HK1168835B - Three phase sulfur separation system with interface control - Google Patents

Description

Three-phase sulfur separation system with interface control

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. non-provisional application No. 12/637,301, filed on 12/14/2009, which is incorporated herein by reference in its entirety.

Technical Field

The present invention relates to a separator system in which three fluid phases are used to separate a denser liquid from a less dense liquid by maintaining a constant pressure in the separator by adding or removing gas from a gas phase zone. More specifically, the present system may be used as an improved sulfur separation system for recovering sulfur from a gas stream and a liquid stream that includes both molten sulfur and redox solution.

Background

Many of the by-products of desulfurization redox processes include solid elemental sulfur suspended in a liquid redox solution. In some liquid redox processes, it is desirable and necessary to use a sulfur furnace to melt solid elemental sulfur to produce a high quality, marketable sulfur product. However, metal ions in redox solutions, such as iron and vanadium, react with hydrosulfide, thiosulfate and bicarbonate ions (collectively referred to as "reactive solutes") at high temperatures to form metal polysulfides. These metal polysulfides are undesirable in the context of producing high quality elemental sulfur. The formation of large amounts of metallic polysulfides can render sulfur unusable and also lead to fouling within the furnace, requiring subsequent cleaning of the furnace tubes.

The rate at which the metal ions react with the sulfur varies with the amount of metal ions in the redox solution, the furnace temperature, the amount of time the sulfur is in contact with the redox solution at elevated temperatures, and the surface area of the interface between the molten sulfur and the redox solution. If more metal ions are present in the solution, more polysulphides will be formed. As the furnace temperature increases, the reactivity between sulfur and metal ions increases, forming more metal polysulfides. As the contact time between the molten sulfur and the redox solution increases at high temperatures, more metal polysulfides will be formed. The interface between the molten sulfur and the redox solution provides constant contact of the molten sulfur with the metal ions. Thus, the smaller interfacial surface area between the molten sulfur and the redox solution will limit the formation of metallic polysulfides.

A filtration/washing/repulping system may be used to reduce metal ions and reactive solutes entering the sulfur furnace. Furthermore, the sulfur furnace may be operated at the lowest possible temperature above the melting point of sulfur. While limiting metal ions and reactive solutes entering the furnace via filtration and operating the furnace at lower temperatures are effective techniques for improving sulfur quality, residence time and interfacial surface area also play an important role in the formation of metal polysulfides and thus sulfur quality. Even when furnace temperatures are maintained at the lowest possible level along with the filtration/washing/repulping system, sulfur quality is degraded when long residence times and large interfacial areas are employed.

With conventional sulfur separation designs, residence time is one of the least controlled variables affecting sulfur quality. The sulfur separator is typically designed to provide a specific residence time for phase separation corresponding to the maximum unit sulfur production, and is primarily determined by the water volumetric flow rate. When the residence time increases beyond that expected during maximum sulfur production, sulfur separation is improved because the sulfur droplets have more time to separate from the redox solution. In addition, the interface level between the redox solution and the molten sulfur is more clearly defined, and thus the interface level control is improved. However, as the holdup increases, the formation of polysulfides increases. Thus, the optimum residence time results from a compromise between these considerations.

Previously known designs control the flow of molten sulfur by maintaining the interface level at a certain vertical level. Examples of these designs are described in U.S. patent nos. 4,730,369 and 5,651,896. These known separators are completely liquid vessels and do not have a gas phase. The principle is to maintain the operating pressure of the vessel at a pressure set point to keep the water phase from evaporating while operating at or above the melting point of sulfur. However, in actual practice, when the molten sulfur control valve (interface level control) or the aqueous solution control valve (pressure control) is opened, the pressure in the vessel drops and a portion of the aqueous phase evaporates. This evaporation leads to serious operational problems such as molten sulfur being carried out of the top of the vessel and clogging due to solidification of the molten sulfur in downstream equipment and piping. The present invention now addresses this and other problems by including a third fluid phase (i.e., the gas phase) in the primary separator vessel with a separation control system to maintain the pressure of the vessel regardless of the level of the aqueous or molten sulfur phase. These and other advantages will be apparent from the following more detailed description of the invention.

