Field of the invention

[001 ] This invention relates to apparatus and methods for feeding and discharging fluids that require high pressure feed into a high pressure process and subsequent pressure reduction upon discharge from the process.

[002] In addition, this invention relates to pipeline pig cleaning apparatus and methods for use with such apparatus and processes.

Background to the invention

[003] There are a number of processes that require high pressure fluid feeds and subsequent product fluid pressure let down after processing.

[004] An example of such a process is the use of supercritical fluids to treat a feed fluid. In such a process, the feed fluid requires a high pressure in-feed into the process and the product fluid requires fluid pressure let down upon discharge from the process. In these processes, the pressure of the feed fluid (the feedstock or fluid fed into the process for treatment) is raised to process pressure, typically to the operating pressure of the processing reactor in which the feed fluid is processed. The processed or product fluid (the fluid product resulting from processing in the process or reactor) is then discharged and the pressure thereof is reduced, typically to ambient pressure. The term "process fluid" is used to describe either or both the feed fluid and the product fluid.

[005] A more specific example of such a process is the use of supercritical water to treat a used oil feedstock or feed fluid at temperatures and pressures at or above the critical point of water. The invention will be described with reference to such apparatus and processes by way of non-limiting examples.

[006] The recycling of used oil, particularly lubrication oil, is essentially a process of recovering the base oil by separating it from the additives, degradation products and contaminants acquired during the use or collection of the oil. Lubricating oils are typically formulated by mixing lubricant bases with additives, some of which contain metals, to confer on the oil the qualities demanded by the application of the oil, including resistance to oxidation, shearing, temperature, emulsification and foaming and low variability of viscosity with temperature to mention but a few properties. During use the oil tends to incorporate degradation products, including lighter petroleum fractions such as naphtha and gas- oil and heavier fractions such as asphalts and coke. In addition, the oil tends to acquire contaminants during use and collection, such as water, cleaning products, solvents and accumulated dirt, including earth and metal particles. [007] I† is an object of this invention to provide apparatus and processes to elevate a feed fluid pressure to reactor pressure; to feed the feed fluid under reactor pressure into the reactor where the feed fluid is processed into a product fluid; to discharge the product fluid from the reactor at reactor pressure; and finally to reduce the product fluid pressure to a desired (lower) end pressure for further use, processing or storage.

[008] In many processes, such as used lubrication oil recycling for instance, the product fluid exits the reactor as a multi-phase liquid with readily separable, substantially immiscible phases. It is a further objective of this invention to provide a pressure let-down mechanism configured to minimise turbulence during pressure let-down with a view to minimising remixing of the liquid phases during pressure reduction of the product fluid.

[009] Several fluid treatment processes tend to develop reactor and pipeline precipitation that is not removed as a result of process fluid circulation and it is yet a further object of this invention to provide pipeline pig apparatus configured for operation with the apparatus and processes of the invention.

Summary of the invention

[0010] This invention provides fluid feed and discharge apparatus for feeding a process fluid to and from a process that operates at a pressure substantially different from the ambient pressure surrounding the process. [001 1 ] For the sake of clarity, the invention is described and claimed with reference to a relatively high pressure process. In addition and regardless of the examples used to illustrate the principles of the invention, it is not the absolute pressure of the process that is relevant. Rather it is the relative pressure of the process taken in relation to the ambient pressure within which the process is operated.

[0012] According to this invention fluid feed and discharge apparatus is provided for feeding a process fluid to and from a relatively high pressure process including a process in-feed and a process out-feed by means of which, respectively, unprocessed process fluid is fed into and processed process fluid is fed from the high pressure process, the apparatus comprising: at least one fluid feed pump for feeding unprocessed process fluid into the high pressure process; at least one fluid discharge pump for discharging the processed process fluid fed from the high pressure process; and a fluid pressure buffer system including at least one buffer fluid pump that is in fluid communication with the fluid feed and fluid discharge pumps.

[0013] The fluid feed and fluid discharge pumps each comprising a free piston reciprocally mounted within a cylinder dividing the cylinder into process fluid and buffer fluid sections, the process fluid sections of the cylinders each having a valve controlled inlet and outlet and the buffer fluid sections of the cylinders being in fluid communication by means of the fluid pressure buffer system.

[0014] The fluid feed pump is connected, at its inlet, to a supply of unprocessed process fluid under relatively low pressure and, at its outlet, to a process in-feed by means of which unprocessed process fluid is fed to the process under process pressure. The fluid discharge pump is connected, at its inlet, to a process out-feed by means of which processed process fluid is fed to the fluid discharge pump under process pressure and, at its outlet, to a processed fluid discharge facility, such as a storage tank, under relatively low pressure.

[0015] The pumps are provided with valve means configured alternately to open and close the process fluid sections of the cylinders to relatively high process pressure and relatively low fluid in-feed and discharge pressure in alternating high- and low-pressure cycles.

