Sulfur rejection reaction

The COt Acceptor Gasification Process is discussed in light of the required properties of the CaO acceptor. Equilibrium data for reactions involving the CO% and sulfur acceptance and for sulfur rejection jit the process requirements. The kinetics of the reactions are also sufficiently rapid. Phase equilibrium data in the binary systems CaO-Ca(OH)t and Ca(OH)jr-CaCOs show the presence of low melting eutectics, which establish operability limits for the process. Data were obtained in a continuous unit which duplicates process conditions which show adequate acceptor life. Physical strength of many acceptors is adequate, and life is limited by chemical deactivation. Contrary to earlier findings both limestones and dolomites are equally usable in the process. Melts in the Ca(OH)2-CaC03 system are used to reactivate spent acceptors. 

Equilibrium Studies and Sulfur Rejection The experimentally determined values for Reaction 5 are given in Table IV. No reliable values for this equilibrium have been available in the literature previously. 

To achieve sulfur rejection in the regenerator, conditions should be maintained so that both CaS and CaS04 can coexist. Sulfur dioxide can now be evolved by Reaction 6. Where an excess of oxygen is used and all the sulfur is in the form of CaS04, sulfur rejection cannot occur owing to the low dissociation pressure of S02 over CaS04. 

It might be expected as well that sulfur rejection would be difficult in a highly reducing atmosphere owing to reduction of the sulfate by the reverse Reactions 7 and 8. 

Experimentally, complete sulfur rejection was always obtained from the acceptor in the continuous unit. Favorable conditions were maintained in the runs without air so that sulfur rejection could occur in the regenerator by Reactions 8 and 6 in succession. 

Sulfur rejection was also observed even when excess air was used in the regenerator in apparent violation of the equilibrium in dissociation of the sulfate. Reducing conditions must have prevailed in some of the dense portions of the bed, thus permitting sulfur to be rejected by Reaction 6. 

Reaction Products. No mercaptan or tetrahydrothiophene intermediates were found in the products of shot reactions over either catalyst. Instead thiophene reacted by rejecting its hydrocarbon portion either as a butadiene-butene mixture (from chromia) or a butene-butane mixture without butadiene (from cobalt molybdate). In both cases, the sulfur was left on the catalyst as adsorbed H2S. No H2S peak was observed following any such reaction and subsequent experiments with H2S showed that the rate of desorption of H2S was negligible until much higher concentrations on the catalyst surface had been reached. 

Experimentally, all three reactions have been shown to be rapid, and equilibrium should be closely approached in actual process operation. The high values for the equilibrium constants in Reaction 5 mean that substantially all the sulfur released from the char in the gasifier will be absorbed by the acceptor as CaS. This sulfur must be rejected as S02 in the regenerator. 

These observations do not allow the full refutation of chemical reinforcement theory. Chemical bonding by acidobasic reaction is clearly rejected, but, following Medalia and Kraus (1994), other chemical reactions could occur in carbon black mixes. For example, elastomeric chain breaking during mixing can lead to radicals that could react with carbon black surface (Le Bras and Papirer, 1979) sulfur-direct bonding of elastomer and carbon black could also be envisaged (Rivin et al., 1968). 

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