Cryogenic Natural Gas Research Articles

Experimental solid–liquid equilibria and solid formation kinetics for carbon dioxide in methane for LNG processing

AbstractIn cryogenic liquefaction processes, CO2 poses a solid‐formation risk even in trace concentrations. We present solid–fluid equilibrium (SFE) data for CO2 in liquid methane at CO2 concentrations from (52 to 500) ppm, extending the available data and indicating that models tuned to existing data over predict the solubility of CO2 at LNG storage temperatures (~112 K) by nearly a factor of 3. The new data are used to improve the SFE model in the ThermoFAST software package. The formation kinetics of CO2 solids in liquid methane are elucidated at conditions relevant to cryogenic gas processing. Repeated, ramped‐temperature formation measurements provide a statistical basis for quantifying solidification risk. Nucleation rates extracted from the ramped‐temperature data, consistent with those measured at fixed temperature, were used to extract parameters describing CO2 solid formation in methane. These results significantly improve the ability to predict CO2 solid formation risk in cryogenic natural gas processing.

Extended Natural Gas Characterization Method for Improved Predictions of Freeze-Out in LNG Production

The formation and the blockage of plant equipment such as heat exchangers by heavy hydrocarbon (HHC) solids is an inherent risk in cryogenic natural gas processing. The accuracy of the gas mixture’s compositional characterization significantly impacts the reliability of solid formaiton temperature predictions. Recently, we showed that complete characterization of the mixture is necessary to obtain accurate predictions of the melting temperature, as current methods based on pseudocomponent characterizations of HHCs are inadequate. Here, we present an improved method of characterizing HHCs that represents each pseudocomponent up to C14+ by a paraffinic, isoparaffinic, naphthenic and aromatic (PINA) composition and allocates an associated defined component to represent these sub-fractions. This new, extended PINA-based characterization of HHC pseudocomponents is derived from 46 different pipeline natural gas samples, and the method is validated against three representative gas samples that were fully characterized. The melting temperatures of the three gas samples based on their full characterizations are 263.2 K (14.1 °F), 260.1 K (8.5 °F) and 248.3 K (−12.8 °F), respectively. Predictions made with the new method match these within (1 to 2) K, while previous correlation methods under-predict them by (10 to 20) K. The improved performance arises from (1) the selection of suitable discrete components to represent each PINA fraction within a pseudocomponent, (2) the more representative distribution of PINA fractions as a function of carbon number, and (3) the use of discrete components to represent the pseudocomponent’s thermodynamic properties in both the fluid and solid phases. These results show how the new characterization method can reliably predict HHC freeze-out conditions, particularly when a full compositional analysis is unavailable. Future research should aim to test the new method on natural gas samples from regions other than the US Gulf Coast.

Retrofitting recycled stripping gas in a glycol dehydration regeneration unit

Abstract Natural gas dehydration is essential in gas processing to avoid serious problems. As a pretreatment in a cryogenic Natural Gas Liquid (NGL) recovery process, it typically uses triethylene glycol (TEG) and followed by a Molecular Sieve dehydration to achieve 1 mg/Sm3 of water moisture in the dehydrated gas. This work studied the retrofitting of the existing dehydration unit to improve its performance in satisfying the gas moisture qualities. The retrofitted process uses recycled stripping gas schemes to achieve high purity TEG while minimizing the use of fresh stripping gas. The results revealed that the recycled stripping gas has provided sufficiently high purity TEG (>99.99%-wt), significantly reduced the heating and cooling duty by 80%, and reduced the electrical duty by 29% compared to the base case. The TAC was reduced by 38.1% from $ 725,245/year to $ 448,670/year. Through this study, the evaluated cases provide similar dehydration results with less equipment, simpler process, more energy-efficient, and better economic numbers. Therefore, a better process was obtained.

