FIELD OF THE INVENTION

The invention relates to an energy recovery system, particularly for use in the extraction of alumina from bauxite. The invention is further suited to any process seeking to reduce the pressure and/or temperature of an abrasive slurry. One such example is the high pressure acid leach process used in the nickel industry.

BACKGROUND OF THE INVENTION The extraction of alumina from bauxite involves dissolving ground bauxite in a sodium hydroxide solution (caustic liquor) within a digestion vessel. Temperature and pressure are applied within the digestion vessel to dissolve the bauxite. The resulting slurry is flash-cooled to atmospheric boiling point by flowing through a series of flash tanks that operate at successively lower pressures. Flash steam generated in the flash tanks is utilised to preheat the incoming caustic liquor prior to its introduction to the digestion vessel. Unstable flash steam temperatures can cause inefficient heat transfer from the flash steam to the caustic liquor and reduce the efficiency of the overall process.

Other problems with current alumina extraction processes include high slurry velocities, high wear rates, unpredictable wear events, poor quality condensate, reduced energy efficiency, and/or process instability.

It is an object of at least preferred embodiments of the present invention to address some of the aforementioned disadvantages. An additional or alternative object is to at least provide the public with a useful choice.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention an energy recovery system comprises: a first flash tank having a first slurry inlet adapted to receive a slurry into the first flash tank, a first slurry outlet adapted to discharge the slurry from the first flash tank, and a first steam outlet adapted to discharge flash steam from the first flash tank; a second flash tank having a second slurry inlet adapted to receive the slurry discharged from the first flash tank; a first slurry pipe connecting the first slurry outlet and the second slurry inlet; a first steam pipe connecting the first steam outlet with a first heat exchanger; a first steam diversion pipe connected to the first steam pipe, the first steam diversion pipe adapted to divert at least part of the flash steam discharged from the first steam outlet away from the first heat exchanger; and a control valve on the first steam diversion pipe operable to control a flow of steam diverted through the first steam diversion pipe.

The term 'comprising' as used in this specification means 'consisting at least in part of. When interpreting each statement in this specification that includes the term

'comprising', features other than that or those prefaced by the term may also be present. Related terms such as 'comprise' and 'comprises' are to be interpreted in the same manner.

In an embodiment the control valve is connected to a pressure sensor adapted to measure a pressure within the first flash tank.

In an embodiment the control valve is adapted, on the pressure sensor detecting a drop in pressure within the first flash tank, to decrease the flow of steam through the first steam diversion pipe.

In an embodiment the control valve is adapted, on the pressure sensor detecting an increase in pressure within the first flash tank, to increase the flow of steam through the first steam diversion pipe.

In an embodiment the control valve is adapted to increase and/or decrease the flow of steam through the first steam diversion pipe when the pressure sensor detects a pressure outside a target pressure range to restore the pressure to within the target pressure range.

In an embodiment the control valve is connected to a level sensor adapted to measure a level of slurry within the first flash tank.

In an embodiment the control valve is adapted, on the level sensor detecting an increase in slurry level within the first flash tank, to decrease the flow of steam through the first steam diversion pipe.

In an embodiment the control valve is adapted, on the level sensor detecting a decrease in slurry level within the first flash tank, to increase the flow of steam through the first steam diversion pipe. In an embodiment the control valve is adapted to increase and/or decrease the flow of steam through the first steam diversion pipe, on the slurry level sensor detecting a slurry level outside a target slurry level range, to restore the slurry level to within the target slurry level range. In an embodiment the control valve is connected to the pressure sensor and the level sensor.

In an embodiment the first steam diversion pipe connects the first steam pipe and the first slurry pipe, the first steam diversion pipe adapted to introduce at least part of the flash steam discharged from the first steam outlet into the slurry discharged from the first flash tank; and the control valve on the first steam diversion pipe is operable to control the flow of steam introduced into the slurry discharged from the first flash tank.

In an embodiment the first slurry pipe is fitted with an orifice plate positioned intermediate the first slurry outlet and the second slurry inlet, the first steam diversion pipe connected at a first end to the first steam pipe and connected at a second end to the first slurry pipe at a point intermediate the orifice plate and the second slurry inlet.

In an embodiment the first steam diversion pipe connects the first steam pipe and the second flash tank, the first steam diversion pipe adapted to introduce at least part of the flash steam discharged from the first steam outlet directly into the second flash tank; and the control valve on the first steam diversion pipe is operable to control the flow of steam introduced directly into the second flash tank.

