A PROCESS FOR REMOVAL OF HYDROGEN SULFIDE FROM GASEOUS MIXTURES WITH SEVERELY STERICALLY HINDERED SECONDARY AMINO COMPOUNDS

Abstract: The removal of H₂S from a normally gaseous stream is accomplished with an absorbent solution containing a sterically hindered secondary amino alcohol having a cumu­ lative -E s value (Taft's steric hindrance constant) greater than about 1.75. In a preferred embodiment the gaseous stream also contains CO₂, and the H₂S is selectively removed from the stream. The most preferred amino alcohol herein is tertiarybutylaminoethoxyethanol.

Description: This invention relates to a process for the removal of H2S from normally gaseous mixtures wherein absorbent solutions containing severely sterically hindered secondary amino alcohols are employed. More preferably, this invention is directed to a process for selectively removing HZS from gaseous mixtures containing H2S and CO2 employing these absorbent solutions. The treatment of acid gas mixtures containing, inter alia, CO2 and H2S with aqueous amine solutions in an absorber tower typically results in the simultaneous removal of substantial amounts of both the CO2 and H2S. For example, in one such process generally referred to as the "aqueous amine process," relatively concentrated amine solutions are employed. A recent improvement on this process involves the use of sterically hindered amines to obtain nearly complete removal of acid gases such as C02 and H2S. This type of process may be used where the partial pressures of the CO2 and related gases are low. Another process often used for specialized applications where the partial pressure of CO2 is extremely high and/or where many acid gases are present, e.g., H2S, COS, CH3SH and CS2 involves the use of an amine in combination with a physical absorbent, generally referred to as the wnonaqueous solvent process." An improvement on this process involves the use of sterically hindered amines and organic solvents as the physical absorbent. It is often desirable, however, to treat acid gas mixtures containing both CO2 and H2S so as to remove the H2S selectively from the mixture, thereby minimizing removal of the CO2. Selective removal of H2S results in a relatively high H2S/C02 ratio in the separated acid gas which simplifies the conversion of H2S to elemental sulfur using the Claus proc#ess. The typical reactions of aqueous secondary and tertiary amines with C02 and H2S can be represented as follows: H2S + R3N# # R3NH+ + SH (1) H2S + R2NH # R2NH2+ + SH (2) C02 + R3N + H2O # R3NH+ + HC03- (3) C02 + 2R2NH # R2NH2+ + R2NCOO (4) wherein R is an organic radical which may be the same or different and may be substituted with a hydroxyl group. The above reactions are reversible, and the partial pressures of both CO 2 and H2S are thus important in determining the degree to which the above reactions occur. While selective 1125 removal is applicable to a number of gas treating operations including treatment of hydrocarbon gases from shale pyrolysis, refinery gas and natural gas having a low H2S/C02 ratio#, it is particularly desirable in the treatment of gases wherein the partial pressure of 1125 is relatively low compared to that of CO2 because the capacity of an amine to absorb H2S from the latter type gases is very low. Examples of gases with relatively low partial pressures of H2S include synthetic gases made by coal gasification, sulfur plant tail gas and low-Joule fuel gases encountered in refineries where heavy residual oil is being thermally converted to lower molecular weight liquids and gases. Solutions of primary and secondary amines such as monoethanolamine (MEA), diethanolamine (DEA), dipropanolamine (DPA), and hydroxyethoxyethylamine (DGA) are known to absorb both 1125 and CO2 gas. Diisopropanolam#ine (DIPA) is relatively unique among secondary aminoalcohols in that it has been used industrially, alone or with a physical solvent such as sulfolanei, for selective removal of H2S from gases containing H2S and CO2. The tertiary amine, methyldiethanolamine (MDEA), has a high degree of selectivity toward 1125 absorption over CO2, but recently it was discovered that aqueous solutions of dialkylmonoalkanolamines, and particularly diethylmonoethanolamine (DEAE), have higher selectivity and capacity for H2S removal at higher loading levels than MDEA solutions. It nas now been discovered that absorbent solutions of a certain class of amino alcohols defined as severely sterically hindered amino alcohols have a high selectivity for H25 compared to C02. These amino alcohols surprisingly maintain their high selectivity at high H2S and CO2 loadings. In general, the present invention relates to a process for the removal of H25 from a normally gaseous stream with an absorbent solution containing a severely sterically hindered secondary amino alcohol having a cumulative-Es value (Taft's steric hindrance constant defined hereinbelow) greater than 1.75. In a preferred embodiment of the invention the process involves the selective absorption of H2S from a normally gaseous stream also containing CO2 by contacting the stream with a solution of the amino alcohol under