and conveyor 40 may be employed as well as other means for effecting different rotational speeds. In the preferred embodiment, the helical conveyor 40 rotates faster than the drum 30 due to the different drive ratios. Since the conveyor 40 rotates faster than the drum 30 in the clockwise direction when viewed in the direction of arrow 12, the conveyor 40 rotates clockwise relative to the drum 30. Also, as noted above, relative rotation between the drum 30 and the conveyor 40 cause the helical blade 44 to push materials along the internal surface 33 of the drum 30. In the illustrated embodiment, clockwise rotation of the conveyor 40 relative to the drum 30 causes the helical blade 44 to push the materials toward the first end 34 of the drum 30. Preferably, the drive motor 50 rotates at a high speed so that each of the drum 30 and the conveyor 40 to rotate at a high speed. The rotational velocities of the drum 30 and the conveyor 40 will typically depend on their respective diameters since larger diameters create substantially greater forces at similar speeds. However, for general illustration purposes, a drum diameter of approximately two feet would typically be rotated at about 2,000 rpm during operation of the decanter centrifuge and the helical conveyor would rotate at a speed of +/- 60 rpm relative to the drum. Accordingly, the helical conveyor would rotate at a speed of either 1,940 to 2,060 rpm depending on the design of the helical blade on the helical conveyor. In the case of the illustrated embodiment, the helical conveyor 40 rotates at the higher speed.

Once the drive motor 50 has reached its design speed and the drum 30 and conveyor 40 have stabilized at their operational speeds, the particulate solids and clean solvent are introduced into the centrifuge 10 via respective inlet conduits 60 and 61. The materials are deposited into their respective inlet chambers 65 and 67 and by the rotation of the scroll tube 42 causes the materials to be pushed out through their respective ports 66 and 69. Focusing on the path of the particulate solids moving through the decanter centrifuge 10, the solids pass through the ports 66 and enter the drum 30 at a medial position therein. Centrifugal forces created by the rotation of the drum 30, causes the particulates to spread out on the internal surface 33 and separate from any liquids mixed therewith. The materials are separated because of their different specific gravities and thus the heavier solids form a solids layer at the internal surface 33 and the lighter liquids form a liquid layer on the solids layer. At the same time the materials are separating, they are being pushed by the helical blade 44 toward the solids outlet 35 at the first end 34 of the drum 30. Along the internal surface 33, two distinct regions are created by the centrifugal forces of the rotating drum 30 and the pushing of the helical blade 40. The first region is called the pond and is indicated by the letter P. The pond P is the region where the solids and liquids are both present and is generally defined to overlie the cylindrical portion 32 of the drum 30. The pond P has a defined depth corresponding to the height of the weirs 37 above the internal surface 33. The other region is called the beach and is referred to by the letter B. The beach B is generally where the liquids are separated from the solids and is generally defined as being along the frustoconical portion 31.

During operation, the helical blade 44 pushes the materials in the pond P toward and up the beach B to the solids outlet 35. However, the frictional forces between the materials and the internal surface 33 have a significantly greater effect on the solids than on the liquids. The liquids tend to respond more to the centrifugal forces and flow outwardly from the axis λ and between the flights of the blade 44 to remain in the pool against the pushing forces of the helical blade 44. The solids, however, do not slide along the internal surface 33 because of the high frictional forces imposed by the centrifugal forces except by the pushing of the helical blade 44 which moves the solids up the beach B toward the solids outlet 35. The solids being moved out of the pool P are initially rather muddy, but as the solids move up the inclined beach B and the centrifugal forces cause the liquid to drain back into the pool P, the solids become rather dry. The liquid draining from the beach B flows in a helical path between the flights of the helical blade 44 toward the weirs 37. Since the conveyor 40, and hence the helical blade 44, is rotating faster than the drum 30, the liquid must also move around the internal surface 33 faster than the blade 44 as the liquid moves to the weirs 37. As such, the fast rotating liquid increases the centrifugal forces in the drum 30 so as to further enhance solid separation from the liquid. Thus, it is preferred to rotate the conveyor 40 faster than the drum 30.

In an alternative embodiment, the conveyor 40 is rotated slower than the drum 30 and in such an arrangement it may be beneficial to have perforations in the blades to enable the liquid to move more directly toward the weirs 37.

