Evaporator with an apparatus for controlling solid particle flow

Abstract: An apparatus for use in an evaporator for selectively controlling the flow of solid particles within the evaporator. The apparatus includes a separation chamber (24) having a plurality of apertures (32), each of which has a critical size. In the separation chamber (24), solid particles which are larger than a critical size are separated from the remainder of the solution. The separated particles exit the separation chamber (24) via a bypass conduit (26) for circulation directly to the sump (14) of the evaporator for recombining with the separated liquid portion which passed through the separation chamber (24) and was subsequently concentrated in the heat exchange tubes (10). The apparatus prevents solid particles from clogging the system and allows for continuous circulation of the solution and solid particles.

Description: The present invention relates to an evaporator for concentrating slurries and solutions containing solid particles and liquid of the type as disclosed in the preamble of claim 1. The invention further relates to a method for concentrating slurries or solutions containing solid particles and liquid of the type as described in the preamble of claim 7. With reference to earlier applications DE-A-38 08 026, FR-A-26 15 607, GB-A-22 05 048 and SE-A-88 01 864, the applicant has voluntarily limited the scope of the present patent, and submitted separate claims for Germany, Great Britain, France and Sweden. An evaporator and a method of the above-mentioned type are disclosed in US-A-4 248 296 (see also figure 1 as hereinafter described), and CH-A-466 221. A wide range of industries employ evaporators for concentrating solutions and slurries, including oil refining, synthetic fuel production, food processing, herbicide and pesticide production, electric generating stations, primary metal refining, pharmaceutical production, and pulp and paper manufacture. The evaporators may be used to increase the concentration of a fluid component, and/or to crystallize a solute component. Various types of evaporators are available and are well known in the prior art. An example for suitable evaporators is the vertical, tube-in-shell, falling film evaporators of the above-mentioned publications. In such evaporators, a solution or slurry is circulated repeatedly through heat exchange tubes. As the solution or slurry passes through the heat exchange tubes, the solvent, i.e., water or organic solvent, is gradually evaporated, leaving a more concentrated solution or slurry, and often causing solute components to precipitate. Evaporators typically include various chambers, apertures, and tubes through which the solution or slurry must pass repeatedly as it is concentrated. Care must be taken to provide for efficient flow of solid particles as well as liquid. Slurries contain solid particles even during initial stages of concentration, while solutions contain solutes, i.e., salts which may precipitate out of solution during concentration. These solid particles can cause clogging in various parts of the evaporator. Deposits may occur along the walls or other surfaces of the evaporator. Deposits may break off in the form of chips or flakes, which can cause clogging of the evaporator and interrupt the flow of the solution or slurry to be concentrated. If the system is clogged and circulation cannot proceed efficiently, the system must be shut down to allow operators to clear and clean the blockage. Deposits occur while the evaporator is in operation and also while the evaporator is shut down for cleaning; as the walls of the evaporator system are allowed to dry out during the cleaning operation, significant amounts of deposits can form. When the system is restarted and circulation is restored, the new deposits often flake off and reclog the system. The clogging problem is most serious when chips lodge themselves within smaller apertures, such as the inlet orifices of the fluid distributors mounted at the top of heat exchange tubes. One such distibutor is described in US-A-4 248 296. Chips interfere with normal flow of the brine, and can effectively remove entire heat exchange tubes from operation. Various attempts have been made to remove the chips or flakes by filtration. For example, screens or strainers have been fitted inside the evaporator system. However, such screens must be cleaned periodically, and in order to do so, the evaporator system must be shut down, resulting in lost time, increased costs of operation, and further formation of deposits. In addition, when the evaporator is restarted, additional chips and flakes may form downstream of the screen or strainer, and reclog the system. It will therefore be appreciated that there has been a significant need to design an evaporator and to provide a method which will facilitate separation of solid particles from liquid, and which will not require the