Riser Pressure Drop Research Articles

MP-PIC simulation of circulating fluidized beds using an EMMS based drag model for Geldart B particles

In this study the multiphase particle-in-cell (MP-PIC) method is used for the simulation of two pilot-scale circulating fluidized beds (CFBs) with quartz sand belonging to Geldart’s group B as bed material. The simulations were performed using a homogeneous drag model as well as a structure dependent drag model based on the energy minimization multi-scale method (EMMS). The results are compared with experimental data from literature as well as experiments. The simulations with the EMMS based drag model show a good agreement of the time-averaged axial solids concentration, circulation rate and riser pressure drop. Furthermore, a lower grid sensitivity is observed compared to the homogeneous drag model. In contrast to the conventional drag model a dense bottom zone is predicted by the EMMS based drag model. An overprediction of the solid concentration in the dense bottom zone is presumably due to an overprediction of the cluster diameter that is calculated using an empirical cluster diameter correlation. This shows the necessity for a new meso-scale cluster correlation for the simulation of Geldart B particles. Furthermore, the results of the time-averaged radial solids concentration differ from the expectations of a core-annulus flow indicating that a mesh refinement at the walls is necessary. Finally, the importance of using a realistic particle size distribution is identified.

Investigation and prediction of severe slugging frequency in pipeline-riser systems

Severe slugging is one of the flow assurance concerns in oil and gas industry because of the undesired fluctuations of pressure and flow rates in the system. Knowledge of severe slugging frequency is essential for the receiving facility design. However, unlike the considerable investigations of normal slug frequency in horizontal and inclined pipes, little work has been done to predict severe slugging frequency. In this work, an experimental study of air-water two-phase flow was conducted in a long pipeline-riser system. According to the riser pressure drop signals and the power spectral density (PSD) distributions, flow patterns were classified into classical severe slugging, transitional severe slugging, oscillating flow and stable flow. The effect of riser-top choking on classical and transitional severe slugging frequency were analyzed. Based on a broad range experimental database from ten flow loops, the effect of dimensionless pipeline length on severe slugging frequency was analyzed and an empirical correlation was developed to predict severe slugging frequency. Good matches with literature data demonstrated the validation of the empirical correlation. The severe slugging frequency correlation was applied to predict severe slugging length and derive a new transition criterion between classical severe slugging and transitional severe slugging in the flow pattern map.

Temperature effects on riser pressure drop in a circulating fluidized bed

Effects of temperature on pressure drop across a riser due to solids holdup in the riser of a circulating fluidized bed (CFB) were investigated at the atmospheric pressure condition. The bed material was a group of sorbent particles and the experimental variables included temperature, gas velocity, and solids flux in the riser. With gas velocity and solids flux held constant, the riser pressure drop decreased as temperature increased. However, temperature effects decreased with increase in gas velocity. The effects of temperature on riser pressure drop were confirmed qualitatively by the same effects on the average ratio of gravity to drag force on a single spherical particle while the particle was accelerated. The pressure drop across the riser increased almost linearly with the ratio of solids flux to gas flux at the given gas velocity. Because gas momentum per unit mass of gas transferred to solids increased, the slope of the linear relationship decreased as temperature increased. This result confirmed the validity of the concept of momentum transfer from gas to particles at high temperature, proposed at ambient temperature in the prior study. The amount of gas momentum per unit mass of gas available to carry over the solid particles was finite; thus as the solids flux increased at the given gas velocity, the gas momentum shared to the unit mass of solids decreased and the mean residence time of solids in the riser, i.e., the pressure drop across the riser increased linearly. The slope of the linear relationship was proportional to the ratio of momentum flux by gravity and buoyancy forces on solids to gas momentum per unit mass of gas by drag force transferred to solids. Correlations were proposed to predict effects of temperature on the pressure drop across the riser and the solids flux in the riser within the range of experimental conditions.

