(375b) A State-Space-Based MINLP Formulation for Integrated Separation Network Design

Separation Network Design with Mass and Energy Separating Agents

Abstract:

Separation operations, which transform chemical mixtures into new mixtures and/or essentially pure components, are of central importance in process industries. Separations involve different modes and one way of classifying these separation processes is based on the nature of separating agents, which take the form of mass separating agents (MSAs) and energy separating agents (ESAs). Traditionally, the tasks of optimizing typical ESA-based and MSA-based processes, especially distillation sequences and the mass exchange networks (MEN), were performed individually (Yeomans and Grossmann, 2000; Proios and Pistikopoulos, 2005; El-Halwagi and Manousiouthakis, 1990; Papalexandri et al., 1994). However, as separation tasks are performed with the aid of either separating agent, or combinations thereof, neglecting either of them may preclude a series of design alternatives, where the optimal solution may actually lies. Despite considerable contributions accomplished by previous works which considered the use of both MSAs and ESAs (Thompson and King, 1972; Nath and Motard, 1981), all these heuristic procedures have a common and serious limitation: they have not addressed the problem of minimizing the total annualized cost (TAC) which is subjected to the thermodynamic constraints. In addition, the economic optimality of these resulting networks cannot be guaranteed because of the inability of these procedures to consider the optimal design of each separator (such as the purity, number of equilibrium stages and/or reflux ratio) and the balances of all cost items.

In this work, an improved state-space superstructure (Bagajewicz and Manousiouthakis, 1992), a framework that takes multi-stream mixing possibilities into consideration, is presented for the design of separation network with both MSAs and ESAs. More specifically, the overall separation network is viewed as a system of two interconnected blocks (see Figure 1). One is referred to as the distribution network (DN), in which all mixers, splitters and the connections between them are embedded. The other is the so-called process operator (PO), which can be further divided into two sub-blocks, i.e., PO-MSA and PO-ESA. All primary and regeneration mass exchange processes are performed in the former sub-block, while all distillation units are placed in the latter. Considering the thermodynamic constraints as well as relevant shortcut models for each type of separator, the overall synthesis problem can be formulated as a mixed-integer nonlinear programming (MINLP) model, where the operating costs (including costs of process and external MSAs, regenerating agents, cold and hot utilities) and equipment cost (including costs of mass exchange units and distillation columns) are minimized simultaneously. Since (1) the state-space representation does not contain any simplifying assumptions in network topologies, and (2) the trade-offs between capital and operating costs, between ESA and MSA costs can be properly carried out, it is reasonable to expect that the TAC of the overall separation network can be reduced. In addition to the dramatic financial savings, better overall conceptual designs have also been brought about by the significant environmental benefits associated with the new design scheme. A benchmark problem already published in the literature has been studied to demonstrate the validity and advantages of the proposed approach.

Keywords: simultaneous synthesis, process integration, separation network design, state-space superstructure, MINLP model

Literature cited:

Yeomans H, Grossmann IE. Optimal design of complex distillation columns using rigorous tray-by-tray disjunctive programming models. Industrial and Engineering Chemistry Research. 2000; 39: 4326-4335.

Proios P, Pistikopoulos EN. Generalized Modular Framework for the Representation and Synthesis of Complex Distillation Column Sequences. Industrial and Engineering Chemistry Research. 2005; 44: 4656-4675.

El-Halwagi MM, Manousiouthakis V. Simultaneous synthesis of mass exchange and regeneration networks. AIChE Journal. 1990; 36: 1209¨C1219.

Papalexandri KP, Pistikopoulos EN. Floudas CA. Mass exchange networks for waste minimization. Chemical Engineering Research and Design. 1994; 72: 279¨C294.

Thompson RW, King CJ. Systematic Synthesis of Separation Schemes. AIChE Journal. 1972; 18: 941-948.

Nath R, Motard RL. Evolutionary Synthesis of Separation Processes. AIChE Journal. 1981; 27: 926-934

Bagajewicz M, Manousiouthakis V. Mass/heat-exchange network representation of distillation networks. AIChE Journal. 1992; 38: 1769-1800.

Figure 1. The improved state-space superstructure