NL2011805C2 - Electrolysis unit for waste water treatment. - Google Patents

Abstract

An electrolysis unit for the separation of hydrocarbons from waste water, comprises a closed electrolysis vessel having at least an electrolysis compartment, a separation compartment, a vessel inlet, a vessel outlet, an air inlet and an air outlet. One or more removable electrode cartridges are located within the electrolysis compartment, each electrode cartridge comprising an electrode pair, separated by a fluid path. The vessel inlet and the vessel outlet are arranged such that a flow entering the electrolysis vessel through the vessel inlet will flow through the fluid paths of the electrode cartridges into the electrolysis compartment and subsequently through the separation compartment to the vessel outlet. The air inlet and the air outlet are arranged at an upper side of the vessel for circulating a flow of air through both the electrolysis compartment and the separation compartment.

Description

ELECTROLYSIS UNIT FOR WASTE WATER TREATMENT

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to waste water treatment by electrolysis and more particularly to an improved electrolysis unit construction and electrode cartridge therefor. The invention also relates to a system and the thereof in order to perform waste water treatment in difficult environments. 2. Description of the Related Art [0002] It has long been known to treat certain waste water using electrolysis.

Waste waters in many cases contain compounds with some surface active properties. One example is that of corrosion inhibitors as used in the industry to protect e.g. piping from corroding. These inhibitors are generally large aliphatic or aromatic compounds with a positively charged amine group. Synthetic surfactants are also used in fracking fluids as well as in the preparation of surfactant polymer mixes for enhanced oil recovery technologies. Natural surfactants are also present in crude oil or gas condensate mixtures. Various mechanisms for treatment by electrolysis may be present either individually or together, including electro-coagulation, electro-oxidation and electro-flotation. The process is particularly applicable for oil in water emulsions containing surfactants although emulsions without surface active agents may also be treated.

[0003] A document that describes the use of electrolysis in this manner is US4439290. According to that document, an electrolysis cell comprises an outer metal casing connected as the cathode and a rod-shaped anode inserted concentrically therein. A feed pump drives the waste water to be treated through the cell in the annular gap between the electrodes. The slurry formed is conveyed together with the de-oiled effluent to a flotation decanter. There the slurry flows to the water surface and overflows into a porous bag where it is collected.

[0004] A disadvantage of such a system is the open nature of the process, whereby gases produced during the electrolysis procedure may be liberated to atmosphere. Additionally, the electrolysis cell construction is complex, especially in the case that inert electrodes are to be used.

[0005] It would therefore be desirable to provide an alternative electrolysis unit that can be easily implemented and utilised.

BRIEF SUMMARY OF THE INVENTION

[0006] According to the invention there is provided an electrolysis unit for the separation of hydrocarbons from waste water, comprising a closed electrolysis vessel having at least an electrolysis compartment, a separation compartment, a vessel inlet, a vessel outlet, an air inlet and an air outlet; at least one removable electrode cartridge located within the electrolysis compartment; the electrode cartridge comprising an electrode pair, separated by a fluid path; wherein the vessel inlet and the vessel outlet are arranged such that a flow entering the electrolysis vessel through the vessel inlet will flow through the fluid path of the electrode cartridge into the electrolysis compartment and subsequently through the separation compartment to the vessel outlet and wherein the air inlet and the air outlet are arranged at an upper side of the vessel for circulating a flow of air through both the electrolysis compartment and the separation compartment. As a result of the proposed construction a robust unit is achieved for use in critical situations such as off-shore or onsite. It will be understood that treatment of waste water containing hydrocarbons may take place in an environment that is critical with respect to risks of explosion. In the present context, closed is intended to require that the electrolysis vessel is closed with respect to the surrounding atmosphere. Air circulation is provided through the air inlet and air outlet whereby the composition of the atmosphere within the electrolysis vessel can be controlled and maintained below required explosion limits. In the present context, although reference is given to air inlet and air outlet, it will be understood that these may also be used for circulation of inert gases such as nitrogen or carbon dioxide. Furthermore, reference to waste water is not meant to be limiting to the particular source or destination of the water but is merely indicative of the fact that it contains hydrocarbons.

