Tracer Applications of Noble Gas Radionuclides in the Geosciences

The noble gas radionuclides, including 81Kr (half-life = 229,000 yr), 85Kr (11 yr), and 39Ar (269 yr), possess nearly ideal chemical and physical properties for studies of earth and environmental processes. Recent advances in Atom Trap Trace Analysis (ATTA), a laser-based atom counting method, have enabled routine measurements of the radiokrypton isotopes, as well as the demonstration of the ability to measure 39Ar in environmental samples. Here we provide an overview of the ATTA technique, and a survey of recent progress made in several laboratories worldwide. We review the application of noble gas radionuclides in the geosciences and discuss how ATTA can help advance these fields, specifically determination of groundwater residence times using 81Kr, 85Kr, and 39Ar; dating old glacial ice using 81Kr; and an 39Ar survey of the main water masses of the oceans, to study circulation pathways and estimate mean residence times. Other scientific questions involving deeper circulation of fluids in the Earth's crust and mantle also are within the scope of future applications. We conclude that the geoscience community would greatly benefit from an ATTA facility dedicated to this field, with instrumentation for routine measurements, as well as for research on further development of ATTA methods

A MODEL TO OPTIMIZE A CRYOGENIC SEPARATION SYSTEM WITH INNOVATIVE HYBRID DISTILLATION MEMBRANE IN SERIES

The study developed a model for hybrid distillation membrane that optimized the energy usage in binary cryogenic separation. In separating air mixture components, distillation columns are often used and these columns consumes very large energy during operation. From analysis, the exergy efficiency and heat transfer of a cryogenic air separation double diabatic column in the distillation process is greater than that of the conventional adiabatic double columns. There is need to discover alternative separation technologies with lesser energy consumption such as membrane separation. However, use of membrane separation alone is constrained to small separation due to large areas needed with the attendant costs. Thus, a hybrid system comprising of distillation column and membrane separator offers the best compromise. To optimize the process, the overhead product from the distillation column was fed to a membrane separator in series in this study. A mathematical Model approach was proposed to improve a hybrid separation system comprising of a distillation column and a Serial novel membrane separation unit. First, a model was introduced that validated if the hybrid system could optimize the process and the order of magnitude of energy that can be expected. Secondly, a superstructure optimization approach was applied and it uses rigorous models for both the column and the membrane. A process simulator, excel and visual basic were used to solve and program the equations. The result showed that significant energy savings was achieved using a novel hybrid separation system with a material membrane

Potential for improving the energy efficiency of cryogenic air separation unit (ASU) using binary heat recovery cycles

In this paper, the potential of improving the energy efficiency of a conventional cryogenic air separation unit (ASU) was investigated through modelling and simulation using Aspen Plus® v 8.1. It is achieved through converting the heat from the compressor effluent to electricity using organic Ranking cycle (ORC). Two different arrangements of combining compressor and waste heat recovery ORC system were compared with the conventional cryogenic ASU which was used as the benchmark. The benchmark is a conventional cryogenic ASU with 3 stages of compression which uses water for intercooling. In the first arrangement the water used as the cooling fluid of the intercooler/after cooler heat exchanger of a conventional cryogenic ASU process was replaced by R134a which also acts as the working fluid for the ORC system (C3WHR) while in the second arrangement, the 3 stages compressor of the conventional process was replaced with a single stage compressor with the same overall pressure ratio as the conventional process and the hot compressor effluent cooled with R134a which also acts as the working fluid of the ORC system (C1WHR). The simulation results based on a cryogenic ASU capable of processing 100 kg/s of atmospheric air at 30 °C as feedstock show that the specific power consumption for the pure products which was 0.32 kWh/kg, 0.37 kWh/kg and 17.35 kWh/kg for oxygen, nitrogen and argon respectively for the conventional cryogenic ASU process was reduced by the addition of the waste heat recovery ORC system. The C1WHR reduced the specific power consumption by an average of 0.2% across the aforementioned pure products while the C3WHR reduced it by an average of 11%. The net power consumption of the conventional cryogenic ASU which was 21826.19 kW was also found to be reduced by the same percentage