[001 6] In the low-pressure cycle: the fluid feed pump inlet and outlet are open and closed respectively to connect the process fluid section of the fluid feed pump to the supply of unprocessed process fluid under relatively low pressure; and the fluid discharge pump inlet and outlet are closed and open respectively to connect the process fluid section of the fluid discharge pump to the discharge facility under relatively low pressure. [0017] With both sets of valves open to to relatively low or ambient pressure, pressure in the cylinders is essentially equalised at low/ambient pressure.

[0018] in the high-pressure cycle: the fluid feed pump inlet and outlet are closed and open respectively to connect the process fluid section of the fluid feed pump to the process by way of the process in-feed; and the fluid discharge pump inlet and outlet are open and closed respectively to connect the process fluid section of the fluid discharge pump to the process by way of the process out- feed.

[0019] With both sets of valves open to to relatively high process pressure, pressure in the cylinders is essentially equalised at process pressure.

[0020] The buffer fluid pump is repeatedly operable: in the low-pressure cycle, to reciprocate the pistons in the fluid feed pump and fluid discharge pump under the relatively low supply pressure of unprocessed process fluid filling the process fluid section of the fluid feed pump and to discharge the processed process fluid from the process fluid section of the fluid discharge pump to the discharge facility; and in the high-pressure cycle, to reciprocate the pistons in the fluid feed pump and fluid discharge pump under the relatively high process pressure of processed process fluid filling the process fluid section of the fluid discharge pump and to feed the unprocessed process fluid in the process fluid section of the fluid feed pump by way of the process in- feed to the process under process pressure.

[0021 ] The free pistons reciprocally mounted within the fluid feed and fluid discharge pumps may conveniently be entirely free-floating and linked only hydraulically to the fluid pressure buffer system.

[0022] Alternatively, the free piston in at least the fluid feed pump may be mechanically linked to the fluid pressure buffer system, the free piston being connected by means of a piston rod to a piston reciprocally mounted within the buffer fluid pump.

[0023] The unprocessed process fluid is preferably fed from a fluid supply assembly comprising at least two pumps driven in a predetermined ratio to mix and feed the unprocessed process fluid in an equivalent ratio.

[0024] To assist in equalising fluid discharge pressures and reducing discharge turbulence, the fluid discharge pump may conveniently include a mini-piston configured to penetrate and occlude an enlarged mini-cylinder formed about the opening of the fluid discharge pump outlet, the mini-piston being configured to enter the mini-cylinder and to raise the pressure in the mini-cylinder between the mini-piston and the fluid discharge pump inlet valve to a pressure at or near process pressure, thereby allowing the valve to open with a reduced pressure drop across its seat.

[0025] The fluid feed and discharge apparatus of the invention is preferably configured to include a pipeline pig cleaning system including a pig insertion station, a fluid feed pump pig holdback assembly, a fluid discharge pump pig holdback assembly and a pig recovery station, the pig system including a plurality of pipeline pigs that are configured for feeding into the fluid lines of the appratus at the pig insertion station and transportation through the fluid lines by the movement of the process fluid through the lines, the fluid feed pump inlet and outlet and the fluid discharge pump inlet and outlet being configured as through-flow, valve controlled lines connected on either side of a combination inlet/outlet line extending from the fluid feed pump and the fluid discharge pump, respectively.

[0026] The pig insertion station preferably includes an intermittently operable, for instance solenoid operated, pig holdback assembly that is configured to release a predetermined quantity of pigs intermittently into the apparatus fluid lines. The pigs are preferably released a single pig per pump stroke. Pig release is preferably timed for the low-pressure cycle of the fluid feed and fluid discharge pumps, the pig release being timed to ensure that the pigs are released into the fluid line only when the fluid feed pump inlet valve is fully open and fluid is flowing to the fluid feed pump.

[0027] The fluid feed and discharge apparatus of the invention may conveniently include at least two additional pumps configured to operate as fluid accumulators, the first pump being open to the process in-feed end of the fluid feed pump and configured to operate as an unprocessed fluid accumulator, the second pump being open to the process out-feed end of the fluid discharge pump and configured to act as a processed fluid accumulator, both accumulators being acted upon by a plunger pump that is in fluid communication with the accumulators and configured to operate the accumulators in contraflow to the fluid feed and fluid discharge pump assembly.

[0028] The invention includes fluid feed and discharge apparatus such as that described above, but then optimised for supercritical water treatment of used oil and preferably used lubrication oil.