conditions whereby H2S is selectively absorbed from the stream, wherein the amino alcohol is additionally characterized by having a PRa value at 200C greater than 8.6. The absorbent solution may additionally contain water, a physical absorbent, preferably sulfolane, or a mixture thereof. In a still more preferred embodiment, after the H2S is selectively absorbed from the stream, the absorbent solution containing the H2S is regenerated at least partially and the regenerated solution is recycled for selective absorption of H2S by recontacting it with the stream. Preferably, the regeneration step is carried out by heating and stripping the solution of any of its absorbed H25 and more preferably by heating and stripping the solution with steam. In the drawings Fig. 1 is a diagrammatic flow sheet illustrating an experimental sparged absorber unit for use in rapid determination of-the selectivity of the amino compound for selective removal of H2S from gaseous streams containing H2S and CO2. Fig. 2 graphically illustrates the selectivity of H2S plotted against moles of H2S and CO2 loading per mole of amine for tertiarybutylaminoethoxyethanol (TBEE) and tertiarybutylaminoethanol (TBE) as compared to methyldiethanolamine (MDEA) as control. Fig. 3 graphically illustrates the selectivity for 1123 plotted against the H2S and CO2 loading for tertiarybutylaminoethoxyethanol (TBEE) as compared to methyldiethanolamine (MDEA), secondarybutylaminoethoxyethanol (SBEE) and isopropylaminoethoxyethanol (IPEE) as control. Fig. 4 graphically illustrates the selectivity for 1123 plotted against moles of H2S and CO2 loading for 3-(tertiarybutylamino)-l-propanol (TBP), 3-aza-2,2dimethyl-1,6-hexanediol (ADMHD), 3-(tertiarybutylamino)-nbutanol (TBB), 2-tertiarybutylamino)-1-propanol (2-TBP) as compared to methyldiethanolamine (MDEA) as control. Fig. 5 is a diagrammatic flow sheet illustrating an absorption-regeneration unit for selective removal of H2S from gaseous streams containing H2S and CO2. Fig. 6 graphically illustrates the H2S leak rate plotted against 1125 loading in mole/mole amine for tertiarybutylaminoethoxyethanol (TBEE) as compared to methyldiethanolamine (MDEA) as control using the unit of Fig. 5. The severely sterically hindered secondary amino alcohols herein may have either acyclic (linear or branched) or cyclic moieties attached to the nitrogen atom(s) of the amino compounds. By the term "severely sterically hindered" it is meant that the nitrogen atom of the amino moiety is attached to one or more bulky carbon groupings. Thus, the severely sterically hindered amino compounds herein have a degree of steric hindrance such that the cumulative -Es value (Taft's steric hindrance constant) is greater than about 1.75 as calculated from the values given for primary amines in Table V in D. F. DeTar, Journal of Organic Chemistry, 45, 5174 (1980). Another means for determining whether a secondary amino alcohol is "severely sterically hindered" is by measuring its 15N nuclear magnetic resonance (NMR) chemical shift. By such measurements it has been found that the "ordinary sterically hindered secondary amino alcohols may have a 15N NMR chemical shift greater than about d +40 ppm, when a 90% by wt. amine solution in 10% by wt. D2O at 350C is measured by a spectrometer using liquid (neat) ammonia at 250C as a zero reference value. For example, severely sterically hindered amino alcohols such as 2-(2-tertiarybutylamino) propoxyethanol, 3-(tertiarybutylamino)-l-propanol, 2-(2-isopropylamino) propoxyethanol and tertiarybutylaminoethoxyethanol had measured 15N NMR chemical shift values of J +74.3, 6+65.9, #+65.7, and 6#+60.5 ppm, respectively, whereas the ordi- nary sterically hindered amine, secondarybutylaminoethoxyethanol,- and the nonsterically hindered amine, n-butylaminoethoxyethanol, had measured 15N NMR chemical shift values of z+48.9 and z35.8 ppm, respectively. When the cumulative E5 values (Taft's steric hindrance constants of these amines) are plotted against the 15N NMR chemical shift values of the amino alcohols mentioned above, a straight line is observed. As evident from the data shown herein, the amino alcohols of the invention referred herein as "severely sterically hindered amino alcohols" were analyzed as having 15N NMR chemical shift values greater than # +50 ppm under the test conditions described above and had a higher 1125 selectivity than those amino alcohols having an 15N NMR chemical shift less than 6+50 ppm. The tertiary amino compound used for comparison herein, methyldiethanolamine, had a measured 15N NMR chemical shift value of C27.4. The amino alcohol of this invention is an amino alcohol or aminoether alcohol and more preferably is of the general formula: EMI5.1 wherein R1 is a primary alkyl or primary hydroxyalkyl radical having 1 to 8 carbon atoms or a branched chain alkyl, branched chain hydroxyalkyl, cycloalkyl or hydroxycycloalkyl radical having a total of 3 to 8 carbon atoms, R2, R3, R4 and R5 are each independently hydrogen or a C1-C4 alkyl or C1-C4 hydroxyalkyl radical, with the proviso that when the carbon atom of R1 directly bonded to the nitrogen atom is