Liquid processing system and method therefor

Abstract

A liquid processing system includes a processing volume configured to receive contaminated liquid, an gas supplying device and an ultrasonic acoustic source The processing volume has a process inlet for receiving liquid and a process outlet for discharging water. The gas supplying device is configured for providing a stream of a gas to the processing volume, the acoustic source coupled to the processing volume and configured to provide acoustic energy to the processing volume. the processing volume is arranged between the process inlet and the process outlet. the gas supplying device is configured to supply a stream of nanobubbles to liquid in the processing volume in a first mode, the nanobubbles having a diameter of 250 nm or less, and to supply a stream of microbubbles to liquid in the processing volume in a second mode, the microbubbles having a diameter of 500 nm or more, and the acoustic source is configured to supply acoustic energy in the liquid in the second mode through an acoustic coupling.

Description

Field of the invention The present invention relates to a liquid processing system. In addition, the invention relates to a method for liquid processing. Background A floating production, storage and offloading (FPSO) vessel is typically used by the offshore oil and gas industry for the processing of hydrocarbon feeds and for storage of hydrocarbon products (liquid and / or gas) from the feeds. For liquid natural gas, LNG, applications such vessel is referred to as FLNG vessel. The vessel is designed to receive a feed comprising hydrocarbons and other components from a nearby offshore platform or directly from a subsea well. The received feed is processed by processing equipment on the vessel to separate the hydrocarbons from the other components. To facilitate the processing, water is used for example for purposes such as steam generation or cooling. Typically on FPSO vessels seawater is taken in, filtered, desalinated if required and supplied to processing equipment and the vessel’s operational equipment. During use, the process water can absorb contaminants from the application. Such contaminants involve for example (solid) particles ranging from a sub-micrometer size to a size of tens of micrometers. After use, the process water is to be cleaned and discharged. It is known that removal of contaminants from the water can be an arduous task since many cleaning methods are not completely effective. From publication CN102863039 a method for cleaning water after its use, i.e., cleaning sewage water, involves the exposure of the contaminated water to a stream of microbubbles typically with diameters in the order of micrometers and optionally larger bubbles. This method is known to clean the water to some extent, but the approach is not very efficient and does not provide cleaning the water from small particles smaller than about 5 micrometer size. Alternatively, various chemicals and traditional filters are used to filter seawater in various stages each separating a certain size range of contaminants. Chemicals are expensive and require storage that takes up space on a processing platform, for example a platform for hydrocarbon processing. Filters also must be periodically changed and flushed out. This increases the cost of operation. It is an object of the present invention to overcome or mitigate one or more of these disadvantages. Summary of the invention The object is achieved by a liquid processing system comprising a container device defining a processing volume and configured to receive contaminated liquid, a gas supplying device and an ultrasonic acoustic source, the container device having a process inlet for receiving liquid and a process outlet for discharging liquid; the gas supplying device configured for providing a stream of a gas to the processing volume in the container device; the ultrasonic acoustic source coupled to the container device and configured to provide acoustic energy to the processing volume in the container device, the processing volume being arranged between the process inlet and the process outlet; the gas supplying device is configured with a bubble generator for generating a stream of nanobubbles during a first mode and for generating a stream of microbubbles during a second mode, so as to supply the stream of nanobubbles to the liquid in the processing volume in the first mode, the nanobubbles having a diameter of 250 nm or less, and to supply the stream of microbubbles to the liquid in the processing volume in the second mode, the microbubbles having a diameter of 500 nm or more, wherein the ultrasonic acoustic source is configured to generate acoustic waves in an ultrasonic frequency range to supply acoustic energy to the liquid in the second mode through an acoustic coupling; wherein the container device comprises a first reactor defining a first volume portion and a second reactor defining a second volume portion; the first and second volume portions being arranged between the process inlet and the process outlet; the second volume portion communicatively coupled to the first volume portion and arranged between the first volume portion and the process outlet; the first volume portion configured for the first mode, the bubble generator being configured with a first bubble outlet coupled with the first volume