Cryogenic Air Separation Plants Research Articles

Introducing and performance evaluation of a novel Multi-Generation process for producing Power, fresh Water, oxygen and nitrogen integrated with liquefied natural gas regasification and carbon dioxide removal process

The objective of this paper is to develop and evaluate a new integrated multigenerational system comprised of cryogenic air separation, hydrate-based desalination, combined cycle and oxyfuel power plants, carbon dioxide amine absorption, and transcritical carbon dioxide power cycle, employing liquefied natural gas cold energy. This integrated multi-generation system, applicable for liquefied natural gas importer terminals with access to seawater, produces approximately 2,533 MW net output power and 1,985 kg/h fresh water. In addition, this integrated multi-generation process produces 16,735,861 kg/h high-purity liquid oxygen (100 mol%), 4,773,633 kg/h liquid nitrogen with 99.71 mol% purity, more than 31,724,465 kg/h high-purity gaseous nitrogen (99.9 mol%), and 90,903,230 kg/h low-purity gaseous nitrogen (83.38 mol%). About 5907.3 MW of produced power is generated by liquefied natural gas expander. The performance of this system is evaluated by energetic methods. The results of energy analysis show the specific energy consumptions of the air separation products are 0.551 kW/kg and 1.93 kW/kg for liquid oxygen and nitrogen, respectively. Moreover, in the hydrate-based desalination subsystem, the specific energy consumption is 0.5 kW/m3 for produced pure water. The combined cycle power plant includes two gas turbines, three steam turbines, and a three-pressure level heat recovery steam generation. The thermal efficiencies of gas turbine cycles (Bryton), steam turbine cycles (Rankine) and combined power cycles are 26.3 %, 29.6 %, and 36.2 %, respectively. The flue gases from power plants enter the carbon dioxide amine absorption subsystem. The separated carbon dioxide is used as a working fluid to produce 372.1 MW power in transcritical carbon dioxide cycle. The results of energy analysis for carbon dioxide amine absorption subsystem show that the specific energy consumption, the ratio of captured carbon dioxide per unit mass of liquefied natural gas and the carbon dioxide removal efficiency are 6.60 kW/kg, 33.91 kg/tone, and 90.06 %, respectively. Approximately 2,905,000 kg/h of liquid carbon dioxide is obtained as the byproduct of this multi-generation process. Additionally, sensitivity analysis is conducted to determine key parameters influencing the main energetic indexes. Finally, the amount of attainable regasified natural gas reaches 257,523 tone/h by applying this multi-generation system with total 86 % energy efficiency.

Scheduling optimization and risk analysis for energy-intensive industries under uncertain electricity market to facilitate financial planning

The planning of energy-intensive processes is intrinsically uncertain due to their dependence on the volatile energy market, with scheduling having a vast impact on the final production cost of these plants. Traditional stochastic methods are mathematically very complex, which translates into a significant computational effort that might prevent a timely response to varying electricity prices. To encounter this uncertainty, we develop a reliable hybrid simulation-optimization approach for optimizing the production plant scheduling, combining scenario analysis with risk analysis. The proposed methodology is demonstrated with real data from a cryogenic air separation plant in Tarragona (Spain). This approach also informs decision-makers about risk or expected shortfall associated with the implied scenario. The generic methodology used here can be easily adapted to schedule facilities in other energy-intensive sectors such as cement, metallurgy or pulp and paper.

Modeling, validation and exergy evaluation of a thermally-integrated industrial cryogenic air separation plant in Colombia. Part II. Optimization under variable products demand

This work deals with the optimization of an existing industrial cryogenic air separation plant with respect to exergy efficiency and argon recovery. A previously validated simulation model of the plant was used, and the operating conditions and specifications of the process correspond to those reported in the historical records of an air separation facility in Colombia. The optimization is carried out considering the variation in the historical demand and the installed capacity of the plant, using an elitist nondominated sorting genetic algorithm (NSGA-II). As exergy efficiency and argon production goals are in conflict, a Pareto front of optimal solutions is obtained for the assessed variables. It is found that under optimal conditions of the pareto fronts, exergy efficiency and argon recovery can be enhanced up to 30 % and 23 %, respectively, with respect to recommended operating conditions from the plant manual. Also, obtained results indicate that the plant operated with larger air feed flowrates than required for the corresponding specific monthly demands. Under the optimal conditions, the plant could operate with a 58 % average reduction in the specific energy consumption (0.53 kW/Nm3O2) and larger argon recoveries. This represents a major improvement in the profitability of the operation and its sustainability indicators.