[0007] In a preferred embodiment of the invention, the unit is provided with a plurality of cartridges arranged in parallel. The cartridges are individually or collectively removable. By providing parallel paths through individual cartridges, should one or more of the cartridges cease to operate, circulation may continue through the other cartridges. The faulty cartridge may be replaced as and when this operation is permitted, without requiring the whole unit to be replaced. It will be understood that in offshore conditions, delivery of a replacement cartridge is considerably simpler than the replacement of a complete unit.

[0008] According to a further aspect of the invention, the unit may further comprise an inlet manifold separating the electrolysis compartment from an inlet chamber and comprising a seat for sealingly engaging an inlet end of the or each electrode cartridge. The cartridge may be screwed into or otherwise attached to the manifold or may merely be engaged against the manifold and held in place by other provisions. Preferably, an O-ring seal between the cartridge and the manifold is provided to ensure that the connection is hermetic. In the case of multiple cartridges, the inlet chamber ensures distribution of the waste water flow to all of the electrode cartridges.

[0009] Most preferably, the electrolysis compartment is separated from the separation compartment by an overflow baffle extending to a position that is higher than an upper extent of the electrode pairs. Treated water that has passed through the electrode cartridge into the electrolysis compartment can overflow into the separation compartment over the overflow baffle. The height of the overflow baffle may be fixed to ensure that the electrodes are always covered by water to avoid exposure and possible overheating of the electrodes. In an alternative, the height of the overflow baffle may be adjustable.

[0010] In the separation compartment, the treated water is allowed to settle so that hydrocarbon droplets that have come out of emulsion or otherwise destabilised can rise to the surface. The relatively cleaner water can be removed from the bottom of the separation compartment in any suitable manner. Preferably, the electrolysis vessel is provided with an underflow baffle and a second overflow baffle located between the separation compartment and the vessel outlet. The second overflow barrier can be used to regulate the liquid level in the electrolysis vessel. By including these additional baffles within the electrolysis vessel, further degassing of the water takes place in a single space allowing for any explosive or toxic gases to be removed via the air outlet.

It will nevertheless be understood that such underflow and overflow provisions could be omitted or provided elsewhere in a separate vessel or vessels.

[0011] Within the separation compartment, the separated hydrocarbon that floats to the surface may be collected. Preferably, this is achieved by a skimmer located in the separation compartment and connected to a hydrocarbon outlet for the separated hydrocarbon portion. The skimmer is preferably adjustable in height and may be maintained in position e.g. by vertical spindles. The height of the skimmer with respect to the water level can be adjusted with these spindles. Floating skimmers may also be used, in particular in the case that the height of the overflow baffle is not constant or if the water level in the separation compartment may vary for other reasons.

Alternatively, active skimmers may be used such as a belt skimmer which may be arranged to transport the floating hydrocarbon portion towards the hydrocarbon outlet.

[0012] According to a still further aspect of the invention, the unit may comprise a temperature sensor located within the electrolysis compartment and adapted to monitor the temperature of the water within the electrolysis compartment. In principle, the temperature sensor may be located at any suitable location within the electrolysis compartment and there may be a plurality of sensors at different locations. Sensors may be provided within or on the electrode cartridges to monitor more closely the flow through the fluid paths.

[0013] The invention also relates to an electrolysis system comprising the electrolysis unit as defined above or hereinafter. The electrolysis system may comprise a suitable energy supply for applying a voltage to the electrode cartridge or cartridges. This may be ensured by one or more rectifier units. Most preferably, the system comprises a plurality of rectifier units, each connected to a respective electrode cartridge. In this manner, individual control of each electrode cartridge is achievable and in the event of an anomaly, one electrode cartridge may be shut down without interfering with the operation of the remaining cartridges. Preferably, the rectifier units are capable of pole-reversal. Pole reversal should preferably be possible at frequencies of from 0.03 Hz to 0.2 Hz i.e. once every 5 to 30 seconds. The skilled person will be aware that energy efficiency may be improved by higher frequencies of pole reversal and that other frequencies may be applicable as circumstances require.

