Alexandra E. Bonet Ruiz

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.

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.

Advantages of Process Integration Evaluated by Gibbs Energy: Biodiesel Synthesis Case

Abstract The present study shows that the potential for process integration is easily evaluated by the Gibbs energy change between input and output streams assuming isotherm and isobaric units. The minimum energy requirements to get the streams to the desired temperature at the entrance and output of the units for the overall process can be later evaluated by the second thermodynamic principle, i.e. Pinch Analysis. In the case study of biodiesel synthesis from oils, a single hybrid reactive extraction unit substitutes a reactor, one extractive column and three distillation columns and therefore avoids the residual streams.

Use of computer dynamic simulation for indoor exposure assessment based on chronogram incident as air pollution source characterization

Abstract Determination of the level of exposure due to a chemical incident is crucial for the assessment of public health risks and environmental impact. This kind of information is also useful for the subsequent legal responsibilities. If the air contaminant concentrations are not quantified during the incident, then its air concentration may fall below detectable levels shortly after the incident has passed. In case of smelly compounds, usually the incident chronogram indicates the time when the smell was detected and when the smell was gone. Unfortunately, an objective and analytical measure of odour is impossible. The present study shows that it is possible to computer simulate the concentration evolution in time for a defined indoor scenario. Comparing the odour detection in the chronogram with the computer simulations, it is feasible to determine the maximum gas contaminant exposure during the incident and its evolution in time. Once the pollution source indoor concentration is characterized, considering the overall air renewal time and a plume model, the effects on the environment around the building can be estimated.

Software Tool for Computing and Visualization of Enhanced Residue Curve Maps

Residue curve maps (RCM) are readily implemented on most chemical engineering commercial simulation software because they are very useful to check the distillation columns feasibility. A section of a residue curve corresponds to a column composition profile of a packed column operated at infinite reflux flow rate between a distillate and bottoms compositions corresponding to the initial and final point of the curve section. The existence of the residue curve between distillate and bottoms assures that the separation is feasible when the mass balances are fulfilled with a high enough reflux. It is also well known that an infinite reflux requires a minimum number of transfer units between distillate and bottoms composition. Unfortunately, no information about the minimum number of transfer units is included in the RCM. This study implements for first time a Visual Basic® program able to generate RCM including the information about the minimum number of transfer units required. In this way, the enhanced RCM provides a valuable information about the difficulty of the separation. Some case studies are provided as examples.

Computational Fluid Dynamics (CFD) Simulation of Fuel Gas and Steam Mixtures to Decrease NOx Emissions of Industrial Burners

Abstract Nowadays, there is a great concern about the NOx emissions from combustion processes due to the adverse effects on health. The NOx emissions are avoided in high flow rate industrial combustion exhaust gases by reaction with ammonia. For the other cases, simpler solutions are proposed and studied using CFD, such as exhaust gas recirculation or improving the burner design. The present study is focused on reducing the NOx by the presence of steam. The presence of steam decreases a bit the peak temperature and greatly affects the reaction mechanism, decreasing the amount of NOx produced. Chemical industry heating services are provided by steam and therefore it is readily available. The introduction of steam in the combustion chamber together with the fuel of the air is a simple and cheap option to decrease NOx emissions. A CFD analysis is provided in the present study, showing that steam addition effect is more intensive when added to the fuel, than added to combustion air. When 25% steam is added in fuel, the quantity of NOx almost halves in comparison to the dry case.

Enhanced Distillation Based on Feed Impurities

Abstract Most of the energy consumed in the chemical industry is due to distillation units. This unit operation is very mature but any improvement in its efficiency leads to important savings. The goal of the present study is to show that the feed impurities, which usually are disregarded in calculation for sake of simplicity, could be useful to enhance the distillation efficiency. Enhanced distillation is only considered when it is required to break an azeotrope or in the presence of a pinch zone, but it could be also used to improve distillation efficiency. The most common enhanced distillation operation is the extractive distillation and the heterogeneous azeotropic distillation. In this case, a third heavy boiling compound (extractive agent) or a compound that forms a light heterogeneous azeotrope respectively (entrainer) is added. The novel method is applied to a case study: the dehydration of the acetic acid from the terephtalic acid production. The results show that methyl acetate and p-xylene would both increase the distillation efficiency and none of them would require an entrainer make up without using any second column for entrainer recovery. The effect of the decanter feed stream flow rate and the column feed stage are evaluated by rigorous simulation using Aspen Plus® v8.6.

Distillation Energy Assessment for Solvent Recovery from Carbon Dioxide Absorption

Abstract An assessment useful to provide a general insight of the chemical CO 2 absorption using different solvents is provided. The absorption operation window where solids and vapours from the liquid phase are avoided is represented. The presence of these phases determines the absorption feasibility but the transport properties have a major impact on sizing of the equipment, therefore the viscosity and CO 2 diffusivity in the solvent are also taken into account. Ammonia (NH 3 ) is studied using three different thermodynamic models: ElectNRTL, Aqueous and MSE and free Gibbs energy is used as a thermodynamic measure of the minimum energy requirements to recover the solvent in the distillation column. For some other solvents, e.g. monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), 2-aminoethoxyethanol (DGA), diisopropanolamine (DIPA), diethanolmethylamine (MDEA), the operation window is also provided.