primary, both R2 and R3 (bonded to the carbon atom directly bonded to the nitrogen atom) are alkyl or hydroxyalkyl radicals and that when the carbon atom of R1 directly bonded to the nitrogen atom is secondary, at least one of R2 or R3 (bonded to the carbon atom directly bonded to the nitrogen atom) is an alkyl or hydroxyalkyl radical, x and y are each positive integers independently ranging from 2 to 4 and z is 0 or or a positive integer ranging from 1 to 4. Preferably, R1 is a tertiary butyl, secondary butyl, isopropyl, tertiary amyl or cyclohexyl radical. Representative of the amino alcohols useful herein are tertiarybutylaminoethoxyethanol, 2-(2-tertiarybutylamino) propoxyethanol, tertiaryamylaminoethoxyethanol, (1-methyl-l-ethylpropylamino) ethoxyethanol, 2-(2-iso propylamino) -propoxyethanol, tertiarybutylaminvethanol, 2-(isopropylamino)-l-propanol, 3-(tertiarybutylamino)-nbutanol, 2-(tertiary-butylamino)-l-propanol, 3- (tertiarybutylamino)-l-propanol, 3-aza-2, 2-dimethyl-l, 6-hexanedial, and 2,5-diaza-2,4,4-trimethyl-7-heptanol. Of these, the most preferred amino compound herein is tertiarybutylaminoethoxyethanol. It is noted that the amino alcohols herein are preferably aminoether alcohols because the indiscriminate increase in molecular weight of tertiary amino alcohols or sterically hindered secondary amino alcohols, for the purpose of lowering volatility, may have an adverse effect on the utility of the non-ether amino alcohol as an absorbent for aqueous gas scrubbing purposes. Simple molecular weight changes may decrease (a) aqueous solubility and/or (b) amine base strength. The volatility losses of amino alcohols without an ether functionality can be controlled or completely eliminated by the alkoxylation of severely sterically hindered secondary amino alcohols. The process of alkoxylation has been found to have a negligible effect on the pK, of the amino alcohol so that the gas scrubbing chemistry which is a function of base strength is not altered. Significantly, it has been found that the increase in molecular mass by alkoxylation does not decrease aqueous solubility. ¯ In fact, in some cases alkoxylation can greatly increase aqueous solubility. The amino alcohols used in the selective absorption process of the present invention are further characterized as having a pK, value at 200C greater than 8.6, preferably greater than about 9.5 and more preferably the pK, value of the amino alcohol will range between about 9.5 and about 10.6. If the pK, is less than 8.6 the reaction with H2S is decreased, whereas if the pxa of the amino alcohol is much greater than about 10.6, an excessive amount of steam is required to regenerate the solution. Also, to ensure operational efficiency with minimal losses of the amino alcohol, the amino alcohol may have a relatively low volatility. For example, the boiling point of the amine (at 760 mm) is typically greater than about l800C, preferably greater than 2000C, and more preferably greater than 225or. The amino alcohols herein may also be -characterized by their high solubility in water at selective H2S removal conditions, and most of the compounds are also generally soluble in polar organic solvent systems which may or may not contain water. The term "absorbent solution" as used herein comprises a solution wherein the amino compound is dissolved in a solvent, preferably selected from water or a physical absorbent or a mixture of water and physical absorbent. Solvents which are physical absorbents (as opposed to the amino compounds which are chemical absorbers) by definition function to solubilize the amino compounds and include, e.g., aliphatic acid amides, N-alkylated pyrrolidones, sulfones, sulfoxides, glycols and the mono- and diethers thereof. The preferred physical absorbents herein are sulfones, and most particularly, sulfolane. The absorbent solution may have a concentration of amino alcohol of about 0.1 to 6 moles per liter of the total solution, and preferably 1 to 4 moles per liter, depending primarily on the specific amino alcohol employed and the solvent system utilized. If the solvent system is a mixture of water and a physical absorbent, the typical effective amount of the physical absorbent employed may vary from 0.1 to 5 moles per liter of total solution, and preferably from 0.5 to 3 moles per- liter, depending mainly on the type of amino alcohol being utilized. The dependence of the concentration of amino alcohol on the particular compound employed is significant because increasing the concentration of amino alcohol may reduce the basicity of the absorbent solution, thereby adversely affecting its selectivity for 1125 removal, particularly if the amino alcohol has a specific aqueous solubility limit which will determine maximum concentration levels within the range given above. It is important, therefore, that the proper concentration level appropriate for each particular amino alcohol be maintained to ensure satisfactory results. The solution of this invention may include a variety of additives typically employed in selective gas removal processes, e.g., antifoaming agents,# antioxidants, corrosion