Wall Fouling Research Articles

Utility of argon as a static charge and wall fouling suppressant in atmospheric gas-solid fluidized beds

This work aimed to study the usability of argon to mitigate electrostatic charging and fouling of polyethylene resin fluidized in a stainless-steel column. Nitrogen, argon, and their binary mixtures of various concentrations were utilized to fluidize polyethylene. Successive and intermittent fluidizations with argon and nitrogen were also carried out to explore the feasibility of using argon in commercial reactors. Fluidization with pure argon resulted in the lowest specific charge and substantially reduced the fouling by 90% relative to pure nitrogen. The results for the mixture trials showed a decreasing non-linear trend as the concentration of argon increased. Even presence of 10 vol% argon in the mixture reduced the wall fouling by 50%. Successive and intermittent fluidization also yielded fouling and specific charge values comparable to pure argon. The results are attributed to argon's relatively low dielectric strength and subsequent localized gas discharge within the particle-dense section of the bed.

Effect of gas properties on particle charging and wall fouling in gas-solids fluidized beds

Effect of gas properties on particle charging and wall fouling in gas-solids fluidized beds

Effect of temperature on triboelectrifcation of polyethylene particles in a pilot-scale pressurized gas-solid fluidized bed

In this work, the influence of temperature on electrostatic charging behaviour of polyethylene (PE) particles and their degree of fouling in a pilot-plant fluidized bed was studied at two pressure levels. PE resin received from a commercial reactor was fluidized in a 0.15 m stainless-steel column at two pressures of 101 and 2600 kPa while the operating temperatures was varied from 24 to 58 °C. The amount of wall fouling and particles charge distribution in various regions of the bed were measured. Overall, it was found that at similar operating conditions, increasing pressure gave rise to a larger amount of wall fouling. At atmospheric pressure, the fouling declined by approximately 30%, when the average bed temperature was doubled from 24 °C. At 2600 kPa, a similar trend was observed although the influence of temperature was significantly greater, with 72.5% decline at the highest temperature tested. The particles within the bulk of the bed acquired less net specific charge (Q/m) at higher temperatures, signifying a lower electrostatic charge generation within the bed with the increase in temperature.

Triboelectric effects of continuity additives and a silica catalyst support on polyethylene fluidized bed wall fouling

In commercial polyethylene gas-phase reactors, continuity additives (CAs) are periodically fed into the reactor to tackle reactor sheeting. In dry feeding of CAs, they are pneumatically conveyed into the reactor separately or doped on the catalyst support. This work investigated the role of five materials reported to act as CA (aluminum-distearate, AS-990, calcium-stearate, zinc-stearate, and CHIMASSORB-944) along with a catalyst support (amorphous silica) on the polyethylene fluidized-bed fouling degree. Powders were conveyed into a 0.1-m diameter stainless-steel atmospheric fluidization column containing polyethylene using a pneumatic conveying line of similar material (SS316) and diameter (4.75 × 10−3 m) to those used in commercial processes. The bed bulk net specific charge and the extent of wall fouling were directly proportional. The degree of bed charge and wall fouling varied by the type/concentration of the CA, emphasizing the importance of CAs surface chemistry. An optimum CA-to-silica ratio was proposed to minimize the wall fouling.

Triboelectric effects of a pneumatically injected silica catalyst support on polyethylene fluidized bed wall fouling

In commercial ethylene gas-phase polymerization, small loads of catalyst particles periodically fed into the reactor. This necessitates studying their contribution on the fluidized bed reactor electrification and sheeting. Prior to testing catalysts utilized in commercial processes, this work focused to investigate the role of a typical amorphous silica catalyst support used in ethylene polymerization on the polyethylene fluidized bed electrification and wall fouling. A 5-m in length and 0.00475-m inner diameter stainless-steel pneumatic conveying line was used to convey 0.1 and 1 g of silica (representing 100 and 1000 ppmw) at various fluidization times of 0, 15 and 30 min into a 1-kg bed of polyethylene fluidized in a 0.1 m diameter stainless-steel atmospheric column. Overall, a reduction was observed in the wall fouling. This was attributed to the silica's negative charge upon entering the bed, silica/polyethylene particle number ratio, and silica's capability to induce negative charge within the bed.

