Valentin Plesu

Broadening the capabilities of pinch analysis through virtual heat exchanger networks

Abstract The present paper proposes a technique to enlarge the capabilities of pinch analysis. Conventional pinch analysis is limited to heat exchange between streams of unchanging composition, whereas in reality, much heat exchange occurs in components such as chemical reactors, mixers etc. This can be overcome by introducing virtual heat exchangers that convert the considered components into equivalent heaters and coolers where the stream compositions remain unchanged. In this way, they are automatically taken into consideration in the current pinch analysis packages, which may lead to different and better optimisation. A client–server interface program has been written to introduce virtual heat exchangers after completion of the process analysis through Aspen Plus and before entering the pinch analyser Super Target. The application of virtual heat exchangers is next illustrated through a simple example for methanol synthesis.

Regeneration of internal streams as an effective tool for wastewater network optimisation

Abstract A significant decrease of water disposal to treatment section from a wastewater network (WWN) can be achieved rising internal water reuse through regeneration, i.e. lowering the concentration of one or more contaminants. Zero discharge is the limit case of complete regeneration, although economic considerations may hamper its full application. In this paper the mathematical model of the WWN is extended to consider partial/total stream regeneration. An improved variant of genetic algorithms is used to solve it. Several concepts are formulated, pinpointing which contaminant is better to be removed for decreasing supply water consumption. A case study with six unit operations, four contaminants, one water source and one regeneration unit is analysed. Four regeneration scenarios are investigated, considering (a) the critical contaminant, (b) the contaminants from the bottleneck island, (c) three contaminants—one being outside the bottleneck island and (d) all the contaminants, according to the zero discharge theory. The results obtained are compared against the base case (without regeneration) to highlight the relationship between supply water reduction and internal regeneration. Increased water reuse is related as well to WWN topology optimisation.

Thermal coupling between crude distillation and delayed coking units

Abstract The paper presents an industrial case study. Crude distillation unit (CDU) and delayed coking unit (DCU) are important plants in oil refineries, presenting huge energy consumption, especially due to high flowrates of process material streams. Any acceptable solution for energy saving is important in this context. The idea of thermal coupling between these two plants is good as they have potential to exchange energy, but the problem is to choose the most appropriate way to do it. The objective of this work is to present the possibility to exchange energy between the two plants, continuing a previous work [Energy saving by integration of CDU-delayed coke plants, third Conference, Process Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction––PRES 2000, P7.72], where a part of the solution was already presented. The difficulty to find a solution arisen from the fact that DCU is working semi-continuous. More insights in the process allowed finding new possibilities, more attractive for rational use of energy, with better applicability. A feasibility study will be performed to give also economic sound of all the implications for the modifications proposed.

Entropy generation reduction through chemical pinch analysis

The pinch analysis (PA) concept emerged, late ‘80s, as one of the methods to address the energy management in the new era of sustainable development. It was derived from combined first and second law analysis, as a technique ensuring a better thermal integration, aiming the minimization of entropy production or, equivalently, exergy destruction by heat exchanger networks (HEN). Although its ascendance from the second law analysis is questionable, the PA reveals as a widespread tool, nowadays, helping in energy savings mostly through a more rational use of utilities. Unfortunately, as principal downside, one should be aware that the global minimum entropy production is seldom attained, since the PA does not tackle the whole plant letting aside the chemical reactors or separation trains. The chemical reactor network (CRN) is responsible for large amounts of entropy generation (exergy losses), mainly due to the combined composition and temperature change. The chemical pinch analysis (CPA) concept focuses on, simultaneously, the entropy generation reduction of both CRN and HEN, while keeping the state and working parameters of the plant in the range of industrial interest. The fundamental idea of CPA is to include the CRN (through the chemical reaction heat developed in reactors) into the HEN and to submit this extended system to the PA. This is accomplished by replacing the chemical reactor with a virtual heat exchanger system producing the same amount of entropy. For an endothermic non-adiabatic chemical reactor, the (stepwise infinitesimal) supply heat δq flows from a source (an external/internal heater) to the stream undergoing the chemical transformation through the reactor, which in turn releases the heat of reaction ΔHR to a virtual cold stream flowing through a virtual cooler. For an exothermic non-adiabatic chemical reactor, the replacement is likewise, but the heat flows oppositely. Thus, in the practice of designing or retrofitting a flowsheet, in order to minimize the entropy production, the chemical reactor should be viewed as a group of two or three virtual heaters/coolers destroying the same amount of exergy. As a result of PA, new operating conditions could be revealed for some or all of the chemical reactors, ensuring a further reduction of the global entropy production of the plant. In this paper, the simple case of the methanol synthesis heat integrated reactor will be analyzed, proving the benefits of the CPA.

