A Simple Criterion for Feasibility of Heat Integration between Distillation Streams Based on Relative Volatilities

Abstract

In a multicomponent distillation configuration, there are numerous sources and sinks of heat, and a potential way to reduce the heat duty requirement is to perform heat integration. Unfortunately, an algorithmic search of the optimal heat integration opportunities is intractable when the required temperatures of intermediate streams are computed via complex models. Instead, in this work, we introduce pressure-scaled pseudo relative volatility, a new metric to compare stream temperatures. We justify the use of pseudo relative volatility by proving that this variable is a monotonically increasing function of the liquid fraction in a saturated mixture stream. Using this metric, we derive a shortcut criterion to check the feasibility of various heat integration opportunities, such as thermal coupling via heat transfer (TCH). The advantage of this approach is that it circumvents the need for explicit temperatures and instead relies on composition, component relative volatilities, and pressurequantities that are readily available in shortcut models for optimization of distillation configurations. Leveraging this fact, we propose a new optimization framework that identifies feasible TCHs that we consider within the formulation while minimizing the total heat duty of a distillation configuration. We demonstrate, on a few examples, that our formulation can identify heat duty efficient configurations, some of which are multieffect configurations. Using this methodology, we discover configurations that are not only simpler than the fully thermally coupled (FTC) configuration but also have a much lower heat duty.

Identifying Heat-Integrated Energy-Efficient Multicomponent Distillation Configurations

“…The composition and relative volatilities (calculated through fitting VLE Data) of the aromatics mixture are given in Table . Although the utility of variable pressure is modeled in the NLP framework, the column pressures are fixed at 1 atm in this case study because we demonstrate the improvement in configuration identification for same case studies investigated by Mathew et al…”

“…When all column pressures were fixed at the same value, the configuration designs presented by Mathew et al were found to be optimal in all the three examples. However, in the alcohols and paraffins examples, when column pressures were varied, the minimum vapor duties of some configurations decreased.…”

“…We remark that when B i > 0 (which is generally the case), ρ can be used as an inverse proxy for temperature. In prior work, the authors reached this conclusion without assuming any specific equation for P i sat such as Antoine or Clausius–Clapeyron. The different sets of A i , B i , and C i for each component may yield different estimates of the mixture’s temperature.…”

“…The different sets of A i , B i , and C i for each component may yield different estimates of the mixture’s temperature. Therefore, in this work, we instead use new parameters, A mix , B mix , and C mix , which are fitted for the mixture rather than a pure species, that is, pseudo-Antoine constants …”

“…The net composition matches the liquid composition at the bubble point and the vapor composition at the dew point. We exploit this relationship with eq to compute ρ bub and ρ dew (see Mathew et al for details). Then, we use these estimates to compute T s bub and T s dew as follows …”