Alkylaromatic process using efficient prefractionation

Details Alkylaromatic process using efficient prefractionation Alkylaromatic process using efficient prefractionation

1999-12-29

2002-11-12

Yildirim, Bekir L. (Department: 1764)

Chemistry of hydrocarbon compounds

Aromatic compound synthesis

By condensation of entire molecules or entire hydrocarbyl...

C585S455000, C585S323000, C208S354000

Reexamination Certificate

active

06479720

ABSTRACT:

FIELD OF THE INVENTION
This invention is an improvement in a process for the production of alkylated aromatic compounds.
BACKGROUND OF THE INVENTION
Nearly forty years ago, it became apparent that household laundry detergents made of branched alkylbenzene sulfonates were gradually polluting rivers and lakes. Solution of the problem led to the manufacture of detergents made of linear alkylbenzene sulfonates (LABS), which were found to biodegrade more rapidly than the branched variety. Today, detergents made of LABS are manufactured worldwide.
LABS are manufactured from linear alkyl benzenes (LAB). The petrochemical industry produces LAB by dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of HF. The linear paraffins are straight chain (unbranched) or normal paraffins. Normally, the linear paraffins are a mixture of linear paraffins having different carbon numbers. The linear paraffins have generally from about 6 to about 22, preferably from 10 to 15, and more preferably from 10 to 12 or from 11 to 13, carbon atoms per molecule.
A preferred method of production of the linear paraffins is the extraction of straight chain hydrocarbons from a hydrotreated kerosene boiling range petroleum fraction. The kerosene boiling range fraction contains a mixture of different hydrocarbons, including mostly paraffinic and aromatic hydrocarbons, but containing also olefinic and naphthenic hydrocarbons. The kerosene boiling range fraction is usually defined as comprising a fraction having a boiling range of from about 300° F. (149° C.) to about 572° F. (300° C.). The initial boiling point of the kerosene boiling range fraction may vary from about 300 to about 374° F. (149 to 190° C.) and the final boiling point may vary from about 455 to about 572° F. (235 to 300° C.). The kerosene boiling range generally includes hydrocarbons having from about 8 to about 17 carbon atoms.
The kerosene boiling range fraction is generally produced by fractionating crude oil. Crude oil is the liquid part, after being freed from dissolved gas, of petroleum, a natural organic material composed principally of hydrocarbons that occur in geological traps. Being derived from a natural material, crude oils vary in composition depending on where the petroleum occurred and other factors. Commercial oil refineries typically receive crude oil from many different sources, and the composition of the crude oil that is charged to the crude oil fractionation unit changes frequently. The paraffinic and aromatic hydrocarbons that make up the bulk of a kerosene boiling range fraction can change as different crude oils are processed in the crude oil fractionation unit. It is common for the volumetric flow rate of the kerosene boiling range fraction to fluctuate by up to 30 vol-% or more, for a given boiling point range of the kerosene boiling range fraction produced from a crude oil fractionation unit. Because such fluctuations in flow rate can make it difficult to control downstream units that process the kerosene boiling range fraction, the operator of a crude oil fractionation unit may intentionally adjust the initial and final boiling points of the kerosene boiling range fraction as permissible within the above mentioned ranges and thereby achieve a more constant flow rate of the kerosene boiling range fraction. It is also common for the kerosene boiling range fraction to fluctuate between liquid phase and a mixture of vapor and liquid phases, since the proportion of vapor phase depends on both the composition of the kerosene boiling range fraction and its temperature, which can also vary. For a given temperature, the lighter the kerosene boiling range fraction, the greater is the proportion in the vapor phase. Accordingly, in commercial practice the composition, the amount, the boiling range and/or the phase of the kerosene boiling range fraction recovered from a commercial crude oil fractionation unit often fluctuate daily, or even hourly.
In order to produce LAB having, for example, from 11 to 13 carbon atoms per linear alkyl group, a stream of linear paraffins comprising C
11
to C
13
hydrocarbons is desired. A suitable stream is a heartcut of the kerosene boiling range fraction suffices, provided that hydrocarbons boiling lower than C
11
linear paraffins and hydrocarbons boiling higher than C
13
linear paraffins must be removed from the kerosene fraction. Generally, this heartcut is produced in a two-step, strip-and-rerun fractionation process. First, the kerosene fraction is introduced into a fractionation column, called a stripper column, which strips overhead the C
10
− hydrocarbons from the kerosene feedstock, producing a bottom stream comprising C
11
+ hydrocarbons. Then, the bottom stream is introduced into a second fractionation column, called a rerun column, which boils overhead the C
11
to C
13
hydrocarbons as a heartcut and produces a bottom stream comprising C
14
+ hydrocarbons. In some commercial units, the overhead condenser of the second fractionation column is a contact condenser. This heartcut is then hydrotreated, and the straight chain hydrocarbons are extracted from the hydrotreated fraction, thereby producing the linear paraffin stream.
Alkylaromatic processes that use the two-step, strip-and-rerun fractionation process to produce the heartcut are inefficient, since they require relatively large amounts of utilities. Thus, alkylation processes are sought in which the heartcut is produced in a more efficient manner that uses fewer utilities than the prior art process.
Over fifty years ago, Wright proposed replacing two distillation columns with a single distillation column having a vertical partition (dividing wall column) within the column that would effect the separation of the column feed into three constituent fractions. It was recognized then that a dividing wall column could minimize the size or cost of the equipment needed to produce overhead, bottoms, and sidedraw products. See U.S. Pat. No. 2,471,134 (Wright). Wright described using the dividing wall column to separate a mixture of ethane, propane, butanes, and a small amount of C
5
and heavier hydrocarbons.
Since then, researchers have studied the dividing wall column and have proposed using dividing wall columns for separating other mixtures, including xylenes (Int. Chem. Engg., Vol. 5, No. 3, July 1965, 555-561); butanes and butenes (See e.g., Trans IChemE, Vol.70, Part A, March 1992, 118-132); methanol, isopropanol, and butanol (See e.g., Trans IChemE, Vol. 72, Part A, September 1994, 639-644); ethanol, propanol, and butanol (Ind. Eng. Chem. Res. 1995, 34, 2094-2103); air (See e.g., Ind. Eng. Chem. Res. 1996, 35, pages 1059-1071); natural gas liquids (Chem. Engg., July 1997, 72-76); and benzene, toluene, and ortho-xylene (Paper No. 34 K, by M. Serra et al., prepared for presentation at the AlChE Meeting, Los Angeles, Calif., U.S.A., November 1997). The Serra et al. paper also describes separating mixtures of butanes and pentane; pentanes, hexane, and heptane; and propane and butanes.
Despite the advantages of the dividing wall column and despite much research and study, the processing industry has long felt reluctant to use dividing wall columns in commercial processes. This widespread reluctance has been attributed to various concerns, including control problems, operational problems, complexity, simulation difficulties, and lack of design experience. See, for example, the articles by C. Triantafyllou and R. Smith in Trans IChemE, Vol. 70, Part A, March 1992, 118-132; F. Lestak and C. Collins in Chem. Engg., July 1997, 72-76; and G. Duennebier and C. Pantelides in Ind. Eng. Chem. Res. 1999, 38, 162-176. The article by Lestak and Collins sets forth some general guidelines and considerations when substituting a dividing wall column for conventional columns. Nevertheless, the literature documents relatively few practical uses of dividing wall columns in commercial plants. See the article by H. Rudd in The Chemical Engineer, Distillation Supplement, Aug. 27,

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