CAPE-OPEN

If I change the input flowrate(eg. of water around 250tonnes/day) of mixer and give solve command it stops working.

Thank you for the problem description. This was caused by a problem in the PH flash introduced with the previous TEA update, and is now fixed. Please update TEA via CUP.

How to design a new unit operation (for eg. packed tower)?
How to get into design mode?

There are several options. I presume you mean a unit operation that cannot be easily simulated with something that is available? (e.g. most packed separation towers can be simulated using ChemSep).

Here is a list of options:

http://www.cocosimulator.org/index_support.html

(scroll down to "How to I add user defined unit operations?")