One of the design considerations of the decanter centrifuge 10 is the rate at which the helical blade pushes the materials up the beach B. It must be slow enough to allow the liquids to flow back down the beach B while not being so slow that the solids flow down the beach B to the pond P. Any liquids mixed with the particulate solids, such as a liquid solvent used to dissolve the contaminant, as will be discussed below, are separated from the solids in the pond P and form a liquid layer on top of the solids layer, as discussed above. Moreover, since the helical conveyor 40 constantly pushes the materials toward the beach B, the opposite end of the pond P is comprised primarily of liquid. Thus, as the pond P overflows the weirs 37, only liquids are obtained in the liquids receiving chamber 24. Therefore, it should be appreciated that the decanter centrifuge 10 separates the solids and liquids of a mixture and directs the solids out of the solids outlet 35 at the first end 34 and the liquids out the liquid outlet 37 at the second end 36. Thus, cleaning of particulate solids contaminated with a solvent soluble contaminant may be accomplished by mixing the particulate solids with a liquid solvent to dissolve the contaminant and separating the solvent with the dissolved contaminant from the solids in the decanter centrifuge 10.

In accordance with the above noted cleaning process, solvent is directed through the passages 69 in the selected portion of the blade 44 generally adjacent the juncture of the frustoconical and cylindrical portions 31 and 32 of the drum 30. The juncture of the frustoconical and cylindrical portions 31 and 32, as best illustrated in Figure 1, is between the solids outlet 35 and the medial position of the drum 30 where the solids are introduced therein. Thus, the solids must traverse the position where the solvent is injected to get to the solids outlet 35 and the injected solvent must traverse the medial position where the solids are introduced into the drum 30 to get to the liquid outlet 37. This is characterized as countercurrent flow of the solids and solvent and provides enhanced cleaning of the solids. The countercurrent flow moreover provides the most efficient use of solvent in decontaminating the solids by initially cleaning the least contaminated solids with the cleanest, most pure solvent and using this slightly contaminated solvent to clean slightly more contaminated solids and successively more contaminated solids until the solvent is saturated with contaminant. By that time, the solvent is extracting contaminant from the most contaminated solids.

Accordingly, the countercurrent technique causes the solvent to clean the particulate solids until it is fully saturated and, moreover, the countercurrent flow technique saves the cleanest, most pure solvent for cleaning the particulate solids that are the most nearly clean. Thus, the cleanest product is obtained with the least amount of solvent.

Referring again to Figure 2, the solvent is not simply deposited in the drum 30 between the medial position and the solids outlet 35, but rather the solvent is delivered to the internal surface 33 of the drum 30. Considering, the centrifugal forces generated by the rotating drum 30, the injected solvent will be quickly displaced from the internal surface 33 by the heavier solids. This creates a further countercurrent flow between the solvent rising in the pond P and the solids moving toward the internal surface 33 to displace the solvent. Moreover, the additional countercurrent flow provides additional mixing of the solvent and solids for enhanced cleaning of the solids. It should also be noted that the centrifugal separation of the liquids and solids also separates the solids into layers on the internal surface 33. More particularly, the heaviest solids will be closest to the surface 33 while the lighter finer particulates will be spaced from the surface 33 but in the solids layer. The delivery of the solvent at the internal surface 33 of the drum 30 at a tangential velocity approximately equal the velocity of the solids on the internal surface minimizes turbulence caused by the injection of the solvent. Such turbulence might disperse the fine particles from the solids layer and into the liquid layer to be carried toward the liquid outlet 37. As such, the delivery of the solvent as disclosed is less likely to impair the clarification of the finer particulate solids at the interface of the liquid and solid layers.

