Method for determining the amount of a gas dissolved in a liquid

Description

[0002] Field of the invention

[0003] The present invention relates to a method for determining the amount of a gas dissolved in a liquid. In particular, the method is intended to measure the amount of hydrogen sulphide gas (H2S) dissolved in water.

[0004]

[0005] The background of the invention.

[0006]

[0007] Farming fish has become a major industry, and in some farms the formation of hydrogen sulphide in the water the fish are raised in has led to significant problems. In particular, this has proven to be a problem in so-called RAS facilities where most of the water is recycled back to where the fish are raised.

[0008]

[0009] It has been shown that hydrogen sulphide is formed when there is sludge in the facilities, as places with poor oxygen conditions are then established. Sulphate-reducing bacteria then convert sulphate into hydrogen sulphide. Seawater contains far greater amounts of sulphate than fresh water, and the problem of formation of hydrogen sulphate is therefore greater in aquaculture facilities with seawater.

[0010]

[0011] Hydrogen sulphide is soluble in water, and a certain amount of hydrogen sulphide gas will be dissolved in the water the fish is in.

[0012]

[0013] There are currently no good practical measuring methods for measuring the amount of hydrogen sulphide in a liquid.

[0014]

[0015] Discussion of prior art.

[0016]

[0017] There are currently sensors for measuring H2S in fresh water. These are used, for example, for the analysis of drinking water. In seawater, there has only recently become a need to measure H2S, and then especially in RAS facilities for marine fish species such as salmon. H2S is highly toxic and easily soluble in water. Salt water contains 2700 mg sulphate per liter compared to 2.2 mg / litre for fresh water.

[0018] Sulfate-reducing bacteria are anaerobic bacteria that form H2S. This happens in areas of the fish tank or water treatment system where there is poor water circulation. It also happens in the biofilter if anaerobic conditions occur. Under thick heterotrophic biofilm, there will also be anaerobic conditions and therefore there will always be a certain background level of H2S in all RAS plants. This level is low, often around 100 ng / litre and is therefore difficult to measure in water. The sensors are also very exposed to the corrosive seawater environment.

[0019]

[0020] Blue-Unit has a method to measure in the range 1000-50000 ng / litre. This instrumentation is very expensive and it is of interest to find solutions to be able to monitor concentrations below 1000 ng / litre to get an early warning that things are starting to happen in the plant

[0021]

[0022] Purpose of the present invention.

[0023]

[0024] It is an aim of the present invention to provide a method for determining the amount of hydrogen sulphide in a liquid. One purpose in this connection is to be able to measure a relative change in the amount of hydrogen sulphide in the liquid, such as an increase or decrease in the amount of hydrogen sulphide in the liquid.

[0025]

[0026] It is also a purpose to be able to determine the amount of other gases dissolved in the liquid.

[0027]

[0028] It is also an object of the invention to be able to determine the amount of hydrogen sulphide and / or other gases dissolved in any type of liquid. It is therefore appropriate to use the method to measure hydrogen sulphide in liquid in a breeding facility, but the method can also be used for other liquids, such as drinking water, treatment plants, etc.

[0029]

[0030] It is also an object of the present invention to provide a system for carrying out the method for determining the amount of hydrogen sulphide in a liquid.

[0031] Summary of the invention

[0032]

[0033] The present invention relates to a method for determining the amount of a gas dissolved in a liquid in a first container, characterized in that said gas and other gases in the liquid are extracted from the liquid in a gas-liquid separation device and where the gases that are separated from the liquid is fed to a sensor device, and where the sensor device measures the amount of said gas.

[0034]

[0035] In one embodiment, the measured amount of said gas in the gas flow is used to calculate the relative or absolute amount of gas that was originally in the liquid.

[0036]

[0037] In one embodiment, the sensor device measures the amount of one or more gases selected from hydrogen sulfide, carbon dioxide, and oxygen.

[0038]

[0039] In one embodiment, said gas is hydrogen sulphide.

[0040]

[0041] In one embodiment, the flow rate and amount of liquid through the gas-liquid separation device is measured or calculated, so that the absolute amount of gas dissolved in the liquid can be calculated.

[0042]

[0043] In one embodiment, the calculation of relative or absolute amount of gas in the liquid is corrected for any added gas, such as air.

[0044]

[0045] In one embodiment (as shown in Figure 1), said gas-liquid separation device comprises a trickle filter, where the liquid is supplied to the trickle filter in a separation chamber, and where a vacuum pump is connected to this separation chamber which extracts the gases from the liquid and leads them to the sensor device, and where the liquid is led from the separation space to a second container.

[0046]

[0047] In one embodiment, the liquid is led from an upper part of the second container to the container.

[0048]

[0049] In one embodiment, said container is arranged vertically lower than the trickle filter, preferably 5-20 meters lower.

[0050] In one embodiment, the vacuum pump generates a reduced pressure of >1 mBar.