inhibitors, and the like. The amount of these additives will typically be in the range that they are effective, i.e., an effective amount. Also, the amino alcohols described herein may be admixed with other amino compounds (i.e., any amines other than those employed as amino alcohols herein) as a blend, preferably with methyldiethanolamine, to form the absorbent solution. The ratio of the respective amino compounds may vary widely, for example, from 1 to 99 weight percent of the severely sterically hindered secondary amino alcohols described herein. Three characteristics which are of ultimate importance in determining the effectiveness of the amino alcohol compounds herein for H2S removal are "selectivity", "loading" and "capacity". The term "selectivity" as used throughout the specification is defined as the following mole ratio fraction: (moles of H2S/moles of CO,) in liquid phase (moles of H2S/moles of C02) in gaseous phase The higher this fraction, the greater the selectivity of the absorbent solution for the H2S in the gas mixture. By the term "loading" is meant the concentraction of the H2S and C02 gases physically dissolved and chemically combined in the absorbent solution as expressed in moles of gas per mole of the amine. The best amino compounds are those which exhibit good selectivity up to a relatively high loading level. The amino compounds used in the practice of the present invention may have a "selectivity of not substantially less than 10 at a "loading" of 0.1 moles, preferably, a "selectivity" of not substantially less than 10 at a loading of 0.2 or more moles of H2S and CO2 per mole of the amino compound. "Capacity" is defined as the moles of 1125 loaded in the absorbent solution at the end of the absorption step minus the moles of 112S loaded in the absorbent solution at the end of the desorption step. High capacity enables one to reduce the amount of amine solution to be circulated, use less heat or steam during regenerations. The amino compounds herein are typically capable of reducing the H2S in gaseous mixtures to a relatively low level, e.g., less than about 200 ppm and typically have a relatively high capacity for H2S, e.g., greater than about 0.2 mole of H2S per mole of amine. The amino compounds may be characterized as having a "kinetic selectivity" for H2S, i.e., a faster reaction rate for 1125 than for C02 at absorption conditions. In addition, they may be characterized as having a higher capacity for H2S at equivalent kinetic selectivity for H2S over CO2. This higher capacity results in the economic advantage of lower steam requirements during regeneration. The normally gaseous stream herein necessarily includes 1125, ordinarily in gaseous form. (The term "normally refers to ambient conditions of temperature and pressure.) It may optionally include other gases such as CO2, N2, CH4, H2, CO, 1120, COS, HCN, C2H4, NH3, and the like. Often such gas mixtures are found in combustion gases, refinery gases, town gas, natural gas, syn gas, water gas, propane, propylene, heavy hydrocarbon gases, etc. The absorbent solution herein is particularly effective when the gaseous mixture is a gas, obtained, for example, from shale oil retort gas, coal or gasification of heavy oil with air/steam or oxygen/steam thermal conversion of heavy residual oil to lower molecular weight liquids and gases, or in sulfur plant tail gas clean-up operations. The absorption step of this invention generally involves contacting the normally gaseous stream with the absorbent solution in any suitable contacting vessel. In such processes, the normally gaseous mixture from which the 1125#is to be removed may be brought into intimate contact with the absorbent solution using conventional means, such as a tower or vessel packed with, for example, rings or with sieve plates, or a bubble reactor. In a typical mode of practising the invention wherein H2S is selectively absorbed from a mixture also containing COZI the absorption step is conducted by feeding the normally gaseous mixture into the lower portion of the absorption tower while fresh absorbent solution is fed into the upper region of the tower. The gaseous mixture, freed largely from the 1125, emerges from the upper portion of the tower, and the loaded absorbent solution, which contains the selectively absorbed H2S, leaves the tower near or at its bottom. Preferably, the inlet temperature of the absorbent solution during the absorption step is in the range of from about 20 to about 1000C, and more preferably from 40 to about 600C. pressures may vary widely; acceptable pressures are between 0.4 and 140.6 kg/cm2, preferably 1.4 and 105.5 kg/cm2, and most preferably 1.8 and 70.3 kg/cm2 in the absorber. The contacting takes place under conditions such that the 1125 is selectively absorbed by the solution. The absorption conditions and apparatus are designed so as to minimize the residence time of the liquid in the absorber to reduce C02 pickup while at the same time maintaining sufficient residence time of gas mixture with liquid to absorb a maximum amount of the H2S gas. The amount of liquid required to be circulated to obtain a