[0014] The system may further comprise a suitable pump connected to circulate a flow of waste water through the unit from the vessel inlet to the vessel outlet. The pump may be otherwise conventional and is preferably located in an inlet conduit leading to the vessel inlet. It will be understood that gravity supply may also be considered.

[0015] According to an important aspect of the invention, an inlet flow monitoring device may be arranged to monitor an inlet flow to the vessel inlet and control or stop power to the electrode cartridges if the inlet flow is below a given value. This may also or alternatively be arranged to only commence power to the electrode cartridge when the inlet flow exceeds a given value. The inlet flow monitoring device may be any form of direct or indirect flow measurement device capable of determining the presence or absence of flow or the amount of flow. The inlet flow monitoring device may be an electromagnetic flow meter, a vortex flow meter, a variable area flow meter or any otherwise conventional flow meter.

[0016] The system preferably also includes a ventilation unit connected to create a flow of air through the air inlet and the interior of the electrolysis vessel to the air outlet and an exhaust location. The ventilation unit is preferably a fan, blower, compressor, pump or the like. The exhaust location is preferably located at a distance from the electrolysis unit and may comprise e.g. a chimney or similar vent or flue. In particular, the ventilation unit avoids any recirculation of air through the electrolysis vessel.

[0017] In one embodiment, the system may comprise a gas composition monitor arranged to determine a concentration of hydrogen within the electrolysis vessel and control the flow of air to keep the concentration below a predetermined percentage of the lower explosion limit. In this manner more effective ventilation may be achieved which assures the correct level of gas dilution. An alarm may be given if the concentration exceeds preset limits, perhaps indicating that the ventilation is ineffective. In this situation, the power to the electrode cartridges may be stopped or reduced. The skilled person will understand that other gases than hydrogen may also be monitored as required.

[0018] In one embodiment, the system may comprise a colorimeter for determining the colour of the water leaving the electrode cartridge. In this manner, a good indication is obtained regarding the effective dosage of oxidants in the treated water. Excess oxidants can cause increased colourisation (in particular, red) because of the presence of iron (III) hydroxide. The current to the electrode cartridges may in this case be reduced. A single colorimeter may be provided for the electrolysis compartment or there may be provided one device per cartridge, allowing individual cartridge control.

[0019] In another embodiment, the system may be provided with turbidity sensors by means of which turbidity measurements may be carried out on the treated water. Preferably the turbidity sensor is provided downstream of the separation compartment and may even be provided at a post treatment location such as after a particulate filter.

In this manner, the actual demulsification can be controlled. An increased turbidity would mean that a higher concentration of oxidants is required from the electrode cartridge.

[0020] The invention also relates to an electrode cartridge for use in the electrolytic separation of hydrocarbons. The cartridge comprises an elongate tubular member of insulating material having a sidewall defining an enclosed channel and having an inlet end and an outlet end; a pair of elongate electrodes facing each other across the channel and separated from each other by a fluid path, wherein the electrodes extend along the channel from the inlet end to the outlet end and are connected to the sidewall. A pair of terminals, each electrically connected to a respective electrode, extends out of the tubular member. Such a construction has been found to be particularly sturdy in withstanding shocks, due to the connection of the electrodes to the sidewall. Additionally, the provision of a tubular member in which the electrodes are contained is a conveniently modular form, which may be easily stored, transported, installed and manufactured. Preferably the tubular member is manufactured of plastics material such as polypropylene or the like.

[0021] The terminals may extend out through the sidewall. Most preferably however, the terminals extend from the outlet end of the tubular member. For a cartridge that is arranged to work in an upright position with the inlet end at the bottom, this allows the terminals to extend out of the upper end, which may be the most accessible.

[0022] According to a still further preferred embodiment of the invention, the terminals are potted in an insulating resin mass. This additionally assists in ensuring that the terminals are insulated from each other and that there can be no loosening or movement of the terminals that might bring them into contact with each other or cause localised hot-spots. It will be understood that for electrolysis use, significant currents may be required and any poor connection to the electrode can cause a region of high electrical resistance and ohmic heating.