Solids circulation in circulating fluidized beds with low riser aspect ratio and varying total solids inventory

This paper presents an experimental study with the aim to understand the relation between the flow conditions – the riser pressure drop and fluidization velocity – in a CFB riser and the net (external) solids flux (Gs [kg/m2s]), applying a riser geometry and overall flow conditions similar to CFB boilers.The experiments are carried out in a CFB unit operated under ambient conditions. The riser has a cross section of 0.7m×0.12m and a height of 8.5m, yielding a riser height-to-width aspect ratio of 10.6 (in the wide dimension), similar to that of CFB boilers. The unit is equipped with densely spaced pressure taps providing a fine resolution of the measured vertical pressure profile along the riser and an automatic system to accurately measure Gs. The experiments cover fluidization velocities of 0.3–7m/s, riser pressure drops of 1.7–10.5kPa and loop-seal fluidization velocities of 0.12–0.54m/s (secondary air flow is not included). These ranges correspond to conditions both with and without a dense bottom region.The results show that Gs is determined by the solids concentration at the riser top, which in turn depends on riser pressure drop and fluidization velocity, and the backflow effect, which depends on the configuration and flow conditions of the loop seal and the exit region. For operating conditions with a dense bottom bed present, Gs is independent of riser pressure drop at any fluidization velocity, whereas when operating without a dense bed an increase in riser pressure drop increases Gs.

Dynamic modeling of the circulating fluidized bed riser

Dynamic modeling is required to understand hydrodynamics of a circulating fluidized bed (CFB) riser as a function of time. The paper develops a dynamic model of the riser from the conservation of solid mass. Solid mass that flows out of the riser requires particle residence time to close the mass balance equation. The mean residence time of solids was derived as the riser length divided by the superficial riser gas velocity normalized by a ratio of solid concentration in the riser outlet to that in the riser itself. For bed materials including high-density polyethylene beads, glass beads, and cork, the percentage of solid concentration in the horizontal crossover pipe connecting the riser exit and the cyclone inlet was found to be 22% of the average concentration of solids in the riser. Upon finding the particle residence time, we calculated the solid flow rate out of the riser and along with the knowledge of solids flowing into the riser column we determined the riser inventory as a function of time. We used the estimated solid inventory to model overall pressure drop across the riser from the conservation of momentum. The momentum equation included the balance of pressure force by the hydrostatic head of solids, and the wall frictions due to gas and solids in a fully developed zone. The prediction of overall riser pressure drop compared well with its measured values at many dynamic conditions. Therefore, our dynamic model is useful for developing advanced control strategy or for improving existing control systems of the CFB plant. The residence time approach is also useful for the design of a transport reactor. The technique also comes handy when setting up the boundary condition for the overall pressure drop in the CFD modeling that can simulate incremental pressures along the riser height.

Hydrodynamic characteristics in cold model of dual fluidized bed gasifiers

Gasification in dual fluidized bed gasifiers (DFBGs) has proven itself as one of the promising technologies. In DFBGs, the bubbling fluidized bed (BFB) acting as a gasifier and a riser applied as combustor are coupled with the provision of solid transfer between them. The hydrodynamic characteristics of these reactors are very important in improving their performance and achieving better operational control. In the present study, a cold model of dual fluidized bed gasifier of configuration – riser as combustor and BFB as gasifier with loop-seal as the upper connection and L-valve as the lower connection – has been constructed. The influence of operating parameters such as fluidization velocity in the riser, aeration velocity in the L-valve as well as the inventory on the pressure drops and solid circulation rate was inspected. Results show that the pressure drop in the riser decreased with the increase of the riser velocity whereas it increased with the increase of L-valve aeration velocity. Besides, the solid circulation rate was found increasing with the increase of both riser velocity and L-valve aeration velocity. However, the increase of the riser pressure drop and solid circulation rate with the increase of L-valve aeration velocity was limited due to the excessive fluidization in L-valve which disrupted the solid flow. This was termed as the maximum L-valve aeration velocity for the stable operation. The results show that the maximum L-valve aeration velocity increased with the increase of solid circulation rate and riser velocity. Furthermore, a correlation was proposed to relate solid circulation rate to riser velocity and L-valve aeration velocity.

