The current IIT Madras academic system offers students a very high degree of flexibility in their choice of courses via the elective system. Students can choose between 40-50% of their courses, including courses from any other department. The percentage varies from department to department so you should refer to the curriculum to find the percentage for chemical engineering.
This year onwards the number of credits a student must complete to graduate has been reduced from 432 to 400, so the curriculum should be updated soon. You can read about other new academic initiatives here.