A real column design exergy optimization of a cryogenic air separation unit

Distillation columns are one of the main methods used for separating air components. Their inconvenient is their high energy consumption. The distillation process is simulated in three types of columns and the exergy losses in the different parts calculated. A sensitivity analysis is realized in order to optimize the geometric and the operational parameters of each type of column. A comparative exergy analysis between the distillation columns considered for cryogenic air separation shows that the exergy efficiency of a double diabatic column, with heat transfer all through the length of the column, is 23% higher than that of the conventional adiabatic double columns.     

Using thermally coupled reactive distillation columns in biodiesel production

Production of methyl dodecanoate (biodiesel) using lauric acid and methanol with a solid acid catalyst of sulfated zirconia is studied by using two distillation sequences. In the first sequence, the methanol recovery column follows the reactive distillation column. In the second sequence, the reactive distillation and methanol recovery columns are thermally coupled. Thermally coupled distillation sequences may consume less energy by allowing interconnecting vapor and liquid streams between the two columns to eliminate reboiler or condenser or both. Here we study the thermally coupled side-stripper reactive distillation and eliminate the condenser of the reactive distillation column. Both the sequences are optimized by using the thermal and hydraulic analyses of the Column Targeting Tools of Aspen Plus simulator. Comparisons of the optimized sequences show that in the thermally coupled sequence, the energy consumption is reduced by 13.1% in the reactive distillation column and 50.0% in the methanol recovery column. The total exergy losses for the columns are reduced by 281.35 kW corresponding to 21.7% available energy saving in the thermally coupled sequence. In addition, the composition profiles indicate that the thermally coupled reactive distillation column operates with the lower concentration of water in the reaction zone which reduces catalytic deactivation.     

Optimization of ethanol fermentation process design

The ethanol fermentation process using molasses as the feedstock has been studied. The process alternatives, compared to the original fermentation process described in the literature, have been modeled using single and double distillation columns. Short-cut and rigorous models for the distillation column were compared and their influence on the process economics indicate the rigorous model to be more exact, the short-cut model to be faster and more convenient for comparative analysis of finding the optimal process structure. The process with the single distillation column was shown to be the optimal variant requiring the lowest equipment and utilities cost. Estimated net present worth value for the single column was 29.6 MUSD and for the double columns 27.1 MUSD. A heat pump use was found to be a possible option for heat integration. The span of temperatures in distillation columns was confirmed to be the decisive factor for the economy of heat pump use. The pay back time was more than 5 years. Thermal integration reduced the total annual costs by 27% as compared to the basic process scheme.     

Planning and optimization of dynamic plant operation

Dynamic optimization is currently gaining attention in the design, synthesis and operation of industrial processes. Recent developments in the optimization algorithms, in the commercial tools and in the computation power of computer systems make it possible for process engineers to optimize the dynamic plant operations. Planning a plant operation can also be made in the scope of optimization. Dynamic optimization is usually used for developing optimal operation policies for new operational situations such as startup procedures, load changes, product changeovers. This paper attempts to give an overview to the methods, available tools and their properties for the planning and dynamic optimization of chemical processes. We will report our experiences in process optimization and show the results achieved in several practical applications: startup for distillation columns with and without sidestreams; heat-integrated distillation columns; multiple-fraction reactive batch distillation. The realization of the developed optimal policies to the real plants shows the benefit and potential of the optimization.     

Heat pump assisted distillation. I: Alternative ways to minimize energy consumption in fractional distillation

A survey is made of the numerous possible ways to minimize the consumption of energy in fractional distillation. Distillation is a highly energy-intensive process in which the actual energy consumed is far greater than the theoretically calculated loss in available work. The methods considered for reducing energy consumption are classified as those which (i) do not require process modifications, (ii) require minor process modifications, (iii) make use of more than one column and (iv) make use of heat pumps.     