system to be shut down for cleaning of the separation apparatus. It is an object of the present invention to separate solid particles from liquid in a solution or slurry undergoing concentration in an evaporator system. It is a further object of the present invention to provide for continuous separation of solid particles from liquid within an evaporator without the need for periodic shutting down of the system to clean the separation apparatus. It is another object of the present invention to provide an evaporator having a bypass route through which solid particles can circulate through the system, rather than being deposited at the site of the separation apparatus. It is still a further object of the present invention to provide an evaporator having an apparatus which enhances the turbulent flow of solid particles within the liquid so as to facilitate breaking up of solid particles into smaller units. The present invention fulfills the above-mentioned need by providing the evaporator of claim 1 and the method of claim 7. Further developments of the invention are contained in the dependent claims. The present invention discloses an evaporator for concentrating slurries or solutions containing solid particles and liquid. Such evaporators may be falling film evaporators, and the like. They include a plurality of substantially vertical heat exchange tubes, a lower reservoir positioned below the heat exchange tubes, a second reservoir positioned above the heat exchange tubes, and recirculation conduit for recirculating slurry or solution from the lower reservoir to the upper reservoir. The evaporator includes a solid particle flow apparatus having a separation chamber which has an inlet and an outlet. The inlet is positionable to receive slurry or solution from the recirculation conduit. The outlet is connected to the bypass conduit, described below. The outlet may be positioned at substantially the lowest portion of the separation chamber to encourage travel of the separated solid particles toward the outlet under the influence of gravity. The separation chamber also has a plurality of apertures which are sized to separate solid particles from liquid. The size of each aperture is a "critical" size - a predetermined size above which the solid particles cause unacceptable clogging of various parts of the evaporator. Solid particles larger than the critical size are preferably separated from the liquid to prevent clogging downstream. The chamber apertures communicate a substantial portion of the liquid to the upper reservoir exterior of the separation chamber for passage through the heat exchange tubes to the lower reservoir. In addition, the solid particle flow apparatus has a bypass conduit which is connected to the outlet of the separation chamber and which is positionable to channel solid particles which are larger than the critical size from the separation chamber to the lower reservoir. The solid particles which are channeled through the bypass conduit do not enter the upper reservoir exterior of the separation chamber. A substantial portion of the liquid exits the separation chamber through the apertures to the upper reservoir exterior of the separation chamber. The liquid is then recombined with the separated solid particles in the lower reservoir after fist passing through the heat exchange tubes. The bypass conduit may be positionable to channel the separated solid particles to one or more of the plurality of heat exchange tubes for travel to the lower reservoir while preventing the separated solid particles from entering the remainder of the plurality of heat exchange tubes. The separation chamber or a portion thereof, may be substantially conical in shape. The separation chamber may be constructed from steel screen material, or from steel material in which apertures have been punched or formed. The separation chamber may be sized to fit between an upper wall of the evaporator and the top of the heat exchange tubes. Further, the separation chamber may be attached for support to an upper wall of the evaporator. The chamber inlet may be positioned in an upper portion of the separation. Similarly, the chamber outlet may be positioned in a lower portion of of the chamber. For example, the chamber may be conical in shape and may have an outlet in a lower portion of the conically-shaped chamber. The chamber may also have tapered sidewalls and may have an outlet positioned at a lower point of the tapered sidewalls. The present invention also includes a method for selectively and continuously separating solid particles having a size larger than a critical size from liquid in a slurry or solution being concentrated in an evaporator. The critical size is a predetermined size above which the solid particles cause unacceptable clogging in various apertures in the evaporator. The method includes the steps of providing at least a first reservoir for the slurry or solution; separating