[0029] The invention includes a method of feeding and discharging a process fluid to and from a relatively high pressure process that includes a process in-feed and a process out-feed by means of which, respectively, unprocessed process fluid is fed into and processed process fluid is fed from the high pressure process, the method comprising the steps of: from at least one fluid feed pump, feeding unprocessed process fluid into the high pressure process; from at least one fluid discharge pump, discharging the processed process fluid fed from the high pressure process; the fluid feed and fluid discharge pumps each comprising a free piston reciprocally mounted within a cylinder dividing the cylinder into process fluid and buffer fluid sections, the process fluid sections of the cylinders each having a valve controlled inlet and outlet and the buffer fluid sections of the cylinders being in fluid communication by means of the fluid pressure buffer system; connecting a fluid pressure buffer system including at least one buffer fluid pump, in fluid communication with the fluid feed and fluid discharge pumps; connecting the fluid feed pump, at its inlet, to a supply of unprocessed process fluid under relatively low pressure and, at its outlet, to a process in-feed by means of which unprocessed process fluid may be fed to the process under process pressure; connecting the fluid discharge pump , at its inlet, to a process out-feed by means of which processed process fluid may be fed to the fluid discharge pump under process pressure and, at its outlet, to a processed fluid discharge facility under relatively low pressure; the pumps including valve means configured alternately to open and close the process fluid sections of the cylinders to relatively high process pressure and relatively low fluid in-feed and discharge pressure in alternating high- and low-pressure cycles and the method including the steps of operating the valves such that: in the low-pressure cycle, the fluid feed pump inlet and outlet are open and closed respectively to connect the process fluid section of the fluid feed pump to the supply of unprocessed process fluid under relatively low pressure and the fluid discharge pump inlet and outlet are closed and open respectively to connect the process fluid section of the fluid discharge pump to the discharge facility under relatively low pressure; and in the high-pressure cycle, the fluid feed pump inlet and outlet are closed and open respectively to connect the process fluid section of the fluid feed pump to the process by way of the process in-feed and the fluid discharge pump inlet and outlet are open and closed respectively to connect the process fluid section of the fluid discharge pump to the process by way of the process out-feed; and operating the buffer fluid pump repeatedly: in the low-pressure cycle, to reciprocate the pistons in the fluid feed pump and fluid discharge pump under the relatively low supply pressure of unprocessed process fluid filling the process fluid section of the fluid feed pump and to discharge the processed process fluid from the process fluid section of the fluid discharge pump to the discharge facility; and in the high-pressure cycle, to reciprocate the pistons in the fluid feed pump and fluid discharge pump under the relatively high process pressure of processed process fluid filling the process fluid section of the fluid discharge pump and to feed the unprocessed process fluid in the process fluid section of the fluid feed pump by way of the process in- feed to the process under process pressure.

[0030] The method may conveniently include the steps of feeding the unprocessed process fluid from a fluid supply assembly comprising at least two pumps and driving the pumps in a predetermined ratio to mix and feed the unprocessed process fluid in an equivalent ratio.

[0031 ] In the transition between high- and low pressure cycles, the pumps and valves may conveniently be operated such that the presssure across the pump pistons and valves is substantially equal at all times.

[0032] The invention is described with reference to positive displacement reciprocating piston pumps, but it will be appreciated that the scope of the invention is not limited to such pumps and alternative and mechanically equivalent pumps may be substituted for the piston pumps described in this specification where the context permits.

[0033] In addition, the terms "piston", "cylinder" and "reciprocal" are normally intended to refer to positive displacement reciprocating piston pumps. It will be appreciated that numerous mechanical equivalents exist for such apparatus, such as rotary pumps and their components. Where the context permits, these mechanical equivalents are intended to be included. Brief Description of the drawings

[0034] The invention will be further described with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic representation of a pump bank constituting the apparatus basic to the high pressure feed and discharge apparatus of the invention;

Figure 2 is a series of similar diagrams illustrating (in Figures 2A to 2L) the manner in which the basic apparatus of Figure 1 operates (for the sake of clarity Figure 21 is omitted);

Figure 3 is a process flow diagram illustrating a first embodiment of a process plant including the high pressure feed and discharge apparatus of the invention;

Figure 4 is a process flow diagram illustrating a second embodiment of a process plant including the high pressure feed and discharge apparatus of the invention;

Figure 5 is a portion of the process flow diagram of Figure 4 illustrating the primary and auxiliary pump assemblies forming part of the plant of Figure 4 in more detail;

Figure 6 is a portion of the process flow diagram of Figure 4 illustrating the primary pump and in-feed assemblies in more detail; and

Figure 7 is a portion of the process flow diagram of Figure 4 illustrating the pipe cleaning pig handling apparatus forming part of the plant in more detail.

Description of embodiments of the invention

[0035] The invention is illustrated and exemplified with reference to supercritical fluid processing plant and particularly supercritical water used oil treatment plant. It will be appreciated that this is done purely for illustrative purposes and is not intended to limit the invention or any part thereof to such plant or equipment.

[0036] A supercritical fluid is any fluid that is heated and pressurised to a temperature and pressure above its critical point, where liquid and gas phases become indistinguishable. The fluid effuses through solids like a gas and dissolves materials like a liquid. Supercritical fluids are used as a substitute for organic solvents in a range of industrial and laboratory processes. Supercritical fluids have solvent powers similar to liquids but the viscosity of the supercritical fluid is substantially lower than the parent fluid while the supercritical fluid density is many times greater than the parent fluid vapour phase at room temperature. Consequently molecular interactions occur at a much faster kinetic rate. The unprocessed process fluid or feed fluid (the fluid fed into the supercritical process for treatment) is typically a blend of feedstock and solvent fluids, the feedstock fluid being the solute. The supercritical process is normally operated to produce a process fluid that is prepared and readily capable of post-processing to extract product and waste streams

[0037] The basic apparatus of the invention, as illustrated in Figures 1 and 2, consists of a bank of positive displacement piston pumps that constitute feed and discharge pumps. The pumps are connected to one another by means of a buffer fluid system that uses a positive displacement plunger pump as a modulating hydraulic control.

