Cryogenic Air Separation Unit Research Articles

Investigation of an integrated system with industrial thermal management options for carbon emission reduction and hydrogen and ammonia production

Abstract A new integrated energy system employing the cement slag waste heat is uniquely proposed in this study. The core focus of the proposed system is to generate clean hydrogen thermochemically and convert it into ammonia. The designed system consists of the copper–chlorine (Cu–Cl) cycle, a cryogenic air separation unit and a steam Rankine cycle while the useful commodities produced by the proposed system are hydrogen, ammonia, oxygen, hot water and electricity. A CO2 emission analysis is also conducted to calculate the emissions which can be avoided by recovering this waste heat. The Aspen Plus simulation software is utilized to model and simulate the proposed integrated system. A thermochemical water splitting process is incorporated into the system for hydrogen production. The cryogenic air separation unit is integrated in order to separate nitrogen from the air. This proposed system also reduces the environmental effects of the flue gas emitted by the cement industry. Multiple parametric studies are performed to investigate the system performance by varying operating conditions and state properties. The energy analysis is implemented on each component of the designed system. The overall energy efficiency of the system is concluded as 30.1%. The amount of CO2 emissions which can be avoided by utilizing this waste heat is 29.64 ktonne/5 years.

A novel cryogenic vapor-recompression air separation unit integrated to oxyfuel combined-cycle gas-to-wire plant with carbon dioxide enhanced oil recovery: Energy and economic assessments

Abstract Oxyfuel carbon capture is both power and capital intensive due to oxygen demand. Consequently, oxyfuel requires the development of more efficient air separation units. This work proposes an alternative cryogenic distillation process for large-scale gaseous oxygen supply. Instead of using different pressure columns, the new air separation unit couples top vapor recompression to a single atmospheric cryogenic air distillation double-reboiler column, whose nitrogen-rich top vapor is compressed to heat the intermediate column reboiler, while the bottom reboiler is heated with compressed saturated air. Several processes for low-pressure oxygen gas supply were simulated and optimized. The power requirement of the new air separation unit producing atmospheric oxygen at 95%mol attained the best value of 139.0 kWh/t (oxygen basis). A sensitivity analysis for oxygen purity was performed showing that, even for higher purities, the new developed process achieves the lowest specific power for low-pressure gaseous oxygen production. The economic leverage of the new air separation unit is proven via successful supply of low-pressure oxygen to oxyfuel natural gas combined-cycle Gas-To-Wire plant. Assuming carbon dioxide destination to enhanced oil recovery, even with an investment about 100% higher than the counterpart of a conventional air-fed combined-cycle Gas-To-Wire plant and despite net efficiency penalty of 6.88%, the oxyfuel combined-cycle Gas-To-Wire solution coupled to the new air separation unit was capable of achieving comparatively superior profitability under carbon taxation above 13.5 USD/t.

Improvement of integrated gasification combined cycle performance using nitrogen from the air separation unit as turbine coolant

An integrated gasification combined cycle (IGCC) power plant uses fuel made by coal gasification for gas turbines. The oxygen required for the gasification reaction is supplied using a cryogenic air separation unit (ASU), and a large amount of nitrogen is generated. In the conventional IGCC, nitrogen generated in the ASU is injected into the combustor to generate more power. However, since the temperature of the injected nitrogen is lower than that of the air supplied to the combustor, injecting nitrogen requires more fuel to maintain a constant outlet temperature (i.e., turbine inlet temperature (TIT)). We propose a novel system configuration to improve the performance of the IGCC and analyzed its effects. We used nitrogen generated in the ASU as a turbine blade coolant instead of compressed air. Since the nitrogen temperature was lower than the temperature of compressed air, the turbine blades can be protected with a lower flow rate of coolant. As a result, both the power and efficiency are increased. We also investigated possibilities for further performance improvement by changing design parameters and determined the design point that maximizes the efficiency.