given degree of H2S removal will depend on the chemical structure and basicity of the amino compound and on the partial pressure of H2S in the feed gas. Gas mixtures with low partial pressures such as those encountered in thermal conversion processes will require more liquid under the same absorption conditions than gases with higher partial pressures such as shale oil retort gases. A typical procedure for the selective H2S removal phase of the process comprises selectively absorbing H2S via countercurrent contact of the gaseous mixture containing H2S and C02 with the aqueous solution of the amino compound in a column containing a plurality of trays at. a low temperature, e.g., below 450C, and at a gas velocity of at least about 9.1 cm/sec (based on "active" or aerated tray surface), depending on the operating pressure of the gas, said tray column having fewer than 20 contacting trays, with, e.g., 4-16 trays being typically employed. Alternatively, packed beds may:. be used in place of trays. After contacting the normally gaseous mixture with the absorbent solution, which becomes saturated or partially saturated with HZS, the solution may be at least partially regenerated so that it may be recycled back to the absorber. As with absorption, the regeneration may take place in a single liquid phase. Regeneration or desorption of the acid gases from the absorbent solution may be accomplished by conventional means such as pressure reduction of the solution or increase of temperature to a point at which the absorbed H2S flashes off, or by passing the solution into a vessel of similar construction to that used in the absorption step, at the upper portion of the vessel, and passing an inert gas such as air or nitrogen or preferably steam upwardly through the vessel. The temperature of the solution during the regeneration step should be in the range from about 50 to 1700C, and preferably from about 80 to l200C, and the pressure of the solution on regeneration should range from about 0.04 to about 7.0 kg/cm2, preferably 0.07 to about 3.5 kg/cm2. The absorbent solution, after being cleansed of at least a portion of the H2S gas, may be recycled back to the absorbing vessel. Makeup absorbent may be added as needed. In the preferred regeneration technique, the H2S-rich solution is sent to the regenerator wherein the absorbed components are stripped by the steam which is generated by re-boiling the solution. Pressure in the flash drum and stripper is usually -0.07 to about 3.5 kg/cm2, preferably 1.05 to about 2.1 kg/cm2, and the temperature is typically in the range from about 50 to l700C, preferably about 80 to 1200C. Stripper and flash temperatures will, of course, depend on stripper pressure; thus, at about 1.05 to 2.1 k#g/cm2 stripper pressures, the temperature will be about 80 to about 1200C during desorption. Heating of the solution to be regenerated may very suitably be effected by means of indirect heating with low-pressure steam. It is also possible, however, to use direct injection steam. The amino alcohols herein are found to be superior to those used in the past, particularly to MDEA and DEAE, in terms of both selectivity and capacity for maintaining selectivity over a broad loading range. Typically, a gaseous stream to be treated having a 1:10 mole ratio of H2S:CO2 from an apparatus for thermal conversion of heavy residual oil, or a Lurgi coal gas having a mole ratio of H2S:CO2 of less than 1:10 will yield an acid gas having a mole ratio of H2S:CO2 of about 1:1 after treatment by the process of the present invention. The process herein may be used in conjunction with another 1125 selective removal process; however, it is preferred to carry out the process of this invention by itself, since the amino compounds are extremely effective by themselves in preferential absorption of H2S. The invention is illustrated further by the following examples, which, however, are not to be taken as limiting in any respect. All parts and percentages, unless expressly stated to be other wise, are by weight. All pKa values, unless otherwise noted, are measured at 200C. The abbreviation b.p. designates boiling point. EXAMPLE 1 Preparation of tertiarybutylaminoethoxyethanol A solution of 41.76 g of tertiarybutylamine and 28.66 g of 2-chloroethoxyethanol in 115 ml of ethanol was charged to a 300 ml metal autoclave and heated at 1500C for 3 hrs. The contents of the cooled reactor was treated with KOH, refluxed for one hour, cooled, filtered, vacuum stripped and distilled. The product, obtained from the distillate in a 77% yield, was analyzed to be tertiarybutylaminoethoxyethanol having a b.p. of l170C at 10 mm. EXAMPLE 2 Selective 1123 removal from a mixture containing U#S and C02 Fig. 1 illustrates the sparged absorber unit, operated on a semibatch mode, used to evaluate the selectivity for H2S removal of the amino compounds of the invention herein. A gas mixture comprised of 10% C02, 1% H2S and 89% N21 expressed in volume percent, respectively, was passed from a gas cylinder (not shown) through line 30 to a meter 31 measuring the rate at which the gas is fed to the absorber. For all examples