Convection Section Research Articles

Microstructure Characteristics of Fatigue Fracture of Furnace Tube in Convection Section of Slurry Bed Fractionator

Microstructure Characteristics of Fatigue Fracture of Furnace Tube in Convection Section of Slurry Bed Fractionator

Thermal deviation mechanisms for coupled heat transfer between the combustion side and the furnace tube side in the tubular heating furnace

Temperature deviation significantly affects the safe operation of tubular heating furnaces. This is attributed to the inevitable temperature difference between the flue gas and the working medium on the tube side. To date, little research has considered the dynamic heat transfer between the furnace body and the tubes in tube furnaces, and even less attention has been paid to the thermal deviation within tube furnaces. To optimize the operational parameters of the heating furnace, a comprehensive heat transfer model is established in this paper to numerically couple the heat transfer between the combustion side and the working medium on the tube side in tube furnaces. A hydrogenation feed heating furnace with a processing capacity of 600,000 tons per year in a chemical enterprise is selected as the subject of study. Using Fluent software, on the basis of coupled simulation, a non-uniformity coefficient of temperature distribution is introduced to characterize the thermal deviation within the furnace. The impact of the thermal load and hydrogen doping in the fuel gas on the thermal deviation within the furnace is analyzed. The numerical results demonstrate that with an increase in thermal load, the flame structure in the furnace gradually becomes concentrated, the average temperature of the flue gas increases, and the outlet temperature of the furnace chamber increases by 19%, with the thermal efficiency decreasing by 5.11%; at the same time, the overall non-uniformity coefficient of the temperature distribution in the furnace fluctuates irregularly, indicating a nonlinear relationship between the thermal load and the thermal deviation within the furnace. With an increase in the hydrogen content in the fuel gas, the flame structure in the furnace becomes more dispersed, the average temperature of the flue gas increases, but the combustion efficiency of the heating furnace exhibits irregular fluctuations; the thermal deviation within the furnace initially increases and then decreases, with a lower non-uniformity coefficient of temperature at 50% hydrogen content, which is 2.48% lower than at 40% hydrogen content. Under both conditions, the thermal deviation primarily leads to localized hot spots in the radiant furnace tubes and uneven temperature distribution in the convection section, with a minimal impact on the thermal efficiency of the heating furnace.

Failure analysis of convection section tube in an ethylene cracking furnace due to metal dusting

To study the failure in the convection section tubes of an ethylene cracking furnace, macroscopic inspections, chemical composition analysis, metallographic microscopy, hardness testing, Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analysis are performed. The experiments confirmed the reason for metal dusting occurrence of 304H austenitic stainless steel that the inner wall Cr2O3 protective film was destroyed by overheating. There were dense circular corrosion pits on the fire side inside the furnace tubes, loose metal oxides and carbide particles were filled in the corrosive pits. Severe carburization occurred on the surface of the pits, the furnace tube experienced metal dusting. The metal dusting resulted in the decomposition of the surface layer of the metal alloy into metal particles and graphite to thin the wall thickness. Then the furnace tube cracked because of the stress concentration appeared in the furnace tube with reduced wall thickness. Finally, some suggestions and countermeasures on the failure of metal dusting of the furnace tube were proposed.

Characteristics of Shallow Flows in a Vegetated Pool—An Experimental Study

Pools are often observed in gravel-bed rivers, together with the presence of vegetation patches. In the present study, a conceptual model of a gradual varied flow with both convective deceleration and acceleration flow sections has been constructed in a flume to study turbulent flow structures. Vegetation patches with extended canopies were planted in the pool sections in order to increase the thickness of the boundary layer inside the inner zone. The effects of different flows (namely decelerating, uniform and accelerating flows) along an artificial pool on flow velocity, shear stress and bursting events have been investigated. In addition, due to the occurrence of secondary currents in shallow streams, the characteristics of turbulent shallow flow have been investigated along two axes that are parallel to the sidewall of the flume. The results showed that the application of the log law should be used with care to estimate shear velocity along a pool with a vegetated bed. The presence of a vegetation patch causes an increase in Reynolds shear stress, especially along the entrance section of the pool where the flow decelerates. The results of the quadrant analysis reveal that the sweep and ejection events have the most dominant influence over the vegetation patch in the pool; however, the contributions of outward and inward events increase near the bed, especially in the entrance section of the pool where the flow is decelerating. The distribution of stream-wise RMS of turbulence intensity along the pool generally presents a convex shape.