Simple Equation for Suitability of Heat Pump Use in Distillation

Abstract A distillation column can be considered as a heat engine that produces separation instead of work. The heat is provided in the reboiler and collected degraded at a lower temperature at the condenser. The energy collected at low temperature at the top of the column can be upgraded back to higher temperatures by means of a heat pump and reused to heat a lower column stage.This can bring saving in terms of the overall amount of energy required. However, the energy required to increase the pressure is of higher quality and price than hot services. The aim of the present paper is to provide an easy way to check, in the early stages of design, when the use of a heat pump can provide a more sustainable distillation process decreasing its energy requirements. After several simplifications, it can be stated that it depends mainly on the efficiency of Carnot. When the efficiency of Carnot is evaluated for the industrial systems where the heat pumps are used, it is concluded that all have Carnot efficiency around 0.1 or lower. Therefore, in the early design of a new distillation column, this criterion is useful to decide when a heat pump is worth to be included in the more rigorous simulations.

Fast solvent screening for counter-current liquid–liquid extraction columns

Liquid–liquid extraction (LLEx) provides the separation of a certain solute present in a stream when adding a solvent that produces a phase split. Then the raffinate is collected free of solute, which in turn is to be found in the extract. The solute is separated from one phase and mixed in another phase from which it is recovered, in most cases, by distillation. The main difficulty of the process is to find a suitable solvent. When an immiscible solvent is not found, in some cases, the extractive distillation is an alternative. The present work compares the LLEx process with extractive distillation process. Furthermore, two parameters are provided to rank the solvents useful for LLEx, one related to the solvent aqueous toxicity impact, and another related to the energy required for solvent recovery. Both parameters for a counter-current LLEx column are calculated using a simple decanter, i.e. a single liquid–liquid equilibrium stage. The first parameter depends on the solvent lost in the raffinate, and its aqueous toxicity is represented by LC50 (lethal concentration required to kill half the members of a tested population). The second parameter depends on the minimum solvent flow rate and its boiling point. Hence, the proposed parameters are suitable to easily check when new proposed solvents are more environmental friendly.

Shortcut assessment of alternative distillation sequence schemes for process intensification

Abstract Finding good process schemes is a difficult task due to its search among multiple alternatives. In literature, several methods are successfully implemented as computer tools, but their use is limited to their developers. There are several useful rules of thumb and heuristics providing useful guidelines, but sometimes these are contradictory. Some authors tried to rank the various alternatives, defining heuristic equations to provide a quantitative parameter to choose among possible solutions. In this paper, mathematical model of distillation columns is simplified, assuming infinite number of stages. The resulting proposed equation has a great similarity to one of the previous heuristic ones. A simple equation not relying on heuristics and easy to use in calculation is provided to evaluate the distillation sequence energy efficiency (DSE) for each alternative. This allows to quantify the advantages derived from process intensification for a given feed composition. The proposed equation is dimensionless, as the Carnot efficiency is used instead of the temperature difference between distillate and bottoms. On the other hand, intermediate results provide also useful information. For instance, the Carnot efficiency of each column indicates when a heat pump or enhanced distillation would be useful. The recovery efficiency for each particular compound allows an easy comparison between alternatives, considering variations on the feed composition. The new equation is verified comparing its results with cases already solved in literature using different methods. The results show that all the methods in literature are able to provide the best sequence, except the heuristics-based ones that are not providing an overall sequence evaluation. The novelty of the proposed DSE method resides in its ease of application, compared to nowadays available methods, and requires only the feed composition and products boiling points.

Reactive distillation process analysis in a divided wall column

Abstract This contribution aims to present a methodology to model and simulate the reactive dividing wall column (RDWC) using commercial software. Feasibility of separation scheme was established with ASPEN DISTIL™. Simulation of flowsheet configuration was performed with ASPEN HYSYS™, using the two columns model. The reactive zone hosted by the prefractionator was modelled and simulated as backward flow CSTR series. Proposed solution is attractive from industrial point of view. As case study isoamylenes (contained in fluid catalytic cracking-FCC-C 5 -fraction) etherification with ethanol (EtOH), to obtain tert-AmylEthylEther (TAEE) in RDWC with structured packing was illustrated. Operation of such arrangement involves very careful solution of the model. Composition profiles demonstrated feasibility of proposed flowsheet.

Topological impact of regeneration unit constraints upon water and wastewater network

Abstract Optimization of water use/reuse gained an increasing attention during the last years based on four major driving forces: higher water demands, supply water cost increase, such as the wastewater treatment cost and more stringent regulatory limits for the disposal of the used water. This means a drastic reduction of the contaminants level of wastewater discharge, which itself has to be reduced continuously. A significant decrease of water disposal can be achieved rising internal water reuse/recycle through regeneration; the concept of “zero discharge” being the limit, although economic considerations are a major impediment in its full application. The topological impact of regeneration unit upon water and wastewater network is studied for three cases: critical component regeneration, partial regeneration and total regeneration (zero discharge concept) . Every case is compared against the optimal water network topology obtained using as objective function the total supply water flow rate.

Modelling and simulation of kinetics and operation for the TAME synthesis by catalytic distillation

The work presents a simulation study of a reactive distillation (RD) unit for tert-amyl-methyl-ether synthesis using HYSYSTM simulation environment. In order to simulate the RD column this was represented as an ensemble of three components corresponding to the software chemical operation modules avialable in HYSYS. The calculated iso-amylenes conversion was compared with measured values on a pilot plant RD unit.