In naphtha cracking process, C4R2 refers to C4 residual obtained after separation of 1,3-butadiene and isobutylene from C4 raffinate stream and which mainly consists of cis- or trans-2-butene 50~60 wt%, 1-butene 10~15 wt%, and n-butane ~20 wt%. Normally C4R2 is a side product in tert-butyl alcohol plant if C4R1 is used for feed.