this rate was 3.6 liters per minute. The gas was then passed through line 32 to a gas chromatography column (not shown) continuously monitoring the composition of the inlet gas and through lines 33 and 34 to a sparged absorber unit 35, which is a cylindrical glass tube 45 cm high and 3.1 cm in diameter charged with 100 ml of the absorbent amine solution 36. The gas was passed through the solution at a solution temperatue of 400C, and 10-ml samples of the solution were periodcially removed from the bottom of the absorber unit through lines 34 and 37 to be analyzed for H2S and C02 content. The 112S content in the liquid sample was determined by titration with silver nitrate. The C02 content of the liquid sample was then analyzed by acidifying the sample with an aqueous solution of 10% HC1 and measuring the evolved C02 by weight gain on NaOHcoated asbestos. While the solution was being periodically withdrawn from the bottom of the absorber unit, the gas mixture was removed from the top thereof via line 38 to a trap 39 which served to scrub out any H2S in the outlet gas. The resulting gas could optionally then be passed via lines 40 and 41 for final disposal or via line 42 to a gas chromatography column (not shown) for periodic evaluation of the composition of the outlet gas to check for system leaks. For purposes of the examples, the H2S and C02 contents of the inlet gas phase were measured and the H2S and C02 contents of the liquid phase were determined as described above. These data were used to calculate selectivity values of the amine as defined above, which were plotted as a function of the loading of the absorbent solution with H2S and C02, in units of moles acid gas per mole of the amino compound. The above procedure was employed using as the absorbent solution aqueous 3M solutions of tertiarybutylaminoethoxyethanol (TBEE), tertiarybutylaminoethanol (TBE) and methyldiethanolamine (MDEA) as control. The selectivity plot for each amine solution is indicated in Fig. 2. As can be seen from Fig 2, both TBEE and TBE are superior to MDEA in terms of H2S selectivity, particularly at high H2S and C02 loadings. It is noted that TBEE is even superior to TBE even though both compounds have a pKa of about 10.3. EXAMPLE 3 The procedure of Example 2 was repeated using as the absorbent solution aqueous 3M solutions of tertiary butylaminoethoxyethanol (TBEE), secondarybutylaminoethoxyethanol (SBEE) as comparison, isopropylaminoethoxyethanol (IPEE) as comparison, and methyldiethanolamine (MDEA) as comparison. The selectivity plot for each amine solution is shown in Fig. 3. The plots clearly show that TBEE is far superior to SBEE, IPEE and MDEA in terms of H2S selectivity. The plots taken together with the summation (Taft) E5 values for the compounds tested also indicate that the severe steric hindrance of TBEE (resulting from the additional carbon atom attached to the carbon atom directly attached to nitrogen atom) has a profound effect on the selective 1125 absorption properties of the compound, EXAMPLE 4 The procedure of Example 2 was repeated using as the absorbent solution aqueous 3M solutions of (l-methyl l-ethyl-propylamino)ethoxyethanol (b.p. 1530C (2mm) pK, = 10.10) and tertiaryamylaminoethoxyethanol (b.p. 1240C (lOmm), pKa = 10.05) . The H2S selectivity using each of these solutions was superior to MDEA, but not quite as good as TBEE. EXAMPLES 5 The procedure of Example 2 was repeated using as the absorbent solution aqueous 3M solutions of 3-(tertiary butylamino) -1-propanol (TBP) , 3-aza-2, 2-dimethyl-l , 6- hexanediol (ADMHD), 3-(tertiarybutylamino)-n-butanol (bob) and 2- (tertiarybutylamino) -1-propanol (2-TBP) . The selectivity plots for these secondary amino alcohols as compared to methyldiethanolamine (MDEA) as control are indicated in Fig. 4. The data in Figs. 2-4 also show that the amino alcohols of the present invention have very high capacity for both H2S and C02 compared to methyldiethanolamine (MDEA) in addition to high H2S selectivities. It will be apparent from an inspection of the data in Figs. 2-4 that if the absorption process is conducted under conditions such that the amino compound has a long contact time with the gases to be absorbed, the selectivity for H2S decreases, but the overall capacity for both C02 and H2S remains rather high. Therefore, one may, in some instances, wish to carry out a "non-selective" absorption process to take advantage of the large absorption capacity of the amino alcohols of the invention. Accordingly, one may carry out a "non-selective" acid gas removal absorption process using the amino compounds of the invention. Such "non-selective" processes are particularly useful in scrubbing natural gases which contain relatively high levels of H2S and low to nil levels of CO2. As such, the amino alcohols of the invention may replace some or all of monoethanolamine (MEA) or diethanolamine (DEA) commonly used for such scrubbing processes. EXAMPLE 6 Several secondary aminoalcohols and aminoether alcohols were further examined to determine their boiling points (b.p.), PKa values at 200C and their cumulative -Es values (Taft's steric