portion, and the second volume portion configured for the second mode, the bubble generator configured with a second bubble outlet coupled with the second volume portion. Advantageously, the invention uses a combination of nanobubbles and microbubbles with a bubble size in the order of 250 nanometer or smaller and 500 nanometer or larger respectively. The combination of nanobubbles and microbubbles is configured to absorb various types of contaminations from a volume of a liquid (i.e., particles that range from submicron to tens of microns, typically comprising a wide range of materials that are typically present in a contaminated liquid, and to form clusters (aggregates) of such nano / microbubbles. In the first mode the exposure of the contaminated liquid to nanobubbles causes absorption of various types of impurities in the liquid to the nanobubbles. In the second mode the addition of the microbubbles to the liquid causes formation of clusters (aggregates) of such nano / microbubbles and the absorbed impurities. These bubble-particle clusters have very little buoyancy and so remain dispersed in the liquid for a long time. In a third step acoustic force generated by an ultrasonic acoustic signal is applied to coalesce the bubble clusters to macroscopic bubbles very rapidly that now have high buoyancy due to their comparatively large size and float up along with the attached impurities leaving the remaining liquid contamination free. According to an aspect, the invention provides a liquid processing system as described above wherein the gas supplying device is configured with a bubble generator for generating the stream of nanobubbles during the first mode and the bubble generator for generating the stream of microbubbles during the second mode. According to an aspect, the invention provides a liquid processing system as described above, wherein the bubble generator comprises a first bubble generator for creating the stream of nanobubbles and a second bubble generator for creating the stream of microbubbles. In the processing volume in the container device, the exposure to nanobubbles and microbubbles occurs consecutively, if the processing volume is a single volume in the container device where the first bubble generator and second bubble generator each are located. According to an aspect, the invention provides a liquid processing system as described above, wherein the container device comprises a first reactor defining a first volume portion of the processing volume and a second reactor defining a second volume portion of the processing volume that are communicatively coupled to each other and arranged between the process inlet and the process outlet.. In the processing volume of the container device, the exposure to nanobubbles and microbubbles occurs in separate first and second volume portions that are coupled to each other to let liquid flow from the first volume portion to the second volume portion. In this manner, the operation of the system can be on basis of a continuous flow. According to an aspect, the invention provides a liquid processing system as described above the process outlet comprises a separator device with a first process outlet conduit and a second process outlet conduit, wherein the first process outlet conduit is configured to create a first flow of liquid containing nanobubbles and / or microbubbles and the second process outlet conduit is configured to create a second flow of liquid substantially void of nanobubbles and / or microbubbles. The separator device divides the liquid flowing through the liquid processing system in a flow containing contaminated liquid that exits through a first outlet conduit and a flow of cleaned liquid that exits through a second outlet conduit separated from the first outlet conduit. According to an aspect, the invention provides a liquid processing system as described above wherein the acoustic source is configured to generate ultrasonic acoustic waves in an ultrasonic frequency range. It is observed that exposure to ultrasonic acoustic energy provides coalescence of the nano- and microbubbles into larger sized buoyant bubbles in an effective manner and at sufficiently high rate. According to an aspect, the invention provides a liquid processing system as described above wherein the first volume portion has a relatively larger volume that the second volume portion. This has the beneficial effect to provide a relatively longer residence time during exposure to nanobubbles to absorb contaminants than during exposure to microbubbles to coalesce the nanobubbles. According to an aspect, the invention provides a seawater processing system as described above wherein the first volume portion has a larger cross-section perpendicular to a flow direction of water than the second volume portion. This has the beneficial effect to have a lower flow rate in the first volume portion than in the second volume portion which results in a relatively longer residence time during exposure to nanobubbles than during exposure to microbubbles. The acoustically driven coalescence process in practical circumstances typically is less than about 10 seconds. The residence time during exposure to nanobubbles is typically at least the same or longer. According to an aspect, the invention provides a liquid processing system as described above further comprising a process inlet volume