Thermodynamic evaluation of the novel distillation column of the air separation unit with integration of liquefied natural gas (LNG) regasification

A cryogenic air separation unit (ASU) with the implementation of vapor recompression column (VRC) and LNG cryogenic energy is proposed and investigated. To this end, three types of cryogenic rectification column for air separation are first illustrated, and their thermodynamic characteristics are evaluated by the first and second thermodynamic laws. The results show the purity and recovery ratio of product are determined by the total amount of heat duty and the scheme of heat distribution. Compared with the conventional double rectification column and internally heat-integrated distillation column, the VRC is the most suitable column for the ASU with use of LNG cryogenic energy as its lower demand of shaft work and minimum energy of separation. Based on the above analysis, adopting the cryogenic energy from product to cool the reflux and feed air is beneficial for producing vapor product in the proposed process. The operating pressure of ASU could be reduced by about 24%, and the cryogenic energy of LNG can be used efficiently and sufficiently with respect to the supercritical thermophysical property. By adding the air-aid refrigeration process, the proposed process also has the ability to produce liquid oxygen. Under the same oxygen purity and recovery ratio, the optimal results show the power consumption is about 6.9% and 11.1% lower than that in the previous studies in producing vapor and liquid product respectively, and the minimum energy of separation is improved by use of LNG cryogenic energy.

Examination of the liquefaction system for the use of different cryogenics in terms of thermodynamic analysis

Energy consumption in the world is increasing day by day. In addition to diversifying energy resources, it is also important to reduce energy consumption. In order to find the actual consumption of a thermodynamic system, energy efficiency as well as exergy efficiency should be done. The purpose of this study is to determine the parameters affecting the exergy efficiency of the cryogenic liquefaction unit integrated into a real cryogenic air separation unit. Cryogenic liquefaction is one of the basic processes between liquefaction methods. In addition to this process, absorption and membrane are used in methods. The main difference in the selection of these methods is the desired purity rates. Cryogenes are defined as fluids used in cryogenic cooling. In this study, five different cryogenes in the air are investigated. The energy and exergy analysis of the liquefaction unit for each cryogen is made. As a result of the study, the highest COPactual value is obtained with 0.3105 hydrogen fluid and the highest COPrev value with 0.8551 oxygen. Exergy of the system is found as 0.48 with hydrogen.

Evaluation of an optimal integrated design multi-fuel multi-product electrical power plant by energy and exergy analyses

A novel integrated multi-fuel multi-product electrical power plant with a net electrical power output of 5.97 × 105 kW is developed and investigated by energy and exergy analyses. The electrical power plant including coal gasification, natural gas solid oxide fuel cell, cryogenic air separation unit, carbon dioxide transcritical and steam cycles, and liquefied natural gas regasification sub-systems. The potential of operating performance improvement by conducted energy and exergy analyses is assessed. Also, the thermal design and heat integration analyses are performed for studying the system sustainability and emphasizing on no requirement for external hot and cold utilities. Effects of the main design parameters such as fuel cell operating parameters, ambient temperature and liquefied natural gas thermodynamic specifications on the system operating performance are examined. The obtained results indicate that the overall energy and exergy efficiencies reach to a maximum value of about 56.4% and 57.9%, respectively; in the case of fuel utilization of solid oxide fuel cell is about 80.0%. Furthermore, increasing the ambient temperature and current density decrease the system operating performance. In contrast, the system performance increases with the liquefied natural gas vapor pressure.

Integrating an oxygen enriched waste to energy plant with cryogenic engines and Air Separation Unit: Technical, economic and environmental analysis

One of the main benefits of using oxygen enriched combustion in Waste-to-Energy plants is the reduction of losses at the stack due to the reduced flue gas production. At the same time, the electricity required by the Air Separation Unit to generate the oxygen, leads to a penalty in energy efficiency that puts at stake its economic feasibility. In order to overcome that criticality, an opportunity is offered by the possibility to exploit one of the main by-products of the Air Separation Units (i.e. liquid nitrogen) by means of a high efficiency open Rankine-cycle expander, namely the Dearman Engine (i.e. cryogenic engine). The proposed research investigates the feasibility of an integrated system - Waste-to-Energy plant, Air Separation Unit and Dearman Engine - in terms of technical, economic and environmental performance indices such as power output, economic savings, incremental income and pollutant emissions reduction. The results show that, under opportune conditions (i.e. liquid nitrogen utilization factor and diesel price higher than 83% and 1.08 USD/kg, respectively) the penalty in energy efficiency coming from the integration between the Waste-to-Energy plant and the Air Separation Unit can be compensated, both economically, with a pay-back time inferior to 10 years, and environmentally (saving 23 kton/year of CO2), by means of the valorisation of the liquid nitrogen through the Dearman Engine.