Analysis of wall fouling and electrostatic charging in gas-solid fluidized beds

This research study focuses on wall fouling and electrostatic charging in gas–solid fluidized beds. Experiments were conducted with glass bead particles with different mass median diameters in an acrylic column at different humidity levels. The coverage ratio of particles on a wall was measured with two different methods. To obtain the local coverage ratio, the number of particles was counted with digital image processing. To achieve the required average coverage ratio, all the particles which adhered to the wall were weighed. The electrostatic charges of these particles and in the dense phase of the fluidized beds were measured individually with the use of a Faraday cup combined with a vacuum device. The surface potential of the wall was also measured with an electrostatic potential meter. The coverage ratio of the particles was high at low humidity because of the electrostatic attraction that was affected considerably by the surface potential of the wall compared with the surface charge density of the particles. The relationship between the wall fouling and electrostatic charging could be explained based on (a) the charge that the wall provided to the particles according to the triboelectric charging and (b) the number of particles that acquired the charge.

Prediction of the Ash Deposition Characteristics of Blended Coals in a 500 MWe Tangentially Fired Boiler

A computational simulation was performed to predict ash deposition in an industrial scale coal-fired boiler. The single-rate model and the improved nth order model were employed for devolatilization and char reaction with the model constants obtained from PC Coal LAB.(1) Ash particles were deposited on the wall by inertial impaction with the capture efficiency in terms of the estimated viscosity. The models for char combustion and ash deposition were implemented as new user defined functions in ANSYS-FLUENT.(2) The results showed that the deposition of blended coals was in good agreement with measurements under the influence of the surrounding flow field in a pilot scale furnace. The same combustion and ash deposition models were applied to the 500 MWe tangentially fired boiler burning blended coals. Ash deposition results for the boiler showed the same tendency as the measured data in terms of the wall fouling resistance. A larger deposition was predicted at the heat exchangers for sub-bituminous coal than for bituminous coal. However, there was no significant influence of the coal types in the furnace region. A parametric investigation was performed on ash deposition with respect to injection location and tilt angle of the burners. Deposition in the ash hopper was affected by the injection location but not by the coal types. The results showed a maximum ash deposition at the heat exchanger section for sub-bituminous coal from the lower burner set. Ash deposition increased in the ash hopper for the downward tilt angle of burners, whereas the maximum deposition occurred at the heat exchangers for the upward tilt angle.

Study of the Effect of Fin-and-Tube Heat Exchanger Fouling on its Structural Performance

ABSTRACTThis paper presents the thermal and structural analysis of high temperature fin-and-tube heat exchanger. Water flowing in tubular space and flue-gas flowing in the intertubular space, were considered as working fluids. The effect of limescale fouling on thermal and structural performance of heat exchanger was studied. The analysis considered an industrial heat exchanger, which failure occur from time to time. The expert inspection, after the failure indicated the existence of fouling layer within the heat exchanger tubes. In order to understand the reasons of heat exchanger failure, a detailed fluid flow analysis (both in the tubular and intertubular spaces) was performed. The analysis indicated that the silicate limescale fouling layer with thermal conductivity of 0.35 W/(mK) and thickness up to 1.5 mm existing in the tube, may increase the tube wall temperature even more than 150°C. The study also includes the impact of outer tube wall surface fouling with thickness of 0.2 mm and heat transfer coefficient of 2 W/(mK). As a result, the compressible stresses may increase over three times compared to the situation where the tube wall fouling does not exist.

Comparison of electrostatic charge generation in gas-solid fluidized beds in turbulent versus pre-turbulent flow regime

In this work, the effect of gas velocity on the electrostatic charge generation in gas-solid fluidized beds was studied with a specific focus on the transition to turbulent flow regime. Experiments were conducted at a pressure of 2600kPa (abs) with fluidizing gas velocities of 1.5, 3, and 5 times of Umf (pre-turbulent regimes) and 7.5 times of Umf (turbulent regime). Increasing the gas velocity and transitioning to the turbulent flow regime improved particle-wall contacts; and thus, augmented the extent of wall fouling, which indicates the increase in bed electrostatic charge generation. The amount of fouling was approximately five times larger in turbulent flow regime (7.5Umf) in comparison to that for the lowest gas velocity examined in bubbling flow regime (1.5Umf). The particles coating on the column wall consisted of a thick bottom layer which extended to a height of approximately 1m above the distributor plate, and a thin top layer which extended to the top of the column near the outlet. The particles net specific charge in the top and bottom layers did not vary with the increase in gas velocity. However, the net charge of these particles increased. The fine particles entrained from the bed had a net negative charge resulting in a net positive charge to be left behind in the bed contributing to the increase in the magnitude of wall fouling at higher gas velocities, especially in turbulent flow regime.