In an embodiment the energy recovery system further comprises: a second slurry outlet adapted to discharge the slurry from the second flash tank; a second steam outlet adapted to discharge flash steam from the second flash tank; and a second steam pipe connecting the second steam outlet with a second heat exchanger. In an embodiment the first steam diversion pipe connects the first steam pipe and the second steam pipe, the first steam diversion pipe adapted to introduce at least part of the flash steam discharged from the first steam outlet into a flow of steam discharged from the second flash tank; and the control valve on the first steam diversion pipe is operable to control the flow of steam introduced into the flow of steam discharged from the second flash tank.

In an embodiment the first steam diversion pipe connects the first steam pipe and an export pipe, the first steam diversion pipe adapted to introduce at least part of the flash steam discharged from the first steam outlet into one or more of a water evaporator or other auxiliary use. In an embodiment the energy recovery system further comprises: a third flash tank having a third slurry inlet adapted to receive the slurry discharged from the first flash tank or the second flash tank, and a third steam outlet adapted to discharge flash steam from the third flash tank; a second slurry pipe connecting the second slurry outlet and the third slurry inlet; a third steam pipe connecting the third steam outlet with a third heat exchanger; a slurry bypass pipe connecting the first slurry outlet and the third slurry inlet; at least one slurry bypass valve in communication with the slurry bypass pipe and the first slurry pipe, the at least one slurry bypass valve adapted to selectively divert the slurry from the first flash tank into either the second flash tank or the third flash tank; a tank bypass pipe in communication with the first steam outlet, the tank bypass pipe adapted to divert at least part of the flash steam discharged from the first steam outlet away from the first heat exchanger; at least one tank bypass valve in communication with the tank bypass pipe and the steam diversion pipe, the at least one tank bypass valve adapted to selectively divert at least part of the flash steam

discharged from the first steam outlet into the tank bypass pipe or the steam diversion pipe; and a control valve operable to control a flow of steam diverted into the tank bypass pipe or the steam diversion pipe from the first flash tank.

In an embodiment the second flash tank is downstream of the first flash tank. The second flash tank operates at lower pressure and temperature than the first flash tank. In an embodiment the third flash tank is downstream of the second flash tank. The third flash tank operates at lower pressure and temperature than the second flash tank.

In an embodiment, the energy recovery system comprises further steam diversion pipes/valves and slurry bypass pipes/valves to enable steam to be diverted into one or more subsequent flash tanks, or to one or more pipes associated with a subsequent flash tank.

In an embodiment the tank bypass valve is adapted to selectively divert at least part of the flash steam discharged from the first steam outlet into the second flash tank or the third flash tank.

In an embodiment the tank bypass pipe is connected to the third flash tank via the second slurry pipe.

In an embodiment the tank bypass pipe is directly connected to the third flash tank.

In an embodiment the tank bypass pipe connects the first steam diversion pipe and the third steam pipe. In an embodiment the tank bypass valve is adapted to selectively divert at least part of the flash steam discharged from the first steam outlet into the flow of steam discharged from the second flash tank or the flow of steam discharged from the third flash tank.

In accordance with a second aspect of the invention, a method of recovering energy comprises: providing a first flash tank, the first flash tank having a first slurry inlet adapted to receive a slurry into the first flash tank, a first slurry outlet adapted to discharge the slurry from the first flash tank, and a first steam outlet adapted to discharge flash steam from the first flash tank; providing a second flash tank, the second flash tank having a second slurry inlet adapted to receive the slurry discharged from the first flash tank; providing a first slurry pipe connecting the first slurry outlet and the second slurry inlet; providing a first steam pipe connecting the first steam outlet with a first heat exchanger; providing a first steam diversion pipe connected to the first steam pipe, the first steam diversion pipe adapted to divert at least part of the flash steam discharged from the first steam outlet away from the first heat exchanger; and providing a control valve on the first steam diversion pipe operable to control a flow of steam diverted through the first steam diversion pipe.

In an embodiment the method comprises providing a pressure sensor adapted to measure a pressure within the first flash tank, the control valve connected to the pressure sensor. In an embodiment the control valve is adapted, on the pressure sensor detecting a drop in pressure within the first flash tank, to decrease the flow of steam through the first steam diversion pipe.

