Water recycling system for process water from a hydrothermal reactor

Abstract

The present inventive concept relates to a process for recovering process water obtained from a hydrothermal reactor. The process comprises the steps of: a) providing a process water stream obtained from a hydrothermal reactor; b) pressurizing the process water stream to form a pressurized stream; c) heating the pressurized stream in at least one heat exchanger to form a heated stream; d) partly evaporating the heated stream to separate water from undesired contaminants to form a concentrated effluent stream and a vapour stream; e) condensing the vapour stream to form a recycle water stream, and f) recycling the recycle water stream back to a water storage tank for reintroduction into the hydrothermal reactor.

Description

[0001] WATER RECYCLING SYSTEM FOR PROCESS WATER FROM A HYDROTHERMAL REACTOR

[0002] Technical field

[0003] The present invention relates to a process for recovering process water obtained from a hydrothermal reactor. The present invention further relates to a system for recovering process water obtained from a hydrothermal reactor.

[0004] Background

[0005] Refineries are experiencing a notable shift as they increasingly embrace the use of alternative feedstocks rather than relying on petroleum. This shift is driven by the numerous advantages offered by renewable, waste, and low-cost feedstocks in the production of hydrocarbon fuels and chemicals. Firstly, renewable feedstocks can be replenished, and their usage does not deplete finite resources and does not contribute to the net increase of carbon dioxide in the atmosphere. Secondly, utilizing waste streams reduces the environmental impact by diverting materials from landfills and reducing greenhouse gas emissions. Moreover, low-cost feedstocks offer cost competitiveness. Overall, the use of these alternative feedstocks supports the transition towards a more sustainable and circular economy, promoting energy and resource efficiency.

[0006] Hydrotreated vegetable oil (HVO) and hydroprocessed esters and fatty acids (HEFA) are types of renewable fuel made from vegetable oil or animal fat. It has similar properties to diesel and aviation fuels, including a high energy density and are compatible with existing engines, infrastructure, and distribution systems. They are considered sustainable options for reducing greenhouse gas emissions in the transportation sector. Vegetable oil or animal fat typically contain phospholipid compounds or complexes, herein referred to as phospholipids. The existence of phosphorus (P) poses two significant issues in the refinery. Firstly, phosphorus acts as a catalyst for coke formation in fired furnaces and heat exchangers, which leads to increased downtime for decoking and maintenance. Secondly, its deactivating effect on hydroprocessing catalysts results in frequent and expensive catalyst replacements, as well as an increased environmental footprint of the production process.

[0007] Other contaminants that need to be removed are metals such as Sodium (Na), Calcium (Ca), Magnesium (Mg), Potassium (K), and Iron (Fe) that also have a deactivating effect on the catalysts. Halides such as fluoride (F) or chloride (Cl) need to be removed to prevent corrosion of processing equipment. Traces of solid contaminants need to be removed to prevent blockage in process equipment.

[0008] Thus, before renewable, waste, and low-cost feedstocks can be utilized in a refinery, a cleanup, i.e. reducing the inorganic and organic contaminants in the feedstocks is necessary. Examples of processes that can be used are chemical degumming, desalting processes, and other chemical extraction or thermal processes.

[0009] A new cleanup process is the hydrothermal cleanup (HCU) as disclosed in WO2016 / 123198. The hydrothermal cleanup process, hereafter only referred to as the HCU process, is a continuous flow process that can be co-located with a conventional refinery. Very short, the HCU process comprises; combining a feedstock having contaminants with a water feed to form a contaminated feedstockwater mixture; feeding the mixture into a hydrothermal reactor, wherein the mixture is subject to conditions that do not cause conversion of the feedstock; maintaining the conditions of the mixture such that inorganic contaminants are liberated from the feedstock in the mixture without feedstock conversion; and separating an effluent of the reactor into an aqueous stream and a product stream having inorganic contaminants.

[0010] The HCU process can reduce the concentrations of salts, metals, and minerals in the feedstock by more than 95% and eliminate the need for vacuum distillation to separate or concentrate products. By using elevated temperatures oils and water becomes miscible which allows for rapid desalting and rapid separation of water and organic phases with or without demulsifying agents. The short residence time of 1 to 10 minutes leads to high capacity without a large footprint in terms of space.