Disclosure of Invention

As demonstrated by the preferred embodiments of the present invention, the present invention relates in a primary aspect to a liquid separator system. The system includes a container having a top and a bottom. The container has a larger diameter at the top than at the bottom, and the cross-section of the container decreases downward from the top to the bottom of the container. The system includes a first inlet for introducing a mixture of two liquids into the vessel, wherein the second liquid is thicker than the first liquid. An inlet/outlet at the top allows for the introduction of a pressurized gas stream into the vessel or the removal of excess pressurized gas to maintain a constant operating pressure of the vessel. Maintenance of constant pressure prevents the aqueous phase from boiling and carrying over a thicker liquid. An outlet near the bottom of the vessel allows the denser liquid to flow out of the vessel. The interface control structure senses the level of the interface between the two liquids and controls the flow of the denser liquid from the outlet. By adjusting the set point of the interface control structure, the vertical height of the interface level inside the vessel can be optimally varied so that the residence time of the denser liquid in the vessel does not increase with decreasing production of the denser liquid. This also reduces the interfacial area of the two liquids as throughput decreases. The interface structure includes a control valve that opens and closes (which may be opened/closed or adjusted as needed by the system) to control the removal of the denser liquid phase. The level of water in the vessel is maintained by level control between the uppermost level of the water phase and the gas interface. The water phase controller is communicated with a control valve which is adjusted to maintain the liquid level of the water phase. The internal pressure of the vessel is maintained at a constant predetermined or desired level regardless of the position of the denser liquid and aqueous phase control valves. This is achieved by regulating the pressure of the gas phase within the vessel by adjusting the gas flow into and out of the vessel. The particular gas used as the gaseous phase is not critical to the invention and may be selected from the group consisting of air, N₂Gas, or any inert, economical, non-condensing gas, at a desired pressure. In certain embodiments of the invention, the system is used to separate molten sulfur from liquid redox solution and/or reslurry water.

It is therefore an object of the present invention to improve the quality of a denser liquid separated from an aqueous liquid using a three-phase separator system. It is also an object of the present invention to improve the quality of sulfur recovered from redox applications. It is another object of the present invention to provide a system wherein the residence time of the molten sulfur in the sulfur separator can be varied depending on the sulfur production. Yet another object of the present invention is an improved sulfur separation device that allows for more precise interface level control while obtaining the benefits of varying interface surface area and residence time. Yet another object of the present invention is a system that allows for varying the interfacial area between the liquid redox solution and/or reslurry water and the molten sulfur. Another object of the present invention is an improved sulfur separation system adapted for use with the prior art. It is yet another object of the present invention to prevent sulfur carryover by maintaining a constant pressure in the vessel. Yet another object of the present invention is a more cost-effective process for recovering high quality elemental sulfur.

The features of the present invention will be better understood upon consideration of the following detailed description of the invention. In the description, reference is made to the accompanying drawings.

Drawings

FIG. 1 is a schematic block diagram of a prior art process for removing elemental sulfur from a slurry;

FIG. 2 is a schematic block diagram of another prior art process for removing elemental sulfur from a slurry;

FIG. 3 depicts a preferred embodiment of the present invention; and

fig. 4 depicts an alternative preferred embodiment of the present invention.

Detailed Description

To demonstrate the context of the present invention, reference is made to fig. 1 and 2, which depict a known process for producing molten sulfur from a liquid redox process. None of these known systems use separators for the three fluid phases. A sulfur slurry from a liquid redox application (shown as 2, which includes solid elemental sulfur suspended in a redox solution) is passed through a sulfur filter 4. The slurry may be a relatively concentrated slurry (about 15 wt%) as made in a concentration device such as a settler, or a relatively dilute slurry (0.1 wt%) as obtained without the use of a concentration device. In the sulfur filtration operation, most of the redox solution is removed and returned to the process as a filtrate, as shown at 8. The solid sulfur remaining after the filtrate is removed is called a sulfur cake or filter cake and is shown at 5. In some applications, wash water is sprayed on the filter cake to achieve better separation between the sulfur cake and the redox solution, as shown at 7. This operation is called "washing". Solid sulfur 5 and some redox solution and wash water enter the reslurry tank 6. Water is added to repulper 6 to produce a sulfur slurry, shown at 10, which is passed through a sulfur furnace or heat exchanger 12. The filtration/washing/repulping systems shown in 4, 6 and 10 help remove metal ions such as iron and vanadium and reactive solutes from the slurry, which in turn will reduce the amount of undesirable polysulfides formed in the melting process. Some systems do not employ a filtration/washing/repulping process, so the slurry from process 2 goes directly to sulfur furnace 12.

