Classification of Refined Petroleum Waxes
Classification of Refined Petroleum Waxes
Classification of Refined Petroleum Waxes
It is difficult to draw up a classification of petroleum waxes that could not be subject to criticism. The classification here given is somewhat different from others that have been proposed; it comprises the various types of waxes produced in American refineries.
(1) Paraffin Wax Group
- Softer paraffin waxes (e.g., slack waxes)
- Intermediate paraffin waxes (e.g., scale waxes)
- Harder paraffins (e.g., fully refined waxes)
(3) Microcrystalline Wax Group
- Medium soft (e.g., “Petrowax”)
- Medium hard (e.g., “Petrosene A”)
- Hard (e.g., “Be Square 190/195”)
These three groups can be distinguished by their crystalline characteristics. Group (1) waxes have relatively large crystals (macro crystals). The slack or subscale waxes contain more oil than scale waxes, and the melting points usually are between 122 and 140°F. The scale waxes have negligible tensile strength and the melting points are between 118 and 135°F. The refined waxes (block paraffines) range in melting point from about 123 to 165°F, but usually from 126 to 146°F.
Group (2) which are not true waxes have crystals so minute because of occluded oil that they can be regarded as amorphoid (amorphous-like) in structure. Not much more than 25 per cent of the hydrocarbons of petrolatum are waxlike.
Group (3) waxes have very minute crystals (micro crystals) and are of high melting point, generally above 145°F and exceptionally as high as 200°F. Waxes which are by-products of the motor oil raffinate have 26 to 42 carbons, melt within the range of 145-175°F, and have an SUS viscosity at 210°F of about 55 seconds. They have needle-like crystals, and are somewhat brittle. These waxes are sometimes referred to as “micro waxes.” Waxes which are prepared from distillation residuals have 36 to 70 carbons, melt within the range of 145-175°F, and have an SUS viscosity at 210°F of about 55 seconds. They have small needle-like crystals, and are flexible. These waxes are sometimes termed “petrolatum waxes.” Waxes which have been recovered from tank bottoms of crude oil have 40 to 75 carbons, melt within the range of 180-200°F, and are hard. They have very small needlelike crystals.
The medium soft microcrystalline waxes have a needle penetration of 25 to 80 (100 g/77°F/5 sec); the medium hard waxes have a value of 10 to 25; and the hard waxes a value of 3 to 10. Penetrations are determined by the ASTM D5-25 method; the values are expressed as mm actual penetration X 10.
Roughly speaking: paraffin waxes come from the dewaxing of paraffin distillates, petrolatum from high-viscosity lube distillates, and microcrystalline waxes from residuum. In dewaxing the paraffin distillates, the various steps along the way, to almost complete freedom from oil, yield slack wax, scale wax, semirefined wax, and finally fully refined paraffine.
Specific Sources of Paraffin Waxes
Paraffin wax associates with paraffinic-base oils in many crudes. The wax content of the wax-bearing raffinate from the crude is about 10 per cent. A pressible Pennsylvania distillate has a gravity of about 34 API, and a viscosity of 70 seconds Saybolt at 100°F or even higher viscosity. To obtain a chillproof high-grade lube oil it is necessary either to recover the wax from the raffinate or to treat it with a pour-point depressant.
Mid-Continent (Arkansas, Louisiana, and Oklahoma) oils provide a better economic source for a paraffin wax by-product than do the Appalachian (Pennsylvania and West Virginia), and are thus extensively employed. A pressible wax distillate of Mid-Continent origin has a gravity of 30 to 31° API. California oils, on the other hand, carry little or no wax, e.g., Midway is almost wax free. Gulf Coast crudes, however, vary from naphthenic oils with almost no wax to oils such as Rodessa which contain more wax (5.2%) than the Appalachian (2.4%). Differences in the physical characteristics of the paraffin waxes can often be ascribed to the origin of the crude, and the particular refinery at which such a crude is handled.
Recovery of Paraffin Wax Group
There are ordinarily three principal types of wax encountered in crude oil, namely paraffin wax, slop wax, and petrolatum. The first is contained in distillates from which it may be pressed. When Pennsylvania crude, or other paraffin wax-yielding petroleum is distilled by the batch type process), using a cylindrical still, there are five steps to this method of distillation. The first out yields light naphtha, the second heavy naphtha, the third high test burning oil, and the fourth low test burning oil; the fifth portion is the undistilled residuum, which is the source of paraffin wax. This residuum is pumped to the tar still, which holds about 200 barrels, and is reheated with high-pressure live steam to the point at which it will again give off the desired vapors. A heavy distillate is taken off which is wax bearing and pressible. About 5 per cent of tar coke remains in the still and is recovered for sale to the industries.