Estimation and reduction of CO2 emissions from crude oil distillation units

Distillation systems are energy-intensive processes, and consequently contribute significantly to the greenhouse gases emissions (e.g. carbon dioxide (CO₂). A simple model for the estimation of CO₂ emissions associated with operation of heat-integrated distillation systems as encountered in refineries is introduced. In conjunction with a shortcut distillation model, this model has been used to optimize the process conditions of an existing crude oil atmospheric tower unit aiming at minimization of CO₂ emissions. Simulation results indicate that the total CO₂ emissions of the existing crude oil unit can be cut down by 22%, just by changing the process conditions accordingly, and that the gain in this respect can be doubled by integrating a gas turbine. In addition, emissions reduction is accompanied by substantial profit increase due to utility saving and/or export.     

Heat integrated distillation operation

Increasing energy demand, consequently high crude oil prices and growing concern for pollution motivated the researchers to explore more energy-efficient and environment-friendly process technologies. Although the heat integrated distillation has been researched for a number of decades, unfortunately it has not yet been commercialized mainly due to high investment cost, complex equipment design, control problem in consequence of severe interaction and nonlinearity, and lack of experimental data at sufficiently large scale to verify the theoretical predictions. It is true that some progress has been made in theory but for practical applications many questions still remain. Among the broader research needs the following areas are identified for heat integrated distillation column: rigorous dynamic modeling, optimal design, multiple steady state analysis, system identification, synthesis and implementation of high-quality nonlinear control, and importantly experimental evaluation. It is also suggested to investigate the feasibility of heat integration in the reactive distillation schemes and in the two distillation columns having no direct connections.     

Retrofit of distillation columns in biodiesel production plants

Column grand composite curves and the exergy loss profiles produced by the Column-Targeting Tool of the Aspen Plus simulator are used to assess the performance of the existing distillation columns, and reduce the costs of operation by appropriate retrofits in a biodiesel production plant. Effectiveness of the retrofits is assessed by means of thermodynamics and economic improvements. We have considered a biodiesel plant utilizing three distillation columns to purify biodiesel (fatty acid methyl ester) and byproduct glycerol as well as reduce the waste. The assessments of the base case simulation have indicated the need for modifications for the distillation columns. For column T202, the retrofits consisting of a feed preheating and reflux ratio modification have reduced the total exergy loss by 47%, while T301 and T302 columns exergy losses decreased by 61% and 52%, respectively. After the retrofits, the overall exergy loss for the three columns has decreased from 7491.86 kW to 3627.97 kW. The retrofits required a fixed capital cost of approximately $239,900 and saved approximately $1,900,000/year worth of electricity. The retrofits have reduced the consumption of energy considerably, and leaded to a more environmentally friendly operation for the biodiesel plant considered.     

Synthesis and heat integration of thermally coupled complex distillation system

Based on stochastic optimization strategy and pinch technique, a method is proposed for optimal synthesis and heat integration of thermally coupled complex distillation column systems comprising simple columns, complex columns with side rectifier and/or side stripper as well as partially or fully thermally coupled (Petlyuk) columns with pre‐fractionators. Three example problems for five‐component mixtures separation have been solved and the optimized parameters and economic benefits of different optimal schemes have been analyzed and compared. The results demonstrate that the annual total cost in energy consumption and investment can be effectively reduced by using the heat‐integrated complex distillation configuration. The solutions of example problems also demonstrate that the proposed method is efficient for the heat‐integrated complex distillation synthesis problem. Copyright © 2009 John Wiley & Sons, Ltd.     

Exergy analysis proves viability of process modifications

Stork Comprimo has developed a commercially available exergy analysis program. With this program several exergy analyses were carried out. Most recently an exergy analysis of a reaction and distillation section within a refinery has been performed. In the reaction section endothermic reactions take place after which the product stream is cooled in a heat exchanger network whereafter the reaction products are separated in a distillation section. From the exergy analysis it can easily be notified that the main part of the losses occurs in the furnaces and distillation columns. A closer look at one of the distillation columns with the largest exergy losses shows that the main part of the losses occurs in the reboiler heated with a furnace. To reduce the exergy losses in this distillation column, Stork Comprimo proposed several process modifications. These modifications are: (1) decreasing operating pressure (lower operating temperature), (2) HP steam reboiling instead of a furnace, (3) splitting feed streams, and (4) recompressing overhead. With these process modifications total exergy losses may be reduced by 70% that directly results in a primary fuel reduction of almost 40% for this column! Splitting of the feed streams has been implemented now and results in an energy saving of 10% and a more stable operation of the column.     