[0011] The HCU process yields two main effluents; a cleaned-up feedstock with reduced concentrations of contaminants, and an aqueous effluent with undesired contaminants from the treated feedstock. The HCU process has a high need of a continuous supply of clean water. However, incorporating a water recovery system to recycle the water for the HCU has major implications; the aqueous effluent must be treated before it is discharged into the environment to comply with strict effluent standards which is cumbersome since traditional processes used for handling wastewater at a refinery, such as gravimetric separation or biological treatment, are often not well-adapted to changing wastewater conditions and the treatment of large volumes of wastewater. Moreover, reducing water consumption is important due to water scarcity, energy conservation, ecosystem preservation, and cost savings.

[0012] Gravimetric separation relies on the differences in density between different components to separate them. It involves the sedimentation of particles or the precipitation of salts by adjusting the pH or adding a precipitating agent, followed by the physical separation of the precipitate through filtration or centrifugation. A drawback with gravimetric separation is a slower process compared to other treatment methods resulting in the need of large process equipment to handle large quantities of process water. To reduce the time centrifuges or other mechanical equipment can be used. However, these machines require significant energy input and might have low efficiency for small particles which may remain suspended. Moreover, gravimetric separation is often not adaptable to changing wastewater conditions.

[0013] Biological treatment uses microorganisms, such as bacteria, fungi, and algae, to break down or absorb the contaminants in the wastewater. A drawback with biological treatment is slower process compared to other treatment methods resulting in the need of large process equipment to handle large quantities of process water. Moreover, microorganisms used in biological treatment are often sensitive to environmental conditions (for example temperature, pH, or toxic substances). The properties of the process water may change with the feedstock processed in the HCU process. In some cases, the growth of microorganisms during biological treatment can lead to biofouling, where the microorganisms form a layer or biofilm on surfaces like pipes or membranes. This can lead to reduced efficiency and increased maintenance requirements. Separation of the microorganism from the treated water is necessary. There is therefore a need for an improved process for the water recycling of an aqueous stream containing organic and inorganic compounds such as the process water from the HCU process treating renewable or non-renewable organic feedstocks, which yields a clean water stream meeting the requirements of the water feed for example for the HCU process.

[0014] According to a first aspect of the present invention, this and other objects are achieved by a process for recovering process water obtained from a hydrothermal reactor. The process comprises the steps: a) providing a process water stream obtained from a hydrothermal reactor; b) pressurizing the process water stream to form a pressurized stream; c) heating the pressurized stream in at least one heat exchanger to form a heated stream; d) partly evaporating the heated stream to separate water from undesired contaminants to form a concentrated effluent stream and a vapour stream; e) condensing the vapour stream to form a recycle water stream, and f) recycling the recycle water stream back to a water storage tank for reintroduction into the hydrothermal reactor.

[0015] It should be noted that the process may comprise the steps a) to f) in the order named.

[0016] The present invention is based on the understanding that by separating undesired contaminants from process water by evaporation, large quantities of contaminated process water may be recovered and the above-mentioned problem with high water and energy consumption during a hydrothermal process can be avoided.

[0017] The process according to the inventive concept has a small footprint in terms of space and makes it possible to handle varying and complex streams of process water from for the example an HCU process.

[0018] In contrast to distillation, evaporators are less energy-intensive due to less heat needed to heat the mixture. Further, in distillation complex mixtures may require multiple distillation stages for complete separation.

[0019] The process water stream of step a) may be a HCU process water stream. The process water stream of step a) may comprise less than 2 wt.% organic material, preferably less than 1 wt.% organic material.

[0020] The process water stream may comprise less than 5 wt.% glycerol, preferably less than 3 wt.% glycerol, and most preferably less than 2 wt.% glycerol.

[0021] The range of water to oil ratio in the process water stream of step a) may be between 10:1 and 1000:1.

[0022] The temperature of the process water stream of step a) may be between 60°C and 100 °C, preferably between 70°C and 90°C.

[0023] The method may comprise a step of I) equalizing the process water stream to form an equalized stream. An advantage of equalizing the process water stream after it is discharged from the hydrothermal reactor is that a consistent composition of the process water is formed before evaporation, which ensures a proper evaporation process. It also allows for intermittent separation and recirculation of small amounts of organic material, such as oil and fat, that have accompanied the process water stream.

