Two-stage Pyrolysis of Vacuum Residues for the Production of Olefins and Gas with High Calorific Value

Abstract

Pyrolysis of Iranian Heavy and Taching vacuum residues has been investigated with the intention to develop a process for producing olefins and a clean gas with high calorific value from vacuum residues. As shown in Fig. 1, the quartz reaction tube was constructed with two zones in which temperatures were controlled at about 450°C and 700-800°C, respectively. The vacuum residue, which was inserted into the low temperature zone in a platinum boat, was pyrolysed into cracked oil (M_W 300-400), which was carried to the high temperature zone by argon flow to undergo subsequent pyrolysis. The procedure is defined as two-stage pyrolysis. In comparison with the case when the boat was inserted into a high temperature zone directly, the yield of olefinic gas increased with the two-stage pyrolysis. Especially ethylene yield was increased by 1.5-1.8 times as shown in Runs 3 and 4 or Runs 12 and 14 as listed in Table 1 and 3. Consequently ethylene was obtained in a high yield of 15% from Iranian Heavy vacuum residue and of 26% from Taching vacuum residue. After fifteen to thirty minutes of pyrolysis of Iranian Heavy vacuum residue at 440°C, 20-30 wt% of feed oil was recovered as residues, which were mostly soluble in carbon disulfide. By raising the temperature at the low temperature zone to about 500°C, ethylene yield was decreased and most of the residue was carbonized. On the other hand, rate of decomposition was extremely slow at temperature below 430°C. Therefore, the optimum temperature of the low temperature zone was determined to be 440°C.