Ranjan K. Malik

Temperature–enthalpy curve for energy targeting of distillation columns

Abstract The temperature–enthalpy ( T – H ) diagram of a distillation column at practical near-minimum thermodynamic condition (PNMTC) or the column grand composite curve (CGCC) is a useful representation for energy targeting studies and may be generated from a converged simulation of a base-case column design. The calculation procedure for the CGCC involves determination of the net enthalpy deficit at each stage by generating envelopes from either the condenser end (top-down approach) or the reboiler end (bottom-up approach). However, the values calculated by the two approaches differ for stages with feeds because existing procedures for CGCC generation do not consider the enthalpy balances at the feed stages. In fact, the net enthalpy deficits at feed stages calculated by both approaches are erroneous even for the simplest case of binary distillation. A feed stage correction (FSC) that rigorously considers the mass and enthalpy balance equations at feed stages is proposed in this work to resolve the discrepancy. Instead of assuming that the compositions obtained from the converged simulation for a feed stage will remain unchanged at PNMTC, the pinched compositions for the feed are determined by the intersection of the equilibrium curve and the feed q -line. Rather than perform an additional flash calculation to establish the pinched feed compositions, a quadratic approximation is developed here for column targeting purposes by assuming the relative volatility obtained from the simulation to remain constant in the neighborhood of the feed stage. The proposed FSC ensures that the CGCC is identical whether the calculations are performed by the top-down approach or the bottom-up approach. The effect of the FSC on the targets for energy conservation by reflux modification, feed conditioning, and introduction of side reboilers/condensers is discussed. As the energy target for reflux modification is determined by the CGCC pinch which typically occurs at or close to the feed location, the significance of the FSC on the reflux modification target is highlighted through several case studies including a complex column featuring multiple feeds and consequently multiple pinch points. The CGCCs for these case studies are generated by a computer program based on the FSC and a single analytical equation for the calculation of the net enthalpy deficits that allows every stage to have a feed, liquid product, vapor product, and side exchanger. The studies show that the reflux modification targets may be erroneous in many cases, if the FSC is ignored.

Optimal reflux rate policy determination for multicomponent batch distillation columns

Abstract This paper describes a procedure to obtain optimal reflux or optimal distillate rate policy for multicomponent batch distillation columns using the Pontryagins continuous maximum principle. For application to multicomponent systems, a reduced order model is used using short-cut procedure developed specifically for analysis and design of batch columns.

Invariant rectifying-stripping curves for targeting minimum energy and feed location in distillation

Invariant rectifying-stripping (IRS) curves are proposed that are independent of the feed location and operating reflux of the distillation column for a given separation problem. IRS curves represent the enthalpy surpluses and deficits in the rectifying and stripping sections, respectively, as a function of temperature for all possible values of reflux and reboil. The IRS curves provide a new representation on the temperature-enthalpy diagram to set distillation column targets prior to detailed design for minimum energy requirement, feed location, feed preconditioning, and side-exchanger loads. The application of the proposed concepts to two binary distillation examples (one featuring a tangent pinch) and a multicomponent distillation example illustrates the usefulness of the IRS curves in properly locating the feed, determining the minimum utility requirements, and reducing the tedium of repeated simulations. The IRS curves are rigorously invariant and provide the absolute minimum utility requirements for binary systems (ideal as well as non-ideal); however, they are near-invariant and predict the near-minimum utility requirements for multicomponent systems (where the pseudo-binary concept of a light and heavy key is employed).

Fischer-Tropsch synthesis using bio-syngas and CO2

While Fischer-Tropsch synthesis (FTS) using coal and natural gas in conventional reactors is an almost well-established technology, the production of liquid hydrocarbons from syngas obtained from biomass is in its preliminary stages of commercialization in countries like Germany. With concerns about global warming and ways of disposing of CO 2 being searched for, CO 2 hydrogenation using FTS to liquid hydrocarbons can act as a CO 2 sink. A brief review of FTS using CO 2 -rich syngas is given in this paper, looking at FTS as a technology that can help reduce global warming and as a process integration alternative. The reverse water gas shift (r-WGS) reaction is vital for CO 2 hydrogenation. We have studied the effect of this using an FT kinetic model and have proposed a new flow sheet alternative for FTS using CO 2 -rich syngas. Simulations suggested that this new process gives better conversion of CO 2 . The product selectivity and yields from an FT plant are vital to make the process viable economically.

