Liquid-vapor Separation Research Articles

On integration of feedback control and safety systems: Analyzing two chemical process applications

This work focuses on two case studies and attempts to elucidate the dynamic interaction between feedback control and safety systems in the context of both model-based and classical control systems. In the first case study, the interaction of a model predictive control (MPC) system with a safety system is studied in the context of the methyl isocyanate (MIC) hydrolysis reaction in a continuous stirred tank reactor (CSTR) to avoid thermal runaway. We develop a specific action for the MPC to take when the safety system is activated due to significant feed disturbances that lead to thermal runaway conditions. In the second case study, we focus on a high-pressure flash drum separator for which the temperature, level, and pressure can be regulated using proportional-integral (PI) controllers. Using a large-scale dynamic process simulator, we demonstrate that modifying the tuning parameters of one of these PI controllers based on the safety system being on or off leads to improved closed-loop performance compared to the case in which the tuning parameters of the PI controller remain the same regardless of the state of the safety system.

Performance investigation of a novel zeotropic organic Rankine cycle coupling liquid separation condensation and multi-pressure evaporation

The decrease in fossil energy reserves and the increase in energy costs have resulted in a strong interest in power generation using renewable heat sources or waste heat. Organic Rankine cycle (ORC) is a promising heat-to-power conversion technology. Although the ORC using zeotropic mixture is superior to ORC using pure fluid in thermodynamic performance due to low irreversibility during the evaporation and condensation of zeotropic mixture, the improvement in thermodynamic performance is usually achieved at the cost of poor economic performance. The studies on improving the thermo-economic performance are limited. In the present study, a novel zeotropic ORC coupling liquid separation condensation and multi-pressure evaporation is proposed. Multi-pressure evaporation is presented to improve heat match between the heat source and working fluid. Liquid separation condensation is applied to control the composition of the mixtures that enter the evaporation processes. Thermodynamic analysis and optimization model of the novel ORC is developed. The superiority of the proposed novel ORC over the traditional simple zeotropic ORC and traditional multi-pressure evaporation zeotropic ORC is elaborated for different mixtures. The contribution of mixture composition adjustment and condensation enhancement through liquid-vapor separation on the cycle performance improvement is investigated. Sensitivity analysis of heat-sink inlet temperature, heat source inlet temperature, vapor quality, heat source specific heat capacity, and heat-sink temperature rise on the cycle performance are conducted. The case results show that net power output of the novel ORC is 13.05–26.18% higher than that of the simple zeotropic ORC. The contribution of mixture composition adjustment on improving the net power output can be up to 3.57% compared with traditional multi-pressure evaporation ORC for mixture R245fa–R365mfc. When the heat transfer enhancement through liquid separation is incorporated into the thermodynamic optimization, the net power output of the novel zeotropic ORC can be increased by 8.22% compared with the traditional multi-pressure evaporation ORC under the same economic constraint.

Visualization investigation on flowing condensation in horizontal small channels with liquid separator

A simultaneous visualization and measurement experiment was carried out to investigate condensation flow patterns and condensing heat transfer characteristics of refrigerant R141b in parallel horizontal multi-channels with liquid-vapor separator. The hydraulic diameter of each channel was 1.5 mm and the channel length was 100 mm. The refrigerant vapor flowing in the small channels was cooled by cooling water. The parallel horizontal multi- channels were covered with a transparent silica glass for visualization of flow patterns. Experiments were performed at different inlet superheat temperatures (ranging from 3°C to 7°C). Mass velocity was in the range of 82.37 kg m-2s-1 to 35.56 kg m-2s-1. It was found that there were three different flow patterns through the multi- channels with the increase of mass velocity. The flow patterns in each channel pass almost tended to be same and all of them were annular flows. The efficiency of the liquid-vapor separator with U-type was related to vapor mass velocity and the pressure in the small channels. It was also found that the heat transfer coefficient increased with the increase of the mass velocity while the cooling water mass flow rate increased. It increased to a top point and then decreased. It increased with the increase of superheat in the low superheat temperature region.

