Selective Absorption of Hydrogen Sulfide from Gas Streams H.D. FRAZIER AND A. L. KOHL The Fluor Corporation. Ltd.. Los Angeles. Calif.
T
HE use of amines for It is frequently desirable to remove the hydrogen sumde step. However, this haa the removal of acidic resulted in serious corrosion completely from a gas stream without removing a major components from gas difficulties and inadequate portion of the carbon dioxide. This paper describes a streams was first described hydrogen sulfide removal. process for accomplishing this result by use of solutions of by Bottoms (3, 4 ) in 1930. In this case an absorber, methyldiethanolamine. Laboratory and pilot plant data Triethanolamine (TEA), presented show that the selectivity of these solutions ie utilizing a selective abwhich was the first of the sorbant, could precede a due to both a large differencein the absorption rates of the ethanolamines to become typical monoethanolamine two acid gases and an equilibrium which favors the abcommercially available, was treating plant from which sorption of hydrogen sulfide. The latter property is unused in the early gasexpected in view of the greater acidity normally attributed there would then be obtreating plants. However, tained a relatively pure carto carbon dioxide. A gas-treating pilot plant is described this amine has been disand data are presented relative to operation under a wide bon dioxide stream. placed to a large extent range of conditions from 45 to 750 pounds per square inch LABORATORYSTUDIES by diethanolamine (DEA) gage and with inlet gases containing from about 4 to 50% and m on oe t h a n ol ami ne acid gas. The methyldiethanolamine has been used both In the course of a labora(MEA) which have the adtory search for new and in aqueous solution and in conjunction with diethylene vantage of lower molecular more efficient gas-treating glycol as a combination treating-dehydrating system. weights and are capable of solutions for use in convenProcess described waa found to offer distinct advantages effecting more complete hytional gas-treating plants, over processes used currently for certain applications, drogen sulfide removal. particularly when a concentrated stream of hydrogen data were obtained which Hutchinson (9) improved indicated that a selective sulfide is desired for conversion to sulfur or sulfuric acid. the amine-type gas-treating process could be developed based on the use of methylprocess by adding a dehydiethanolamine (MDEA). drating agent such as diethylene glycol (DEG) to the amine solution, thereby enabling These data and the subsequent progression of the investigation from laboratory through the pilot plant stage are here presented. the same installation to both purify and dehydrate thc gas stream. From over fifty different amines which were examined for possible A number of such plants, which utilize a monoethanolamine-diapplication, methyldiethanolamine was selected as the most ethylene glycol misture for the treatment of natural gas, are o p promising for a selective process on the basis of capacity for erating. Commercial application of this process has been dehydrogen sulfide, selectivity in the presence of carbon dioxide, scribed by Blohm and Chapin ( 1 ) . availability, cost, thermal stability, and physical properties. Of the three amines mentioned, monoethanolamine is generally Figure 1is a plot of data obtained with simple bubblers containpreferred where applicable because of its ability to reduce the ing test solutions through which sour gas was passed while samples hydrogen sulfide concentration in gas streams to estremely low were withdrawn at periodic intervals for later analysis. For comlevels. In combination with diethylene glycol, monoethanolparison, the results obtained with a monoethanolamine solution amine has been shown to produce gas containing less than 0.01 are presented with those of a methyldiethanolamine solution of grain of hydrogen sulfide per 100 standard cubic feet ( 1 4 ) in a approximately the same strength on the basis of molar equivalents contactor operating at approximately 600 pounds per square of amine. Since the gas was bubbled through a simple open tube inch gage. Although no references have been noted in the literaat 77' F. and atmospheric pressure, the efficiency of the apparatus ture to aqueous monoethanolamine solutions attaining this high degree of gas purity, it is generally accepted that such solutions are as a liquid-gas contactor was poor so that the curves clearly demonstrate differences in absorption rates. The rate of absorpcapable of producing purer gas with regard to hydrogen sulfide tion of carbon dioxide in methyldiethanolamine is observed to be than diethanolamine or triethanolamine solutions under the same extremely slow compared to that of hydrogen sulfide or of either conditions. acid gas in monoethanolamine solution. Hydrogen sulfide frequently occurs in gas streams in the Figure 2 is a plot of data taken under similar conditions except presence of large quantities of carbon dioxide. When processes that the open tube was replaced by a fritted glass disk to increase utilizing monoethanolamine and diethanolamine are operated so the contact efficiency. Similar relative absorption rates are obthat complete hydrogen sulfide removal is attained, they also served and, in addition, the approximate equilibrium conditions absorb essentially all the carbon dioxide (IS). There are freare indicated. Although not shown in Figure 2, this procedure quently instances when i t is desirable to leave a major portion of was continued for over 300 minutes or 25 cubic feet with no the carbon dioxide in the treated gas stream even though good appreciable change from the conditions indicated