WO1994001754A1

WO1994001754A1 - Method for analyzing gas in liquid media and equipment for carrying out the method

Info

Publication number: WO1994001754A1
Application number: PCT/NO1993/000104
Authority: WO
Inventors: Erik Sletholt
Original assignee: Norsk Hydro A.S
Priority date: 1992-07-03
Filing date: 1993-06-30
Publication date: 1994-01-20
Also published as: AU4515493A; NO176496B; NO922627L; NO176496C; NO922627D0

Abstract

The invention relates to a method for analyzing the content of gaseous components in liquid media and equipment comprising means for sampling liquid and a gas analyzer. The liquid sample is mixed with an acid or base to adjust its pH sufficiently for releasing the gaseous component, and a carrier gas is supplied to said mixture, whereupon the released gases are mixed with additional amounts of carrier gas and then stripped from the liquid phase and transferred to a gas analyzer. For releasing hydrogen sulphide or hydrogen cyanide the liquid sample is preferably mixed with sulphuric acid. Air can be applied as carrier gas and part of it be supplied to the liquid mixture at or in a pump ahead of a mixer for acid or base, liquid sample and air. The sampling means comprises a collecting/settling vessel (14) into which liquid sample is sucked through pipe (13) by means of vacuum in pipe (25) and transferred through pipe (15) to a sample receiver (16) by means of pressurized air, and further in pipe (1) to be mixed with acid or base from pipe (2) and carrier gas from pipe (3) to a mixer (5). The invention is especially applicable for analyzing gas like H2S in liquids containing particulate matter.

Description

Method for analyzing gas in liquid media and equipment for carrying out the method

The present invention relates to a method for analyzing the content of gaseous components in liquid media. The method comprises collecting liquid samples. The invention further relates to equipment comprising a means for sampling liquid and gas analyzer for carrying out the method.

The toxic gas H2S is often formed in sewage and waste water and several methods for removal and/or prevention of H₂S formation are known. However, the optimal effects of these methods depend to a great extent on reliable monitoring and analysis of H₂S. Another toxic gas that might be present in liquid media and which requires accurate analysis is hydrogen cyanide (HCN) . It is important that the cost of the method and means for obtaining reliable analysis for these toxic gases are acceptable. The sampling constitutes a specific area within the concept that might cause a great problem. Growing and plugging of the sampling equipment will inherently complicate the subsequent analysis.

There are a number of methods for analyzing hydrogen sulphide and the choice of methods depends to a great extent on the presence of other components in the sample to be analyzed. From U.S. Patent No. 4,380,586 there is known a method and apparatus for photometrically monitoring low level concentration of hydrogen sulphide in alkanol a ine. This method comprises isolating a first reference sample of the stream and removing the hydrogen sulphide from the reference sample until a second, lower pre¬ determined concentration remains. Thereafter the first reference sample is photometrically analyzed and the reference signal representative of the ultraviolet radiation-absorbing character¬ istics of the first reference sample is produced. The reference signal is thereafter used in the photometric analysis of the second sample to produce a signal representative only of the hydrogen sulphide concentration in the second sample. The hydrogen sulphide can be removed from the reference sample by displacement using hydrogen chloride.

For low concentrations in alkanol amine this method is applic¬ able, but rather complicated. The patent does not teach anything about how the method should be used on other H₂S-containing liquids. Further, there is no teaching with regard to the sampling problems experienced with sewage and similar liquids.

Also for other gaseous components in liquid media reliable analysis is in great demand. Examples of such components are hydrogen cyanide, carbon dioxide and ammonium or ammonia.

The object of the present invention was to develop a reliable and simple method for analyzing gaseous components in liquid media like sewage, waste water etc. using sturdy equipment.

A second object was to develop a method which could analyze H₂S or HCN in representative samples taken continuously or inter¬ mittently.

A further object was to be able to analyze the content of ammonium and/or ammonia in liquid media.

The inventors were concerned about finding a method which could utilize sturdy, simple and cheap equipment and still giving reliable results. The method should also be flexible with regard to concentration range of the gaseous components like hydrogen sulphide, hydrogen cyanide etc. In view of these pre-requisites they decided to analyze this component in gaseous state.