Detection of chemicals in water using an oxide nanowire transistor covered with an aerogel microsphere thin film as a liquid-vapor separation filter

In this study, we developed a method to detect small amounts of chemical substances contained in water by using a SnO2 nanowire transistor covered with an aerogel microsphere thin film. The aerogel microsphere thin film acts as a liquid-vapor separation filter, allowing the passage of chemical vapor while blocking liquid chemicals and water. The concentration of chemical substances, such as toluene, nitric acid, benzene, and chloroform, in water could be estimated by evaluating the current changes in the SnO2 nanowire transistor covered with the aerogel microsphere thin film. The time-dependent response of the device depending on the chemicals in water was repeatedly observed for multiple cycles, which demonstrated that the response and recovery properties were reproducible.

Technical and economic evaluation of flare gas recovery in a giant gas refinery

Flare gas recovery (FGR) is known as an option to reduce greenhouse gases (GHG) from oil and gas refineries. The major concerns about environmental impacts of GHG emission lead refineries to deploy different FGR methods, most of which requiring new equipment and high cost of design and construction. In this paper, feasibility of three methods for FGR is evaluated in a giant gas refinery in Iran. The first two methods considered liquefaction and LPG production by implementing flare gases as feed for existing LPG unit. Different parameters were studied in feed liquefaction and LPG production. The third studied option is using a three-stage compression unit to compress the flare gases. Finally, an economic evaluation was performed to find the most profitable method. Based on simulation results, the 0.75-barg pressure of flash drum leads to maximum LPG production. For FGR, the 1-barg pressure of flash drum recovered the most CO2-equivalent from releasing into the atmosphere, which is more than 205ton/day. The economic evaluation shows that rate of return (ROR) for liquefaction unit and LPG production unit are more than 200% for different scenarios and are higher than compression.

Treatment costs of ammonia recovery from biogas digestate by air stripping analyzed by process simulation

Ammonia can be recovered from liquid biogas digestate as ammonium sulfate by air stripping. A full-scale plant treating 30,000 kg h−1 of digestate was simulated in Aspen Plus.The classical sequence stripper–absorber was modified by introducing a flash drum to reduce the buffer capacity. The stripped digestate was mixed with NaOH up to a pH value of 9 and fed to the stripping column using air at 90 °C. The air flow rate was optimized considering the possible recycle from the absorption column. The complete process was modeled using the process simulator Aspen Plus considering a recovery of ammonia equal to 95%. The total capital investment was evaluated by means of Aspen Plus Process Economic Analyzer in about 5.5 million US$, and the costs of energy and chemicals were quantified in less than 1 million US$ year−1. A benefit of 1.6 US$ for m3 of digestate was obtained considering an ammonium sulfate price of 522 US$ ton−1.

Development of an Efficient Methanol Production Process for Direct CO2 Hydrogenation over a Cu/ZnO/Al2O3 Catalyst

Carbon capture and utilization as a raw material for methanol production are options for addressing energy problems and global warming. However, the commercial methanol synthesis catalyst offers a poor efficiency in CO2 feedstock because of a low conversion of CO2 and its deactivation resulting from high water production during the process. To overcome these barriers, an efficient process consisting of three stage heat exchanger reactors was proposed for CO2 hydrogenation. The catalyst volume in the conventional methanol reactor (CR) is divided into three sections to load reactors. The product stream of each reactor is conveyed to a flash drum to remove methanol and water from the unreacted gases (H2, CO and CO2). Then, the gaseous stream enters the top of the next reactor as the inlet feed. This novel configuration increases CO2 conversion almost twice compared to one stage reactor. Also to reduce water production, a water permselective membrane was assisted in each reactor to remove water from the reaction side. The proposed process was compared with one stage reactor and CR from coal and natural gas. Methanol is produced 288, 305, 586 and 569 ton/day in CR, one-stage, three-stage and three-stage membrane reactors (MR), respectively. Although methanol production rate in three-stage MR is a bit lower than three stage reactors, the produced water, as the cause of catalyst poisoning, is notably reduced in this configuration. Results show that the proposed process is a strongly feasible way to produce methanol that can competitive with a traditional synthesis process.

