Characterisation and utilisation of acid tar waste from crude benzol processing for environmental sustainability

Abstract

Acid tar waste is produced from the purification of benzene, toluene and xylene using concentrated sulphuric acid or oleum. The acid tar waste increases sulphate ion concentration in ground-water if stored in ponds as is the current practice. In view of this, the study will investigate the use of acid tar waste for pavement binding. The acid tar waste studied was obtained from two process streams namely the benzene purification labelled benzene effluent stream acid tar and the toluene and xylene purification labelled toluene and xylene effluent stream acid tar. The acid tar wastes were characterised separately. The parameters were: viscosity, organic compound identification using Gas Chromatography-Mass Spectroscopy (GC MS) and Fourier Transform Infrared (FTIR), sulphuric acid composition determined by titration, metal and anion content using Inductively Coupled Plasma/ Atomic Emission Spectroscopy (ICP/AES). The acid tar waste was rheologically characterised using the method developed by Frolov et al (1981) and later refined by Stanciu (2014). The acid tar waste was neutralised with calcium hydroxide and calcium oxide with the objective of determining the best agent in terms of producing a more insoluble product with less gas emissions during neutralisation. The acid tar waste was modified to produce a material with high flow resistance that could be used in pavement construction. Rubber from tyres, calcium oxide and used engine oil were used to modify acid tar to produce material that is suitable for use as pavement binders. There were five materials produced from the two acid tars. The modified acid tar waste was compared with standard 60/70 bitumen grade material in terms of viscoelastic, stiffness and the rutting resistance parameter. Benzene effluent stream acid tar viscosity measured average value of 28.6mPa.s while that of toluene and xylene effluent stream acid tar determined as 183.0mPa.s. The toluene and xylene effluent stream acid tar contained pyrrole, carboxylic acid, ketone and phenol while benzene effluent stream acid tar contained quinoline, ketone and phenol which are some of the functional groups found in asphalt used for pavement binders. The sulphuric acid percentage concentration (wt) was found to be 24.2 and 23.7 for the benzene effluent stream acid tar and toluene and xylene effluent stream acid tar, respectively. Calcium oxide salt for both acid tar waste materials had lower solubility (10.3%) after heating as compared to calcium hydroxide salt for both acid tar wastes which was approximately 20%. The benzene effluent stream acid tar behaved as a Herschel-Bulkely fluid while the toluene and xylene effluent stream acid tar displayed a flow characteristic resembling a Bingham fluid. Of the five modified materials produced from the two acid tar wastes, two of the products behaved like bitumen grade 60/70 and other asphalt binders found in literature. The two were designated Benzene effluent stream acid tar with Rubber and Calcium oxide (BRC) and Toluene and xylene effluent stream acid tar with Calcium oxide (TC). The two acid tar waste streams were different in the physical and chemical characteristics, thus the two acid tar wastes have to be treated differently. Calcium oxide was the best neutralising agent and is recommended to be used in neutralising the acid tar waste before further modification. The inference drawn is that fewer chemicals will be leached to the environment if calcium oxide is used to neutralise the acid tar waste. The modified acid tar waste products need further adjustment in the viscoelastic behavior by adding bitumen 60/70 grade.