低温位废热回收与液化天然气冷能利用的集成研究

Two novel thermal cycles based on Brayton cycle and Rankine cycle are proposed, respectively, which integrate the recovery of low-level waste heat and Liquefied Nature Gas （LNG） cold energy utilization for power generation. Cascade utilization of energy is realized in the two thermal cycles, where low-level waste heat,low-temperature exergy and pressure exergy of LNG are utilized efficiently through the system synthesis. The simulations are carried out using the commercial Aspen Plus 10.2, and the results are analyzed. Compared with the conventional Brayton cycle and Rankine cycle, the two novel cycles bring 60.94% and 60% in exergy efficiency, respectively and 53.08% and 52.31% in thermal efficiency, respectively.     

天然气基乙炔工艺 分析与改进

This article presents an acetylene production process by partial oxidation/combustion of natural gas. The thermodynamic performance and exergy analysis in the process are investigated using the flow-sheeting program Aspen Plus. The results indicate that the most important destruction of exergy is found to occur in the reactor and water quenching scrubber, amounting to 8.23% and 10.39%, respectively, of the entire system. Based on the results of thermodynamic and exergy analysis, the acetylene reactor has been retrofitted. The improvement ratios of molar 02 to CH4 and molar CO to CN4 are 0.65 and 0.20, respectively. An improvement of the acetylene production system is proposed. Adopting the improvement operation conditions and using oil to realize the reaction heat recovery, the feedstock of natural gas is reduced by 9.88% and the exergy loss in the retrofitting process is decreased by 19.71% compared to the original process.     

一个新α-函数用于Peng-Robinson状态方程:应用于天然气(英文)

Cubic equations of state （EOSs） are simple and easy at calculation. One way of improving the accuracy of a cubic EOS is through the modification of temperature-dependent energy parameter by using alpha-function. The industrial applications of natural gas are very wide and as a result, prediction of thermodynamic properties and phase behavior of natural gas is an important part of design for such processes. In this work we develop a newα-function for the Peng-Robinson （PR） EOS with the parameters optimized especially for natural gas components. The parameters are generalized as a linear function of acentric factor. The results are compared to the predictions from original PR EOS and otherα-functions in literature. It is shown that the newα-function presents a good accu-racy with the average deviation of 1.42%for natural gas components.     

Soft-sensing modeling and intelligent optimal control strategy for distillation yield rate of atmospheric distillation oil refining process

It is a challenge to conserve energy for the large-scale petrochemical enterprises due to complex production process and energy diversification. As critical energy consumption equipment of atmospheric distillation oil refining process, the atmospheric distillation column is paid more attention to save energy. In this paper, the optimal problem of energy utilization efficiency of the atmospheric distillation column is solved by defining a new energy efficiency indicator — the distillation yield rate of unit energy consumption from the perspective of material flow and energy flow, and a soft-sensing model for this new energy efficiency indicator with respect to the multiple working conditions and intelligent optimizing control strategy are suggested for both increasing distillation yield and decreasing energy consumption in oil refining process. It is found that the energy utilization efficiency level of the atmospheric distillation column depends closely on the typical working conditions of the oil refining process, which result by changing the outlet temperature, the overhead temperature, and the bottom liquid level of the atmospheric pressure tower. The fuzzy C-means algorithm is used to classify the typical operation conditions of atmospheric distillation in oil refining process. Furthermore, the LSSVM method optimized with the improved particle swarm optimization is used to model the distillation rate of unit energy consumption. Then online optimization of oil refining process is realized by optimizing the outlet temperature, the overhead temperature with IPSO again. Simulation comparative analyses are made by empirical data to verify the effectiveness of the proposed solution.     