Modeling, simulation, and optimization of multiproduct cryogenic air separation unit startup

AbstractThe startup of multiproduct cryogenic air separation plants takes several hours, during which time limited revenue is generated with high costs incurred due to the highly energy‐intensive nature of these operations. This motivates the development of high‐fidelity dynamic models to capture the complexity of the startup process to aid decision‐making. This article focuses on the development of a startup model for a multiproduct air separation unit (ASU), and its use in dynamic simulation and optimization. To accomplish this, a first‐principles based dynamic ASU model is extended by including various discontinuities using smooth approximations, adding dynamics to the primary heat exchanger, and extending the handling phase change within process streams. Dynamic simulations demonstrate plant response behavior during startup, including a failed startup resulting from an injudicious choice of input trajectory. In addition, improvement of startup operation is demonstrated through the incorporation of the model within a dynamic optimization framework.

Modeling, validation and exergy evaluation of a thermally-integrated industrial cryogenic air separation plant in Colombia

This work deals with the modeling, computer simulation, validation, and exergy evaluation of a ~40 ton/h industrial air separation plant located in Colombia. The model was implemented in Aspen Hysys® at the corresponding operating conditions of the facility. Initially, the reliability of the model was verified by comparison with data reported in the plant manual (EDM) from the base operating point. An additional validation was performed by comparison with historical records which involved a statistical analysis of plant metadata. The model showed good agreement with both sets of data, displaying deviations< 10%, and it was used to identify that compressors (1325 kW), multichannel heat exchanger (519 kW) and the high-pressure column (448 kW) exhibit the largest exergy destruction rates. By mean of a sensitivity analysis, main controlled variables affecting the process performance were identified. This enabled to detect the feasible operating zones, and a local minimum of exergy destruction for each variable. The overall exergy efficiency was estimated in 0.18, and the specific power consumption with respect to produced oxygen was 1.26 kWh/Nm3 O2. Results indicate that plant performance needs to be highly improved to reduce energy intensity and that the obtained model is suitable for further optimization.

Strategizing practical implementation of additional condenser-reboilers (CR) to reduce power consumption of a dual purity cryogenic single column gaseous oxygen plant

A basic single column cryogenic air separation plant refluxed by nitrogen has a single condenser-reboiler (CR) that produces gaseous oxygen simultaneously at dual purities of 90% and 99.5% at 10 bar and 40 bar, respectively. This basic plant consumes about 42,000 kW in an integrated steel plant of 5 million tons per year. Saving power even by a small percentage holds a great promise to reduce the production cost of steel. In place of pressurizing the entire recycled nitrogen flow to 5.1 bar that feeds the condenser-reboiler of the basic plant, some portions of nitrogen in the modified plant is required at 4.3 bar and 2.8 bar to feed the additional condenser-reboilers. Nitrogen compressor accounts for 20–25% of the net power consumption and putting additional condenser-reboilers reduces it by 15–23% that leads to an overall reduction of the power of 5.4%. As a single column plant can produce pure and impure oxygen simultaneously at different proportions, this paper determines the additional number of reboilers to be installed, their locations in the column and other plant parameters keeping oxygen recovery intact above 99%. The analysis evolves the strategy of operating recycled nitrogen compressors at multiple pressures in modified plant having three condenser-reboilers.

Reducing power consumption in a cryogenic air separation plant for filling oxygen cylinders in-situ by utilizing thermal energy of pumped liquid oxygen

• Energy-efficient air separation plants filling oxygen cylinders in situ are simulated. • Cold of liquid oxygen that gets wasted in external vaporisation is now captured. • Utilisation of liquid oxygen cold reduces net plant specific power input by 23–29%. • Optimum operational method generates all 3 pressures of oxygen from a single plant. • Penalty of 2–3% in specific power has to be accepted with a single-plant option. Cryogenic air separation plants produce oxygen at 99.5% purity at 150, 230 and 300 bar to fill cylinders. From the production location, liquid oxygen is often transported to a cylinder-filling plant located at a far-off location where it is pumped to high pressure and gasified in ambient vaporizers. Among the methods of cylinder filling, external vaporization is the safest, operationally most flexible, and allows the main heat exchanger to be designed at low pressure. Despite these advantages, external vaporization is not preferred for filling cylinders “in situ” because of high power consumption attributed to the loss of the entire cold exergy of pumped liquid to the environment. An appropriate modification of the plant helps to utilize the thermal energy of pumped liquid oxygen that is externally vaporized to fill cylinders at the plant site. A reduction of specific power consumption by 23% to 29% is possible while the main heat exchanger still operates at low pressure. Cylinders can also be filled at any of these pressures from the same plant with a penalty of 2% to 3% in SPC. The paper shows ways to adjust the operation of the air compressor to obtain oxygen at varying pressures with utilization of the thermal energy of liquid oxygen.