An experimental study of dynamic jet behaviour in a scaled cold flow spray dryer model using PIV

AbstractResearch on the dynamic flow behaviour in spray dryers has a long history. Interest in describing these flows originates from problems like roof and wall fouling. The aim of the present study is to experimentally investigate the dynamic jet behaviour and turbulent flow in a scaled‐down cold flow model of a spray dryer in order to better understand and optimize spray drying units. Dynamic jet behaviour and turbulent flow features (i.e., RMS velocities) were studied by particle image velocimetry (PIV) using water as the continuous phase. To obtain more insight in the jet dynamics, we analyzed the turning point, the width and shape, and the velocity profiles of the turbulent jet at different heights and the turbulence characteristics. We found that at higher Reynolds numbers, the jet penetrates further along the downward direction with a time‐averaged profile which is symmetric at the centre. In addition, we investigated the effect of the expansion ratio via proper orthogonal decomposition (POD). Outcomes of different characteristics of the dynamic jet, like steady, transient, regular, and complex precession, can be collapsed by proper scaling. These results can be used for validation of computational fluid dynamics simulations and facilitate the design (identification of jet operation boundaries) of new spray dryer configurations.

Softening of polyethylene powders at reactor conditions

Polymer softening is amajor problem in industrial gas phase polymerizations, because it enhances particle agglomeration and reactor wall fouling. It is thus desirable to investigate this phenomenon and identify safe operating conditions. We developed anew experimental method for the characterization of polyolefin softening and proposed atheory qualitatively correlating this phenomenon with the Young modulus. The developed laser dilatometry technique is based on the measurement of polyolefin powder deformations under a constant load at elevated temperatures. Our method enables to measure the softening of polyolefin powders in air and in hydrocarbon gases/vapors, which means that polyolefin softening can be studied at industrially relevant conditions. In the experimental part, we present results obtained for a broad set of polyethylene samples in various atmospheres. The results are in very good agreement with our theoretical expectations and thus verify the proposed effects of polymer properties and penetrant sorption on the softening of polyethylene.

Investigation of electrostatic charge distribution within the reactor wall fouling and bulk regions of a gas–solid fluidized bed

Abstract The distribution of charge within the wall fouling region and bulk of a fluidized bed reactor was investigated. Experiments were conducted in a 0.1 m in diameter carbon steel fluidization column under atmospheric conditions. Polyethylene particles were fluidized with extra dry air at 1.5 the minimum fluidization velocity (bubbling flow regime) for 1 h. Using an online Faraday cup measurement technique, the net charge-to-mass ratio ( q / m ), as well as the size distribution of all particles adhered to the column wall and those in the bulk of the bed was determined. The wall particles were found to be predominantly negatively charged while those which did not adhere to the wall were predominantly positively charged. The charge distribution within each region was then investigated by a custom made charged particle separator that separated the particles according to their charge magnitude and polarity. It was determined that although the net charge of the wall layer particles was negative, a significant amount of positively charged particles existed within each sample and therefore the entire wall particle layer. This suggests that the wall layer was formed through layering between positively and negatively charged particles. Particles in the bulk of the bed also consisted of bipolarly charged particles.

Effect of gas relative humidity on reactor wall fouling generated due to bed electrification in gas-solid fluidized beds

Abstract By having a thorough understanding of the methods involved in reducing electrification within gas-solid fluidized beds, additional techniques for reduction or prevention can be developed. In this study, the effect of fluidizing gas relative humidity was investigated with the focus on its influence on charge distribution within a fluidized bed, and the amount of particle wall fouling within the bubbling and slugging fluidization flow regimes. Gas relative humidities of 0%, 20%, 40%, 60% and 80% were evaluated with polyethylene resin received from a commercial reactor. The effect on charging was examined for particles in three regions of the bed: elutriated fines, bulk particles and those particles that adhered to the fluidization column wall. Particle charge, mass, and size distributions were measured for all regions. It was found that the effect of relative humidity on wall fouling was only apparent in the bubbling flow regime where the particle layer on the wall reduced at 60% and 80% relative humidity. A similar trend was also observed for these particle charges while their charge-to-mass ratio remained constant. In the slugging flow regime for those particles that adhered to the column wall a decline in particle net charge-to-mass ratio was obtained for only at high humidity of 80%. Other variations in charge and charge-to-mass ratio were observed in both flow regimes but did not follow any trends with respect to the gas relative humidity.

