Wastewater treatment by evaporation and fouling prevention using cleaning particles

Abstract

The present invention is directed to a process for evaporating a wastewater stream comprising a mixture of inorganic and organic matter, said process comprising passing the wastewater stream together with cleaning particles through a shell-and-tube heat exchanger of a forced circulation evaporator in order to reduce scaling on the internal surfaces of the heat exchanger through which the wastewater stream flows. In another aspect, the present invention relates to a forced circulation evaporator comprising a fluidized bed heat exchanger suitable for carrying out the process according to any one of the preceding claims, said evaporator being located after a chemical process plant, preferably after a dye manufacturing plant, which produces wastewater comprising a mixture of inorganic and organic matter.

Description

[Technical field]

[0001] The present invention is in the field of process wastewater treatment. In particular, the present invention is directed to a process and an apparatus for evaporating wastewater streams containing a mixture of inorganic and organic matter, such as wastewater from a dye production process. [Background technology]

[0002] In various chemical production processes, wastewater streams are obtained that contain a mixture of organic and inorganic matter. They may originate from upstream processes, such as extraction and washing or addition of acidic or basic solutions to neutralize the reaction liquid, or from reaction liquids that contain, for example, organometallic compounds. Also, various wastewater streams may be combined for further combined processing, such as concentration by evaporation of water, and finally deposition of dry material, for example after a post-processing step.

[0003] Forced circulation evaporators are widely used to treat aqueous wastewater streams, especially when the liquid is known to foul the heat transfer surfaces. Forced circulation evaporation involves the use of both one or more heat exchangers and one or more flash separation units with circulating water to avoid boiling in the heat exchanger tubes, which would increase fouling, and removing the vapor by flashing after the heat exchanger. Such heat exchangers are generally of the shell-and-tube type, comprising a shell through which a bundle of tubes passes, the shell generally being provided with an inlet and an outlet for condensing water vapor on the shell side of the tubes, the tubes being arranged to pass a recirculating process liquid to receive the heat of condensation via the inner surface of the tubes.

[0004] A typical waste treatment scheme involving multi-effect evaporation is shown in Figure 1. After pre-treatment of the wastewater in a pre-treatment plant, the water stream is concentrated in a multi-effect evaporator plant so that the wastewater stream can finally be dried in a post-treatment step such as a spray dryer plant.

[0005] Generally, evaporators, including those of the forced circulation type that apply shell-and-tube heat exchangers, have the disadvantage of fouling the inner surfaces of the heat exchanger through which the wastewater stream flows. Fouling is generally caused by deposition, such as precipitation or crystallization of components in the wastewater stream. The formation of a fouling layer has a negative effect on the operating efficiency of the forced circulation evaporator. For example, a multiple-effect forced circulation evaporator (MEE) plant operated to treat wastewater streams from a dye production process is usually operated for a period of 12 to 15 days and then needs to be shut down for 5 to 7 days for cleaning, generally by high-pressure water jet cleaning. In most cases, cleaning is very difficult and some layer still remains. After cleaning, the capacity of the plant is only 80% on the first day and drops to 50% before the next cleaning. Companies use various solutions such as chemical addition to reduce crystallization, chemical cleaning, high-pressure water jet cleaning, and duplication of equipment to deal with fouling. However, these solutions cause downtime of the equipment and additional costs in terms of CAPEX and OPEX.

[0006] Fouling in forced circulation evaporators is particularly problematic in evaporators that treat wastewater streams containing a mixture of inorganic and organic matter, as such streams result in the formation of fouling layers that are known to be highly adhesive and hard. The fouling layers in such cases are generally insoluble in water and even in 10% by weight hydrochloric acid solutions. It is believed that the hardness and adhesion of this fouling layer is at least partly due to its chemical composition, and in particular the presence and nature of organic matter. Organic matter, especially from dye production processes, can be thermally reactive compounds and therefore can change properties when heated resulting in stronger adhesion and increased hardness. Despite the long-standing need for solutions, particularly in this field, no satisfactory method has been provided to fully address the fouling problem. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 8,075,740 [Patent Document 2] WO 02 / 25201 [Non-patent literature]

[0008] [Non-Patent Document 1] Vallespin et al., “Heat Exchanger Fouling and Cleaning”, (2017), pp. 293-298 [Non-Patent Document 2] Klaren et al., “Self-Cleaning Fluidized Bed Heat Exchangers for Severely Fouling Liquids and their Impact on Process Design, Heat Exchangers-Basic Design Applications,” edited by Mitrovic, J. Summary of the Invention [Means for solving the problem]

[0009] The inventors have surprisingly found that inorganic and organic fouling build-up on the internal surfaces of a heat exchanger through which a wastewater stream flows can be reduced by passing said wastewater stream through the heat exchanger together with cleaning particles. Advantageously, the invention provides a self-cleaning principle, thereby eliminating the need to shut down the process to perform a separate cleaning.