[0038] Referring to Figure 1 , the bank 10 of pumps comprises: a positive displacement piston pump configured to operate as a fluid feed pump 12; a positive displacement piston pump 14 configured to operate as a product fluid discharge pump; and a buffer system including a plunger pump configured to operate as a buffer pump 16 that is in fluid communication with the pumps 12, 14 by way of fluid lines 18 that are open (not valve controlled) to the feed and discharge pumps 12, 14, the buffer pump 16 and circuitry 18 constituting a modulating hydraulic control mechanism for the pumps 12, 14.

[0039] The feed pump 12 comprises a feed cylinder 20 within which a free piston 22 is mounted for reciprocating movement between the ends of the feed cylinder 20. The feed cylinder piston 22 divides the feed cylinder 20 into a buffer fluid section 24 and a process fluid section 26.

[0040] The product fluid discharge pump 14 comprises a discharge or product cylinder 28 within which a free piston 30 is mounted for reciprocating motion between the ends of the cylinder 28, the piston 30 separating the cylinder 28 into a buffer fluid section 32 and a process fluid section 34.

[0041 ] The buffer fluid system (made up by the buffer fluid pump 16 and buffer fluid lines 18) is open only to the buffer fluid sections 24, 32 of the feed and discharge pumps 12, 14.

[0042] The buffer pump 16 comprises a plunger pump made up of a plunger 36 mounted for reciprocating motion within a buffer fluid cylinder 38, the plunger being acted upon by a free piston 40 reciprocally mounted within a hydraulic control cylinder 42, the operation of which is controlled by a set of modulating valves 44.

[0043] The process fluid section 26 of the feed pump 12 is provided with an inlet 46 controlled by means of a control valve 48, the inlet 46 being connected to a supply of feed fluid under relatively low pressure (not shown in Figure 1 ). The process fluid section 26 is provided with an outlet 50 controlled by a valve 52, by means of which feed fluid is supplied to a process under relatively high process pressure (the process not being shown in Figure 1 ). The outlet constitutes a process in-feed by means of which product fluid is fed to the process.

[0044] The process fluid section 34 of the discharge pump 14 is provided with an inlet 54 controlled by a valve 56 by means of which the product fluid is fed from a process out-feed (not shown).

[0045] The process out-feed is the feed of product fluid that, after processing, is fed under process pressure from the high pressure process to the fluid discharge pump 14. [0046] The terms "in" and "out" in the terms "process in-feed" and "process out-feed" refer, respectively, to a feed into and out of the process, regardless of the flow direction of the process fluid into or out of a pump 12, 14.

[0047] The pump 14 is provided with an outlet 58 controlled by a valve 60 by means of which the product fluid in the fluid discharge pump 14 is discharged under relatively low pressure to a product fluid store (not shown).

[0048] The feed and discharge cylinders 20, 28 are sealed except for the inlets and outlets 46, 50, 54, 58 and the pistons 22, 30 in each cylinder are hydraulically connected by the buffer fluid in the buffer fluid circuit 16, 18 which provides a hydraulic link between the feed and discharge pumps 12, 14.

[0049] The operation of the apparatus is illustrated in Figures 2A to 2L.

[0050] In Figure 2A, the bank of pumps 10 is illustrated at the beginning of the discharge cycle, with the discharge pump 14 filled with product fluid previously supplied from the process. The discharge pump inlet valve 56 is closed, thereby isolating the discharge pump 14 from high process pressure. The process fluid section 26 of the feed pump 12 is essentially empty and the outlet valve 52 is closed, thereby isolating the feed pump 12 from process pressure. Buffer fluid within the buffer fluid circuit 18 equalises pressures within the buffer fluid sections 24, 32 of the pumps 12, 14 and balances the fluid pressure within the entire bank 10, which, in this state, is relatively low since the pumps 12, 14 are isolated from process pressure by virtue of the valves 52, 56 being closed.

[0051 ] Moving from Figure 2A through Figures 2B and 2C, feed fluid is supplied to the process fluid section 26 of the feed pump 12 by means of a low pressure feed pump (not shown). The resulting pressure differential between the process fluid section 26 of the feed pump and the process fluid section 34 of the discharge pump 14 causes the feed pump piston 22 to reciprocate automatically to allow the process fluid section 26 to fill up with feed fluid.

[0052] Due to the hydraulic linkage provided by the buffer fluid circuit 18, the feed pump piston 30 is reciprocated in the opposite direction to discharge the product fluid in the process fluid section 34 by way of the outlet 58 and open valve 60 to the product fluid store (not shown).

