H2SO4 Alky unit solids

[longeron]

I'm having diffuculty with an H2SO4 Alky Unit reactor... Tar like solids which I understand to be the product of a reaction of the acid and Isobutane seem to end up downstream. The pumps that move the product downstream, called effluent pumps, also supply the flush for the reactor agitator/mixer seal. These solids seem to clog the flush line starving the mechanical seal and bringing the reactor down for maintenance.

I'm obviously not a ChemE, I'm the mech seal rep.

Am I barking up the wrong tree, are these solids what I think they are, has anyone had experience with this problem?

Thanks.

 

[25362]

I haven't had personal experience on the subject of your query. Anyway, I may advance some thoughts on the factors that may influence the formation of the clogging solids and ways to cope with these factors.

1. Acid. The acid strength should be kept in the range of 88-91%. 98-99 % acid is added, and spent acid removed to control the system inventory. The lower limit of system acid strength is dictated by the danger of dropping into a region of excessive polymerization and ester formation, whilst the upper limit is dictated by the economics of acid consumption and octane requirement. Acid consumption is strongly dependent on temperature, on the type of olefin feedstock, and the contaminant levels.

2. Butadiene. With butene feedstocks, acid consumption varies between 35 and 60 kg per m³ of alkylate, but is much influenced by traces of butadiene, always present (about 0.3% mol) in FCC-C₄ streams.
Diolefins react with concentrated acid to form tars, SO₂, CO₂, and water, and it has been estimated that 1 kg of butadiene in alkylation feedstock will result in about 10 kg of 98% sulfuric acid degraded to a strength of 90%.

3.Feed contaminants. H₂S, mercaptans and moisture also contribute to acid consumption.
Feedstock drying, alkanolamine/caustic treats and partial hydrogenation steps are sometimes applied.

4. Agitation. A good degree of vigorous mixing is required due to the higher viscosity of the acid at the reaction temperature range of 3-12^oC, and the large difference in densities between the acid and the reactants. Otherwise there would be a strong tendency of polymerization and the formation of acid complexes, which would also lead to alkylate quality decline and an increase in acid comsumption.

Good luck.