[0010] Therefore, the present invention is directed to a process for evaporating a wastewater stream containing a mixture of inorganic and organic matter, said process comprising passing the wastewater stream together with cleaning particles through a shell-and-tube heat exchanger of a forced circulation evaporator to reduce fouling build-up on the internal surfaces of the heat exchanger through which the wastewater stream flows.

[0011] Fouling mitigation using cleaning particles has been previously performed in condensers using seawater or brine from oil and gas extraction processes, where scaling of inorganic substances such as calcium carbonate has been observed (see, for example, J. Am. Soc. 1999, 143: 1111-1123, and U.S. Patent No. 5,311, 1999, which are incorporated herein in their entirety). However, these fouling layers are known to be less hard and adherent than mixed inorganic and organic fouling layers, and therefore the mild scraping effect of the cleaning particles is sufficient to clean the fouling layers from the surface, even when layers of 3 mm or more in thickness have formed.

[0012] However, the inventors have surprisingly found that in the wastewater treatment according to the invention, the dirt deposits can be particularly effectively reduced if the scraping off of the dirt deposits is carried out at an early stage, i.e. before a thick layer of contamination is formed. Thus, even in cases where the mild scraping effect of the particles is insufficient to remove a hard, adherent dirt deposit, said dirt can be unexpectedly reduced by preventive or initial cleaning. Therefore, in a preferred embodiment of the invention, the wastewater flow is passed through the heat exchanger together with the particles before the layer thickness of the dirt deposit grows to more than 3 mm, preferably before it grows to more than 1 mm, more preferably before it grows to more than 0.5 mm. In particular, the wastewater flow is passed through the heat exchanger together with the particles from the beginning of the process in order to remove the dirt on the surface of the tubes from the beginning, i.e. when the dirt starts to accumulate.

[0013] Without being bound by theory, it is believed that the particular hardness and adhesiveness of scaling originating from wastewater streams containing inorganic and organic matter is at least partially due to further reaction of the organic matter at high temperatures on the surfaces of the heat exchanger. Typical operating temperatures of heat exchangers on the shell side, i.e. the side where steam condenses on the outside of the tubes of the heat exchanger, are 110°C to 130°C. The formation of a fouling layer at these temperatures may cause the organic matter to react, for example by polymerization, to form a hard, adhesive material. It is believed that the inclusion or incorporation of inorganic matter further hardens the scaling. It is believed that, according to the present invention, the reaction of the organic matter can be limited or even prevented by cleaning the surfaces with cleaning particles at an early stage.

[0014] The invention proves to be particularly suitable for the treatment of wastewater streams originating from dye manufacturing processes. Dyes such as reactive, disperse, acid, vat, basic, sulphur and direct dyes generally contain thermally reactive organic compounds, in particular aromatic compounds and / or compounds with reactive groups. These dyes or molecular parts thereof may be present in wastewater streams and cause scaling. The inorganic matter generally comprises one or more salts selected from the group consisting of ammonium, potassium, sodium, magnesium and calcium salts, preferably from the group consisting of calcium carbonate, calcium sulphate, sodium chloride, sodium sulphonate, magnesium sulphonate and magnesium carbonate. It is therefore believed that the combination of thermally reactive organic matter and inorganic matter results in a particularly hard and adherent scaling due to the inclusion of the inorganic matter in the organic reaction products formed at high temperature scaling. The thermal reactivity of the compounds should be evaluated in the operating conditions according to the invention, thus at the steam temperature for example at which the wastewater stream is heated.

[0015] The present invention, especially the early stage cleaning and scaling reduction, has the advantage that the process is suitable for a variety of wastewater stream compositions. In a typical chemical production facility, the composition of the wastewater can vary greatly due to changes in the chemical processes carried out in the plant and changes in intermediate treatment steps of the wastewater stream. The present invention is a robust and reliable method since it does not use specific chemical products for only one specific type of scaling. In addition, even very tough and adherent scaling can be reduced. The present invention is particularly suitable for reducing scaling that is not soluble in 10% by weight hydrochloric acid.