Paraffin wax is also recovered by the continuous-type process . A continuous tower still (pipe still with fractionating column ) is a less expensive process since it yields a wider range of products in the same run, and has a much lower cost of maintenance. With a tower still it is not necessary to pump the residuum to a separate tar still. The heavy distillate is taken off at the first trap; it is wax-bearing, and in the last part of the run is known as wax failings. An “intermediate distillate” or raffinate is taken off at the second trap, and a “light distillate,” which is a raffinate of high test oil, such as BOO oil, is taken at the third trap.
The heavy distillate above referred to is one which distills over at 315 to 125°C (600-800°F), and contains not less than 6 to 8 per cent of wax. This wax-bearing distillate goes to the “chill rooms,” where cooling to a temperature of 25 to 30°F is effected by pumping the distillate through tubes jacketed by cold brine and provided with helical scrapers. The resultant crystal mush is subjected to powerful pressure by plate and frame filter presses set in refrigerator rooms, and the heavy oil raffinate which drains off from the press is collected as lubricating oil. It is known as “spindle oil” and has a gravity of 32° Baum6 and a pour point of plus 6 to 7°C (20°F).
The press cake (“slack wax”) which may contain as much as 50 per cent or more of wax may be broken up, melted, chilled, and sweated to form scale wax, which contains 1 to 6 per cent of oil, and foots oil, made up of lower melting point wax plus oil. The scale wax is then refined to paraffin waxes of different melting points, leaving a slop oil which is of little or no value.
Paraffin waxes consist of mixtures of purely paraffinic compounds, whereas the dewaxed stocks (e.g., slop oil) from the heavy distillate consist essentially of compounds containing up to eight aromatic or naphthenic rings with paraffin side chains.
Softer Paraffin Waxes
The softer paraffin waxes include slack wax and sweat wax, the latter a more oily by-product of the former. In the refinery the crude oil raffinate known as “paraffin distillate” (wax distillate) has a boiling point range of 170-310°F when distilled at a very low pressure. To separate the paraffin wax from the oil, the wax distillate may be chilled until the wax crystallizes, and the resultant slurry may be filter pressed. If the wax forms a filter cake from which the oil drains easily the wax distillate is said to be “pressible.” If the oil cannot be separated well from the wax crystals in the pressing operation, the wax distillate is “unpressible.” The wax left on the filter presses is called slack wax. The consistency of slack wax may be soft or semisolid and the oil content is normally 10 to 35 per cent.
Slack Wax. The paraffin distillate at a temperature of 80 to 100°F is pumped to the paraffin sheds (wax plant), where it is allowed to repose in tanks to promote settling at the lowest room temperature (between 0-and 32°F). It is then pumped through a bank of cooling units (wax chillers) to hydraulic presses which squeeze out the wax from the chilled distillate. The cooling units, usually “double-pipe horizontal” or “double-pipe vertical” chillers, consist of a series of pipe coils placed in a steel jacket of circulating liquid ammonia. One section of a double-pipe chiller will reduce the temperature of 30 barrels of distillate from 90 to 15°F in twenty-four hours.
Some wax plants use a vertical double-shell tank chiller, known as the “Gray unit;” the distillate enters at the top of the inner shell and leaves at the bottom. The refrigerant enters at the bottom of the outer shell, passes around the inner shell, and leaves at the top. Through the inner shell extends a shaft filled with arms carrying scrapers; these are thrown against the inner surface as the shaft revolves driven by an overhead mechanism.
In the hydraulic presses, usually of horizontal type, there are locked hundreds of canvas-covered circular metal plates and rings, arranged so that a space is formed between each two plates when they are pressed together. Chilled wax distillate comes into each space; the oil it contains flows out through the canvas blanket, whereas the wax remaining on the blanket is removed. The wax filter cake produced is then acid treated, and the sludge withdrawn; it is then neutralized and washed. The resultant product is slack wax. The slack wax is slowly melted and charged into sweat ovens where it is chilled to solidify it somewhat, and then is slowly heated. If oil and low-melting waxes drain away from the slack wax during the sweating operation and leave a high yield of wax with low oil content, the slack wax is “sweatable;” otherwise it is “unsweatable.” Slack wax is marketed as such to a limited extent. A product containing about 19 per cent of oil has been marketed as a “luster cleaner.”
Several samples of slack wax give the following melting point characteristics: drop points (ASTM) 135-145°, softening point (CC&S) 112-122°, ball and ring 130-140°, Fisher-Johns spread 117-130°, maximum 135-145°F. The penetration values (100 g/5 sec/25°C) ranged between 60 and 80.