on the graph a t hydrogen sulfide treating may still be essential. Where the 7 cubic feet. It is evident that for both amines, hydrogen sulfide is treating plant regenerator off-gas is to be utilized for the producabsorbed initially to a concentration somewhat higher than the tion of sulfur or sulfuric acid, for example, the advantage of the equilibrium value and is subsequently liberated from the solution more concentrated hydrogen sulfide stream produced by such a as carbon dioside continues to be absorbed. The equilibrium process is obvious. Another example involves plants producing capacities of both solutions are approximately the same with redry ice from the carbon dioxide content of sour natural gas. In gard to hydrogen sulfide but are widely different with regard to an existing plant of this type (6), iron oxide slurry in soda ash carbon dioxide. The fact that approximately the game quantity solution is utilized for the preliminary hydrogen sulfide removal 2288
November 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
of carbon dioxide as hydrogen sulfide is absorbed in the methyldiethanolamine solution at equilibriumis unexpected in view of the higher carbon diovide concentration in the gas and the generally accepted concept of higher acidity of carbonic acid. The selectivity of methyldiethanolamine is apparently not only due to absorption rate effects but to equilibrium considerations m well. Since the preceding data were based on simple laboratory tests, a bench-size pilot plant wm constructed of glass equipment t o carry out the dynamic proceas encountered in a conventional treating plant. Figure 3 is a diagrammatic sketch of this apparatus. The absorption column consisted of a variable number of Oldershaw bubble-plate sections of three, six, nine, or fifteen plates per section (fa). Two of the three-plate sections were used in the still column. Figure 4 is a photograph of the &9 used u+th fifteen
VOLUME OF GAS PASSED, SCF
2289
VOLUME OF 6AS PASSED, SCF
Figure 1. Abso tion Characteristics of Figure 2. Absorption Characteristics MDEA and &Solutions with Open- of MDEA and MEA Solutions with Tube Bubbler Fritted Glaw Bubbler
trays in the absorption column. No attempt was made with this equipment to determine the maximum gas purity obtainable, although in Table I data are shown that indicate that, even at atmospheric pressuro, gas containing approximately 1 grain of hydrogen sulfide pel. 100 cubic feet could be obtained. This table shows data obtained with various solutions including different strengths of aqueous solutions and glycol-amine systems. All data presented were obtained with a 24tray contactor and an inlet gas containing 13.3% carbon dioxide and 5.1% hydrogen sulfide, although other factors such M gas and solution rates to the contactor were variable. These data indicate that over a fairly wide range of concentration, methyldiethanolamine solutions absorbed practically all the hydrogen sulfide while absorbing only about 15 to 20% of the carbon dioxide which entered the column; the amount of carbon dioxide absorbed varies, of course, with operating conditions. Table 11presents a comparison of the performance of three dif-
ferent amines in the g l w pilot plant, including monoethanolamine, triethanolamine, and methyldiethanolamine of commercial gadea tested at absorber temperatures of 77’ F. and about a t mospheric pressure. This shows that although triethanolamine is a tertiary amine, &B is methyldiethanolamine, it is not only less selective than methyldiethanolamine and haa a lower capacity, but it is also not as satisfactory from the standpoint of production of sweet gas. Monoethanolamine, which has adequate capacity and produces sweet gas, was found to remove carbon dioxide completely. These results with respect to triethanolamine and monoethanolamine agree qualitatively with field experience Although triethanolamine has been used in gas-treating processes to some considerable extent, no applications are known to the authors in which such solutions have produced lead acetate sweet gas even a t high pressures although numerous applications of monethanolamine solutions have been reported to yield gas having less than 0.1 grain of hydrogen sulfide per 100 standard cubic feet (IO).
+ SWEETGAS TO ANALMICAL TRAIN
PILOT PLANT
WORK
At this stage of the investigation, the process was considered to be sufficiently well developed
EXCHANGER
COOLING ,420.*
Figure 3. Diagrammatio Sketoh of Glass Apparatus Used for Dynamic studies
to be advanced to the pilot plant stage for further development and testing. Figure 5 shows a front view of the pilot plant used for this testing. The contactor is the taller vessel on the left-hand side of the picture and the still column is shown on the right. Figure 6 is B view of the pilot plant structure from the rear showing the steam condensate accumulator and solution pump shed in the foreground and the analytical bench for the continuou. gas analysis system on the right. The photo on page 2287 is a general view of the pilot plant area showing some of the heat exchangers and gas surge vessels. This photograph was taken from the cooling tower used for dissipating hest load from the reflux ‘Ondenser and the and gas coolers.
2290
INDUSTRIAL AND ENGINEERING CHEMISTRY Table I.
Run
Vol. 42, No. 11
Glass Pilot Plant Operation-MDER Solutions
(24-Tray Contactor; Inlet Gas: 1 3 . 3 % COz, 5.1y0 HzS) Flow Rates Outlet Gas Acid Gab: in Rich Soln. Outlet gas, Solution HIS, g r a i n d l 0 0 C02,stand. Has, stand. stand. cu. ft./hour gal./llou; cot, % stand. cu. ft. CU. ft./cu. ft. cu. ft./cu. f t .
Cot Removed. c/o of Inlet
15% Aqueous Methyldiethanolamine
0.4 0.4
7.9 8.9 12.7
G1 G2 G3
12.1 12.0 12.0
n.4
XU