hindrance constants) by using the Taft -Es value system presented by D. F. DeTar, Journal Organic Chemistry, 45, 5174 (1980) in Table V. The results of these evaluations are shown in Table I. TABLE I Amino Compound Pka b.P., C -E (mm) cH3 | CH3-C-NH-CH2CH2OH (TBE) 10.2 90(25) 2.10 C113 CH3 CH3-C-NH-CH2CH2-O-CH2CH2OH (TBEE) 10.3 117(10) 2.10 CH3 CH3 CH3-C-NH-CH2CH2-CH2OH(TBp) 11.05 106(20) 2.13 C113 CH3 CH3 CH3CH-NH-CHCH20E 10.40 75(22) 1.86 CH3 HO-CH2-C-NH-CH2CH2CH20H (ADMHD) 10.05 125(0.8) 2.13 CH3 CH3 CH3-C-NH-CHCH2CH20H 10.8 90(10) 2.7 CH3 CH3 CH3 CH3-C-NH-CHCH20H 10.63 85(20) 2.67 CH3 CH3CH-NH-CHCH20CH2CH20H 10.4 119(20) 1.86 CH3 CH3 CH3C-NH-CHCH2OCH2CH20H 10.6 122(19) 2.67 CH3 Comparative Compounds CH3 | CH3CH-NH-CH2CH20CH2CH20H (IPEE) 10.3 104(7) 1.29 C113 CH3CH2CH-NH-CH2CH20CH2CH20H (SBEE) 10.3 118(10) 1.32 CH3N-(CH2CH3)2 (MDEA) 0.79 It can be seen from the values in the table that the severely sterically hindered amino alcohols of the invention have cumulative (Taft) -Es values greater than 1.75. Those secondary amino compounds having cumulative -Es values below 1.75 have been found to lack the ability to selectively absorb H2S versus C02 at a level greater than MDEA apparently due to the propensity of the amine to react with the COZ. EXAMPLE 7 Continuous Process for Selective HnS Removal This example demonstrates a comparison of the severely sterically hindered secondary aminoether alcohol, tertiarybutylaminoethoxyethanol (TBEE), with methyldiethanolamine (MDEA) using a unit such as that shown in Fig. 5. An aqueous 1.5M solution of tertiarybutylaminoethoxyethanol (TBEE) was charged via pipe 5 into the upper portion of a gas-liquid countercurrent absorption column 2 having a lower section 3 and an upper section 4 and being segregated by three packed beds. The gas to be purified was a synthetic mixture containing 1% H2S, -11% C02 and 88% N2, expressed in volume percent, respectively. The gas was introduced through line 1 into the lower portion of column 2 at a gas flow rate of 67 standard cubic feet per minute. The absorber pressure was adjusted to 2.2 kg/cm2. The gas freed from H2S left through pipe 6 and was analyzed. The aqueous amine solution containing 1125 and C02 was flowed toward the lower portion of the absorber, from which it was discharged through pipe 7. The solution was then passed via pump 8 through a simulated heat exchanger and cooler 9 disposed in pipe 7, which allowed the hot solution from the regenerator 12 to equilibrate in temperature with the cooler solution from the absorber column 2. The hot rich solution was then entered, via pipe 7, to a flash drum 10 equipped with a line (not shown) which vents to line 13 and then introduced by pipe 11 into the upper portion of the regenerator 12, which is equipped with several plates and effects desorption of the H2S gas. The pressure of the regen erator was set at 2.3 kg/cm2. The H2S gas was passed through a pipe 13 into a condenser 14 wherein cooling and condensation of water and amine solution from the gas occurred. The gas was then entered into a separator 15 wherein further condensation took place. The condensed solution was returned through pipe 16 to the upper portion of the regenerator 12. The gas remaining from the condensation, which contains H2 and some CO2, may be removed through pipe 17 for final disposal (e.g., to a vent or incinerator or to an apparatus which converts the H2S to sulfur, such as a Claus unit or a Stretford conversion unit (not shown)). The solution liberated from the gas which it contained was flowed downward through the regenerator 12 and -left ' through pipe 18 at the bottom of the regenerator for transfer to a reboiler 19. Reboiler 19r equipped with a steam injection pipe 20 and a condensate exit pipe (not shown), vaporized a portion of this solution to drive additional 1125 therefrom. The H2S driven off was returned via pipe 21 to the lower section of the regenerator 12 and left through pipe 13 for entry into the condensation stages of gas treatment. The solution remaining in the reboiler 19 was drawn through pipe 22, cooled in heat exchanger 9, and introduced via the action of pump 23 through pipe 5 into absorber 2. The flow rate was slowly adjusted downward until the H2S leak rate showed a dramatic increase. The experiment was repeated using in place of tertiarybutylaminoethoxyethanol (TBEE), an aqueous 3M solution of methyldiethanolamine (MDEA) as control. The results of both experiments were plotted wherein the H2S leak rate, in parts per million by volume (Vppm), was plotted against H2S loading in moles/mol amine. The results are shown in Fig 6. These results show that tertiarybutylaminoethoxyethanol gives far greater cleanup, even at higher H2S loadings, in a continuous selective H2S removal process than methyldiethanolamine (MDEA) In summary, this invention is seen to provide a process for removing H2S from gaseous streams utilizing a special class of amino compounds characterized as severely sterically hindered secondary amino compounds which have a high selectivity for H2S in preference to C02 which selectivity is maintained at high H2S and C02 loading levels.