communicatively coupled between the process inlet and the first volume portion and a second ultrasonic acoustic source coupled to the process inlet volume for generating a second acoustic signal in the process inlet volume, wherein the second acoustic signal comprises an acoustic signal in the ultrasonic range. In an embodiment, the contaminated process liquid is treated with acoustic energy to have acoustically driven clustering of particles before entering the first volume portion. Advantageous embodiments are further defined by the following description of the embodiments. Brief description of drawings Embodiments of the present invention will be described hereinafter, byway of example only, with reference to the accompanying drawings which are schematic in nature and therefore not necessarily drawn to scale. In the drawings, identical or similar elements are indicated by the same reference number or sign. The scope of the invention is only limited by the definitions presented in the appended claims. Figure 1 shows a liquid processing system according to an embodiment of the invention; Figure 2 shows a liquid processing system according to an embodiment of the invention. Detailed description of embodiments The liquid processing system relates to a liquid cleaning system on a floating offshore structure to be located on a body of water and that supports process equipment in which the liquid cleaning system is configured to receive waste liquid from the processing equipment. Such liquid may be fresh water, seawater, desalinated seawater, water mixtures, hydrocarbon based liquid, water-based solutions, or any other processing liquid used within the processing equipment. Figure 1 shows a liquid processing system according to an embodiment of the invention. The liquid processing system 100 is typically installed onboard the FPSO / FLNG vessel (not shown) and is configured to receive waste liquid processed / used on the vessel. The liquid processing system 100 comprises a first reactor defining a first volume portion 10, a second reactor defining a second volume portion 20, a process inlet conduit 5 and a process outlet conduit 30. The liquid processing system 100 comprises a bubble generator 15, coupled to the first volume portionlO and a second bubble generator 25 coupled to the second volume portion 20. Each of the bubble generators is coupled to a gas supplying device 17 that is configured to supply gas, optionally pressurized gas, to each bubble generator. Further, the seawater processing system comprises an ultrasonic acoustic source 28 that is acoustically coupled to the second volume portion. The process inlet conduit 5 is configured to receive waste liquid from the process equipment / vessel 1 and is connected to an inlet side 11 of the first reactor such that during operation the waste liquid enters the first volume portion 10. At an outlet side 12 thereof, the first reactor (first volume portion 10) is connected to an inlet side 21 of the second volume 20 of the second reactor, such that water can flow from the first volume portion 10 to the second volume portion 20. At an outlet side 22 of the second reactor, the second volume portion is connected to the process outlet conduit 30. The process outlet conduit 30 is configured with a first and second outlet 32, 34 for liquid from the second volume 20. The process outlet conduit 30 is provided with a separator 31 configured for separating the liquid received from the second volume portion into a cleaned portion of liquid through the first outlet 32 and a contaminated portion of liquid holding the impurities through the second outlet 34. During operation, the seawater processing system 100 receives waste liquid from the process equipment I vessel 1 at the process inlet conduit 5. The waste liquid is transported through the process inlet conduit 5 into the first volume portion 10. In the first volume portion, the waste liquid is exposed to a stream of nanobubbles of gas or air from the bubble generator 15. The term nanobubbles refers to air or gas bubbles with a diameter of typically about 250 nm or less , which due to their size are electrically charged, which allows nanobubbles to be an extremely efficient source of absorber of contaminations of various types and sizes. Impurities of size in the range from a few submicron to about ten microns are thus captured / decorated by nanobubbles. Typically the bubble generators are configured to mix gas into a stream of liquid to obtain a stream of liquid enriched with nanobubbles or microbubbles. The terms nanobubbles and microbubbles refer to air or gas bubbles. Microbubbles are defined to have a size of 500 nm or more, typically about 1 micron, whereas nanobubbles have typically a size of 250 nm or less. Alternatively, nanobubbles or microbubbles maybe generated by a cavitation process in which the water is exposed to acoustic signals in a frequency range between about 30kHz and about 100kHz. It is observed that typically cavitation threshold goes down significantly as the frequency is increased beyond 100 kHz. For the ultrasonic coalescence of the nano- and microbubbles as described here, cavitation is avoided as typically ultrasonic frequencies above 100 kHz are used. In an embodiment the ultrasonic frequency is between about 100 kHz and about 1 MHz. In an alternative embodiment the ultrasonic frequency is between