Numerical analysis of an ion transport membrane system for oxy–fuel combustion

Ion transport membranes (ITM) have been studied as a promising air separation unit (ASU) technology for oxy–fuel combustion owing to their high oxygen permeability. Even though the power consumption of the ITM is lower than that of cryogenic ASU, it still consumes a high proportion of the overall system power.In this study, a numerical analysis of the ITM system has been conducted using Aspen Plus® to determine the optimal system design for minimizing the power consumption to separate oxygen from air. Since the oxygen permeation through the ITM is driven by the oxygen partial pressure gradient between feed and permeation side, three ITM systems that have different pressure gradients across the membrane have been presented and their performances compared. The effects of the contributing parameters, such as thickness, pressure, temperature, and air flow rate on the oxygen permeation rate have been investigated. ITM performances of the counter and parallel flow configurations have been compared. The system that operates under atmospheric pressure at the feed channel and under vacuum pressure at the permeate channel yields the lowest power consumption for obtaining the same oxygen permeation rate among other pressure conditions.

Novel design of integrated gasification combined cycle (IGCC) power plants with CO2 capture

Abstract To develop the new integrated gasification combined cycle (IGCC) power plants with CO2 capture, first the integrated intermittent chemical looping air separation (IICLAS) unit is added to improve the coal gasification and carries out the oxy-fuel combustion, second the CO2 compression and sequestration (CCS) unit is added to achieve deep reductions in emissions of CO2 to the atmosphere. Compared to the conventional cryogenic air separation (CAS) unit, the IICLAS unit continuously produces the high-purity oxygen without the solid looping cycles and it also generates the additional electricity by virtue of the heating jacket for the oxy-fuel combustion of methane (OCM) and the waste heat recovery mechanism. A few comparisons show that the IICLAS-based IGCC + CCS power system (Design 2) can effectively increase 12.2% of the gross power compared to the CAS-based IGCC + CCS power system (Design 1). For the dual IICLAS-based IGCC + CCS power system (Design 3), the OCM units are removed and the flue gas is split to reduce the circulated steam/water. Finally, Design 3 can ensure the lowest oxygen supply rate by 50.67 kg/s, the highest net efficiency by 40.25%, and the largest KCC factor by 0.58.

Influence of Selected Gasification Parameters on Syngas Composition From Biomass Gasification

In this study, the syngas composition exiting a biomass gasifier is investigated to determine the effect of varying selected gasification parameters. The gasification parameters considered are the mass flow rate of steam, the gasification agent, the mass flow rate of oxygen, the gasification oxidant, and the type of biomass. The syngas composition is represented by its hydrogen, carbon monoxide, carbon dioxide, and water fractions. The oxygen fed to the gasifier is produced using a cryogenic air separation unit (CASU). The gasifier and the air separation unit are modeled and simulated with aspenplus, where the gasification reactions are carried out based on the Gibbs free energy minimization approach. Finally, the syngas composition for the different types of biomass as well as the different compositions of the three types of the biomass considered are compared in terms of chemical composition. It was found that for each type of biomass and at a specified steam flow rate there is an air to the air separation unit where the gasification of the biomass ends and biomass combustion starts and as the volatile matter in the biomass increases the further the shifting point occur, meaning at higher air flow rate. It was found for the three considered biomass types and their four mixtures that, as the volatile matter in the biomass increases, more hydrogen is observed in the syngas. An optimum biomass mixture can be achieved by determining the right amount of each type of biomass based on the reported sensitivity analysis.

