reaction intermediates. Treating systems include washing with fresh acid and
aqueous caustic (as shown). Caustic and water washes, bauxite, and KOH pellets
have also been used.
Fractionation
After effluent treating, the balance of reactor isobutane requirement is distilled from
the alkylate and n-butane as overhead from the deisobutanizer tower (DIB). Most
refiners charge saturated butanes from other units to the DIB for isobutane/n-butane
splitting. Normal butane is distilled from the alkylate for the control of product RVP
in a debutanizer, and in some cases, an n-butane vapor draw from the DIB. Finally, in
a few units, aviation alkylate is produced by removing heavy ends in a rerun column.
Process Variables in H 2 SO 4 Alkylation
• Temperature – maximum octanes for butenes and pentenes are obtained at about
35–40
F and for propylene at about 50
F. Typically, octane decreases at about
0.04–0.06 RON per 1
F at higher temperatures.
• Isobutane concentration – reaction selectivity and octane increase with isobutane
concentration. Octane typically increases by about 0.1 RON per 1 vol% isobutane in effluent.
• Acid strength – octane is maximized at about 93 wt% acidity. It declines at
about 0.2–0.3 RON per 1 % acidity decline below that. The economics of
acid cost usually require lower final spending strengths (as low as 88 wt%), so
multi-reactor alkylation units typically feed fresh acid to several reactors in parallel
operating at 93 % then reuse the 93 % as makeup to other reactors in series.
• Space velocity – octane decreases with the charge rate of olefin. Alkylation
reactors are typically designed at 0.15–0.30 LHSV, which is defined as the volume
of olefin charged per hour per volume of acid. Operations at much higher throughputs are possible, but low octanes and high acid consumption generally result.
• Acid fraction – octane increases with acid fraction and peaks at about 60 vol%
acid in the emulsion.
• Power per unit volume – octane depends upon the mass transfer rate of isobutane
from the hydrocarbon phase into the acid phase. According to Sprow (1969),
reaction selectivity and octane increase with interfacial area which depends upon
the power per unit volume to the 0.25 power.
H 2 SO 4 Consumption and Contaminants
Acid consumption is generally expressed as the pounds of acid diluted from 99 % to
90 % by conjunct polymer, water, and other contaminants per gallon or barrel of
450
D.A. Nafis et al.
aqueous caustic (as shown). Caustic and water washes, bauxite, and KOH pellets
have also been used.
Fractionation
After effluent treating, the balance of reactor isobutane requirement is distilled from
the alkylate and n-butane as overhead from the deisobutanizer tower (DIB). Most
refiners charge saturated butanes from other units to the DIB for isobutane/n-butane
splitting. Normal butane is distilled from the alkylate for the control of product RVP
in a debutanizer, and in some cases, an n-butane vapor draw from the DIB. Finally, in
a few units, aviation alkylate is produced by removing heavy ends in a rerun column.
Process Variables in H 2 SO 4 Alkylation
• Temperature – maximum octanes for butenes and pentenes are obtained at about
35–40
F and for propylene at about 50
F. Typically, octane decreases at about
0.04–0.06 RON per 1
F at higher temperatures.
• Isobutane concentration – reaction selectivity and octane increase with isobutane
concentration. Octane typically increases by about 0.1 RON per 1 vol% isobutane in effluent.
• Acid strength – octane is maximized at about 93 wt% acidity. It declines at
about 0.2–0.3 RON per 1 % acidity decline below that. The economics of
acid cost usually require lower final spending strengths (as low as 88 wt%), so
multi-reactor alkylation units typically feed fresh acid to several reactors in parallel
operating at 93 % then reuse the 93 % as makeup to other reactors in series.
• Space velocity – octane decreases with the charge rate of olefin. Alkylation
reactors are typically designed at 0.15–0.30 LHSV, which is defined as the volume
of olefin charged per hour per volume of acid. Operations at much higher throughputs are possible, but low octanes and high acid consumption generally result.
• Acid fraction – octane increases with acid fraction and peaks at about 60 vol%
acid in the emulsion.
• Power per unit volume – octane depends upon the mass transfer rate of isobutane
from the hydrocarbon phase into the acid phase. According to Sprow (1969),
reaction selectivity and octane increase with interfacial area which depends upon
the power per unit volume to the 0.25 power.
H 2 SO 4 Consumption and Contaminants
Acid consumption is generally expressed as the pounds of acid diluted from 99 % to
90 % by conjunct polymer, water, and other contaminants per gallon or barrel of
450
D.A. Nafis et al.