| 摘要 |
<PICT:0847781/III/1> In the steam cracking of virgin naphtha fractions to form C2-C6 unsaturated hydrocarbons the cracked products are cooled by mixing with a quench oil and then passed to a quenching and fractionating zone where they are further cooled by countercurrent contact with quench oil to separate material boiling above 430 DEG F., residual oil is withdrawn from the base of the quenching zone, cooled and recycled and a portion purged and replaced by a gas oil of relatively lower polymer-tar forming reactivity. In a preferred embodiment a naphtha boiling in the range 0-330 DEG F. is introduced by line 1 and sufficient steam is added through line 4 to produce a mixture containing 60-90% (mol) of steam. The naphtha is cracked in the furnace 3 at 1350-1500 DEG F. and a total pressure of 1-5 atmospheres, the residence time being less than 1 minute. Quench oil from line 7 at a temperature of 300-400 DEG F. is added to the cracking coil effluent at one or more points at a rate of 3-6 parts by weight per part of effluent to cool the effluent to 450-650 DEG F. The cooled effluent is then fed to quench tower 6 where it is further cooled by contact with downflowing quench oil, it being arranged that all material boiling above 430 DEG F. is condensed but no water vapour is condensed. The light ends leave by line 29 and are processed conventionally. Quench oil leaves by line 10 and is cooled by heat exchanger 13 to 300-400 DEG F. Part of the cooled oil passes by line 43 to a mid point of the quench tower; a further portion, about 1-10% of the total stream is purged by line 16. The amount passed by line 7 to line 5 as described amounts to about half the total stream. The remainder is further cooled in cooler 18 to 150-200 DEG F. and recycled by line 19 to the top of tower 6. Part may be bled off through line 20 for use as a sponge oil to remove higher boiling components from an ethylene containing stream in processing the light ends and is then returned by line 21 to line 22 which feeds an intermediate stream to tower 6 at a temperature of 175-200 DEG F. The stream fed to the top of the tower by line 24 is 5-50 DEG F. cooler. The total feed of quench oil to tower 6 is preferably about half the total circulating quantity and 3-6 times the weight of hydrocarbons in the cracking coil effluent. Make-up oil may be introduced by line 8 or preferably line 9 and may be a virgin or catalytically cracked gas oil boiling in the range 450-700 DEG F. Part of the quench oil may be removed by line 25, clay treated to remove tar, and returned by line 26. |