Significant thermal energy reduction in lactic acid production process

Lactic acid is widely used as a raw material for the production of biodegradable polymers and in food, chemical and pharmaceutical industries. The global market for lactic acid is expected to reach 259 thousand metric tons by the year 2012. For batch production of lactic acid, the traditional process includes the following steps: (i) esterification of impure lactic acid with methanol in a batch reactor to obtain methyl lactate (ester), (ii) separation of the ester in a batch distillation, (iii) hydrolysis of the ester with water in a batch reactor to produce lactic acid and (iv) separation of lactic acid (in high purity) in a batch distillation. Batch reactive distillation combines the benefit of both batch reactor and batch distillation and enhances conversion and productivity (Taylor and Krishna, 2000 [1]; Mujtaba and Macchietto, 1997 [2]). Therefore, the first and the last two steps of the lactic acid production process can be combined together in batch reactive distillation (Fig. 1) processes. However, distillation (batch or continuous) is an energy intensive process and consumes large amount of thermal energy (via steam). This paper highlights how significant (over 50%) reduction in thermal energy consumption can be achieved for lactic acid production process by carefully controlling the reflux ratio but without compromising the product specification. In this paper, only the simultaneous hydrolysis of methyl lactate ester and the separation of lactic acid using batch reactive distillation is considered.     

Economic feasibility of heat pumps in distillation to reduce energy use

An i-butane/n-butane mixture was selected to analyze several distillation assisted heat pump processes when compared to conventional distillation. This conventional process, along with top vapour recompression, bottom flashing and absorption heat pumps, were simulated using the HYSYS software platform, in order to determine economically the best alternative.Distillation with both top vapour recompression and bottom flashing heat pumps allows reduction of operation (energy) costs by 33% and 32%, respectively. This improves the economic potential (incorporating capital costs) by 9% and 10%, respectively. Due to the large steam consumption, when compared to the conventional case, the absorption heat pump is not suitable for this system.     

Significance of ortho-para hydrogen conversion in the performance of hydrogen liquefaction process

Hydrogen is an energy vector and is produced just like electricity. In order to overcome the shortcomings associated with its low molecular weight and energy density per unit volume, hydrogen is liquefied for storage and transportation purposes. The liquefaction of hydrogen differs from that of other substances as it involves the reactive transformation of its isomeric states. At 25 °C, molecular hydrogen consists of 75% orthohydrogen and 25% of parahydrogen. As the normal boiling point, hydrogen essentially exists in the para-state, which is preferred because of its lower boil-off gas rate. However, the conversion of ortho-to-para hydrogen is an exothermic reaction, and this enthalpy of conversion enhances the total reversible work by about 15%. Little work has been done regarding ortho-to-para hydrogen conversion from the process systems point of view. Therefore, parametric analysis of this vital conversion reaction was studied with potential impact on the performance of cryogenic heat exchangers, reactors configuration and mode of operation, and probable impact on the energy efficiency of the liquefaction process. An alternate approach to simulate the reaction is also proposed. The results show that the current approaches to process design need to be changed. The study opens avenues for more in-depth analysis and optimization approaches to present a holistic framework for future integrated energy systems.     