Energy, exergy and pinch analyses of an integrated cryogenic natural gas process based on coupling of absorption–compression refrigeration system, organic Rankine cycle and solar parabolic trough collectors

The natural gas entering the liquefaction cycle usually consists of nitrogen, ethane, propane and also heavier hydrocarbons which are economically explainable to be separated from methane, considering that their heating values are higher than methane. In this paper, a hybrid system is developed and analyzed for liquefied natural gas, natural gas liquids and power tri-generation using LNG/NGLs recovery system, absorption–compression combined refrigeration, organic Rankine cycle and solar parabolic trough collectors. This integrated structure produces 54.12 kg s−1 NGLs, 66.52 kg s−1 LNG and 278.5 MW net power output. Specific power consumption, thermal and exergy efficiencies of the hybrid system are 0.3771 kWh kg−1 LNG, 78.38% and 84.47%, respectively. The pinch method is used to extract the heat exchanger network related to the multi-stream heat exchanger of the hybrid system. To simulate the integrated structure, MATLAB programming, HYSYS and TRNSYS software with the weather conditions of Bandar Abbas city in Iran are used. The effect of natural gas composition entering the cycle on system parameters is studied and reported. Results show that with the reduction in methane percentage in natural gas to 55 mol%, specific power consumption increases to 0.6004 kWh kg−1 LNG, and thermal efficiency decreases to 71.61%. The integrated structural behavior at different operating conditions is used to investigate the sensitivity analysis.

A generic concept for Helium purification and liquefaction plant

This study describes and evaluates the performance of producing a pure Helium fraction from Helium extraction facility designed for cryogenic natural gas plants. A generic concept for obtaining a Helium pure fraction, which has relatively lower capital and operating costs should be provided. In order to achieve this objective, a new concept for obtaining a Helium pure fraction from a crude Helium fraction, is proposed based on simulations run under diverse process conditions regarding crude Helium gas’ temperature, pressure and composition. This concept is characterized by; reducing the plant safety requirements due to the extensive separation of combustible components, and compact layout of Helium extraction plant. Further re-purification is included in the subsequent Helium liquefaction step through selective adsorption, hence then increasing the purity of the Helium product and reducing the plant energy consumption required for liquefying Helium-rich fraction and the valuable Helium boil-off routed from the storage facility. The Nitrogen-rich fraction is routed to Nitrogen liquefaction installation. Liquid Nitrogen is generated within Helium recovery facility for liquid Helium shielding and container cooling. Surplus gaseous Nitrogen either can be liquefied and used within cryogenic natural gas plant as process coolant or be vented to atmosphere.

Advanced predictions of solidification in cryogenic natural gas and LNG processing

The formation and deposition of solids during the cryogenic processing of natural gas is a perennial risk for operators. Several tools are available for predicting the temperatures at which heavy hydrocarbon solids will form in cryogenic processing equipment; of these the Kohn and Luks Solids Solubility Program (KLSSP) from GPA Midstream has become an industry standard tool for predicting solid-fluid equilibria (SFE) in cryogenic processes. However, although it describes well many of the data sets generated as part of the GPA’s research program in the 1970s and 1980s, the KLSSP suffers from limitations including fixed ranges of temperature, mixture composition, no dependence on pressure and a limited set of possible freeze-out components. Here, a new software tool called ThermoFAST is presented, which overcomes these limitations and has been endorsed by GPA Midstream to replace the KLSSP. This model uses a cubic equation of state and efficient flash algorithms to enable rapid calculations of solid-liquid, solid-vapour, and solid-liquid-vapour equilibrium conditions in addition to normal vapour-liquid phase envelopes. The model has been tuned to the available solid-fluid equilibrium literature data for 58 binary mixtures and is able to represent them with an average root-mean-squared temperature deviation of only 1.7 K. It was thoroughly tested against available SLE data for multi-component LNG mixtures, and improved the accuracy of predicted melting temperatures by a factor of 7 relative to KLSSP, with an average rms deviation of only 2.0 K. The rapid flash algorithm was used to identify 26 distinct transition pathways involving solidification phenomena in LNG-relevant fluids: this includes the prediction of retrograde solidification in multiple systems (including the methane + benzene binary) where a decrease in system temperature produces a reduction in the amount of solid phase present.