[0023] According to a most preferred embodiment of the invention, the tubular member is a circular cylinder and the electrodes are D-shaped having flat surfaces which face each other. The curved rear surface of the electrodes can have the same curvature as the channel of the cylinder. This ensures that the electrodes seat firmly, flush against the sidewall, reducing any gap where waste water could pass through without being treated. The skilled person will understand that the tubular member may have other shapes including square, rectangular and oval tubes.

[0024] The electrodes may be connected to the sidewall in any convenient manner, including gluing, welding or by mechanical means. Preferably, the electrodes are bolted through the sidewall at a plurality of locations along their length.

[0025] Depending on the precise detail of the separation process, the electrodes may have any composition. Most preferably, the electrodes are both carbon based, in particular they may comprise a mixture of graphite and acrylic or epoxy. Use of such inert electrodes prevents release of metal ions from the electrodes, and allows for stable operation over time.

[0026] In one preferred configuration, the inlet end is axially open into the channel and the outlet end is open radially from the channel through the sidewall. Such a radial outflow allows more space at the end of the cartridge for the terminals and also ensures that the outflow is not directed upwards out of the electrolysis compartment. The axial inflow maximises the flow area and also facilitates the design with respect to engagement e.g. with a suitable seat. The inlet end may also comprise a sealing element around the tubular member for engagement with a seat.

[0027] The invention is applicable to electrode cartridge of any convenient dimensions to the extent that they are suitable for the intended use of hydrocarbon separation. Preferably, the elongate member is between 30 cm and 150 cm in length, more preferably around 90 cm in length. The cartridge may be between 25 mm and 150 mm in diameter, preferably around 70 mm in diameter.

[0028] The dimensions of the fluid path will also depend upon the precise components to be separated and on the fluid flow rating and the power rating of the system. Most preferably, the fluid path has a constant width over the length and breadth of the channel in order to avoid local concentrations or hot-spots. The width of the fluid path may be between 6 mm and 10 mm, most preferably around 8 mm.

[0029] The invention also relates to a method of carrying out electrolytic treatment of saline waste-as described above, the method comprising: supplying the waste water to the vessel inlet to cause a flow through the electrode cartridges, the electrolysis compartment, the separation compartment and to the vessel outlet, applying a voltage to the electrode pairs to cause current to flow across the fluid path; and causing a flow of air or other gases into the air inlet and through the vessel to the air outlet, the flow of air being adapted to the current flowing between electrode pairs to ensure a concentration of hydrogen at the air outlet that is below a preset explosive limit. Preferably, the limit is set at a value of less than 1 %, more preferably, less than 0.4%. This may be achieved by monitoring directly or indirectly the hydrogen concentration or can be based on the theoretical maximum hydrogen production according to Faraday’s first law of electrolysis and based on the current supplied.

[0030] The method may also comprise monitoring a temperature of the waste water in the electrolysis compartment and ceasing application of the voltage when the temperature exceeds a predetermined value. In this manner, boiling of the water and/or overheating of the electrodes may be avoided. Preferably the predetermined maximum temperature is 50o C, although other temperature limits may be used as required.

[0031] The method may also or alternatively comprise monitoring a current supply to each electrode cartridge and ceasing application of voltage to a cartridge in the event that the current supply shows an anomaly. Although it is possible to shut down the whole unit, preferably only the cartridge drawing excess current is shut down, while maintaining a voltage across the remaining cartridges. Various alternatives may be used for monitoring the current supply. This may include comparing the current supplied to one cartridge with the current supplied to a number of cartridges, comparing the current with an absolute value or monitoring changes in the current over time. Additionally the method may comprise preventing flow through a cartridge in the event that a voltage is no longer applied to that cartridge. In this manner, untreated waste water may be prevented from passing through the shut-down cartridge. Flow may be prevented by an automatic valve or the like or may be interrupted manually.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The features and advantages of the invention will be appreciated upon reference to the following drawings of a number of exemplary embodiments, in which: [0033] Figure 1 shows a perspective view of an electrolysis unit according to the present invention; [0034] Figure 2 shows a schematic view of an electrolysis system according to the present invention; [0035] Figure 3 shows a side view of one of the electrode cartridges of Figure 1; [0036] Figure 3A shows a transverse cross-section through the electrode cartridge of Figure 3 in direction A-A; [0037] Figure 3B shows a longitudinal cross-section through the electrode cartridge of Figure 3 in the direction B-B indicated in Figure 3 A; [0038] Figure 3C shows a detail of the electrode cartridge of Figure 3B; [0039] Figure 3D shows in detail one of the terminals of the electrode cartridge of Figure 3; and [0040] Figure 3E shows a transverse cross-section through the electrode cartridge of Figure 3 in direction E-E.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0041] Figure 1 shows a partially cut-away perspective view of an electrolysis unit 10 according to the invention. The electrolysis unit 10 comprises an electrolysis vessel 12 made of stainless steel and internally coated with an epoxy coating to prevent current leakage and provide corrosion resistance. The electrolysis vessel 12 is closed by a lid 14, which is tightly clamped down by lid clamps 15. The lid 14 is also provided with air inlets 16 and an air outlet 18. On a lower side of the electrolysis vessel 12 are arranged a vessel inlet 20 and a vessel outlet 22.