Experimental study of the effect of friction phenomena on actual and calculated inventory in a small-scale CFB riser

Accurate information concerning riser inventory in a fluidized bed is required in some applications such as the calcium looping process, because it is related to the CO2 capture efficiency of the system. In a circulating fluidized bed (CFB), the riser inventory is normally calculated from the riser pressure drop; however, the friction and the acceleration phenomena may have a significant influence on the total riser pressure drop. Therefore, deviation may occur in the calculation from the actual mass. For this reason the magnitude of the friction and the acceleration pressure drop in the entire riser is studied in small-scale risers. Two series of studies were performed: the first one in a scaled cold model riser of the 10kWth facility, and the second one in the 10kWth fluidized bed riser under process conditions. The velocities were chosen to comply with the fluidization regimes suitable for the calcium looping process, namely, the turbulent and the fast. In cold-model experiments in a low-velocity turbulent fluidization regime, the actual weight (static pressure drop) of the particles is observed more than the weight calculated from a recorded pressure drop. This phenomenon is also repeated in pilot plant conditions. In the cold-model setup, the friction and acceleration pressure drop became apparent in the fast fluidization regime, and increased as the gas velocity rose. Within calcium looping conditions in the pilot plant operation, the static pressure drop was observed more than the recorded pressure drop. Therefore, as a conservative approach, the influence of friction pressure drop may be neglected while calculating the solid inventory of the riser. The concept of transit inventory is introduced as a fraction of total inventory, which lies in freefall zones of the CFB system. This fraction increases as gas velocity rises.

GAS-SOLID FLOW BEHAVIOR IN A PRESSURIZED HIGH-FLUX CIRCULATING FLUIDIZED BED RISER

Gas-solid flow behavior was investigated in a high-flux circulating fluidized bed (HFCFB), whose riser is 0.068 m ID and 5.2 m high. Experiments were carried out at different pressures (0.1–0.5 MPa), solids mass fluxes (122–1015 kg/m2 · s), and standard state superficial gas velocities (6–40 m/s). Geldart group B particles 137 µm in diameter and 2490 kg/m3 in density were used as bed materials. Information on pressure drop, apparent solids holdup, average slip velocity, and solid-to-air mass flow ratio were systematically tested at elevated pressures. The results showed that increase of the operating pressure led to higher total riser pressure drop and solids holdup, lower slip velocity, more homogeneous pressure drop, and longitudinal solids holdup distribution. As a result, a high-density condition with Geldart group B particles, i.e., solids holdup in the riser over 10%, could be successfully formed in the HFCFB at elevated pressure. Similar to the trend at atmosphere, the dimensionless slip velocity at pressurized condition increased with increasing solids holdup for any given superficial gas velocity, and the total riser pressure drop increased approximately linearly with solids-to-air mass flow ratio at the given operating pressure.

Operation of a 100 kW chemical-looping combustor with Mexican petroleum coke and Cerrejón coal

This study describes the design and operation of a 100kW chemical-looping combustor for solid fuels. Six experiments of continuous operation, varying between 8 and 32min in length, have been conducted. The fuels investigated were a Mexican petroleum coke and a bituminous coal from Cerrejón in Colombia. Overall, it was found that operation was stable and loss of char to the air reactor was small, meaning that the CO2 capture efficiency was high (up to 90% at temperatures close to 950°C in the fuel reactor). Gas concentration measurements showed the presence of unconverted CO, H2 and CH4 corresponding to an oxygen demand of around 20%, depending on the fuel reactor temperature. In addition, a residence-time analysis was conducted from a batch experiment, enabling an estimation of the mass flow of oxygen carriers through the system using the riser pressure drop in the air reactor.

Hydrodynamic analysis of a 10 kW th Calcium Looping Dual Fluidized Bed for post-combustion CO 2 capture

Calcium Looping (CaL) in a Dual Fluidized Bed (DFB), utilizing a carbonator and a regenerator, is a post-combustion CO 2 capture technology currently under development. At IFK, University of Stuttgart, a 10 kW th CaL DFB system has been built consisting of a carbonator riser and a Bubbling Fluidized Bed (BFB) regenerator. A major novelty of this facility is the implementation of a cone valve to control the sorbent looping rate between the two beds. This study presents detailed results of tests conducted on a hydrodynamically scaled cold model of the 10 kW th CaL DFB facility. The performance of the cold model was compared with CaL process boundary conditions in order to determine the suitability of the 10 kW th CaL DFB system. The resulting qualitative conclusions regarding DFB hydrodynamics may be of aid to other DFB processes, such as Chemical Looping Combustion (CLC) and Fast Internally Circulating Fluidized Bed (FICFB) gasification. All important operational parameters of the cold model DFB system, namely the Total Solid Inventory (TSI), riser superficial velocity, loop seal aeration, BFB overpressure, cone valve opening and mean particle size were varied in order to fully characterize the DFB operation. A stable operating region, bordered by two unstable regions, has been identified for the cold model riser. The cold model riser pressure drop profile, solid fraction profile, solid flow structure and their variation with respect to operational parameters have been analyzed in order to draw conclusions regarding axial inventory allocation and gas–solid contacting which are important criteria for the CFB carbonator's CO 2 capture efficiency. Finally, empirical correlations regarding the cold model riser entrainment and the solid looping rate have been derived.

