Separation of bio-oil by molecular distillation

Abstract

In this study, KDL5 molecular distillation apparatus manufactured by the UIC Corporation was adopted to separate bio-oil, which came from a bench-scale fluidized-bed fast pyrolysis reactor at a feeding rate of 1 kg/h. A maximum distillate yield of 85% was obtained without obvious coking or polymerization during the molecular distillation process. The effect of distillation temperature on physical and chemical characterization of each bio-oil fraction was investigated. Statistical calculations showed that molecular distillation was successful in the separation of bio-oil. A separation factor was proposed to reflect the ability of isolating the chemicals contained in the bio-oil using molecular distillation.

Introduction

Bio-oil obtained from fast pyrolysis of biomass has the potential to substitute for fossil liquid fuels after it is upgraded by catalytic hydrogenation, catalytic cracking or steam reforming. Bio-oil can also provide a source of valuable and useful chemicals by using separation technologies such as distillation and/or extraction. There have been more than 400 compounds identified in bio-oil derived from biomass fast pyrolysis. The developmental focus in this field has been on recovering products from whole bio-oil or from the major, relatively easily separated fractions [1].

In the past, researchers have taken advantage of almost all conventional separation or purification technologies to isolate bio-oil, such as liquid chromatography, extraction, centrifugation and distillation. Liquid chromatographic separation is an effective method in isolating aromatic and saturated hydrocarbons. However, high consumption of solvents and silica gel solid phase regeneration result in high cost [2]. Extraction is often used for phenol recovery under alkaline conditions, which unavoidably resulted in a large amount of flocculation [2]. Centrifugation was a simple pretreatment technique [3], [4]; however, there were limitations with the homogeneous bio-oil. The conventional distillation includes atmospheric distillation, vacuum distillation, flash distillation and steam distillation. Although atmospheric distillation and vacuum distillation are relatively inexpensive and simple, due to long residence time, high temperature and low efficiency, thermo-sensitive bio-oil cannot be completely vaporized by these methods [5]. Flash distillation is a process where mixtures are heated to partial vaporization in the distillation column in order to separate the vapor phase and liquid phase [2], [6]. Compared with flash distillation, steam distillation is special for temperature-sensitive materials such as natural aromatic compounds, because it achieves distillation at temperatures lower than the normal boiling point by adding water vapor [7]. Flash and steam distillation avoid secondary reactions because of short residence time, but they are only suitable for pre-separation where high purity is not required. Applications are currently limited to the laboratory and difficult to scale up.

Molecular distillation (or short path distillation) is usually used for the distillation of thermally unstable materials, and it is the most economically feasible method of purification [8]. It means that in the high vacuum condition, the distance between the evaporation surface and the condensation surface is less than or equal to the mean free path, namely, the molecules which escape from the evaporation surface easily reach and are condensed on the surface before they collide with each other, i.e. without any resistance. Molecular distillation equipment is widely used in the fine chemical, petrochemical, pharmaceutical, oil and grease, food processing and light industries as well as in scientific research to concentrate and purify organic chemicals of high molecular weight, high boiling point, high viscosity and poor heat stability. Since the petroleum source is progressively decreasing, the demand for upgrading heavy fractions is increasing, and thus molecular distillation has been used for heavy petroleum processing and characterization [9], [10], [11]. Molecular distillation was also applied to produce high purity biodiesel, in which the methyl ester content was above 95% [12]. However, molecular distillation has never been used to fractionate bio-oil. Bio-oil is a complex mixture of many compounds with a wide range of boiling points. It is thermo-sensitive and easily undergoes reactions such as decomposition, polymerization, and oxygenation. Molecular distillation is not limited by these poor properties and is suitable for the separation of bio-oil.

In this study, molecular distillation was used to isolate bio-oil obtained from biomass fast pyrolysis. The separating ability of molecular distillation was evaluated according to the analysis of physical and chemical properties of the fractions, and the advantages of molecular distillation for bio-oil pre-isolation were discussed.