[0053] The reciprocation of the pistons 22, 30 continues through the positions illustrated in Figure 2C to the position illustrated in Figure 2D in which the product fluid section 34 of the discharge pump 14 is essentially empty and the feed fluid section 26 of the feed pump 12 is essentially full.

[0054] Throughout this discharge cycle (Figures 2A to 2D), the fluid pressure within the bank 10 of pumps remains relatively low. The discharge pressure is also relatively low and discharge turbulence is kept to a minimum.

[0055] At this point, the bank 10 of pumps cycles from discharge mode (Figures 2A to 2D) to charge mode (Figures 2E to 2J).

[0056] In Figure 2E, the feed pump inlet valve 48 is closed and the outlet valve 52 is opened, exposing the process fluid section 26 of the feed pump 12 (which is filled with product fluid) to process pressure. At the same time, the discharge pump outlet valve 60 is closed while the inlet valve 56 is opened, exposing the process fluid section 34 of the discharge pump 14 to process pressure. As a result, the pressure in the bank 10 of pumps 12, 14 rises to the process pressure. With the pressure in the pumps 12, 14 essentially balanced, the buffer plunger 36 is withdrawn, thereby allowing product fluid flowing in through the inlet valve 56 to charge the process fluid section 34 of the feed pump 14, a process that continues through the stages illustrated in Figures 2F, 2G and 2H up to the end of the charge cycle illustrated in Figure 2J. During this time, the plunger 16 in the buffer cylinder 38 oscillates to modulate system pressure within the bank 10 of pumps.

[0057] As the bank 10 of pumps reaches the end of the charge cycle (Figure 2J), all the valves (48, 52, 58, 60) close (as illustrated in Figure 2 ) to isolate the feed and discharge pumps 12, 14 from process pressure and the buffer cylinder plunger 36 is withdrawn to reduce pressure prior to opening the discharge valves as illustrated in Figure 2L, which brings the bank 10 of pumps back to the state illustrated in Figure 2A ready to commence the discharge cycle described above with reference to Figures 2A to 2D.

[0058] In a typical implementation, the apparatus of the invention will have at least two banks of cylinders to smooth out the flow to and from the high pressure process.

[0059] Such an implementation is illustrated in Figure 3 in which a first bank 10.1 of pumps is interconnected with a second bank 10.2 of pumps. The banks 10.1 , 10.2 of pumps are supplied with feed fluid from a set of low pressure feed pumps 62. The high pressure process is represented by a reactor 64 and the product fluid is discharged into a product fluid tank 66.

[0060] The buffer fluid circuits 18.1 , 18.2 of the pump banks 10.1 , 10.2 are provided with cooling coils 68 by means of which the buffer fluid can be cooled if necessary. The interiors of the pistons 22, 30 in the feed and discharge cylinders 20, 28 of the pump banks 10.1 , 10.2 are open to the buffer fluid in the buffer fluid sections 24, 32 to allow the ingress of cooled buffer fluid if cooling should become necessary to preserve the piston seals.

[0061 ] In the state illustrated in Figure 3, the valves of the pump banks 10.1 , 10.2 are set to ensure that the pumps of the one bank 10.1 will discharge when the pumps of the other bank 10.2 are charging and vice versa, thereby ensuring essentially continuous flow.

[0062] In reactor charge mode, the cylinders are under reactor process pressure, but the differential pressure across the feed and discharge pistons 22, 30 equates substantially to the pressure drop across the reactor system. In most implementations, this pressure drop will be relatively small, thereby ensuring automatically that the piston seals have little pressure to resist.

[0063] Referring to Figure 1 , the product pumps 14 include a mini- piston 31 configured to penetrate and occlude an enlarged mini- cylinder formed about the opening of the product fluid inlet 54. When the discharge piston 30 reaches the end of its piston stroke in the process fluid section 34, the mini-piston 31 enters the mini-cylinder, which raises the back pressure in front of the product fluid inlet valve 56, thereby allowing the valve 56 to open with a reduced pressure drop across its seat.

[0064] As illustrated above, the inlet valves 56 to the discharge pumps 14 shut at the end of the charge stroke whilst the outlet valve 60 remains closed. The buffer cylinder plunger 16 then withdraws fractionally, thereby reducing the pressure in the discharge cylinder 28. Once the pressure in the discharge cylinder 28 has reduced, the outlet valve 60 to the product fluid store 66 has no differential pressure across the valve as it opens. In this manner, the discharge cycle allows the product fluid to be transferred to the product fluid tank 66 with minimal turbulence.

[0065] A second embodiment of the invention (essentially a development of the plant of Figure 3) is illustrated in Figures 4 to 6 which show a plant made up of a plurality of functional process assemblies, including: a reactor assembly 100; a feed fluid supply assembly 101 ; a feed cylinder pig holdback assembly 102; a primary pump assembly 103; a production cylinder pig holdback assembly 104; a pig recovery station 105; a product storage assembly 106; an auxiliary pump assembly 107 which is equivalent to and takes the place of the buffer system and buffer pump of Figures 1 to 3; and an accumulator pump assembly 108; which, together, make up a process plant 109.