Energy and environmental sustainability assessment of a crude oil refinery by thermodynamic analysis

This study presents the assessment of energy and environmental sustainability metrics for a crude oil refinery consisting of three distillation columns. The assessments of the current operation and the retrofits for possible improvements are suggested by the thermodynamic analysis and energy analyzer. The main objective is to explore the scope of reducing the thermal energy consumption and CO₂ emissions for a more sustainable refinery operation. Thermodynamic analysis is carried out by using the thermal analysis capability of ‘column targeting tool’ to address the ‘energy intensity metrics’ and the ‘energy analyzer’ to design and improve the performance of the heat exchanger network system for process heat integration. Environmental pollution impact metrics are estimated from the ‘carbon tracking’ options with a selected CO₂ emission data source of US‐EPA‐Rule‐E9‐5711 and using crude oil as a primary fuel source for the hot utilities. The results indicate that column targeting tool, energy analyzer, and carbon tracking can estimate the energy and environmental sustainability metrics of an existing design and determine the scope of considerable improvements for reducing the costs of thermal energy required and emissions of carbon dioxide in a crude oil refinery operation. Copyright © 2015 John Wiley & Sons, Ltd.     

Energetic and exergetic analysis of steam production for the extraction of coniferous essential oils

Bioenergy production is optimal when the energy production process is both efficient and benefits from local resources. Energetic and exergetic analyses are applied to highlight efficiency differences between small-size systems that are based on the co-generation of heating and power (CHP) versus the co-generation of heating and power with steam production (CHP-S). Both systems use the Organic fluid Rankine Cycle (ORC).The recovery of heat from flue gases is considered to be a way of increasing energy efficiency. In the CHP-S case, steam (at low pressure) is used to extract essential oils from fresh twigs and needles of coniferous trees throughout a steam distillation process. When the systems work at a thermal combustion power of 1350 kW, energetic analysis shows that the energy efficiency of the CHP-S plant (89.4%) is higher than that of the CHP plant (77.9%). Exergetic analysis shows that the efficiency of the CHP-S plant is 2.2% higher than that of the CHP plant.     

Fuzzy TOPSIS method for ranking renewable energy supply systems in Turkey

Multi-Criteria Decision Making (MCDM) techniques are gaining popularity in energy supply systems. The aim of this paper is to develop the multi-criteria decision support framework for ranking renewable energy supply systems in Turkey. Given the selection of renewable energy supply systems involves many conflicting criteria, multi criteria decision methods (Fuzzy TOPSIS) were employed for the analysis. The Interval Shannon's Entropy methodology was used to determine weight values of the criteria. In this study, α = 0.1, 0.5 and 0.9 values based sensitivity analysis were performed. Three α-cutting levels were identical to the sequence of alternatives. According to result, the first criterion in preference ranking of renewable energy sources in Turkey is the Amount of Energy Produced, followed by the ranking systems Land use, Operation and maintenance cost, Installed capacity, Efficiency, Payback period, Investment cost, Job creation, and Value of CO₂ emission. Thus the multi-criteria analysis showed that the Hydro Power Station is determined to be the most renewable energy supply system in Turkey. Additionally, the Geothermal Power Station, Regulator and Wind Power Station are determined to be the second, third and fourth, respectively. The government of Turkey should invest, in order of priority, in these systems. The government should also evaluate the projects, which are related to these renewable energy resources.     

Design of heat integrated distillation systems for a light ends separation plant

This paper presents an industrial case-study: the synthesis of partially thermally coupled and heat-integrated distillation systems applied to the light ends separation section of a crude distillation plant. The distillation systems presented in this work employ the thermal coupling and the heat-integration principles to significantly reduce the heat requirements with respect to the traditional simple column train.The work started from the simulation of the existing plant, by which the parameters of the system were identified. Then the possible sequences of simple columns with sharp splits were identified for the considered application, and all the columns of the configurations were designed. The corresponding thermally coupled sequences were obtained by using a simple procedure derived from the literature and the heat-integrated partially coupled configuration (HIPC) was also considered.In order to verify the examined distillation systems, all the simple and complex configurations were simulated by rigorous numerical models. On the basis of the numerical simulations, the energy requirements for each configuration were evaluated. A rating of the different plants was then performed, based on the total annual cost, allowing to identify the best plant configuration. A thermally coupled configuration showed the best performances for the considered separation.     