[0016] The invention is also suitable for evaporation of wastewater with high chemical oxygen demand (COD) and / or biological oxygen demand (BOD). Thus, in certain embodiments of the invention, the wastewater stream has a COD of up to 1,000 mg / L, but the COD values ​​possible with this process can be greater, such as up to 10,000 mg / L, or even up to 100,000 mg / L, or even more. No lower limit for the COD value is necessary, but since the wastewater stream contains organic matter, the COD will always be greater than 0 mg / L. A feed of wastewater from a chemical plant (e.g., a dye plant) may have a COD of 25,000 mg / L to 30,000 mg / L. Typical CODs for wastewater streams are from about 5,000 mg / L to about 100,000 mg / L. It is believed that higher COD levels may increase the hardness and adhesion of the contaminated layer over time, especially when the organic matter includes thermally reactive organic matter.

[0017] The washing particles may be fed to the tubes from a fluidized bed of particles, for example a fluidized bed maintained at the lower inlet of the heat exchanger. The fluidized bed may be maintained at a quasi-steady state so that the washing particles exit the fluidized bed, flow with the wastewater stream through the tubes, and are later returned to the fluidized bed. The washing particles may be separated from the wastewater stream, for example using a gravity separator connected to the upper outlet of the heat exchanger, and recirculated separately from the wastewater stream by using gravity to return the wastewater stream through a downcomer to the lower inlet. The wastewater stream free of the washing particles may then pass through a flash chamber, and the liquid portion of the wastewater stream may be returned to the heat exchanger.

[0018] Typical dimensions of cleaning particles suitable for the present invention are 1 mm to 5 mm, about 3 mm for ceramics and 2 mm for stainless steel, preferably 2 mm to 4 mm. The cleaning particles can be based on various materials such as metals and ceramics. For cost-effectiveness, ceramic cleaning particles are preferred. However, after detailed evaluation of various particle materials, the inventors have surprisingly found that particularly good results are obtained when said cleaning particles comprise stainless steel particles (i.e. particles based on, containing or consisting essentially of stainless steel), especially in terms of low erosive properties of the particles. Although stainless steel particles are generally less economically attractive than ceramic particles, it has been found that the better resistance to erosion of stainless steel particles leads to an overall preference for stainless steel particles, due to the reduced scaling of the hard adhesive properties according to the present invention.

[0019] The cleaning particles are typically present in the flow through the heat exchanger at 2 vol% to 10 vol%, based on the total volume of the wastewater stream containing the cleaning particles. In the art, the volume fraction of fluid per volume of the heat exchanger tube through which the wastewater flows is commonly referred to as the porosity of the wastewater stream. The remainder of this volume is occupied by particles. Thus, the wastewater streams of the present invention typically have a porosity of 90% to 98%, preferably 92% to 96%, such as about 94%. This porosity has been found to be an excellent balance between effective scaling mitigation and pressure drop across the total system.

[0020] Another process parameter that has been found to be a balance between effective scaling mitigation and preventing or limiting equipment erosion is the flow velocity of the wastewater stream through the heat exchanger. While very high flow velocities are not necessary, lower flow velocities have generally been found to be less effective at mitigating scaling. Therefore, a flow velocity of about 0.5 m / s to about 0.7 m / s is preferred.

[0021] The cleaning particles in the wastewater stream passing through the tubes apply an abrasive action, thereby preventing the build-up of dirt deposits on the inside of the tubes. The steam used in the shell side of the first effect of the heat exchanger may conventionally be at a temperature of 110°C to 130°C. It has been found that this cleaning is so effective that the temperature of the steam supplied to the first effect may be significantly higher than in the prior art without the build-up of a fouling layer, for example 130°C to 160°C. This higher temperature allows the required heat transfer area to be significantly reduced, thus making the MEE plant more compact. Thus, the steam temperature according to the present invention may be from about 110°C to about 160°C, preferably from 130°C to about 160°C.

[0022] Steam at this temperature at the flow rates described above will increase the temperature of the waste water by typically 2° C. to 4° C. Thus, since the temperature increase is relatively small, boiling of the waste water in the heat exchanger tubes is suppressed by the water column pressure present, for example, by the arrangement involving the particle separator and the flash chamber. The evaporator can thus provide a concentration of the waste water of about 25% to 40% by weight.