Thermodynamic design of a cascade refrigeration system of liquefied natural gas by applying mixed integer non-linear programming

Liquefied natural gas (LNG) is the most economical way of transporting natural gas (NG) over long distances. Liq-uefaction of NG using vapor compression refrigeration system requires high operating and capital cost. Due to lack of systematic design methods for multistage refrigeration cycles, conventional approaches to determine op-timal cycle are largely trial-and-error. In this paper a novel mixed integer non-linear programming (MINLP) model is introduced to select optimal synthesis of refrigeration systems to reduce both operating and capital costs of an LNG plant. Better conceptual understanding of design improvement is illustrated on composite curve (CC) and exergetic grand composite curve (EGCC) of pinch analysis diagrams. In this method a superstruc-ture representation of complex refrigeration system is developed to select and optimize key decision variables in refrigeration cycles (i.e. partition temperature, compression configuration, refrigeration features, refrigerant flow rate and economic trade-off). Based on this method a program (LNG-Pro) is developed which integrates VBA, Refprop and Excel MINLP Solver to automate the methodology. Design procedure is applied on a sample LNG plant to illustrate advantages of using this method which shows a 3.3% reduction in total shaft work consumption.     

利用扩展Tao-Mason状态方程预测天然气混合物容积特性(p-V-T)

A statistical-mechanical-based equation of state(EOS)for pure substances,the Tao-Mason equation of state,is successfully extended to prediction of the(p-v-T)properties of fourteen natural gas mixtures at temperatures from 225 K to 483 K and pressures up to 60.5 MPa.This work shows that the Tao-Mason equation of state for multicomponent natural gas is predictable with minimal input information,namely critical temperature,critical pressure,and the Pitzer acentric factor.The calculated results agree well with the experimental data.From a total of 963 data of density and 330 data of compressibility factor for natural gases examined in this work,the average absolute deviations(AAD)are 1.704%and 1.344%,respectively.The present EOS is further assessed through the comparisons with Peng-Robinson(PR)equation of state.For the all of mixtures Tao-Mason(TM)EOS outperforms the PR EOS.     

A Two-step Design Method for Shaft Work Targeting on Low-temperature Process

In low-temperature processes, there are interactions between heat exchanger network （HEN） and refrig-eration system. The modification on HEN of the chilling train for increasing energy recovery does not always coor-dinate with the minimum shaft work consumption of the corresponding refrigeration system. In this paper, a sys-tematic approach for optimizing low-temperature system is presented through mathematical method and exergy analysis. The possibility of＂pockets＂, which appears as right nose section in the grand composite curve （EGCC） of the process, is first optimized. The EGCC with the pockets cutting down is designed as a separate part. A case study is used to illustrate the application of the approach for a HEN of a chilling train with propylene and ethylene refrig-erant system in an ethylene production process.     

EQQUATION OF STATE BASED HYDRATE MODEL FOR NATURAL GAS SYSTEMS CONTAINING BRINE AND POLAR INHIBITOR

A new thermodynamic model for gas hydrates was established by combining the modified Patel-Teja equation of state proposed for aqueous electrolyte systems and the simplified Holder -John multi -shell hydrate model. The new hydrate model is capable of predicting the hydrate formation/dissociation conditions of natural gas systems containing pure water/formation water (brine) and polar inhibitor without using activity coefficient model. Extensive test results indicate very encouraging results.     

Simulation of Fuel Ethanol Production from Lignocellulosic Biomass

Models for hydrolysis, fermentation and concentration process, production and utilization of biogas as well as lignin gasification are developed to calculate the heat demand of ethanol production process and the amounts of heat and power generated from residues and wastewater of the process. For the energy analysis, all relevant information about the process streams, physical properties, and mass and energy balances are considered. Energy integration is investigated for establishing a network of facilities for heat and power generation from wastewater and residues treatment aiming at the increase of energy efficiency. Feeding the lignin to an IGCC process, the electric efficiency is increased by 4.4% compared with combustion, which leads to an overall energy efficiency of 53.8%. A detailed sensitivity analysis on energy efficiency is also carried out.     

