AMMONIA GENERATOR GEA-01

Boudovitch V. L*, Konopelko L. A.**, Polotniouk E. B.*
*CHROMDET/Analytical Instruments
** D. I. Mendeleev Istitute For METROLOGY (VNIIM)

INTRODUCTION

A great variety of gas analytical equipment that is manufactured and used nowadays requires for calibration mixtures having different qualitative and quantitative composition. Up to now a few ways to obtain calibrating gas mixtures are available. The most widespread one is to store the mixtures in gas cylinders. Generally gas cylinders with such mixtures are manufactured at special plants and purchased to the customers. In spite of obvious break-through in manufacturing such mixtures, which occurred during last the few years for the majority of substances the main drawbacks remain as before. The gas cylinders are very successful for the mixtures of constant gases (CH₄, C₂H₆, C₂H₂, CO, CO₂ etc.). But for a number of compounds gas cylinders do not guarantee execution of the principal demands, which are the correctness of the content value of the working substance and its short and long-term stability. Tests of gas mixtures from commercially available gas cylinders often show significant discrepancy between different cylinders and change of the concentration value in time as well as, especially when the gas of interest is reactive and the concentration is low. Only insignificant part of the volatile compounds that are used in industry and have to be monitored is available in gas cylinders. To obtain different values of concentration several cylinders or precise high-cost dilution device are required. In russia the problem of calibrating equipment is complicated by the long distances between manufacturers of analitical equipment and customers, that poses major problems for cylinder transportation. An alternative to store gas mixtures is to generate the working substance when necessary. Permeation tubes are very accurate sources of working substance for calibrating mixtures. However their output is incapable of generating mixtures with high content of working substance, they have long warm-time, are difficult in automation and can not be used in the field. Electrochemical generators are good for generating inorganic vapours: Cl₂, H₂, HCN, H₂S. But they are powerless for the other substances. Recently the new method of generating gas mixtures of known concentrations has been developed and the generator, which is accurate and easy to use, has --been built.

PRINCIPLE OF GENERATOR OPERATION

The principle of generator operation is based on dynamic dilution of the flow of working substance vapour with the flow of cleaned ambient air.As a source of the vapour the working substance dissolved in water (ammonia, ethanol and other soluble in water components) or absorbed with inert sorbent (hydrocarbons and other water-insoluble components) is used. There are no any other limitations for the working substances besides their stability and volatility. The key elements of the generator are shown in the picture. The working substance in reservoir is evaporated, the flow of equilibrium vapour is drawn away with the pump into the dilution unit where it is mixed with the flow of cleaned ambient air. The concentration obtained is monitored with a sensor.

The electrical signal of the sensor is sent to the computer, which controls the dilution conditions. Direct measurement of the concentration of working substance in calibrating mixture and feed back with computer displace the control of parameters of generator operation (including special control of temperature, flow-rates, dilution coefficient and the amount of working substance in the reservoir for evaporation). The calculation of the value of concentration obtained is not necessary also. However, this approach poses strong demands on the measuring sensor in respect to its stability, response time, reproducibility, measuring range. Besides the sensor must be nondestructive. The most of sensors for substance measuring in air do not fit all these demands. Thermocatalitic sensors are not stable enough. Their measuring range is not suitable for the mixtures with low content of working substance. The measuring range of electrochemical sensors is as required, but their response time is too high (no less than 20 s) and stability is too low. Flame-ionization detector fits all the demands but it is a destructive one. Nowadays the only sensor that can be used in the generator is photoinization detector (PID)*, having stability no less than 2%, response time no more 1 c, measuring range 0 – 10000 ppm, life-time of photoionization lamp is about 10000 hours. PID is highly sensitive to the majority of organics, ammonia, hydrogen sulfide, carbon sulfide etc. It can be easily calibrated with commercially available gas mixtures (for example isobutylene). To measure the concentration of different substances the correlation coefficients are used that are inherent to photoionization lamp and are very stable. The novel method of dilution (patent pending) used in generator allows to change continuously the concentration of working substance in the mixture in a wide range. The flow rate of calibrating mixture is maintained constant in the wide range of concentrations without using additional dilution means or by-pass discharge. The minimum concentration that can be generated depends on the PID detection limit and the degree of air purity. If the air is cleaned with activated carbon PID allows to obtain mixtures with content of aromatics from 0,1 ppm up to low percent level. Due to the absence of thermostated volumes generator does not need much warm-up-time and time to pass from one concentration to another. The operation of generator with ammonia as working substance has been studied in detail. The aqua ammonia with different ammonia content was used as a source of ammonia vapour. It was found that the deviation of ammonia concentration in calibrating mixture from preset value depends on the concentration of ammonia in aqua ammonia solution. The curves demonstrating this dependence are shown in Fig 2 and Fig. 3.

At low concentration of ammonia in working liquid (0,1 – 0,2%) more accurate values of ammonia concentration in the calibrating mixture are available in the range of tens ppm. 1% aqua ammonia solution provides the operation of generator in the range of 10 – 2000 ppm. Thus when the “correct” solution is used it is possible to reach the value of deviation as small as 2% in the range from 10 to 2000 ppm. The specifications of the ammonia generator GEA-01 are given in the table below.

AMMONIA GENERATOR OPERATION PARAMETERS

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