[0023] Another aspect of the invention is directed to a forced circulation evaporator placed after a chemical process plant producing wastewater containing a mixture of inorganic and organic matter. The forced circulation evaporator may be placed so as to be able to treat such wastewater leaving the plant directly and / or to treat wastewater after pre-treatment or after intermediate treatment. In a preferred embodiment, the forced circulation evaporator is placed after a dye manufacturing plant. Such placement after a chemical process plant, in particular after a dye manufacturing plant, may be without pre-treatment but advantageously includes pre-treatment equipment, e.g. sedimentation and filtration devices such as pressure filters. [Brief description of the drawings]

[0024] [Figure 1] Common waste treatment systems are presented, including multi-effect evaporation. [Diagram 2] 1 shows a schematic diagram of the general configuration of a forced circulation evaporator according to the invention. [Diagram 3] An evaporator (100) in accordance with a preferred embodiment of the present invention is shown. [Figure 4] Computational fluid dynamics (CFD) analysis results are presented for the flow distribution and velocity of heated effluent liquid with ceramic 3 mm diameter spherical washing particles in a particle separator. [Diagram 5] 1 shows a schematic representation of an MEE according to the present invention. [Figure 6] The evaporative capacity of the plant in its original configuration and after retrofitting the first effect with a self-cleaning fluidized bed heat exchanger is shown as a function of operating time. [Figure 7] The change in evaporation capacity upon further operation of the plant is shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention will now be described more fully with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and / or cross-sectional illustrations of possible idealized embodiments and intermediate structures of the invention. Like numbers refer to like elements throughout the description and drawings. Relative terms, as well as their derivatives, should be construed as referring to the orientation then being described, or shown in the drawings under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation, unless specifically stated otherwise.

[0026] In Fig. 2, a general configuration of a forced circulation evaporator according to the invention is shown diagrammatically. The shell-and-tube heat exchanger (1) is arranged upright, in particular vertically (allowing suppression of initial water boiling by the water column pressure) and comprises an inlet compartment (11) at the bottom adapted to maintain a uniform fluidized bed of cleaning particles, so that a part of the particles of the fluidized bed can leave the fluidized bed and flow upwards through the heat exchanger tubes towards the outlet (12) at the top of the heat exchanger. The cleaning particles in the wastewater stream passing through the tubes can then apply a scouring action to prevent the accumulation of dirt deposits on the inside of the tubes. The heat exchanger further comprises an inlet (13) for allowing the inflow of the wastewater stream (WW). The heat exchanger further comprises an inlet (14) and an outlet (15) for water vapor. In this example, the temperature of the water vapor at the inlet (14) can be between 130°C and about 160°C. The shell-and-tube heat exchanger therefore preferably comprises a fluidized bed heat exchanger. Examples of fluidized bed heat exchangers suitable for the present invention are described in US Pat. No. 5,393,311, US Pat. No. 5,393,625, and US Pat. No. 5,493,625, which are incorporated herein in their entireties.

[0027] In FIG. 2, the outlet 12 is connected to a solid-liquid separator (2), e.g. a gravity separator (see below), configured to separate at least most of the washing particles from the heated waste water (HW) entering through the solid-liquid separator inlet (21). The solid-liquid separator particle outlet (22) and liquid outlet (23) allow a concentrated washing particle stream (CP) and a heated waste water liquid (HL), respectively, to exit the solid-liquid separator. The concentrated washing particle stream is returned under gravity through a downcomer to the chamber 11 by a control stream (4'), which is a small part of the main liquid circuit. The particle recycle stream replenishes the fluidized bed at the inlet and keeps it in a quasi-steady state. The liquid outlet (23) of the solid-liquid separator is connected to the flash chamber (3) via a flash chamber inlet (31). The flash chamber further comprises a vapor outlet (32) and a liquid outlet (33). A vapor outlet 32 ​​allows the discharge of a discharge vapor (DV) from the flash chamber, and a liquid outlet 33 allows a concentrated waste water stream (CW) to leave the flash chamber and enter a liquid circuit (4) to which a waste water liquid outlet (41) allows the discharge of a concentrated waste water liquid (DL) from the forced circulation evaporator and a waste water liquid inlet (42) allows the discharge of fresh waste water (FW) to the forced circulation evaporator are connected. A circulation pump device (5) is connected to the liquid circuit (4) and is configured to pump the concentrated waste water stream (WW) to the inlet 13 of the heat exchanger. Furthermore, the particle outlet (22) is connected to the fluidized particle distribution compartment (11) via a particle inlet (16) so that a concentrated particle stream (CP) can be recirculated into the heat exchanger.