[0042] The interior of the electrolysis vessel 12 is divided into a number of zones or compartments. A first overflow baffle 24 divides the interior of the electrolysis vessel into an electrolysis compartment 26 adjacent the vessel inlet 20 and a separation compartment 28. An inlet manifold 30 separates the electrolysis compartment 26 from the vessel inlet 20 and defines an inlet chamber 32. Between the separation compartment 28 and the vessel outlet 22 is an outlet chamber 33. The separation compartment 28 is separated from the outlet chamber 33 by an underflow baffle 34 and a second overflow baffle 36, the function of which will be explained in due course. Above the first overflow baffle 24 is a strainer 38. The inlet manifold 30 is manufactured of non conductive HDPE. The other baffles are manufactured from stainless steel. The skilled person will be aware that other materials may be chosen for these elements as appropriate under the circumstances.

[0043] The inlet manifold 30 comprises a number of seats 40 in which are located inlet ends 42 of electrode cartridges 44. Outlet ends 46 of the electrode cartridges 44 are retained by a hold-down plate 48 through which they protrude. Cables 50 extend from the outlet ends 46 and exit through the lid 14 of the electrolysis vessel 12. A temperature sensor 52 extends through the wall of the electrolysis vessel 12 into the electrolysis compartment 26. Also in Figure 1 is shown a skimmer 54 in an upper portion of the separation compartment 28. The skimmer 54 is in the form of a tray having a tubular connection 56 to a hydrocarbon outlet 58 at the outside of the electrolysis vessel 12.

[0044] Figure 2 shows schematically an electrolysis system 100 including the electrolysis unit 10 of Figure 1. A pump 102 connects a source S of waste water to the vessel inlet 20 via an inlet conduit 104 having a non-return valve 106. A flow monitor 108 is included along the inlet conduit 104, capable of emitting a signal indicative of the flow rate within the inlet conduit 104. The vessel outlet 22 is connected by an outlet conduit 110 to a post treatment apparatus P which will not be further described herein. The air outlet 18 is connected to a fan 112 which is vented to a safe location. A gas sensor 114 is provided at the air outlet 18. The gas sensor 114 is capable of detecting the concentration of hydrogen gas at the air outlet 18. The air inlets 16 receive conditioned air from an otherwise undisclosed location. Cables 50 are connected individually to rectifiers 116. A controller 120 is connected to receive signals from the temperature sensor 52, the flow monitor 108 and the gas sensor 114. The controller 120 is also operatively connected to the pump 102, the fan 112 and the rectifiers 116.

[0045] Figure 3 shows one of the electrode cartridges 44 of Figure 1. The electrode cartridge 44 comprises an elongate tubular member 60 of polypropylene (PP) having a sidewall 62 extending from the inlet end 42 to the outlet end 46. The outlet end 46 is provided with an end cap construction 49 through which the cables 50 protrude. Also visible in Figure 3 are bolts 72, the function of which will be explained below and outlet slot 47, extending radially outwards from the interior of the cartridge 44. At the inlet end 42, O-ring seals 43 are provided around the sidewall for sealing engagement with the manifold seat 40.