Assessing the energy intensity of alternative chemical and cryogenic natural gas purification processes in LNG production

The production of Liquefied Natural Gas (LNG) from natural gas reservoirs with high content of acidic compounds is expected to be a strategic and crucial issue for the development of the natural gas market in future decades. Therefore, the identification of alternative suitable processes for the synergistic natural gas purification and LNG production, when the amount of CO2 in the raw gas feed is high, and their comparative thermodynamic assessment, is necessary to foster a cleaner and efficient production of LNG.In this paper, the energy intensity of the classical chemical absorption units using aqueous solutions of Methyldiethanolamine (MDEA) and the novel Dual Pressure Low-Temperature Distillation process (DPD) operating in different national contexts is assessed by means of the Net Equivalent Methane analysis and the Energy Life Cycle Assessment. These methods return respectively the equivalent methane and the embodied non-renewable energy invoked by the two alternative processes in one operating year. In general, the primary energy required by the construction phases of both the systems results negligible compared to the operation phase, and most of the latter is due to the consumption of energy utilities, which is strongly dependent by the considered national context. According to the results obtained by means of both the methodologies, the DPD process is generally less energy intensive compared to MDEA: the DPD process results to be an efficient and promising technique to perform the synergistic natural gas purification and LNG production.

Structural, operational and economic optimization of cryogenic natural gas plant using NSGAII two-objective genetic algorithm

Combined pinch and exergy analysis is herein employed to design and optimize a complex integrated structure of cryogenic natural gas plant based on the dual mixed refrigerant refrigeration cycle. Exergy composite curves are applied to reveal possible process improvement. Process intensification can reduce the number of required equipment as well as energy consumption. This integrated structure has specific power of 0.339 (kWh/kg LNG) and exergy efficiency of 66.14%. Sensitivity analysis is used to identify the important parameters affecting on the integrated processes and study behavior of the integrated structure against plant disturbances as well. After identifying important parameters, two single-objective and a NSGAII two-objective genetic algorithms are used to minimize thermodynamic properties and economic values of the integrated structure, simultaneously. Optimization procedure is conducted by structural and operational methods. The amounts of specific power and period of return are decreased by 6.68% and 5.65% by implementing the NSGAII two-objective genetic algorithm.

Global cost optimization of a mini-scale liquefied natural gas plant

Cryogenic natural gas liquefaction plant has huge capital and operating expenses corresponding to operating equipment and energy utilization. Considering ever-increasing energy price, therefore, minimization of energy consumption rate for a better profit is highly required. However, any un-engineered energy cut off would result in larger surface area of heat-exchanger and hence bigger capital cost. Here, the net profit of establishing a mini 50 ton/day liquefied natural gas facility, operating for 25 years, is optimized via Genetic Algorithm technique. Poly Refrigerant Integrated Cycle Operations (PRICO) process is simulated in HYSYS environment and linked to MATLAB software for subsequent maximization. The simulation resulted in total consumed power, heat exchanger area and total profit by 2745.33 kW, 3285.58 m2 and 1266.64 million$, respectively. In order to determine unit efficiency and plant irreversibility rate, exergy analysis is performed on individual equipment. Basically, thirteen independent variables are considered for optimization of objective function. Sensitivity analysis for objective function is considered by altering each variable. Final results indicate 9.26% rise in total profit (1383.95 million$) by 59% reduction in energy utilization (1127.68 kW) and 37.50% in heat-exchanger size (2053.7 m2). Meanwhile, the total and heat-exchanger exergy losses are decreased by 65.8% and 80.7%, respectively.

