Some Generalizations about Processes to Absorb Acid Gases and Mercaptans Ian M. J. Rlchardson and John P. O’ConnelP Department of Chemical Engineering, University of FlotWa, Geinesville, Florida 326 I 1
Pearson’s categories of hard and soft acids and bases can be applied to the chemicals involved in processes for absorbing the acid gases HpS, Cop, mercaptans, COS, and CSp. These can rationalize the different levels of removal presently obtained. Ideas for new absorbents including potential compounds for liquid streams, as well as general limitations on heavier compounds, might be developed from the concepts.
Removing acid gases and other impurities from natural gas, synthetic natural gas, or refinery gas, and liquid streams has involved much time and effort. This paper discusses some general qualitative concepts which allow a simple classification of process constituents for absorbing sulfur compounds and C02 and for suggesting new alternatives.
Present Processes for Removing Acid Gases Table I lists many of the processes that have been commercially successful for removing hydrogen sulfide, carbon dioxide, mercaptans, carbonyl sulfide, and carbon disulfide from gas streams (Wall, 1973, 1975). Also given is a subjective indication of which impurities each process removes satisfactorily, based on a variety of information sources. Listed are various alkanolamines and carbonate solutions which have been used to remove hydrogen sulfide to very low levels by strong chemical reaction with H2S in an absorption tower. However, these treatments do not remove other impurities to low enough levels for many purposes. In addition, large heats of regeneration and limited capacities are found for H2S and C02 (Hochgesand, 1970). Since 1960 there has been much commercial use of “physical” absorption processes (Swaim, 1970) where an aqueous solution absorbs the H2S and C02 with considerably less energy change than in the chemical absorption processes, and mercaptans, carbonyl sulfide, and carbon disulfide generally can be removed as well. Occasionally, however, except with special additional compounds or multistage contacting, the H2S levels using only the physical solvent may not be sufficiently low to meet pipeline or reactor specifications. Thus, for example, the Sulfinol process involves a mixture of a chemical absorbent such as diisopropanolamine with a physical component (tetrahydrothiophene dioxide) (Frazier, 1970) to accomplish removal of all compounds to low levels. Both absorption (Wall, 1973, 1975) and oxidation (Hoffmann, 1974; Kasai, 1975) treatment processes for natural gas streams which contain only H2S and C02 are presently quite adequate since there are several that readily meet H2S pipeline specifications. Hawever, in most cases, the treating processes used for natural gases cannot be used for refinery gases and liquids due to the presence of the other sulfur impurities which are not removed well enough, and more importantly, the physical absorbents also take out paraffins heavier than Cd and aromatics, both of which usually constitute a sizable proportion of the streams which should be retained. For example, to get the total sulfur content down to levels low enough to prevent catalyst poisoning, it is usually necessary to follow the alkanolamine treat with a caustic (sodium hydroxide solution) wash for further
removal of H2S, COz, and the light mercaptans that may be present. For heavier streams, reactions to change the form of sulfur, such as in merox and mercapfining systems (catalytic oxidation of mercaptans to products such as disulfides which may or may not be subsequently removed) and hydrodesulfurization (hydrogenation of sulfur compounds to form H2S) are often used (Hoffman, 1974). Subsequent treatment with caustic satisfactorily removes H2S but can take out methyl and ethyl mercaptans adequately only with difficulty (relatively large volumes of caustic and multiple contacting) and will remove very little of the heavier mercaptans. Merox systems remove mercaptans adequately, but they require a prewash to remove H2S in the hydrocarbon stream. Unfortunately, all sodium hydroxide treatments leave sour caustic to be disposed of, creating a nontrivial pollution problem, as well as wasting resources. Obviously, it would be desirable to find an absorbent for refinery streams that will do the combined job of alkanolamine, caustic, and physical absorbents while reducing the cost for regeneration of the absorbent (lower heating requirement) and eliminating caustic purchasing and disposal costs. In order to assist in the development of potentially better systems we have attempted to classify the gases and absorbents and make some generalizations about the various gas-absorbent interactions involved.