Determining design criteria to reduce power and cost in filling high-pressure oxygen cylinders directly from cryogenic air separation plants

Cryogenic air separation plants produce pressurized oxygen gas at 150 bar and 99.8% purity to fill oxygen cylinders directly. It is either produced by compression in oxygen compressor (external compression) or pumping and vaporizing liquid oxygen in main heat exchanger by incoming air (internal compression). Small and medium-scale plants differ in the types of main heat exchanger, compression and expansion devices owing to limitations of market availability. Levelized cost of production of oxygen gas is about 9% less in external compression, though internal compression process is far more fire-safe. Design and operating parameters applicable for internal compression plants supplying oxygen at 40 bar cannot be extended to 150 bar plants due to the differences of fluid properties between super-critical and sub-critical oxygen. UA (surface area times overall heat transfer coefficient) of main heat exchangers, flow and pressure of incoming air are optimized for minimum capital and operating costs. If the chosen flow and pressures deviate substantially from the optimum points, size of main heat exchanger may even increase five-fold. Ideally, 27–29% of total air intake should be compressed between 110 and 100 bar in a medium-scale internal compression plant to keep levelized cost of production at the minimum.

Novel cryogenic argon recovery from the air separation unit integrated with LNG regasification and CO2 transcritical power cycle

Two novel air separation units at cryogenic temperature were proposed to reach high purity nitrogen, oxygen, and argon. The first process refers to a three-column cryogenic air separation plant without using an external refrigeration system. An integrated process including cryogenic air separation, combined-cycle power plants (e.g., transcritical CO2 cycle and gas turbine), and LNG regasification was presented and analyzed as the second process to produce liquid oxygen and vaporize LNG without using external refrigeration source. Results of the first proposed process demonstrate that the specific energy consumption of high purity nitrogen, oxygen, and argon reduces to 18.7%, 13%, and 12% respectively when compared with the conventional processes. Specific energy consumptions and exergy efficiency for the second plant improved by nearly 33% and 16% in comparison with the first process. Also, the gas turbine and CO2 power cycle efficiencies were almost 35% and 45% in the second process. Exergy analysis on both systems demonstrated that expansion valve V-2 (99.42%), high-pressure distillation columns (99.41%), and argon recovery section (98.34%) have the lowest irreversibility and highest exergy efficiency. Meanwhile, the highest exergy destructions in the first and second proposed plants belong to the low-pressure distillation tower of the first process and the combustion chamber of the second process with around 3400 kW and 24,000 kW respectively.

Heat calculation unit in the mathematical model of air separation plant evaporator-condenser

The thermal calculation, which is part of the mathematical model of a condenser-evaporator of cryogenic air separation plants, is considered. The calculation is based on solving the criteria equations of heat exchange for cases of condensation outside of the pipe and evaporation inside the pipe. The thermal calculation is preceded by a hydraulic calculation, the results of which are used as input quantities. The solution is carried out by a multistep (iterative) method. The heat flow density on the evaporation side is used as a criterion for convergence of iterations. The calculation scheme, divided into successive steps, is shown. The calculation results are used to determine the geometric configuration and the main characteristics of the condenser-evaporator.

Optimal design for cryogenic structured packing column using particle swarm optimization algorithm

Large scale cryogenic air separation is the most efficient and cost-effective approach to produce high-purity air products by present. Structured packing columns (SPC) are widely focused and applied due to their characteristics of high efficiency and energy saving in the cryogenic distillation process. The optimal design of the SPC is to reduce the energy consumption and initial investment, while it is a highly nonlinear and multivariable problem. The coexistence of real variables and integer variables, such as the flow rates and the positions of materials at the inlets/outlets, makes the optimization become a typical mixed integer nonlinear programming (MINLP) problem. The purpose of this paper is to study the optimal design method for the cryogenic SPC using the particle swarm optimization (PSO) algorithm. A modified PSO for handling the MINLP problem (MI-PSO) is proposed. A multi-objective optimal design for the SPC in cryogenic air separation plant with the capacity of 17,000 Nm3/h is investigated as a instance. By MI-PSO algorithm, the total exergy loss theoretically reduces 36.3% and the main condenser heat load decreases 5.4% after optimization, which can provide test prediction for the cryogenic distillation experiment.

