1. Field of the Invention
This invention relates to photometric analysis of hydrogen sulfide in alkanol amines and, in particular, to a method and apparatus adapted to measure and to compensate for background photometric absorption caused, for example, by impurities in the amine stream so that an accurate measurement of the concentration of hydrogen sulfide may be obtained.
2. Description of the Prior Art
The amine absorption process by which hydrogen sulfide is removed from refinery "sour" gas is well-known. The mechanism by which hydrogen sulfide is eliminated is understood and relates to the bonding of hydrogen sulfide with an alkanol amine. Typically, a "lean" amine, that is, an amine having an available site at which to bond with hydrogen sulfide, is introduced near the top of a countercurrent absorption tower, or absorber, while a sour gas stream is introduced near the bottom of the absorber. A chemical reaction within the absorber occurs wherein the hydrogen sulfide is bonded to the alkanol amine. The resultant bonded amine, or so-called "rich" amine, is then pumped to the inlet of a stripping column wherein the high temperature associated with high pressure steam is used to strip the hydrogen sulfide from the bonded amines. The stripped amines are conveyed, after cooling, to the absorption tower and the cycle is completed.
The amine-absorption desulfurization process is described in a paper written by R. S. Saltzman entitled "Continuous Photometric Analysis for Measurement and Control in Sulfur Recovery Operations" presented at the Texas A&M University Instrumentation Symposium for Process Industries, January 1977.
For various economic reasons the amount and cost of energy utilized to produce high pressure steam used in the stripping column has become the focus of conservation activity. When energy costs and availability were less important considerations it was the typical practice to use steam to lower the concentration of hydrogen sulfide to a level below that necessary for efficient absorption in the absorber. However, as the cost of energy has increased, this practice is no longer economical. It is, accordingly, important to provide some indication as to the level of hydrogen sulfide remaining in the effluent from the stripping column so that the efficiency of the stripping column may be optimized from an energy consumption standpoint.
To this end it is known that photometric analyzers have been utilized to provide an indication of the amount of hydrogen sulfide remaining in the effluent of the stripping column. One such apparatus is described in the above-mentioned article and is sold by E. I. du Pont de Nemours and Company as the 400 Photometric Analyzer. Such apparatus relies upon the strong ultraviolet radiation absorption characteristic of the alkanol amine-hydrogen sulfide bond to measure the hydrogen sulfide concentration in the stripping column effluent. In general, two problems have been observed with regard to obtaining accurate measurements of the hydrogen sulfide concentration.
The first problem relates to the acute temperature sensitivity of the absorption characteristic of the amine-hydrogen sulfide bond. At higher temperatures, more ultraviolet radiation absorption occurs. Accordingly, it has been necessary to provide a stabilizing heat exchanger in the monitoring path upstream of the photometric analyzer to insure that the sample of the effluent from the stripping column introduced into the sample cell of the analyzer is maintained at a predetermined constant temperature. By this expedient it has been found that the effect of temperature on the ultraviolet radiation absorption characteristic of the amine-hydrogen sulfide bond is minimized.
A further problem relates to the effects of recycled impurities in the amine loop. As the amines become recycled repeatedly it is possible that dirt, corrosion inhibitors, or other contaminants produced by chemical breakdown may occur to form such impurities. Such impurities behave as absorbers of ultraviolet radiation at the same wavelengths as are absorbed by an amine-hydrogen sulfide bond. Accordingly, incorrect measurements of the concentration of hydrogen sulfide in the effluent from the stripping column can occur. This could result in the unnecessary increase in steam use with a concomitant increase in energy consumption. Increased energy consumption under these circumstances would not be economically justified and would, therefore, be disadvantageous. It should be noted that the application of the device mentioned in the referenced article was on specially controlled processes where impurities in the stream were maintained at low levels and the sample temperature was closely regulated.
To obtain a measurement of the hydrogen sulfide concentration in a stream it is known also to introduce a sample of the stream that contains bonded hydrogen sulfide-amine into a vessel and to thereafter introduce a predetermined amount of hydrogen chloride into the vessel. The hydrogen chloride replaces the hydrogen sulfide and the hydrogen sulfide is measured to provide an indication of the concentration of hydrogen sulfide in the sample.