The term “sweat wax” is sometimes confused with “slack wax” from which it differs in that the sweat wax is a mixture of oil and paraffins of a rather limited range of melting points in the lower melting point series.
In the sweating of slack wax the oil which is drawn off contains a large quantity of wax crystals. This mixture when chilled at only a moderately low temperature will yield on sweating the paraffins of the higher melting points and a sweat oil containing crystals that melt at 60°F and also some that melt at somewhat higher and lower temperatures. Sweat wax therefore is a solid mass at 60°F or thereabouts but appears to have 60 per cent or more of oil at 80 or 90°F.
Sweat wax is used whenever it is essential to secure extreme wax penetration, as in the textile and paper industries. It is used in wax emulsion with naphtha, or without it. A mixture of 87 parts of sweat wax; 11 parts of bentonite, whiting, or other mineral filler; and 2 parts of pigment, makes a paint suitable for closing the mesh of woven fabric, imparting color, and rainproofing.
Tent cloth can be treated with sweat wax using naphtha as a carrier. In water emulsions it is of value in waterproofing krafts. Bags for holding chemicals can be prevented from rotting by weaving the material with some form of scale wax or sweat wax treated thread.
Intermediate Paraffin Waxes
Scale wax can be considered a paraffin wax intermediate between softer waxes, such as slack wax, and the harder paraffines. It is derived by a process of sweating the greater part of oil from slack wax. Grades of crude scale wax which are marketed contain up to 6 per cent of oil. Crude scale waxes are obtainable in both yellow and white, the yellow usually containing less than 4 per cent of oil, and the white less than 2 per cent. They are produced in high as well as low melting points, although their softening points are invariably low.Refined Scale Waxes.
Ordinarily a white crude scale wax of 124-126°F melting point will contain several per cent of oil and moisture; however, a “second pudging and pressing” will bring the oil content down to below 1 per cent. Such a wax is referred to as a special refined white scale wax, or a semirefined paraffine.
Crude scale fractions when passed through bauxite beds at relatively high temperatures lose their oil content, at least down to less than 0.5 per cent, thus greatly improving the high light and stability of the scale wax.
Refined scale waxes of 126/130 grade gave the following melting points: drop 126-130°F (ASTM); softening 96-lll°F (CC&S); ball & ring 126- 130°F (ASTM); Fisher-Johns spread 117-122°F, maximum 126-130°F. Penetration values (100g-5 sec-25°C) were 23-33.
White scale waxes of the Atlantic Refining Company are furnished in 124-126, 128-130, and 133-135°F (AMP) grades, with an optimum expressed oil and moisture content of 0.5 per cent, and a Saybolt color of 19 average.
In the fabrication of cotton duck and canvas the thread is treated with a crude scale wax of 124-126°F (AMP) grade to impart waterproofing. A refined scale wax of 133-135°F (AMP) grade is used with material where a drier, nongreasy, finished product, which is not very susceptible to atmospheric change, is desired.
Higher-melting scale waxes are sometimes required for superior waterproofing, and to accomplish this purpose small quantities of other material, such as carnauba wax, gilsonite, or high melting asphaltume, are incorporated in the scale wax. Such adjusted scale waxes are referred to as “textile proofere.” They work out economically since a thinner application will give the article the required degree of waterproofing.
Refined scale wax is used in the formulation of certain grades of artificial ceresin where lubricity to the wax is required. The use of a scale wax with much oil, or of a slack wax, may yield a product with large macrocrystals of paraffin tending to destroy the flexing or elongation. The hardness of an artificial ceresin which has been prepared by adding carnauba wax to paraffin-ozokerite mixtures can be offset by the addition of several per cent of low-melting refined scale wax.
Scale waxes of 124-126°F. (AMP) grade are used in the match industry. Scale waxes of high melting point are used in the kraft paper industry, e.g., for builder’s papers, cement bag stock, roofer’s felt, car liners, and heavy kraft wrappings. Petrolatums of high melting point (140°F) are used in much the same way as the scale waxes for the waterproofing of kraft papers, cotton ducks, and canvas. Scale waxes of very low oil content are required in the manufacture of crayons.
Harder Paraffin Waxes
The harder paraffin waxes are referred to as paraffines, customarily accompanied by a designation of the melting point, such as “paraffine 128- 130°F (AMP)”; or as fully refined paraffin waxes. The term block paraffine is sometimes used to designate a harder paraffin wax, one which, can be molded into a hard firm block. The production of harder paraffin waxes constitutes by far the greater proportion of the refinery output of petroleum waxes in the United States, approaching a billion pounds per year.