An experimental-based energy integrated process for biodiesel production from waste cooking oil using supercritical methanol

Biodiesel has been recognised as one of the effective, green, renewable and sustainable fuels. It is derived from renewable living resources either animal fats or vegetable oils. Biodiesel production in the absence of catalyst using supercritical methanol has recently been receiving significant attention. Non-catalytic transesterification reaction eliminates the difficulties of catalyst preparation and separation processes. Although it has shown high conversion for the reactants with relatively short reaction time in comparison with the conventional catalytic transesterification processes, it has some disadvantages including higher reaction temperature and pressure, large excess of methanol to oil (M:O) molar ratio and higher energy consumption. In an attempt to mitigate these problems, an experimental study followed by process design/integration for biodiesel production from waste cooking oil (WCO) has been performed. A low-quality WCO collected from local restaurants has been selected as a feedstock for the reaction. The experimental phase of the transesterification reaction together with an optimisation procedure resulted in the optimised conditions of M:O molar ratio of 37:1, reaction temperature of 253.5 oC, reaction pressure of 198.5 bar in 14.8 min. The maximum yield was 91%. In addition, kinetics of the reaction has been studied concluding an irreversible pseudo first order reaction with a reaction rate constant of 0.0006 s-1. Moreover, thermodynamics of the reaction has been studied at a temperature range of 240 - 270 oC with resulting frequency factor and activation energy of 4.05 s-1 and 50.5 kJ/mol. A kinetic reactor has been simulated using the experimentally determined kinetic and thermodynamic data. The enthalpy content of the reactor product stream has been used to separate most of the unreacted methanol in an adiabatic flash drum. Finally, a scheme has been developed for an energy integrated process in order to maximise the heat recovery. Energy savings resulted from the developed heat exchanger network (HEN) have been concluded as 32.2 % and 23.8 % for both heating and cooling energies respectively, in comparison with an existing process energy requirements in the literature. The amount of heat exchanged for each unit has been determined in addition to composition, temperature and pressure of the streams. Vacuum distillation column has been designed to separate the unreacted triglycerides from biodiesel in order to fulfil the quality restrictions of the final biodiesel product.

Comprehensive experimental study on a transcritical CO2 ejector-expansion refrigeration system

A comprehensive experimental study on the ejector and cooling performances of a transcritical CO2 ejector-expansion system is presented. The ejector performance of the entrainment ratio, pressure lift ratio, and efficiency were investigated under various primary flow pressure, secondary flow pressure, and back pressure conditions. A new coefficient of mass balance, β, was introduced based on the liquid mass balance in and/or out of the vapor–liquid separator, which was able to estimate the deviation between current operating state and a steady state. The experiment results show that the coefficient of performance (COP) ratio of the ejector-expansion refrigeration system to the basic system decreases from 18.9% to −11% with the coefficient of mass balance increasing from −0.1 to 0.1. Both the entrainment ratio and COP decrease with an increase in the coefficient of mass balance.

