Optimal operation of cryogenic air separation systems with demand uncertainty and contractual obligations

Cryogenic air separation is an efficient technology for supplying large quantities of nitrogen, argon, and oxygen to chemical, petroleum and manufacturing customers. However, numerous uncertainties make effective operation of these complex processes difficult. This work addresses the problem of determining an optimal operating strategy to maximize the total profit of a cryogenic air separation process while considering demand uncertainty and contractual obligations. A rigorous process model is included as constraints in a nonlinear programming formulation. Uncertain demands are assumed to be normally distributed with known mean and standard deviation, and expected profit in the objective function is evaluated using the standard loss function. A probabilistic fill-rate expression, also based on the loss function, is used to model the contractual obligations by providing a lower bound on the expected product sales. In the single period case with one customer satisfaction constraint, the nonlinear programming formulation can be solved efficiently using the general purpose nonlinear optimization package, Ipopt. This formulation is then extended to include multiple time periods, the potential for product storage, and customer satisfaction constraints on multiple products. To solve the large-scale nonlinear programming formulation that considers a seven-day operating horizon, a tailored parallel nonlinear programming algorithm is used. This approach makes use of a Schur complement decomposition strategy to exploit the block structure of the problem and allow efficient solution in parallel. Using these tools, we solve for a set of optimal operating strategies over the complete space of different fill rates. This produces planning figures that identify key trade-offs between profitability and contractual obligations.     

A flexible air separation process: 1. Design and steady-state optimizations

The flexible operation of energy-intensive processes, such as cryogenic air separation, has economic potential due to increasing fluctuations of the electricity markets. Multiproduct air separation processes with high ratios of liquid product are very promising for flexible operation due to storable products. We present a process design with an integrated liquefication cycle and liquid assist operation, that facilitates a high liquid product ratio and a flexible process operation. We use a mechanistic dynamic process model in steady-state process optimizations covering the wide operational range of the proposed process. The optimization results show that the power demand can be varied in a range from 3.5 to 28 MW without violating operational constraints by changing the nitrogen and oxygen production rates. Thus, the proposed process is a promising air separation candidate for flexible operation with respect to fluctuating electricity markets.     

MPEC strategies for cost optimization of pipeline operations

This study develops a mathematical program with equilibrium constraints (MPECs) approach for efficient operation of gas pipelines. The resulting model handles time dependent operations in order to determine minimum energy consumption and operating cost over a given time horizon. The MPEC structure also allows flow reversals, flow transitions and other nonsmooth elements to be incorporated within the approach. Applied to industrial gas pipelines, this approach can also deal with customer demand satisfaction in the presence of compressor outages and minimize recovery time for systems that are unable to meet customer demands at all times. A large-scale oxygen pipeline case study is considered to demonstrate this approach and complex energy pricing schemes are also applied to this problem. These schemes include time of day electricity pricing, along with extensions to Real Time Pricing and Day Ahead Pricing. Compared to flat rate and minimum energy optimizations, respectively, we observe operating cost savings up to 5.13% for time of day electricity pricing and up to 12.85% for Real Time Pricing.     

深冷空分循环冷却水的处理

油、水、电是空分生产装置中最关键的3个因素,水系统结垢会造成热交换和生产效率的降低,甚至引起装置停车."冷结垢"是深冷空分装置中冷却水系统普遍存在的故障,困扰着许多深冷空分生产企业.通过对"冷结垢"产生原因进行分析,适当调整循环冷却水系统的运行条件,并使用合理的水处理方案和配套药剂,有效解决了多家生产企业存在的"冷结垢"现象.     

