GCC (generalized cutpoint calculation) is a first principles based inferential package for CDU and other main fractionators – FCC, Coker, Hydrocracker, etc. Fractionator feed contains hydrocarbons of wide boiling range, which are to be separated to LPG, naphtha, kerosene, diesel, heavy diesel and heavy boiling bottoms. The products must satisfy certain specifications such as distillation 90% points, flash points, freeze and cloud point. The fractionator feed rate and quality is not constant, and occasionally changes quite abruptly, for example upon crude switch or coker drum switch. At such times of uncertainty, the operator is challenged to run the unit optimally while keeping the products on specification.
GCC helps the operator cope, working as a reverse simulation. It reads unit conditions: pressure, temperature and flows from the DCS, and comes up with an estimate (inference) of product qualities. Being a first principle package, GCC is as reliable as the instrumentation. It typically does not drift and does not have a crude-to-crude bias. The GCC code resides on a host computer and inferential results are written into DCS tags. Operators trend the results and they can see how product qualities change and take corrective action.
APC (advanced process control) when implemented works to optimize the unit against constraints, while satisfying product specifications. To accomplish this goal APC must also rely on high fidelity inferential models, and GCC does provide the adequate level of reliability. Over the last 25 years Petrocontrol has installed about 100 fractionator inferential packages to run in support of APC applications.