Field of the Invention

This invention relates to evaporator vessels which are otherwise known as flash tanks, generally used for evaporating or flashing of volatile gas phase from a slurry, usually under pressure differential. In particular, this invention relates to improvements in flash tank design, to obviate the effects of erosion caused by fluid.

Background Art

The following discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application.

Evaporator vessels are used in the evaporation of steam from superheated slurries, to cool the slurry, and to recover heat from the spent steam.

Evaporator vessels are arranged with a fluid inlet, a fluid outlet, and a vapour outlet, and in use, connected together serially, with the fluid outlet of an upstream vessel connecting to the fluid inlet of a downstream vessel. Flow control may be incorporated between the fluid outlet of the upstream vessel and the fluid inlet of the downstream vessel, to control the rate of discharge from the upstream vessel. In use, a fluid, usually a slurry of ore and a reactant is superheated, and proceeds through the series of evaporator vessels with vapour (usually steam) at decreasing temperatures being recovered from respective vapour outlets heading downstream along the series of evaporator vessels. The vapour recovered from upstream vessels is usually superheated by 2°C to 10 ⁰ C (relative to the boiling point of pure water at a given pressure), and all vapour recovered is used, where possible, for heating in other parts of a processing plant, in accordance with known engineering principles.

There are three principal ways in which evaporator vessel fluid inlets and fluid outlets have normally been arranged. These are side entry with bottom outlet as illustrated in Figures 5 and 6, top entry with side outlet as illustrated in Figures 7 and 8, and bottom entry with bottom outlet as illustrated in Figures 9, 10, 11 and 12.

i The side entry with bottom outlet and top entry with side outlet configurations are similar in that they both introduce the incoming slurry from the upper section of the vessel and inject the slurry vertically down toward the slurry surface.

The bottom entry with bottom outlet configuration in contrast, introduces the incoming slurry from the bottom of the vessel. The slurry is directed upward through an internal pipe and deflected back down toward the slurry surface using a "diffuser" or impact plate.

The side entry with bottom outlet configuration shown in figures 5 and 6 was adopted by many of the older style alumina refineries built in the 1950's and 1960's. It minimised the mechanical complexity of arranging slurry discharge into the vessel but suffers from several defects. The first defect concerns the pipework external to the evaporator vessel. As illustrated in Figure 5, the slurry in the upstream evaporator vessel 11 is at equilibrium with its vapour (ie saturated vapour pressure) or in other word is at its vapour pressure, or put more simply, at the vapour/liquid interface the slurry is at its boiling point. It is only the static height of slurry within the vessel that suppresses the boiling of the slurry through the downstream piping system 13. For example, at the bottom of the vessel 11 the pressure of the slurry is equal to its vapour pressure plus the additional static pressure resulting from the slurry level in the vessel 11. This suppression of boiling or 'flashing' is maintained until the frictional pressure losses through the downstream piping 13 and valves 15 equals the static pressure of slurry in the upstream vessel. (Note that the restriction imposed by the valve 15 controls the slurry level in the upstream vessel 11 ) At this point in the piping, two phase flashing flow will commence. For the arrangement of vessels shown in Figures 5 and 6, the onset of two phase boiling or flashing flow could at best be delayed if the piping system were frictionless. Boiling of the slurry would then commence when the rise in slurry elevation through the downstream piping exceeded the slurry level in the upstream vessel, indicated at position 17 in the downstream piping leading to the downstream evaporator vessel 19.

In reality however, piping systems are not frictionless, and flashing of the slurry will occur before it reaches a height in the piping equivalent to the upstream vessel slurry level. It is evident therefore, that unless the upstream vessel is staged or elevated significantly above the downstream vessel such that the upstream vessel slurry level is above the pipe discharge elevation (thus providing additional static pressure to prevent boiling), this arrangement of vessels and piping must incur the development of two phase flashing flow in the external piping. This vapour development leads to increased fluid velocities in the piping. Commensurate with increased fluid velocities is an increase in erosive wear of the piping and fittings in the system requiring continual inspection and replacement.