[0046] Figure 3 A shows a transverse cross-section through the cartridge 44 in the direction A-A in Fig 3. Figure 3B is a longitudinal cross-section of the cartridge 44 taken in the direction indicated B-B in Figure 3 A. As can be seen from Figure 3B, within the channel 64 are located a pair of elongate electrodes 66A, B. The electrodes 66A, B have flat faces 68 A, B which face each other across a fluid path 70. The electrodes 66A, B are connected to the sidewall 62 by bolts 72, which are screwed through the sidewall 62 and into the respective electrode 66A, B. This construction ensures support for the electrodes 66A, B over their full length making the cartridge 44 more resistant to shocks and damage.

[0047] As can be seen in Figure 3A, the electrodes 66A, B are D-shaped or semicircular in cross-section and fit flush against the sidewall 62. The flat faces 68A, B are parallel to each other so that the fluid path 70 is a generally rectangular slot having a width of 8 mm [0048] Figure 3C shows in greater detail the outlet end 46 of the electrode cartridge 44. As can be seen, the channel 64 is closed by a plug 74, also of PP.

Threaded brass rods 76A, B extend through plug 74 and are screwed into the electrodes 66A, B and secured with electrically conducting adhesive. The threaded brass rods 76A, B form part of terminals 75A, B, one of which is shown in greater detail in Figure 3D. Each terminal 75A, B comprises a number of nuts 78 and washers 79 that retain a cable lug 80 in position on the threaded brass rod 76A. The cable lug 80 is connected to the stripped end of a respective cable 50. Each terminal 75A, B is assembled to ensure maximum conductivity of the assembly, whereby local resistivity and heating is avoided.

[0049] Figure 3C also shows spacing bolt 82 which is threaded into the plug 74 between the terminals 75A, B. The spacing bolt 82 is made of nylon and ensures that a maximum spacing is maintained between the terminals 75A, B and particularly their respective cable lugs 80. Figure 3E is a transverse cross-section through the cartridge 44 in the direction E-E in Figure 3, showing the spacing bolt 82 located between the terminals 75A, B. The end cap construction 49 is welded to the elongate tubular member 60 and the space within the end cap construction is completely filled with an insulating potting filler 84.

[0050] Operation of the electrolysis system 100 will now be explained with reference primarily to Figures 1 and 2. In order to put the electrolysis system into operation, the electrolysis vessel 12 must initially be filled with water at least sufficient to cover the overflow baffle 24. Actuation of the pump 102 is commenced and waste water is supplied via inlet conduit 104 from source S to the vessel inlet 20. The waste water contains a mixture of saline water and hydrocarbons in a stable emulsion due to the presence of surface active compounds. The waste water flows into the inlet chamber and is distributed by the inlet manifold 30 to the electrode cartridges 44, where it flows upwards through the fluid paths 70 and out of the outlet slot 47. Once the pump 102 is actuated, the flow monitor 108 provides a signal to the controller 120 that flow has been detected and the rectifiers 116 are switched on. A DC voltage is applied to each of the electrode cartridges 44. The rectifiers are controlled to reverse the voltage with a frequency of around 0.05 Hz.

[0051] The waste water passing through the electrode cartridges is subject to electrolysis as a result of the applied potential and the ionic character of the saline solution. Preferably, the waste water should have a salt concentration of at least 3 g/1. For waste water having concentration below this value, additional salt and/or sea-water can be added. Without wishing to be bound by theory, it is understood that the electrolysis process causes hypochlorite production, which serves as an oxidant, in particular for subsequent reaction with surface active nitrogen compounds. As a result of this process, the emulsified hydrocarbons are destabilised and return to their immiscible state. The oxidised reaction products are in general insoluble and precipitate out. Additionally, hydroxyl radicals cause reduction of dissolved organics such as benzene, toluene, ethylbenzene, and xylenes (BTEX). The controller 120 adjusts actuation of the pump 102 to maintain a flow rate through the cartridges of between 0.2 and 2 m/s. This flow rate has been found adequate to avoid laminar flow over the electrode, which can lead to undesirable reactions at the electrode surface.

