a
THE MANUFACTURE OF \\ETHYLH ANTIKNOCK COMPOUND CHRISTOPHER C. VOGEL Ethyl Corporation, New York, New York
Wmiv A MOTORIST drives into a service station for gasoline, the chances are better than three to one that the motor fuel he buys has been improved with "Ethyl" antiknock compound. For more than three-fourths of all gasoline sold in the United States is treated with this antiknock compound, whose active antiknock ingre dient is tetraethyl lead. "Ethyl" antiknock compound im~rovesthe c~ualitvof easolme bv increasine the fuel's octane number or ability to produce power. Before the antiknock properties of tetraethyl lead were discovered, automotive progress was limited by the inherent tendency of gasoline to knock. This was because the temperatures and pressures being exerted upon the gasoline during combustion were too great for the fuel to stand. As a result, instead of normal combustion, a final unburned portion of the gasoline vapor suddenly ignited in a spontaneous explosion known as detonation, or more popularly called knock. The addition of only a few centimeters of tetraethyl lead per gallon of gasoline, it was found, was enough to take the knock out of most gasoline and restore normal combustion. Under these conditions, the gasoline burned evenly and completely across the combustion chamber, delivering a smooth power thrust to the engine. Gasoline treated with "Ethyl" antiknock fluid first went on sale in 1923; since 1933 t h e antiknock fluid has been used to improve regular as well as premium grades of motor fuel, and it is also used in aviation gasoline. Today, a gallon of motor fuel usually contains between one and three ml. of antiknock compound. As small as its average concentration per gallon of gasoline may be, the manufacture of the antiknock fluid draws upon many raw materials, entails a series of complicated chemical operations, and involves a plant investment running well into the millions of dollars. "Ethyl" antiknock compound, which is manufactured and sold by Ethyl Corporation to gasoline reh e r s , consists primarily of tetraethyl lead, ethylene dibromide, and ethylene dichloride. To these are added a small amount of kerosene and dyes, the latter purely for identification purposes. Tetraethyl lead constitutes around two-thirds, by weight, of the finished antiknock fluid. The halides, ethylene dibromide, and ethylene dichloride, are incorporated in the compound to act as scavenging agents during the combustion of lead-treated gasoline, combining with the lead vapors given off and carrying them off in the exhaust gases in the form of lead bromide and lead chloride. Tetraethyl lead is a clear and colorless organo-metalA
"
-
-
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lic compound with molecular weight of 323.45 and a density of approximately 1.65. It is completely soluble in oil and gasoline but only very slightly soluble in water. It is produced by the reaction of ethyl chloride with a lead-sodium alloy, expressed in the following equation: 4PbNa
+
4C2H6C1+
Pb(C
+
4N8.C1
+
3Pb
In the actual manufacturing process, the above reaction does not go quite to completion. As a practical matter, slightly less tetraethyl lead and slightly more lead are formed than the quantities indicated stoichiometrically in the equation. Three major processes are involved in tetraethyl lead manufacture. These are the so-called sodium process, the ethyl chloride process, and the final reaction to make tetraethyl lead. In the sodium process salt brine is piped into the Ethyl plant from wells 20 miles away. In the first step, the impurities are precipitated out and the brine is evaporated to dryness. The dry salt is then charged to a battery of electrolytic cells which are a modification of the Downs cell, that has been described in the literature. These cells contain a bath of molten sodium chloride and another molten salt which is added to lower the melting temperature. During the electrolysis of the salt the chlorine is gathered a t the anode of each cell and piped under a vacuum to the liquefaction system consisting of washing and drying towers, pumps, and condensers. The sodium released a t the cathode is forced out of the cell through a pipe by virtue of the hydrostatic head. It is then gathered into a heated collector and, after filtering, is transferred to the lead-sodium alloy plant or to storage tanks. In the alloy plant, themolten sodiumis combmed with molten lead to form the lead-sodium alloy which, after being cooled and ground, is transferred to the tetraethyl lead buildings for the final reaction. The lead-sodium alloy is brittle and readily oxidizes. Approximately 170,000 pounds of sodium and 262,000 pounds of chlorine are produced daily in the Baton Rouge plant. Ethyl's sodium operations, which entail the consumption of tremendous amounts of electricity, account for nearly half of all the sodium produced in the United States. Because of the need for it in its manufacturing operations, Ethyl Corporation is the world's largest producer of ethyl chloride. Ethyl chloride is made by two new processes in use
JOURNAL OF CHEMICAL EDUCATION
56