[0024] The method comprises a step of b) pressurizing the process water stream to form a pressurized stream. The pressure of the pressurized stream may be between 1 to 5 bar, or between 2 to 4 bar. An advantage of pressurizing the process water before the heating in step c) is that the boiling point temperature is increased such that no premature evaporation is taking place before the separation vessel.

[0025] The method comprises a step of c) heating the pressurized stream in at least one heat exchanger to form a heated stream. The energy used for step c) may be recovered from the heat in the concetrated effluent stream and / or in the recycle stream. To this end, the process may further comprise the step of: g) feeding the recycle water stream to the at least one heat exchanger to heat the pressurized stream during step c). Step g) may be conducted between step e) and f).

[0026] The method comprises a step of d) partly evaporating the heated stream to separate water from undesired contaminants to form a concentrated effluent stream and a vapour stream.

[0027] An advantage of using evaporation as separation method is that it is cost effective and requires minimum of energy. Unlike some chemical separation methods, evaporation doesn't introduce additional chemicals or pollutants. The evaporation of step d) may be conducted by flash evaporator. To this end, the process may further comprise the step of: k) saturating the heated stream of step c) before step d).

[0028] The energy for the evaporation of step d) may be supplied by a Mechanical Vapour Recompression (MVR).

[0029] The process may further comprise the step of: h) anaerobic digesting the concentrated effluent stream to form a digestate and biogas.

[0030] The concentrated effluent stream may thus be used as a feedstock for biogas production and / or be recycled into agriculture as soil amendment or fertilizer.

[0031] The process may further comprise the step of: i) recovering treated feedstock in the process water stream between step a) and b) and returning the recovered treated feedstock to the hydrothermal reactor for treatment together with untreated feedstock. To this end, the system may comprise an oil recovery vessel.

[0032] The method comprises a step of e) condensing the vapour stream to form a recycle water stream. The energy obtained in step e) may be reused in step d) to partly evaporate the heated stream. Alternatively, if the concentrated effluent stream is partly evaporated a second time, the energy obtained in step e) may be reused in this second partial evaporation of the effluent stream, as will be described below.

[0033] The evaporation may be conducted in at least two, three or four consecutive evaporation stages. Stated differently, evaporation may be conducted in two, three, four or five consecutive evaporation stages. This means that, step d) and step e) may be repeated for the concentrated effluent stream before step f). Hereby, the method may comprise the step of j) partly evaporating the concentrated effluent stream to separate additional water from undesired contaminants in the concentrated effluent stream and to form a second vapour stream. An advantage of using more than one evaporation stage is that more recycle water can be obtained from the process water and the total solids content of the effluent stream can be increased. The total solid content of the concentrated effluent stream may be from 10 to 70 wt.%, preferably from 30 to 60 wt.%. The temperature of the heated stream obtained in step c) may be at least 75°C, preferably at least 90°C.

[0034] The method comprises a step of f) recycling the recycle water stream back to a water storage tank for reintroduction into the hydrothermal reactor. Hereby, the recycling water may be reused in the HCU process.

[0035] According to a second aspect of the present invetive concept, a system for recovering process water obtained from a hydrothermal reactor is provided. The system comprises: an evaporator feed pump configured to pressurize the process water stream and to form a pressurized stream; at least one heat exchanger configured to heat the pressurized stream and to form a heated stream; an evaporator package comprising at least one evaporator stage configured to separate water from undesired contaminants in the heated stream, the evaporator stage comprising a plate evaporator configured to saturate the heated stream and forming a saturated stream, and a separator vessel configured to separate the saturated stream into a vapour stream and a recycle water stream.

[0036] This aspect may exhibit the same or similar features and technical effects as the first aspect, and vice versa.

[0037] The term saturate in this context means heating to the boiling temperature.

[0038] The evaporator package may further comprises a recirculation pipe configured to recirculate the vapour stream to the plate evaporator to heat the heated stream.

[0039] The evaporator package may further comprise a recompression unit configured to increase the pressure of the vapour stream and to form a recompressed vapour stream that is fed to the plate evaporator of the at least one evaporator stage.