[0052] The treated waste water overflows the overflow baffle 24 from the electrolysis compartment 26 into the separation compartment 28. Foaming within the electrolysis compartment 26 due to the formation of gaseous components is retained by the strainer 38. Foam formation tends to be a process with a stable equilibrium between production and adsorption and the total foam present within the electrolysis compartment tends to remain constant. In the separation compartments 28, the treated waste water is allowed to settle, such that the released hydrocarbon droplets and other precipitates can rise to the surface as a mixed slurry. The slurry overflows into the skimmer 54, whence it flows to the hydrocarbon outlet 56 for collection.

[0053] From the separation compartment 28, the treated water passes under the underflow baffle 34 and over the second overflow baffle 36 to the outlet chamber 33. The height of the second overflow baffle 36 is adjustable in order to determine the level of the waste water within the separation compartment 28. The treated water flows from the outlet chamber 33 out of the vessel outlet 22 and through the outlet conduit 110 to the post treatment apparatus P. It will be understood that this may include filtration and recirculation steps and may involve the addition of further products to the treated water. Although not further discussed herein, it will also be understood that recirculation may also be provided from the electrolysis compartment 26 to the inlet chamber 32. Although not shown, the vessel outlet 22 may also be connected directly to the separation compartment 28 with the under- and overflow baffles 34 and 36 removed.

[0054] During operation of the electrolysis system 100, a number of additional precautions are taken. The controller 120 constantly monitors the temperature sensor 52 to determine the temperature within the electrolysis compartment 26. In the event that the temperature exceeds 50o C, power to the electrode cartridges 44 is prevented by the rectifiers 116. Furthermore, during operation, fan 112 is operated to maintain a flow of air from the air inlet 16 over the surface of the waste water within the electrolysis vessel 12 to the air outlet 18. The air flow is around 5 m3/hr per electrode cartridge and can be adjusted based on the momentary current being applied to the electrode cartridges 44. Additionally, the controller 120 monitors the gas sensor 114 to determine the concentration of hydrogen gas in the air flow at the air outlet 18. This is maintained at all times below 0.4%. In the event that a higher concentration is detected, the fan speed is increased. In the event that the maximum air flow is reached and the hydrogen concentration remains above the desired limit, the rectifiers 116 are shut down.

[0055] The controller 120 monitors the current supplied by each rectifier 116. In the event that current to one of the cartridges falls outside of a set range, that rectifier is shut down in order to avoid possible short circuit conditions.

[0056] In the event that the electrolysis system 100 is to be stopped, the rectifiers 116 are shut down while continuing to circulate both waste water and air. Thereafter, the pump 102 is stopped. The non-return valve 106 prevents backflow of water from the inlet chamber 32, which could otherwise cause the electrodes 66 to be uncovered.

In this way, it is ensured that the electrodes 66 remain submerged at all times.

[0057] Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. Many modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

Claims (28)