Optimization based Conceptual Design of Reactive Distillation for Selectivity Engineering

This work presents some representative reactive distillation (RD) models to obtain attainable region for multi-component reaction systems when feed contains significant amount of inerts in it. One can achieve maximum selectivity for a quantitative conversion unlike conventional reactors with semibatch reactive rectification (SRR) as the best RD configuration when the inert is more volatile than the reactant. The other plausible RD configurations were suggested for different volatility patterns of the components and the inert. The MINLP optimization technique is also applied to industrially important case of dimerization of isobutylene for maximizing the selectivity towards di-isobutylene and the results were in agreement to those obtained from conceptual design. This work highlights the potential of optimization as a tool to complement the conceptual design method for selectivity engineering in RD columns.

Efficient feed preheat targeting for distillation by feed splitting

Abstract Thermal condition of the feed may be altered to reduce the reboiler duty. However, only a portion of the thermal energy given to the feed reduces the reboiler duty. By splitting the feed and altering the thermal condition of a part of the feed, it is possible to achieve reduction only in the reboiler duty (100% preheat efficiency). Based on the thermodynamic analysis of a distillation column, a methodology is developed to target split fraction of the feed for preheating to obtain reduction only in the reboiler duty.

A Conceptual Design Algorithm for Single-Feed Hybrid Reactive Distillation Column involving azeotropic systems

Abstract Reactive Distillation (RD) which combines reaction and distillation can be advantageously used to obtain desired selectivities in case of multi-reaction system. In case of azeotropic systems, the presence of complex vapor liquid equilibrium and distillation boundaries, shrink the feasible stage composition region and thereby increase design complexity for RD systems. Therefore an attempt is made here to develop a conceptual design algorithm based on boundary value method for the synthesis of single feed hybrid reactive distillation (HRD) column to obtain desired selectivities in case of single reactant complex reaction scheme (vande Vusse reaction) involving azeotropic systems. This work is the continuation of our earlier work on ideal multicomponent system ( Hasan et al., 2013 ). We believe that the conceptual design algorithm developed here can be extended to more complex schemes involving multiple reactants.

Solution multiplicity in multicomponent distillation - a computational study

Abstract Rigorous models of staged distillation processes are formulated by setting up material balance equations, equilibrium relations, summation equations, and enthalpy balance equations (MESH equations). In these models, the extent of nonlinearity may be very severe, particularly for azeotropic and reactive distillation systems. MESH system based mathematical models can thus yield multiple solutions (multiple steady states), a fact which has been observed by many researchers. Multiple steady states (MSS) are not detected automatically by the state-of-the-art commercial simulators, despite the fact that extensive capabilities are available for complex distillation columns and the solution algorithms used are very robust and efficient. Since the implications of multiplicity could be numerous, it is of great importance to detect MSS in distillation by a systematic procedure. The approach proposed in this paper uses a recent algorithm that can track, the multiple solutions of nonlinear algebraic equations (NLAE) starting from only one initial point. This optimization-based algorithm is interfaced with the MESH system to solve two case studies, an ideal distillation system with two products, and an azeotropic distillation system. For both the case studies, the proposed methodology looks very promising as it successfully tracks the MSS from a single starting point

A Design methodology for Internally HeatIntegrated Distillation Columns (IHIDiC) with side condensers and side reboilers (SCSR)

Abstract Distillation is the most widely used but an energy-intensive separation technology which consumes huge amount of thermal energy as a separating agent. Because of low thermal efficiencies of distillation columns, various energy integration methods have been explored in the past in order to reduce the energy requirements for a given separation task. Among them, Internally Heat Integrated Distillation Columns (IHIDiC), in which the rectification zone of the column is at a higher pressure than the stripping zone, are reported to be the best alternative to the conventional columns that give up to 50% energy savings when the rectifying and stripping zones are configured in an annular fashion. However, the structural complexity of IHIDiC draws one’s attention to search for an alternative which involves minimum design effort. In this work, a methodology has been proposed for an IHIDiC with side condensers and side reboilers (SCSR). This configuration introduces minimum structural complexity as the integrated side condensers and side reboilers are external to the columns. The side reboilers of the stripping column are driven by the side condensers of the rectifying column. The problem, however, is to determine the number of such units, their heat loads, and their locations that lead to an optimal design. Thus, a design methodology based on the well known Column Grand Composite Curves (CGCC) has been proposed algorithmically and tested on two separation problems of close-boiling mixtures, namely, Propane-Propylene and Styrene-Ethyl Benzene splitters. A case study of a multi-component mixture has also been analysed. Economic evaluation studies show that the optimized configuration of the IHIDiC with SCSR for the close-boiling mixtures resulted in attractive payback periods and significant savings in operating expenses. The energy savings in the case study of multi-component system, however, is less attractive due to much higher capital investment.