In an embodiment the control valve is adapted, on the pressure sensor detecting an increase in pressure within the first flash tank, to increase the flow of steam through the first steam diversion pipe.

In an embodiment the control valve is adapted to increase and/or decrease the flow of steam through the first steam diversion pipe when the pressure sensor detects a pressure outside a target pressure range to restore the pressure to within the target pressure range. In an embodiment the method comprises providing a level sensor adapted to measure a level of slurry within the first flash tank, the control valve connected to the level sensor.

In an embodiment the control valve is adapted, on the level sensor detecting an increase in slurry level within the first flash tank, to decrease the flow of steam through the first steam diversion pipe. In an embodiment the control valve is adapted, on the level sensor detecting a decrease in slurry level within the first flash tank, to increase the flow of steam through the first steam diversion pipe.

In an embodiment the control valve is adapted to increase and/or decrease the flow of steam through the first steam diversion pipe, on the slurry level sensor detecting a slurry level outside a target slurry level range, to restore the slurry level to within the target slurry level range.

In an embodiment the control valve is connected to the pressure sensor and the level sensor. In an embodiment the first steam diversion pipe connects the first steam pipe and the first slurry pipe, the first steam diversion pipe adapted to introduce at least part of the flash steam discharged from the first steam outlet into the slurry discharged from the first flash tank; and the control valve on the first steam diversion pipe is operable to control the flow of steam introduced into the slurry discharged from the first flash tank. In an embodiment the first slurry pipe is fitted with an orifice plate positioned intermediate the first slurry outlet and the second slurry inlet, the method comprising connecting the first steam diversion pipe at a first end to the first steam pipe; and connecting the first steam diversion pipe at a second end to the first slurry pipe at a point intermediate the orifice plate and the second slurry inlet. In an embodiment the first steam diversion pipe connects the first steam pipe and the second flash tank, the first steam diversion pipe adapted to introduce at least part of the flash steam discharged from the first steam outlet directly into the second flash tank; and the control valve on the first steam diversion pipe is operable to control the flow of steam introduced directly into the second flash tank. In an embodiment the method of recovering energy further comprises: providing a second slurry outlet adapted to discharge the slurry from the second flash tank;

providing a second steam outlet adapted to discharge flash steam from the second flash tank; and providing a second steam pipe connecting the second steam outlet with a second heat exchanger. In an embodiment the first steam diversion pipe connects the first steam pipe and the second steam pipe, the first steam diversion pipe adapted to introduce at least part of the flash steam discharged from the first steam outlet into a flow of steam discharged from the second flash tank; and the control valve on the first steam diversion pipe is operable to control the flow of steam introduced into the flow of steam discharged from the second flash tank.

In an embodiment the first steam diversion pipe connects the first steam pipe and an export pipe, the first steam diversion pipe adapted to introduce at least part of the flash steam discharged from the first steam outlet into one or more of a water evaporator or other auxiliary use.

In accordance with a third aspect of the invention a refining system comprises at least one flash tank having an energy recovery system according to the first aspect.

In an embodiment, the refining system further comprises at least one flash tank having a control valve on its associated slurry line.

The invention in one aspect comprises several steps. The relation of one or more of such steps with respect to each of the others, the apparatus embodying features of construction, and combinations of elements and arrangement of parts that are adapted to affect such steps, are all exemplified in the following detailed disclosure. This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features.

To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. Where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. In addition, where features or aspects of the invention are described in terms of Markush groups, those persons skilled in the art will appreciate that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

As used herein, '(s)' following a noun means the plural and/or singular forms of the noun.

As used herein, the term 'and/or' means 'and' or 'or' or both. It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9, and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5, and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed.

These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents or such sources of information is not to be construed as an admission that such documents or such sources of information, in any jurisdiction, are prior art or form part of the common general knowledge in the art. Although the present invention is broadly as defined above, those persons skilled in the art will appreciate that the invention is not limited thereto and that the invention also includes embodiments of which the following description gives examples.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred forms of the energy recovery system and method will now be described by way of example only with reference to the accompanying figures in which :

Figure 1 shows an example of a prior art energy recovery system.

Figure 2 shows an example of an energy recovery system in accordance with the first aspect of the invention. Figure 3 shows an embodiment of the energy recovery system of figure 2 that enables a flash tank to be optionally bypassed.

Figure 4 shows an example of an energy recovery system in accordance with the third aspect of the invention.