Effect of fluidizing particle size on electrostatic charge generation and reactor wall fouling in gas–solid fluidized beds

Electrostatic charge generation from particle–particle and particle–wall contacts in gas–solid fluidized beds is virtually unavoidable and generally undesired. Charged particles are known to cause problems including particle agglomeration and particle–reactor wall adhesion. The polymerization industry has been plagued by reactor wall fouling for many years, but the actual mechanism behind this problem is not well understood. The wide particle size distribution present in polyolefin fluidized bed reactors is believed to play an important role. This work studied the effect of particle size of a polyethylene resin received directly from industrial reactors on electrostatic charge generation and reactor wall fouling. The resin was sieved into five different narrowed particle size fractions and fluidized at two different gas velocities; 1.5 and 4 times the minimum fluidization velocity (umf) representing the bubbling and the slugging flow regimes. The fluidization was performed in a 0.1m carbon steel fluidization column with the charge measurement technique described by Sowinski et al. (2010). In the bubbling flow regime significant particle wall adhesion was found for particles sizes up to 600μm, after which very little reactor wall fouling was observed. In the slugging flow regime, there was significant particle wall adhesion for all particle size fractions tested with the exception of the 600–710μm range, which at times resulted in significant particle–wall adhesion and in other times none was obtained. Overall, it was found that the smaller particles had a higher charge and resulted in more reactor wall fouling. With the non-sieved resin, the particles that adhered to the column wall were approximately of the same size as the smallest size fraction tested (300–425μm). This study found that the measurement of the net charge-to-mass ratio inside a fluidized bed is not an indication of the amount of reactor wall fouling.

Comparison of the effect of grounding the column wall in gas-solid fluidized beds on electrostatic charge generation

In gas-solid fluidized beds as particles are fluidized, they continuously come into contact with other particles, as well as the fluidization column wall. This generates electrostatic charges by means of triboelectrification and frictional charging, leading to particle agglomeration, reactor wall fouling, and eventually process downtime and large financial losses. Grounding the fluidization column has been considered as a means of helping electrostatic charge dissipation within fluidized beds; however, in industrial applications despite the process vessels being grounded, the electrostatic problem still persists. This work focused on the effect of fluidization column grounding on particle wall fouling. Experiments were conducted in an atmospheric system consist of a 0.1 m in diameter carbon steel fluidization column. The mass and charge-to-mass ratio (q/m) of the particles that remained adhered to the column wall upon the completion of one hour fluidization period were measured in an electrically isolated and grounded columns to quantitatively determine the amount of reactor wall fouling. Polyethylene particles with different particle size ranges (300– 1000 μm) were fluidized with extra dry air at 1.5 times their respective minimum fluidization velocity (umf). Results obtained in the grounded fluidization column were not significantly different from those in the isolated column for all particle size ranges tested where the particles mass collected and q/m and were found to be generally similar.

Oil–water flows through sudden contraction and expansion in a horizontal pipe – Phase distribution and pressure drop

In this paper, change of flow patterns during the simultaneous flow of high viscous oil and water through the sudden contraction and expansion in a horizontal conduit has been studied. It is noted that these sudden changes in cross-section have a significant influence on the downstream phase distribution of lube oil–water flow. The observation suggests a simple technique to establish core flow as well as a way to prevent pipe wall fouling during the transportation of such oil. A number of interesting differences have been noted during low viscous oil–water flow through the same test rigs. While several types of core annular flow are observed for the former case, a wider variety of interfacial distribution characterizes kerosene–water systems. The pressure profiles during the simultaneous flow of lube oil and water through the sudden contraction and expansion are also studied and compared with low viscous oil–water flows. The pressure profiles are found to be independent of liquid viscosity and the loss coefficients are observed to be independent of flow patterns in both the cases.

