Literature on Petroleum Waxes:

The quantity of waxes obtained from crude petroleum has increased continuously for two reasons:

1.      the demand for lubricating oils with low pour points and

2.      the large proportion of paraffinic crudes in total crude oil production that have to be dewaxed for the production of lubricating oils.

This development has increased the need to find more industrial applications for petroleum waxes. When the processing of lubricating oil began in the second half of the 19th century, waxes were still inconvenient side products. Since then, worldwide consumption of petroleum waxes has increased to 3×10⁶ t/a. This consumption corresponds to more than 95 % of the waxes of all types produced worldwide. Depending on their natural occurrence and their crystallinity, petroleum waxes are divided into:

Table belowgives a detailed classification of paraffin and microcrystalline waxes based on origin and method of refining.
 

Classification of waxes from crude petroleum

 Paraffin waxes are obtained from light and middle lubricating oil cuts of vacuum distillation. Paraffin waxes also include waxes from heavy lubricating oil distillates, which are intermediates between macrocrystalline and microcrystalline waxes with regard to structure and composition (intermediate waxes).

Microcrystalline waxes originate from vacuum residues and from the sediments of paraffinic crude oil (settling waxes). Waxes that are liquid at room temperature are mostly contained in diesel oil or gas oil fractions (®  Heating Oil, ®  Aviation Turbine Fuels) and can be isolated from them. These are not dealt with here.

 

4.2. Macrocrystalline Waxes (Paraffin Waxes)

4.2.1. Chemical Composition and General Properties

Chemical Composition. Paraffin waxes consist predominantly of mixtures of straight-chain alkanes in a typical distribution of the homologous series whose molar masses depend on the boiling range of the lubricating oil distillate from which they are obtained. Long-chain, weakly branched isoalkanes are present in a much lower proportion, along with a very small fraction of monocyclic alkanes.

The intermediate waxes have a similar composition, but the molar masses of the n- and isoalkanes are higher. Intermediate waxes contain a higher proportion of cycloalkanes and isoalkanes; the latter more strongly branched than those in paraffin waxes.

According to the European Wax Federation (EWF), paraffin waxes have a C-number distribution of n-alkanes from 18 to 45 and a total content of iso- and cycloalkanes of 0 – 40 %. Typical data for the intermediate waxes are an n-alkane C-number of 22 to 60 and a total content of iso- and cycloparaffins of 30 – 60 %.

General Physical Properties. Paraffin waxes are insoluble in water and sparingly soluble in low molar mass aliphatic alcohols and ethers. They are more soluble in ketones, chlorohydrocarbons, petroleum spirit, solvent naphtha, benzene, toluene, xylene, and higher aromatics, especially at elevated temperature. The solubility decreases markedly with increasing molar mass (higher melting point) of the waxes.

Chemical Properties. Paraffin waxes are extremely unreactive under normal conditions. Oxidation reactions occur only at elevated temperatures (e.g., on storage and processing above 100 °C), particularly in the presence of oxygen and catalytically active metals. These reactions can be recognized from the burnt odor produced and the yellow to brown coloration of the waxes. Nevertheless, under certain thermally and catalytically controlled conditions, these waxes can undergo chemical reactions such as chlorination (®  Chlorinated Hydrocarbons), oxidation, dehydrogenation, and cracking (® Oil Refining – Residue Hydrocracking (Hydroconversion), ®  Oil Refining), of which chlorination and cracking are important in industry.

 

4.2.2. Division into Product Classes

Depending on the degree of refining, paraffin waxes are divided into the following product classes:

Table below gives an overview of the change in physical characteristics with degree of refining.
 