about 400 kHz and about 700 kHz. In an embodiment a single bubble generator may be adjustable to create either nanosized bubbles or micron-size bubbles. Nanobubble (and microbubble) generators are known in the art and can be based on various bubble generation methods: see for example, US 10,814,290. It is observed that nanobubbles have no significant buoyancy in water and therefore remain substantially floating / dispersed in the water. Thus, the liquid in the first volume portionlO can be regarded as a mixture of liquid in which the nanobubbles are dispersed. In the first volume portion 10, the liquid is exposed to the nanobubbles. After exposing the waste liquid to the nanobubbles stream, the mixture of waste liquid and nanobubbles is entering the second volume portion 20 In the second volume portion 20, the mixture of waste liquid and nanobubbles is exposed to a stream of microbubbles that is created by the second bubble generator. The microbubbles have the effect of causing nanobubbles to cluster with microbubbles. As a result, the impurities that were captured / absorbed / decorated by the nanobubbles are taken up by the bubble clusters, i.e. the clusters of nano- and microbubbles. Due to the clustering the bubbles obtain a net buoyancy, which results in an upward flow from the second volume portion and the probability that the bubbles burst and the impurities are released from the clusters increases. To avoid that the impurities are released into the liquid, the nanobubble / microbubble clusters are exposed to an ultrasonic acoustic signal from an ultrasonic acoustic source that promotes coalescence of the clusters into larger bubbles (with a size in the order of millimeters). The ultrasonic acoustic source 28 is configured to generate ultrasonic acoustic waves with a frequency in the ultrasonic range from about 80 kHz to about 500 kHz, preferably in a range above 100 kHz, more preferably from above 100kHz to about 1 MHz, even more preferably in a range between 400 kHz and 700kHz. The ultrasonic waves can be applied in a continuous manner but can also be pulsed or intermittent depending on the volume of containment and the flow of the water. As a result of exposure to the acoustic signal, the bubble clusters coalesce and form larger buoyant bubbles that at high rate move upward from the liquid in the second volume portion 20 and exit into the outlet conduit 30. Separator 31 in the outlet conduit 30 is configured to separate the liquid flow incoming from the second volume portion 20 into a contaminated portion of the liquid flow enriched with impurities and a clean portion of the liquid flow depleted from impurities. In an embodiment the separator 31 is operative based on the density difference between liquid and coalesced bubbles. Nanobubbles have very low buoyancy because of their size. However, the aggregated and coalesced microbubbles have size that is more than two orders of magnitude larger and much higher buoyancy. This higher buoyancy helps these coalesced bubbles to rise rapidly through the liquid to the top in the separator. The density difference here is substantially liquid vs. air or gas (large air or gas bubbles). In an embodiment, in the first volume portion 10 the liquid has a relatively longer residence time than in the second volume portion 20. The flow rate in the first volume portion 10 is relatively lower than in the second volume portion 20, which can be embodied by having a first volume portion 10 that is relatively larger than the second volume portion 20 or for example by having a first volume portion 10 with relatively larger cross-section (perpendicular to the flow direction) than the second volume portion 20. Figure 2 shows a seawater processing system according to an embodiment of the invention. In a further embodiment the liquid processing system 100 comprises a secondary ultrasonic acoustic source 40. The secondary ultrasonic acoustic source is arranged at the process inlet conduit 5 and configured for exposing the waste liquid entering from the process equipment / vessel 1 to a secondary acoustic signal in the frequency range above at least 30 kHz and as high as about 3 MHz (adjustable). The frequency can be set in this range depending on the contamination level and volume of the waste liquid to be treated. For very low contamination level, a higher frequency range is used, the secondary acoustic source being configured to generate a second acoustic signal with an adjustable frequency. The second acoustic signal from the secondary acoustic source can be continuous or pulsed depending on the volume of waste liquid to be treated. The pulsed source can be used with adjustable different duty cycles to control the electrical excitation energy and to prevent undesirable heating of the acoustic sources. The secondary acoustic signal has the effect that impurities in the waste liquid are clustered into particles of larger size upon entry of the first volume portion. Clustering the impurities before entering the first volume portion 10, advantageously improves the capture of the impurities by the exposure to the nanobubbles. The operation of the liquid processing system in this embodiment is substantially similar as described with reference to Figure 1. The invention has been described with reference to the preferred embodiment. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims.