Claims: C L A I M S: 1. A process for removing H2S from a normally gaseous stream with an absorbent solution, said solution comprising a solvent and a sterically hindered secondary amino alcohol having a cumulative E5 value greater than 1.75. 2. A process according to claim 1 wherein the solvent is water, a physical absorbent or a mixture thereof. 3. A process according to either of claims 1 and 2 wherein the normally gaseous stream also contains C02 and the H2S is selectively absorbed from the stream. 4. A process according to any one of claims 1 to 3 wherein the amino alcohol has a pKa value at 20-C greater than 8.6. 5. A process according to any one of claims 1 to 4 wherein the total concentration of amino alcohol in the absorbent solution is in the range of from about 0.1 to 6 moles per litre, the process is conducted at a temperature ranging from about 20 to lOOC and at a pressure ranging from 0.4 to 140.6 kg/cm2, the absorbent solution is regenerated by heating at a temperature ranging from 50 to about 1700c and at a pressure ranging from 0.07 to about 3.5 kg/cm2 and the solution is stripped of its absorbed H2S. 6. A process according to any one of claims 1 to 5 wherein the amino alcohol is defined by the general formula EMI22.1 wherein R1 is a primary alkyl or primary hydroxyalkyl radical having 1 to 8 carbon atoms or a branched chain alkyl, branched chain hydroxyalkyl, cycloalkyl or hydroxycycloalkyl- radical having a total of 3 to 8 carbon atoms, R2, R3, R4 and R5 are each independently hydrogen or a C1-C4 alkyl or C1-C4 hydroxyalkyl radical, with the proiviso that when the carbon atom of R1 directly bonded to the nitrogen atom is primary, both R2 and R3 are alkyl or hydroxyalkyl radicals and that when the carbon atom of R1 directly bonded to the nitrogen atom is secondary, at least one of R2 or R3 is an alkyl or hydroxyalkyl radical, x and y are each positive integers independently ranging from 2 to 4 and z is 0 or a positive integer ranging from 1 to 4. 7. A process according to claim 6 wherein the amino alcohol is tertiarybutylaminoethoxyethanol. 8. An absorbent solution comprising a physical solvent and a sterically hindered secondary amino alcohol having a cumulative -E8 value greater than 1.75. 9. An absorbent solution comprising a physical absorbent and an amino alcohol defined by the general formula: EMI23.1 wherein R1 is. a primary alkyl or primary hydroxyalkyl radical having 1 to 8 carbon atoms or a branched chain alkyl, branched chain hydroxyalkyl, cycloalkyl or hydroxycycloalkyl radical having a total of 3 to 8 carbon atoms, R2, R3, R4 and R5 are each independently hydrogen or a C1-C4 alkyl or C1-C4 hydroxyalkyl radical, with the proiviso that when the carbon atom of R1 directly bonded to the nitrogen atom is primary, both R2 and R3 are alkyl or hydroxyalkyl radicals and that when the carbon atom of R1 directly bonded to the nitrogen atom is secondary, at least one of R2 or R3 is an alkyl or hydroxyalkyl radical, x and y are each positive integers independently ranging from 2 to 4 and z is 0 or a positive integer ranging from 1 to 4. 10. A solution according to claim 9 wherein the amino alcohol is tertiarybutylaminoethoxyethanol.

Inventors: SARTORI, GUIDO, SAVAGE, DAVID WILLIAM, STOGRYN, EUGENE LEO
Applicants: EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1983
Main classification: B01D53
IPC codes: B01D53