[0051]

[0052] In one embodiment (as shown in Figure 2), the liquid is fed via a gas venting unit and back to the first container and where the gases that are separated from the liquid are fed to a separation unit that is in communication with a vacuum fan that carries the gases on to the sensor device, while residues of the liquid is transferred to a second container.

[0053]

[0054] In one embodiment, said vacuum fan establishes a reduced pressure in the range 350-50 mBar.

[0055]

[0056] In one embodiment, ejectors generate microbubbles to circulate liquid and to vent the gases in the gas venting unit.

[0057]

[0058] In one embodiment, the gases are led from the separation unit via an aquaduct.

[0059]

[0060] In one embodiment (as shown in Figure 3), liquid is transferred from the first container to a gas exchange chamber comprising a diffuser, arranged so that gas from the diffuser circulates in a closed circuit by means of an air pump via the sensor device, as the gases in the gas phase are constantly in equilibrium with the gases dissolved in the liquid.

[0061]

[0062] In one embodiment (as shown in figure 4), the liquid in the first container is pumped into a separation device by means of a pump, and where an ejector adds air bubbles to the liquid in the separation device, and where gases leaving the liquid are led to the sensor device circulating in a closed circuit .

[0063]

[0064] Description of figures

[0065]

[0066] Preferred embodiments of the invention will be described in more detail below with reference to the accompanying figures, where:

[0067]

[0068] Figures 1-4 schematically show different designs for removing gases dissolved in a liquid from the liquid, and measuring the content of the various gases (which have been in the liquid) in a gas phase in a sensor device.

[0069]

[0070] Figure 5 shows measurements of H2S and CO2.

[0071] Description of preferred embodiments of the invention.

[0072]

[0073] As mentioned above, there are no solutions to be able to measure H2S in a solution that is sufficiently sensitive to, for example, detect levels of H2S that are harmful to fish.

[0074]

[0075] This problem is solved with the present invention by extracting all gases that are dissolved in a given liquid, and then measuring the amount of the individual gases in a gas phase (and not in a liquid phase). In order to carry out the method, one must be able to extract the gases in a liquid, and a number of systems are also described for doing this. Some of these systems are described in other patent applications that the applicant has.

[0076]

[0077] Figure 1 shows an embodiment where water 10 from the vessel 11 is sucked up by means of a vacuum pump 24 and passed through a throttle valve 13 in order to be able to regulate the amount of water. The water flows down through a trickle filter 20, which in this embodiment consists of a number of balls with a large rough surface, and further into a sump 30. The height between the sump and the trickle filter 20 is considerable, for example more than 10 m, so that a absolute vacuum above the water column. The gases that are dissolved in the water will then be extracted and passed through the filter 15 and vacuum pump 24 before being passed through the sensor box 14. The sensor box 14 contains all the necessary gas sensors as well as a gas quantity meter, temperature sensor, humidity sensor, etc. This device provides a direct measure of the amount of gas that has been dissolved in the water, as all the gases that have been dissolved in the water are extracted from the water and further to the sensor box 14 by establishing an approximately 100% vacuum in the trickle filter 20.

[0078]

[0079] Figure 2 shows an embodiment of the invention where water 10 from the vessel 11 is drawn by means of vacuum into an aqueduct 40. Air is added to the aqueduct 40, preferably in the form of microbubbles in the air supply device 60. This creates circulation in the device and the water flows down into the same vessel 11, or transferred to another vessel (not shown in figure). On the horizontal part of the aqueduct 40, vertical aeration towers are arranged which are connected to a fan 52 via a cyclone 50. Gas and foam in the aqueduct 40 are led into the cyclone 50 where water goes down into the sump 30, while the gases go up through the fan 52. A partial flow of the gas out of the fan 52 passes through the sensor box 14 where all the gas sensors are arranged, in addition to sensors for gas quantity, temperature, humidity, etc. With this device we add a known amount of air to the water and a equilibrium between air / water through the aqueduct 40. The values ​​of gases, such as H2S, in ppm and ppb which are measured by the sensors in the sensor box must then be converted back to real values ​​in water based on chemical formulas, Henry's constant and equilibrium considerations. By making many measurements, empirical constants will be found for this conversion from concentration in air to concentration in water.

[0080]

[0081] Figure 5 shows a plot for data recorded from this type of sensor. The figure shows a concentration of H2S in gas of 200 ppb. Based on theory and estimated empirical factors, this gives a H2S concentration in water of around 290 ng / litre. This is a level below what known methods can register, and the method according to the invention is thus far more sensitive as it can measure lower levels of H2S dissolved in liquid than solutions according to known techniques. It is crucial for a breeding facility to be able to follow the development of H2S, so that measures can be taken when the amount of H2S dissolved in the water in the cage rises, or exceeds a given predetermined threshold value.