[0028] In a typical embodiment of the forced circulation evaporator, the fluidized bed heat exchanger is connected to a solid-liquid separator to separate the washing particles from the heated wastewater to allow separate recirculation of the washing particles to the heat exchanger. To achieve this purpose, various solid-liquid separators may be suitably employed. For example, mesh plates, hydrocyclones and gravity separators are suitable. However, the inventors have found that for the present invention, mesh plates are generally less preferred since they may have problems with sieve clogging due to fouling. This is surprising since mesh plates have been successfully used in conventional evaporator systems used, for example, to treat saltwater from oil and gas extraction processes. It has been found that gravity separators may be particularly suitable for the present invention. Although hydrocyclones would generally be preferred due to their small dimensions, they are in practice less preferred for use in the present invention due to erosion reasons. The solid-liquid separator of the present invention generally needs to separate about 20 kg / s to 30 kg / s of solid particles from the liquid. To achieve this purpose, gravity separators have been found to be optimal.

[0029] Therefore, the evaporator (100) of the present invention, in a preferred embodiment shown in FIG. - a fluidized bed heat exchanger (1) configured to receive a particle-laden wastewater stream (LW) and to heat the wastewater stream together with the cleaning particles to form a heated wastewater stream (HW) containing said cleaning particles; - a solid-liquid separator (2) configured to receive a heated wastewater stream (HW) containing said cleaning particles and to separate a concentrated cleaning particle stream (CP) from a heated wastewater liquid (HL); - a flash chamber (3) configured to receive a heated drain liquid (HL) and to release water vapor (DV) and a concentrated drain stream (CW) into the liquid circuit (4); - a liquid circuit (4) connected to a waste liquid outlet (41) configured to discharge a concentrated waste water (CW) and to a waste liquid inlet (42) configured to direct fresh waste water (FW) into the liquid circuit (4) to form a waste water flow (WW); Equipped with the solid-liquid separator comprises a gravity separator.

[0030] The computational fluid dynamics (CFD) analysis results of the flow distribution and velocity of the heated wastewater liquid with the ceramic 3 mm diameter spherical washing particles in the particle separator are shown in Figure 4. It is evident from the results that above the inlet, the flow velocity is below the settling velocity of the ceramic washing particles, and any particle carryover to the flash chamber can be essentially avoided.

[0031] For thermodynamic efficiency or good steam economy, the steam generated in the forced recirculation evaporator can be reused. In particular, in one embodiment not shown, the steam generated in the flash chamber can be (re)compressed in a single effect forced circulation evaporator using, for example, the technology of mechanical vapor compression (MVC) or mechanical vapor recompression (MVR) and fed to the steam side of the shell at the steam inlet (14). The steam generated in the flash chamber can also be reused in further stages of a multiple effect evaporator (MEE) plant, which includes a series of forced circulation evaporators operating in subsequent stages, in order to utilize the generated steam more efficiently. In particular, the steam generated in the flash chamber can be reused in the steam outlet (32) and fed to the steam inlet (14) on the shell side of a further shell-and-tube heat exchanger.

[0032] Thus, the forced circulation evaporator described herein above can be a suitable part of an MEE plant. The inventors have surprisingly found that in an MEE plant, using the forced circulation evaporator and the process according to the invention, it may be sufficient to pass the wastewater stream together with the cleaning particles only through the shell-and-tube heat exchanger in the first effect (i.e. the first forced circulation evaporator in the MEE plant). Surprisingly, the active mitigation of scaling in the first effect also mitigates or strongly reduces these foulings, even if the subsequent effects are based on conventional heat exchangers and do not have a fluidized bed heat exchanger. Therefore, another aspect of the invention is an MEE plant, in which at least one effect only, preferably the first effect, comprises a fluidized bed heat exchanger as described herein, and the subsequent effects may comprise conventional heat exchangers without cleaning particles.

[0033] In general, if only fouling reduction is required, it would be advantageous to equip the MEE plant or to carry out a multi-effect evaporation process with a fluidized bed of washing particles as described herein. In other words, if it is not required to equip certain effects of the MEE plant or to carry out a multi-effect evaporation process with a fluidized bed of washing particles to reduce fouling in these effects, since no fouling occurs, it is preferable to omit the fluidized bed of washing particles from these effects. This reduces investment costs. In fact, the surprising finding mentioned above that actively reducing scaling in the first effect also reduces fouling in subsequent effects of the MEE plant is not necessarily limited to the first and subsequent effects. One reason for this is that it is not necessarily the first effect where the most fouling may occur. For example, in an MEE process applying a forward feed, it may be the last effect where the concentration of the mixture containing organics and inorganics is highest (but the temperature is lowest), which is a situation where fouling may be dominant in some cases. It can therefore be seen that in general, application of a fluidized bed of cleaning particles according to the present invention can be advantageous in that one effect is equipped with cleaning particles rather than all effects.