Model-based Design, Operation and Control of Pressure Swing Adsorption Systems

This thesis is concerned with the design, operation and control of pressure swing adsorption (PSA) systems, employing state of the art system engineering tools. A detailed mathematical model is developed which captures the hydrodynamic, mass transfer and equilibrium effects in detail to represent the real PSA operation. The first detailed case study presented in this work deals with the design of an explicit/multi-parametric model predictive controller for the operation of a PSA system comprising four adsorbent beds undergoing nine process steps, separating 70 % H2, 30 % CH4 mixture into high purity hydrogen. The key controller objective is to fast track H2 purity to a set point value of 99.99 %, manipulating time duration of the adsorption step, under the effect of process disturbances. To perform the task, a rigorous and systematic framework is employed comprising four main steps of model development, system identification, the mp-MPC formulation, and in-silico closed loop validation, respectively. Detailed comparison studies of the derived explicit MPC controller are also performed with the conventional PID controllers, for a multitude of disturbance scenarios. Following the controller design, a detailed design and control optimization study is presented which incorporates the design, operational and control aspects of PSA operation simultaneously, with the objective of improving real time operability. This is in complete contrast to the traditional approach for the design of process systems, which employs a two step sequential method of first design and then control. A systematic and rigorous methodology is employed towards this purpose and is applied to a two-bed, six-step PSA system represented by a rigorous mathematical model, where the key optimization objective is to maximize the expected H2 recovery while achieving a closed loop product H2 purity of 99.99 %, for separating 70 % H2, 30 % CH4 feed. Furthermore, two detailed comparative studies are also conducted. In the first study, the optimal design and control configuration obtained from the simultaneous and sequential approaches are compared in detail. In the second study, an mp-MPC controller is designed to investigate any further improvements in the closed loop response of the optimal PSA system. The final area of research work is related to the development of an industrial scale, integrated PSA-membrane separation system. Here, the key objective is to enhance the overall recovery of "fuel cell ready" 99.99 % pure hydrogen, produced from the steam methane reforming route, where PSA is usually employed as the purification system. In the first stage, the stand-alone PSA and membrane configurations are optimized performing dynamic simulations on the mathematical model. During this procedure, both upstream and downstream membrane configuration are investigated in detail. For the hybrid configuration, membrane area and PSA cycle time are chosen as the key design parameters. Furthermore, life cycle analysis studies are performed on the hybrid system to evaluate its environmental impact in comparison to the stand-alone PSA system

Efficient Nonlinear Optimization with Rigorous Models for Large Scale Industrial Chemical Processes

Large scale nonlinear programming (NLP) has proven to be an effective framework for obtaining profit gains through optimal process design and operations in chemical engineering. While the classical SQP and Interior Point methods have been successfully applied to solve many optimization problems, the focus of both academia and industry on larger and more complicated problems requires further development of numerical algorithms which can provide improved computational efficiency. The primary purpose of this dissertation is to develop effective problem formulations and an advanced numerical algorithms for efficient solution of these challenging problems. As problem sizes increase, there is a need for tailored algorithms that can exploit problem specific structure. Furthermore, computer chip manufacturers are no longer focusing on increased clock-speeds, but rather on hyperthreading and multi-core architectures. Therefore, to see continued performance improvement, we must focus on algorithms that can exploit emerging parallel computing architectures. In this dissertation, we develop an advanced parallel solution strategy for nonlinear programming problems with block-angular structure. The effectiveness of this and modern off-the-shelf tools are demonstrated on a wide range of problem classes. Here, we treat optimal design, optimal operation, dynamic optimization, and parameter estimation. Two case studies (air separation units and heat-integrated columns) are investigated to deal with design under uncertainty with rigorous models. For optimal operation, this dissertation takes cryogenic air separation units as a primary case study and focuses on formulations for handling uncertain product demands, contractual constraints on customer satisfaction levels, and variable power pricing. Multiperiod formulations provide operating plans that consider inventory to meet customer demands and improve profits. In the area of dynamic optimization, optimal reference trajectories are determined for load changes in an air separation process. A multiscenario programming formulation is again used, this time with large-scale discretized dynamic models. Finally, to emphasize a different decomposition approach, we address a problem with significant spatial complexity. Unknown water demands within a large scale city-wide distribution network are estimated. This problem provides a different decomposition mechanism than the multiscenario or multiperiod problems; nevertheless, our parallel approach provides effective speedup