[0066] In Figures 4 to 6, like elements found in both embodiments of the invention (Figures 1 to 3 and Figures 4 to 6) are given similar numbering, but then with the second embodiment elements (illustrated in Figures 4 to 6) numbered in 100s. For instance the Figure 6 equivalent of the Figure 1 feed pump 12 is the Figure 6 feed pump 1 12.

[0067] The plant 109 is a process plant for the treatment of used lubrication oil using supercritical water, the solute being used fuel oil and the solvent being supercritical water.

[0068] The two fluids (used oil and water) are pumped into the process by means of the feed fluid supply assembly 101 in which a motor 101 .1 drives a pair of pumps 101 .2, 101 .3 in a predetermined ratio to mix and feed the feedstock and solvent fluids from supply tanks 101 .4 and 101 .5 respectively. The feed fluid supply assembly 101 raises the feed fluid pressure to a predetermined fluid supply pressure (approximately 600kPa in the used fuel oil example). An accumulator 101 .6 is used to regulate and maintain the feed fluid supply pressure. Additional chemicals and catalysts may be dosed into the feed fluid mixture.

[0069] The primary pump assembly 103 raises the feed fluid (and therefore the process fluid) pressure to a predetermined process pressure that is substantially greater than the fluid supply pressure (produced by the feed fluid supply assembly 101 ). In the used fuel oil treatment process 109, the primary pump assembly 103 increases the pressure of the feed fluid (the feedstock/water mixture) to anything from 22mPa to 25MPa. At this point, the pressure of the mixture must be higher than the supercritical pressure of water, which is approximately 22mPa at 420°C. In the preferred form of the invention, in use as a used fuel oil treatment process, the primary pump assembly 103 increases the pressure of the feed fluid (the feedstock/water mixture) to 24MPa.

[0070] The reactor assembly 100 illustrated in Figure 1 comprises a countercurrent heat exchanger 100.1 , the input side of which is operated as a pinch heater, an electrical resistance heater 100.2 a contact tower 1 10.3 and a water cooled heat exchanger 100.4.

[0071 ] Together, the two heaters 100.1 , 100.2 raise the temperature of the process fluid (the feed fluid fed to the contact tower 100.3) to a predetermined process temperature. In the preferred form of the invention, in use as a used fuel oil treatment process, the process fluid temperature is raised to between 375°C and 440°C and the process fluid pressure is kept at around 24MPa.

[0072] The contact tower 100.3 is operated as a supercritical reactor. [0073] The product fluid exits the reactor 100.3 as a multi-phase liquid (essentially a two-phase fluid with readily separable fractions). Whilst the phases or fractions are substantially immiscible, it will be appreciated that excessive turbulence in the exit stream will remix the product fluid to a greater or lesser degree which will cause commensurate complication in subsequent separation of the product fluid. The apparatus and processes downstream of the reactor 100.3 are therefore configured to minimise turbulence is keep process flows as non-turbulent as possible.

[0074] After processing in the reactor 100.3, the product fluid (or product) is fed to the countercurrent section of the countercurrent heat exchanger 100.1 where heat energy is recovered from the product to pre-cool the product and pre-heat the incoming feed fluid.

[0075] After pre-cooling, the product is fed to the water cooled heat exchanger 100.4.

[0076] Fluid circulation in the process 109 is driven by the primary pump assembly 103, which is a bank 1 10 of positive displacement piston pumps 1 12, 1 14 similar to the pumps 12, 14 described with reference to Figures 1 to 3. The pumps 1 12, 1 14 are configured to operate as a fluid feed pump 1 12 and a product (product fluid) discharge pump 1 14 respectively.

[0077] Fluid pressure in the primary pump assembly 103 is adjusted by means of a plunger pump configured to operate as a buffer pump 1 16. This pump 1 16 and its associated control- and reticulation circuitry or piping constitutes a modulating hydraulic control similar to the buffer fluid circuit 16, 18 described with reference to Figures 1 to 3).

[0078] The feed pump 1 12 comprises a feed cylinder 120 within which an essentially free piston 122 is mounted for reciprocating movement between the ends of the feed cylinder 120. The feed cylinder piston 122 divides the feed cylinder 120 into a buffer fluid section 124 and a feed fluid section 126.

[0079] The product fluid discharge pump 1 14 comprises a discharge or product cylinder 128 within which a free piston 130 is mounted for reciprocating motion between the ends of the cylinder 128, the discharge cylinder piston 130 separating the cylinder 128 into a buffer fluid section 132 and a product (product fluid) section 134.

[0080] The buffer fluid system (which is made up by the buffer fluid pump 1 16 and connecting buffer fluid lines 1 18) is open only to the buffer fluid sections 124, 132 of the feed and discharge pumps 1 12, 1 14. The buffer pump 1 16 comprises a plunger pump made up of a plunger 136 mounted for reciprocating motion within a buffer fluid cylinder 138, the plunger being acted upon by a piston 140 reciprocally mounted within the buffer fluid cylinder 138, the operation of which is controlled by a set of modulating valves.