A comprehensive approach toward utilizing mixed refrigerant and absorption refrigeration systems in an integrated cryogenic refrigeration process

Abstract This study progresses absorption, mixed refrigerant and mixed fluid cascade refrigeration systems in the integrated cryogenic natural gas plant. It also presents a comprehensive systematic method for designing of refrigeration cycles based on synthesis of mathematic and thermodynamic viewpoints. The possibility of using absorption refrigeration system in lieu of precooling stage of mixed fluid cascade refrigeration cycle with the aim of reducing in energy consumption is investigated. In this way, the high amount of energy consumption in these units because of eliminating of some parts of mixed fluid cascade system has been reduced, and the possibility of using wasted energy of plant has been provided. Then, energy, exergy and economic analyses are carried out using simulated data. Economic analysis is carried out using annualized cost of system for the two structures. Using absorption refrigeration systems as an alternative to compression refrigeration system in the integrated structure causes reduction in the amount of specific power, capital cost, and prime cost of product by 38.94%, 31.9%, and 15.31%, respectively. Sensitivity analysis of economic parameters with respect to utilities price as well as products price on the market has been done.

Purification of helium from a cryogenic natural gas nitrogen rejection unit by pressure swing adsorption

Abstract Helium is produced conventionally from cryogenic processes in conjunction with LNG/NRU units. However, cryogenic processes are expensive and energy demanding. Pressure swing adsorption (PSA) was investigated as a potential replacement for the cryogenic processes to enrich dilute He (1 vol%) downstream of the NRU. The feasibility of a PSA process utilizing zeolite 13X was studied by dynamic simulation of 3-bed 3-step, 3-bed 5-step and 4-bed 7-step PSA cycle schedules. The static equilibrium parameters and dynamic mass transfer coefficients were estimated experimentally and the model was validated using dynamic breakthrough curves and PSA cyclic tests. The validated model was used to study the effect of adding equalization steps on the PSA process performance of the 3-bed 3-step cycle. Adding one equalization step (3-bed 5-step) effectively improved both the purity and recovery, while adding two equalization steps (4-bed 7-step) increased the capital costs with only minor improvements in purity and recovery. The best PSA process performance was obtained from the 3-bed 5-step PSA cycle, wherein feeding 1 vol% He resulted in a He purity of 94.3% at 62.1% He recovery and a feed throughput of 746.4 L(STP)/(h kg).

Cascade refrigeration systems in integrated cryogenic natural gas process (natural gas liquids (NGL), liquefied natural gas (LNG) and nitrogen rejection unit (NRU))

Heavy components in the natural gas itself can feed downstream units and also due to the low temperature process may be formed solid. Therefore heavy components separation is a necessity and can produce useful products. Virtually all natural gases are containing nitrogen that would lower the heating value of natural gas. This study investigates design and optimization of integrated process recovery of natural gas liquids, natural gas liquefaction, and nitrogen remove unit. In this integrated process, design of low temperature processes is started from the core process and continued by heat exchangers network design and cooling system based on MFC. Design and integration processes of units at the same time reduces the number of required equipment and energy consumption. The results show that the new integrated process has specific power around 0.343–0.33 (kW-h/kg-LNG) and its thermal efficiency equal to 62.82%, compared to other integrated systems have the lowest and highest values. Exergy analysis shows that towers has the highest Exergy destruction among other equipment. Sensitivity analysis shows that the structure of the integrated process capable of removing nitrogen from natural gas at a concentration of between 5% and 15%. By analyzing the operating parameters shows reduction in the Total specific power from 19.5% to 24% and the Specific power from 2.57% to 11%, yet surging in the Ethane recovery from 2.5% to 17%. Sensitivity analysis is the method to identification of the Decision variables, finally Genetic Algorithm used to identify optimum of objective function (minimization of Specific Power) and reduction of it to 6%.