Variation in physical data with degree of refining of paraffin wax (starting material : slack wax from a medium machine oil)

Crude waxes (slack waxes) [64742-61-6] consist of a mixture of alkanes, which can be solid, semisolid, or liquid at room temperature; alkyl-substituted cyclopentanes and cyclohexanes (naphthenes); and alkyl-substituted aromatics. They also contain, as impurities, the typical contents of the lubricating oil cuts from which they originate, such as asphaltenes, resins, olefins, and sulfur and nitrogen compounds. The oil content of slack waxes is usually between 5 and 12 %, but can be as high as 25 %.

n-Paraffins and isoparaffins predominate in slack waxes in terms of quantity; both are present in continuously distributed homologous series. The chain-length spectrum (C-number distribution) is determined from the width of the boiling range, the distribution maximum (C-number maximum) from the boiling level of the lubricating oil fraction. In isoparaffins, compounds with terminal methyl branches predominate, followed by other methyl-substituted alkanes. The concentration of these compounds decreases as the branching point moves toward the middle of the chain. Other components are compounds with terminal ethyl branches and multiply branched structures, which can be detected only in slack waxes from heavy vacuum distillates.

Table below shows the composition of some slack waxes as a function of origin.
 

Composition of slack waxes from various lubricating oil distillates of the same origin (80 % Arabian, 10 % German, and 10 % Norwegian crude oil)

Physical Properties: Depending on the origin and production process, slack waxes are light- to dark-brown, soft, and unctuous to semisolid materials without a clear crystal structure because of their high oil and soft wax content. The congealing points lie between 35 and 65 °C; the drop points, between 35 and 68 °C; the needle penetration at 25 °C (a reciprocal measure of the hardness) is between 40 and 80 (at greater oil content, even higher); the viscosities at 100 °C are between 3 and 10 mm²/s; the densities at 70 °C, between 775 and 815 kg/m³; and the flash points between 190 and 250 °C.

Scale Waxes. Normally being physically decolorized slack waxes, scale waxes [90669-78-6] essentially have the same composition as the raw materials from which they are obtained. Only the dark material contents of the slack waxes (asphaltenes and resins) are removed by the decolorization process, and the content of alkylaromatics, and of sulfur and nitrogen compounds, is reduced. In physical properties, scale waxes are similar to slack waxes. Decolorization renders the products white to pale yellow.

Deoiled Slack Waxes (Crude Hard Waxes, Raw Waxes). In deoiled slack waxes [8002-74-2] the predominant proportion of the hydrocarbons that are liquid (oils) under normal conditions, and a certain proportion of the semisolid ones (soft waxes), have been removed. Liquid and semisolid waxes consist mainly of low molar mass n- and isoparaffins and of isoparaffins with centrally located and strongly branched side chains, as well as naphthenes and alkylaromatics. Therefore, in deoiled slack waxes the n- and weakly branched isoparaffins are enriched. By removal of the low molar mass portion a narrower C-number range results along with a more pronounced C-number maximum.

Table below shows these relationships with two waxes from heavy spindle oil and medium machine oil used as examples.
 

Composition of deoiled slack waxes from two lubricating oil distillates

^*  From heavy spindle oil.
 

^**  From medium machine oil.
 
Physical Properties: In structure and consistency, deoiled slack waxes are coarse to medium crystalline, brittle to weakly plastic and depending on the degree of deoiling, hard to very hard. These waxes are light to dark brown and darken on heating. The congealing points are between 48 and 72 °C; the drop points between 48 and 75 °C; and the needle penetrations at 25 °C between 10 and 30. At the same degree of deoiling, the course of the penetration — temperature curve (increase in penetration in the penetrogram [essentially depends on the origin of the waxes.

Crude soft waxes [64742-67-2] are sometimes known as foot oils and are formed in the deoiling of slack waxes. They consist predominantly of low molar mass n-paraffins and strongly branched isoparaffins; higher molar mass, weakly branched isoparaffins; naphthenes; and alkylaromatics. The n-paraffin content of soft waxes is generally <25 wt %.

Soft waxes are very soft, unctuous to inhomogeneous materials at room temperature because of their high oil content, which can be 30 wt % or greater. The oil is included in voids of the crystal lattice of the solid hydrocarbons. Soft waxes have a light- to dark-brown color and congealing points <40 °C. Their viscosities at 100 °C are between 3 and 12 mm²/s; their flash points, between 180 and 220 °C.