In known processes, a sulfur slurry is heated in sulfur furnace 12 to a temperature above the melting point of sulfur by indirect heat exchange with steam or a heat transfer fluid. Thus, the sulfur is melted and the hot solution exiting the sulfur furnace 12 contains the aqueous redox solution and reslurry water and molten sulfur. Molten sulfur is immiscible in and thicker than the redox solution and reslurry water. The hot solution (also called furnace effluent) then enters the vessel or sulfur separator 14 or 40 via an inlet. Inside the sulfur separator, the denser molten sulfur droplets are separated by gravity from the less dense redox solution and reslurry water, and the denser molten sulfur droplets fall to the bottom of the sulfur separator. The denser molten sulfur forms an interface with the redox solution or reslurry water, as depicted by line 36.

Molten sulfur flows from the bottom of the sulfur separator and is removed through outlet valve 28 or 62. Unlike the present invention, the pressure in the sulfur separator of known systems is controlled by a pressure control valve 24 or 47, which controls the flow of the aqueous phases (i.e., reslurry water and redox solution) from the separator. This is to prevent the water boiling which in turn can lead to molten sulphur being carried out into the process line where it solidifies and leads to plugging. The reslurry water flowing from the sulfur separator is returned to the reslurry tank or sent to disposal. In applications where a filter/wash/repulp system is not employed, the liquid leaving the aqueous phase region of the sulfur separator will be a redox solution, where it will be returned to the unit. The flow of molten sulfur out of the sulfur separator is controlled by an automatic control valve, which is typically a plug valve with a steam jacket for on/off control or a v-ball valve with a steam jacket for regulatory control. An interface level control unit that indirectly measures the level of molten sulfur in the separator controls the control valve. Previous designs, as illustrated in fig. 1, used a single vessel large enough to phase separate at the design aqueous phase flow rate at the design sulfur loading. Generally, the flow rate of the aqueous phase determines the desired size. Since the interfacial level control uses the same diameter as the separation step, the sulfur residence time in these vessels is extremely long. Furthermore, at deeper dips (turndown), the separation volume is larger than necessary, but the volume cannot be changed. In addition, the volume of sulfur below the interface level is constant regardless of production. Therefore, the residence time of sulfur exposed to the operating temperature under descending conditions is extremely long.

In the second prior art as shown in fig. 2, the narrow sleeve 46 becomes the liquid level control section; the tapered conical portion 44 allows for varying residence time in the separation region, but requires the operator to first change the interface level set point to bring the interface in the conical region. At very low production, the interface may be physically located in the casing, thereby minimizing sulfur residence time.

Referring now to FIG. 3, which depicts one of many preferred embodiments of the present invention, the system includes a vessel or sulfur separator 100 that receives a sulfur furnace effluent containing molten sulfur redox solution, preferably two separate streams 102 and 103, from a sulfur furnace 101. Since the consistency of the sulphur droplets is nearly twice that of water droplets, an initial separation between the two liquid phases will occur in the heat exchanger outlet end, namely stream 102 (which will be mainly aqueous solution) and stream 103 (which will be mainly molten sulphur). By removing these two streams separately from the heat exchanger, more efficient separation will occur in the separator 100. The sulfur separator 100 includes three zones, a gas phase zone 105, an aqueous phase zone 106, and a denser liquid phase zone 107. Both the gas and water phase regions are cylindrical and have a larger diameter than the denser liquid phase region.