Control optimization to achieve energy-efficient operation of the air separation unit in oxy-fuel combustion power plants

Cryogenic air separation unit (ASU) is considered as the currently available commercial oxygen production method for oxy-fuel combustion power plants; however, this method leads to significant energy penalty and economic cost. Real-time optimizing system operations during dynamic processes (such as flow rate change, oxygen product purity change, and flexible operation) are expected to achieve remarkable energy savings. Dynamic exergy provides a powerful indicator for real-time evaluating the system thermodynamic performance and quantifying the impact of a control strategy. In this work, some important transient exergy parameters of ASU systems under typical dynamic operating scenarios were first obtained through combining steady-state and dynamic process simulations. Next, control penalty and cost for internal control structures (layers and loops) were determined for the optimizations of control strategy and operation. Feedforward-feedback control structure and ASU-following control strategy are more suitable for ASU regulation and flexible operation, respectively, because more efficient thermodynamic performance is achieved during the investigated operating scenarios. The control structure, layer and loop play different roles in terms of energy behavior and require reasonable regulation to optimize energy behavior. This study provides an important insight into using control optimization aided by the dynamic exergy method to implement energy-efficient operations for industrial plants.

Analysis and assessment of a hydrogen production plant consisting of coal gasification, thermochemical water decomposition and hydrogen compression systems

A novel hydrogen production plant is proposed, including a thermochemical water decomposition cycle, a pressurized entrained flow gasifier, a water gas shift membrane reactor, a cryogenic air separation unit, a hydrogen-fueled combined cycle for power production and a hydrogen compression system. The syngas produced by the pressurized entrained flow gasifier undergoes the shift reaction in the water gas shift membrane reactor. The stripping of hydrogen is done simultaneously with the shift reaction in the water gas shift membrane reactor to capture more hydrogen and increase the shift reaction conversion percentage. The remaining syngas is combusted in the Brayton cycle and power is produced through Brayton cycle gas turbine. The hot exhausts exiting the Brayton cycle gas turbine goes to the heat recovery steam generation unit where steam is produced. The generated steam goes to the copper-chlorine cycle to produce hydrogen. Part of the power generated by the Brayton cycle is used for the electrolysis reactor in the copper-chlorine cycle. Then, a small part of the hydrogen produced passes to the combined cycle to overcome the needed work rate by the components in the plant. The remaining larger portion of the hydrogen produced goes to the compression system to compress hydrogen to 700 bar for storage. The hydrogen production plant is developed and modeled in the Aspen Plus software package. Energy and exergy analyses are performed on the hydrogen production integrated system. The overall energy and exergy efficiencies of the proposed hydrogen production plant are found to be 51.3% and 47.6% respectively.

Thermodynamic assessment of an integrated biomass and coal co-gasification, cryogenic air separation unit with power generation cycles based on LNG vaporization

A novel self-heat system which consists of a coal and biomass co-gasification process, low-temperature air separation based on the LNG vaporization, steam cycle, supercritical CO2 power generation unit and cryogenic carbon dioxide capture section is introduced and analyzed. Some new aspects of this study can be categorized as follows. There is no need to implement external heat sources and refrigeration units. Suitable process integration significantly decreases energy consumption and the heaters and coolers can be eliminated. Co-gasifier produces high yield synthesis gas to supply power and heat energy. The required pure oxygen (99.99%) for gasification is supplied from ASU. The specific energy required for high purity oxygen production is about 0.11 kW h/kg. The results show that the cold gas efficiency of gasifier is 81%. Also due to sufficient integration between LNG vaporization and other subsystems, 40459.53 kW energy is saved in the system. In cryogenic CO2 capture unit, 0.10 kW h/kg-CO2 is required to separate 99% carbon dioxide. Moreover, exergy and sensitivity analyses are carried out on the integrated process. Exergy efficiency and exergy destruction have been calculated for the main equipment of the system. The overall exergy destruction rate of the process is around 1.007 × 105 kW and heat exchangers have the highest portion of irreversibility. Finally, the effect of some significant parameters on the performance of the units is studied.