[0034] Therefore, one aspect of the present invention is directed to a multiple-effect evaporation process for evaporating a wastewater stream containing a mixture of inorganic and organic matter, said process comprising a plurality of forced circulation evaporator effects in series, some but not all of which include subsequently passing the wastewater stream along with cleaning particles through a shell-and-tube heat exchanger of the forced circulation evaporator to reduce fouling on the internal surfaces of the heat exchanger through which the wastewater stream flows.

[0035] Another aspect of the invention is a multiple effect evaporator (MEE) plant comprising a plurality of forced circulation evaporator effects in series, consistent with the MEE process described above, where one or more, but not all, of the effects comprise a shell-and-tube heat exchanger having a fluidized bed of scrubbing particles configured to pass through the tubes with the wastewater stream.

[0036] Typically, when wash particles are used in an effect, the wash particles are recirculated while the concentrated effluent stream continues to the next effect. Separation of the wash particles and the heated or concentrated effluent stream may be performed, for example, in a solid-liquid separation device as described herein.

[0037] In certain MEE processes, fouling, if not treated, may occur primarily in the effect with the most extreme operating temperature. Thus, in certain embodiments, it is preferred that those effects (e.g., 1, 2, 3, or more than 3 effects) having the most extreme operating temperature for all effects of the multiple-effect evaporation process include the step of passing the wastewater stream with the scrubbing particles through a shell-and-tube heat exchanger of a forced circulation evaporator. By most extreme operating temperature herein is meant the highest or lowest temperature to which the respective wastewater stream is heated as it passes through the heat exchanger.

[0038] Similarly, in certain MEE processes, fouling, if not treated, may occur primarily in effects with higher concentrations of inorganic and organic mixtures. Therefore, this parameter may also be considered to determine which effects to equip. Alternatively or in addition, the operating temperature is considered to be in accordance with the previous paragraph. Therefore, in one embodiment, those effects (e.g. 1, 2, 3, or more than 3 effects) with the highest concentration of said mixtures in the wastewater stream for all effects of a multiple-effect evaporation process may preferably include said step of passing the wastewater stream with the cleaning particles through a shell-and-tube heat exchanger of a forced circulation evaporator.

[0039] FIG. 5 shows a schematic of an MEE according to the invention. Although FIG. 5 includes four effects (100, 200, 300, 400), the invention is not necessarily limited to this number of effects. The first effect (100) comprises a forced circulation evaporator with flowing cleaning particles as shown in FIG. 3. For clarity, reference numbers and flow labels have been omitted. The further effects in FIG. 5, namely the second effect (200), the third effect (300) and the fourth effect (400), are conventional forced circulation evaporators. As shown in FIG. 5, a condensate stream is provided from all heat exchangers. This condensate stream can optionally be used to preheat the incoming wastewater feed or can be fed to the shell of the heat exchanger of the next effect where it is rapidly evaporated to provide additional heat input. This may be called condensate flashing and can be used to improve energy efficiency. The steam produced in the last, or fourth, effect (400) is condensed in a condenser. A portion of the steam in the condenser can be used for additional preheating.

[0040] Also included in Figure 5 is an optional thermal compressor (TVR), whose primary purpose is to further improve efficiency. The TVR is a thermal compressor that can use fresh steam at a relatively high pressure and aspirate lower pressure steam to provide a steam mix that is at a higher pressure and temperature than the steam coming from the first effect, allowing it to be used as a heat source in the shell of the heat exchanger. The use of the TVR has (almost) the same effect on steam economy as adding an effect to the MEE.

[0041] The MEE according to the invention can be used with or without a TVR. Also, in the case of a single effect evaporator according to the invention, a TVR may or may not be used. If a TVR is not used, fresh steam is fed directly to the heat exchanger of the first effect. When a TVR is not used, the steam is usually reduced in pressure and temperature to keep the heat exchanger wall temperature limited in order to reduce the fouling tendency. Advantageously, with the technology according to the invention, a higher steam temperature can be used, since the higher fouling tendency is mitigated by the cleaning particles. A higher steam temperature allows a more compact plant to be built.

[0042] According to the present invention, the conventional heat exchanger in the forced circulation evaporator can be retrofitted with a fluidized bed heat exchanger, which allows easy adaptation of existing systems without incurring large costs.