Application of Hard and Soft Acid Base Theory One simplistic, but useful, theory that does give some qualitative insight into the important aspects of various absorption treating systems, and some oxidation processes, is the hard and soft acid-base theory described mainly by Pearson (1963, 1965, 1967). Nearly all chemical reactions can be considered as either generalized acid-base reactions, where pairs of electrons or a proton can be partially shared by the two species, or as oxidation-reduction reactions, where the valence states of the species change. The fundamental acid-base theory of G. N. Lewis states that a Lewis base is an electron donor, Le., an atom, molecule, or ion which has at least one pair of electrons which is not already shared in a covalent bond. A Lewis acid is an electron acceptor, with a vacant electron orbital where a pair of electrons can be accommodated. The typical acid-base reaction where a new compound is formed is A + :B A:B where A is an acid and B is a base. The AB product is referred to as a coordination compound, an adduct or an acid-base complex. All anions are Lewis bases, whereas all cations and metal atoms are Lewis acids. Refining the acid-base concept, Pearson defines a soft base as a base in which the valence electrons are easily distorted, polarized, or removed, whereas a hard base is one which holds onto its valence electrons more tightly (and, in
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Ind. Eng. Chem., Process Des. Dew, Vol. 14, No. 4, 1975
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our view, may even attract protons to it rather than donating electrons). A soft acid is defined as an acid in which the acceptor is of large size, has small or zero positive charge, or has several valence electrons which are easily distorted or polarized, while a hard acid is one in which the acceptor is of small size, has high positive charge or has no valence electrons that are easily distorted. Pearson’s (1967) general principle of reactions for hard and soft acids and bases is as follows: Hard acids prefer t o coordinate with hard bases, and soft acids prefer t o coordinate with soft bases. Although this general principle has only qualitative application and ignores many aspects of molecular interactions including some charge-transfer compounds between bases, delineating the various groups does help to explain present experience and invites speculation about new processes for acid gas absorption. Some of the components found in gas and liquid hydrocarbon streams and present and potential treating processes can be categorized as follows (R is a short hydrocarbon group): hard acids: ions of the families I, 11, I11 of the periodic table, C02; soft acids: Cu+, Cd2+, Hg2+, RS+, 12, Br2, paraffinic and napthenic groups attached to organic hard bases, trinitrobenzene, quinones, tetracyanoethylene; borderline acids: SOz, Fez+, Cu2+, Zn2+, Pb2+, Sn2+, B(CH&; borderline bases: aniline, pyridine, S032-; soft bases: sulfides, mercaptans, RS-, C O , olefins, aromatics; hard bases: water, OH-, Sod2-, hydroxyl (alcohols), carbonyl (ketones), oxy (ethers), nitrogen (amines), cwboxylates (organic acid anions). Most of the above categorizations were given by Pearson, who also included other species of less relevance here. For our purposes, H2S has been classified bifunctional. It acts as a hard acid when it reacts readily with hard bases such as NaOH. However, it seems unlikely that this character comes from accepting electrons; probably it comes from the hydrogen atoms (protons) being stripped off. It also acts as a soft base in donating electrons from the sulfur atoms as in oxidation processes involving metals (iron in the Konox process, Kasai, 1975; zinc in the GASYNTHAN process, Jockel and Triebskorn, 1973; vanadium in the Stretford process, Wall, 1973, 1975; arsenic in the Giammarco-Vetrocoke-sulfur process, Wall, 1973, 1975). We have classified carbon disulfide as a soft base since the sulfur electron clouds are easily polarizable and CS2 is soluble to some deagree in several hard acids. However, it is not clear that the CS2 molecule does not attract electrons, so it also could have some soft acid character. In addition, we can classify COS as bifunctional with the oxygen having excess tightly held electrons for hard-basic character and the sulfur having a depleted, yet polarizable, electron cloud for soft-acidic character. Classifications of the impurities to be removed from the hydrocarbon streams are given in Table 11. Some clarifying observations about absorption processes can now be made. For years, attempts have been made to use caustic (NaOH), a hard base, to remove soft-basic mercaptans, COS, and CS2, from hydrocarbon streams (Williams, 1964). However, removal to low levels has been successful only for low molecular weight mercaptans, and this success results only when multistage counter-current contacting of caustic and hydrocarbon is carried out. This is consistent with the concept that the larger the molecule containing an active group becomes, the softer it is, and the less able it is to react with hard acid or base. The heavier the compound, the more soft-base character it has and the less likely it is to be removed by caustic which is a hard base., Mercaptans and perhaps CS2 and COS, should be more successfully removed using a component with some softacid character. Now, many of the physical absorbents in
Table 11.Acid-Base Classification of Acid-Gas Components Classification Impurity Name
Formula
Acid Hard
Base Soft
Hard
Soft
Hydrogen H2S Xa X sulfide Carbon co2 X dioxide X Carbon cs2 ? disulfide Carbonyl cos X X sulfide Mercaptans RSHb X X a Probably proton donor, not electron acceptor. R is hydrocarbon group such as CH3, CzHs, etc.