Techno-economic assessment of different routes for olefins production through the oxidative coupling of methane (OCM): Advances in benchmark technologies

This paper addresses the techno-economic assessment of two technologies for olefins production from naphtha and natural gas. The first technology is based on conventional naphtha steam cracking for the production of ethylene, propylene and BTX at polymer grade. The unused products are recovered in a boiler to produce electricity for the plant. The plant has been designed to produce 1 MTPY of ethylene.In the second case, ethylene is produced from natural gas through the oxidative coupling of methane (OCM) in which natural gas is fed to the OCM reactor together with oxygen from a cryogenic air separation unit (ASU). The overall reactions are kinetically controlled and the system is designed to work at about 750–850 °C and close to 10 bar. Since the overall reaction system is exothermic, different layouts for the reactor temperature control are evaluated.For the naphtha steam cracking plant, the energy analysis shows an overall conversion efficiency of 67% (with a naphtha-to-olefins conversion of 65.7%) due to the production of different products (including electricity), with a carbon conversion rate of 70%. The main equipment costs associated with naphtha steam cracking are represented by the cracker (about 30%), but the cost of ethylene depends almost entirely on the cost associated with the fuel feedstock.In case of the OCM plant, the overall energy conversion efficiency drops to maximally 30%. In the studied plant design, CO2 capture from the syngas is also considered (downstream of the OCM reactor) and therefore the final carbon/capture efficiency is above 20%. The cost of ethylene from OCM is higher than with the naphtha steam cracking plant and the CAPEX affects the final cost of ethylene significantly, as well as the large amount of electricity required.

Process integration of coal fueled chemical looping hydrogen generation with SOFC for power production and CO2 capture

Chemical looping hydrogen generation (CLHG) is integrated with solid oxide fuel cell (SOFC) and combined cycle for high-efficiency power production with CO2 sequestration. To avoid energy penalty imposed by cryogenic air separation unit for oxygen production, the CLHG is directly fed by coal. The coal gasification and iron oxide reaction occur in the same reactor and no gaseous oxygen is used in gasification. The CLHG operates at an auto-thermal condition by solid circulation. The proposed system could achieve a net power efficiency of 41.59% at a baseline case. Sensitivity analysis is conducted as well on the basis of heat balance. Varying the temperatures of CLHG reactors and iron oxide circulation rate influences the system's efficiency. The increase of the fuel cell's temperature and fuel utilization factor favors the system's net power efficiency. In addition to high net power efficiency, the CO2 capture rate of the proposed system is up to ∼100%, resulting in a virtual carbon-free power plant.

Dynamic optimization of a cryogenic air separation unit using a derivative-free optimization approach

Optimal dynamic product transition is a challenging and important issue in manufacturing plants. When a reliable dynamic model is available, gradient-based optimization algorithms can be used to achieve this aim. However, in some cases a first principles dynamic model may not be available. In this work, we will assume that only input-output information from a dynamic model embedded in a dynamic process simulator is available for optimal product transitions between products. We present a derivative-free optimization trust region approach to deal with the product dynamic optimization problem of an air separation unit used in several processing plants to obtain pure oxygen. High-purity oxygen is required in intensive energy applications such as steel plants and in combustion processes. A closed-loop model predictive control strategy is used where the system to be optimized is embedded in the ASPEN Dynamics simulation environment. The results demonstrate that black-box dynamic models can be dynamically optimized when model of the dynamic model and/or its gradient information are not available. We have successfully applied the non-linear model predictive control and derivate-free approach to several oxygen composition and productivity transition issues.

Optimal design and operations of a flexible oxyfuel natural gas plant

We co-optimize the design and operations of a flexible semi-closed oxygen-combustion combined cycle (SCOC-CC) carbon capture plant under time-varying electricity prices. The system consists of a cryogenic air separation unit, liquid oxygen storage, a gas turbine, a heat-recovery steam generator, and a steam turbine. The gas turbine is modeled allowing part-load operation. Computational optimization is used to maximize net present value (NPV) in order to examine the potential benefits achievable through upfront investments in increased flexibility (i.e., allowing price arbitrage between times of low and high price). Case studies of Germany and California are examined. Flexible SCOC-CC systems are not profitable in either region under current electricity prices. With electricity prices ≈2 times current prices, we find systems with positive NPVs. Oxygen storage is used in days with extreme price variability. Optimal designs favor constant operation, without over- or under-sizing system components and without additional oxygen storage. Sensitivity analyses show that external factors such as mean electricity price (± 200%), natural gas price (± 150%), and nominal discount rate (± 50%) have the strongest effect on NPV. Electricity price variability, which is thought to increase with increased penetration of renewables, does not strongly impact system design and profitability.