[0040] In other words, the evaporator package may further comprise a recompression unit configured to increase the pressure of the vapour stream and a recirculation pipe leading the recompressed vapour stream to the plate evaporator of the at least one evaporator stage. The plate evaporator may be a thin film evaporator or a flash evaporator. Non-limiting exampels of thin film evaporators are rising film evaporator and falling film evaporator.

[0041] The evaporator package may comprise at least two consecutive evaporation stages. Alternatively, the evaporator package may comprise at least three, or at least four consecutive evaporation stages.

[0042] The system may further comprise a recirculation pipe configured to recirculate the recycle water stream back to the at least one heat exchanger to heat the pressurized stream.

[0043] The system may further comprise an equalization tank configured to equalize the process water to form an equalized stream.

[0044] Brief of the

[0045] These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention.

[0046] Fig. 1 is a schematic illustration of a system for recovering process water obtained from a hydrothermal reactor according to at least one example embodiment of the inventive concept;

[0047] Fig. 2 is a schematic illustration of a system for recovering process water obtained from a hydrothermal reactor according to at least one example embodiment of the inventive concept;

[0048] Fig. 3 is a schematic illustration of hydrothermal treatment with water recycling and biogas production according to at least one example embodiment of the inventive concept;

[0049] Fig. 4 is a process flow diagram of a pilot HCU system;

[0050] Fig. 5 illustrates a methane yield curve in BMP of concentrate and cellulose as control; Fig. 6 is a schematic illustration of hydrothermal treatment with rising film evaporator according to at least one example embodiment of the inventive concept.

[0051] Detailed description

[0052] Fig. 1 illustrates a system 10 for recovering process water obtained from a hydrothermal reactor 1 according to at least one example embodiment of the inventive concept. The hydrothermal reactor 1 may be comprised in a HCU process which is characterised in that the process water has low organic content. The process water may for example comprise less than 2 wt.% organic material, preferably less than 1 wt.% organic material.

[0053] As shown in Fig. 1, heated and contaminated process water obtained from the hydrothermal reactor 1 is optionally discharged to an equalization tank 2. The temperature of the process water stream may typically be between 60°C and 100 °C, preferably between 70°C and 90°C. The primary function of the equalization tank 2 is to equalize or balance the characteristics and temperature of the incoming process water stream before it enters the evaporator package 5. The recovering process can be run at neutral or acidic conditions.

[0054] The system may optionally comprise an oil recovery vessel 13 to which a part of the equalized stream (or the process water stream if no equalizer is present) is fed. In the oil recovery vessel 13, recovered treated feedstock is separated from water. The recovered treated feedstock is fed back to the hydrothermal reactor and the water is fed back to the equalization tank 2. The equalized process water exits the equalization tank 2 at atmospheric pressure and enters into an evaporator feed pump 3 where it is pressurized to a pressure e.g. of between 1 to 5 bar, or 2 to 4 bar. It should be noted that the process water stream may be fed directly to the evaporator feed pump 3 without prior equalization.

[0055] The pressurized stream is then heated by a heat exchanger 4 (pre-heater) to a temperature of e.g. at least 75°C, preferably at least 90°C.

[0056] In Fig. 1 one heat exchanger is illustrated to heat the pressurized stream. However, the water recovery system may comprise two or three heat exchangers configured to heat the pressurized stream. The heated stream then enters the evaporator package 5. The evaporator package 5 comprises an evaporator stage 6 configured to separate water from undesired contaminants in the heated stream. In the evaporator package 5, the heated stream first enters a plate evaporator 7 configured to saturate the heated stream. The plate evaporator 7 has an inlet for the heated stream and an exit for the saturated stream. The plate evaporator 7 may be a thin film evaporator or a flash evaporator. Non-limiting examples of thin film evaporators include rising film evaporator and falling film evaporator.

[0057] The saturated stream that exits the plate evaporator 7 is fed to a separator vessel 8 configured to at least partly evapourate the saturated stream to separate water from undesired contaminants and to form a concentrated effluent stream (liquid phase) and a vapour stream (gaseous phase). The separator vessel 8 has an inlet for the saturated stream, an exit for the concentrated effluent stream, and an exit for the vapour stream.