Slack wax which comes from the hydraulic presses, already described, contains 10 to 15 per cent oil, and this oil as well as the softer waxes must be removed in order to produce block paraffines. This is customarily done by “sweating” beyond the stage employed in producing slack wax, or intermediate waxes.
Sweating is a process by which the oil and very soft waxes can be liquated from the firmer, harder paraffins by means of supplying heat to a previously chilled slack wax at a carefully regulated temperature. The solubility of paraffin wax in oil increases very markedly with even slight elevation of heat at temperatures 10 degrees below the melting point of the wax, thus permitting the separation of the components of slack wax by sweating. Low-melting waxes are more soluble in oil than higher-melting paraffins. Mixtures of high- and low-melting waxes have solubilities between those of the components, which makes it necessary to resweat the liquates after they have been chilled, if further recoveries of high-melting wax are to be made.
In the refining of paraffin wax, the pudged and pressed wax retains about 2 per cent of the oil at room temperature; but the amount retained at the temperature at which final sweating is carried out is too small to be detected. The presence of this small amount of oil in solution offers an explanation for some of the difficulties encountered in the separation of oil from wax. The greatest loss in wax in the sweating process occurs at the temperature at which most of the oil is removed. The wax separated from slack wax by chilling and then sweating in the initial stage is known as scale wax, and the by-product made up of lower-melting wax plus oil as foots oil. The oil sweated out in subsequent pudgings and pressings is known as slop oil.
Production of Paraffin Wax at the Refinery
The following process and equipment has been prescribed for the refining of block paraffine from an Eastern Texas crude wax distillate. Basically the sweating process follows the Henderson modification of the early methods of Price’s Candle Company.
The slack wax from the chill room is melted and pumped to shallow steel pans in the sweater house, which houses several sweating ovens. The latter are well insulated rooms which accommodate one or two stacks of the long pans or trays. The pans are 8 by 20 feet and about 8 inches deep at the ends, but slope to a depth of 14 inches at the center, the pans are usually stacked eight high.
A rack of light angle or tee iron is laid in the pan and carefully leveled. A very coarse galvanized iron mesh is stretched across the rack; and over this is placed a brass wire gauze which is securely fastened to the rack, and which supports the cooling coil. There is a 2-inch space between the bottom of the pan and the gauze into which the cold water is pumped and circulated. The melted wax is run onto the surface where it congeals to a sheet of 4 to 6 inches thickness. After solidification the pans are tilted, the oven is lightly closed, and the water run off.
The sweater oven has steam coils along the walls and under the pans for controlling the interior air temperature, and is equipped with vertical ducts between the frames to allow for air circulation. By a thermostatic regulator the temperature is raised 1 or 2 °F per hour, and the oil and soft wax that sweats out is drained off from time to time. The first running, or light foots oil, is used as a plant fuel, but the next, or heavy foots oil fraction, is pumped back to the wax distillate, and subsequent fractions are resweated, or used later for blending before the acid treatment. The end product is a block wax of 130-132°F melting point, which still requires a finishing treatment to remove color.
Thereafter the wax is melted and run into agitators (lead-lined and steam-jacketed), treated with strong sulfuric acid, violently agitated for 30 minutes, and settled for one hour. The acid sludge is then drawn off and the melted wax is run into another agitator, in which it is washed with hot water and then with sodium hydroxide and finally with a hot water spray. After removing traces of water by long settling, the wax is dried finally by blowing air through it.
The melted wax at this stage is not entirely colorless, and hence is run through vertical filters of hot fuller’s earth. The filtered, bleached, refined wax is run into a slab mold, 40 feet or more in length, which consists of a tank-like structure having a sealed bottom, open top, and a large-size turnscrew at one end. In this housing is a series of compartments formed by chill plates, each plate is filled with chilled brine and connected to the other with a flexible hose so as to provide a continuous circulation. The plates are about 2 inches thick and spaced inches apart. The mold is filled with melted wax and the wax seeks its level by filling each compartment. When the wax shrinks on cooling, an extra portion of melted wax is released into the mold. After the wax is congealed, the screw at the end of the mold is unturned to loosen the plates, the plates are manually pried apart sufficiently to lift out the slabs of wax. After stripping, surplus wax is removed from the end of the slab. These slabs constitute the fully refined paraffine of commerce. They are 12" x 18" x 1H" in size, weigh about 10 pounds each, and are packed in bags or cartons stamped with the AMP melting point. The paraffin is also packed loose or on pallets and shipped in paper-lined freight cars.
