Hydrothermal clean-up process comprising recycling of process water

Abstract

The present inventive concept relates to a process for recovering process water obtained from a hydrothermal reactor (1). The process comprises the steps of: a) providing a process water stream obtained from a hydrothermal reactor (1); b) pressurizing the process water stream to form a pressurized stream;c) heating the pressurized stream in at least one heat exchanger (4) 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, andf) recycling the recycle water stream back to a water storage tank for reintroduction

Description

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.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.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.The process water stream of step a) may comprise less than 2 wt.% organic material, preferably less than 1 wt.% organic material.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.The range of water to oil ratio in the process water stream of step a) may be between 10:1 and 1000:1.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.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.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.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).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.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).The energy for the evaporation of step d) may be supplied by a Mechanical Vapour Recompression (MVR).The process may further comprise the step of:h) anaerobic digesting the concentrated effluent stream to form a digestate and biogas.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. 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.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.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 ofj) 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.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.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 comprisinga plate evaporator configured to saturate the heated stream and forming a saturated stream, anda separator vessel configured to separate the saturated stream into a vapour stream and a recycle water stream.This aspect may exhibit the same or similar features and technical effects as the first aspect, and vice versa.The term saturate in this context means heating to the boiling temperature. The evaporator package may further comprises a recirculation pipe configured to recirculate the vapour stream to the plate evaporator to heat the heated stream.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.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.to a part, discharged to a water storage tank via a recycle water pipe 14 for reintroduction into the hydrothermal reactor 1.The recycle water meets the requirements to be used in for example the HCU process.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.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.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.Hydrothermal cleanupThe 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.Table 2. Operating conditions of HCU.Water recycleFour 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 method, complexometric titration with EDTA and endpoint titration with Eriochrome Black T.Table 4. Organic acids in recycle water in water recycling Example 1.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%.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).

Claims

6. The process according to any one of the preceding claims, further comprising the step of:h) 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) comprisinga plate evaporator (7) configured to saturate the heated stream and forming a saturated stream, anda separator vessel (8) configured to separate the saturated stream into a vapour stream and a recycle water 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.