Semi-refined, filtered (decolorized), and fully refined waxes are similar to deoiled slack waxes in the composition of their main components. Through refining they are freed from more highly condensed alkylaromatics and naphthenes, olefins, and sulfur and nitrogen compounds. Depending on the origin, their content of n-paraffins varies between 92 wt % (East-Asian waxes) and 45 wt % (waxes from heavy machine oil distillates of German origin). The composition of the refined waxes is determined not only by the origin of the crude, but also by the boiling range of the starting distillate. As the boiling ranges of the lubricating oils rise, the molar masses of the hydrocarbons contained in waxes obtained from them increase. The concentration ratio of iso- and cycloparaffins to n-paraffins also increases. Thus, for example, a refined wax from Tuismasy (Ural) crudes contains 81.6 % n-paraffins with an average molar mass of 360 g/mol (average C-number 25.7) and 9.3 % isoparaffins (average molar mass 384 g/mol; average C-number 27.4). The naphthenes (9.1 %) consist of 81.5 % monocyclic and 15.5 % bicyclic alkanes (average C-number 28.9).
 

Composition of fully refined waxes (congealing point 62 °C in each case) from different origins

 
Physical Properties. Waxes in this group differ from deoiled slack waxes and from each other in their residual oil content and, thus, in hardness, color, and color stability.

Semirefined waxes have an oil content of 1.5 – 3 %, and the needle penetration at 25 °C can be 20 – 60. They are somewhat plastic and kneadable, colorless to white, have good color stability under light, and are virtually odorless and tasteless.

Filtered (decolorized) waxes have an oil content of 1.5 % (max.) and a needle penetration at 25 °C of 10 – 26. They have a whitish color and are relatively color stable and generally odorless.

Fully refined waxes (oil content and penetration similar to decolorized waxes) are crystalline, no longer kneadable, pure white, transparent to slightly opaque, color stable, light-resistant, odorless, and tasteless. They fulfill purity criteria for the production of packaging materials for foods and for the formulation of cosmetics and pharmaceuticals. Detailed information on the chemical composition of fully refined waxes and methods for their analytical determination can be found in the CONCAWE report.

According to a more recent definition, fully refined wax products are those that fulfill various national purity requirements. The oil content, which was once limited to 0.5 % (max.), is no longer a criterion since the oils contained in the wax also consist of n- and isoparaffins of the required purity. Depending on the literature, the oil content of fully refined waxes can be 1.5 % (max.) or 2.0 % (max.).

Occurrence of Raw Materials and Processing

Paraffin waxes are contained in crude petroleum and are obtained during oil refining. Depending on the source of the petroleum, the content of solid waxes can vary between 2 % (Romanian origin) and 30 % (Indonesian origin); generally, however, it is between 3 and 15 % in crude petroleum from the main production areas.

Because of their higher molar masses and thus higher boiling ranges, waxes become enriched in the residues from atmospheric distillation (long residues). In processing the residues to lubricating oils, waxes are obtained in all fractions of the vacuum distillation. Because of their poor solubility, they give rise to the high pour points of the basic oils and must therefore be removed. The yields of wax vary depending on the origin and quality requirements of the oils. For a wax content of crude petroleum of ca. 5 %, 8 – 18 % solid waxes in the individual lubricating oil fractions can be expected.

Slack waxes are produced in the dewaxing of lubricating oil distillates (®  Lubricants and Lubrication). The type of process used and process parameters used are according to the desired quality of the lubricating oils.

Dewaxing Lubricating Oil Distillates

The principle of dewaxing lubricating oil distillates is based on the different crystallization temperatures of straight-chain and weakly branched paraffins and the oil phase. The wax-containing solvent-neutral oils (i.e., the raffinates from solvent refining) are mixed with suitable solvents such as propane, naphtha, chlorohydrocarbons, ketones, or (most frequently) a toluene – methyl ethyl ketone mixture. The oil – solvent mixture is subsequently warmed to obtain a homogeneous solution and then cooled continuously in scraping chillers to obtain the wax crystals in a loose suspension. In the subsequent filtration, crystallized wax is separated in vacuum rotary filters, and washed with fresh solvent; the low-wax solvent-neutral oil is then freed from the solvent by distillation. The solvent-containing wax is either separated from the solvent by distillation if it is to be obtained as such or fed directly into the deoiling process for production of hard waxes.