Design analysis of low pressure distillation column for cryogenic air separation

Cryogenic air separation units are widely used in industry, which consume a large energy and need to produce a huge amount of need of nitrogen, oxygen and argon products with high purity. High purity distillation columns are critical unit operations in cryogenic air separation plants. In this paper the low pressure distillation column is investigated by using a steady state model. The first principles mathematic model of the column is firstly developed, and the pressure, temperature and static liquid nitrogen, oxygen and argon concentrations in the upper column are presented. The obtained results give important information on the variation of temperature, pressure, component concentrations and molar flows within the distillation column. The study highlighted the importance of considering air as ternary mixture. The separation process performance is significantly influenced by the location of side draw of argon and reflux ratio at the top of the column

Comparisons of thermodynamic and economic performances of cryogenic air separation plants designed for external and internal compression of oxygen

Cryogenic air separation plants producing pressurized gaseous oxygen (PGOX) near 40 bara at purities 99.8% and 95% and gaseous nitrogen at 5 bara above 99.99% purity are analyzed. Oxygen is compressed either by external compression process (EC) or by internal compression (IC). IC is gasification of pumped liquid oxygen in the main heat exchanger (MHX). IC plants require appropriate combinations of pressure and flow of air and size of the MHX to vaporize PGOX with reasonable specific power consumption (SPC). Based on exergy and economic analyses of the ASU after simulating the plant in Aspen HYSYS®™8.6, the values of parameters which give high exergy efficiency with low capital and operating expenditures are determined. SPC of IC is 7–8% higher than that of EC. Addition of crude argon column improves the purity of oxygen from 96.3% to 99.8%. Recovery of oxygen improves by 5% with 3–4% reduction of SPC for both types of plants. Capital requirement for both EC and IC plants is about 50% higher with argon separation than without it. HP air between 33% and 31% of total air flow at 70 bara and 80 bara respectively is the most preferred range of operation of IC plants for 95% purity of oxygen. The corresponding flow of HP air is 5% lower for 99.8% purity of oxygen.

Hydraulic calculation unit in the mathematical model of air separation plant evaporator-condenser

The hydraulic calculation, which is part of a mathematical model of a condenser-evaporator of a cryogenic air separation plant, based on the multi-step (iterative) method of equation of the hydrodynamic balance of a circuit with natural circulation.

Pressure-driven dynamic simulation of start up and shutdown procedures of distillation columns in air separation units

Changing power generation from fossil energy sources to renewables poses several challenges to the energy grid. Major energy consumers such as cryogenic air separation plants need to adapt in terms of operational agility and flexibility. Within this project, detailed dynamic models of all air separation unit (ASU) components are presented, which are an important tool to gain a deeper understanding of flexible plant operations. The main focus of this work, which is an extended version of Kender et al. (2018), lies in the dynamic modeling of the cryogenic distillation columns. Hence, a full-order stage-wise model (FOSM) is presented. All flows are pressure-driven, i.e. they result from a quasi-stationary momentum balance. Using this approach, specific scenarios for start-up or load change procedures, such as zero flow condition and reverse flow, can be simulated. To cope with the complexity of this modeling task, an innovative simulation framework is introduced. As case studies, the warm start up and a plant shutdown procedure are presented to demonstrate the ability to simulate the full operating range of an ASU. Warm start up means to initialize the model inventory at ambient conditions with zero flow. The shutdown procedure starts at steady state operation. The feed inlet valves are closed and the simulation is continued until a state of cold stand-by is reached.

Pressure-driven dynamic simulation of distillation columns in air separation units

At the moment, the change in power generation from fossil energy sources to renewables poses several challenges to the energy system and major energy consumers such as cryogenic air separation plants. Due to a more volatile power generation and the lack of storage systems, the energy price fluctuates and power intensive processes need to adapt regarding their operational agility and flexibility.Currently, air separation units are designed for steady state operation at their optimal operating point with a minimum amount of load change procedures during their lifetime.Within this work, detailed dynamic models of all plant components are developed which are an important tool to get a deeper understanding of flexible plant operations. Based on this, the process design can be adapted to allow for a more flexible mode of operation. In addition, those models can be used to develop and optimise advanced load change control strategies, which are more agile and consume less equipment lifetime compared to the state-of-the-art load change procedures. Hence, air separation plants are able to perform more frequent load changes and participate in a fluctuating energy market. Furthermore, these tasks are particularly challenging for air separation units, since they are mainly characterised by a high degree of process integration, high demands on product purity and non-linear column responses during plant start-up.The “warm” start-up procedure is chosen as a representative scenario for any load change procedure, since it is the numerically most challenging simulation. “Warm” start-up means to initialise the model inventory at ambient conditions with zero flow. Based on this scenario, more technically relevant scenarios like the “cold” start-up after a plant shutdown as well as advanced control strategies can be investigated to allow a more flexible and agile operation.

