(42j) Energy-Saving Design and Control of a Methyl Methacrylate Separation Process

In the industrial process to produce methyl methacrylate (MMA), a mixture including MMA, methanol (MeOH) and water (H2O) needs to be separated. The MMA and water are designed to be drawn out of this separation process as two product streams while methanol as an excess reactant should be recycled back to the esterification reactor. Wu et al. (2011) devised a design flowsheet including a distillation column with a bottom decanter and another stripper. The bottom composition of the distillation column was designed to be near MMA/H2O azeotrope so that a decanter can be designed to obtain aqueous and organic streams. The aqueous stream is pure enough to draw-out of the system while another small stripper was needed to purify the organic stream to MMA product specification. This design flowsheet was further simplified by combining two columns into one column with a middle decanter.

The drawback of the above design is that the bottom composition of the distillation column needs to be placed near a saddle point (MMA/H2O azeotrope) of this ternary system. Thus, larger reboiler duty was required at the distillation column to satisfy this bottom purity requirement. In this paper, an alternative design using the same process units (a distillation column, a decanter and another stripper) is developed. In this alternative design, the fresh feed is first mixed with some water and condensed stripper top stream so that the composition of the mixture is pulled within the LLE envelope. After liquid-liquid separation in the decanter, the aqueous stream can further be separated in a distillation column with top product of mostly MeOH (at MeOH/MMA azeotrope) and the bottom product of pure water. The MMA product can be obtained from further purify the organic stream in a small stripper. Note that in this alternative design, top and products of the distillation column are designed to be at unstable node and stable node, respectively. Thus, signification saving in the steam cost (16.3%) can be realized by using this alternative design with the same process units. Another benefit of this alternative design is that the MMA product loss through the water outlet stream is also less than the design in Wu et al. (2011). This represents another 9.6% savings of the operating cost for this alternative design.

Furthermore, in this research, a suitable and simple control structure for this alternative design is also devised based on both open loop and closed loop sensitivity tests. By controlling temperature difference at two trays of the distillation column and another tray temperature at stripper, both MMA and water products can be maintained at high-purities despite large variations in feed flow rate and feed composition changes.

Reference:

Wu, Y. C.; Hsu, C. S.; Huang, H. P.; Chien, I. L.; Design and Control of a Methyl Methacrylate Separation Process with a Middle Decanter, Ind. Eng. Chem. Res., 2011, 50, 4595-4607.