Correlations Between Geological and Cryogenic Natural Gas Storage Capacity and the Characteristics of the Gas Industry in the World

Correlations Between Geological and Cryogenic Natural Gas Storage Capacity and the Characteristics of the Gas Industry in the World

Optimum design of integrated liquid recovery plants by variable population size genetic algorithm

AbstractIncrease in the price of energy sources as well as economic problems have caused cryogenic natural gas plants to become more complex and efficient. After selecting the process configuration, the flow rate, pressure, and temperature of the process fluid streams are determining factors which should be tuned in order to find the optimum condition. Products specification and operating costs of the plant are two significant parameters which should be considered in an optimal design. Moreover, process design limitations contribute to the problem being more difficult. This paper shows how the optimal operating point in an integrated NGL recovery plant can be found through solving a complex constrained optimization problem. A Variable Population size Genetic Algorithm (VPGA) was used for optimization. As well, the role of VPGA algorithm parameters in solving the process design problems is investigated in this study. The analysis showed that the VPGA method has better performance compared to the general GA methods. The plant‐wide net profit increases 12493360 $/year only by changing the selected operating conditions to its optimal value.

Analysis of the dispersion of a fixed mass of LNG boil off vapour from open to the atmosphere vertical containers

Dynamics of the dispersion of a fixed mass of cryogenic natural gas (NG) vapour produced from LNG (liquefied natural gas) boil off is examined. The vapour is considered to have been suddenly exposed to an overlaying atmosphere within open vertical cylindrical enclosures with a negligible pressure difference. A 2-D axis-symmetric CFD model was used to resolve the resulting complex temporal changes of the concentration and temperature fields. Focus was made on the features of the flammable regions, from their formation to dissipation, both inside and immediate outside of the enclosure. It was observed that with adiabatic walls, the structure of the flow changes from diffusion driven to buoyancy driven as methane warms up on mixing with the air to the atmospheric temperature. The effect of heat transfer from the walls on the evolution of the flammable cloud is discussed. The associated potential fire hazard is assessed based on the propagation rate of the lower flammability limit of the NG vapour cloud.

Experimental study on liquid/solid phase change for cold energy storage of Liquefied Natural Gas (LNG) refrigerated vehicle

The present paper addresses an experimental investigation of the cold storage with liquid/solid phase change of water based on the cold energy recovery of Liquefied Natural Gas (LNG) refrigerated vehicles. Water as phase change material (PCM) was solidified outside the heat transfer tubes that were internally cooled by cryogenic nitrogen gas substituting cryogenic natural gas. The ice layer profiles were recorded in different cross-sections observed by digital cameras. The temperatures of cryogenic gas, tube wall and bulk region were measured by embedded thermocouples continuously. The results of the smooth tube experiments and the thermal resistance analysis prove that the main thermal resistance occurs in the gaseous heat transfer fluid (HTF) inner the tube. The enhancement of the inner heat transfer is achieved by adding wave-like internal fins. Besides, the results show that the ice layer not only increases in radial direction but also propagates in axial direction. It distributes in parabolic shape along the tube length due to the parabolic axial distribution of the tube wall temperatures. This investigation provides valuable references for the design and optimization of the cold energy storage unit of LNG refrigerated vehicles and for the numerical study on the unsteady two-dimensional conjugated heat transfer with phase change.

Simulation of Coyote series trials—Part I:: CFD estimation of non-isothermal LNG releases and comparison with box-model predictions

This paper deals with the computational simulation of atmospheric cloud dispersion resulting from liquefied gas spills. Experimental data were taken from the Coyote series trials that involved large scale cryogenic natural gas releases. The data were used for checking the validity of the results obtained through the computational fluid dynamics code CFX. Moreover, using the experimental data-set of the non-isothermal gas dispersion, a comparative evaluation was performed between the code and two popular box-models (SLAB and DEGADIS) on the basis of specific statistical performance measures. The results showed that CFX approximated gas concentration histories with a reasonably good agreement and predicted correctly the behavior of gas cloud during dispersion. In particular, it predicted that the cryogenic dense cloud produced by the flash boiling would disperse as a heavy rather than a light gas, although the natural gas is lighter than air at normal conditions. On the other hand, the statistical treatment revealed that box-models may also give acceptable results even for the complex case of non-isothermal gas dispersion. Nevertheless, the CFX code unambiguously demonstrated considerably better accuracy.