Accordingly, it was decided to transfer the H₂S, HCN, ammonium etc. from the liquid sample into a carrier gas in which it should be analyzed. It was then found that the H₂S, HCN, ammonium etc. could be removed from the liquid sample by adjusting its pH sufficiently for releasing the gas. This could simply be obtained by adding a cheap acid to the liquid sample in the case of H₂S or HCN. Mineral acids like sulphuric acid were found to be most suitable. The addition of acid also secures desired pH of the sample regardless of the pH in the liquid media. However, in order to secure complete removal of H₂S or HCN and get a suitable concentration range of the toxic gas, air was mixed into the liquid mixture of acid and sample from sewage etc. In addition to attain intensive mixing, this air reduces the risk for growing or plugging of the equipment. Air together with expelled H₂S or HCN could then be stripped from the liquid and brought into a gas analyzer. The most suitable gas analyzing technique was found to comprise application of an electrochemical gas detector. Equipment for such analysis could easily be calibrated and could be operated continuously, intermittently or by controller. The accuracy would be about 10% of full-scale and measuring range would be minimum 0-5 mg H₂S/1 and maximum 0-50 mg/H S/l. Corresponding measuring ranges could be applied for HCN. In that case the same procedure was used, but the H₂S-gas analyzer was substituted with a HCN-gas analyzer or measuring cell. Analysis of HCN will especially be useful for waste water from nickel plating plants.

The same principle could be used for releasing gases like NH₃ and C0₂ from the liquid media, but in those cases the pH of the liquid had to be raised. Cheap bases like NaOH solutions would be applicable in such cases. Also in these cases the measuring cell would be one designed for the special gas to be analyzed. In connction with the development of the analyzing method it was also found that it was quite essential to have a reliable method and equipment for supplying the analyzer with representative samples of the liquid to be analyzed.

One problem often experienced with the liquids in question is the presence of particulate matter like solid particles etc. It should also be easy to transfer the sample from the sampling site to the analyzer. Firstly, it was decided to apply pressurized gas (air) and vacuum for collecting and transporting samples. These considerations lead to development of a method and equipment comprising the following: Suction of sample into a vessel in which large solid particles can settle before the liquid in the upper part of the vessel is transferred by pressurized air to a sample receiver located at an analyzer in the plant or in a laboratory. The remaining part of the sample contaminated with the settled particles could be returned to the sewer or process by means of pressurized air before collection of a new sample started. The distance between the sampling point and the sampling receiver could be as much as 100 m and the lifting height up to 30 m. Before the sample was supplied to the analyzing system it would preferably pass through a filter for additional removal of solids and sludge. This filter could be placed close to the sample receiver and be self-cleaning, i.e. air can be blown intermittently through the filter.

The scope of the invention is as defined in the attached claims.

The invention is further explained in connection with the description of the drawings (Figs. 1 and 2) .

Fig. 1 shows a flowsheet of the analyzing system. Fig. 2 shows a system and equipment for supplying the analyzing system with samples. In Fig. 1 is shown a system for carrying out the analyzing method. A liquid sample containing the gaseous components like H₂S, HCN, NH4, NH₃, C0₂ etc is fed to a pump 4 through pipe 1. Acid, preferably sulphuric acid, is fed to pump 4 through pipe 2. If a base is used for pH adjustment, it is supplied through pipe 2 instead of the acid. Carrier gas, preferably air, is also fed to pump 4 through pipe 3. The two liquid streams and air are then pumped through a coil 5 for being mixed together and transferred through pipe 6 and mixed with air supplied through pipe 3 in which is a flow meter 18 for regulating the amount of stripping gas. This mixture is fed through pipe 7 into stripping column 8. Liquid 9 is drawn from the column 8 and transferred to a discharge vessel 11 and then returned through pipe 12 to the liquid from which the sample was drawn. The gas mixture (air and gas to be analyzed, H₂S etc.) stripped from the sample is fed to a special gas analyzer for the actual gas expelled from the liquid sample. This analyzer is situated in block 10.

In Fig. 2 is shown a preferred system and equipment for supplying the gas analyzing system A (Fig. 1) with samples. Samples from the sewer liquid to be analyzed are sucked by means of vacuum in pipe 25 into a vessel 14 through pipe 13. A self-cleaning filter 23 can be placed in pipe 13. This sampling operation is con¬ trolled by a valve VI operated by pressurized air/vacuum. Air is supplied from the compressor 19 and the suction is obtained by means of the vacuum source 20 and pipe 25. The sample is left in the vessel 14 for sufficient time to allow the large solid particles to settle. Then the liquid in the upper part Δh of vessel 14 is transferred through pipe 15 into a sample receiver 16. The sample is transferred by means of pressurized air controlled by the valve V2. The sample then will be transferred from the sample receiver 16 through a filter 17 into pipe 1. This pipe 1 corresponds to pipe 1 in Fig. 1. The filter 17 can be cleaned intermittently by means of pressurized air supplied through pipe 21. Part of the sample in receiver 16 flows through a by-pass 22 and pipe 12 back to the liquid process stream. The lower part of receiver 16 can be drained through valve V3 into pipe 12.