Performance enhancement of water bath heater at natural gas city gate station using twisted tubes

Natural gas is transported from producing regions to consumption regions by using transmission pipelines at high pressures. At consumption regions, the pressure of natural gas is reduced in city gate stations(CGSs). Before the pressure reduction process, the temperature of natural gas is increased usually by using a water bath heater,which burns natural gas as fuel, to protect against freezing of natural gas. These types of heat exchangers have a low efficiency and consume a lot of fuel to generate the required heat. In the current study, the twisted configuration of the heating coil is proposed and investigated to enhance the heat transfer through a water bath heater with a nominal capacity of 1000 m~3·h~(-1). Firstly, the implementation procedure is validated with data collected from the CGS of Qaleh-Jiq(located in Golestan province of Iran). A very good agreement is achieved between the obtained results and the real data. Then, three different twist ratios are considered to examine the twisting effects. The proposed technique is evaluated in the terms of velocity, temperature, and pressure variations, and the results are compared with the conventional case, i.e. straight configuration. It is found that both the heat transfer rate and the pressure drop augment as the twist ratio is raised. Finally, it is concluded that the twisted tubes can reduce the length of the gas coil by about 12.5% for the model with low twist ratio, 18.75% for the model with medium twist ratio, and 25% for the model with high twist ratio as compared to the straight configuration.     

液化天然气冷能构成及其利用方式探讨

液化天然气(LNG)在汽化过程中会释放大量冷能,如果这部分冷能被成功回收利用,其节能效果和对系统效率的提高都十分显著。文中对LNG冷能从冷量和冷量的角度进行分析,把LNG冷能回收方式分为冷量回收与冷量回收,揭示了目前各种LNG冷能回收利用形式的能量利用实质:发电、空分中主要是利用LNG的冷量;冷藏、空调和制干冰利用了LNG的冷量。最后对不同的冷能回收系统提出指导性建议:动力回收系统中,应充分利用其在低温下的高品质能量;冷量回收系统中应减少跑冷。     

利用LNG冷能的冷库工艺模拟及分析

LNG卫星站由于负荷波动较大等问题,其LNG的冷能大多未经过利用。针对这一问题,提出了适用于LNG卫星站的LNG冷能用于冷库的冷能利用工艺。通过与电压缩制冷工艺对比可知,对于制冷负荷为306.7kW的冷库,工艺每小时可节电约为193kW,具有较好的节电效果。同时,火用分析结果表明,工艺的能量利用效率远优于电压缩制冷工艺。随着我国LNG产业的发展,工艺将拥有广阔的应用前景。     

空气分离与天然气液化组合循环

提出了利用空分系统冷量液化天然气的联合流程思想,并介绍了一种采用氮气膨胀循环的空气分离与天然气液化联合方案,在得到液化天然气（LNG）的同时,也生产氮气、氧气等多种产品。通过流程模拟计算并采用有效能分析原理,确定了高、低温膨胀量比对传热温差及有效能损失的影响规律,得到了不同液化量下的优化膨胀量之比。结果表明,基于热负荷总组合曲线的有效能分析原理能够方便、有效地研究整个传热过程的温差分布及有效能损失规律,对实际低温过程系统优化有重要意义。     

利用液化天然气冷能捕集CO_2的动力系统的集成

为提高液化天然气(LNG)冷能的利用效率和CO2近零排放动力循环的发电效率、降低CO2减排的能耗,在对CO2近零排放动力循环利用LNG冷能进行火用分析的基础上,提出了一个以天然气为介质的Rankine循环与CO2近零排放动力循环进行集成的动力系统模型,可以在保持CO2预冷和液化所需冷能不变的情况下,将深冷部分的LNG冷火用转换为电能。研究结果表明,集成后动力系统中LNG冷火用的利用效率从34.9%提高到55.7%,整个动力循环的火用效率可达到57.9%。同时,对影响以天然气为介质的Rankine循环发电效率的参数进行了分析。     

几种典型的煤层气液化流程计算及分析比较

针对3种典型的煤层气液化流程方案,采用Visual Fortran和物性程序,并结合流程分析软件Aspen Plus对状态点参数进行优化,计算得到流程各状态点状态参数等数据,进而对3种循环的损失、能耗进行了比较和分析。结果显示,丙烷预冷的N2-CH4单级膨胀液化循环方案的损失和能耗2项指标比N2-CH4串联双级膨胀液化循环和N2-CH4并联双级膨胀液化循环方案小。得到了天然气液化流程计算和优化的有效方法和途径,对实际工程有积极的指导意义和预测作用。     