Partial miscibility behavior in cryogenic natural gas systems

Three-phase fluid equilibria (liquid-liquid-vapor) can be observed under certain conditions for cryogenic natural gas mixtures. The appearance of the second liquid phase necessarily affects the liquified natural gas (LNG) process design for these systems. The occurrence of liquid-liquid-vapor (LLV) phenomena in natural gas systems is reviewed, and candidate mixtures for LLV behavior are identified. Experimental results on ten ternary natural gas systems studied by the investigators suggest a classification system for LLV behavior in natural gas mixtures.

Design and Operating Characteristics Of the Sunflower Helium Plant

The plant described here is designed to process feed gas having a broad range of compositions. Tolerating a wide seasonal flow demand, the plant produces crude helium, recovers 80 percent of C3+, and rejects nitrogen to produces crude helium, recovers 80 percent of C3+, and rejects nitrogen to maintain tail gas at a constant heating value. Introduction The Sunflower Helium Plant, located at Scott City, Kans., is jointly owned by Helium Inc., a subsidiary of the Kansas-Nebraska Natural Gas Co., and by Cities Service Cryogenics, Inc., a subsidiary of Cities Service Co. Each subsidiary owns a 50 percent undivided interest in the plant. The plant was designed, engineered and constructed by the M. W. Kellogg Co. and began operation in Sept., 1968. The Sunflower Helium Plant has been designed to accept a wide range of feed compositions, pressures, flow rates and ambient temperature conditions; as a result, it is one of the most versatile helium plants ever put into operation. Variations in feed conditions inherently produce a series of interactions of operating variables. To cope with both the feed variations and the operating interactions the plant has been designed to give the operator a wide selection in mode of operation and control. The plant can process as little as 110 MMscf/D of natural gas in the spring and fall, and as much as 200 MMscf/D in the winter. This wide range in processing capability is for the purpose of optimizing the annual yield of products and return on investment. The plant has been designed to produce crude helium (80-percent purity), pure (99.5 percent) nitrogen, and tail gas purity), pure (99.5 percent) nitrogen, and tail gas (945 Btu/cu ft) and to recover 80 percent of the C3+. At this point let us consider the fact that the helium concentration in the plant feed is only about 0.5 mol percent. As a result, most of the equipment (with the percent. As a result, most of the equipment (with the attendant operating problems) in a helium plant is concerned with the handling of the bulk natural gas; and the part of the plant where helium finally appears as a major component is a relatively small section. Basis of Design Basically, the Sunflower Helium Plant consists of two natural gas liquids recovery plants operating in parallel. Only one plant produces helium and we shall parallel. Only one plant produces helium and we shall continue to refer to it as the helium plant. It is designed to process 75 MMscf/D of feed gas at a constant daily rate throughout the year. In parallel with the helium plant is a cryogenic natural gas liquids (NGL) plant is a cryogenic natural gas liquids (NGL) recovery plant that is designed to process 93 MMscf/D; however, it processes as little as 35 MMscf/D in spring and fall and as much as 125 MMscf/D in the winter. When the flow exceeds 93 MMscf/D the liquids recovery percentage suffers. To illustrate the problem of varying load, Fig. 1 shows the nominal flow rate through the facility during the various months of the year. One line shows the helium plant operating at 75 MMscf/D and another shows it as 168 MMscf/D. The latter line represents the design basis for propane and ethylene refrigeration capability installed in the plant. It is evident that the design for 168 MMscf/D is inadequate to cope with winter loads and maintain maximum recovery of hydrocarbons. In the spring and fall when the load is relatively light, there is excess propane and ethylene refrigeration capacity. Note that there is an increase in gas demand in the hot summer months; this is because of air conditioning and farm product drying requirements. JPT P. 1098