Claims

1. A liquid processing system comprising a container device defining a processing volume and configured to receive contaminated liquid, a gas supplying device and an ultrasonic acoustic source, the container device having a process inlet for receiving liquid and a process outlet for discharging liquid;the gas supplying device configured for providing a stream of a gas to the processing volume in the container device;the ultrasonic acoustic source coupled to the container device and configured to provide acoustic energy to the processing volume in the container device, the processing volume being arranged between the process inlet and the process outlet;the gas supplying device is configured with a bubble generator for generating a stream of nanobubbles during a first mode and for generating a stream of microbubbles during a second mode, so as to supply the stream of nanobubbles to the liquid in the processing volume in the first mode, the nanobubbles having a diameter of 250 nm or less, and to supply the stream of microbubbles to the liquid in the processing volume in the second mode, the microbubbles having a diameter of 500 nm or more, wherein the ultrasonic acoustic source is configured to generate acoustic waves in an ultrasonic frequency range to supply acoustic energy to the liquid in the second mode through an acoustic coupling;wherein the container device comprises a first reactor defining a first volume portion and a second reactor defining a second volume portion; the first and second volume portions being arranged between the process inlet and the process outlet; the second volume portion communicatively coupled to the first volume portion and arranged between the first volume portion and the process outlet; the first volume portion configured for the first mode, the bubble generator being configured with a first bubble outlet coupled with the first volume portion, and the second volume portion configured for the second mode, the bubble generator configured with a second bubble outlet coupled with the second volume portion.

2. The liquid processing system according to claim 1, wherein the bubble generator comprises a first bubble generator for creating the stream of nanobubbles at the first bubble outlet and a second bubble generator for creating the stream of microbubbles at the second bubble outlet.

3. The liquid processing system according to any one of the preceding claims 1 - 2, wherein the system is configured to carry out operations of the first mode and the second mode, consecutively in the processing volume.

4. The liquid processing system according to any one of the preceding claims 1 - 3, wherein the process outlet comprises a separator device with a first process outlet conduit and a second process outlet conduit, wherein the first process outlet conduit is configured to create a first flow of liquid containing nanobubbles and / or microbubbles andthe second process outlet conduit is configured to create a second flow of liquid substantially void of nanobubbles and / or microbubbles.

5. The liquid processing system according to any one of the preceding claims 3-4, wherein the first reactor volume defining the first volume portion has a relatively larger volume than the second reactor volume defining the second volume portion.

6. The liquid processing system according to any one of the preceding claims 3-5, wherein the first reactor volume defining the first volume portion has a larger crosssection perpendicular to a flow direction of the liquid than the second reactor volume defining the second volume portion.

7. The liquid processing system according to any one of the preceding claims, further comprising a process inlet volume portion communicatively coupled between the process inlet and the first volume portion and a second ultrasonic acoustic source coupled to the process inlet volume portion for generating a second ultrasonic acoustic signal in the process inlet volume portion,8. The liquid processing system according to any one of the preceding claims, wherein the ultrasonic acoustic source is configured to output ultrasonic acoustic signals in a frequency range above 100 kHz, preferably in a frequency range between 100 kHz and 1MHz, more preferably in a frequency range between 400 kHz and 700kHz.