Modeling and simulation of ethane cracker reactor using Aspen Plus

The main aim of the work was to investigate if simple reactor blocks could be combined to adequately represent complex reactor configurations; with the objectives of selecting an appropriate reaction scheme, creating a working simulation and validating the results obtained from the simulation. An ethane cracker reactor is simulated using Aspen Plus, for a molecular reaction scheme, made up of seven reactions and eight hydrocarbon components; using the kinetics and reactor geometrics. Several assumptions were made such that the fired furnace reactor could be modeled as a series of two plug flow reactors using Aspen Plus. Modeling of a fired furnace reactor for the cracking of ethane is made difficult by the inherent separation between the convection and radiation sections; a difficulty which was overcome by separating the convection region and the radiation region of the fired furnace reactor into two simulations such that temperatures were solved for iteratively. A firebox simulation was created with the intent of finding the optimal fuel and air flow rates that would supply the fired furnace rector with the required 2490 kW; these flow rates were found to be 190 kmol hr−1 and 60 kmol hr−1.

Three-Dimensional CFD Model Development and Validation for Once-Through Steam Generator (OTSG): Coupling Combustion, Heat Transfer and Steam Generation

The current research studies the coupled combustion inside the furnace and the steam generation inside the radiant and convection tubes through a typical Once-Through Steam Generator (OTSG). A 3-D CFD model coupling the combustion and the two-phase flow was developed to model the entire system of OTSG. Once the combustion simulation was converged, the results were compared to field data showing a convincing agreement. The CFD analysis provides the detailed flow behavior inside the combustion chamber and the stack, as well as the two-phase flow steam generation process in the radiant and convective sections. The flame shape and orientation, the velocity, the species, and the temperature distribution at the various parts of the furnace, as well as the steam generation and the steam distribution inside the pipes were investigated using the developed CFD model

A Review on the Production of Light Olefins Using Steam Cracking of Hydrocarbons

Light olefins are the main building blocks used in the petrochemical and chemical industries for the production of different components such as polymers, synthetic fibers, rubbers, and plastic materials. Currently, steam cracking of hydrocarbons is the main technology for the production of light olefins. In steam cracking, the pyrolysis of feedstocks occurs in the cracking furnace, where hydrocarbon feed and steam are first mixed and preheated in the convection section and then enter the furnace radiation section to crack to the desired products. This paper summarizes olefin production via the steam cracking process; and the reaction mechanism and cracking furnace are also discussed. The effect of different operating parameters, including temperature, residence time, feedstock composition, and the steam-to-hydrocarbon ratio, are also reviewed.

Multi-objective Optimal Design of Gas-fired Heater Based on Modified Design Model of Fired Heater Taking into Account Exergy, Economic and Environmental Factors

Heaters are one of the central parts of natural gas reduction stations using turboexpanders to prevent the formation of hydrate and corrosion failure. This study intends to design a fired heater by applying a combustion sub-model to derive an optimal model for this kind of application. This model is developed to accurately consider all subsections of the fired heater namely radiation, convection, and shield sections, as well as flue gas composition, and its volume. Within this context, a multi-objective optimization is employed to identify the optimal design of the gas-fired heater in the natural gas reduction station for the Ramin power plant case study. The total economic and environmental costs, together with modified exergy efficiency, are selected as objective functions. Multi-criteria-decision-making-method is employed on Pareto frontiers optimal curve to suggest the optimal solution. Results show that the developed model can outperform previous models in thermal efficiency with relatively similar costs. Besides, the optimal point in Pareto suggested by the decision-making-method accounts for a higher modified exergy efficiency (1.3%) than the counterpart, which thermal efficiency is regarded as an objective function. At the same time, its total cost remained almost constant. The effects of changes in each of the design parameters on the objective functions are also evaluated.