After the furnace effluent enters the sulfur separator, the denser sulfur droplets settle to the bottom of the sulfur separator by gravity, while the less dense aqueous solution rises to a level near the middle of the sulfur separator. Although the inlet for the furnace-containing effluent can be located anywhere in vessel 100, it is preferred to split the effluent using a single flow inlet optimally located where the sulfur separator has a relatively large diameter. This reduces the upper velocity of the redox solution as the effluent is introduced into the sulfur separator and allows the sulfur droplets to settle and coalesce into larger droplets and eventually form a continuous molten sulfur phase at the bottom of the sulfur separator. The second effluent inlet is preferably located near the middle of the denser liquid phase region of the separator vessel. The molten sulfur forms an interface with the liquid redox solution, as indicated by dashed line 108. The bottom of the container 100 may have a sleeve as shown in fig. 3 and 4, or a tapered bottom terminating in a larger taper in the sleeve as shown in fig. 2.

The pressure in the sulphur separator 100 is controlled by a pressure controller 109 operating control valves 110 and 111. The pressure controller senses the pressure in the separator 100 and opens or closes the valve 110 or 111 as necessary to maintain a predetermined or desired pressure, i.e., a set point pressure. If the pressure controller senses a pressure below the set point pressure, it will open valve 110 to introduce a pressurized gas at the desired pressure, preferably a gaseous fluid selected from the group consisting of air, nitrogen, gas, or any other non-condensing gas. If the sensed pressure is above the set point pressure, the pressure controller will close valve 110 and open valve 111 to remove or vent gas from the gas phase region of the separator. This removed gas can be used in other processes or burned off with a flame.

The system of the present invention also uses two level controllers 112 and 113 that control valves 114 and 115, respectively. Controller 112 senses the liquid level between gas phase zone 105 and water phase zone 106 and controller 113 senses the liquid level between water phase zone 106 and denser liquid phase zone 107. Valve 114 controls the flow of aqueous solution from the sulfur separator and prevents boiling of the aqueous solution within the separator. The aqueous solution exiting the sulfur separator via outlet 117 is returned to the unit or repulper or disposed of. The interface level control mechanism used in the present system may be any type of reliable control mechanism. The preferred mechanism measures the interface in the sulfur separator by measuring the pressure with pressure sensors located above and below the interface level of the gas phase and the aqueous phase and below and above the interface level of the aqueous phase and the denser liquid phase. The pressure difference between the two sensors indicates the interface level.

The system also includes an outlet 116 through which molten sulfur flows from the sulfur separator through a control valve 115. The flow rate of molten sulfur from the sulfur separator determines the sulfur production. The amount of time between the introduction of molten sulfur into the sulfur separator at inlets 102 and 103 and the removal of molten sulfur via outlet 116 determines the residence time of the molten sulfur in the system. The flow rate of molten sulfur is controlled by an outlet valve 115 controlled by an interface level control mechanism 113. The interface level control mechanism 113 and the outlet valve 115 together form an outlet control mechanism or structure. The interface level control mechanism 112 and the control valve 114 together form another control mechanism. When operating the unit at design sulfur production, it is preferable to maintain the interface 108 near the smaller diameter top of the separator 100. When sulfur production decreases, the interface level 108 is lowered by adjusting the set point of the interface level control mechanism 113.

The present invention may also include variations in the shape of the sulfur separator, such as the horizontal shape of the sulfur separator illustrated in FIG. 4. Alternatively, the slope of the inner wall of the separator may determine the conical transition between the denser liquid phase region and the aqueous phase region. The system may also be used for liquids other than redox solutions, such as reslurry water. Likewise, the system may also be used with liquids other than molten sulfur.

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation.

The means, materials, and steps for performing various disclosed functions may take a variety of alternative forms without departing from the invention. Thus, the expressions "means" and "means" or any method step language as found in the above specification or the above claims and the functional statements that follow are intended to define and cover any structure, physical, chemical, or electronic elements or structures, or any method steps, which may exist now or in the future, which performs the described function, whether or not precisely equivalent to one or more of the embodiment(s) disclosed in the above specification, i.e., other means or steps may be used to perform the same function; and it is intended that such statements be given the broadest interpretation within the scope of the terms of the appended claims.