9. A method for processing liquid comprising:- receiving a flow of contaminated liquid in a processing volume defined by a container device through a process inlet, wherein the processing volume comprises a first reactor volume defining a first volume portion and a second reactor volume defining a second volume portion; the first and second volume portions beingarranged between the process inlet and the process outlet; the second volume portion communicatively coupled to the first volume portion and arranged between the first volume portion and the process outlet; the first volume portion configured for the first mode;- exposing the flow of contaminated liquid in the first volume portion of the processing volume in a first mode to a stream of nanobubbles, the nanobubbles having a diameter of 250 nm or less, to create dispersed nanobubbles in the seawater;- exposing the flow of contaminated liquid from the first mode in the second volume portion of the processing volume, to a stream of micro bubbles in a second mode, the micro bubbles having a diameter of at least 500nm to create bubble clusters in the liquid with the nano bubbles;- exposing the flow of contaminated liquid from the second mode to an ultrasonic acoustic signal with ultrasonic frequency to coalesce the bubble clusters to form buoyant bubbles gathering in a top level of the processing volume.

10. The method according to claim 9, further comprising:- separating in the top level the gathered bubbles in a first separated flow of liquid and in a second separated flow of liquid substantially void of nanobubbles and / or microbubbles;- discharging the first separated flow from a first process outlet conduit, and discharging the second separated flow from a second process outlet conduit.

11. The method according to claim 9 or claim 10, wherein in the first mode the method is configured for capturing contaminants in the contaminated liquid by the nanobubbles.

12. The method according to any one of the preceding claims 9-11 or the system according to any one of the preceding claims 1 - 8, wherein the liquid is selected from a group comprising fresh water, seawater, desalinated seawater, water and fluid mixtures, hydrocarbon based liquid, and processing liquid used within the processing equipment.

13. A method for manufacturing a liquid processing system according to any one of the preceding claims 1-8, comprising:- providing a process volume defined by a container device with a process inlet and a process outlet, wherein the processing volume comprises a first reactor volume defining a first volume portion and a second reactor volume defining a second volumeportion; the first and second volume portions being arranged between the process inlet and the process outlet; the second volume portion communicatively coupled to the first volume portion and arranged between the first volume portion and the process outlet; the first volume portion configured for the first mode,;- providing a gas supplying device;- providing an ultrasonic acoustic source;- arranging in the process volume a bubble generator coupled to the gas supplying device, the bubble generator being configured with a first bubble outlet coupled with the first volume portion, and the second volume portion configured for the second mode, the bubble generator configured with a second bubble outlet coupled with the second volume portion;- arranging an acoustic coupling of an output from the ultrasonic acoustic source with the processing volume, whereinthe processing volume is configured to receive contaminated liquid at the inlet and to discharge processed liquid at the outlet;the bubble generator is configured to provide nanobubbles to liquid in the first volume portion of the processing volume, the nanobubbles having a diameter of 250 nm or less,the bubble generator is configured to provide microbubbles to liquid in the second volume portion of the processing volume, the microbubbles having a diameter of 500 nm or more, wherein the acoustic coupling of the output from the ultrasonic acoustic source is configured to supply acoustic energy from the ultrasonic acoustic source into the liquid in the processing volume.

14. The method according to claim 13, further comprising:-- providing at the process outlet a separator device with a first process outlet conduit and a second process outlet conduit for separating the liquid into a fraction of liquid enriched with nanobubbles and / or microbubbles and a remainder fraction of liquid substantially void of nanobubbles and / or microbubbles.

15. The method according to any one of the preceding claims 13-14, wherein the ultrasonic acoustic source is configured to output ultrasonic acoustic signals in a frequency range above 100 kHz, preferably in a frequency range between 100 kHz and 1MHz, more preferably in a frequency range between 400 kHz and 700kHz.

Images