Mercaptans, adsorption

Loaded Adsorbents. Where highly efficient removal of a trace impurity is required it is sometimes effective to use an adsorbent preloaded with a reactant rather than rely on the forces of adsorption. Examples include the use of 2eohtes preloaded with bromine to trap traces of olefins as their more easily condensible bromides 2eohtes preloaded with iodine to trap mercury vapor, and activated carbon loaded with cupric chloride for removal of mercaptans. 

Sweetening. Another significant purification appHcation area for adsorption is sweetening. Hydrogen sulfide, mercaptans, organic sulfides and disulfides, and COS need to be removed to prevent corrosion and catalyst poisoning. They ate to be found in H2, natural gas, deethanizer overhead, and biogas. Often adsorption is attractive because it dries the stream as it sweetens. 

Whereas other metal salts, especially lead stearates and srdfates, or mixtures of Groups 2 and 12 carboxylates (Ba—Cd, Ba—Zn, Ca—Zn) ate also used to stabilize PVC, the tin mercaptides are some of the most efficient materials. This increased efficiency is largely owing to the mercaptans. The principal mechanism of stabilization of PVC, in which all types of stabilizers participate, is the adsorption of HCl, which is released by the PVC during degradation. This is important because the acid is a catalyst for the degradation, thus, without neutralization the process is autocatalytic. 

Fig. 4.59. Raman spectrum of methyl mercaptan (a) and SERS spectrum of methyl mercaptide (b) formed by adsorption ofthe mercaptan on a silver surface. The surface reaction is proven by the disappearance ofthe S-H stretching and bending bands at 2575 cm" and 806 cm", respectively. The Raman shift ofthe C-S stretching band at approximately 700 cm" is reduced during adsorption by withdrawal of electron density from the C-S, because of bonding to the silver. The symmetric methyl stretching appears above 2900cm" [4.303].

Unavailable because experimental methods for estimation of this parameter for mercaptans are lacking in the documented literature. However, its high solubility in water suggests its adsorption to soil will be nominal (Lyman et ah, 1982). 

The block copolymers were modified by introducing carboxylic groups into the butadiene sequences to provide sites for selective adsorption of one end onto the Ti02 surface. The modification was accomplished by adding thioglycolic acid, making use of the well-known addition of mercaptans to carbon-carbon double bonds. 

Adsorptions of CS2 (102, 117), S02 (118), (CH3)2S (104, 119, 120), (C2H5)2S (105), n-propyl mercaptan (120, 121), thiophene (120), and isopropyl, n-butyl, isobutyl, and tert-butyl mercaptans (121) on Ni have also been investigated. It is speculated that CS2 adsorbs dissociatively on Ni surfaces at room temperature (117). The interaction of CS2 with Ni is limited to the surface at 193 K, but above 298 K bulk sulfidation is observed (102). Sulfur dioxide chemisorbs rapidly and irreversibly on Ni at 193 K, but extensive incorporation into the bulk is not observed below 373 K. 

ON THE ROLE OF WATER IN THE PROCESS OF METHYL MERCAPTAN ADSORPTION ON ACTIVATED CARBONS... 

Adsorption of methyl mercaptan in moist conditions was performed on numerous samples of activated cartons of various origins. Methyl mercaptan adsorption was tested by a dynamic method. The amount of products of surface reaction was evaluated using thermal analysis. The results revealed that the main product of oxidation, dimethyl disulfide, is adsorbed in pores smaller than SO A. There is apparent competition for adsorption sites between water (moist conditions) and dimethyl disulfi. The comp ition is won by the latter molecule due to its strong adsorption in the carbon pore system. Althou dimethyl disulfide has to compete with water for the adsorption sites it can not be formed in a significant quantity without water. Water facilitates dissociation of methyl mercaptan and thus ensures the efficient removal process. 

The amount of methyl mercaptan (MM) adsorbed (and converted to dimethyl disulfides (DMDS) depends on the sur e pH [ 1, 2], and the presence of various impregnants, such as potassium iodide, potassium iodite, potassium carbonate or ammonia [3,4], It has also been pointed out in the literature that different functional groups on the carbon sur ce or/and metal ions such as iron can catalyze oxidation of mercaptans to disulfides [3-6]. As we have found recently, there is an indication of a competition for high-energy adsorption sites between dimethyl disulfide and water molecules when adsorption occurs in the presence of moisture [1,2]. This happens as a result of big differences between water and DMDS in the strength of adsorption forces and their incompatibility (DMDS has very low solubility in water) [7]. 

An objective of this paper it to describe the results of our further investigation of the competition for adsorption sites between water and dimethyl disulfide molecules during methyl mercaptan adsorption on activated carbons. Moreover, we attempt to indicate the apparent borderlines between the conditions of adsorption processes leading to different adsorption/oxidation paths. Those working conditions have a significant effect on the feasibility of methyl mercaptan removal. 

Bashkova, S., Bagreev, A. and Bandosz, T.J., Adsorption of methyl mercaptan on activated carbons. Environ. ScL Technol. 36 (2002) 2777. 

Bagreev, A., Bashkova, S. and Bandosz. T. J., Dual role of water in the process of methyl mercaptan adsorption on activated carbons. Langmuir, in press. 

Single-component adsorption equilibria on activated carbon of the n-alkanes Q-C4 and of the odorant tert-butyl mercaptan were measured at the operating conditions expected in a large-scale facility for adsorbed natural gas (ANG) storage. The experimental data were correlated successfully with the Adsorption Potential theory and collapsed into a single temperature-independent characteristic curve. The obtained isotherm model should prove to be very useful for predicting the adsorption capacity of an ANG storage tank and to size and optimize the operation of a carbon-based filter for ANG applications. 

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