Evaluation of the performance and economic viability of a novel low temperature carbon capture process

A novel Advanced Cryogenic Carbon Capture (A3C) process is being developed using low cost but high intensity heat transfer to achieve high CO2 capture efficiencies with a much reduced energy consumption and process equipment size. These characteristics, along with the purity of CO2 product and absence of process chemicals, offer the potential for application across a range of sectors. This work presents a techno-economic evaluation for applications ranging from 3% to 35%vol. CO2 content. The A3C process is evaluated against an amine-based CO2 capture process for three applications; an oil-fired boiler, a combined cycle gas turbine (CCGT) and a biogas upgrading plant. The A3C process has shown a modest life cost advantage over the mature MEA technology for the larger selected applications, and substantially lower costs in the smaller biogas application. Enhanced energy recovery and optimization offer significant opportunities for further reductions in cost

Analysis of steady state Cryogenic Air Separation unit of Rourkela Steel Plant and simulation of Fixed Bed Adsorption Separation of Air

Atmospheric dry air contains approximately 78% nitrogen, 21% oxygen, and 1% argon plus low concentrations of noble gases like carbon dioxide, hydrocarbons and other impurities. An air separation unit divides atmospheric air into the three pure gaseous components (nitrogen, oxygen and argon). Further separation may be performed on some plants to produce other gases such as krypton, neon and xenon. Other gas components of atmospheric air, such as carbon dioxide, water vapour and hydrocarbons must be removed to ensure safety, product quality and efficient plant operation. Nitrogen, oxygen and argon are used by industry in large quantities and hence termed industrial gases. The current work aim is to simulate the cryogenic air separation unit including adsorber and cryogenic distillation. Simulation of absorber is carried out using ADSIM of Aspen Tech to remove carbon dioxide (CO2) and water vapour (H2O). The breakthrough curves of carbon dioxide (CO2) and water vapour on 5A molecular sieve and activated alumina respectively are found at different Reynolds number. The study helps to find out schedule time adsorber/desorber unit. ASPEN Plus simulator is used to simulate cryogenic air separation into nitrogen, oxygen and argon. The steady-state simulation results (purity) are compared to Rourkela steel plant real data

Optimized Design of Shale Gas Processing and NGL Recovery Plant under Uncertain Feed Conditions

Shale gas is an increasingly booming resource and it has been predicted to increase from 1% in 2000 to 40% in 2035 of the total US domestic gas produced. Since shale gas is both industrially economical and environmentally clean compared to oil or coal as a resource, many studies are focused on developing technologies to monetize shale gas. However, one of the key challenges in utilizing shale gas is its fluctuating flow rate and compositional behavior. The flow rate of a shale gas well dwindles over a period of time and the composition of shale gas differs from well to well in the same shale play. This provides a challenge in designing a plant of optimum size for a shale gas processing and NGL recovery plant. In this study, this uncertainty in shale gas feed flow rate and composition is addressed while designing a shale gas processing and NGL recovery plant. First, different shale gas flow rates are chosen over a period of shale gas well life based on the average shale gas rate declination curve of a shale play. Second, two different process flow sheets are developed (i) using conventional technology and (ii) using novel technology. In the novel technology, the NGL recovery section of the conventional technology is modified to accommodate novel changes such as using a divided wall column or pre-fractionated sequence to separate methane, ethane, and propane. Later, these process flow sheets are simulated in Aspen plus for comparing the economics of different plant sizes. Furthermore, heat integration and optimization of individual units of the process flow sheets are carried out using pinch and sensitivity analyses, respectively. Lastly, the economic analysis of a plant of optimum size with constant feed flow rate over its plant life is evaluated. In this case, shale gas from different wells is collected in a header and adjusted such that the shale gas flow rate is constant to the plant. Environmental impact of the process is also observed. From the economic analysis of various cases for conventional and novel technology, it is observed that case-3 provides the optimum plant design with highest ROI percentage compared to other cases and for case-3, novel technology ROI is 4.17% more compared to conventional technology. Finally, constant production rate case, at the flow rate of case-3, the ROI percentage is observed to be more than minimum requirement implying that this processing plant is economically viable to implement