An electrolysis unit for separating hydrocarbons from salt water, comprising: a closed electrolysis vessel that has at least one electrolysis compartment, a separation compartment, a vessel inlet, a vessel outlet, an air inlet and an air outlet; one or more removable electrode cassettes that are located within the electrolysis compartment; wherein each electrode cassette comprises a pair of electrodes separated by a fluid path; wherein the vessel inlet and the vessel outlet are arranged such that a current entering the electrolysis vessel through the vessel inlet will flow through the fluid path from the electrode cassette to the electrolysis compartment, and then through the separation compartment to the vessel outlet, and wherein the air inlet and the air outlet are arranged on an upper side of the vessel to cause an air flow to flow through the electrolysis compartment and the separation compartment. The unit of claim 1, which comprises a plurality of parallel arranged electrode cassettes. The unit of claim 2, further comprising an inlet tubing that separates the electrolysis compartment from an inlet chamber and which includes a seat suitable for sealingly engaging an inlet end of each of the electrode cassettes. A unit according to any of the preceding claims, wherein the electrolysis compartment is separated from the separation compartment by a blending partition that extends to a position that is higher than an upper extent of the pairs of electrodes. A unit according to any of the preceding claims, further comprising a blot and a second blot located between the separation compartment and the vessel outlet. A unit according to any of the preceding claims, further comprising a skimmer located in the separation compartment and connected to a hydrocarbon outlet for the separated hydrocarbon portion. A unit according to any of the preceding claims, further comprising a temperature sensor located within the electrolysis compartment. An electrolysis system comprising an electrolysis unit according to any of the preceding claims, further comprising one or more rectifier units each connected to a respective electrode cassette. The system of claim 8, further comprising a pump connected to cause a stream of waste water to flow through the unit from the vessel inlet to the vessel outlet. The system of claim 8 or claim 9, further comprising an inlet flow monitor device adapted to monitor an inlet flow going to the vessel inlet and control or terminate the supply of power to the one or more electrode cassettes if the inlet flow is less than a given value. The system of any one of claims 8-10, further comprising a non-recoil valve on, or upstream of, the vessel inlet, which prevents effluent from the vessel inlet. A system according to any of claims 8-11, further comprising a ventilation unit connected to cause an air flow to flow through the air inlet and the interior of the electrolysis vessel to the air outlet and an ejection location. A system according to any of claims 8-12, further comprising a gas composition monitor adapted to determine a concentration of hydrogen within the electrolysis vessel and control the air flow to keep the concentration below a predetermined percentage of the lower explosion limit . An electrode cassette for use in a unit or system according to any of the preceding claims, comprising an elongated tubular member of insulating material that has a side wall defining an enclosed channel and having an inlet end and an outlet end; a pair of elongated electrodes facing each other across the channel and separated from each other by a fluid path, the electrodes extending along the channel, from the inlet end to the outlet end, and connected to the side wall; and further comprising a pair of terminals, each of which is electrically connected to a respective electrode and extends beyond the tubular member. The electrode cartridge of claim 14, wherein the connections extend from the outlet end of the tubular member. Electrode cassette according to claim 14 or claim 15, wherein the connections are cast in an insulating resin mass. Electrode cassette according to any of claims 14-16, wherein the tubular member is a circular cylinder and the electrodes are D-shaped, with flat surfaces facing each other. The electrode cartridge of any of claims 14-17, wherein the electrodes are bolted through the side wall at a plurality of locations along their length. The electrode cartridge of any of claims 14-18, wherein the electrodes are carbon electrodes. The electrode cartridge of any of claims 14-19, wherein the inlet end is axially open to the channel and the outlet end is radially open from the channel through the side wall. The electrode cartridge of any of claims 14-20, further comprising a sealing element around the inlet end of the tubular member adapted to engage on a seat. Electrode cassette according to any of claims 14-21, wherein the elongate member has a length between 30 cm and 150 cm, preferably a length of about 90 cm, and a diameter between 25 mm and 150 mm, preferably a diameter of about 70 mm. The electrode cartridge of any of claims 14 to 22, wherein the fluid path has a constant width over the length and width of the channel in the range of 6 mm to 10 mm, preferably about 8 mm. A method for performing an electrical treatment of waste water containing hydrocarbon emulsions, the method comprising: providing a unit or system according to any of claims 1-13; supplying the waste water to the vessel inlet to cause a current to flow through the one or more electrode cassettes, the electrolysis compartment, the separation compartment and to the vessel outlet, applying a voltage to the pairs of electrodes to cause current to flow via the fluid path; and flowing an air flow to the air inlet and through the vessel to the air outlet, the air flow being adapted to the flow flowing between pairs of electrodes to ensure that there is a concentration of hydrogen at the air outlet of less than 0.4 %. The method of claim 24, further comprising monitoring the temperature of the waste water in the electrolysis compartment and terminating the voltage application when the temperature exceeds a predetermined value. A method according to claim 24 or claim 25, wherein a plurality of cassettes are provided, and the method further comprises: monitoring a power supply to each of the electrode cassettes, and terminating the supply of voltage in the event that the power supply exhibits an anomaly while maintaining a voltage across the remaining cassettes. The method of claim 26, wherein monitoring a power supply comprises comparing the current supplied to one cassette with the current supplied to a plurality of cassettes. The method of claim 26 or claim 27, further comprising preventing flow through a cassette in the event that a voltage is no longer applied to that cassette.