[0043] As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly indicates otherwise. The term "and / or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprising" and / or "comprising" specify the presence of stated features but do not exclude the presence or addition of one or more other features.

[0044] Although each feature is described herein as part of the same or separate embodiment for clarity and conciseness of description, it will be understood that the scope of the invention may include embodiments having all or any combination of the described features.

[0045] The invention may be illustrated by means of the following examples. EXAMPLES

[0046] An existing conventional full-scale evaporator for treating wastewater from a dye manufacturing company was modified with a fluidized bed setup according to the present invention. The modification involved the addition of some specific components for the operation of a fluidized bed, such as an inlet channel for particle distribution and a gravity separator for separating the particles from the circulating fluid. As the first effect of the multiple effect evaporator (MEE) plant, the configuration shown in Figure 2 was obtained. The steam temperature into the first effect averaged 115°C. The maximum evaporation capacity reached with this plant was nearly 4.8 m 3 / h.

[0047] The composition of the wastewater in the infeed may, for example, have the following composition: COD levels of at least 20,000 mg / L; BOD levels of at least 9,000 mg / L; A TDS level of at least 40,000 mg / L; and K, Na, CaCO3 (hardness), Cl, and SO4, all greater than 1,000 mg / L.

[0048] A typical wastewater was found to have the following composition:

[0049] [Table 1]

[0050] Plant performance was monitored by daily charts of input feed, collected condensate, and product obtained from the MEE. Daily average evaporation rates were calculated from the daily charts. 3 The evaporation capacity is calculated in units of 1000000000 / h. Figure 6 shows the evaporation capacity of the plant in its original configuration and after retrofitting the first effect with a self-cleaning fluidized bed heat exchanger as a function of operating time. The data presented below was received from the plant operations team.

[0051] With the fluidized bed, during the first 400 hours of operation, the evaporation capacity has an increasing gradient compared to operation with the conventional heat exchanger. The heat exchanger tubes were cleaned using high pressure water jet cleaning before the retrofit, but not all tubes were 100% clean. Thus, over the course of these 400 hours, probably some part of the existing contamination layer was removed by the abrasive action of the fluidized bed. This hypothesis that the tubes were not 100% clean was confirmed during later inspections when tubes were found to have some contamination layer. This existing contamination layer also had a negative effect, as some of the loosened contamination layer clogged some tubes and later filled with particles.

[0052] Other effects were also inspected during the inspection. The tubes of effects 2, 3 and 4 were found to be cleaner than before the first effect modification. A possible explanation for the observed reduction in fouling in the other unmodified effects could be that the application of the fluidized bed reduced the size of the solids in the system due to the grinding effect of particle motion. The reduction in solids size helped reduce particulate fouling in the other effects of the MEE plant.

[0053] Taking into account the observations made during the inspection, the heat exchanger tubes were cleaned once more to remove the old layer and particles stuck in the tubes were removed. After the inspection and cleaning of the tubes, the heat exchanger was restarted. Figure 7 shows the evolution of the evaporation capacity during further operation of the plant.

[0054] Figure 7 confirms that the evaporation capacity of this plant remains stable over long periods of operation. The curve fits are only shown for the 400th hour of operation after cleaning the tubes, and the curves demonstrate that the evaporation is constant. [Explanation of symbols]

[0055] 1. Shell and tube heat exchanger, fluidized bed heat exchanger 2 Solid-liquid separator 3. Flash Chamber 4 Liquid circuit 4' Control flow 5. Circulation Pump Device 11 Entrance room 12 Exit 13 Entrance 14 Steam inlet 15 Water vapor outlet 16 Particle Inlet 21 Entrance 22 Particle outlet 23 Liquid outlet 31 Flash Room Entrance 32 Steam outlet 33 Liquid outlet 41 Drain liquid outlet 42 Drain liquid inlet 100 Evaporator, utility tank 200-function can 300-function can 400-function canister

Claims

1. A multiple-effect evaporation process for evaporating a wastewater stream containing a mixture of inorganic and organic matter, comprising a plurality of forced circulation evaporation effects in series, one or more of which, but not all of the effects, include passing the wastewater stream along with cleaning particles through a shell-and-tube heat exchanger of a forced circulation evaporator to reduce fouling on an internal surface of the heat exchanger through which the wastewater stream flows; With respect to all effects of the multiple-effect evaporation process, one or more effects, but not all, having the most extreme operating temperature and / or the highest concentration of the mixture in the waste water stream include the step of passing the waste water stream together with the washing particles through the shell-and-tube heat exchanger of the forced circulation evaporator. Multiple effect evaporation process.