[0082]

[0083] Figure 3 shows a version of the invention where a small amount of water is led from a vessel 30 via a gas exchange chamber 80 to a sump 30. In this chamber 80 there is a diffuser 82 which pushes air in the form of microbubbles through the water. The air is sucked up and passed through a sensor box 14 and on through pump 84 to then return through the diffuser 82 and thus circulate in a closed circuit where the water flows through. There will always be an equilibrium between gas and liquid in the gas exchange chamber 80. Changes in the concentration of the various gases will then be registered in the sensor box 14 when the gas is circulated.

[0084]

[0085] Figure 4 shows an embodiment of the invention which is based on the same principle as in Figure 3, except that in this case the water 10 is pumped onto an ejector 94 which is in the gas exchange chamber 90. Air from a closed air loop through a sensor box 14 is circulated by it is sucked into the ejector 94 by means of pump 92 where microbubbles are created and mix with the water. The gas rises and constantly achieves equilibrium with the water and the concentration is recorded by sensors in the sensor box 14. The water goes to the drain via pipeline 32.

[0086]

[0087] In fig.3 and 4, the air is circulated in a closed circuit, where in fig.3 diffusers 82 are used to create microbubbles and in fig.4 ejectors 94 are used. The air that circulates is a measuring medium that is constantly in gas balance with the water. Changes in the water are transferred to changes in air, which are then registered in the sensor box. The recorded measurements for gases in the gas phase that are measured in the sensor device 14 are then used to calculate how much of the individual gases were originally dissolved in the water 10.

[0088]

[0089] In the sensor box 14, conventional H2S sensors can be used to measure the amount of H2S gas in a gas phase. For example, sensors from Spec Sensors (www.spec-sensors.com) can be used, which are electrochemical sensors.

[0090]

[0091] Figure 5 shows the measurement of CO2 (indicated in ppm) and H2S (indicated in ppb) measured in the sensor box, i.e. in the gas phase, on an embodiment of the invention as indicated in Figure 2.

Claims

Claim 1. Method for determining the amount of a gas dissolved in a liquid (10) in a first container (11), characterized in that said gas and other gases in the liquid (10) are extracted from the liquid (10) in a gas-liquid separation device and wherein the gases separated from the liquid are fed to a sensor device (14) and wherein the sensor device (14) measures the amount of said gas.

2. Method according to claim 1, wherein the amount of said gas in the gas stream is used to calculate the relative or absolute amount of gas that was originally in the liquid (10).

3. Method according to claim 1, characterized in that the sensor device (14) measures the amount of one or more gases selected from hydrogen sulfide, carbon dioxide and oxygen.

4. Method according to claim 3, characterized in that said gas is hydrogen sulfide.5.Method according to claim 1, characterized in that the flow rate and the amount of liquid through the gas-liquid separation device (12) are measured or calculated, so that the absolute amount of gas dissolved in the liquid (10) can be calculated.

6. Method according to claim 1, characterized in that the calculation of the relative or absolute amount of gas in the liquid is corrected for any added gases, such as air.

7. Method according to claim 1, characterized in that said gas-liquid separation device (12) comprises a trickle filter (20), where the liquid (10) is supplied to the trickle filter (20) in a separation space (22), and where a vacuum pump (24) is connected to this separation space (22) which extracts the gases from the liquid (10) and directs them to the sensor device (14), and where the liquid is directed from the separation space (22) to a second container (30).

8. Method according to claim 7, characterized in that the liquid is directed from an upper portion of the second container (30) to the container (11).9.Method according to claim 6, characterized in that said container is arranged vertically lower than the trickle filter (20), preferably 5-20 meters lower.

10. Method according to claim 7, characterized in that the vacuum pump (24) generates a reduced pressure of >1 mBar.

11. Method according to claim 1, characterized in that the liquid (10) is passed via a gas venting unit (40) and back to the first container (11) and where the gases separated from the liquid (10) are passed to a separation unit (50) which is in communication with a vacuum fan (52) which passes the gases on to the sensor device (14), while the remains of the liquid (10) are passed to a second container (30).

12. Method according to claim 11, characterized in that said vacuum fan establishes a reduced pressure in the range of 350-50 mBar.

13. Method according to claim 11, characterized in that ejectors (60) generate microbubbles to circulate liquid and to vent the gases in the gas venting unit (40). 14.Method according to claim 11, characterized in that the gases from the separation unit (50) are conducted via an aquaduct (70).

15. Method according to claim 1, characterized in that liquid (10) from the first container (11) is transferred to a gas exchange chamber (80) comprising a diffuser (82), arranged so that gas from the diffuser (82) circulates in a closed circuit by means of an air pump (84) via the sensor device (14), the gases in the gas phase being constantly in equilibrium with the gases dissolved in the liquid.

16. Method according to claim 1, characterized in that the liquid (10) in the first container (11) is pumped into a separation device (90) by means of a pump (92), and where an ejector (94) supplies air bubbles to the liquid in the separation device (90), and where gases leaving the liquid are fed to the sensor device (14) circulating in a closed circuit.