The countercurrent cleaning process, as discussed above, forms the second stage in the preferred embodiment of a two stage cleaning process. Briefly stated, the first stage comprises mixing the solids with a solvent and delivering the mixture to the decanter centrifuge 10 through the solids inlet conduit 60. The solvent in the mixture dissolves a substantial portion of the contaminant which is carried away with the liquid solvent upon separation of the liquid and solids. The second stage takes place concurrently in the same decanter centrifuge 10 and, by countercurrent flow, performs a secondary cleaning for removing residual contaminant adhering to the solids. Referring now to Figure 3, the decanter centrifuge 10 is utilized in a system, generally referred to by the number 100, for cleaning particulate solids. The system 100 in particular is adapted to clean drill cuttings from a well drilling operation which are soaked with oil-based drilling fluid. The drilling fluid is used as a lubricant for the drill bit and as a means for flushing cuttings from the borehole. Once the cuttings are carried up the borehole to the well head, it is desired to separate the drilling fluid therefrom for further use and to dispose of the drill cuttings in an economical and environmentally sound manner. In offshore drilling installations, it is considerably more economical to be able to dispose of the drill cuttings by returning them to the seafloor. However, the cuttings must first be cleaned of the oil-based drilling fluid adhering to the particulate cuttings prior to being discharged into the sea. It should be clearly understood, however, that the system 100 may be fully suitable for cleaning other materials or may be adapted to clean other particulate solids.

The system 100 operates in a continuous manner and comprises an incoming line 105 carrying particulate drill cuttings mixed with oil-based drilling fluid to a mechanical separator, such as shale shaker 110. The shale shaker 110 comprises a vibrating screen which continuously separates the free or loosely adhering contaminant from the particulate solids. The oil-based drilling fluid recovered in the shale shaker 110 is removed through a fluid line 111 for recycling in the drilling process. The contaminated drill cuttings are conveyed by a contaminated solids line 112 into a slurry tank 120. In the slurry tank 120, the drill cuttings are mixed with a solvent to dissolve the oil-based drilling fluid and form a slurry. The solvent is introduced to the slurry tank 120 via a solvent inlet line 121 so as to thoroughly mix with the drill cuttings and dissolve as much contaminate as possible. Solvent make up line 122 provides additional or make up solvent for the slurry tank 120. The slurry is delivered from the slurry tank 120 to the decanter centrifuge 10 via a slurry line 123. Since the slurry is pumpable, a pump 124 may be used to pump the slurry into the decanter centrifuge 10.

The decanter centrifuge 10 separates the particulate drill cuttings from the oily liquid solvent. Moreover, as discussed above with respect to the decanter centrifuge, the drill cuttings are subjected to a second cleaning as the cuttings progress toward the beach B prior to being discharged from the drum 30 (Figure 1) . As such, relatively contaminant free liquid solvent is directed into the drum 30 through the solvent feed line 125 which is connected to the conduit 61 (Figure l) .

The cuttings being discharged from the decanter centrifuge 10 still have residual solvent moisture therein although they have been separated from the bulk of the solvent. Accordingly, the system 100 further includes a dryer which for purposes of illustration is depicted as heated auger 130. The solids fed to the auger may be introduced entirely at the entry end thereof or a multiple of locations along its length through line 131. The auger 130 is heated by any conventional means to provide temperatures which are at least efficient to vaporize the residual solvent present in the solids. Such heating means may include an internally heated auger or an auger with a heated jacket which utilizes circulated heating fluids or perhaps electrical resistance means. As the solids continuously move through the dryer 130, any residual solvent is volatilized and serves to further strip the contaminates which may not have been dissolved and removed during earlier solvent treatment. The volatilized solvent together with stripped contaminants are removed from the dryer through a vapor line 132. The dried cuttings are discharged through a dried particulates line 133 and are ready to be discharged from the system 100. The economic considerations of the system 100 make it desirable to maximize the use of the solvent for cleaning the particulates cuttings. For example, the vaporized solvent recovered in the dryer is relatively clean and pure without a lot of contaminant dissolved therein. Accordingly, the vapors are condensed by condenser 140 and directed to a decanting tank 145 through a condensate line 141. The decanting tank permits any water to be separated from the solvent and removed from the system 100 through a liquid impurities line 146. The liquid solvent is then pumped by pump 147 through a condensed solvent return line 148 which branches into solvent inlet feed line 121 and solvent inlet conduit 125. To the extent that additional solvent is necessary, fresh solvent is provided through fresh solvent inlet 161.