Process conditions are controlled in such a wax that high throughput rates (i.e., short filtration times) and as low an oil content of the slack waxes as possible are achieved [88].

The selectivity of the dewaxing process is controlled by the cooling temperature in the scraping chiller and by the oil – solvent ratio. For common lubricating oil fractions the cooling temperature is between –50 and –20 °C at oil yields of 65 – 85 %. The most important process parameter is the targeted pour point of the dewaxed oil.

In dewaxing, all hydrocarbons with crystallization temperatures above the chosen cooling temperature are removed. n-Paraffins are thus almost completely separated, along with a suitable fraction of the long-chain, weakly branched isoparaffins. Even under optimum filtration and washing conditions, impurities consisting of highly branched isoparaffins and cycloparaffins are present in slack waxes, thus giving rise to their oil content.

A second process for producing lubricating oils with the required low-temperature properties is catalytic dewaxing. n-Paraffins are preferentially cracked on zeolite catalysts to give low molar mass hydrocarbons. Wax thus cannot be obtained from this process. Catalytic dewaxing is said to be very selective in the case of heavy distillates, whereas in light distillates the branched paraffins also react (loss of yield). Since no solvents are used in this process it does not require solvent recovery and is therefore environmentally friendly. 

Deoiling Slack Waxes:

Refining slack waxes begins with deoiling to obtain harder, higher-value products (hard waxes). The two main processes are solvent and sweat deoiling. The oils produced as byproducts, which contain a higher or lower proportion of soft waxes, are known as foot oils or soft waxes, depending on the consistency and the process used.

Solvent deoiling is the most commonly used process, involving one of three possible techniques: pulping, crystallization, or spray deoiling. Starting materials for all deoiling processes are either molten slack waxes (if deoiling is carried out in a location other than that of slack wax production) or slack wax – solvent mixtures, formed in dewaxing lubricating oil distillates. 

Processes for deoiling slack waxes

^*  From the dewaxing step.

Slack waxes from spindle oils to heavy machine oils, and to some extent petroleum as well, can be deoiled by the three solvent deoiling methods. The yields of deoiled slack waxes depend on their origin, the process used, and the degree of deoiling. Yields are between 80 % for spindle oil distillates and ca. 60 % for heavy machine oil distillates, based on the slack wax used.

Pulping Process. The inhomogeneous mixture of crystallized wax, oil, and solvent is diluted or repulped by addition of the same solvent or a solvent mixture (normally by using part of the wash filtrates produced later). The oils, soft waxes, and part of the low molar mass hard waxes thus dissolve. The pulp is cooled with stirring in scraping chillers, and the temperature is adjusted to that necessary for the desired degree of deoiling. Hard waxes, which are partly undissolved and have partly recrystallized, are removed on vacuum rotary filters and washed with solvent; the solvent is subsequently distilled off.

The quality of the hard waxes produced is determined by the deoiling temperature and the quantity of solvent used. With a 1,2-dichloroethane – methylene chloride mixture as solvent (Edeleanu Process), for example, typical deoiling temperatures are –10 °C for slack waxes from heavy spindle oil, –5 °C for those from a light machine oil, and +15 °C for those from medium to heavy machine oil.

Crystallization Process. The molten slack wax or solvent-containing wax crystallizate from the dewaxing process is dissolved completely, or almost completely, in solvents by warming, and the mixture is cooled to a given temperature in one or several steps, depending on the desired degree of deoiling. The temperature is chosen such that only the hard wax crystallizes. The crystallizate is filtered and washed, and the solvent is removed by distillation.

In both the pulping and the crystallization processes, technical problems arise in attempting to produce readily filterable hard wax crystals that allow high filtration capacity with good washability. These problems can be solved in various ways: e.g., through the nature and quantity of the solvent; the construction of the crystallization apparatus; successive solvent dosing during cooling; addition of cold solvent to the warm, intensively stirred wax – solvent mixture; and use of filter aids. Other improvements have been brought about by the development of economical cooling and filtration systems.

Spray Deoiling. Molten slack wax is sprayed into a countercurrent of cold air, and precipitated wax particles are washed with solvents in mixers whereby the oil is largely diluted. After settling the washed wax particles are centrifuged and washed, and the adhering solvent is distilled off. The washing temperature is between 5 and 15 °C.