Synthesis and optimization of membrane cascade for gas separation via mixed‐integer nonlinear programming

Currently, membrane gas separation systems enjoy widespread acceptance in industry as multistage systems are needed to achieve high recovery and high product purity simultaneously, many such configurations are possible. These designs rely on the process engineer's experience and therefore suboptimal configurations are often the result. This article proposes a systematic methodology for obtaining the optimal multistage membrane flow sheet and corresponding operating conditions. The new approach is applied to cross‐flow membrane modules that separate CO₂ from CH₄, for which the optimization of the proposed superstructure has been achieved via a mixed‐integer nonlinear programming model, with the gas processing cost as objective function. The novelty of this work resides in the large number of possible interconnections between each membrane module, the energy recovery from the high pressure outlet stream and allowing for nonisothermal conditions. The results presented in this work comprise the optimal flow sheet and operating conditions of two case studies. © 2017 American Institute of Chemical Engineers AIChE J, 63: 1989–2006, 2017     

Sidestream Analysis and Optimization of Full Tower Internal Thermally Coupled Air Separation Columns

An appropriate sidestream is the key to improve the yields and purities of products in the cryogenic air separation process. The sidestream of a full tower internal thermally coupled air separation column is characterized. Sensitivity analysis indicates that heat duty distribution, purities, and yields of the products are strongly influenced by the withdrawn position and the flow rate of the sidestream. Optimization models are proposed by adopting the minimum flow rate of the low‐purity liquid nitrogen product and with maximum flow rates of the oxygen product or nitrogen product as optimization target. The optimization research allows selecting the optimum operating conditions with different production requirements.     

Optimal operation of the pressure swing adsorption (PSA) process for CO<Subscript>2</Subscript> recovery

The operation of PSA (Pressure Swing Adsorption) processes is a highly nonlinear and challenging problem. We propose a systematic procedure to achieve the optimal operation of a PSA process. The model of the PSA process for CO₂ separation and recovery is developed first and optimization is performed to identify optimal operating conditions based on the model. The effectiveness of the model developed is demonstrated by numerical simulations and experiments using CO₂ and N₂ gases and zeolite 13X. Breakthrough curves and temperature changes in the bed are computed from the model and the results are compared with those of experiments. The effects of the adsorption time and reflux ratio on the product purity and the recovery are identified through numerical simulations. The optimization problem is formulated based on nonlinear equations obtained from simulations. The optimal operating conditions identified are applied to experiments. The results show higher recovery of CO₂ under optimal operating conditions.     

Improving the load flexibility of industrial air separation units using a pressure-driven digital twin

Air separation units are one of the prime examples for studies on demand side management and (non-)linear model predictive control due to their high power consumption and energy storage potential. These plants separate ambient air into its main components, nitrogen, oxygen, and argon, by means of cryogenic distillation at different pressure levels. Approximately two thirds of the industrially operated air separation units consider the separation of argon either as a value product or for reasons of energy efficiency. However, most of the studies in literature neglect the separation of argon since this requires additional equipment, increases the heat and process integration and, thus, the complexity of process control. In this work, a digital twin of an air separation unit with argon system is used to analyze and to improve load change procedures. Moreover, the potential of applying the digital twin as a soft sensor is demonstrated.     

Air Fractionation by Adsorption

Abstract

Pressure swing adsorption (PSA) processes for air separation differ by the modes and conditions of operation of the adsorption, the desorption, and the complementary steps, as well as by the types of adsorbents used. Three commercial PSA processes for air separation are reviewed and compared. The first process uses a zeolitic adsorbent and produces only an oxygen-enriched product gas. The second process uses a carbon molecular sieve and produces only a nitrogen-enriched product gas. The third process uses a zeolite and simultaneously produces both oxygen-and nitrogen-enriched product gases. The performance and separation efficiency of the last process, called the ‘vacuum swing adsorption (VSA) process’, are reported to be superior to the others.     

Microbubble Generation with the Aid of a Centrifugal Pump

For saturation of water treated with atmospheric air in a dissolved air flotation (DAF) system and the production of microbubbles, air was introduced through the suction pipe of a centrifugal pump. The operating conditions of the centrifugal pump were adjusted to avoid the occurrence of cavitation phenomena. The novel technique was applied for the treatment of oily water in a DAF pilot prototype. A central composite rotational design (CCRD) was used to determine the high efficiency of the separation process. In addition to this high degree of oil‐water separation, the technique reduces space and energy costs in comparison to conventional processes of saturation of the effluent treated with compressed air.     