The liquid solvent recovered from the liquid outlet 25 of the decanter centrifuge 10 may also be recycled. For example, the liquid solvent with the dissolved oil-based contaminant is removed from the decanter centrifuge 10 through a liquid outlet line 151 to a surge tank 150 where the volume of spent or contaminated solvent in the system 100 may be controlled by increasing or decreasing the liquid stored in the surge tank 150. The spent solvent is removed from the surge tank 150 through a discharge line 152 to be directed to a fraction distillation unit where, by known processes, the solvent is separated from the contaminants. The solvent derived from the distillation process is reintroduced into the system 100 via the fresh solvent inlet line 161. Some of the spent solvent, however may be recycled, as illustrated in Figure 3 as make up solvent for slurrying the particulate solids. The oily solvent recovered at the surge tank 150 is directed through a solvent make up return line 153 which is connected to solvent inlet line 122 leading into the slurry tank 120. It is recognized that reuse of the oily solvent may not clean the maximum amount of contaminant from the particulate drill cuttings at the slurry tank 121, but reuse does provide full utilization of the solvent.

In the foregoing drawings and specification, there has been set forth a preferred embodiment of the invention. Although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. A method of cleaning solids contaminated with solvent-soluble contaminant in a decanter centrifuge of the type generally comprising a hollow drum with a helical conveyor coaxially positioned in the drum, wherein hollow drum includes a frustoconical peripheral wall portion, a solids outlet at the small end of the frustoconical portion and a liquid outlet at the opposite end of the drum, and wherein the helical conveyor includes a generally continuous helical blade extending toward and proximately spaced from the internal surface of the hollow drum, the method comprising the steps of: delivering solids contaminated with solvent-soluble contaminant into the decanter centrifuge; directing a relatively contaminant free liquid solvent via a selected portion of the helical blade to the internal surface of the drum to mix with the solids and dissolve contaminant therefrom; rotating the drum at a sufficient rotational speed so as to form a layer of solids along the internal surface of the drum, separate the liquid solvent from the solids within the rotating drum, and move the liquids toward the liquid outlet; rotating the helical conveyor at a rotational speed slightly different than the rotational speed of the drum so that the helical blade pushes the layer of solids toward the solids outlet; discharging solids from the drum at the solids outlet of the decanter centrifuge; and discharging liquid solvent from the drum at the liquid outlet of the decanter centrifuge.

2. The method according to Claim 1 wherein the step of delivering the solids to the decanter centrifuge comprises mixing the solids with liquid solvent to form a slurry and delivering the slurry to the decanter centrifuge.

3. The method according to Claim 1 wherein the step of delivering the solids into the decanter centrifuge comprises depositing the solids in a generally medial position of the drum and the wherein the step of directing the liquid solvent comprises directing the solvent via a selected portion of the helical blade which is selected to be between the medial position of the drum and the solids outlet so as to form a countercurrent flow between the solids moving toward the solids outlet and the solvent moving toward the liquid outlet.

4. A method of cleaning drill cuttings contaminated with oil in a centrifuge of the type generally comprising a hollow drum with a helical conveyor coaxially positioned in the drum, wherein hollow drum includes a frustoconical peripheral wall portion, a solids outlet at the small end of the frustoconical portion and a liquid outlet at the opposite end of the drum, and wherein the helical conveyor includes a generally continuous helical blade proximately spaced from the internal surface of the hollow drum, the method comprising the steps of: delivering solids contaminated with solvent-soluble contaminant into the decanter centrifuge; directing a relatively contaminant free liquid solvent via a selected portion of the helical blade to the internal surface of the drum to mix with the solids and dissolve contaminant therefrom; rotating the drum at a sufficient rotational speed so as to form a layer of solids along the internal surface of the drum, separate the liquid solvent from the solids within the rotating drum, and move the liquids toward the liquid outlet; rotating the helical conveyor at a rotational speed slightly different than the rotational speed of said drum so that the helical blade pushes the layer of solids toward the solids outlet; discharging solids from the drum at the solids outlet of the decanter centrifuge; and discharging liquid solvent from the drum at the liquid outlet of the decanter centrifuge.

5. The method according to Claim 4 wherein the step of delivering the solids to the decanter centrifuge comprises mixing the solids with liquid solvent to form a slurry and delivering the slurry to the decanter centrifuge.

