A rapid screening methodology for chemical processes

Highlights

• The CFSTR Equivalence Principle can be used to obtain the most optimistic raw materials costs.

• By incorporating estimates of separations work, we can determine estimates of economic metrics.

• The Feinberg Decomposition can be used to generate target values of NPV and NPV_%.

Abstract

Techno-economic analysis is an important part of evaluating potential chemical manufacturing ventures. A proposed conceptual process design gives the design engineer most of the information required to conduct these analyses. However, developing conceptual designs can be time consuming and expensive, especially given that most conceptual designs never see commercialization. In this work, we present a rapid screening methodology to generate screening-level economic metrics (NPV and NPV_%) without dedicating significant time or resources to conventional chemical process design. This methodology uses ultimate bounds on reaction selectivity combined with the minimum work of separations to bypass the design procedure altogether. The methodology presented here provides a means to rapidly screen chemistries for their economic merit, compare a proposed process design to the best-case economic scenario, and compare potential projects competing for the same funding sources. In this article, we present the necessary theory and demonstrate the methodology.

Introduction

The main goal of chemical processes is to take starting materials and upgrade them into more useful and valuable products, which can be sold for a profit. When considering a new chemical venture, a techo-economic analysis must be completed to ensure not only that the conversion of raw materials into the desired products is possible, but also to ensure that doing so is economical. A very large number of flowsheets can be generated for a process designed to achieve a certain task (e.g., 10⁴ to 10⁹) (Douglas, 1988), which allows design engineers a tremendous amount of creativity. However, at the beginning stages of a project, when management must decide to either fund or halt the project, expoloring even a small fraction of these design alternatives can be prohibitively expense and time consuming. Furthermore, only a small percentage of chemical process concepts are commercialized (Douglas, 1988). Being able to quickly analyze the potential profitability of a proposed process is crucial to determining which projects should receive more funding and consideration for research and development. Here, we present a rapid screening methodology that allows design engineers to quickly determine if a process will be economical using only the reaction kinetics for the chemistry of interest. Furthermore, we estimate economic metrics including net present value and net present value percent (Mellichamp, 2013) that allow one to compare multiple processes competing for the same funding.

In analyzing the economics of a proposed chemical venture, the process can be divided up into two main sections – the reaction section and the separations section. The reaction section of a chemical process has significant impacts on the economics as raw materials requirements are a large expense, typically accounting for up to 85% of total variable operating costs (Douglas, 1988). In a process with an inadequately designed reactor (and low reaction selectivity), a significant fraction of raw materials react to generate undesired byproducts and wastes instead of the desired product. This is a poor use of raw materials and often leads to an uneconomical process. In addition to operating costs, the reaction section contributes to the capital expense of the project through the cost of the reactor (including catalyst, if needed), which can become quite large if trying to reach high conversions.

The separations systems also greatly affect the process economics. Separations account for approximately 60% of energy requirements in chemical process, 95% of which (on an energy basis) is accomplished through distillation (Eldridge et al., 2005). As such, utilities for the condensers and reboilers can have a large impact on the operating costs of the process. Additionally, the capital expenditure for separation systems can be very large, especially for multicomponent distillation, which may require several columns along with the associated reboilers and condensers.

With information about the reaction and separations sections of a proposed chemical process, one can conduct a techno-economic analysis; however, this normally requires a conceptual process design. One common approach to chemical process design is the hierarchical method presented by Douglas (1988). In this methodology, process design is completed in systematically increasing levels of detail. In the first level, one decides on a continuous or a batch process. The second level defines the general input-output process flowsheet, showing only fresh feeds of raw materials and exit streams of products, byproducts, and wastes, as shown in Fig. 1a. At Level 3, the reaction section is designed, and the general recycle structure is decided upon, as shown in Fig. 1b, but no separation equipment is designed or specified. Perfect separations are assumed at this level of design. In Level 4 of the hierarchy, the vapor and liquid separation and recovery systems are designed. Finally, in Level 5 the heat exchanger network is designed and optimized through a pinch analysis.

After the completion of each design level, an analysis is completed to determine the economic potential of the process (i.e., the profit before taxes). The Level 2 economic potential (EP2) is simply the difference between the value of the products and valuable byproducts and the cost of the required raw materials for assumed values of reaction selectivity. At Level 3, the economic potential (EP3) takes into account the reactor design, the resulting selectivities, and decisions made about recycle. EP4 incorporates the design of the separations system. Finally, Level 5 economic analysis incorporates heat integration in the calculation of economic metrics.

Level 2 of the Douglas process synthesis methodology can be completed relatively quickly; however, Levels 3 and 4 take significantly more work. In Level 3, a wide variety of reactor types and operating conditions exist, and finding the best combination can be time consuming and difficult. At Level 4, the design engineer has many choices to make because the required separations can be achieved in variety of ways. The most common method is distillation, but even after the method is decided upon many more decisions exist (e.g., column sequencing in distillation). Each of these levels of design requires more information (such as phase equilibria data for separations), which can be time consuming to obtain. Thus, a methodology allowing one to bypass these steps to make a rapid assessment would be very useful.

Here, we present a methodology to determine estimates on economic metrics by using (1) the Continuous Flow Stirred Tank Reactor (CFSTR) Equivalence Principle (Feinberg and Ellison, 2001) (which allows one to exactly represent any and every candidate reactor system for a chemistry using a single model) combined with (2) the minimum work for separations. The only information required in this methodology are the kinetics for the chemistry of interest. Using this methodology, we can avoid both the detailed reactor design and the synthesis/conceptual design of a separations system. This in turn eliminates a significant amount of information that needs to be gathered, particularly for the separation system (e.g., information about phase equilibrium, presence of azeotropes, etc.) and allows for the rapid techno-economic analysis of potential chemical processes. The economic metrics we obtain are the net present value (NPV) and net present value percent (NPV_%) (Mellichamp, 2013). These metrics take into account reaction and separation, but they do not correspond to a specified conceptual design. We therefore refer to these metrics as “screening level” metrics. This methodology provides essentially a “stage 0” front-end loading (FEL) analysis (Perry and Green, 2008) with more rigor than can normally be achieved using conventional methods. The screening methodology is demonstrated on two examples, production of phthalic anhydride from o-xylene (see later in this article), and on butane alkylation (see SI).