一种发电和天然气再液化相结合的LNG冷能利用系统

针对冷能回收再利用问题,提出了一种结合LNG和燃煤废气发电与天然气再液化的冷能利用系统并对系统进行了改进。对原系统和系统改进部分进行了热力学计算,详细分析了蒸发压力、蒸发温度对系统热力性能的影响,分析了天然气液化率对系统净输出功的影响,确定了发电循环的最佳蒸发压力、蒸发温度及天然气液化率的范围。结果表明：以回收1000 kg·h^-1的LNG冷量（火用）计算,发电系统最大净输出功为69.6 kW·h,系统冷（火用）回收效率为41.43%;液化系统LNG液化率最大值为24%;系统改进后,发电系统净输出功和冷（火用）回收效率提高了17.85%,液化系统LNG液化率提高至28%。为日后LNG气化供气过程中的冷能利用提供一种新的思路。     

液化天然气过程的热力学分析

利用R K S方程建立了天然气和液化天然气焓火用计算的热力学模型;对 2×104 m3 /d液化天然气液化过程进行了模拟计算;计算了各设备的火用损失和液化过程的火用效率;热力学计算分析结果表明,装置的最大火用损环节是循环压缩机,其次是透平膨胀机和气波制冷机。本装置利用自身所产尾气作为燃气发动机的燃料,进而利用燃气发动机带动循环压缩机,节省了大量电能,回收了排放尾气的能源,有效地解决了压缩机的高能耗问题。     

一种天然气液化和CO2捕集相结合的余热回收发电系统

针对余热回收和能源利用的问题,以液化天然气(LNG)作为冷源,稠油开采废气作为热源,提出了一种结合天然气液化和废气发电与CO₂捕集的余热回收利用系统。分析了关键热力学参数对系统热力学性能的影响。结果表明：对于有机朗肯循环和制冷循环,增加透平膨胀机的进口温度,降低其出口压力以及减少制冷循环压缩机进出口的压缩比,可获得最大净输出功为454.9 kW,余热回收效率为34.2%。对于天然气液化系统,采用C++进行非线性约束优化计算,以氮膨胀制冷循环压缩机总功耗为目标函数进行优化,得到压缩机最优总功耗为101.54 kW。降低天然气压缩机(K110)进口温度,氮气膨胀机(T3)出口压力以及氮气质量流量,可获得最大LNG调峰量为378.8 kg/h,反之,CO₂捕集量可提高28.6%。     

高含硫天然气净化装置 分析

针对高含硫天然气净化装置运行能耗高的问题,本文建立了高含硫天然气净化过程中各类 值计算方法,并对离子溶液体系的 值计算方法进行了修正,使其适用于酸气吸收过程中醇胺溶液的 值计算。在天然气净化过程模拟软件ProMax建立的净化过程全流程模型的基础上,采用 分析方法对高含硫净化装置的全流程进行用能分析。分析结果表明,净化装置全流程的 效率为54.2%,其中硫黄回收单元和尾气处理单元 效率最高,分别为66.8%和66.1%;脱酸气单元的 损失最高,占全流程总 损失的43.5%,这是由于净化装置处理的原料气中H2S含量很高,需要更大溶剂循环量才能使净化气达到商品气标准,这导致吸收溶剂再生过程的能耗大大增加。本文研究成果可指导高含硫天然气净化装置的用能评价及节能改造。     

小型撬装式LNG装置的流程模拟

为了使小型撬装式LNG（液化天然气）装置的流程研究具有普遍意义,通过对液化流程的评价,分别从混合制冷剂液化流程和膨胀机液化流程中选择了极具代表性、性能最佳的丙烷预冷混合制冷剂液化流程和N2-CH4膨胀机液化流程,并结合液化流程的发展趋势,综合多种液化流程的优点,提出了节能新型混合制冷剂液化流程,对以上液化流程进行了模拟计算,并比较了流程的关键参数.结果表明,节能新型混合制冷剂液化流程简便灵活、能耗低、液化率高,适应于小型撬装式LNG装置.