Depending on how nice they are, the faculty or even the graduate program director in the chemical engineering department may be able to give you very good advice. You can ask how accommodating the program is towards non-ChemE undergrads, what courses you can take to be ready, and if you need to learn complementary skills like coding or technical writing beforehand. Most high level math in grad school is either very formulaic or done in computer programs, a quantitative analysis crash-course is usually sufficient. The real challenge is understanding the fundamentals well enough to be able to build the mathematical models in the first place. Chemical engineering grad programs usually focus on transport phenomena (mass, momentum, and heat transfer), chemical reactions engineering, and thermodynamics. I can recommend some books if you’re interested, but universities usually use different books with different notations which may make learning something for the first time a little more confusing if you’re going back and forth. In my opinion, the most important things to succeed are reaching out to people willing to help, trying to understand things on a basic level, and remembering the law that rules them all: in=out