Experimental research on liquid-vapor two-phase flow separation of zeotropic mixtures at an impacting T-junction

Impacting T-junctions could work as a partial vapor-liquid separators by enhancing the mal-distribution of phase that occurs naturally therein. Previous studies have mainly concerned the phase splitting of air-water and pure refrigerants but here the performance of a vertical impacting T-junction using zeotropic mixtures as test fluids is reported. Experiments were conducted to investigate the effect of parameters, including inlet mass velocity, vapor quality, splitting ratio, outlet branch diameter and circulating composition of mixtures, on the liquid separation efficiency, based on mixtures R134a/R245fa and R290/R600a. The inlet flow patterns of each test fluid were also identified by using a high-speed camera under various test conditions. It was found that the mixture composition had an obvious effect on phase splitting at high separation loads, but the separation efficiency would not change linearly versus the R134a mass fraction in mixture R134a/R245fa. Overall, pure refrigerant showed better separation performance than zeotropic mixtures, under the same inlet and outlet conditions. Besides, the vaporization due to pressure drop at T-junction was also discussed by conducting a comparison study between air-water mixture and organic fluids. Calculation results indicated that an increase in pressure drop would promote mass transfer from liquid to vapor phase at the impacting T-junction.

Thermo-economic analysis and optimization of a zoetropic fluid organic Rankine cycle with liquid-vapor separation during condensation

Organic Rankine cycle (ORC) is a promising thermal-to-power technology that uses low-temperature heat from various sources including renewable energy and waste heat. A zeotropic fluid ORC offers thermodynamic-performance advantages over pure fluid ORC because of the relatively low irreversibility during the heat transfer process. Nonetheless, zeotropic fluid ORC may incur higher cost than pure fluid ORC because the former has high mass transfer resistance and small temperature difference in the heat exchanger. Limited research has been carried out to enhance thermo-economic performance of zeotropic ORC. In the present study, an ORC that uses zeotropic fluids and undergoes liquid–vapor separation during condensation is proposed. A thermo-economic analysis is developed based on a new optimization model for the proposed ORC. The objective function of the optimization model is the minimization of the specific investment cost. A genetic algorithm is used to solve the optimization model. A case study is then solved to illustrate the advantages of the proposed ORC and validate the proposed thermo-economic optimization method. The results of the thermodynamic analysis show that the condenser area of the proposed ORC is 17.6% lower than that of the conventional ORC under the same working conditions. Thermo-economic optimization results show that the specific investment cost of the proposed ORC is 13.3–18.4% lower than that of the basic ORC. Meanwhile, the second law efficiency of the proposed ORC is 4.2% higher than that of the conventional ORC. A sensitivity analysis is carried out to assess the dependence of the ORC performance on the temperatures of the heat source and the surrounding environment.

Improvement of condenser performance by phase separation confirmed experimentally and by modeling

This paper presents results of an experimental study to determine the effect of vapor–liquid separation in a header of microchannel condenser for a MAC system. R134a is used as the working fluid. A condenser with separation and a baseline condenser identical on the air side have been tested to evaluate the difference in the performance due to separation. Two categories of experiments have been conducted: the heat exchanger-level test and the system-level test. In the heat exchanger-level test, it is found that at the same inlet and outlet temperatures the separation condenser generates 1.6% to 7.4% more condensate flow rate than the baseline. The separation condenser also lowers the refrigerant exit temperature compared with the baseline at the same condensate flow rate. The improvement in the separation condenser confirms the results by a model. In the system-level test, COP is compared under the same superheat, subcooling and refrigerating capacity. System with separation condenser shows up to 6.6% a higher COP than the system with baseline condenser.

ESTUDO DA REMOÇÃO DE INIBIDORES DE FERMENTAÇÃO APÓS TRATAMENTO ÁCIDO E ENZIMÁTICO DE PALHA DE CANA

Neste trabalho estudou-se a remoção do furfural, inibidor tóxico à fermentação, formado no pré-tratamento ácido de lignocelulósicos. Experimentalmente estudaram-se no pré-tratamento diferentes concentrações de H2SO4 (1%, 2,5% e 5%) e tempos de reação (15, 30 e 60 minutos). Definiu-se como condição mais adequada 1 % m/m de ácido sulfúrico e 15 minutos como tempo de reação. Com os dados experimentais realizou-se a simulação da remoção do furfural e da concentração dos açúcares. As correntes contendo açúcares foram denominadas de C5 (hidrolisado ácido – xilose e furfural) e C6 (hidrolisado enzimático - glicose). A corrente C5 foi purificada e concentrada através de uma coluna de enchimento integrada a evaporadores múltiplos com cinco efeitos, com gasto energético de 0,85 kW/kg de xilose. Para concentrar a corrente C6 de 14 a 20% de açúcares utilizaram-se evaporadores múltiplos com quatro efeitos com gasto energético requerido de 0,34 kW/ kg de glicose.