СОВЕРШЕНСТВОВАНИЕ КРИОГЕННЫХ ВОЗДУХОРАЗДЕЛИТЕЛЬНЫХ УСТАНОВОК МАЛОЙ ПРОИЗВОДИТЕЛЬНОСТИ

A comparative analysis of cryogenic and non-cryogenic air separation methods has been carried out. It is shown that the main advantage of membrane and adsorption air separation plants (ASPs) is their relatively low cost and, consequently, a short payback period. Despite significant progress in improving adsorption and membrane ASPs achieved in the last decade, cryogenic air separation plants remain unsurpassed in a number of parameters. To increase the competitiveness of cryogenic ASPs of low capacity, it is necessary to radically reduce their cost. This can be achieved by using gas cryogenic machines to compensate for cold losses in cryogenic ASPs. They do not have the most expensive elements of the Claude-cycle units: medium pressure compressor and expander. We conducted simulation of an air separation plant in the stream simulator «SOSO / ChemSep» for obtaining oxygen at a pressure of 16 MPa, using a gas cryogenic machine operating on the Stirling cycle. Energy consumption to produce one nm3 of production oxygen in such a facility is 1,2 kWh. To obtain one kilogram of liquid nitrogen, 1,32 kWh of electric power is consumed.

Fault-diagnosis of an operating cryogenic air separation plant through steady state simulation

Cryogenic air separation plant is expected to produce nitrogen, oxygen and argon in gaseous and/or liquid forms. Performance of such plants depend on number of components which may malfunction. When an operating plant has a reduced rate of production, increased power consumption and off-the-mark purity of products, the questions that haunt the operators are: which component is malfunctioning? Which parameters ought to be tuned? As the manufacturer almost never shares the geometric details of plant manufacturing, it becomes a challenge for the simulating person to guess the design details. However, once operating parameters match with simulated plant, it may be termed as recreation of the design. In the present work estimated design parameters were used for simulating plant in Aspen HYSYS® (a commercial chemical process simulator) platform until the results matched with output parameters. Results could serve as guidance to the operating engineers to preempt the decrease of plant performance.

Dynamic modeling and collocation‐based model reduction of cryogenic air separation units

High purity distillation columns and multi‐stream heat exchangers (MSHXs) are critical units in cryogenic air separation plants. This article focuses on modeling approaches for the primary section of a super‐staged argon plant. A full‐order stage‐wise model for distillation columns in air separation units (ASUs) that considers key process phenomena is presented, followed by a reduced‐order model using a collocation approach. The extent of model reduction that can be achieved without losing significant prediction accuracy is demonstrated. A novel moving boundary model is proposed to handle MSHXs with phase change. Simulation results demonstrate the capability of the proposed model for tracking the phase change occurrence along the length of the heat exchanger. Dynamic simulation studies of the integrated plant show that the thermal integration between the feed and product streams captured in the primary heat exchanger is critical to accurately capture the behavior of ASUs. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1602–1615, 2016

Production Scheduling of an Air Separation Plant

Cryogenic air separation plants consume a large amount of electricity to produce various gaseous and liquid products; they can reduce their operational cost by proper exploitation of energy contracts and intelligent utilization of liquid products. In this paper, we propose a State Task Network(STN) based model which replicates a representative air separation plant. This STN based representation owing to its significantly more granular modelling can reap higher benefits in terms of improved decision making than the approaches proposed in literature. Production Scheduling based on this model computes the optimal operating conditions for the plant under variable power pricing options and demand scenarios. The modelling framework is very rigorous and includes almost all the real world limitations and constraints in air separation plant operation. This unit wise scheduling approach provides more flexibility and also pave the way for integration of the scheduling layer with the lower layered (hierarchically) control system. The proposed framework has been evaluated on the representative air separation plant and the results demonstrate the benefits of optimal plant operation.