Residence time optimised choice of tube diameters and slit heights in distribution systems for non-Newtonian liquids

In order to minimise thermal degradation effects such as yellowing and losses in molecular weight, distribution systems for polymer melts and solutions should be designed for residence times as small as possible for a given value of the total pressure loss. Tube and slit flow distributors that can be considered as mainly one-dimensional systems are analysed with respect to an optimal ratio of technical expenditure (pressure loss) and gain in polymer quality (reduction in residence time, i.e. in thermal degradation). Assuming steady, fully developed, laminar flow and excluding changes in the rheology of the liquid with residence time, this leads to mathematical optimisation problems which may be solved by the methods of the calculus of variations. The definition of average residence time turns out to be ambiguous for systems with more than one inlet or one outlet. The problem may be defined in two alternative ways, using different measures of residence time. One of these time measures, called “travel time”, turns out to be superior as a criterion for the optimisation of distributors for polymeric liquids. Based on this criterion, a general design rule for optimising slit and tube distributors is derived. This rule states that, in order to minimise the travel times within a system, it is sufficient to choose the diameter of the tubes or the slit width in such a way that the local apparent wall shear rate is uniform throughout the whole system. This rule has a broad range of applicability. It holds for arbitrary non-linear flow curves of the liquid under consideration and for arbitrary total flow rates. In addition, the choice of a uniform wall shear rate and, hence, a uniform wall shear stress safeguards the highest level of protection against wall fouling throughout the system at the lowest possible cost (pressure loss).

Flexibly using results of CFD and simplified heat transfer model for pulverized coal‐fired boilers

The efficient use of pulverized coal is crucial to the utility industries. The use of computational fluid dynamics (CFD)-based numerical models has an important role in the design of new boiler furnaces or in retrofitting situations. The results of CFD simulations can be used to better understand the complex processes occurring within the boiler furnace. The use of these results to support boiler operation and training of operators requires that the CFD models can be easily accessed and the results are easily analysed. This paper discusses two ways to simulate the heat transfer process in boiler furnaces. The method directly applying CFD results is employed, in which the grid for solving the energy equation is the same as the flow grid in the CFD simulation while radiation heat transfer is solved in another relatively coarse grid. Comparison of the prediction results between CFD and Heat Transfer code (Simple model) is performed under boiler full load (100%) with one side wall fouling, as well as for different boiler loads (100, 98 and 95 per cent boiler full load, respectively). Finally, the flexible use of the results of CFD and the simple model for pulverized coal-fired boilers is presented. To facilitate the use of the system, a user-friendly interface was developed which enables the user to manipulate new calculations and to view results, namely performing ‘what–if’ analysis. Copyright © 2000 John Wiley & Sons, Ltd.

Flexibly using results of CFD and simplified heat transfer model for pulverized coal-fired boilers

The efficient use of pulverized coal is crucial to the utility industries. The use of computational fluid dynamics (CFD)-based numerical models has an important role in the design of new boiler furnaces or in retrofitting situations. The results of CFD simulations can be used to better understand the complex processes occurring within the boiler furnace. The use of these results to support boiler operation and training of operators requires that the CFD models can be easily accessed and the results are easily analysed. This paper discusses two ways to simulate the heat transfer process in boiler furnaces. The method directly applying CFD results is employed, in which the grid for solving the energy equation is the same as the flow grid in the CFD simulation while radiation heat transfer is solved in another relatively coarse grid. Comparison of the prediction results between CFD and Heat Transfer code (Simple model) is performed under boiler full load (100%) with one side wall fouling, as well as for different boiler loads (100, 98 and 95 per cent boiler full load, respectively). Finally, the flexible use of the results of CFD and the simple model for pulverized coal-fired boilers is presented. To facilitate the use of the system, a user-friendly interface was developed which enables the user to manipulate new calculations and to view results, namely performing ‘what–if’ analysis. Copyright © 2000 John Wiley & Sons, Ltd.

Wet‐dry process of HCL removal from flue gas: experimental study on operating parameters

Wet‐dry process of hydrogen chloride removal from flue gas has been studied at pilot scale. Hot gas produced by methane burner, added with gaseous hydrogen chloride, is contacted by an aqueous suspension of calcium hydroxide in a cylindrical steel vessel 0.25 m I.D. and 4 m high. The effects of feed ratio Ca(OH)2/HCl, hydrogen chloride concentration in the inlet gas, operating temperature and residence time on the amount of hydrogen chloride removed have been exstensively investigated. The effect of inside wall fouling by Ca(OH)2 has also been evaluated. The experimental results show that the overall amount of hydrogen chloride removed increases when the feed ratio and the gas inlet concentration are increased and the operating temperature is decreased. However, the HCL removal does not seem affected by the residence time within the short interval investigated.