Chiral separation using a novel combination of cooling crystallization and a membrane barrier: Resolution of DL-glutamic acid

Chiral separation of racemic mixtures is essential in the production of many pharmaceutical compounds. The present work describes a novel chiral separation technique that combines cooling crystallization and a membrane separation that is used in DL-glutamic acid resolution. The process utilizes two crystallization chambers that are separated by a membrane that prevents transport of crystals from one chamber to another. Importantly, conditions must be controlled so that only a pure species crystallizes in each of the chambers. This is done by appropriate addition of seed crystals to each chamber and by restricting the formation of new crystals to secondary nucleation mechanisms. The seed crystals may grow or participate in secondary nucleation, but conditions must be controlled so as to prevent primary nucleation, which would result in the formation of both crystal species in each chamber. Experiments were conducted with different amounts of seed crystals to determine operating conditions that produce the high product yield and purity. The results show that this novel chiral separation process is promising: the product purity was over 94% (with a separation factor of 16) and the product yield was increased by as much as 56% more than could be obtained with simple cooling crystallization.     

由灵武煤新法干馏半焦制备炭分子筛的研究

以灵武不粘煤的固体热载体新法干馏半焦为原料，综合使用碳化法和碳沉积法进行了空分富氮用炭分子筛的制备研究，考察了碳化和碳沉积工艺条件对产品空分性能的影响，并用ＦＴＩＲ和ＸＲＤ技术对产品炭分子筛进行了表片。结果表明，灵武煤的新法干馏半焦是制备炭分子筛的优良产；在最佳工艺条件下制得的炭分子筛的空分性能可与进口的同类产品相媲美；炭分子筛是典型的无定形炭，其表面含氧官能团主要是Ｃ－Ｏ键和Ｃ＝Ｏ键。     

HT—L粉煤加压气化示范项目空分装置简介

介绍了与航天炉粉煤加压气化示范装置配套的空分装置的工艺流程选择、设备及产品的用途。膨胀空气进上塔挂小粗氩塔流程具有能耗低、运行费用少、安全可靠、操作简单、维护工作量小等优点。整套空分装置投运1年多来,运行正常、稳定,为航天炉粉煤加压气化装置连续稳定运行提供了保障。     

Solvent-based fatty alcohol synthesis using supercritical butane: Flowsheet analysis and process design

The liquid-phase hydrogenolysis of fatty esters to fatty alcohols is an important step in the industrial manufacture of surfactants and detergents. High operating pressures are necessary, due to the low solubility of hydrogen in fatty esters feeds. In principle, these high operating pressures might be overcome by use of a suitable solvent, but only at the expense of large solvent recycle and cumbersome product-solvent separation. The employ of supercritical solvents may resolve these drawbacks, as an elegant solvent-product separation is possible by reverting to the subcritical regime. In the present work the hydrogenolysis of methyl palmitate in supercritical butane is investigated by simulation. Operating conditions are analyzed on the basis of vapor liquid equilibrium data and chemical equilibrium considerations. Separation and recycle problems are evaluated and discussed on the basis of a flowsheet analysis. It is demonstrated that an efficient hydrogenolysis process may be developed by using supercritical butane as solvent. A moderate operating pressure (9 MPa) and temperature (470 K) lead to high conversion levels and high product purity. A hydrogen to ester molar ratio of 4∶1 in the feed is achievable, which compares favorably to existing liquid-and gas-phase processes, and allows recycle streams to be reduced.     

Use of Neural Networks in the Simulation and Optimization of Pressure Swing Adsorption Processes

In this work a method for the simulation and optimization of a pressure swing adsorption process for the separation of nitrogen from air by using neural networks was developed. The model is used to obtain a prediction for the process performance, namely, the specific product and yield, over a wide range of operating conditions. These results are compared with the predictions from a mass tranfer model, and a very good agreement is found. The network developed is also used to minimize a cost objective function, and it is shown that it can easily be used in process optimization and/or control.     

