Extraction of tar acids from tar oils by aqueous acetamide

Abstract

Aqueous acetamide has been found to be a good solvent for the extraction of tar acids from low temperature carbonisation tar oils. Saturated acetamide solution at 15-30" in the molar ratio acetamide/tar acids of 3 : 1 effects a recovery of about 90% of tar acids originally present in the oil in two extraction stages. The tar acids are obtained in 77% purity, the impurities consisting of neutral oils and tar bases. Re-extraction with light petroleum can raise the purity of tar acids to 94%. Tar acids can be separated from the purified acetamide extract by suitable dilution with water.The system tar acids-(bases+neutral oils) -saturated aqueous acetamide has been studied. It has been shown that three theoretical counter-current extraction stages are required to effect a recovery of 95% of tar acids from a feed oil containing 45% tar acids. 98% recovery of the tar acids could be achieved i n four stages. TntroductionThe use of solvents for the extraction of tar acids from tar oils has been discussed in an earlier paper.' It has been shown that acetic acid can be used for extraction of tar acids. The disadvantage is the contamination with tar bases and neutral oils.2 Attempts at improving the process revealed that aqueous acetamide solution could be used with advantage for extraction. The solubility of tar bases and neutral oils in aqueous acetamide was comparatively low, while the solubility of tar acids was high.The results of investigations carried out on the extraction of tar acids from tar oils using aqueous acetamide as the extracting solvent are given below.The only reference to the use of amides for the extraction of tar acids is in a paper by Kowalski et 0 1 .~ They found that compounds like urea, thiourea, amides of lower fatty acids and similar compounds and their solutions in water had only limited solvent power for phenols as compared with known extracting agents like alcohols, acetone etc. The main interest of these workers, however, was to develop urea as a solvent for tar acids. ExperimentalThe feed oil used was crude tar oils obtained by the distillation of tar obtained from the low temperature carbonisation pilot plant (externally heated vertical retorts) at the Central Fuel Research I n s t i t~t e .~ The tar oil fraction boiling up to 270" was used for experiments. This oil had the composition: tar acids 42-32, tar bases 4.36, neutral oil 53.32%.For phase equilibrium studies, the oils were separated into neutral tar oils, tar bases and tar acids, and these fractions were distilled under vacuum. Suitable compositions were made by blending these fractions. The solvent used was a solution of acetamide in water. Experimental procedureThe requisite quantities of aqueous acetamide and Par oils were well mixed by stirring in a temperature controlled separating funnel for 30 min. Stirring was then stopped and the layers allowed to separate for 1 h. The layers were separated and analysed for tar acids, bases and neutral oils by methods essentially the same as described in an earlier paper.2 Fig. 1 shows th...