Hello,
No it's not that difficult to do Mtech in chemical engineering after msc in chemistry. If you are determined enough, then you can achieve anything.
you are eligible for M-TECH course because the criteria of M-TECH course is
1)Bachelor degree holders in Engineering/ Technology/ Architecture (4 years after 10+2) and those who are in the final or pre-final year of such programmes.
2)Master degree holders in any branch of Science/ Mathematics/ Statistics/ Computer Applications or equivalent and those who are in the final or pre-final year of such programmes.
3)Candidates in the second or higher year of the Four-year Integrated Master degree programme (Post-B.Sc.) in Engineering/ Technology or in the third or higher year of Five-year Integrated Master degree programme and Dual Degree programme in Engineering/ Technology
so you have completed M.SC in chemistry so you are eligible for M-TECH chemical engineering.
Hope it helps