A model for the pressure balance of a low density circulating fluidized bed

Abstract The pressure balance along the solid circulation loop of a circulating fluidized bed equipped with a solid flux regulating device has been modelled and the influence of the pressure balance on the riser behaviour has been predicted. The solid circulation loop has been divided into many sections, where the pressure drop was calculated independently: riser, cyclone, standpipe, control device and return duct. A new theoretical model, that is able to predict the pressure losses in the return path of the solid from the standpipe to the riser, has been built. A new correlation for cyclone pressure loss with very high solid loads has been found on the basis of experimental data. The pressure loss in the riser has been calculated by imposing the closure of the pressure balance, ΣΔ P = 0. Once the riser pressure drop had been calculated, the holdup distribution along the riser was obtained by imposing a particular shape of the profile, according to the different fluid-dynamics regimes (fast fluidization or pneumatic transport). In the first case, an exponential decay was imposed and the bottom holdup was adjusted to fit the total pressure drop, in the second case, the height of the dense zone was instead varied. The experimental data was used to develop the sub-models for the various loop sections have been obtained in a 100 mm i.d. riser, 6 m high, CFB. The solid was made of Geldart B group alumina particles. The tests were carried out with a gas velocity that ranged between 2 and 4 m/s and a solid flux that ranged between 20 and 170 kg/m 2 s. A good agreement was found between the model and experimental data.

Subsea Processing and Boosting - Technical Challenges and Opportunities

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18261, "Subsea Processing and Boosting - Technical Challenges and Opportunities," by R.M. Bass, Shell Intl. E&P, prepared for the 2006 Offshore Technology Conference, Houston, 1-4 May. Subsea oil-pressure boosting often enhances subsea deepwater oil developments. Currently available subsea processing and boosting solutions, global portfolio needs for subsea oil boosting, and the limitations of existing technologies in meeting these needs are described. Introduction Fig. 1 shows how subsea-processing applications have increased in recent years. Subsea processing can include the following.Subsea oil-pressure-boosting systems.Subsea oil processing.Bulk water separation.Sales-quality oil polishing.Subsea raw-seawater injection for reservoir-pressure support.Subsea gas compression.Subsea gas dewpointing and dehydration.Flow assurance.Sales-quality processing. Oil Systems Subsea oil-pressure boosting can enable ultradeepwater light-oil production, can enable deepwater heavy-oil production, can increase ultimate recovery for deepwater light oil, and may be used to reduce flow-assurance costs. In all cases, subsea boosting reduces or eliminates the back-pressure on the wells resulting from the riser hydrostatic head and the riser and flowline pressure drop caused by high viscosity. Although many reservoirs have sufficient pressure to produce to the seafloor without the use of downhole artificial lift, many ultradeepwater light-oil and deepwater heavy-oil reservoirs have pressure that is near hydrostatic pressure and cannot produce to the sea surface for more than a short period of time. Moderate-depth light-oil wells frequently are economical without artificial lift. However, as pressure declines in late life, reducing the backpressure can add significantly to ultimate recovery. Subsea oil-pressure boosting can add value to deepwater oil projects. At the time this paper was written, 10 applications were in place, and an additional five had been announced. Subsea oil-pressure boosting has been proved in water depths to approximately 3,000 ft, depending on desired boost pressure, gas content, and viscosity. Significant cost or technical challenges remain for light and heavy oil in deeper water. Bulk subsea water separation can be used to debottleneck water-processing capacity where topside debottlenecking cannot be accomplished, thus enabling additional production to an existing host. Because topside debottlenecking is possible for many situations, subsea water separation has selected applicability. Bulk subsea water separation has been used without solids-handling equipment for relatively shallow water depths and easily separated oils. Solids-handling components have been tested onshore and will be incorporated into an announced project. Water-depth limits can be extended with moderate changes in separator-vessel design. Electrostatic coalescers are being tested in topside facilities to increase separation efficiency for difficult oils. Separators for deep water remain a technical challenge. Subsea raw-seawater injection can be used to debottleneck topside facilities when additional topside injection capacity cannot be accommodated. Because topside debottlenecking is not needed for many situations, subsea raw-seawater injection has selected applicability, with two projects announced. These systems will use proven subsea injection pumps and redesigned versions of solids-filtering components that have been proved in topside application.

