The general character of the treatment can be shown by comparing the two common proceSses known as "running to tar" and "running to cyl inder stock." The main difference between these two processes is that the former gives the largest possible yield of illuminating oils and a small yield of heavier products for lubricating. The second process is intended to yield a maximum amount of the lubricating oils, with the illuminating oils of secondary consideration. In general, therefore, one process is the direct reverse of the other in so far as its chief object in view is concerned.
Both processes start with crude oil heated in the still, and the vapors passing off into the condenser. The most volatile of these vapors begin to appear before much of any heat is applied to the still. They can be condensed only by special processes at temperatures near the freezing point, consequently in the ordinary course of distillation they pass off into the air through the escape pipe from the con denser or are led under the still to serve as fuel. The first distillate which condenses and passes through the V tube to the stillhouse is a clear, colorless light oil, but, as the process goes on, the stream of oil entering the separating box becomes heavier, and the color gradually changes through yellow to darker shades. The stillman tests the density of the oil from time to time, and on the basis of these tests and the color, he turns the stream into different tanks, by simply closing and opening convenient valves.
The stream passing through the separating box is continuous as long as the still contains any oil which can be vaporized, hence the stillman's divi sions of the stream of distillates, or his " cuts," as they are called, are an exceedingly important part of the process. The first cut is usually made when oils of the naphtha class cease to appear. The second cut is the illuminating oil. In the "run ning to tar" process, the method known as "crack ing " is employed after about two thirds of the cut of illuminating oil has passed over, its object being to increase the proportion of illuminating oils obtained.
The exact changes which take place in the still during this "cracking" process are only partly un derstood. The process was discovered accidentally in 1861 by a stillman at Newark, N. J., who left his post one day after about half the contents had passed off, building a strong fire under the still to last until he returned, as he expected, a half hour later. Several hours elapsed, however, before he did return, and then, to his amazement, he found issuing from the condenser a lighter distillate than was being obtained when he left, whereas it should normally have been much heavier. Such an entirely unheard-of thing led
immediately to experiments, in which it was found that a portion of the heavy distillate, normally com ing through the condenser last, had condensed on the cooler upper portion of the still, and dropping back onto the highly heated liquid had encountered a temperature hot enough to cause decomposition of some sort, so that a lighter oil was the final re sult. Many different devices have been invented to aid in this cracking process, and, though some re fineries use it but little, cracking has been of enor mous benefit in the case of certain petroleums, naturally yielding only a small percentage of kero sene, yet rich in the grades heavier than kerosene, and not heavy enough to be high quality lub ricating oils. By cracking many of these inter mediate grades are broken up, and become valuable illuminating oil.
After cracking has given as much kerosene as can be secured the fires are checked, and the tar process stops so far as the first still is concerned. A cer tain amount of thick residue or "tar" always re mains in the still and must be removed before the still can receive another charge of crude oil. This tar usually goes to a second still, where further dis tillation gives lubricating oils, paraffin wax, and coke. The cuts of naphtha and illuminating oils are also either redistilled or subjected to further treatment to purify them and separate them into different commercial grades.
The process known as "running to cylinder stock" is essentially the same as the other up to the point where cracking would begin, except that it is usually applied to crude oils naturally adapted to the manufacture of lubricants. The important difference consists in heating the still by free super heated steam within, as well as by the usual fire underneath the still. The presence of the steam causes a more even distribution of the heat, and more completely vaporizes the volatile lighter oils from the whole charge without having to subject it to such a high temperature. When the distillate in this process appears too heavy for kerosene, in stead of the cracking treatment, a third cut, known as the "wax slop," is often made. Different methods of handling this cut yield special brands of oil for a great variety of purposes, from the headlight oil of locomotives to the thin "spindle oils" used to lubricate light machinery. The en tire elimination of the cracking process leaves a greater residue in the still after the "wax slop" cut is made and this residue, known as "cylinder stock," forms the basis for the manufacture of a host of lubricating oils.