TECHNOLOGY
Alkylation reactions probed at Purdue Workers there propose new chemical mechanisms, get better understanding of processes ical mechanism for the several intermediate reactions of the alkylation process when sulfuric acid is used as the acid catalyst. Even though numerous details in understanding alkylation are incomplete, the results so far are valuable in understanding other reaction systems. For example, Dr. Albright is also investigating nitration of aromatics using mixed acids. Such a reaction is typical of the phenomena occurring at or close to the interface between two liquid phases. The character of the emulsion at the interface particularly is important, Dr. Albright says, because of the effect of the emulsion on the physical transfer of reactants in systems of two liquid phases. Controlling. In the alkylation proc-
PETROLEUM
Work at Purdue University on alkylation holds promise for refiners that they will be able to make gasoline more efficiently. The results could lead to better understanding also of processes involving two liquid phases. Dr. Lyle F. Albright and Dr. Roger E. Eckert of Purdue, and Dr. K. W. Li, now with Continental Oil, have found that physical transfer of the reactants from the hydrocarbon phase to the acid phase or to the interface between the two phases is the most controlling step of the complex alkylation reaction. They propose a new chem-
ln alkylation, trimethylpentanes form by this proposed mechanism 1. Most important sequence
— C~ •C-C
CH-
t
*
R CH
r
*-9-:
3 isobutyiene or n-butenes
(K)
tertiary cation
larger red oil cations (moderately stable)
in red oil
some decomposition to isobutyiene
or
+
+R,H (formation of hydride ion)
—C — \
aliyiic cation in red oil
trtmethylpentane
Cti^
*, + c l V
CH t l R . H + c H - c I
'
=> \
isobutane 2. Secondary sequence
^ M I 3 z
IT +
CH - C - + 3
j £U
tert-butyl cation
56
tert-butyl cation
i CH
^ - f
C & E N S E P T . 15, 1969
C
^ ~
R,H-
-
3 isobutyiene C
trimethylpentyl cation H ~ CW*
hydride transferion
trimethylpentane
ess to make high-octane gasoline components—the trimethylpentanes being the most important of these—transfer of isobutane across the interface is the most controlling factor of the process. Agitation, ratio of the reactants in the feed, and residence time are interchangeable in effecting this physical transfer step. Agitation is generally the single most important operating variable in sulfuric acid alkylation, Dr. Albright says. Large increases in alkylate quality (octane number) can generally be obtained. Refineries may find it advantageous to increase agitation in existing alkylation units. Temperature and acid strength also affect the physical transfer steps of alkylation and influence to a degree the chemical steps of the reaction. Temperature, for example, affects the mass transfer steps through changes in solubilities, viscosities,, and diffusivity, as well as in kinetic rate constants for chemical steps. The correlations which are developing from the Purdue work probably will find most use in predicting changes of alkylate quality and composition, as operating conditions and feeds change. Experimental conditions used in this work produced, in addition to the usual range of values, some minimum (about 83 octane number) and maximum (about 98 octane number) values of alkylate quality not before reported for experimental programs. Such information could be used to improve operation of existing alkylation units, Dr. Albright points out. Influence. Future design of alkylation units could be influenced by several findings of the Purdue group. Among these results are a better understanding of the importance of secondary reactions in production of trimethylpentanes and of the strength of the acid in its effect on the overall reaction. Previously, designers and engineers apparently assumed that the especially desirable trimethylpentanes were found almost exclusively by primary reactions. Now the Purdue group finds that much acid-soluble organic material reacts with isobutane after all olefin has reacted. The acid-soluble material, known to refiners as "red oil," is made up largely
of intermediates and conjunct polymers in the complicated alkylation process. Red oil now is clearly known to enter into various reactions and, in some cases, leads to trimethylpentanes, says Dr. Eckert. Red oil contains some of the complicated ions which are important in the mechanism proposed by the Purdue group. These ions include tertiary (or secondary) cations, which form from a proton added to double bonds; allylic cations, which form because of conjugated double bond structures of the red oil; and protons present in the acid phase. Numerous reactions occur between the ions and olefins to form large red oil cations, often tertiary, but sometimes secondary. These cations react reversibly with isobutane to form tert-buty\ ions. At the same time, some of the large red oil ions decompose to release isobutylene, which reacts with