Wax oils are products which are commercially available for converters who wish to employ blends of wax and oil for the impregnation of paper. These oils have been so refined that papers treated with them will impart no taste nor odor to foodstuffs. The wax oils contain 15 to 30 per cent of wax, and have a high pour point of 80 to 110°F. If used alone the paper sheet is soft and oily. Blends with petrolatum will impart less oiliness and more grease resistance for meat wrapping paper.
Two types of wax oils marketed by the Socony Mobil Oil Company (New York) known as “S/V Wax Oil No. 1” and “S/V Wax Oil No. 2” have the following physical characteristics: No. 1 gravity (API) 28.8, specific gravity (d15-8) 0.883, bulks 7.35 lb per gal, pour point 80°F, flash point 395°F, Saybolt viscosity 126 at 100°F, color (ASTM) light 2; No. 2 gravity (API) 32.1, specific gravity (da-e) 0.865, bulks 7.2 lb per gal, pour point 110°F, flash point 405°F, Saybolt viscosity 63 at 130°F, color (ASTM) light 2.
Refined Paraffin Wax
A refined mixture of associated solid paraffin hydrocarbons or paraffins is commonly designated as “paraffin wax.” The word “paraffin” appears to be a French derivative of the Latin parum, meaning “little,” and affinis, meaning “affinity”; in other words, a chemically inactive substance. The high-melting grades are customarily produced free from impurities and are referred to as “fully refined paraffin waxes,” or by the simple designation or English term “paraffines”; for example, Asiatic paraffines are said to contain paraffins in the range of Cn to C3<. Reichenbach obtained paraffin wax from wood tar in 1830, and later by the distillation of lignite, as patented by James Young in 1850. In the United States paraffin wax was first obtained from petroleum in 1868. Today, paraffin wax is produced from many “crudes” that are designated “paraffin-base oils” because of their wax content.
The wax-bearing crudes of America contain on the average less than 5 per cent of merchantable wax. Those of Galicia yield less than 3 per cent, and those of Burma 8-10 per cent. Canadian crude oils yield less than 3 per cent of paraffin wax. Much of the Russian petroleum contains no paraffin wax; an exception is the so-called “mazouts” of Northwest Caucasus which are wax-bearing.
Fully Refined Paraffin Wax
The term “fully refined” is given to paraffin waxes which are water white, free from more than a mere trace of oil, odorless and tasteless, and hard or firm in consistency. The oil plus moisture content should be well within the allowable optimum of 0.5 per cent.
Fully refined paraffin wax has a friable, coarse, fibrous crystalline structure which is translucent to opaque white in appearance, particularly noticeable when a slab of the wax is broken. Microscopically its structure when crystallized from a solvent shows more or less circular or oval aggregates or platelets of cells; the fibrous characteristic in large masses appears to be due to superimposed aggregates of platelets.
Paraffines have a melting point range of 118-155°F (ASTM D-87), but ordinarily the graded melt in the range of 126-137°F, or considerably lower than microcrystalline wax. A paraffine has a color of +21 or lighter (ASTM D-156).
When melted the average grade of paraffine has a viscosity of 40-50 seconds, Saybolt at 210°F, equivalent to 4.2-7.3 centistokes, or appreciably lower than the viscosity of microcrystalline wax. Fully refined paraffin wax (m. ca. 130°F) has a specific gravity of 0.910; a flash point not less than 177°C (350°F) and usually over 400°F, and a specific heat of about 0.694 calorie per gram. Its mean refractive index (nd) is 1.5277-1.538 at 25°C, and 1.430-1.433 at 100°C. The average paraffin wax has a molecular weight of 350-420 and contains normal hydrocarbons with a mean value of C25 to C27. Waxes prepared from mixed base type crudes are said to have a small proportion of side-chain hydrocarbons (side-chain alkanes).
A specimen of Esso brand paraffin wax 128/130°F (ASTM) melted at 129.5°F, and had an oil content of 0.3 per cent, color +30 Saybolt, tensile strength 300 psi, modulus of rupture 400, and flexibility 0.001" units-50.
The demand for American paraffines of the higher melting points, 130/132,133/135, and 135/137°F AMP, has greatly increased, and the use of lower melting point paraffines has correspondingly decreased. There is only a restricted demand for paraffines melting between 143 and 150°F. These higher melting point waxes may constitute 25 per cent of the total yield of a modem refinery, and hence they are used largely to blend with paraffin waxes melting below 128°F to obtain marketable grades. A paraffine of 151°F melting point has a SITS viscosity of 43 at 210°F, a tensile strength of 151 psi at 70°F, 195 psi at 40°F, a color of 30 Saybolt, a stain pass at 135°F, and an oil content (MEK) of less than 0.5 per cent.