Sweat Deoiling. Molten slack wax is charged to chambers equipped with sieve bottoms and heating coils. The wax is solidified by cooling and then warmed very slowly. The oil — and, at higher temperatures, the low-melting soft waxes, as well — sweat from the wax block and run through the perforated bottom (run-off slack wax). At the end of the process the remaining hard wax is melted to remove it from the sweating chamber.

No new plants are being built for this classical deoiling process because of the low selectivity (poor yields of hard wax), time-consuming warmup, need to use a batch process, and inapplicability of the method to strongly oil-binding slack waxes from medium and heavy machine oil distillates. In existing plants an attempt is being made to improve yields of hard waxes by partially recycling the run-off slack.

Overview of Processes in Current Use: Of the deoiling plants operating in Germany, half employ solvent deoiling (using mainly the Edeleanu dichloroethane – methylene chloride process) and the others use spray deoiling (Edeleanu) and sweat deoiling.

In the United States, more than 90 % of the existing plants use solvent deoiling. The most common solvents are methyl ethyl ketone and methyl ethyl ketone – aromatics mixtures. Propane and methyl isobutyl ketone are also used.

Refining Deoiled Slack Waxes

Deoiled slack waxes whose maximum oil content is 0.5 %, 0.5 – 1.5 %, or 2 – 3 %, depending on the use anticipated, still contain impurities and are mostly dark in color. To improve their quality they are purified further by adsorbents (decolorizing) or chemically. The choice of refining process depends on economic factors and the quality of raffinates required. Process combinations have also been introduced into industry. The preferred process in new plants is hydrotreating.

Refining with adsorbents (decolorizing) (®  Lubricants and Lubrication) can be performed batchwise, semicontinuously, or continuously. The decolorizing temperature can be between 70 and 120 °C, depending on starting material.

In batch decolorizing, molten wax is stirred in heated vessels with decolorizing clays (chemically activated clays, bentonite, bauxite) in one or more steps until the desired lightening of color is achieved. The used clay is removed in heated filter presses and thermally regenerated.

Percolation Process. In the percolation process, molten wax flows downward through a tower containing 10 – 50 t of decolorizing clay, depending on plant size. After the clay has been exhausted, the wax flow is stopped; wax adhering to the clay is washed off with naphtha; the remaining solvent is driven off with steam; and the clay is burnt off in a separate tower, activated, and reused.

The semicontinuous decolorizing process operates with two or three percolator towers, which are in different phases of the adsorption cycle. While the adsorption process occurs in one tower, in the others the used decolorizing clay is washed, stripped, removed for regeneration, and replaced by new activated clay.

In the continuous process the regenerated clay flows downward through the adsorption tower in countercurrent to the molten wax. The loaded clay passes in the form of a sludge to a washing tower where adsorbed wax is extracted with naphtha and then to a second tower where naphtha is driven off wich steam. The clay is then burnt off in a rotary kiln, activated, and fed back to the top of the adsorption tower after cooling.

The decolorized waxes are then stripped with steam in packed columns at 110 – 170 °C, either under vacuum (0.6 – 1.6 kPa), under reduced pressure (up to 80 kPa), or under pressure (up to 0.8 MPa) to remove odorous substances. The weight ratio of steam to wax varies between 0.5 : 1 and 1 : 1 at detention times up to 15 min. To improve the aging resistance of the waxes, apparatus with aluminum or stainless steel lining is recommended for stripping.

Depending on the starting material, the wax qualities listed in Table below are obtained by the decolorizing process.

Wax qualities obtained by decolorization

 
The quantity of decolorizing clay required depends on the origin and desired quality of the waxes and is normally between 2 and 4 %. Refining with adsorbents removes dark substances, condensed alkyl aromatics, compounds containing hetero atoms, and metallic impurities.

Nevertheless, producing fully refined waxes fulfilling all of the purity by adsorption is possible only using extremely complex processes (multistage decolorizing) and large quantities of decolorizing clay, particularly in the case of the high molar mass wax fractions. To produce these types of high-purity waxes, chemical refining is the process of choice.

