To reduce carbon dioxide emissions for chemical processes, one should make them as reversible as possible. Patel et al. [Ind. Eng. Chem. Res. 2005, 44, 3529−3537] showed that, in some cases, one can analyze processes in terms of their work and heat requirements. In particular, for processes, there exists a temperature that is called “the Carnot temperature”, at which one can satisfy the work requirement for the process, using the heat that must be added or removed. The analogy of a heat engine and Carnot temperature is applied to chemical processes, particularly on reactive processes, using a graphical approach. This approach looks at chemical processes holistically, where only the inlet and outlet streams are considered. The process is represented in a ΔH−ΔG space. Chemical processes are classified in different thermodynamics regions as defined in the ΔH−ΔG space, and their feasibility in terms of heat and work requirement is discussed. This approach allows one to determine whether heat at an appropriate temperature is sufficient to meet the work requirement of a chemical process, or if other means should be considered. The approach is used to investigate and discuss the possibility of combining reactive chemical processes classified in different thermodynamic regions in the ΔH−ΔG space, with the purpose of making infeasible processes possible, or to minimize, or even eliminate, the work requirement of the combined process.
This paper describes a new technique to analyze processes with a positive change in the Gibbs free energy, ∆G, based on the second law of thermodynamics. In particular, the application of a two-stage process, consisting of an endothermic high-temperature first stage and an exothermic low-temperature second stage, has been investigated. This paper considers chemical reaction processes as heat engines and that by the appropriate flow of heat at a specific temperature (and, hence, with a specified exergy level) work can be added or removed from a process. The technique also investigates the integration of such processes in terms of work flows. The technique is useful in the early stages of the design process as well as for retrofitting. It helps identify opportunities and set targets for the process. The method does not require detailed information regarding the process and is based only on thermodynamic properties of the system.
It is common practice in chemical engineering to design processes sequentially. The type of product desired determines the choice of the feed materials that are introduced into the reactor networks. These in turn lead into the separation networks. The flows of heat and work are the final part of the sequence to be considered, with the application of heat exchanger networks, and any deficiency or excess in these flows is usually compensated for with the use of utilities. Although the ongoing research into reactor, separation, and heat exchanger optimization is of indubitable value, an aspect that is often overlooked in conventional research is the question: How do changes to one of the elements in the sequence affect the others? Most process designers do not address such matters until the next optimization of the sequence begins. The result of this sequential approach to design is that processes may contain a few very efficient units, but may also have others that are highly inefficient. A graphical technique that incorporates the flows of heat and work into the design of the process at a very early stage is proposed. The technique can be used to prepare flow sheets that represent a synthesized version of the elements that make up the complete process, rendering each component highly efficient. This new design tool uses the thermodynamic properties of enthalpy (representative of process heat requirements) and Gibbs free energy (representative of process work requirements) to develop process flow sheets that operate as close to reversibly as possible, and can be used as a foundation for more detailed refinements to achieve the best possible result. A case was described in a previous paper in which the graphical technique was applied to gasification. The application of the technique to the production of syngas by the steam reforming of natural gas is detailed. We show that the steam reforming process can be operated with increased reversibility and can actually consume carbon dioxide, thus representing a process with a carbon efficiency of greater than 100%, if the way in which all the process units interact with one another is used to utmost advantage. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3714–3729, 2013
A method for setting performance targets for a chemical process, based on mass, energy, and entropy, is presented. These targets can be determined in the early stages of the design process to aid in the synthesis of a suitable flowsheet. These targets are “global” targets, because the overall process is considered and does not consider the individual processing units. A method of determining the minimum amount of inputs (mass, energy, and work) is illustrated by setting various targets (for example, no CO2 emissions, no energy emitted from the process, or ΔH = 0 and ΔG = 0). The approach allows one to screen the various options easily and thus determine processes that are efficient and environmentally friendly. This approach also allows opportunities of mass, heat, and work integration to be determined and applied at the earliest stages of the design process. Initially, a graphical solution is illustrated, and thereafter, the approach is generalized using a linear programming formulation. The approach is demonstrated by identifying opportunities in a methanol production plant.
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