Mass Exchange Network Synthesis Research Articles

Synthesis of combined heat and reactive mass-exchange networks

In this work we introduce the problem of synthesizing combined heat and reactive mass exchange networks “CHARMENs”. The synthesis task involves the preferential transfer of a certain species from a set of rich streams to a set of lean streams and at the same time accomplish a specified heat transfer task in a cost effective manner. The economic objectives of mass exchange and heat recovery are in general incompatible and need to be addressed simultaneously. The proposed work introduces a systematic and generally applicable procedure to trade-off the two objectives and synthesize a CHARMEN. First, we derive a number of important properties of the problem. Based on these characteristics, a two-stage targeting procedure is developed. In the first stage, the minimum operating cost “MOC” of the network is identified via a mixed-integer nonlinear program “MINLP”. A shortcut linear formulation is also devised to identify a near-optimum MOC solution which may be used as an initial guess for the MINLP. The optimum solution to the MOC problem results in a natural decomposition of the original CHARMEN problem into two independent subproblems that may be solved to identify the minimum number of heat and reactive mass exchangers. The applicability of the synthesis procedure is demonstrated by tackling a case study on the desulfurization of gaseous wastes with simultaneous heat recovery.

Waste reduction through multicomponent mass exchange network synthesis

We have developed the “mass exchange network (MEN)” synthesis concept to provide solutions for waste-minimization problems. Often, waste reduction problems require the removal of multiple toxic species from plant streams. Aside from the transfer of multiple chemical species multicomponent problems also encompasses simultaneous heat and mass exchange. To address this problem, we employ the recently proposed “State Space Approach.” The State Space Approach decomposes the network synthesis problem into the evaluation of an operator—where the actual unit operations are defined, and the distribution network—which contains all the flows to and from the operator. The entire network is optimized simultaneously. The use of distinct subnetworks for the flows and unit operations gives complete flexibility to choose over all possible configurations. The proposed approach is illustrated through an example from recycle-reuse tasks; the removal of nitrogenous compounds from the waste streams of a fertilizer plant.

Minimum utility cost of mass exchange networks with variable single component supplies and targets

Mass exchange network (MEN) synthesis is considered, with streams whose supply and target compositions are allowed to vary between upper and lower bounds. The design task is to determine the minimum mass separating agent (utility) cost needed for the transfer of a component from the rich to the lean streams. The mathematical formulation of this synthesis problem leads to a mixed integer nonlinear program (MINLP). This program is shown to possess certain properties which, in turn, are used to develop a solution procedure. This procedure is guaranteed to converge to the global optimum. Two examples, illustrating that utility cost savings can be achieved over the fixed composition MEN synthesis problem, are discussed

Synthesis of reactive mass-exchange networks

The synthesis of a network of reactive mass exchangers involves the same challenges as in synthesizing physical MENs. The problem is further compounded by virtue of the reactivity of the MSAs. Typically, reactive MSAs have a greater capacity and selectivity to remove an undesirable component than physical MSAs. Furthermore, because they react with the undesirable species, it may be possible to convert pollutants into other species that may either be reused within the plant itself or sold. The problem of synthesizing rea ctive m ass- e xchange n etworks (REAMENs) was developed, along with systematic techniques for its solution. The first step in synthesizing a REAMEN is to establish the conditions for which the reactive mass exchange is thermodynamically feasible. To establish the conditions for thermodynamic feasibility of reactive mass exchange, it is necessary to invoke the basic principles of mass transfer with chemical reactions. After a procedure for establishing the corresponding composition scales for the rich-lean pairs of streams has been outlined, it is possible to develop the CID. However, it should be noted that the conversion among the corresponding composition scales may be more laborious because of the nonlinearity of equilibrium relations. The necessary thermodynamic concepts are covered in the chapter along with chemical equilibrium, which is tackled in a manner that renders the REAMEN synthesis task close to the MEN problem. Finally, an optimization-based approach is presented and illustrated by a case study.

Automatic synthesis of mass-exchange networks with single-component targets

This paper addresses the problem of automatically synthesizing mass-exchange networks in which the mass of a single (key) component is exchanged between a set of rich streams and a set of lean streams. In the first part of this paper we present a two-stage synthesis procedure that employs one minimum allowable composition difference for all possible rich-lean stream pairs. In the first stage, a linear programming problem is solved to determine the minimum cost of mass-separating agents and to locate thermodynamic bottlenecks (pinch points) which restrict the exchange of mass between the rich and the lean streams. This formulation can also preclude (preassign) any specified forbidden (compulsory) matches between streams. In the second stage, a mixed-integer linear program is solved to yield minimum-utility cost networks in which the number of mass-exchanger units is minimized. In the second part of this paper we present a more general, yet computationally intensive, procedure that employs one minimum allowable composition difference for each possible rich-lean stream pair. These degrees of freedom are then used to minimize the annualized total cost of the network. An additional merit of the latter methodology is its potential as an improved approach for the synthesis of optimal heat-exchange networks. Several examples with industrial relevance are solved to demonstrate the usefulness of the notion of synthesizing mass-exchange networks and to illustrate the computational effectiveness of the proposed synthesis strategy.

Synthesis of mass exchange networks

AbstractThe purpose of this work is to introduce the notion of synthesizing mass‐exchange networks (MEN's). A systematic, two‐stage, procedure is proposed for the synthesis of cost‐effective MEN's. In the first stage, a thermodynamically‐oriented procedure is used to identify the thermodynamic bottlenecks (pinch points) that limit the extent of mass exchange between the rich and the lean process streams. Preliminary networks, that feature maximum mass exchange, are generated at this stage. The objective of the second stage is to improve the design of these preliminary networks so as to develop a final configuration of the MEN that satisfies the assigned exchange duty at minimum venture cost. This approach is applied to the synthesis of MEN's with single‐component targets as well as multicomponent, compatible targets. An illustrative example on the sweetening of coke‐oven gas is presented to demonstrate the applicability of the proposed synthesis procedure.