2. 2. The process of claim 1, wherein only one effect includes the step of passing the wastewater stream with the cleaning particles through the shell-and-tube heat exchanger of the forced circulation evaporator.

3. At least a first effect of the multiple-effect evaporation process includes the step of passing the wastewater stream together with the cleaning particles through the shell-and-tube heat exchanger of the forced circulation evaporator.

2. The process of claim 1.

4. 4. The process of claim 1, wherein the wastewater stream is passed through the heat exchanger together with the cleaning particles from the start of the process to remove fouling as it begins to build up on the surfaces of the tubes.

5. 5. The process of claim 1, wherein the organic matter comprises a thermally reactive organic compound.

6. 6. The process of any one of claims 1 to 5, wherein at the start of the process the wastewater stream has a Chemical Oxygen Demand (COD) of 5,000 mg / L to 100,000 mg / L.

7. 7. The process of claim 1, wherein the inorganic matter comprises one or more salts selected from the group consisting of ammonium salts, potassium salts, sodium salts, magnesium salts and calcium salts.

8. 8. The process of claim 1 , wherein the wastewater stream comprises wastewater from a dye production process.

9. The process of any one of claims 1 to 8, wherein the cleaning particles comprise stainless steel particles of a size between 1 mm and 5 mm.

10. 10. The process of any one of claims 1 to 9, wherein the effluent exiting the heat exchanger is directed to a solid-liquid separation device to provide a stream of heated effluent liquid and concentrated washed particles.

11. 11. The process of any one of claims 1 to 10, wherein the heated effluent stream exiting the heat exchanger is directed to a solid-liquid separation device to provide a heated liquid stream and a concentrated particle stream, the heated liquid stream is directed to a flash chamber to produce a vapor blowout stream and a liquid blowout stream, and at least a portion of the vapor blowout stream and / or liquid blowout stream are directed to a further forced circulation evaporator step as part of a multiple effect evaporation process.

12. 12. The process of any one of claims 1 to 11, wherein the waste water stream flows through the heat exchanger at a flow velocity of from 0.5m / s to 0.7m / s.

13. 13. The process of any one of claims 1 to 12, wherein the wastewater stream with particles has a porosity of 90% to 98%, porosity being defined as the volume fraction of fluid per volume of the heat exchanger tubes through which the wastewater stream with particles flows.

14. 14. The process of any one of claims 1 to 13, wherein the heat exchanger is heated by supplying steam having a temperature of from 110°C to 160°C.

15. A multiple effect evaporator (MEE) plant suitable for carrying out the process of any one of claims 1 to 14, comprising a plurality of forced circulation evaporator effects in series, one or more, but not all, of which comprise a shell-and-tube heat exchanger having a fluidized bed of washing particles arranged to pass through the tubes with a wastewater stream; for all effects of the multiple effect evaporator plant, one or more of the effects adapted to carry out the process having the most extreme operating temperature and / or one or more of the effects adapted to receive the highest concentration of inorganic and organic matter in the wastewater stream comprise a fluidized bed of the washing particles; Multiple effect evaporator (MEE) plants.

16. A multiple effect evaporator (MEE) plant as described in claim 15, wherein only one effect of the multiple effect evaporator plant is provided with a fluidized bed of the cleaning particles.

17. The method of claim 1, wherein at least a first effect of the multiple effect evaporator plant comprises a fluidized bed of the cleaning particles.

16. A multiple effect evaporator (MEE) plant according to claim 15.

18. One or more of the evaporators comprises a shell-and-tube heat exchanger having a fluidized bed of scrubbing particles, the shell-and-tube heat exchanger comprising: - a fluidized bed heat exchanger (1) configured to heat a waste water stream (WW) together with cleaning particles to form a heated waste water stream (HW) containing said cleaning particles; a solid-liquid separator (2) adapted to receive the heated wastewater stream (HW) containing the cleaning particles and to separate a concentrated cleaning particle stream (CP) from a heated wastewater liquid (HL); a flash chamber (3) adapted to receive said heated effluent liquid (HL) and to release water vapor (DV) and a concentrated effluent stream (CW) into the liquid circuit (4); - said liquid circuit (4) configured to discharge a concentrated waste water stream (CW) and to direct a fresh waste water stream (FW) into said liquid circuit (4) to form a waste water stream (WW); Equipped with The solid-liquid separator comprises a gravity separator; 18. A multiple effect evaporator (MEE) plant according to any one of claims 15 to 17.

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