[0081 ] The feed fluid section 126 of the feed pump 1 12 is provided with an inlet 146 controlled by means of a control valve 148, the inlet 146 being connected to the feed fluid supply provided by the feed fluid supply assembly 101 . The feed fluid section 126 is provided with an outlet 150 controlled by a valve 152 by means of which unprocessed feed fluid is supplied †o the reactor assembly 100 under relatively high process pressure. The outlet constitutes the process in-feed by means of which product fluid is fed to the process constituted by the reactor assembly 100.

[0082] The valves 148, 152 form part of a feed cylinder pig holdback assembly 102 that is described I more detail below.

[0083] The product section 134 of the discharge pump 1 14 is provided with an inlet 154 controlled by a valve 156 by means of which the product fluid is fed from the process out-feed of the reactor assembly 100 under process pressure to the fluid discharge pump 1 14. An outlet 158 from the pump 14 is controlled by a valve 160 by means of which the product (product fluid) is discharged under relatively low pressure to the product storage assembly 106.

[0084] The feed and discharge cylinders 120, 128 are sealed except for the inlets and outlets 146, 150, 154, 158 and the pistons 122, 130 in each cylinder are hydraulically connected by the buffer fluid in the buffer fluid circuit 1 16, 1 18 which provides a hydraulic link between the feed and discharge pumps 1 12, 1 14.

[0085] The plant 109 includes a pipeline pig cleaning system including a pig insertion station 200, a feed cylinder pig holdback assembly 102, a production cylinder pig holdback assembly 104 and a pig recovery station 105. The pig system uses a plurality of pipeline pigs 202 that are fed into the fluid lines of the plant 109 at the pig insertion station 200 and transported through the plant fluid lines by the movement of the fluid through the lines.

[0086] At the pig insertion station 200, a solenoid actuated feed is mounted on a valve controlled bypass line 204 that is intermittently pressurised and de pressurised. The pigs 202 are fed from a pig feed hopper 206 into the bypass line 204 and from there into the feed fluid inlet line 101 .7. The pig feeder comprises a solenoid operated pig holdback assembly that is operated to ensure that the pigs are held back and released intermittently to feed a single pig 202 per pump stroke into the feed fluid line 101 .7. The pigs 202 must not move into the feed cylinder inlet valve 148 until this valve is fully open and fluid is flowing to the feed cylinder 1 12.

[0087] The pig recovery station 105 includes a catch grid 105.1 and a hopper 105.2 that collect the pigs 202 exiting the process 109 at the point where the processed product is discharged to the product storage assembly 106. From the pig recovery station 105 the pigs 202 are returned to the pig feed hopper 206.

[0088] The pig holdback assemblies 102, 104 are virtually identical and will be described mainly with reference to the production cylinder pig holdback assembly 104 (Figure 4) for the sake of brevity and clarity. The pig holdback assembly 104 includes a pair of solenoid actuated holdback devices 104.1 , 104.2, each mounted in-line on a fluid feed line of the plant 109 extending to/from the input/output end of the discharge cylinder 1 14. The solenoid actuated holdback devices of the feed cylinder pig holdback assembly 102 (Figures 1 and 3) are mounted in-line on a fluid feed line extending to/from the input/output end of the discharge cylinder 1 14.

[0089] The fluid lines are constructed to ensure the minimum of internal projections, to ensure uninterrupted pig transport through the fluid lines. The fluid lines are joined by means of T-pieces fitted with baffles and ratchets (not shown) that serve to deflect and direct the pigs during movement and fluid transport of the pigs 202 through the fluid lines.

[0090] The solenoid actuated holdback devices have a solenoid- controlled gate 104.3 (Figure 4) mounted to partially occlude a bypass tube 104.4 (Figure 4), the solenoid being actuable selectively to operate the gate 104.3 to interrupt or allow pigs through the bypass tube in dependence on the process cycle. The bypass tube 104.4 ensures that the gate 104.3 never occludes the fluid line in which it is mounted. Technically therefore, the gate 104.3 does not close the fluid line. For purposes of pig transport and movement however, the gate 104.3 does close or open the fluid line to permit or prevent the passage of the pigs 202 and in this sense the gate 104.3 is said to be "open" or "closed" respectively.