Figure 5 shows an embodiment of the energy recovery system of figure 4 that enables a flash tank to be optionally bypassed. DETAILED DESCRIPTION

Figure 1 shows an example of a prior art energy recovery system 100 within a digestion flash train. The system 100 is particularly suited to the extraction of alumina from bauxite. A digestion vessel 102 receives ground bauxite and a solution of sodium hydroxide (caustic liquor). By applying steam and pressure within the digestion vessel 102, the bauxite dissolves. The alumina released from the bauxite as it dissolves reacts with the sodium hydroxide to form sodium aluminate. The remaining solids form a waste product known in the industry as 'red mud'. Following digestion the material within the digestion vessel 102 typically comprises a suspension of silicates, iron oxides and titanium oxides. The slurry is flash-cooled to atmospheric boiling point by flowing through a series of flash tanks that operate at successively lower pressures.

The term 'flash tank' as used in the specification means a pressure vessel. It is common in the industry to refer to an unpressurised 'tank' and a pressurised 'vessel'. However it is also common to refer to a pressurised 'flash tank'.

One such flash tank is shown as a first flash tank 104. The tank 104 has a slurry inlet 106 adapted to receive the slurry discharged from the digestion vessel 102 through a slurry pipe 108. A control valve 110, referred to as a back pressure control valve, is fitted to slurry pipe 108. The slurry pipe 108 further includes an orifice plate 112 to reduce the differential pressure across the control valve 110.

The tank 104 has a slurry outlet 114 that is adapted to discharge the slurry from tank 104 into a slurry pipe 116. Slurry pipe 116 is fitted with an orifice plate 118 to reduce the pressure between flash tank 104 and flash tank 130. Some slurry pipes, particularly downstream slurry pipes that operate at lower temperatures and pressures, may have control valves instead of orifice plates. Slurry pipe control valves may not be practical for high temperature/pressure flash tanks because valves that are able to withstand the required temperature and pressure are very expensive, and the harsh conditions can lead to high maintenance cost and reliability issues. Flash steam is generated within slurry pipe 108 and/or flash tank 104. The tank 104 includes a steam outlet 120 adapted to discharge the flash steam into a steam pipe 122.

A heat exchanger 124 is connected to the steam pipe 122. Examples of heat exchangers include shell and tube heat exchangers and/or direct steam injection heaters. The heat exchanger 124 is adapted to preheat caustic liquor and/or slurry entering the digestion vessel 102 through pipe 126. Condensate from the heat exchangers, indicated for example at 128 is used for boiler feed water and/or for washing waste mud.

A second flash tank 130 has a slurry inlet 132 adapted to receive the slurry discharged from flash tank 104. In an embodiment the slurry pipe 116 connects slurry outlet 114 of flash tank 104 to slurry inlet 132 of flash tank 130. Flash tank 130 has a slurry outlet 134 that is adapted to discharge the slurry from tank 130 into a slurry pipe 136. In an embodiment the slurry pipe is connected to a further flash tank.

The flash tanks are arranged so as to create a series of flash tanks that operate at successively lower pressures. For example, flash tank 130 operates at a lower pressure than flash tank 104. The orifice plate 118 ensures that there is sufficient differential pressure between flash tank 104 and flash tank 130.

Flash steam is generated within slurry pipe 116 and/or flash tank 130 in the same way as in slurry pipe 108 and/or flash tank 104. Flash tank 130 includes a steam outlet 138 adapted to discharge the flash steam into a steam pipe toward a heat exchanger in the same manner as for flash tank 104.

The pressure drop through the slurry pipe 116 from flash tank 104 to flash tank 130 is a function of the mass flow of slurry and steam. As the steam fraction in the slurry pipe 116 increases, the volumetric flow increases which in turn increases the pressure drop. With an increasing pressure drop there is also an increasing amount of steam evolved from the slurry.

The performance of the heat exchangers, for example heat exchanger 124, determines the amount of steam condensed at each flash tank stage. This in turn affects the steam balance for each flash tank and the amount of steam exiting the flash tank with the slurry. The heat exchanger performance also changes with flash tank pressures, spent liquor temperature feeding the heat exchanger, and the condition of the heater tubes.