AN INTEGRATED MODEL TO STUDY THE EFFECTS OF OPERATIONAL PARAMETERS ON THE PERFORMANCE AND POLLUTANT EMISSIONS IN A UTILITY BOILER

A gas fired tangentially boiler was modeled under full load conditions. Furnace was simulated by CFD and then was joint with two mathematical models to calculate heat transfer in the convective section, and metal temperature of waterwall tubes. Effects of changing the combustion excess air (0 to 20%) and burners tilt angle (-30° to +30°) were studied. Results showed that the boiler efficiency is optimum if excess air= 10% and the burners have a negative angle. However, these optimum settings cannot produce a superheated and reheated steam of 538 °C which is desirable. Indeed, a zero or positive tilt angle with 10% excess air, or a negative burner angle with 15% excess air lead to highest efficiency by considering the potential of generating superheated steam of 538 °C. In addition, CO emission in low excess air values growths by increasing the burner tilt angle. NOx emission in low and high excess air ratios is lower at positive burner angles while a moderate excess air (10%) needs a zero tilt angle to minimize NOx emission. Furthermore, a critical fouling thickness was computed, considering boiler's circulation ratio, in which the metal temperature of the waterwall exceeds the short overheating threshold. With a certain thickness of scale layers inside the tubes, a burner tilting equal to 0° or 30° postpones tube rupture. These results could be utilized by operating engineers to keep their utility boilers in the most efficient state and avoiding overheating and tube rupture.

Cause analysis of corrosion leakage in convection section of ethylene cracking furnace

Convection tubes of an ethylene cracking furnace at a petrochemical plant had multiple leaking points along longitudinal directions after a fraction of the anticipated tube life span. To study the cause of the failure, the failed tubes were investigated with macro inspections, chemical composition analysis, metallographic microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Through analysis of the failed tube samples, we found no evidence of scaling corrosion caused by S and Cl and no significant corrosion contributed by carburisation of the inner wall. By combining cavitation characteristics observed from the leaking tube inner wall and the feed composition changed by the plant recently, we highly suspected that cavitation was the main reason causing the tube to leak. A simulation model was built to illustrate the possibility of liquid cavitation at the leaking region of the tube. After the plant controlled the heavy component in the light hydrocarbon suspected of causing the liquid cavitation, the investigated tube section did not show any similar leaking problem.

Oxy-Fuel Combustion Characteristics of Pulverized Coal under O2/Recirculated Flue Gas Atmospheres

Oxy-fuel combustion is an effective technology for carbon capture and storage (CCS). Oxy-combustion for coal-fired power stations is a promising technology by which to diminish CO2 emissions. Unfortunately, little attention has been paid to the oxy-combustion characteristics affected by the combustion atmosphere. This paper is aimed at investigating the oxy-fuel combustion characteristics of Australian coal in a 0.3 MWth furnace. In particular, the influences of various oxygen flow rates and recirculated flue gas (RFG) on heating performance and pollutant emissions are examined in O2/RFG environments. The results show that with increases in the secondary RFG flow rate, the temperatures in the radiative and convective sections decrease and increase, respectively. At a lower oxygen flow rate, burning Australian coal emits lower residual oxygen and NO concentrations. In the flue gas, a high CO2 concentration of up to 94.8% can be achieved. Compared to air combustion, NO emissions are dramatically reduced up to 74% for Australian coal under oxy-combustion. Note that the high CO2 concentrations in the flue gas under oxy-coal combustions suggest great potential for reducing CO2 emissions through carbon capture and storage.

An in-house built code incorporated into CFD model for the simulation of boiler's convection section

This work presents an in-house built code and its incorporation into an ANSYS Fluent® Computational Fluid Dynamics (CFD) model for the simulation of the convection section of a boiler without needing numerous thermodynamic or experimental data, sophisticated numerical grids or assumptions that decrease the validity of the model. This model has been applied in a recently retrofitted lignite-fired boiler's convection section. Due to the lack of experimental data after the retrofit, the investigation follows a parametric approach, assuming three different distributions of the ash fouling thickness. The three distributions have been suitably selected to fit the CFD results of the in-house built model in the full-load case with: 1) the CFD results using the Macro Heat Exchanger Model provided by ANSYS Fluent®, 2) the results of an ASPEN Plus® process model and 3) experimental data before the retrofit actions. The only necessary input data for the in-house built code are the inlet temperature, the average pressure and the mass flow rate of the working medium for each heat exchanger. Based on the results, it can be concluded that the in-house built model is robust, consistent and validated for the simulation of a boiler's convection section.