Claims (12)

1. A liquid separator system comprising, in combination:

(a) a vessel having a top, a bottom, a gas phase region, an aqueous phase region, and a denser liquid region, the vessel having a larger diameter at the top than at the bottom;

(b) an inlet for introducing a first liquid and a second liquid into the vessel, wherein the second liquid is thicker than the first liquid and wherein the vapor region is directly above the first liquid;

(c) a first outlet proximate the bottom of the container that allows flow of the second liquid from the container;

(d) a second outlet in fluid communication with the aqueous phase region;

(e) a pressure controller in communication with the gas phase region that controls a gas inlet valve and a gas outlet valve, each valve in fluid communication with an inlet/outlet proximate the top of the vessel; and

(f) an interface control structure constructed and arranged for sensing an interface level between the first liquid and the second liquid within the container bottom, and for controlling the flow of the second liquid from the first outlet, and for raising or lowering the interface level in response to a corresponding increase or decrease in the flow.

2. The liquid separator system of claim 1, wherein the first liquid is a redox solution and reslurry water from a redox operation and the second liquid is molten sulfur.

3. The liquid separator system according to claim 1 wherein the top of the vessel is cylindrical and the bottom of the vessel is cylindrical.

4. The liquid separator system of claim 1, wherein the residence time of the second liquid in the vessel does not increase with decreasing flow from the outlet.

5. The liquid separator system of claim 1, wherein second interface control structure is constructed and arranged for sensing an interface level inside the vessel between the aqueous phase region and the vapor phase region, and for controlling the flow of the first liquid from the second outlet, and for raising or lowering the second interface level in response to a corresponding increase or decrease in the flow of the first liquid.

6. A sulfur separator system for separating sulfur from a mixture containing a first liquid and a second liquid, comprising, in combination:

(a) a vessel having a top, a bottom, a gas phase region, a redox solution region, and a molten sulfur region, said vessel having a larger diameter at said top than at said bottom;

(b) an inlet for introducing a mixture of the redox solution from a sulfur furnace and molten sulfur into the vessel, the molten sulfur being thicker than the redox solution, wherein the mixture forms an interface between the redox solution and the molten sulfur at a vertical level within the vessel and wherein the vapor phase region is located directly above the redox solution;

(c) a first outlet near the bottom of the vessel that allows a flow of the molten sulfur from the vessel that determines a sulfur yield;

(d) a second outlet in fluid communication with the redox solution zone allowing flow of the redox solution from the vessel;

(e) a pressure controller in communication with the gas phase region that controls a gas inlet valve and a gas outlet valve, each valve in fluid communication with an inlet/outlet proximate the top of the vessel; and

(f) an interface control structure constructed and arranged for sensing the vertical level of the interface within the bottom of the vessel and for raising or lowering the interface level in response to a corresponding increase or decrease in the sulfur production.

7. The sulfur separator system of claim 6, wherein the first inlet is in communication with the redox solution zone and a second inlet is in communication with the molten sulfur zone.

8. The sulfur separator system of claim 7, wherein the mixture from the sulfur furnace is split between the first inlet and the second inlet.

9. The sulfur separator system of claim 6, wherein the residence time of the molten sulfur in the vessel does not increase as the sulfur production decreases.

10. A method of separating molten sulfur from a fluid comprising the steps of, in combination:

(a) introducing a liquid mixture comprising the fluid and the molten sulfur into a vessel, the molten sulfur sinking into the vessel and forming an interface with the fluid at a vertical height;

(b) monitoring the pressure within the vessel, and regardless of the vertical height of the interface, adding or removing gas from a gas phase region in the vessel directly above the fluid to maintain a predetermined pressure within the vessel, wherein the gas is added through a gas inlet valve and removed through a gas outlet valve, both controlled by a pressure controller;

(c) removing molten sulfur from the vessel; and

(d) changing the vertical height of the interface in response to a rate of removal of the molten sulfur from the vessel.

11. The method of claim 10, wherein the fluid is a liquid redox solution or reslurry water.

12. The method of claim 10, wherein the residence time of the molten sulfur in the vessel does not increase as the production of molten sulfur decreases.