[0058] The vapour stream obtained in the separator vessel 8 is fed via a recirculation pipe 9 to the plate evaporator 7 to heat the heated stream. In the plate evaporator 7, which then also acts as a condensor, the vapour stream is condensed to a recycle water stream.

[0059] The evaporator package 5 may optionally comprise a recompression unit 11 configured to increase the pressure of the vapour stream before it enters the plate evaporator 7. The function of the recompression unit 11 is to increase the energy of the vapour stream to enhance the partial evapourization of the heated stream in the plate evaporator 7.

[0060] The concentrated fluid effluent that exits the separator vessel 8 may have a total solids content of 10 - 70% w / w and a high ash content. The temperature of the concentrated effluent stream may be nearly 100°C.

[0061] The heat in both the concentrated effluent stream and the recycle water stream may be recovered. The heat in the concentrated effluent stream may be recovered in two steps from 99.8 °C and 3.44 bar to 65 °C and 3.00 bar.

[0062] The recycle water stream that exits the plate evaporator 7 may be fed to the heat exchanger 4 to preheat the pressurized stream before it enters the evaporator package 5. After passing the heat exchanger 4, the recycle water stream is, at least to a part, discharged to a water storage tank via a recycle water pipe 14 for reintroduction into the hydrothermal reactor 1.

[0063] The recycle water meets the requirements to be used in for example the HCU process.

[0064] The system according to the inventive concept may comprise more than one evaporation stage, e.g. two, three or four consecutive evaporation stages. Fig. 2 illustrates such a system 20 where the evaporation package comprises three consecutive evaporation stages. An advantage of using more than one evaporation stage is that more recycle water is obtained from the process water.

[0065] The set up for the process in Fig. 2 is the same as that of Fig. 1 except that the concentrated effluent stream that exits the first separator vessel 8a of the first evaporator stage 6a is fed to a second evaporator stage 6b and a third evaporator stage 6c. Similar to the first evaporator stage 6a, the streams that exit the second plate evaporator 7b are a second vapour stream and a second concentrated effluent stream. The second concentrated effluent stream obtained in the second plate evaporator 7b has a higher total solids content then the first concentrated effluent stream obtained in the first evaporator stage 6a. The second vapour stream obtained in the second evaporator stage 6b may be fed to the recompression unit 11 and any of the plate evaporators 7a, 7b, 7c to make use of energy provided thererin.

[0066] The concentrated effluent stream that exits the second separator vessel 8b of the second evaporator stage 6b is fed to a third evaporator stage 6c where it enters a third plate evaporator 7c. The streams that exit the third plate evaporator 7c are a third vapour stream and a third concentrated effluent stream. The concentrated effluent stream obtained in the third evaporator stage 6c has a higher total solids content then the concentrated effluent stream obtained in the second evaporator stage 6b. The vapour stream obtained in the third evaporator stage 6c may be fed to the recompression unit 11 and any of the plate evaporators 7a, 7b, 7c to make use of energy provided thererin. Example 1: Hydrothermal treatment with water recycling and biogas production

[0067] Example 1 is schematically illustrated in Fig. 3. A mix of contaminated waste fats was treated in the HCU process. Evaporation was used to separate the contaminants from the process water yielding recycle water. The concentrate of contaminants was aerobically digested to produce methane.

[0068] Untreated feeds tock

[0069] The untreated feedstock was a blend of 35.8% animal fats (AF), 53.7% used cooking oil (UCO), and 10.5% crude soybean oil (SBO) with chemical and physical properties listed in Table 1. The feedstock was filtered to 1pm.

[0070] Table 1. Properties of the untreated feedstock. Values are given as an interval for all batches used.

[0071] Hydrothermal cleanup

[0072] The hydrothermal cleanup was performed at 3HCU Pilot at System Applied Research Associates (ARA) Inc. in Panama City, Florida, as described in Figure 4. The system is configured for lipid feed rates from ~20 L / h to ~40 L / h. Actual operating conditions of HCU are provided in Table 2.

[0073] Table 2. Operating conditions of HCU.