Investigation on Heat Exchanger Pipe Failure

Convection heat exchangers are commonly used for heat transfer in high-temperature furnaces and processes. Pipes from U-bundles of a convection section of a MIDREX direct reduction ironmaking unit heater which carries reducing gas have been investigated for failure analysis. Repeated failures had occurred at a specific location. Samples of the failed pipe were subjected to microscopic and metallographic characterization. Inner diameter erosion and metal dust, carbides, oxides, and carbon were found at the failure location. Optical microscope revealed severe carburization on the inner surface that was in contact with heat exchanger gas. Coarse carbides at the grain boundaries accelerated the wear and corrosion by intergranular cracking. The reducing gas composition (with CO), dust composition (with carbides), and exposed temperatures (> 400 °C) confirmed that metal dusting is the cause of the failure. The high-temperature corrosion of steel is caused by hydrocarbon gases which form thermodynamically metastable carbides of the base metal. Such failures can be avoided by maintaining the pipe surface temperatures below 400 °C or using pipes manufactured from high-temperature-resistant steels.

Fouling in a Steam Cracker Convection Section Part 2: Coupled Tube Bank Simulation using an Improved Hybrid CFD-1D Model

Performing an adequate fouling study for the heat exchangers in the convection section of a steam cracker requires reliable data on circumferential tube wall temperature profiles. A hybrid Computational Fluid Dynamics (CFD)-1D convection section model, developed to perform coupled flue gas/process gas side simulations of convection sections, is improved by the implementation of flue gas radiation modeling and extended to include typical tube banks. A complete naphtha cracker convection section is simulated with the improved hybrid CFD-1D model. All tubes show distinct maximum heat fluxes on the tube walls due to the high flue gas velocity. Based on the calculated circumferential heat flux profiles, the maximum heat flux value is calculated to be 1.8 times the average tube heat flux value. As computational costs associated with a hybrid CFD-1D simulation are high, a convective heat flux profile reconstruction scheme is developed. Using the scheme, circumferential heat flux profiles are reconstructed, based on the heat fluxes calculated when performing a fully 1D coupled convection section simulation. The heat flux reconstruction profile scheme enables fast retrieval of circumferential heat flux profiles and, thus, tube wall temperature profiles. Optimization and/or design of a steam cracker convection section becomes less computationally demanding.

A Study on the Failure of AISI 304 Stainless Steel Tubes in a Gas Heater Unit

This paper investigates a failure in convection section tubes of a gas heater unit in a petrochemical plant. Tubes are made of AISI 304 stainless steel. The failure is reported after 5 years of service at working temperature 500 °C. The failure is in the form of circumferential cracks in the vicinity of the weld. Various characterization techniques, including optical and electron microscopes as well as energy-dispersive X-ray spectroscopy (EDS), were used to study the failure. Results showed that that the damage has initiated in the heat affected zone (HAZ) area parallel to the weld/base metal interface. Cracks have propagated alongside grain boundaries, resulting in an intergranular fracture. The main cause of failure was concluded to be attributable to the grain boundary sensitization and intergranular grain boundary attack due to improper welding and long time exposure of tubes to high temperature. Possible mitigation strategies to minimize similar failures will be discussed.

Assessment of energy saving potential of an industrial ethylene cracking furnace using advanced exergy analysis

Steam cracking furnace is a high energy-consuming equipment in the ethylene plant. Reducing the exergy destruction and losses associated with the steam cracking furnace can increase the thermodynamic efficiency of the system and thereby reducing energy penalties. This paper aims to quantitatively evaluate thermodynamic performance of an industrial steam cracking furnace through conventional and advanced exergy analysis in order to assess its energy saving potential. A steady state simulation of an industrial steam cracking furnace with a total feed capacity of 12 t/h was carried out. The simulation was validated by comparing the model prediction results with the industrial data. The conventional exergy analysis shows that the overall exergy efficiency of the steam cracking furnace is found to be 43.43% and the combustion process in the radiation section exhibits the largest exergy destruction followed by the tube reactors in the radiation section. The advanced exergy analysis shows that the combustion process has the highest unavoidable exergy destruction. Moreover, the tube reactors in the radiation section has the highest avoidable exergy destruction, followed by the combustion process and the feed-steam mixture superheater in the convection section. Therefore, there is high energy saving potential in the tube reactors, combustion process and feed-steam mixture superheater. The advanced exergy analysis also indicates that efforts on improving the radiation and convection sections should be dedicated to themselves while the thermodynamic performance of the quench system should be improved by reducing the exergy destruction of other interacting components.

