Establishing standards for consumption of caustic soda in treating liquefied gases (LPG) to remove mercaptans

Abstract

1. On the basis of experimental data, regression equations have been obtained, giving an adequate description of the specific consumption of caustic soda as a function of mercaptan content in the original feedstock and the initial concentration of the NaOH solution, under specific conditions of temperature, treating ratio (caustic solution/feedstock), and degree of treating. 2. The specific consumption of NaOH (kg of 100% NaOH per metric ton of feedstock), for a given degree of treating, is directly related to the initial caustic solution concentration and to the initial mercaptan concentration in the feedstock; hence, both these factors must be taken into account in calculating standards for caustic soda consumption. 3. The limiting saturation of the caustic solution with mercaptans for a given degree of treating (this saturation representing the discard point for the caustic) does not depend on the initial mercaptan content in the feedstock, but is a function of the initial concentration of the caustic solution. 4. With the aim of providing stable quality for LPG meeting the requirements of TU 38101498-74, the existing method for monitoring the depletion level of caustic soda in treating mercaptan-containing LPG should be reviewed, on the basis that the primary criterion for depletion is the degree of saturation of the caustic, expressed in moles S−− per mole NaOH. 5. In reviewing the requirements on LPG quality with respect to content of sulfur compounds, from the standpoint of imposing more severe requirements, it must be considered that this will involve a considerable increase in specific consumption of caustic soda. Hence we see a great need for the rapid commercialization of catalytic processes for demercaptanization.