Design of the recovery section of the extracted aromatics in the separation of BTEX from naphtha feed to ethylene crackers using [4empy][Tf2N] and [emim][DCA] mixed ionic liquids as solvent

Abstract Aromatic hydrocarbons are mainly obtained from reformer and pyrolysis gasolines by liquid-liquid extraction. In these streams, the content of aromatics is between 40 wt.% and 70 wt.%. However, several streams as the naphtha feed to ethylene crackers, in which the aromatics are between 10 wt.% and 25 wt.%, have not available technology to obtain a commercial stream of BTEX. In our previous contribution, the BTEX removing from a naphtha model with the minimum aromatic content, 10 wt.%, was studied using ionic liquids (ILs) as solvents. The high aromatic purity in the extract stream and the good extraction yields obtained suggested liquid-liquid extraction using ILs as a good technological election. To confirm this, in this work the vapor-liquid separation of all hydrocarbons from the solvent in the extract stream have been determined at several temperatures by using a headspace – gas chromatography (HS-GC) technique. After that, the operating conditions were simulated to maximize the purity and recovery of the aromatics. The proposed recovery section to selectively separate the aromatics from the extract stream consists in three flash distillation units that operate at low temperatures and high vacuum conditions, achieving an aromatic-rich stream with a purity of 98.5 wt.%.

Experimental investigation of multi-effect regenerator for desiccant dehumidifier: Effects of various regeneration temperatures and solution flow rates on system performances

We have experimentally measured the thermal performances (PR) of vacuum assisted multi-effect regenerator, which employs hydrophobic membrane to separate water vapor from LiCl solution with higher evaporation rates. We have modified the working principles of the conventional multi-effect membrane distillation (MEMD) by the transfer of heat from one stage to the other stage of regenerating module such that more energy can be recovered through multi-effect evaporation generated in each regenerating module with enhanced liquid vapor separation in the membrane. The PR and percentage change in desiccant concentration are measured for the temperatures ranging from 50 °C to 75 °C. The experimental results show that the conventional MEMD is not suitable for dehumidification applications due to higher feed concentration. We have modified the conventional MEMD system employing (i) one-steam-three-solution passes and (ii) two-steam-four-solution passes, which handles higher feed concentrations. The PR is found 0.7 for one steam three solution passes system.

Detection of chemicals in water using a three-dimensional graphene porous structure as liquid-vapor separation filter

In this study, we report the utilization of a three-dimensional (3D) porous graphene structure as liquid–vapor separation filter, which allows the passage of chemical vapors while blocking liquid chemicals and water. The blockage of liquid chemicals and water from semiconducting sensing regions is required to avoid abnormal transistor characteristics. In order to impart omniphobic characteristics, a (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane (HDF-S) self-assembled monolayer was coated on the surface of the 3D porous graphene structure. The concentration of chemical agents in water could be consistently detected by observing the shift in the threshold voltage in the oxide nanowire transistor covered by the HDF-S self-assembled 3D graphene structure. The proposed monitoring method is expected to offer means for application in different environments by providing a stable sensing performance.