Flow maldistribution at bubble cap distributor in a plant‐scale circulating fluidized bed riser

AbstractFlow maldistribution was investigated in an 8.5 m high circulating fluidized bed with a rectangular cross section of 1 × 0.3 m using a 14‐bubble‐cap distributor and a 33‐bubble‐cap distributor. Experimental results indicate that the bubble caps in the center region have a large flow rate and the bubble caps at the wall region have a small flow rate. The extent of the flow maldistribution is quantitatively characterized by the ratio of the maximum flow rate through a bubble cap to the average flow rate through all bubble caps (maldistribution number, Vmax/Vav). A correlation is proposed for predicting flow rate distribution, maldistribution number, in the bubble cap distributor by the ratio of distributor pressure drop to riser pressure drop (pressure‐drop ratio, ΔPd/ΔPr). © 2005 American Institute of Chemical Engineers AIChE J, 2005

Factorial Tests on Process Operating Conditions and Bed Fines on the Circulating Fluid Bed Performance

A cold-flow circulating fluid bed (CFB) was operated using coke breeze with a packed-bed standpipe over a range of riser and standpipe air flows. The bed materials were selected to simulate solids flow in a CFB gasifier (carbonizer) but are generally relevant to most CFB processes. CFB tests were conducted primarily in the transport mode with sufficient gas velocity to achieve a uniform axial riser pressure profiles over most of the riser height. The independent variables tested included the riser gas velocity, aeration at the base of the standpipe, and concentration of fines (average particle size). The solids inventory and riser outlet pressure were maintained constant. Factorial tests were conducted in randomized order and in duplicate to provide and an unbiased estimate of the error. Fines were tested as a blocked variable. The gas velocity, standpipe aeration, and relative amount of fine particles were all found to be significant factors affecting both the riser solids holdup and solids flux. The riser pressure drop and mass circulation increased at the higher level of fines contrary to some earlier reports in the literature. The riser pressure drop was fitted using the general linear model (GLM), which explained more than 98% of themore » variation within the data, while a GLM for the mass circulation rate explained over 90%. The uncertainty of process operating variables was characterized independently through a series of duplicated flow proving experiments.« less

Detection and Modeling of the Core-Annulus Transition in Industrial and Pilot Plant Risers

The core-annulus structure is essential to the modeling and optimization of riser reactors such as used in Fluid Catalytic Cracking. This paper presents results of measurements taken with various probes in a pilot plant and in an industrial riser. The instantaneous probe signals were analyzed with sophisticated signal analysis methods based on the detection of cycles and the determination of the correlation dimension.In a pilot plant riser, a core-annulus structure was identified with optical and momentum probe measurements. Cycle analysis of the optical probe measurements showed that the annulus was unstable: its thickness fluctuated with an average cycle time of 0.3 s. There were waves at the core-annulus boundary. In an industrial riser, a similar core-annulus structure could be identified with temperature and momentum probe measurements. Local temperature measurements are much easier to perform in an industrial riser than momentum probe measurements but can provide, with cycle analysis, the location of the core-annulus transition. Analysis of the momentum probe and temperature signals showed that the thickness of the wavy transition layer between core and annulus was about the same in the pilot plant and the industrial riser, meaning that the relative range of fluctuations in annulus thickness was much smaller in the larger industrial riser.A model was developed to predict the time-averaged transition between core and annulus. This model, which had been successfully used to predict the annulus thickness in dilute-phase vertical pneumatic transport lines, assumes that the annulus thickness is such that the riser pressure drop is minimized. Measurements and model predictions were in good agreement.