from the liquid solid particles having a size larger than a critical size; channeling to the first reservoir the solid particles which were removed from the liquid; and channeling the concentrated liquid to the first reservoir. The separation step may also include the steps of providing a separation chamber having apertures with a size substantially equal to the critical size or smaller; providing a second reservoir in fluid communication with the heat exchanger; positioning the separation chamber within the second reservoir; and recirculating the slurry or solution from the first reservoir into the separation chamber. The step of channeling the solid particles to the first reservoir may further include the step of providing a bypass conduit connected to and leading from the separation chamber to the first reservoir without passage of the separated solid particles through the heat exchanger. An embodiment of the present invention is shown in the figures, wherin Figure 1 is a schematic of a prior art evaporator. Figure 2 shows an evaporator the present invention. Figure 3 shows a detailed view of the evaporator of the present invention. Figure 4 is a flow chart schematically illustrating the steps of the method taught by the present invention. The solid particle flow apparatus is designed for use with an evaporator for concentrating slurries and/or solutions. A typical prior art falling film evaporator is shown in Figure 1. Such evaporators usually include a plurality of heat exchange tubes 10 arranged substantially vertically. The evaporator also has an upper reservoir or flood box 12, a lower reservoir or sump 14, and a recirculation means 16 for recirculating the slurry or solution from the lower reservoir to the upper reservoir. The recirculation means typically includes at least one recirculation conduit 20, and a recirculation pump 18. Also typically present in evaporators, but not shown for clarity, are heating means, means for introducing slurry or solution to the evaporator, and means for removing concentrated slurry or solution from the evaporator. In operation, an evaporator such as the one depicted in Figure 1 continuously circulates the slurry or solution from the lower reservoir 14, through the recirculating means 16, back to the upper reservoir 12. The slurry or solution repeatedly passes through the heat exchange tubes 10, where solvent is evaporated, leaving a more concentrated slurry or solution. The heat exchange tubes 10 may be fitted with distributors 22 which direct the slurry or solution as it enters the heat exchange tubes. An example of distributors is described in United States patent number 4,248,296, issued to Louis J. Jezek and entitled "Fluid Distributor for Condensor Tubes". Distributors, such as those described in the Jezek patent, introduce the slurry or solution into the heat exchange tubes in a film form, thus enhancing evaporation. Such distributors have small apertures which must remain open in order for the evaporator to run most efficiently. Solid particles, such as chips or flakes in the slurry or solution can block and clog the apertures in the distributor, preventing normal thin film flow and limiting performance of the heat exchange tubes, or if completely blocked, effectively remove the tube from service. The solid particle flow apparatus is designed to prevent such blockage, in distributors or elsewhere in the system, by selectively controlling the flow of solid particles. Figure 2 shows the evaporator of Figure 1 fitted with this apparatus. The solid particle flow apparatus includes a separation chamber 24 and a bypass conduit 26. Figure 3 is a more detailed illustration of the solid particle flow apparatus. The separation chamber 24 is fitted within the upper reservoir 12 and is positioned below an outlet 27 of the recirculation conduit 20. The separation chamber is preferably attached to a cover 28 of the upper reservoir by bolts, screws, pins or other means of attachment 30. The separation chamber 24 includes a plurality of apertures 32 which are sized to separate solid particles from liquid. Each chamber aperture 32 is of a "critical size." The critical size may vary from system to system but should be equal to or smaller than the apertures downstream of the separation chamber and upstream of the heat exchange tubes. As such, the chamber apertures 32 should be of such dimensions as to separate from the liquid all solid particles which are large enough to block apertures downstream of the separation chamber and upstream of the heat exchange tubes, for example, the apertures in the distributors 22. The separation chamber 24 may be constructed from sheet steel into which holes, preferably round ones, have been punched or otherwise formed. It also may be constructed from steel screen or mesh material. Preferably, the separation chamber is conical in shape, or at least a lower portion 33 of the separation chamber is