[0074] Water recycle

[0075] Four aliquots of approximately 4 liters, totalling 16.02 kg of the process water from hydrothermal cleanup were concentrated using a rotavapour (Buchi R-220) with a 10-L flask. The bath temperature was set to 60 °C and the pressure was stepwise reduced from 110 to 91 mbar. Water and other volatile components were distilled at a rate of approximately 1 L / h. After concentrating the fourth portion, the empty containers that originally held the water stream, were rinsed with 3.5 L distillate warmed to 50 °C. The rinse water was also concentrated. All distillates were combined and analysed as one aliquot, here referred to as recycle water.

[0076] The properties of the process water and recycle water are described in Table 3. A total of 97 % of the process water was recovered as recycle water, while also achieving a significant reduction in undesired components.

[0077] Table 3. Properties of process water and recycle water in water recycling Example 1.

[0078] The recycle water's pH level remained acidic, although the concentration of citric acid in the water was low. It is worth noting that the low pH was caused by other organic acids, see Table 4. The hardness is measured according to Preem internal method, complexometric titration with EDTA and endpoint titration with Eriochrome Black T.

[0079] Table 4. Organic acids in recycle water in water recycling Example 1.

[0080] Table 5 demonstrates the successful removal of metals (present in concentrations above 0.1 mg / L) in the process water. The metal content of the process water is calculated as a mean of five samples. Total metals include Ag, Al, As, Ba, Ca, Cd, Cr, Cu, Fe, K, Mg, Na, Ni, Pb, Se, Si, V, and Zn. The concentration in the water was reduced by more than 99 % for calcium, iron, magnesium, potassium, sodium, and zinc. The concentration of silicon was reduced by at least 90%.

[0081] Table 5. Reduction of major metals (present in concentrations above 0.1 mg / L) in recycle water compared with process water in water recycling example 1. Samples analysed with Inductively Coupled Plasma - Mass Spectrometry (ICP-MS).

[0082] Anaerobic digestion

[0083] The concentrates from the four evaporations as well as the concentrate from the rinsing of the containers were combined and analysed as one aliquot, here referred to as the concentrate.

[0084] The concentrate may be used as a substrate for biogas production via anaerobic digestion. Biochemical methane potential (BMP) tests of the concentrate were carried out in 0.5 L flasks using the AMPTS II system from Bioprocess Control (Lund, Sweden). The inoculum was collected from the anaerobic digestion process at a mesophilic sewage treatment plant biogas facility. Cellulose was used as a positive control. The concentrate, control and blank were all assessed in triplicate. Table 6 and Table 7 list the characteristics of concentrate and inoculum, as well as conditions in the BMP assessment. Table 6. Characteristics of concentrate and inoculum in BMP assessment.

[0085] Table 7. Conditions used in BMP assessment. Results of anaerobic digestion of concentrate are shown in Fig. 3 and Table 8. Fig. 5 illustrates Methane yield curve in BMP (NmL CH4 / g VS, 1 atm and 0 °C) of concentrate (circle) and cellulose as control (triangle). No toxic effects or inhibition of the process were observed.

[0086] Table 8. Biochemical methane potential (BMP) for concentrate. Methane production is expressed as normal gas volume (1 atm and 0 °C) produced per kg of total solids (NL CH kg TS), produced per kg of volatile solids (NL CH kg VS), and per kg of substrates fed into the digesters in their wet form (NmLCH gww).

[0087] Example 2: Hydrothermal treatment with rising film evaporator and flash evaporator

[0088] Example 2 is schematically illustrated in Fig. 6. Process water from hydrothermal cleanup described in Example 1 was evaporated in two consecutive rising film plate heat evaporators and a flash evaporator with a total heat transfer area of 32 m2. The feeds and condensates of the three stages are described in Table 9. Total amount of process water treated was 2000 kg and 1920 kg of recycle water was produced. The total water recovery using the three-stage rising film plate heat evaporators / flash evaporator was 98%. Stages one and two were rising film plate heat evaporators and stage three was a flash evaporator.

[0089] Table 9. Water recycling with a three-stage plate heat evaporator / flash evaporator.

[0090] Table 10 demonstrates the successful removal of metals (present in concentrations above 0.1 mg / L) in the process water, wherein each value is calculated as a mean of five samples. The concentration in the water was reduced by more than 98 % for calcium, iron, magnesium, and potassium. The concentration of silicon and sodium was reduced by at least 89%. The high sodium concentration in the recycle water is due to the addition of sodium hydroxide for neutralization prior to evaporation. If sodium is excluded 98.8 % of the metals in the process water was removed. Total metals include Ag, Al, As, Ba, Ca, Cd, Cr, Cu, Fe, K, Mg, Na, Ni, Pb, Se, Si, V, and Zn. The COD condensate is a calculated value.