VY1-VY3 regulate the corresponding valves V1-V3 by means of pressurized air/vacuum.

The gas-analyzing method according to the invention gives an accurate and reliable analysis which is representative for the concentration of gases like H₂S, HCN, NH₃, C0₂ and the like in a liquid, sewer, waste water etc. The method allows application of simple and sturdy equipment which can be operated continuously, intermittently or by controller. The method is flexible with regard to measuring range. Detection limits can for instance for H₂S and HCN be below 0.1 mg H₂S/1 or 0.1 mg HCN/1, respectively. The only chemical necessary is a cheap acid, preferably H₂Sθ4 or a base and carrier gas, preferably air. To switch from H₂S to HCN analysis for instance only requires substitution of the gas measuring cell.

Representative liquid samples can easily and safely be supplied to the analyzing system by means of pressurized air/vacuum. If the liquid to be analyzed contains solid particles or sludge, this will not cause any problems as it will be removed from the sample by settling and self-cleaning filters. The removed particles and sludge are returned to the liquid to be analyzed. The liquid sample will never be in direct contact with the gas analyzer. Sludges or particles in the liquid to be analyzed will accordingly not do any harm to the analyzer itself or interfere with the analysis. The sample can be collected far from the analyzing system and even be lifted to heights of about 30 m. The preferred sampling system is durable, sturdy and requires little maintenance and can be cleaned by compressed air once per sample cycle. The sample sequence may easily be modified. A complete sampling sequence will be about 2 minutes depending on distance between sampling point and sample receiver.

Claims

1. Method for analyzing the content of gaseous com¬ ponents in liquid media, comprising collecting liquid samples, c h a r a c t e r i z e d i n that the liquid sample is mixed with an acid or base to adjust its pH sufficiently for releasing the gaseous component, and that a carrier gas is supplied to said mixture, whereupon the released gases are mixed with additional amounts of carrier gas and then stripped from the liquid phase and transferred to a gas analyzer and that the liquid phase preferably is returned to the liquid media.

2. Method according to claim 1, c h a r a c t e r i z e d i n that the liquid sample is mixed with a mineral acid, preferably sulphuric acid for releasing hydrogen sulphide or hydrogen cyanide from the liquid sample.

3. Method according to claim 1, c h a r a c t e r i z e d i n that air is applied as carrier gas and that part of the air is supplied to the liquid mixture at or in a pump ahead of a mixer for acid or base, liquid sample and air.

4. Method according to claim 1, c h a r a c t e r i z e d i n that an electrochemical gas detector is applied for analyzing the hydrogen sulphide or hydrogen cyanide.

5. Method according to claim 1, c h a r a c t e r i z e d i n that the liquid sample is sucked into a vessel in which large solid particles are removed by settling, whereupon the purified sample is transferred by means of pressurized gas to a sample receiver from which the sample is transferred and thereupon is mixed with acid for pH adjustment.

6. Method according to claim 1, c h a r a c t e r i z e d i n that ammonium and/or ammonia is released from the liquid sample by means of a base, preferably diluted sodium hydroxide, and that the released and stripped gas is analyzed in an ammonia gas analyzer.

7. Means for sampling liquid to be analyzed according to claim 1, c h a r a c t e r i z e d i n that said means comprises a collecting/settling vessel (14) into which liquid sample is sucked through pipe (13) by means of vacuum in pipe (25) and transferred through pipe (15) to a sample receiver (16) by means of pressurized gas, preferably air, and further in pipe (1) to be mixed with acid or base from pipe (2) and carrier gas from pipe (3) to a mixer (5) .

8. Means according to claim 7 , c h a r a c t e r i z e d i n that between the sample receiver (16) and the pipe (1) is situated a self-cleaning filter (17) which intermittently can be cleaned by means of pressur¬ ized air.

9. Means according to claim 7, c h a r a c t e r i z e d i n that solids having settled in vessel (14) can be re¬ turned to the liquid media through pipe (13) .

10. Means according to claim 7, c h a r a c t e r i z e d i n that a self-cleaning filter (23) is situated below vessel (14) in pipe (13) .