Enriched Air or Pure Oxygen as Oxidant for Gas‐to‐Liquid Process with Microchannel Reactors

AbstractThe syngas production step is one of the most costly steps in a gas‐to‐liquid plant. Commonly, oxygen is used as an oxidant in the reforming step. However, through the introduction of microchannel reactors, the use of enriched air may be justified. The merits of using enriched air versus pure oxygen are analyzed by utilizing an autothermal reformer, with microchannel reactors in the once‐through Fischer‐Tropsch (FT) step. Pure oxygen is provided by a cryogenic air separation unit (ASU) and enriched air by use of air separation membranes. Pure oxygen requires a smaller FT reactor volume, which means lower reactor costs at the expense of having a costly cryogenic ASU to produce pure oxygen. The operating cost of the ASU is lower than that of the air membrane, but the installed cost is higher.

Parametric study of a hybrid one column air separation unit (ASU) and CO2 power cycle based on advanced exergy cost analysis results

In this study advanced exergy and exergoeconomic analyses are performed on the integrated cryogenic air separation unit (ASU), oxy-fuel carbon dioxide power cycle and LNG vaporization process. Advanced exergy results show that endogenous part of exergy destruction is more than exogenous part which means interactions between the process components is not significant. CR-4 compressor with 190,095 kW exogenous exergy destruction rate has the highest share of this type of exergy destruction among the process components. Advanced exergoeconomic analysis results show that interactions between the process components are weak. That is because of large values of endogenous part towards the exogenous part. Among investigated process components unavoidable exergy destruction cost rate of heat exchanger (H-1) and combustion chamber is higher than the avoidable part. Avoidable/endogenous exergy destruction cost rate of P-1, CR-1 and H-8 components is the highest and consequently these elements will have the highest priority for the optimization. The capital investment and operating and maintenance cost rate of pump (P-1) is 10,018 $/h. This value reveals that saving the money from the operating and maintenance costs will make up the replacement cost of this component with the high-tech ones. Based on the parametric analysis, the optimum design parameters of pump (P-1), compressor (CR-1) and heat exchanger (H-8) are ηis=82%, rp=6.195 and ΔTmin=2K, respectively.

Development of an integrated system for electricity and hydrogen production from coal and water utilizing a novel chemical hydrogen storage technology

A coal gasification-based integrated system is proposed to produce electrical power and hydrogen. The hydrogen produced is stored in a chemical storage medium, which is ammonia. The integrated system contains a water gas shift membrane reactor, a hybrid thermochemical water decomposition cycle based on the chemical couple copper and chlorine and a multistage ammonia production system. A hydrogen fueled supporting combined cycle is used to meet the electrical requirement of the integrated system. Coal is gasified, and the resulting syngas is water shifted in a membrane reactor, which produces hydrogen. The remaining syngas is combusted to generate power through a gas turbine, and the turbine hot exhaust is used to provide the required thermal energy of the water decomposition cycle. The hydrogen outputs from the coal gasification and the water decomposition cycle are fed to the ammonia production system and the supporting combined cycle. The ammonia production system contains multiple stages to achieve a high conversion percentage of hydrogen. The nitrogen fed to the ammonia reactor is provided by the cryogenic air separation unit, which also provides oxygen to the gasifier. The proposed system is simulated with the process simulation software Aspen Plus. The system performance is evaluated through energy and exergy efficiencies. The integrated system is found to have an energy efficiency of 48.7% and an exergy efficiency of 48.4%. The system is capable of producing 0.18kg/s of hydrogen and 1.2MW of power per 1.5kg/s of coal.