use not only remove mercaptans, but they also remove most of the hard-acidic impurities in the hydrocarbons they are treating. The reason they are successful in doing both jobs is that they are bifunctional, that is, they have both a hard-basic center for removal of hard-acids and soft-acidic portion for removal of soft bases. An example is the Selexol process where the dimethylether of polyethylene glycol is used as the solvent. The oxygen of the ether group of the molecule acts as a hard-basic center and the CH3 and the pair of CH2 groups act as soft-acidic portions. Table I includes most presently available commercial processes classified according to their functional groups. All of the chemical absorbents which work on H2S do not remove anything else except perhaps COz and COS to the desired levels. This is because they are only hard bases. On the other hand, all the physical solvents absorb all the impurities, although not all remove H2S to low levels without special treatment or additional chemicals because the hard-basic character may not be proton accepting. In the case of oxidizing systems such as the GASYNTHAN and Giammarco-Vetrocoke-sulfur process, the softbasic sulfur replaces the hard-basic oxygen to form a more stable compound with a soft-acid ion. Thus, in these cases as well, the present concept describes the process. It is not clear to us, however, that the subtle shifts of oxidation state in the Konox and Stretford processes are readily explainable by the acid-base ideas. In any case, we can assert the following desired pattern for absorption; to remove all of the undesirable sulfur compounds and C02, the system should optimally have (1) a hard-basic proton acceptor group or a reacting soft-acid group for H2S, (2) a hard-basic electron donor group for C02, and (3) a soft-acidic group for all the others. Unfortunately, as mentioned above, the usual physical solvent processes cannot be used for liquid streams because they absorb too much of the C4 and higher paraffins, napthenes and aromatics. However, this is not surprising, since their soft-acidic group which removes mercaptans, COS, and CS2, is a hydrocarbon group which would be expected to dissolve heavy hydrocarbons. Thus, a better answer for absorption treating of liquid streams may lie in using softacid groups which are ionic or inorganic. Some examples from Pearson’s lists, which may or may not be feasible, include tetracyanoethylene, K2Co(CN)5, organic cations such as RSe+, RTe+, RO+, R3C+ (where R is methyl or ethyl), and compounds from the metals thallium, gallium, and indium. These would have to be used in conjunction with one Ind. Eng. Chem., Process Des. Dev., Vol. 14. No. 4, 1975
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of the hard-base substances such as an alkanolamine to remove all the desired species either as a mixture or in successive contacting. An alternative might be to choose oxidation reactants from the list of borderline and soft acids. Unfortunately, there will always be some absorption of aromatics in any solvent which removes mercaptans because they are both in the same (soft-base) category. Although the hard and soft-acid base generalizations are overly simplistic and without further development cannot be quantified to the point of predicting effective activity coefficients, they do classify the processes in a consistent pattern and could be helpful for screening new absorbents and oxidants for potential use in removing acid gas impurities of fluid hydrocarbons.