Combination of Sorption-Enhanced Steam Methane Reforming and Electricity Generation by MCFC: Concept and Numerical Simulation Analysis

Abstract

Reformed gas made by the steam methane reforming(SMR) process is used as fuel feed to MCFC, but it is not as good as pure hydrogen due to the presence of CO₂ and CO. The sorption-enhanced steam methane reforming(SE-SMR) process can reduce CO₂ and CO to a low level and produce high purity hydrogen. Considering the merits of similar operating temperatures (about 500°C) and carbon dioxide recycle, a novel concept of a six-step sorption-enhanced steam methane reforming (SE-SMR) combined with electricity generation by molten carbonate fuel cell (MCFC) is proposed. In the present paper, a cycle of the SE-SMR process, which include the steps of reaction/adsorption, depressurization, gas purges (nitrogen and reformed gas, respectively), and pressurization with reformed gas, is modeled and analyzed. The process stream in the SE-SMR process is used as anode feed in MCFC. According to the result of numerical simulation, a fuel cell grade hydrogen product (above 80% purity) at the SE-SMR temperature of 450°C can be obtained. A carbon dioxide recycle mechanism is developed for cathode feed of MCFC from flue gas by burning with excess air to achieve a proper CO₂/air ratio (about 30:70). The novel electricity generation system, which can operate at lower energy consumption and high purity hydrogen feed is helpful for the MCFC'S performance and life time.     

Optimization of Drying of Olive Leaves in a Pilot-Scale Heat Pump Dryer

Recently, the interest in olive leaf has increased due to its high phenolic content. It has a high potential for industrial exploitation in food industry and the main process in olive leaf treatment is drying. Drying affects the product quality and is an energy-intensive process, so the use of heat pumps in drying processes that have low operating cost has attracted the attention of the investigators. In this study, response surface methodology was used to optimize operating conditions of drying of olive leaves in a pilot-scale heat pump conveyor dryer. The independent variables were air temperature, air velocity, and process time, and the responses were total phenolic content and antioxidant activity loss, final moisture content, and exergetic efficiency. Optimum operating conditions were found to be temperature of 53.43°C, air velocity of 0.64 m/s, process time of 288.32 min. At this optimum point, total phenolic content loss, total antioxidant activity loss, final moisture content, and exergetic efficiency were found to be 9.77%, 44.25%, 6.0% (w.b.), and 69.55%, respectively.     

Analysis of the Behavior of the Simulated Moving Bed Reactor in the Sucrose Inversion Process

Abstract

The simulated moving bed reactor (SMBR) is a device in which reaction and separation processes take place simultaneously. The separation of products allows higher conversion and high‐purity product can be also obtained. In this work, a mathematical model has been presented to predict the behavior of the SMBR in the sucrose inversion process. For this process, the triangular region which defines operating conditions to recover high‐purity products in SMBR has been obtained using two modeling strategies. The set of partial differencing equations is solved by finite volume method. The influence of some operation conditions on the reactor performance is analysed for the sucrose inversion process.     

Global optimization of an industrial gas network operation

Air Liquide operates several industrial gas pipeline networks around the world, connecting air separation plants to customers of industrial gases. The operation of such a network of plants, pipelines, and customers is complicated due to fluctuating electricity prices and customer demands. We describe a complex industrial problem for real‐time optimization of network operations in the presence of these challenges. We then summarize a concerted modeling and algorithmic effort toward global optimization of this model. The resulting advances include development of a regression‐based fully‐deterministic nonconvex optimization model, a tool for diagnosing infeasibilities during model development, reformulations and scaling to make the model more amenable for optimization, and development of strengthened relaxations for its efficient solution. We provide details on the development of these tools and techniques that facilitated the solution of this model in a reasonable computational time with the global solver BARON. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3215–3224, 2016     

Evaluating the demand response potential of ammonia plants

Demand-side management/demand response (DSM/DR) are key strategies for mitigating the inherent variability in electricity generation rates by renewable sources. This article represents—to our knowledge—the first foray into assessing the DR potential of ammonia plants. Ammonia plants are interesting candidates for DR initiatives because of their significant electricity use (for operating compressors driving the synthesis loop) and the ability to store the ammonia product relatively easily and safely. Our approach is based on formulating and solving an optimal DR scheduling problem for an ammonia plant while accounting for the process dynamics. To this end, we introduce a new Hammerstein–Wiener-inspired modeling framework based on injecting linear dynamics in a first-principles static nonlinear model of the process. The results are encouraging; for the cases considered, peak-time power consumption decreases between 3.57% and 7.40%, coupled with 1.39% to 3.70% reductions in operating cost.