The changing heat exchanger performance has the potential to alter the steam balance existing within the flash tank. The amount of steam present in the slurry therefore changes, which in turn alters the pressure drop, which then changes the steam temperatures and heat exchanger performance. All of these effects are linked, which has the potential to produce a process cascade that is highly variable. A small change in the process conditions results in a large change in pressures along the flash tank train as the system finds a new equilibrium. Determining a single fixed orifice plate 118 for slurry pipe 116 to suit the complete range of process conditions is very difficult. Traditionally the principal compromise involves accepting high steam flow rates through the slurry piping. High steam flow rates can increase the maintenance requirement for the slurry pipe 116. Figure 2 shows an example of an energy recovery system 200 that is intended to address some of the disadvantages of the prior art system shown above in figure 1. The system 200 includes a digestion vessel 202, a first flash tank 204, and a second flash tank 230 that are similar to tanks 102, 104 and 130 respectively from figure 1.

The first flash tank 204 has a slurry inlet 206 adapted to receive the slurry discharged from the digestion vessel 202 through a slurry pipe 208. A slurry outlet 210 is adapted to discharge slurry from tank 204 to a slurry pipe 212.

A steam outlet 214 is adapted to discharge flash steam into a steam pipe 216 connected to a heat exchanger 224 in the same manner as flash tank 104 from figure 1.

The second flash tank 230 has a slurry inlet 232 adapted to receive the slurry discharged from flash tank 204. In an embodiment the slurry pipe 212 connects slurry outlet 210 of flash tank 204 to slurry inlet 232 of flash tank 230.

As shown in figure 2, flash tank 230 is connected to a further flash tank adapted to receive slurry discharged from flash tank 230.

In an embodiment slurry pipe 208 and slurry pipe 212 are fitted with orifice plates 240 and 242 respectively. The orifice plates are intended to ensure that there is a sufficient pressure drop between flash tanks to provide the required pressure at each stage. The overall sizing of the orifice plates is chosen to provide an even pressure distribution through the system. The actual size chosen will depend on the piping layout of an individual plant. A steam diversion pipe 250 is connected to steam pipe 216. The steam diversion pipe 250 is adapted to divert at least part of the flash steam within the steam pipe 216. In an embodiment a control valve 252 is fitted to steam diversion pipe 250. The control valve 252 is connected to a sensor 254.

In an embodiment the sensor 254 is a pressure sensor adapted to measure a pressure within the flash tank.

In an embodiment the sensor 254 is a level sensor adapted to measure a level of slurry within the flash tank. In an embodiment the sensor 254 comprises a pressure sensor and a level sensor.

In an embodiment the steam diversion pipe 250 is adapted to divert at least part of the flash steam discharged from the steam outlet 214 away from the heat exchanger 224.

In an embodiment the steam diversion pipe 250 is adapted to introduce at least part of the flash steam discharged from the steam outlet 214 of the flash tank 204 into the slurry discharged from the first flash tank 204. The steam diversion pipe 250 in one embodiment is connected at a first end to the steam pipe 216 and is connected at a second end to the slurry pipe 212. The steam diversion pipe 250 for example is connected to the slurry pipe 212 at a point in the slurry pipe 212 between the orifice plate 242 and the slurry inlet 232.

In an embodiment the steam diversion pipe 250 is adapted to introduce at least part of the flash steam directly into flash tank 230 in addition to, or as an alternative to, introducing the flash steam into slurry pipe 212. In an embodiment the control valve 252 on the steam diversion pipe 250 is operable to control the flow of steam introduced directly into the second flash tank 230.

In an embodiment the steam diversion pipe 250 connects the steam pipe 216 and an export pipe, the steam diversion pipe 250 adapted to introduce at least part of the flash steam discharged from the steam outlet 214 into another part of the plant. For example, flash steam could be introduced into a water evaporator or other auxiliary use. The control valve 252 in an embodiment is adapted to control steam flow through the slurry pipe 212. For example, as the sensor 254 detects a drop in pressure within flash tank 204, the control valve 252 closes to decrease a flow of steam through the first steam diversion pipe 250 and to increase the steam flow through the slurry pipe 212 via the outlet 210. Similarly, as the sensor 254 detects an increase in pressure within flash tank 204, the control valve 252 opens to increase a flow of steam through the first steam diversion pipe 250 and to decrease the steam flow through the slurry pipe 212.