Chemical Refining: Concentrated sulfuric acid, fuming sulfuric acid (oleum), or hydrogen is used as chemical refining agent.

Sulfuric acid refining can only be carried out batchwise because of the slow separation of the wax and acid sludge phases. Molten wax is mixed with sulfuric acid at 80 – 140 °C for some time (between 0.5 and 2 h, depending on the origin of the wax). After separation of the heavier acid sludge, the wax is washed with alkali, treated with decolorizing clay, and stripped with steam to remove the last residues of impurities and odorous substances.

All compounds that react chemically with the aggressive acid, such as unsaturated aliphatic and aromatic hydrocarbons, metal compounds, and those containing hetero atoms, are removed by sulfuric acid refining. Labile quaternary carbon atoms are also attacked, with bond cleavage and formation of aliphatic sulfonic acids.

The process can be optimized by variation of the number of reaction steps, degree of acidity, reaction temperature, ratio of wax to acid used, reaction time, and intensity of mixing. The higher the average molar mass of the wax fraction, the slower is the separation between the wax and the acid sludge phase.

The process has several disadvantages such as the necessity for batch operations, poor yields of raffinates, formation of polluting byproducts, and waste gas and corrosion problems. For these reasons, almost all new plants are based on hydrotreating.

Hydrotreating Deoiled slack wax is heated to the required temperature together with fresh and recycyled hydrogen in the preheater (b) and passed over a sulfur-resistant fixed-bed catalyst (c). After cooling, the reaction mixture and hydrogen are separated in the high-pressure gas separator (d), and hydrogen is recycled into the process. After depressurizing in the low-pressure gas separator (e), the wax passes into the stripper (f), where all the light crack products, odorous substances, and reaction gases are stripped off completely with compressed steam under vacuum. The wax is subsequently dried with nitrogen (g). A combination of metals from groups 6 and 10 on an inert carrier is generally used as the catalyst (e.g., nickel – tungsten or nickel – molybdenum on neutral aluminum oxide).

Reaction conditions can vary within wide limits, depending on starting material, degree of refining required, and composition of the catalyst — e.g., between 200 and 350 °C, 20 and 200 bar, and throughputs of 0.2 – 0.81 L of wax per liter of catalyst and hour. Yields are almost 100 %, based on starting wax.

Under the severe reaction conditions of high-pressure hydrogenation, all aromatic compounds are hydrogenated to naphthenes, all dark substances are decomposed, and all sulfur and nitrogen atoms are removed as hydrogen sulfide and ammonia. Metallic impurities become bonded to the catalyst.

The process can be used to refine all classes of wax (paraffin waxes, intermediate waxes, and microwaxes).

4.2.4. Storage, Transportation, Commercial Forms, and Producers

Paraffin waxes are transported in liquid form (molten) in road tank trucks, rail tankers, or liquid containers. Solid waxes are marketed in slabs in small and large cartons and sacks, and as pastilles, flakes, and powder in sacks or big bags. They are also supplied in fiberboard and steel drums and in molded cartons.

Even refined waxes can become yellow or decompose to form odorous substances on prolonged heating, particularly in the presence of air or catalytically active metals. On storage, transportation, and processing, the waxes are therefore preferably heated in vessels, fitted with warm water equipment, and the containers and piping are well insulated. Storage tanks are often blanketed with inert gas. In some cases, antioxidants are added to fully refined waxes in quantities up to 0.01 %. Deoiled and refined waxes are differentiated according to melting range (gradation). Some examples of commercial waxes are given below.

^*  AMP = American Paraffin Wax

Important producers of paraffin waxes include: Shell (United Kingdom, United States, Germany), Texaco (United States), BP (France, Germany), Mobil (United States, United Kingdom), Lützendorf (Germany), DEA (Germany), Wintershall (Germany), CFP (France), Total (France), Empetrol (Spain), Petrogal (Portugal). In Germany, for example, the companies either process the waxes to refined waxes (DEA, Wintershall) themselves or sell them to wax refineries (H. O. Schümann). Waxes are sold mostly by DEA, H. O. Schümann, and Wintershall.

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