[0091 ] The operation of the primary pump assembly 103 is as follows -

Cycle A - the feed cylinder inlet valve 148 opens and the feed cylinder piston 122 moves from right to left (directions taken from the drawings); a single pig 202 is dropped into feed line 101 .7 by means of the pig insertion station 200; the discharge cylinder outlet valve 160 opens and the discharge cylinder piston 130 moves from left to right; PHB1 is open; PHB4 is open; feed fluid flows into the feed cylinder 1 12 and product flows out of the discharge cylinder 1 14, through the pig recovery station 105 to the product storage assembly 106 - buffer fluid pressure: 10 bar

Cycle B - the feed cylinder piston 122 is located at the left hand end of the feed cylinder 1 12; the feed cylinder inlet and outlet valves 148, 152 are closed; the discharge cylinder inlet and outlet valves 156, 160 are closed; the discharge cylinder piston 130 is at the right hand end of the discharge cylinder 1 14; all the production cylinder pig holdback assemblies (PHB1 ; PHB2; PHB3; PHB4) are closed); buffer fluid pressure is raised to 250 bar by the auxiliary pump assembly 107 - buffer fluid pressure: 250 bar

Cycle C - the feed cylinder piston 122 moves from right to left; the feed cylinder inlet valve 148 is closed; the feed cylinder outlet valve 152 is opened; the discharge cylinder inlet valve 156 is opened; the discharge cylinder outlet valve 160 is closed; the discharge cylinder piston 130 moves from right to left; PHB3 is open; PHB4 is closed; feed fluid flows out of the feed cylinder 1 12 to the reactor assembly 100; product fluid is discharged from the reactor assembly 100 into the discharge cylinder 1 14; the auxiliary pump assembly 107 is used to maintain the pressure at system pressure: buffer fluid pressure - 250 bar

Cycle D - the feed cylinder piston 122 is at the right hand end of the feed cylinder 1 12, feed cylinder inlet valve 148 and feed cylinder outlet valve 152 are closed; discharge cylinder inlet valve 156 and discharge cylinder outlet valve 160 are closed; the discharge cylinder piston 130 is at the left hand side of the discharge cylinder 1 14; PHB1 ; PHB2; PHB3; PHB4 are closed; the auxiliary pump assembly 107 is used to reduce the system pressure: buffer fluid pressure - 10 bar

Repeat - Cycle repeats in this description: PHB1 is the pig holdback assembly on the feed cylinder inlet 146; PHB2 is the pig holdback assembly on the feed cylinder outlet 150; PHB3 is the pig holdback assembly on the discharge cylinder inlet 154; and PHB4 is the pig holdback assembly on the discharge cylinder outlet 158.

[0092] The buffer fluid and the auxiliary pump assembly 107 are used to develop, distribute, equalise and balance pressures within the buffer fluid sections 124, 132 of the pumps 1 12, 1 14.

[0093] To facilitate the use of a pipeline pig cleaning system, the process 109 achieves fluid flow-through continuity with the use of a second bank of positive displacement piston pumps 108.1 , 108.2 that are operated as fluid accumulators. The first pump 108.1 is open to the outlet 150 (to the reactor assembly 100) of the feed cylinder 1 12 and acts as a feed fluid accumulator. The second pump 108.2 is open to the inlet 154 (from the reactor assembly 100) of the product cylinder 1 12 and acts as a product fluid accumulator. Both accumulator cylinders 108.1 , 108.2 are acted upon by a plunger pump 108.3 that is hydraulically interlinked with the auxiliary pump assembly 107 to operate the feed fluid accumulator 108.1 and the product fluid accumulator 108.2 in contraflow to the primary pump assembly 103.

[0094] The accumulator arrangement including the pumps 108 obviates the need for the mini-piston 31 arrangement used in the product fluid discharge pumps 14 of Figure 1 .

[0095] In addition to the basic pump strokes, sequences will be programmed in to provide that, before the discharge cylinder inlet valve 156 opens to open the discharge cylinder 1 14 to reactor pressure, the buffer fluid pressure will be raised by the auxiliary pump assembly 107 (to 250 bar in the examples given above). This will ensure that the fluid pressure on either side of the discharge cylinder inlet valve 156 is substantially equal.

[0096] A similar sequence will reduce the buffer fluid pressure (to 10 bar in the examples given above) before opening the discharge cylinder outlet valve 160, to reduce the turbulence on the outflow of product fluid through the outlet 158. To do this, the inlet valve 156 to the discharge pump 1 14 shuts at the end of the charge stroke whilst the outlet valve 160 remains closed. The buffer cylinder plunger 136 then withdraws fractionally, thereby reducing the pressure in the discharge cylinder 128. Once the pressure in the discharge cylinder 128 has reduced, the outlet valve 160 to the product storage assembly 106 has no differential pressure across the valve 16 as it opens. In this manner, the discharge cycle allows the product fluid to be transferred to the product storage assembly 106 with minimal turbulence. [0097] The apparatus described in the examples above finds particular application in feeding and discharging process fluids to and from high pressure reactors with the benefit that the fluid discharge volume varies in direct proportion to feed fluid input, which permits automatic volumetric discharge variation. In addition, the apparatus of the invention creates total separation between the feed and the discharge fluid streams through the use of separate feed and discharge pumps separated by one another by means of a buffer fluid.

[0098] Notwithstanding such separation, the fact that the apparatus of the invention can be combined in multiple pump banks enables virtually continuous flow operation with the added benefit that the buffer system and valving allows the product fluid to be transferred to the product fluid store with minimal turbulence.