In another example, as the sensor 254 detects an increase in slurry level within flash tank 204, the control valve 252 closes to decrease a flow of steam through the first steam diversion pipe 250 and to increase the steam flow through the slurry pipe 212 via the outlet 210. Similarly, as the sensor 254 detects a decrease in slurry level within flash tank 204, the control valve 252 opens to increase a flow of steam through the first steam diversion pipe 250 and to decrease the steam flow through the slurry pipe 212.

In an embodiment the control valve 252 is adapted to increase and/or decrease a flow of steam through the steam diversion pipe 250 when the sensor 254 detects a pressure in flash tank 204 that is outside a target range to restore the pressure to within the target range.

In an embodiment the control valve 252 is adapted to increase and/or decrease a flow of steam through the steam diversion pipe 250 when the sensor 254 detects a slurry level in flash tank 204 that is outside a target range to restore the slurry level to within the target range.

The control valve 252 has the potential to control a pressure drop between flash tank 204 and flash tank 230 without the use of a control valve in the slurry pipe 212. The addition of the steam pipe 250 with control valve 252 between flash tank 204 and flash tank 230 has the potential to balance the steam for each of flash tanks 204 and 230 so that when the steam demand reduces in flash tank 204, the excess steam can be directed to the downstream flash tank 230. The energy can be recovered by the heat exchanger associated to the flash tank 230.

Flash tank pressure and slurry level are inversely related. Diverting steam from the steam outlet 214 reduces the pressure in the first flash tank 204 and causes the slurry level in the first flash tank 204 to increase. This reduces the steam fraction within the slurry pipe 212 and reduces the pressure drop between the first flash tank 204 and the second flash tank 230.

Slurry level is also related to the size of the orifice plates. The orifice plates should be sized so that at the highest slurry flow rate, the pressure drop across the orifice plate is sufficient to generate a slurry level in the flash tank that the slurry is discharged from. For example, orifice plate 242 should be sized so that at the highest slurry flow rate the pressure drop across orifice plate 242 generates a slurry level in flash tank 204.

A higher slurry level in the first flash tank 204 increases the static head in the first flash tank 204 and increases the pressure at outlet 210, increasing the pressure drop between the first flash tank 204 and the second flash tank 230.

A slurry level will not necessarily be present in a flash tank, depending on the

configuration of the system. It is important to maintain a stable slurry level within a flash tank. If the slurry level varies over a short period of time, the flow rates between the tanks also vary, which affects the overall mass and energy balance of the system. This can make the system unstable, difficult to control, and inefficient. There is a slurry level upper limit above which the separation performance of a flash tank is negatively affected. The slurry level may approach the slurry level upper limit during periods of higher flow rates. It is advantageous to utilise a control system to control the flow of steam through control valve 252 to maintain the slurry level below the slurry level upper limit. An exemplary control system could operate as follows: 1. Control the flow of steam through control valve 252 by opening/closing the

control valve 252 as required to maintain the steam balance/pressure drop within a range of a pressure setpoint.

2. Monitor a slurry level upper limit, and if the slurry level approaches the limit, start closing the control valve 252 so that the slurry level upper limit is not exceeded.

As shown in figure 2, flash tank 230 is fitted with a steam diversion pipe, control valve and pressure sensor similar to steam diversion pipe 250, control valve 252 and pressure sensor 254 respectively. In an embodiment at least some of the flash tanks are fitted with similar steam pipes. In an embodiment the flash tanks closest to the digestion vessel 202 are each fitted with steam pipes.

Figure 3 shows an embodiment of an energy recovery system 200 that enables flash tank 230 to be optionally bypassed, for example so that the system can continue to operate if flash tank 230 is taken offline.

The second flask tank 230 has a slurry outlet 234 adapted to discharge the slurry from the second flash tank 230, and a steam outlet 238 adapted to discharge flash steam from the second flash tank 230.

A third flash tank 260 is similar to the first flash tank 204 and the second flash tank 230. The third flash tank 260 has a slurry inlet 262 adapted to receive the slurry discharged from the first flash tank 204 or the second flash tank 230. In an embodiment, the third flash tank 260 is fitted with a steam diversion pipe, control valve and pressure sensor similar to steam diversion pipe 250, control valve 252 and pressure sensor 254 respectively. In an embodiment, flash tank 260 is connected to a further flash tank adapted to receive slurry discharged from flash tank 260.

A slurry pipe 236 connects slurry outlet 234 of the second flash tank 230 to a slurry inlet 262 of the third flash tank 260. A slurry bypass pipe 276 connects the slurry outlet 210 of the first flash tank 204 and the slurry inlet 262 of the third flash tank 260.