6. The method according to Claim 5 wherein the step of delivering the solids into the decanter centrifuge comprises depositing the solids in a generally medial position of the drum and wherein the step of directing the liquid solvent comprises directing the solvent via a selected portion of the blade which is selected to be between the medial position of the drum and the solids outlet so as to form a countercurrent flow between the solids moving toward the solids outlet and the solvent moving toward the liquid outlet.

7. An apparatus for cleaning solids contaminated with solvent-soluble contaminant, comprising: a generally elongate hollow drum having a frustoconical peripheral wall portion, a solids outlet at the small end of said frustoconical portion and a liquid outlet at the opposite end of said drum and an internal surface extending from said solids outlet to said liquid outlet; a helical conveyor coaxially positioned in said drum and having a generally continuous helical blade with its peripheral edge proximately spaced from said internal surface of said drum and extending substantially the length of said drum; means for delivering solids contaminated with solvent-soluble contaminant into said drum; means for directing relatively contaminant free liquid solvent via a selected portion of said helical blade to said internal surface of said drum; means for rotating said drum at a sufficient rotational speed so as to form a layer of solids along the internal surface of said drum; and means for rotating said conveyor at a rotational speed slightly different than the rotational speed of said drum so that said helical blade moves the solids toward said solids outlet.

8. The apparatus according to Claim 7 wherein said means for directing said solvent via said selected portion of said blade comprises passages within said blade which extend to said peripheral edge thereof so as to provide the relatively contaminant free liquid solvent to said internal surface.

9. The apparatus according to Claim 7 wherein said means for delivering solids includes means for delivering solids to a medial portion of said elongate drum and wherein said means for directing solvent into said drum further comprises means for directing solvent to said drum between said solids outlet and said medial position so as to arrange a countercurrent flow between the solids and the liquid solvent.

10. The apparatus according to Claim 7 wherein said drum further comprises a cylindrical peripheral wall portion extending coaxially from the large end of said frustoconical portion and wherein said medial position is along said cylindrical portion.

11. The apparatus according to Claim 10 wherein said selected portion of said helical blade is adjacent the juncture of said frustoconical portion and said cylindrical portion.

12. The apparatus according to Claim 7 wherein said helical conveyor further comprises a hollow scroll tube with said helical blade attached to and extending radially from said scroll tube and wherein said means for delivering solids into said drum comprises means within said scroll tube for defining a particulate feed chamber therein and ports extending through said scroll tube from said particulate feed chamber for particulate solids to exit said particulate feed chamber and enter said drum.

13. The apparatus according to Claim 12 wherein said means for delivering solids into said drum further comprises a particulate solids inlet conduit extending coaxially into said scroll tube to provide particulate solids into said particulate feed chamber.

14. The apparatus according to Claim 13 wherein said means for directing liquid solvent into said drum comprises: means within said scroll tube for defining a solvent feed chamber spaced from said particulate feed chamber therein; and passages extending through said scroll tube and said selected portion of said helical blade from said solvent feed chamber for liquid solvent to exit said solvent feed chamber and enter said drum; and wherein said means for directing liquid solvent to said internal surface of said drum further comprises a liquid solvent inlet conduit coaxially overlying said particulate solids inlet conduit for carrying liquid solvent in the annular space between said conduits to said solvent feed chamber.

15. The apparatus according to Claim 14 wherein said particulate solids inlet conduit extends further into said scroll tube than said liquid solvent inlet conduit and said particulate solids inlet conduit includes a peripheral deflector attached to the periphery thereof to deflect liquid solvent from the liquid solvent inlet conduit into the solvent feed chamber.

