Benzene content in washing oil

The benzene content in the lean gas is one of the main factors determining the amount of benzenoid hydrocarbons in the gas downstream of the tower. With other conditions constant, the higher the content of lean oil in crude benzene entering the tower, the greater the losses downstream of the tower. If the benzene hydrocarbon content at the exit of the tower is 2 g/m3, assuming that the pressure of the gas at the outlet of the benzene washing tower is p = 107.19 kPa, the relative molecular weight of the washing oil is M = 160, and the saturated vapor pressure of crude benzene at 30°C is p0 = 13.466 kPa, it is possible to calculate the crude benzene content in the washing oil that is in equilibrium with this condition: w = 2.24 × (2 × 107.19) ÷ (160 × 13.466) = 0.22. The calculations show that, in order to keep the loss after the tower at no more than 2 g/m3, the maximum benzene content in the lean oil should be 0.22%. To maintain a certain absorption driving force, w should be divided by the equilibrium shift coefficient n, which is generally between 1.1 and 1.2. If n=1.14 is used, the allowable benzene content in the lean mixture is w=0.22%/1.14=0.193%. In fact, since the composition of crude benzene in lean oil contains low levels of benzene and toluene, with the majority being xylene and solvent oils, its vapor pressure is only 20–30% of that of the benzenoid hydrocarbons in gas at the same temperature; therefore, the actual content of crude benzene in lean oil can be as high as 0.4–0.6%, while still ensuring that the benzenoid hydrocarbon content in the gas exiting the tower remains below 2 g/m3.

The benzene content in the low-oil content of our factory is around 0.34%.

The benzene content in the lean oil should be as low as possible; here, the benzene level after the tower is basically below 1.2. Lean oil contains about 0.3% benzene, and the level can be lower than 0.2% in some cases.

In fact, since the composition of crude benzene in lean oil consists mainly of xylene and solvent oils, with low levels of benzene and toluene, its vapor pressure is only 20–30% of that of the benzenoid hydrocarbons in gas at the same temperature; therefore, the benzene content in lean oil can reach 0.4–0.6%, yet this still ensures that the benzenoid hydrocarbon content in the gas exiting the tower remains below 2 g/m3. If the content of crude benzene in the lean oil is further reduced, although this helps to lower the losses after the tower, it will increase the steam consumption during benzene removal distillation. As a result, the yield of the crude benzene product at 180°C decreases, which in turn increases the amount of solvent oil produced in the crude benzene and raises the consumption of washing oil.