Evaluation of retrofitting of an industrial steam cracking furnace by means of CFD simulations

Ethylene is one of the most important building blocks of current chemical industry with a global production of 150 million tonnes in 2016. Most ethylene nowadays is produced via thermal cracking with steam of fossil feedstocks in cracking furnaces. This process accounts for approximately 8% of the sector’s primary energy consumption, making it the single most energy-consuming process in the chemical industry. Therefore, an improvement of energy efficiency in cracking furnaces is both environmentally and economically driven. A CFD model of a full scale cracking furnace, including the radiative section equipped with wall burners and the convective sections, has been developed and validated with plant measurements under six different operation scenarios. Modifications of burner configuration and furnace geometry have been considered and simulated with the CFD model to study their effect on overall energy efficiency. Results show that reduction in the number of operative burners leads to higher inhomogeneity in coil temperatures in the radiant section and that a reduction of 25% in crossover length leads to a 2% increase in energy efficiency. An additional post-processing step has been applied to estimate furnace NOx emissions. The resulting outlet concentrations are slightly underestimated although in the correct order of magnitude (100 ppm).

Analytical models for heat transfer in the tube bundle of convection section in a steam cracking furnace

In steam cracking furnaces, about 45% of the heat released from fuel combustion is recovered by preheating cracking feedstock and high pressure steam. This is achieved mainly by indirect convective heat transfer from flue gas to feedstock and steam in a series of tube bundles located in the so-called convection section. Radiation also plays in an important role in lower bundles where flue gas temperature is high. Thus, accurate heat transfer simulation of the convection section is crucial for operation optimization and control of steam cracking furnace. This paper aims to develop a heat transfer analysis (HTA) model for combined convective and radiative heat transfer calculation in tube bundles with both inline and staggered layouts. Computational fluid dynamics (CFD) was employed to validate the HTA model in the first stage. Based on the HTA model, coupled steady-state simulations of the convection section were performed for an industrial steam cracking furnace with naphtha feed capacity of 30 t/h. The predictions of the HTA model are in good agreement with the design data. The study shows that convective heat transfer is significantly enhance by fins in the tube bundles where the flue gas temperatures are too low to effectively heat up the feedstock. Radiative heat transfer at the bottom of the convection chamber is much larger than the convective heat transfer and thus cannot be ignored in heat transfer calculation.

Fouling in a Steam Cracker Convection Section Part 1: A Hybrid CFD-1D Model to Obtain Accurate Tube Wall Temperature Profiles

To study fouling in steam cracker convection section tubes, accurate tube wall temperature profiles are needed. In this work, tube wall temperature profiles are calculated using a hybrid model, combining a one-dimensional (1D) process gas side model and a computational fluid dynamics (CFD) flue gas side model. The CFD flue gas side model assures the flue gas side accuracy, accounting for local temperatures, while the 1D process gas side model limits the computational cost. Flow separation in the flue gas side at the upper circumference of each tube suggests the need for a compartmentalized 1D approach. A considerable effect is observed. The hybrid CFD-1D model provides accurate tube wall temperature profiles in a reasonable simulation time, a first step towards simulation-based design of more efficient steam cracker convection sections.

Modeling and optimization of process fired heaters

In this study, a mathematical model associated with genetic algorithms (GAs) is developed to optimize the design of process fired heaters where the objective function is the total annual cost. The mathematical model considering operational and geometric constraints is used to design all subsections of a furnace including the radiant chamber, the convection and stack sections. The proposed model allows the economic optimization using the MATLAB genetic algorithm toolbox. This procedure is implemented to design and optimize fired heaters. Two case studies are considered. First, by comparing the results with literature, it is shown that the developed model can be successfully used in design with acceptable accuracy. In the first case study, based on TAC (total annual cost) minimization, two optimization scenarios with two sets of decision variables using GA are applied to determine the optimal economic design. It is demonstrated that by considering the tube diameter as a decision variable (scenario B), the TAC minimization approach does not present a realistic optimal design, because the crude oil pressure drop is out of a permissible range. Finally, by including the pumping cost into the operating cost and modifying the objective function, another optimization approach based on MTAC minimization is applied for the same optimization scenarios. In this case, scenario B results in the economic/realistic optimal design by up to 2.48% cheaper than Original Design.