A technical and economic assessment of variants for CO2 separation from synthesis gas

ABSTRACTAn analysis of carbon dioxide (CO2) removal process from synthesis gas has been carried out in this work. CO2 removals by physical and chemical solvents are the most often used. The most important factors that affect cost and performance of a CO2 recovery unit are the number of flash drum separation units, and the pressure drop pattern among them. Different variations of this process were simulated using Hysys software, and the capital and operational expenditures for these plants were estimated. The focus was to reach an optimum process to minimize the total unit cost of separated CO2. The results indicate that for a fixed number of separation units, the case with uniform pressure drop pattern has the minimal cost and energy consumption. On the other hand, it was revealed that among the cases with uniform pressure drop pattern, the one with four separation units will be the optimum case.

Effects of evaporator superheat on system operation stability of an organic Rankine cycle

The stability of organic Rankine cycle (ORC) is important for the monitoring and controlling of the system. This paper analyzes the effects of the outlet superheat of evaporators on an ORC system using R245fa as working fluid. The experimental apparatus used in this research is an installed 500W ORC system integrated with a plate evaporator and a shell-and-tube evaporator in parallel. Test results indicate that the superheat at the evaporator’s outlet is the main parameter correlated with the system’s operation stability. When using the plate evaporator, with a superheat of 1.8°C, instability due to liquid entrainment was observed through a sight glass. However, if the superheat is increased to 8.7°C, the system became quite stable. In the case that the shell-and-tube evaporator was used, no unstable oscillation was observed even when a superheat was as low as 0.2°C. In addition, the relationship of the vapor dryness, the entrainment quality fraction, and the superheat was analyzed. It is suggested that installing a vapor-liquid separator or maintaining a relatively high superheat could be an effective way to avoid the liquid entrainment and hence to have a better operation stability.

Availability analysis of safety critical systems using advanced fault tree and stochastic Petri net formalisms

Failure scenarios analysis constitutes one of the cornerstones of risk assessment and availability analysis. After a detailed review of available methods, this paper identified two distinct formalisms to analyze failure scenarios and systems' availability: generalized stochastic Petri nets (GSPN) and Fault tree driven Markov processes (FTDMP). The FTDMP formalism is a combination of the Markov process and the fault tree. This aims to overcome fault tree limitations while maintaining the use of deductive logic. The GSPN is a Petri net with probabilistic analysis using Monte Carlo simulation. The effectiveness of both methods is studied through an emergency flare system including a knockout drum. It is observed that GSPN provides a robust and reliable mechanism for accident scenario analysis. It provides additional information such as events' frequencies at operating and failing modes and expected occurrence timing and durations resulting from different complex sequences. Even for multi-state variables which could be used to design a safety management system. Although FTDMP is a powerful formalism, it provides limited information.

Separation of aromatics from n-alkanes using tricyanomethanide-based ionic liquids: Liquid-liquid extraction, vapor-liquid separation, and thermophysical characterization

Ionic liquids (ILs) have been extensively studied as replacements to sulfolane in the separation of aromatics from alkanes. The employment of ILs could reduce energy requirements and operating costs of the aromatic extraction unit as a result of their nonvolatile character. However, the ILs studied so far have shown mass-based aromatic distribution ratios lower than the sulfolane values, which would increase the solvent-to-feed ratio in the extractor. To overcome this drawback, we tested the performance of the 1-butyl-3-methylimidazolium tricyanomethanide ([bmim][TCM]) and the 1-butyl-4-methylpyridinium tricyanomethanide ([4bmpy][TCM]) in the separation of toluene from n-heptane, exhibiting the [4bmpy][TCM] mass-based toluene distribution ratios and toluene/n-heptane selectivities higher than those of sulfolane. We also studied the vapor-liquid recovery of the extracted hydrocarbons from the ILs, obtaining relative volatilities of n-heptane from toluene substantially higher in the presence ILs than those without ILs. A thermophysical characterization of the ILs was also made by measuring their densities, viscosities, thermal stabilities, and estimating their maximum operation temperatures. Finally, the regeneration and reuse of the ILs was studied on successive recovery cycles. After five recovery cycles, ILs have shown the same extractive capacity.