Framework for analysis of solar energy systems in the built environment from an exergy perspective

Exergy analysis is a more powerful tool than mere energy analysis for showing the improvement potential of energy systems. Direct use of solar radiation instead of degrading other high quality energy resources found in nature is advantageous. Yet, due to physical inconsistencies present in the exergy analysis framework for assessing direct-solar systems commonly found in literature, high exergy losses arise in the conversion process of solar radiation in direct-solar systems. However, these losses are disregarded in indirect-solar systems.In this paper, contradictions and physical inconsistencies which result from including the conversion of solar radiation only for direct-solar systems are shown. An evaluation framework physically coherent for systems making direct and indirect use of solar radiation is derived and its physical correctness is thoroughly discussed. Results from case studies using the proposed framework are presented and compared with the conventional approach, enabling their direct comparison and better understanding of the benefits and correctness of the proposed method. The new method allows recognizing clearly the suitability of direct-solar systems, being appropriate for highlighting more sustainable energy supply systems.Although this paper focuses on building systems, the framework might be used for exergy analysis of direct-solar systems in the context of other energy uses.     

Production of anhydrous ethanol using oil palm empty fruit bunch in a pilot plant

Bioethanol production from lignocellulosic biomass for use as an alternative energy resource has attracted increasing interest, but short-term commercialization will require several technologies such as low cost feedstock. The huge amount of oil palm empty fruit bunches (EFB) generated from palm oil industries can be used as a raw material for cheap, renewable feedstock for further commercial exploitation. Using a pilot-scale bioethanol plant, this study investigated the possibility of utilizing oil palm empty fruit bunches as a renewable resource. All bioethanol production processes such as pretreatment, hydrolysis, fermentation, and purification were constructed as automatically controlled integrated processes. The mass balance was calculated from operational results. Changhae ethanol multiexplosion pretreatment with sodium hydroxide was conducted to improve the enzymatic hydrolysis process, and a separate hydrolysis and fermentation process was used for producing bioethanol at an 83.6% ethanol conversion rate. In order to purify the ethanol, a distillation and dehydration facility was operated. Distillation and dehydration efficiencies were 98.9% and 99.2%, respectively. The material balance could be calculated using results obtained from the operation of the pilot-scale bioethanol plant. As a result, it was possible to produce 144.4 kg anhydrous ethanol (99.7 wt%) from 1000 kg EFB. This result constitutes a significant contribution to the feasibility of bioethanol production from lignocellulosic biomass and justifies the pilot plant's scale-up to a commercial-scale plant.     

Ethylene separation by feed-splitting from light gases

Separation of ethylene from light gas mixture is one of the most energy intensive separations in petrochemical processes, which uses distillation columns up to 100 m tall and containing over 100 trays due to very small differences in the relative volatilities and very large reflux ratios and also due to the need for sub-ambient temperatures. In recognition of these costs, several attempts have been made in the past to develop processes with less energy and equipment costs. In distillation columns with condenser temperatures significantly below room temperature, such as in ethylene separation towers, it is essential to minimize the expensive energy requirements of the refrigeration cycle that produces the tower reflux. In this work, a solution has been found by expanding the gaseous distillate to decrease its temperature. Moreover, additional solutions applied to conventional ethylene fractionation columns have been implemented here in order to study this behavior. In this contribution, an outlet stream of an Oxidative Coupling of Methane (OCM) reactor, which has been previously stripped of its CO₂ content, is also introduced in a demethanizer tower to remove almost all of its CH₄ content before entering the ethylene fractionating column. Then, it is cooled exchanging its heat with the distillate stream of the ethylene tower, warming the distillate. The main goal is to reduce the condenser heat duty. This objective is achieved reducing a significant amount of heat required by the condenser, maintaining the mandatory product purity. Due to this improvement, it was also possible to reduce the reboiler heat in almost the same percentage amount that is achieved with the condenser. In addition, the reflux of ethylene column decreases. The sensitivity analysis and the corresponding simulations results will be discussed in order to show the efficiency of the presented approach. These results have also been used for the design of the pilot plant which is now being built at our department.