This paper deals with the problem of synthesizing mass exchanger networks (MEN's) for processes with some streams having semi-continuous characteristics. Recently, some advances on the synthesis of the continuous mass exchanger networks have been proposed . In contrast to previous works that have simplified the solution by decomposition based on the concept of pinch points, a suprstructure-based representation of MEN's has been proposed by Chen and Hung (2005a,b). Therein the MEN synthesis problem was formulated as a mixed-integer nonlinear programming optimization problem with both network operating and investment costs being optimized simultaneously. The impact of simultaneously minimizing operating and investment costs to waste minimization problems were demonstrated via several numerical examples. Until now, however, there are only few papers investigated the synthesis problems for semi-continuous mass exchanger networks.

The semi-continuous means inlet streams of a continuous process come from another batch process, and thus the available inlet streams have different starting and ending times. Foo et al. (2004) presented the semi-continuous MEN's synthesis problem involving mass separating agents (MSAs) other than water. On the basis of external utility target, Foo et al. (2004) used time-dependent composition interval table (TDCIT) to calculate the consumption of external MSAs of the semi-continuous coke oven gasses (COG) problem with three operating modes, including the single operation without storage tanks, the single operation with storage tanks, and the cyclic operation with storage tanks. In their later work, Foo et al.(2005) addressed the network structure with storage tanks based on utility target, provided a skill of storing and discussed the relationship between units numbers and among of utility. A systematic approach for network synthesis, however, is still desirable for semi-continuous processes. In this paper, our previous works for synthesis of MEN's will be extended to the synthesis problem for semi-continuous MEN's. The stage-wise superstructure for continuous-time MEN synthesis proposed by Chen and Hung (2005a,b) will be extended to include storage tanks and a periodical partition-based representation will be proposed to handle the time-dependent operational characteristics for semi-continuous MEN's. The superstructure-based configuration for semi-continuous MEN synthesis problems can be formulated as a mixed-integer nonlinear program (MINLP), where the consumption of the external lean MSAs is targeted. The stage-wise superstructure-based representation can handle multiple components straightforwardly and can easily consider single as well as cyclic operation modes. The coke oven gases (COG) problem from the literature will be supplied to demonstrate the applicability of the proposed MEN synthesis method for semi-continuous processes.

Regerences 1. Chen, C. L.; Hung, P. S., Simultaneous synthesis of mass exchange networks for waste minimization. Computers and Chemical Engineering. 2005, 29(7), 1561. 2. Chen, C. L.; Hung, P. S.,Retrofit of mass-exchange networks with superstructure-based MINLP formulation. Computers and Chemical Engineering. 2005, 44(18), 7189. 3. Foo, C. Y.; Manan, Z. A.; Yunus, R. M.; Aziz, R. A., Synthesis of mass exchange network for batch processes -- Part II: Utility targeting. Chemical Engineering Science. 2004, 59(5), 1009. 4. Foo, C. Y.; Manan, Z. A.; Yunus, R. M.; Aziz, R. A., Synthesis of mass exchange network for batch processes -- Part II: Minimum units target and batch network design. Chemical Engineering Science. 2005, 60(5), 1349.