a t the Baton Rouge plant. The lirst of these processes is based on the hydrochlorination of ethylene, which is represented by the equation, C2H1 HC1-+C2HaC1. In this process the raw material, ethylene, is first obtained by cracking refinery gases, usually propane and ethane, and then fractionating the cracked gases at low temperatures to concentrate the ethylene formed. The ethylene is then treated with hydrogen chloride in the presence of a catalyst, resulting in ethyl chloride, which is then purified by fractionation. The second method yields ethyl chloride by the reaction of chlorine with waste products from the ethylene process. Developed during the war, this process is used in a plant which was put in operation in 1945. A third, and older, method is sometimes used. Tbis is based upon the hydrochlorination of alcohol, in which ethyl alcohol vapor and hydrogen chloride gas react in the presence of a catalyst to form ethyl chloride. This is expressed in the equation, CzH50H f HC1+ CzH6C1 HzO. The mixture then passes through a series of water coolers and scrubbing and drying towers, which remove any remaining hydrochloric acid and alcohol as well as the remaining water. In the electrolysis of salt approximately one and a half pounds of chlorine are given off with every pound of sodium. Although part of the chlorine output enters directly into one of the three ethyl chloride operations, another part is first converted into hydrogen chloride gas by burning it with hydrogen. The hydrogen chloride gas is then passed through cooling and drying towers, prior to its use in the manufacture of ethyl chloride. Additional supplies of hydrogen chloride are obtained by the output of four Mannheim furnaces which were built during the war. In this familiar operation a mixture of salt and sulfuric acid is heated to a high temperature, resulting, finally in hydrogen chloride and sodium sulfate. The hydrog m chloride enters into the ethyl chloride operations. The sodium sulfate, commonly known as "salt cake," is sold to the kraft paper industry in the South. In the final tetraethyl lead operation the ethyl chloride and lead-sodium alloy are allowed to react at moderate pressures and temperatures. At the completion of the reaction the product is distilled with steam to separate the tetraethyl lead from the fixture. The use of four pounds of metallic lead is required for each pound that appears in the finished product. At the conclusion of the tetraethyl lead reaction the remaining three pounds of lead are recovered in the form of sludge, and resmelted to pig lead. This is done in the lead recovery building which adjoins each of the five tetraethyl lead manufacturing units. The completed tetraethyl lead is finally transferred to the blending plant where it is mixed with ethylene dibromide, ethylene dichloride, kerosene, and dye, to form finished "Ethyl" antiknock compound. The ethylene dichloride for the antiknock fluid is obtained by the reaction of chlorine with ethylene, according to the equation Clz CzH~-C1H4Cl~. Ethyl-
+
+
+
ene dichloride is a colorless liquid with a molecular weight of 98.97, a density of 1.25, and is soluble to the extent of 0.88 per cent by weight in water. The ethylene dibromide used in the finished antiknock compound was originally obtained from bromine extracted from the salt brine wells of Michigan. However, it soon became apparent that supplies from this source would be insufficient for indicated requirements. As a rcsult, Ethyl Corporation, in cooperation with the Dow Chemical Go.,pioneered in the research which led to the first commercially successfulprocess for recovering bromine from sea water. Bromine is ordinarily found in sea water in the ratio of 67 parts to every million parts of sea water. In order to recover the bromine in pure form an elaborate extraction process is required. In the plant a t Freeport, Texas, sea water flows into an incoming flume and on to a pumping basin. There, a series of pumps lifts the brine to the extraction plant where, after an acid treatment, chlorine is added. Liberated bromine, dissolved in the brine, is removed by forcing a blast of air upward through a tower while the brine is flowing down. The resulting bromine "mist" is absorbed to form a concentrated solution of 80,000 parts per million. Substantially pure bromine is recovered from this solution and combined with ethylene to form ethylene dibromide. The completed product, which is a colorless liquid, has a molecular weight of 187.68, a density of 2.18, and is practically insoluble in water. The first bromine extraction plaut was constructed a t Kure Beach, North Carolina, during the 1930's. Its output was later supplemented by the Freeport, Texas, plant, which was built during the war. Since the war's end the Kure Beach plant has been placed in a standby condition, for enough ethylene dibromide comes from the Freeport plant. The Freeport plant can treat as much as 550 million gallons of sea water a day, which is about equal to the combined daily water consumption of Cleveland, St. Louis, and Boston. Most shipments of antiknock fluid are pumped from the blending plant a t Baton Rouge directly into specially designed railroad tank cars. Arriving at a refinery, these tank cars transfer their shipments in the same manner to another type of blending plaut wherein the antiknock compound is now blended with gasoline. Because of the toxicity of concentrated antiknock compouud the entire system of transporting, handling, and blending the antiknock fluid with gasoline is completely enclosed and subject to comprehensive safety precaution. Since it was first marketed in 1923 "Ethyl" antiknock compound has come to be used to improve the great majority of gasolines. For the eighteen years since records of gasoline gallonage have been kept, more than 50 billion gallons of premium grade gasoline and more than 200 billion gallons of regular grade gasoline have been improved by this antiknock compound. Of corresponding importance is the fact that it has contributed to the broad progress in fuels and engines
JANUARY, 1948
that has been seen in recent decades. Originally, by reducing the degree of fuel knock, it permitted the development of more and more powerful engines with higher compression ratios, thereby ushering in a great era of automotive progress. At the same time it has enhanced the contribution of each advance in refining tech-
57
nology, until today even the most powerful aviation gasolines in use are improved with the addition of "Ethyl" antiknock compound. What such progress has meant to motorists is seen in the fact that the average car owner today runs his car on a far better grade of gasoline than Lindbergh bad to fly the Atlantic.
