Comparative study on separation and characterization of high melting point macro- and micro-crystalline waxes

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Abstract

In order to separate and characterize the high melting point macro- and micro-crystalline waxes from two different waste crudes (El-Ameria heavy slack wax and Alexandria crude petrolatum, respectively), one stage fractional crystallization has been done to separate these waxes by using different solvents at different fractionating solvent ratios ranging from 4:1 to 10:1 by weight and at constant washing solvent ratio of 2:1. The solvents used are butyl acetate, tertiary butyl acetate and cyclohexane at ambient temperature of 20 °C. Also, the fractionating temperature was studied in the range of 20–40 °C. The resulting products were subjected to bauxite adsorption treatment technique to bleach and purify the waxes. Also, different tools of analysis such as GC, XRD, DSC & TGA, H NMR, and SEM were used to characterize and compare the produced waxes. Data revealed that, butyl acetate solvent at solvent ratios of 6:1 and 8:1 by weight at fractionating temperature of 40 °C were selected as the most suitable conditions for obtaining the highest melting point macro- and micro-crystalline waxes having different n-paraffin contents, crystallinities, thermal characteristics, and degrees of branching and owning different crystal sizes. These waxes are considered as ceresins and can be used in different industrial applications.

Introduction

In this century, both macro- and micro-crystalline waxes have been widely used in many applications such as candles, polishes, treatment of paper and cardboard, printing inks, production of laminated foils and papers for the food industry, cosmetics and pharmaceutical industries, and other industrial purposes (Crause and Nieuwoudt, 2003).

Generally, macro-crystalline waxes (paraffin waxes) are derived from low-boiling wax distillate fractions (light and middle ones). They are solid and crystalline mixtures of hydrocarbons; they are usually produced in the form of relatively large crystals and consist mainly of n-paraffins ranging from C16 to C30 and possibly higher. Varying proportions of slightly branched-chain paraffins (C18 to C36) and naphthenes are present (Concawe, 1999, Freund et al., 1982, Gottshall and McCue, 1973, Letcher, 1984, Mazee, 1973).

Micro-crystalline waxes are usually derived from heavy petroleum distillates or residues or tank bottoms. They are capable of retaining more oil than macro-crystalline waxes and extremely small crystals or microscopic in size and sometimes called amorphous. They are petroleum waxes containing substantial proportion of hydrocarbons other than normal paraffins. They consist mainly of highly branched chain paraffins, cyclo-paraffins and small amounts of n-paraffins and alkylated aromatics (Concawe, 1999, Gottshall and McCue, 1973, Mazee, 1973).

The most predominant deoiling process is the wax re-crystallization (fractional crystallization) which is sometimes called wax fractionation process and can be used to fractionate or de-oil all types of waxes (Richter, 2000, Sequeria, 1994).

Our previous studies revealed that some grades of paraffin waxes can be produced by fractional crystallization of light, middle and heavy slack waxes at ambient temperature by using different solvents. Also, the most suitable solvents for separating paraffin waxes are ethyl and butyl acetates, MIBK and the mixture of MEK, B & T (60:20:20 by weight, respectively) (Mohamed et al., 2008).

The differentiation between fractional crystallization and solvent percolation technique was studied for separation of paraffin waxes from light, middle and heavy slack waxes. The study indicated that, fractional crystallization technique is more suitable; for deoiling the heavy slack wax than the percolation technique. While, percolation technology is a preferable technique for separation of paraffin waxes from light and middle slack waxes (Zaky et al., 2007).

In another study, micro-crystalline waxes with different standard specifications were separated from two different crude petrolatums by multistage fractional crystallization technique using n-hexane as the main solvent containing 40 and 30 wt. % of absolute ethyl alcohol (anti-solvent), respectively, at ambient fractionating temperature of 20 °C and at dilution and washing solvent ratios of 4:1 and 2:1 by weight, respectively (Mohamed and Zaky, 2004).

Petersson et al. (2008) studied the characterization of two petroleum waxes (petrolatum and micro-crystalline wax) with regard to their microstructure, mechanical properties and crystallization behavior. They found from the results of infrared spectroscopy that the two waxes consisted only of saturated alkanes. Confocal laser scanning microscopy showed that the petrolatum sample had a more open microstructure with coarse crystals separated from each other than the micro-crystalline wax, which appeared to have a more network-like crystalline structure consisting of somewhat finer crystals. The crystallization behavior of the two waxes was different. Both waxes crystallized over a broad temperature range and their crystallization characteristics were quite different owing to the difference in their oil contents. From the modulated DSC and rheological measurements, it was found that the micro-crystalline wax crystallized through a two-step process, whereas the petrolatum crystallized through only one step.

Thus, the present work deals with a comparative study on separation of high melting point macro- and micro-crystalline waxes from waste petroleum by-products (Heavy slack wax and crude petrolatum, respectively) and characterizing them by using different tools of analysis such as GC, XRD, DSC, TGA, H-NMR and SEM.

Section snippets

Materials

Two appropriate crude waxes; heavy slack wax and crude petrolatum obtained from El-Ameria Refining Company and Alexandria Petroleum Company, respectively; with different characteristics are used in this study for isolation of macro- and micro-crystalline waxes.

Isolation of macro- and micro-crystalline waxes

The two crude waxes were subjected to one stage fractional crystallization technique using butyl acetate, tertiary butyl acetate and cyclohexane solvents at different solvent feed ratios of dilution (S/F, by weight) ranging from 6:1 to

Characterization of crude waxes

The physical characteristics and the molecular type composition for the two crude waxes are represented in Table 1. Data indicate that the congealing point, kinematic viscosity, refractive index and density of crude petrolatum are higher than those of heavy slack wax. This is due to the high boiling point range and/or mean molecular weight of the crude petrolatum.

Although, the oil content of crude petrolatum is lower than that of heavy slack wax its needle penetration value is high. Molecular

Conclusions

The study shows that butyl acetate solvent at fractionating temperature of 40 °C and at dilution solvent ratio of 6:1 and 8:1 are the most suitable conditions for separation of high melting point macro- and micro-crystalline waxes from heavy slack wax and crude petrolatum, respectively. The macro- and micro-crystalline waxes are differed in their n-paraffin contents, crystallinities, thermal characteristics, and degrees of branching and owning different crystal sizes. These high melting point

Acknowledgements

The authors wish to express their deep gratitude to Prof. Dr. Amal Said Farag the Professor in Egyptian Petroleum Research Institute (EPRI), for her continuous support and collaboration that substantially contributed for completion of this research.

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