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• The designs of vapor-recompression distillation columns for the ethylene-ethane separation with high or low pressures in the auxiliary condenser are compared. • High pressure reduces refrigeration cost in the condenser but increases compressor cost. • The high-pressure design has 11% lower total energy cost but requires more capital investment. • The return on investment is quite attractive. A recent paper studied the separation of ethylene and ethane using several types of distillation systems in which cryogenic cooling is required. The economics of a conventional column using a very low-temperature expensive refrigerant in the condenser were compared with several configurations using process compressors. The alternative configurations involve the pressure in the auxiliary condenser and the fraction of the column overhead vapor sent through the compressor. Low pressure in the condenser requires a very low-temperature expensive refrigerant but has lower compressor costs. The purpose of this paper is to show that all of the vapor from the column should be compressed and then split, with some going to the reboiler and the rest going to an auxiliary condenser that operates at high pressure so that a higher-temperature less-expensive refrigerant can be used. [ABSTRACT FROM AUTHOR]

• The general problem of developing profitability optima is considered at the conceptual level for chemical process designs. • Industrial estimates often employ IRR (Internal Rate of Return) to measure profitability, with a "traditional" hurdle rate to specify the required IRR (ROI). • The NPV % measure provides a more effective basis. Its parameters are adjustable to match the process, the financial environment, and profitability requirements. • Viewing long-term profitability (NPV project), short-term risk [(NPV %)−1] and total capital invested (TI), the first should be maximized ... the last two, minimized or at least subject to bounds. • A new graphical method, with NPV % plotted versus NPV project , couples the three objectives, thus avoiding a Pareto-style optimization with its arbitrary weightings. • The optimum design is at the point of diminishing returns for LT profitability. Any lower value of TI reduces max (NPV project); any larger value isn't fully recovered. Recent fundamental work with discounted cash flow methods shows that an alternative metric for evaluating profitability and risk, one different from Internal Rate of Return (IRR) —namely the annualized and non-dimensionalized Net Present Value (NPV %)— can make evaluating project profitability easier yet more informative. It provides a simple and transparent strategy to optimize a design's key financial dependent variables (capital investment, profitability, risk). 1 1 Model form was originally referred to as "inside-out," but is now called "exo-parametric," i.e., switching from the vernacular to a more scientific and widely-used terminology (Mellichamp, 2018). Both traditional and exo-parametric modeling methods developed by the author are linked to this metric, providing an efficient and precise mechanism to organize and carry out the financial/optimization search procedures in process design. Design engineers want to know what is the optimum size of investment (TI) that meets projected market conditions? What is the expected long-term (NPV) return? What metric can be used to maximize them, consistent with the need to recover capital as quickly as reasonably possible (i.e., to bound the "short-term risk")? The proposed approach answers all three questions simultaneously, with a precise min-max solution obtained transparently. [ABSTRACT FROM AUTHOR]