conical in shape, such as is shown in Figure 3. This shape helps to funnel the solid particles down to an outlet 34 of the separation chamber 24 which is located at the bottom of the lower portion 33 of the separation chamber. The chamber outlet communicates with the bypass conduit 26. The bypass conduit is attached to the chamber outlet 34 by conventional attachment means 36. The bypass conduit extends from the separation chamber outlet into at least one of the heat exchange tubes 10. Preferably, the bypass conduit is sized to fit within the heat exchange tube. The heat exchange tube into which the bypass conduit is fitted has no distributor in it. The apparatus may be incorporated into evaporators during original construction, or, may be retrofitted to existing evaporators. For example, many evaporators have doors, or manholes, in the top of the upper reservoir 12 to allow operators to enter the upper reservoir to permit its cleaning and maintenance. The apparatus may be constructed outside the evaporator and then installed by an operator entering the upper reservoir through the existing doors. Not only does the apparatus improve the efficiency of the conventional evaporator, the apparatus is economical to construct and install as original equipment or as a retrofit on existing evaporators. In operation, the slurry or solution circulating in the evaporator undergoes concentration. It is circulated from the lower reservoir 14 to the upper reservoir 12 through the recirculation conduit 20. The separation chamber 24 receives the recirculated slurry or solution from the outlet 27 of the recirculation conduit. Upon entering the separation chamber, the slurry or solution undergoes physical separation, whereby solid particles which are larger than the critical size are separated from the liquid, and a substantial portion of the liquid component exits the separation chamber through the chamber apertures 32. The exiting liquid enters the upper reservoir exterior of the separation chamber and then enters the heat exchange tubes 10 where it is further concentrated. Solid particles which are larger than the critical size and will not pass through the chamber apertures 32 exit the separation chamber through the chamber outlet 34 into the bypass conduit 26. The solid particles then pass directly to one of the heat exchange tubes 10 into which the bypass conduit is fitted for travel to the lower reservoir 14. In addition to its straining function, the separation chamber 24 also helps to reduce the size of the solid particles through turbulence and impact of the solid particles against the wall of the separation chamber. When the size of the solid particles is sufficiently reduced, the solid particles will no longer cause clogging problems in the evaporator. The present invention also includes a method for selectively controlling the flow of solid particles within the evaporator. The following discussion of the method of the present invention is better understood with reference to the flow chart shown in Figure 4. A slurry or solution undergoing concentration in the evaporator is held in the reservoir. The slurry or solution has a liquid component and solid particles, the solid particles usually being of varying sizes. The first step of the method of the present invention is to separate from the liquid component the solid particles which are larger than the critical size. Particles which are larger than the critical size and which, therefore, will not flow through downstream apertures will be removed from the slurry or solution before the slurry or solution continues downstream, i.e., enters the distributors of the evaporator. A second step in the method is to recirculate the separated solid particles back to the reservoir containing the slurry or solution. Preferably, this step of recirculation is achieved by washing the solid particles through a conduit with the aid of some of the liquid from the slurry or solution. A third step in the method is to direct the separated liquid to the heat exchange tubes to the evaporator. The liquid may contain solid particles; however, they will all be of a size smaller than the critical size and hence will not create blockage problems in the distributors or heat exchange tubes. Within the heat exchange tubes, the liquid undergoes concentration. A final step in the method is to direct the concentrated liquid back to the reservoir containing the slurry or solution where it is again mixed with the solids which were removed in the first step. The method of the present invention provides for continual circulation of solid particles, and does not provide for a building up of solid particles at any point in the system, such as would a conventional filter. As the solid particles are constantly separated from the liquid and recirculated, at least a portion of them will decrease in size as a result of the turbulence, and will pose less of a clogging problem.