At least one slurry bypass valve 278 is in communication with the slurry bypass pipe 276 and the slurry pipe 212 of the first flash tank 204. The at least one slurry bypass valve 278 is adapted to selectively divert the slurry from the first flash tank 204 into either the second flash tank 230 or the third flash tank 260. A tank bypass pipe 264 is in communication with the steam outlet 214 of the first flash tank 204. The tank bypass pipe 264 is adapted to divert at least part of the flash steam discharged from the steam outlet 214 away from the heat exchanger 224. In the embodiment shown, the tank bypass pipe 264 connects the steam diversion pipe 250 with the third flash tank 260 via the slurry pipe 236 that discharges slurry to the third flash tank 260. In an embodiment, the tank bypass pipe 264 is directly connected to the third flash tank 260.

At least one tank bypass valve 266 is in communication with the tank bypass pipe 264 and the steam diversion pipe 250, to selectively divert at least part of the flash steam discharged from the steam outlet 214 of the first flash tank 204 into the tank bypass pipe 264 or the steam diversion pipe 250.

In an embodiment the at least one tank bypass valve 266 is adapted to selectively divert at least part of the flash steam discharged from the first steam outlet 214 into the second flash tank 230 or the third flash tank 260.

A control valve 252 is operable to control the flow of steam diverted into the tank bypass pipe 264 or the steam diversion pipe 250 from the first flash tank 204.

The tank bypass pipe 264 and the slurry bypass pipe 276 allow the energy recovery system to operate if a flash tank is removed from the system, for example for maintenance purposes.

In an embodiment the energy recovery system 200 comprises further steam diversion pipes/valves and slurry bypass pipes/valves to enable steam to be diverted into one or more subsequent flash tanks, or to one or more pipes associated with a subsequent flash tank. In an embodiment at least some of the steam diversion pipes are fitted with similar flash tank bypass pipes and tank bypass valves. Figure 4 shows another embodiment of an energy recovery system 300. The energy recovery system 300 has similar features and functions to those described above in relation to the energy recovery system 200, except as described below. In particular, the steam diversion pipe 350 of energy recovery system 300 diverts steam to a steam pipe 356 of the second flash tank 330. Like numbers are used to indicate like parts with the addition of 100.

The steam pipe 356 connects the steam outlet 338 of the second flash tank 330 with a heat exchanger 368.

Steam diversion pipe 350 connects the steam pipe 316 of the first flash tank 304 and the steam pipe 356 of the second flash tank 330.

Similar to the embodiment of figure 2, steam diversion pipe 350 is adapted to divert at least part of the flash steam within the steam pipe 316 away from the heat exchanger 324. Instead of introducing at least part of the flash steam discharged from the steam outlet 214 of the first flash tank 204 into the slurry discharged from the first flash tank 204, the steam diversion pipe 350 is adapted to introduce at least part of the flash steam discharged from the steam outlet 314 of the first flash tank 304 into the flow of steam discharged from the steam outlet 338 of the second flash tank 330.

In an embodiment a control valve 352 is fitted to steam diversion pipe 350. The control valve 352 is connected to a sensor 354. The control valve 352 is operable to control the flow of steam introduced in the flow of steam discharged from the second flash tank 330.

This embodiment has the potential to be easier to retrofit than the embodiment of figures 2 and 3. It may be useful where heat loss through the pipes cause the steam in the steam pipe 356 to be near saturation temperature when it reaches heat exchanger 368. Where this is not the case, the embodiment of figure 2 may lead to greater system efficiencies.

Figure 5 shows an embodiment of an energy recovery system 300 that enables flash tank 330 to be optionally bypassed, for example so that the system can continue to operate if flash tank 330 is taken offline. This embodiment has similar features and functions to those described above in relation to the tank bypass embodiment of figure 3 above, except as described below. In particular, the tank bypass pipe 364 connects the steam diversion pipe 350 with a steam pipe 372 of the third flash tank 360. Like numbers are used to indicate like parts with the addition of 100. Third flash tank 360 has a steam outlet 370 adapted to discharge flash steam from the third flash tank 360. A steam pipe 372 connects the steam outlet 370 with a third heat exchanger 374.

A slurry bypass pipe 376 connects the slurry outlet 310 of the first flash tank 304 and the slurry inlet 362 of the third flash tank 360.

