ThioSolv, LLC

410 Grand Oaks
Spring Branch, TX 78070
U*S*A

PHONE: (832)443-0952
FAX: (830) 228-4410

Michael.Ray@ThioSolv.com

Alkylation with Sulfuric Acid

Consider:

  1. Under the Tier II regulations, Alkylate will be the ideal gasoline component because it contains no sulfur, no aromatics, no olefins, high octane number.
  2. Alkylate is the logical candidate to replace the octane barrels lost with the banning of MTBE.
  3. Cost of acid replacement is the big disadvantage compared to HF alkylation.

With ThioSolv technology,

  1. Reduce sulfuric acid consumption by 50% or more
  2. Improve alkylate octane
  3. Reduce cost of acid regeneration, free up regeneration capacity

Control acid diluents independently

Operators of sulfuric acid alkylation units replace acid catalyst with fresh acid in order to maintain "acid strength". However, it is oversimplification to think only in terms of "acid strength". The acid catalyst becomes diluted by three distinct classes of materials and it important to distinguish among them because each has a different effect on the equipment and on the reaction.

Red Oil is the hydrocarbonaceous diluent produced by various irreversible reactions that compete with the desired alkylation of C4 or C5 tertiary paraffin with one olefin. Red oil is a necessary component of the catalyst system and contributes to the reaction. It has a vital role as a hydrogen transfer medium to transform isoparaffin molecules into cations that can react with the olefin. It also increases the solubility of hydrocarbons in the acid phase in which the reaction takes place, favoring the local ratio of isoparaffin to olefin at the point of reaction. It also acts as a surfactant to increase interphase surface area, which also contributes to increasing the iso/olefin ratio in the acid phase. Operation at Red Oil concentrations as high as 18% have been described in the patent literature.

Another diluent in the acid is hydrocarbon ester, an intermediate in the alkylation reaction formed by reaction of sulfuric acid with olefin. Although the alkylation reaction proceeds by way of ester formation, the concentration of ester in both the hydrocarbon effluent from the reactor system and in the spent acid should be zero, as ester in either stream represents a loss of both acid and potential alkylate. Acid wash of the DIB feed converts dialkyl esters, which are soluble in hydrocarbon, to monoalkyl esters that extract into the wash acid, recovering the acid value and returning the hydrocarbon part of the molecule to the reactor. To minimize the concentration of ester in the spent acid, the reactor system should be designed so that the hydrocarbon with which the spent acid is last in contact is isoparaffin free of olefin. This contact allows the ester to complete its reaction to alkylate, leaving the acid in the acid phase and converting the hydrocarbon part of the ester to alkylate before leaving the alkylation reaction section.

The third major acid diluent is water, which enters the reactor system with feed streams, with iso recycle, with the makeup acid, and as a product of oxidation of hydrocarbon by sulfate, as well as sometimes by leaks or from the caustic and water wash sections during abnormal conditions. Water in the acid phase is helpful insofar as it forces the reaction

H2O + SO3 H2SO4
to the right, preventing the reaction of SO3 with hydrocarbon. However, water increases the polarity of the acid phase and thereby reduces solubility of hydrocarbon, particularly isoparaffin, in the phase where alkylation takes place. And, of course, it is the water concentration that increases the corrosiveness of the acid to carbon steel.

Ideally, the alkylation unit would provide means to control each of these diluents to its respective optimum value. Ester at the reactor outlet is controlled by the feed distribution and by reactor system design, but can be reduced by an inexpensive retrofit. Conventionally, the red oil and water are not controlled independently; in fact, most discussion of acid strength does not even distinguish between them, because the operator has only one variable to manipulate to control both of them. However, ThioSolv has developed a means that allows the operator to control the concentrations of red oil and water independently so that the water may be held in its optimum range of 1 to 4% while allowing the red oil to cycle up to its optimum.

Increase unit capacity inexpensively

Most of the energy consumed in the alkylation process is used for two purposes:

  1. Transfer the heat of reaction of alkylation to the environment (refrigeration)
  2. Recycle isobutane to sustain a high ratio of isoparaffin to olefin in the acid phase where the reaction takes place.

The heat of reaction is proportional to the amount of alkylate produced, so the manipulable variable in energy consumption to transfer heat is the compression ratio required in the iso vapor compressor to reject the heat. Some reactor designs allow significantly lower compression ratio.

Proper design of the process can achieve the necessary local ratio of iso to olefin with less recycle of isobutane.

ThioSolv can identify opportunities in both of these areas.