tert-butyl ions to make trimethylpentyl cations. These cations isomerize and add a hydride ion to form trimethylpentanes. A second sequence of reactions to form trimethylpentanes occurs, as has been postulated in the past, out this sequence is of less importance than the first. The tert-butyl cation in the acid phase reacts with isobutylene and 2butene as the latter transfers from the hydrocarbon phase to the acid phase. A trimethylpentyl cation forms which isomerizes and picks up a hydride ion to form a trimethylpentane. In a third and still less important sequence, trimethylpentanes form primarily from olefins and red oil. Self-alkylation. A fourth sequence —self-alkylation of isobutane—is of most relative importance when isobutane is alkylated with propylene or C 5 olefins. In this sequence, the acid and red oil dehydrogenate part of the isobutane to isobutylene,, which reacts with tert-butyl ions as before to form trimethylpentanes. The Purdue group has analyzed available results for alkylations catalyzed by hydrogen fluoride. The limited data indicate that H F alkylate differs from sulfuric acid alkylate. For example, H F alkylates obtained when isobutylene is used as the olefin contain more trimethylpentanes, especially 2,2,4-trimethylpentane, but less light ends; and when 1-butene is used, H F alkylates have much higher amounts of dimethylhexanes, especially 2,3dimethylhexane. Certain features of the alkylation mechanism still need verification and added details, Dr. Albright says. For example, more work is needed on the chemistry of the acid phase, especially that of the organic materials dissolved in the acid phase. More H F alkylation studies also are needed to further compare the two types.
SAFE. Charles A. Farish, executive director of the National Sanitation Foundation, says that plastic pipe will not affect the appearance, taste, or odor of drinking water
Plastic pipe is safe for drinking water WATER, AIR & WASTE Plastic pipe is safe and will not affect the appearance, taste, or odor of drinking water, according to Charles A. Farish, executive director of the National Sanitation Foundation (NSF). Mr. Farish does have some reservations about the chemicals used to stabilize plastic pipe, however. Lead compounds, he says, should not be used. While this warning isn't necessary in the U.S., since lead stabilizers are not used in plastic pipe for potable water here, it does have implications for Europe, where lead-stabilized plastic pipe has been used extensively. Mr. Farish told the Forum on Environmental Quality (Water) held jointly with the Divisions of Analytical and Industrial and Engineering Chemistry that a series of tests performed by NSF on lead-stabilized plastic pipe show that total soluble lead extracted from samples exceeds the maximum limit for lead in drinking water established by the Public Health Service. Although the comparative study is continuing, "the data acquired to date do not support the use of leadstabilized plastic pipe for transporting potable water," he explains. NSF, a nonprofit, nonofRcial agency for research, education, and service in areas concerned with man's health and environment, now authorizes 220 plastic producers and fabricators to use its seal. The seal indicates that the plastic pipe is nontoxic, will not adversely affect the appearance,
taste, or odor of water, and conforms to applicable physical standards under which it is produced and identified. Five types. Plastic pipe was first introduced in Europe in 1930 and in the U.S. in 1940. Five major types of thermoplastics are used in manufacturing the pipe in the U.S.: acrylonitrile-butadiene-styrene (ABS), polyethylene ( P E ) , polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), and polybutylene (PB). In recent years, plastic pipe has assumed a major role in potable water transportation. More than 200,000 miles of plastic pipe and fittings are now in domestic service. Among its numerous advantages are light weight, high strength, economical installation, and excellent corrosion resistance. While lead, cadmium, strontium, lithium, and antimony can be used in plastics formulations as stabilizers, lead and cadmium are particularly hazardous, Mr. Farish points out. All soluble lead salts are toxic. Even small concentrations of lead continuously present in drinking water may cause serious illness or death. Lead poisoning is not normally a problem in hard water since lead carbonate and sulfate are insoluble. Lead is a cumulative poison because elimination, which is through the kidneys, is very slow. Cadmium accumulates in the soft tissues of the body, resulting in anemia, poor metabolism, arterial changes in the liver, and, at high concentrations, death. Acceptable upper limits for lead and cadmium in drinking water are 0.05 and 0.01 mg. per liter, respectively, according to drinking water standards published by the Public Health Service. SEPT. 15, 1969 C&EN
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