Literature Cited Frazier. J.. Hydrocarbon Process., 49 (4), 101 (1970). Hochgesand, G., I d . Eng. Chem.. 62 (7), 37 (1960). Hoffmann, H., Hydrocarbon Process., 53 (9), 103 (1974). Jockel. H., Triebskorn, B. E., Hydrocarbon Process., 52 (l),93 (1973). Kasai. K., Hydrocarbon Process.,54 (2). 93 (1975). Pearson, R. G., Chem. Br., 3 (3), 103 (1967). Pearson, R. G.. J. Amer. Chem. SOC.,85,3533 (1963). Pearson, R. G., Chem. Eng. News, 43 (26), 90 (1965). Swaim. C. C., Jr.. Hydrocarbon Process., 49 (3), 127 (1970). Wall, J., Hydrocarbon Process., 52 (4), 87 (1973). Wall, J., Hydrocarbon Process., 54 (4), 79 (1975). Williams, W. W., Pet. Refiner, 43 (7), 121 (1964).
Received for review September 12,1974 Accepted May 26,1975
Kinetics of the Activated Carbon-Steam Reaction Herbert E. Klei,' James Sahagian, and Donald W. Sundstrom Department of Chemical Engineering, University of Connecticut, Storrs, Connecticut 06268
A bed of activated carbon was reacted with steam at temperatures between 1400 and 150OoF and steam concentrations between 10.7 and 53.4 mol YO to determine the kinetic rate equations which predict the reaction rate. In addition, since activated carbon is a porous solid, the effect of gas flow rate through the bed was studied to see if mass transfer was important. The reaction rate was found to vary with the 0.58 power on the steam concentration with an activation energy of 63.6 kcallg-mol. The gas rate did have some effect on the reaction rate but it appeared not to be sufficient to shift the controlling step to mass transfer.
Introduction Activated carbon with its high surface area and affinity for water contaminants is becoming widely used in the purification of wastewater. In order to lower costs, the carbon is regenerated in a furnace near 1500OF in an atmosphere of steam and inert gases. During this regeneration step, the adsorbed organic materials are burned off the carbon restoring its adsorptive capacity. Unfortunately about 10% of the carbon is consumed per pass by reacting with the steam to form a variety of gaseous products. Several pilot plant studies have been done noting the effect of temperature and gas composition on activated carbon losses and on the absorptivity of the regenerated activated carbon (Loven, 1973; Battelle, 1970). Generally these studies were empirical and the kinetics of the reaction were not determined. For the steam-carbon system, however, a considerable amount of kinetic data are available. In contrast with activated carbon, the carbon or graphite used was nonporous and had a well-defined surface area for reaction. For the steam-carbon system the principal reactions are C
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Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4, 1975
sure is small compared to the steam (Walker, 1959; Long, 1948; Gadsby et al., 1946). Since the overall steam-carbon reaction is endothermic, difficulties in maintaining the isothermal conditions throughout the reaction zone were encountered (Derman et al., 1960; Malinauskas, 1970; Abel and Holden, 1962). Activation energies for this reaction range from 19.8 to 88 kcal/g-mol (Walker, 1959; Derman et al., 1960; Malinauskas, 1970; Long, 1948; Pilcher, 1955; Shchebrya et al., 1965). It is generally accepted that the hydrogen directly inhibits the rate of the steam-carbon reaction by occupying sites that would otherwise enter into the reaction. The effect of carbon monoxide is uncertain with some data showing inhibition while other data show no effect. Since the activated carbon is a porms solid, the steam reaction may be controlled by one or more of the transfer steps involving mass transfer across the stagnant gas film around the particle, pore diffusion within the particle, and surface reaction within the pore. At low temperatures the reaction rate is controlled by the chemical reactivity of the solid with an effectiveness factor equal to 1. At medium and high temperatures, steam concentration gradients exist within the activated carbon and the effectiveness factor is much less than 1. This paper presents a study to determine whether mass transfer is important in the reaction of steam with the activated carbon and the kinetic rate equations which predict the reaction rate.
Experimental Section Equipment. A thin layer of activated carbon (Nuchar
