Performance assessment of control strategies with application to NGL separation units

In this contribution, the problem of NGL separation control is addressed by dealing with the most common process schemes. The main goal is to achieve a specified ethane recovery as well as maintaining certain levels of methane impurity in the demethanizer column. An indirect control of composition through the temperature control in the column is proposed. A cascade arrangement between the column temperature control and the controller that maintains a constant ratio of boil-up to column bottom product is proposed for the improvement of methane impurity levels. Additionally, an “inferential” control approach based on Antoine's law is formulated and tested to enhance the ethane recovery control. The performance indexes calculated for ethane recovery and methane impurity show the superiority of the proposed control structure in each NGL separation process scheme. When the feed flowrate is reduced by 10%, the proposed control strategy allows a lower deviation from the target and a smaller offset with a reduction of 73.7% for ethane recovery and 72.7% for the methane concentration in the conventional process, 86.6% for ethane recovery and 96.4% for methane concentration in the GSP, and 97.1% for ethane recovery and 91.1% for methane concentration in the CRR process. In case of sinusoidal variations of inlet flowrate, the integral square error is reduced by 99.33% for methane bottom concentration in the GSP process scheme, while ethane recovery shows a reduction of 82.69% in the CRR scheme.