The second defect of the side entry configuration is the physical arrangement of slurry discharge into the downstream vessel. As shown in figure 5, the boiling slurry discharges into the vessel vapour space 21 of the downstream evaporator tank 19, generating a high population of liquid droplets that must coalesce at the slurry surface, for ideal operation of the downstream evaporator tank 19. The vessel vapour space 21 however must serve the function of vapour/liquid/solid disentrainment. One of the vessels principal purposes is to produce vapour (or steam) that is relatively free of liquid or solids contamination as this steam is used in heat transfer equipment (heat exchangers) connected to the evaporator vessel. Excessive contamination of the steam can lead to malfunction of the heater equipment or corrosion/erosion of the equipment and associated piping. The vapour must traverse an upward path to the vapour outlet nozzle located at the top of the vessel whilst disengaging from the liquid and solids particles discharging downward in the slurry stream.

The contacting of upward flowing vapour with downward flowing slurry is not conducive to minimising the contamination of the vapour with entrained liquid and solids particles. The side entry therefore offers a limited capacity to produce clean vapour.

In the top entry and side outlet configuration illustrated in figures 7 and 8, the slurry is directed to a control valve 23 located at the top of the downstream evaporator vessel 19. The restriction imposed by the control valve 23 controls the slurry level in the upstream vessel 11. Two phase flashing flow may be induced by the control valve 23 or an orifice located downstream of the valve 23. The slurry is then directed through a short internal tube before discharging into the vessel vapour space 21. This top entry arrangement is subject to the same hydraulic performance characteristics as described above for the side entry arrangement of figures 1 and 2 and similarly suffers from limited vapour disentrainment capacity.

The second defect in the top entry and side outlet configuration resides in the side outlet configuration ensuring that a minimum fluid inventory is retained in the vessel at all times. This physical arrangement is of benefit where the fluid level is used to absorb the momentum of the downward flowing inlet slurry stream however its use is restricted to slurry applications that will not solidify and coalesce to form solid residues. For slurries that exhibit the physical characteristics of particle coalescence and cementation (such as in the alumina industry), the use of the top. entry and side outlet configuration would result in complete blockage of the non flowing area of the vessel i.e the bottom 'head' of the vessel. This would require frequent cleaning cycles for the evaporator vessel significantly reducing its operational availability.

Three arrangements incorporating the bottom entry and bottom outlet configuration are illustrated in figures 9, 10, 11 and 12. Figures 9, 10 and 12 represent an evaporator vessel with a dished (hemispherical) bottom head whilst figure 11 represents an evaporator vessel with a conical bottom. Hemispherical heads may be used for low pressure or high pressure applications and have the benefit of a reduced wall thickness in relation to the cylindrical shell. This is particularly advantageous where excessive head thicknesses would otherwise be required for a conical bottom head which would present fabrication difficulties and increased material costs. The conical bottom heads are used more generally for low pressure applications where excessive material thicknesses are avoided. They have the geometric advantage over the dished head of minimising the potential formation of stagnant fluid zones in relation to the discharge of fluids from the evaporator, particularly where the fluid retains an affinity for cementation.

The saturated or subcooled slurry is introduced to the downstream evaporator vessel 19 via an inlet nozzle 25 located at the bottom of the vessel 19. This takes advantage of the available static pressure provided by the operating level in the upstream vessel (refer to figure 9). The benefit of this design is that two phase flashing (boiling) flow within the interconnecting piping can be prevented. The external interconnecting piping is then exposed only to moderate fluid velocities and therefore minimal erosive wear. Depending on the pressure differential to be dissipated, pressure reduction may be induced by an orifice 27 or nozzle or simply an upstream control valve 29. As in the previous described arrangements, the restriction imposed by the control valve 29 controls the slurry level in the upstream vessel 11. Following the commencement of two phase flashing flow at the evaporator inlet nozzle 25, the slurry is directed upward via an internal pipe 31 to an impact plate 33. The impact plate 33 is used to absorb the momentum of the flashing slurry as it exits the internal pipework 31 and deflect it down toward the slurry level 35 in the vessel. Here, the slurry level is utilised as a 'wet scrubber' reducing the population of liquid (and any entrained solids) particles carried upward with the vapour flow. If the design allows for full flashing and vapour development to be completed within the internal piping prior to slurry discharge into the vessel vapour space, minimal vapour contamination can be achieved, as the flash vessel then serves the less onerous duty of vapour/liquid disentrainment only, without the turbulence that is induced in the disentrainment space by further boiling within that space.