16. An apparatus for cleaning solids contaminated with solvent-soluble contaminant, comprising: a generally elongate hollow drum having a frustoconical peripheral wall portion, a cylindrical peripheral wall portion coaxially extending from the large end of said frustoconical portion, a solids outlet at the small end of said frustoconical portion and a liquid outlet at the opposite end of said drum and an internal surface extending from said solids outlet to said liquid outlet; a helical conveyor coaxially positioned in said drum and having a hollow scroll tube, a generally continuous helical blade attached to and extending radially from said scroll tube with its peripheral edge extending toward and proximately spaced from said internal surface of said drum and extending substantially the length of said drum; means for delivering solids contaminated with solvent-soluble contaminant into said drum at a medial position thereof and comprising means within said scroll tube for defining a particulate feed chamber at a generally medial position therein and ports extending through said scroll tube from said particulate feed chamber for particulate solids to exit said particulate feed chamber and enter said drum; means for directing relatively contaminant free liquid solvent via a selected portion of said helical blade to said internal surface of said drum between said solids outlet and said medial position of said drum and comprising means within said scroll tube for defining a solvent feed chamber spaced from said particulate feed chamber therein and passages extending through said scroll tube and within said selected portion of said helical blade from said solvent feed chamber for liquid solvent to exit said solvent feed chamber and enter said drum; means for rotating said drum at a sufficient rotational speed so as to form a layer of solids along the internal surface of said drum; and means for rotating said conveyor at a rotational speed slightly different than the rotational speed of said drum so that said helical blade moves the solids toward said solids outlet and liquid solvent moves toward said liquid outlet in countercurrent flow.

17. A system for cleaning drill cuttings from a well drilling operation which is drilling a well into the earth for the production of hydrocarbons, wherein the drill cuttings are contaminated with oil-based drilling mud, the system comprising: means for separating the cuttings from the drilling mud; means for conducting the cuttings from the separating means to a generally elongate hollow drum having a frustoconical peripheral wall portion, a solids outlet at the small end of said frustoconical portion and a liquid outlet at the opposite end of said drum and an internal surface extending from said solids outlet to said liquid outlet; a helical conveyor coaxially positioned in said drum and having a generally continuous helical blade with its peripheral edge proximately spaced from said internal surface of said drum and extending substantially the length of said drum; means for delivering the cuttings contaminated with solvent-soluble oil-based drilling mud into said drum; means for directing relatively contaminant free liquid solvent via a selected portion of said helical blade to said internal surface of said drum; means for rotating said drum at a sufficient rotational speed so as to form a layer of cuttings along the internal surface of said drum; means for rotating said conveyor at a rotational speed slightly different than the rotational speed of said drum so that said helical blade moves the cuttings toward said solids outlet; and means for discharging the clean cuttings from the cleaning system.

18. The apparatus according to Claim 17 wherein said means for directing said solvent via said selected portion of said blade comprises passages within said blade which extend to said peripheral edge thereof so as to provide the relatively contaminant free liquid solvent to said internal surface.

19. A system for cleaning drill cuttings from a well drilling operation which is drilling a well into the earth for the production of hydrocarbons, wherein the drill cuttings are contaminated with oil-based drilling mud, the system comprising: means for separating the cuttings from the drilling mud; means for conducting the cuttings from the separating means to a generally elongate hollow drum having a frustoconical peripheral wall portion, a cylindrical peripheral wall portion coaxially extending from the large end of said frustoconical portion, a solids outlet at the small end of said frustoconical portion and a liquid outlet at the opposite end of said drum and an internal surface extending from said solids outlet to said liquid outlet; a helical conveyor coaxially positioned in said drum and having a hollow scroll tube, a generally continuous helical blade attached to and extending radially from said scroll tube with its peripheral edge extending toward and proximately spaced from said internal surface of said drum and extending substantially the length of said drum; means for delivering cuttings contaminated with solvent-soluble oil-based drilling mud into said drum at a medial position thereof and comprising means within said scroll tube for defining a particulate feed chamber at a generally medial position therein and ports extending through said scroll tube from said particulate feed chamber for the cuttings to exit said particulate feed chamber and enter said drum; means for directing relatively contaminant free liquid solvent via a selected portion of said helical blade to said internal surface of said drum between said solids outlet and said medial position of said drum and comprising means within said scroll tube for defining a solvent feed chamber spaced from said particulate feed chamber therein and passages extending through said scroll tube and within said selected portion of said helical blade from said solvent feed chamber for liquid solvent to exit said solvent feed chamber and enter said drum; means for rotating said drum at a sufficient rotational speed so as to form a layer of cuttings along the internal surface of said drum; means for rotating said conveyor at a rotational speed slightly different than the rotational speed of said drum so that said helical blade moves the solids toward said solids outlet and liquid solvent moves toward said liquid outlet in countercurrent flow; and means for discharging the clean cuttings from the cleaning system.