Total sulfur in propylene in the gas phase
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Hello, experts. I have a question: after catalytic refining, the total sulfur content in liquefied gas is generally low, at 7–9 ppm. However, the total sulfur content in the propylene produced by the gas separation unit has recently risen to 5–7 ppm, with a carbonyl sulfide content of 1.5–3 ppm. (The propylene produced by this facility is of polymer-grade; downstream polypropylene plants require a total sulfur content of ≤1 ppm and a carbonyl sulfide content of ≤0.2 ppm.) Amine solution plus alkaline solution fiber membrane technology is used for desulfurization of liquefied gas – how can we reduce the sulfur content in propylene?
We’ve also encountered situations where in the end we didn’t know what was best for ourselves. First, the amine solution is used to remove hydrogen sulfide, while the alkaline solution is used to remove thiol; neither of these methods removes hydroxysulfur. Hydroxysulfur has a density similar to that of propylene, so it ends up mixed with the propylene. We are working in a catalytic cracking unit, and later we adjusted the feedstock and reaction temperature to fix this issue
Doesn’t the polypropylene plant have a refining system?
Generally, improved desulfurization technologies can be employed to reduce the total sulfur content and carbonyl sulfide content in propylene. If a multi-loop counter-current desulfurization process is adopted, with backflow desulfurization technology implemented, copper catalysts and other impurities in the liquefied gas can be reduced ； Using the alkali recombination method for desulfurization can improve the desulfurization efficiency, optimize the process parameters for desulfurization, and adjust the ratios of liquid absorbent, alkali, and acid ； The use of recirculating desulfurization technology can improve desulfurization efficiency and enhance the performance of catalysts, among other things.
The total sulfur content in the propylene produced by our plant is <8 ppm, and the moisture content is less than <16 ppm. It has now been revised to a total sulfur content of <4 ppm and a moisture content of less than <8 ppm; under normal conditions, the carbonyl sulfide content is <0.2 ppm. This indicator has been refined. If improvement is desired, adding an adsorption tower before the feed to the vapor fractionator can also yield good results; take a look into it! There are many desulfurization agents, and they all work very well. When the carbonyl sulfide content is 1.5–3 ppm, even small-scale reactions do not work well, and the impact on large-scale polymerization is even greater.
If the goal is purely to reduce sulfur content, install a zinc oxide desulfurization system, with the two desulfurization units used in rotation. Then it can be desulfurized to the ppb level. If such sulfur content does not affect the operation of downstream processes, there is no need to add additional equipment or measures.