The prevailing issue with these evaporator vessel arrangements of figures 9 to 11 is that once flashing (boiling) of the slurry has been induced at the evaporator inlet nozzle 25, the slurry direction is deflected at a bend 37 in the internal pipe 31 to a vertical upflow inside the vessel 19. This internal change of two phase flashing fluid flow induces erosion at the inside face of the internal elbow. This erosion has typically required the internal inlet piping to be replaced in periods of between 4 to 18 months.

Furthermore, the complex fluidynamic mechanisms and localised eddies associated with this flow regime can result in the life of the internal piping components being unpredictable. Once the internal piping has eroded, the vapour quality is significantly compromised and the evaporator vessel must be taken offline for maintenance. This has occurred in many circumstances, well before the evaporator's scheduled outage.

The evaporator vessel illustrated in figure 12 is the latest evaporator arrangement to be installed in recent alumina refinery expansions. It attempts to redress the internal piping erosion experienced with evaporator designs of figures 9 to 11 by ensuring no change in fluid flow direction of the inlet piping, following the initiation of two phase flashing slurry flow. The inlet pipe 39 enters the evaporator vessel 19 vertically upward, directing the fluid directly into the deflection cap 41 via the vertical inlet riser tube 43. To accommodate the altered configuration of the inlet piping, the evaporator slurry outlet port 45 is offset to one side of the evaporator bottom head. This new configuration restricts the choice of evaporator bottom heads to a hemispherical bottom. This is required so as to avoid solids particle accumulation and blockage that would otherwise occur if a conical bottom head were installed.

Whilst this design has proven successful at eliminating the erosion of the internal piping 31 associated with the designs of figures 9 to 11 , it has contributed to the potential to form fluid stagnation ('dead') zones in the bottom of the evaporator directly opposite the position of the slurry outlet port. In these areas, solids particles may accumulate and cement together. This cementation effect can result in multiple blockages of the outlet piping. As with the two phase flashing fluid flow mechanisms associated with the inlet piping, such cementation formations in the bottom of the evaporator vessel have proven unpredictable and have led to evaporator outages anywhere from 3 months to 8 months in operation. Both periods remain well short of the 12 to 24 month (typical) required evaporator life before scheduled outage.

It is an object of this invention to provide an alternative slurry inlet and slurry outlet arrangement in an evaporator tank that will overcome shortcomings inherent in the above described arrangements

Throughout the specification unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Throughout the specification unless the context requires otherwise, the word "include" or variations such as "includes" or "including", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure of the Invention

In accordance with one aspect of the present invention there is provided an evaporator vessel having a fluid inlet and a fluid outlet, and having a vapour outlet located above said fluid inlet and fluid outlet, said fluid inlet extending linearly to a position within the interior of said evaporator vessel and having a deflector mounted near to and spaced from the terminating end of said fluid inlet, said fluid outlet commencing at a position distal from the terminating end of said fluid inlet, and said fluid outlet extending around the circumference of said fluid inlet for a portion of the length of said fluid inlet, whereafter said fluid outlet diverges away from said fluid inlet, said fluid inlet passing through part of the wall of said fluid outlet where said fluid outlet diverges.

The fluid outlet may be divided into portions provided by a plurality of pipes circumferentially surrounding said fluid inlet, but in the most preferred form of the invention the fluid outlet and fluid inlet form a coaxial configuration for said portion of the length of said fluid inlet.

Preferably said fluid inlet is located substantially centrally within said fluid outlet and surrounded concentrically by said fluid outlet.

Preferably said fluid inlet passes through a sleeve which forms part of the wall of said fluid outlet, said sleeve being shaped to define part of the internal wall of said fluid outlet.

Preferably said fluid inlet lowermost end is shielded by an annular insert, said annular insert performing an anti-wear function, said fluid inlet lowermost end being sandwiched in coaxial arrangement between said annular insert and said sleeve.