[0091] Table 10. Reduction of major metals (present in concentrations above 0.1 mg / L) in recycle water compared with process water in water recycling Example 2. Samples analysed with Inductively Coupled Plasma - Mass Spectrometry (ICP-MS). Further concentration increase to a solids content of 54 % \N / \N was done by evaporation on bench equipment. The viscosity of the concentrate of less than 10 Pa-s (at 20 °C) was still at a level where a plate evaporator can efficiently process it. Achieving a final concentration of 54 wt.% of total solids using a four-stage plate heat evaporator implies that a 99% water recovery can be achieved.

[0092] The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Claims

CLAIMS1. A process for recovering process water obtained from a hydrothermal reactor, comprising the steps of: a) providing a process water stream obtained from a hydrothermal reactor; b) pressurizing the process water stream to form a pressurized stream; c) heating the pressurized stream in at least one heat exchanger to form a heated stream; d) partly evaporating the heated stream to separate water from undesired contaminants to form a concentrated effluent stream and a vapour stream; e) condensing the vapour stream to form a recycle water stream, and f) recycling the recycle water stream back to a water storage tank for reintroduction into the hydrothermal reactor.

2. The process according to claim 1, wherein the process water stream of step a) comprises less than 2 wt.% organic material, preferably less than 1 wt.% organic material.

3. The process according to claim 1 or 2, wherein the temperature of the process water stream of step a) is between 60°C and 100°C, preferably between 70°C and 90°C.

4. The process according to any one of the preceding claims, further comprising the step of: g) feeding the recycle water stream to the at least one heat exchanger to heat the pressurized stream during step c).

5. The process according to any one of the preceding claims, further comprising the step of: h) anaerobic digesting the concentrated effluent stream to form a digestate and biogas.

6. The process according to any one of the preceding claims, further comprising the step of: i) recovering treated feedstock in the process water stream between step a) and b) and returning the recovered treated feedstock to the hydrothermal reactor for treatment together with untreated feedstock.

7. The process according to any one of the preceding claims, wherein the method further comprises the step of: j) partly evaporating the concentrated effluent stream to separate additional water from undesired contaminants in the concentrated effluent stream and to form a second vapour stream.

8. The process according to any one of the preceding claims, wherein the temperature of the heated stream obtained in step c) is at least 75°C, preferably at least 90°C.

9. The process according to any one of the preceding claims, wherein the method further comprises the step of: k) saturating the heated stream of step c) before step d).

10. A system (10, 20) for recovering process water obtained from a hydrothermal reactor (1), the system comprising: an evaporator feed pump (3) configured to pressurize the process water stream and to form a pressurized stream; at least one heat exchanger (4) configured to heat the pressurized stream and to form a heated stream; an evaporator package (5) comprising at least one evaporator stage (6) configured to separate water from undesired contaminants in the heated stream, the evaporator stage (6) comprising a plate evaporator (7) configured to saturate the heated stream and forming a saturated stream, and a separator vessel (8) configured to separate the saturated stream into a vapour stream and a concentrated effluent stream.

11. The system (10, 20) according to claim 10, wherein the evaporator package (5) further comprises a recirculation pipe (9) configured to recirculate the vapour stream to the plate evaporator (7) to heat the heated stream.

12. The system (10, 20) according to claim 10 or 11, wherein the evaporator package (5) further comprises a recompression unit (11) configured to increase the pressure of the vapour stream and to form a recompressed vapour stream that is fed to the plate evaporator (7) of the at least one evaporator stage (6).

13. The system (10, 20) according to any one of claims 10 to 12, wherein the plate evaporator (7) is a thin film evaporator or a flash evaporator.

14. The system (10, 20) according to any one of claims 10 to 13, wherein the evaporator package (5) comprises at least two consecutive evaporation stages (6).

15. The system (10, 20) according to any one of claims 10 to 14, further comprising a recirculation pipe (12) configured to recirculate the recycle water stream back to the at least one heat exchanger (4) to heat the pressurized stream.

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