At least one slurry bypass valve 378 is in communication with the slurry bypass pipe 376 and the slurry pipe 312 of the first flash tank 304. At least one slurry bypass valve 378 is adapted to selectively divert the slurry from the first flash tank 304 into either the second flash tank 330 or the third flash tank 360. A tank bypass pipe 364 is in communication with the steam outlet 314 of the first flash tank 304. The tank bypass pipe 364 is adapted to divert at least part of the flash steam discharged from the steam outlet 314 away from the heat exchanger 324. In the embodiment shown, the tank bypass pipe 364 connects the steam diversion pipe 350 with the steam pipe 372 of the third flash tank 360. At least one tank bypass valve 366 is in communication with the tank bypass pipe 364 and the steam diversion pipe 350, to selectively divert at least part of the flash steam discharged from the first steam outlet 314 into the tank bypass pipe 364 or the steam diversion pipe 350.

In an embodiment at least one tank bypass valve 366 is adapted to selectively divert at least part of the flash steam discharged from the first steam outlet 314 into the flow of steam discharged from the second flash tank 330 or the flow of steam discharged from the third flash tank 360.

A control valve 352 is operable to control the flow of steam diverted into the tank bypass pipe 364 or the steam diversion pipe 350 from the first flash tank 304. Similar to the embodiment of figure 3 above, the tank bypass pipe 364 and the slurry bypass pipe 376 allow the energy recovery system to operate if a flash tank is removed from the system, for example for maintenance purposes.

In an embodiment the energy recovery system 300 comprises further steam diversion pipes/valves and slurry bypass pipes/valves to enable steam to be diverted into one or more subsequent flash tanks, or to one or more pipes associated with a subsequent flash tank. In an embodiment at least some of the steam diversion pipes are fitted with similar flash tank bypass pipes and tank bypass valves. A refining system can comprise a combination of the energy recovery system 200 (figures 2 and 3) and the energy recovery system 300 (figures 4 and 5) associated with different flash tanks. An appropriate combination of embodiments may be selected to generate the highest efficiencies depending on system parameters. In an embodiment, the refining system further comprises at least one flash tank having a control valve on its associated slurry line.

In an embodiment the techniques described above are adapted to maintain a level within a flash tank. For example, when the level in the flash tank falls, the excess steam can be fed to a downstream flash tank thereby restoring the level. In practice the flash tank pressure will fluctuate with the overall slurry flow rate. In this scenario, a reduction in slurry flow will reduce the pressure drop between the flash tanks and ultimately reduce the pressure and temperature of the first high-pressure flash tank. The pressure of the highest pressure flash tank ultimately determines how much energy can be recovered from the slurry, therefore a drop in flash tank pressure is not always desirable. In an embodiment, a flash tank is operated with little or no level. The system is then controlled by setting a pressure of the flash tank and adjusting the steam control valve to maintain this set point. As the pressure of the flash tank falls, the steam valve closes thereby forcing more steam through the slurry piping and/or steam piping. With an increased steam fraction in the slurry the pressure drop between the flash tanks will increase and the pressure of the flash tank will be maintained.

By controlling the pressures of the flash tanks the cascade of heater performance-steam balance-slurry/steam flow-flash tank pressures-heat exchanger performance is interrupted which produces a more stable system. With a more stable system and fixed flash tank pressures the process range that the orifice plates need to be sized for is much smaller. This has the potential to make the design process easier and greatly reduces the amount of steam that passes through the slurry piping.

Potential benefits arising from implementation of the above techniques include:

• stable pressure profile between flash tanks making the task of sizing orifice plates much easier · the set point of individual flash tank pressures can be adjusted to suit process conditions, for example a flash tank coming off line • the highest pressure flash tank, for example the flash tank closest to the digestion vessel, maintains a constant pressure that ensures that overall energy recovery is maintained

• reduced amount of steam passing through the slurry piping reduces the velocities that in turn reduce the overall rates of wear

• more stable operation with reduced fluctuations in slurry velocities, meaning that erosion rates become more predictable and allows for more effective preventative maintenance

• a balanced steam load across the flash tanks can be maintained, thereby

protecting the vessel internal components from damage and ensuring the production of clean steam and condensate

• better quality condensate improves the life of the heaters by reducing fouling of the shell side of the heater.

The foregoing description of the invention includes preferred forms thereof. Modifications may be made thereto without departing from the scope of the invention.