In accordance with another aspect of the present invention there is provided a combined fluid inlet and fluid outlet for an evaporator vessel, said combined fluid inlet and fluid oulet having a central fluid inlet and a fluid outlet disposed circumferentially therearound, said combined fluid inlet and fluid outlet being provided with a connector to connect to the bottom of said evaporator vessel; said fluid inlet extending linearly to a position so as to be disposed within the interior of said evaporator vessel being connectable to a deflector located near to and spaced from the terminating end of said fluid inlet, said fluid outlet commencing at a position distal from the terminating end of said fluid inlet, and said fluid outlet extending around the circumference of said fluid inlet for a portion of the length of said fluid inlet, whereafter said fluid outlet diverges away from said fluid inlet, said fluid inlet passing through part of the wall of said fluid outlet where said fluid outlet diverges.

Preferably said fluid inlet passes through a sleeve which forms part of the wall of said fluid outlet, said sleeve being shaped to define part of the internal wall of said fluid outlet.

Preferably said fluid inlet lowermost end is shielded by an annular insert, said annular insert performing an anti-wear function, said fluid inlet lowermost end being sandwiched in coaxial arrangement between said annular insert and said sleeve. Brief Description of the Drawings

Two preferred embodiments of the invention will now be described with reference to the drawings, in which:

Figure 1 is a diagram of a high pressure flash tank according to the first embodiment;

Figure 2 is an exploded view of part of the high pressure flash tank of figure 1 ;

Figure 3 is a diagram of a low pressure flash tank according to the second embodiment; and

Figure 4 is an exploded view of part of the low pressure flash tank of figure 3.

Best Mode(s) for Carrying Out the Invention

Referring to figure 1, a high pressure evaporator vessel in the form of a flash tank 111 is illustrated, which has a cylindrical body 113 with hemispherical lower end 115 and hemispherical upper end 117. The hemispherical upper end 117 has a vapour outlet and pipe 118 through which steam may be vented to a heat exchanger or other plant equipment as required. The hemispherical lower end 115 has a slurry inlet/outlet port 119 which comprises a fluid inlet conduit 121 which extends vertically upward in linear fashion (having no bends) from a restrictor in the form of an orifice plate 123 to the terminating end 125 above which a deflector assembly 127 is mounted. The deflector assembly 127 is mounted spaced from the terminating end, and is constructed so that fluid emerging from the terminating end is prevented from continuing upward within the flash tank 111 , and is deflected sideways by the deflector assembly 127. In particular the function of the deflector assembly is to prevent slurry from fouling the interior of the flash tank 111 or entering any vapour outlet or fittings. Typically the deflector assembly 127 will be mounted near or proximal to the terminating end 125 of the fluid inlet conduit 121 , but not so spaced from the terminating end 125 that any divergence in the inlet stream flow could result in part of the inlet stream flow not impacting the deflector assembly 127.

The inlet/outlet port 119 has an outlet port 129 which commences at a drain 131 which extends circumferentially around the fluid inlet conduit 121 at the bottom of the hemispherical lower end 115. The inlet/outlet port 119 includes a Y-piece 133 which directs fluid flowing from the drain 131 to an outlet 135 to which an outlet pipe 137 is connected. An annular insert 139 is press-fit in the Y-piece 133, and is shaped to cleanly divert flow from the drain 131 to the outlet 135. The Y-piece 133 includes flanged connectors 141 , 143, 145; flanged connector 141 provided to connect to a flange 147 mounted to the bottom of the hemispherical lower end 115, and flange 145 mounting to a flange 149 on the outlet pipe 137.

The bottom of the fluid inlet conduit 121 is received within the insert 139 in coaxial arrangement with a fluid inlet protective sleeve 151 clamped to the orifice plate 123 by a flanged assembly 153 which joins an inlet pipe 155 to flange 143. The fluid inlet protective sleeve 151 is received within (inside) the fluid inlet conduit 121.

In effect, with this described arrangement, the fluid inlet passes through a part of the wall of the fluid outlet. The fluid inlet conduit 121 is restrained against movement that may be caused by forces imparted by boiling slurry by bracing struts 156 located radially around the drain 131 at the bottom of the flash tank, and radial struts 158 secured to the fluid inlet conduit 121 near the terminating end 125 of the fluid inlet conduit 121 , underneath the deflector assembly 127, and attached to inside of the flash tank 111. The radial struts 158 are generally raked downward in order to also restrain against movement in an axial direction imparted by impact forces on the deflector assembly 127.

The hemispherical upper end 117 of the flash tank 111 includes a flanged connector 161 providing a vapour outlet to which is connected a vapour outlet pipe 163 which provides steam and is directed in controlled manner to a heat exchanger for heating elsewhere in the plant in which the flash tank is used.