Claims: Claims for the following Contracting States : AT, BE, CH, ES, GR, IT, LI, LU, NL An evaporator for concentrating slurries and solutions containing solid particles and liquid having a plurality of generally vertical heat exchange tubes (10), a lower reservoir (14) positioned below the heat exchange tubes (10), an upper reservoir (12) positioned above the heat exchange tubes (10), and a recirculation conduit (20) for recirculating slurry or solution from the lower reservoir (14) to the upper reservoir (12), characterized by an apparatus (24,26) for selectively controlling the flow of solid particles having a size larger than a critical size, the critical size being a predetermined size above which the solid particles cause unacceptable clogging of the evaporator, comprising: a removal means (24) for removing from the slurry or solution solid particles having a size larger than the critical size; said removal means (24) communicating a substantial portion of the liquid to the upper reservoir (12) exterior of said removal means (24) for passage through the heat exchange tubes (10) to the lower reservoir (14); and a bypass means (26) connected to said removal means (24) and positionable to channel from said removal means solid particles which are larger than the critical size to the lower reservoir (14) without entering the upper reservoir (12) exterior of said removal means (24). The evaporator of claim 1 wherein said removal means comprises a chamber (24) having apertures (32) with a size substantially equal to the critical size or smaller. The evaporator of claim 2 wherein said chamber (24) has an inlet and an outlet (34) and is positionable within the upper reservoir (12), and wherein said chamber inlet is positioned to receive slurry or solution from the recirculation conduit (20). The evaporator of claim 3 wherein said chamber outlet (34) is positioned at substantially the lowest portion of said separation chamber (24) to encourage travel of the separated solid particles toward said chamber outlet (34) under the influence of gravity. The evaporator of any one of claims 1 to 4 wherein said bypass means (26) includes a conduit positionable to circulate the separated solid particles to one or more of the plurality of heat exchange tubes (10) for travel to the lower reservoir (14) while preventing the separated solid particles from entering the remainder of the plurality of heat exchange tubes (10). The evaporator of any one of claims 2 to 5 wherein said separation chamber (24) being sized to fit between an upper wall of the evaporator and the heat exchange tubes (10), said separation chamber (24) being attached for support to an upper wall (28) of the evaporator. A method for concentrating slurries or solutions containing solid particles and liquid in an evaporator in which the slurry or the solution flows from a heat exchanger (10) to a first reservoir (14), characterized in that for selectively and continuously controlling the flow of solid particles having a size larger than a critical size, the critical size being a predetermined size above which the solid particles cause unacceptable clogging of the evaporator, solid particles having a size larger than the critical size are separated from the liquid and channelled to the first reservoir (14), the liquid from which solid particles were removed is channelled to the heat exchanger (10) for concentration of the liquid; and the concentrated liquid is channelled to the first reservoir (14) for recombining with the separated solid particles. Claims for the following Contracting States : DE, FR, GB, SE An evaporator for concentrating slurries and solutions containing solid particles and liquid having a plurality of generally vertical heat exchange tubes (10), a lower reservoir (14) positioned below the heat exchange tubes (10), an upper reservoir (12) positioned above the heat exchange tubes (10), and a recirculation conduit (20) for recirculating slurry or solution from the lower reservoir (14) to the upper reservoir (12), characterized by an apparatus (24,26) for selectively controlling the flow of solid particles having a size larger than a critical size, the critical size being a predetermined size above which the solid particles cause unacceptable clogging of the evaporator, comprising: a separation chamber (24) having an inlet connected to an outlet (34) of said recirculation conduit (20) for receiving said slurries or solutions, said chamber (24) having apertures (32) with a size substantially equal to the critical size or smaller for removing from the slurry or solution solid particles having a size larger than critical size; said chamber (24) communicating a substantial portion of the liquid to the upper reservoir (12) exterior of said chamber (24) for passage through the heat exchange tubes (10) to the lower reservoir (14); and a bypass means (26) connected to an outlet (34) of said chamber (24) and positioned to channel from said chamber (24) solid particles which are larger than the critical size to the lower reservoir (14) without entering the upper reservoir (12) exterior of said chamber (24). The evaporator of claim 1 wherein said chamber (24) is positioned within the upper reservoir (12). The evaporator of claim 1 or 2 wherein said chamber outlet (34) is positioned at substantially the lowest portion of said separation chamber (24) to encourage travel of the separated solid particles toward said chamber outlet (34) under the influence of gravity. The evaporator of any one of claims 1 to 3 wherein said bypass means (26) includes a conduit positioned to circulate the separated solid particles to one or more of the plurality of heat exchange tubes (10) for travel to the lower reservoir (14) while preventing the separated solid particles from entering the remainder of the plurality of heat exchange tubes (10). The evaporator of any one of claims 1 to 4 wherein said separation chamber (24) being sized to fit between an upper wall of the evaporator and the heat exchange tubes (10), said separation chamber (24) being attached for support to an upper wall (28) of the evaporator.

Inventors: BOSTJANCIC JOSEPH J, BOSTJANCIC, JOSEPH J.
Applicants: RESOURCES CONSERVATION CO, RESOURCES CONSERVATION COMPANY
Application year: 1989
Main classification: F28F19
IPC codes: B01D1, F28F19, C02F1