Control structure selection for the deethanizer column from Mongstad/Statoil

The selection of a good control structure for a distillation column is important for the plant operation because it can affect its operability. In this work, it is studied the control structure selection of a deethanizer distillation column from Mongstad/Statoil. This is a multicomponent distillation column with a partial condenser and a vapor distillate flow. Having a very small distillate flow rate (1 % of the feed flow rate), this column presents some difficulties for pressure control. The simulations are done in ASPEN Plus and ASPEN Dynamics. The main objective is to find a structure that minimizes the effect of the disturbances and implementation error (noise and bias) in the top and bottom compositions (indirect composition control). The results show that it is better to control temperatures in the top section where the gains are higher and, as this is a multicomponent column, the two-point temperature control does not improve the control system performance, confirming the results obtained by Hori and Skogestad (2007b). As the most important disturbance for this column is the feed flow rate, the results also show that we should ratio the available manipulated variables with feed flow to improve the control performance. Also, the simulations show that it is possible to control the pressure using the distillate flow rate if we also close a temperature loop.