In use, super heated slurry enters the flash tank 111 via the inlet pipe 155, in controlled manner. Flow may be controlled by a valve (not shown). Initiation of boiling evaporative fluid flow commences at the orifice plate 123 which is located immediately downstream of the inlet pipe 155. The boiling slurry flow is directed vertically upward without change in direction, through the flanged assembly 153, fluid inlet protective sleeve 151 and fluid inlet conduit 121 (riser tube).

The boiling slurry flow is then redirected back into the fluid level within the flash tank 111 evaporator by the deflector assembly 127 which is attached to the terminating end 125 of the fluid inlet conduit 121. Slurry exits the flash tank 111 in a vertically downward direction via the annular passage of the drain 131 passing along the exterior of the fluid inlet conduit 121. The specialized geometry of the interior of the hemispherical lower end 115 and the annular configuration of the drain 131 ensures no particulate accumulation occurs within the bottom of the flash tank 111.

The slurry continues its downward annular flow path entering the Y-piece 133 which directs fluid flowing from the drain 131 to the outlet 135 and outlet pipe 137. Particulate accumulation is prevented from occurring in the bottom annular section of the Y piece 133 by the insert 139. The insert may be attached directly to the flanged assembly 153 or in an alternative embodiment may comprise a separately flanged component. The axis of the outlet 135 of the Y piece 133 may be disposed at an angle of from 30 degrees to 60 degrees offset from vertical, with from 30 degrees to 45 degrees being optimal.

The difference between the low pressure flash tank shown in figures 3 and 4 and the high pressure flash tank shown in figures 1 and 2 are that the low pressure flash tank has a conical lower end 165 instead of the hemispherical lower end 115. In a further alternative embodiment, the low pressure flash tank may also have a conical upper end instead of the hemispherical upper end 117. In addition, the low pressure flash tank is larger in overall dimensions than the high pressure flash tank, but in other respects is substantially the same.

Materials of construction will vary dependent on the chemical fluids used in the processing plant. For example, in the processing of a lateritic nickel ore, sulphuric acid is used to leach (or dissolve) the nickel from the ore into solution. The residue solids particles and acid solution form the fluid slurry within the evaporator vessel. Titanium clad steel vessels with silica brick internal linings are used for this industry.

For the processing of bauxite ore as used for the alumina refining industry, sodium hydroxide solution is used to leach (or dissolve) the alumina mineral out of the bauxite ore and into solution. Mild steel (carbon steel) is relatively resistant to corrosion for this chemical reagent. For an evaporator application in this industry, typical materials of construction are illustrated in the Table below.

The combined slurry inlet/outlet port 119 provided in a flash tank 111 , according to the invention, ensures that there is no alteration of the fluid flow direction associated with the fluid inlet piping following initiation of the boiling slurry flow, and also avoids fluid stagnation zones in the bottom of the flash tank.

The combined slurry inlet/outlet port 119 incorporates a specially fabricated piping component in the form of the Y-piece 133 providing a passage through which the fluid inlet conduit 121 passes.

The fluid inlet conduit 121 continues in a vertical up-flow orientation through the Y- piece 133 inside the flash tank 111 , vessel directing the flashing fluid into the deflector assembly 127. In this manner, erosion of the internal piping is mitigated and clean quality vapour (steam) is produced within the vessel.

In parallel, the annular nature of the drain 131 leading the outlet port 129 in the combined slurry inlet/outlet port 119 prevents the formation of eccentric fluidynamic forces in the bottom of the flash tank, enabling the fluid/slurry to discharge evenly from all directions from within the vessel bottom. The new Y-piece 133, annular drain 131 and internal fluid inlet conduit 121 combine to present an annular passage for fluid flow that forms the primary channel for discharging slurry downward and evenly from the evaporator bottom head. This mitigates the potential formation of fluid stagnation zones and consequential particle cementation that in an off centre outlet nozzle geometry, may promote blockage of the evaporator outlet piping.

Changes to the above described arrangements will be readily appreciated by a person skilled in the art, and such changes may be made without departing from the spirit and scope of the invention. It should be appreciated that the scope of the invention is not limited to the specific embodiments described herein.