Riser Column Research Articles

Transient hydrodynamics and heat transfer behaviour in a pressurized circulating fluidized bed during abrupt changes in operating pressure

Pressurized circulating fluidized beds (PCFB) are subjected to various fluctuating conditions due to fluctuations in load demands owing to its vast utilization as a combination of heat and power generating unit. As such, analysis of transient heat transfers and hydrodynamics during these fluctuations in PCFB is very important. The present work includes experimental investigation on the transient behaviour of a PCFB unit during abrupt changes in operating pressure. The experiments are performed on a laboratory scale PCFB set up consisting of a stainless-steel riser of height 2 m and an inner diameter of 54 mm. The operating pressure is regulated by a pressure regulator and is measured using a piezo resistive pressure sensor attached to the inlet of the riser. Static pressures and temperatures are also measured along the riser column at various heights. Experiments are performed with 500 g sand as bed inventory at two operating pressure of 2 and 3 bar. Drastic changes in voidage values were observed mostly in the upper region of the riser column for transient conditions in the operating parameters. The experiments showed that with a sudden drop in operating pressure, the heat transfer coefficient momentarily increase and subsequently decrease steadily. The reverse is observed for a sudden increase in operating pressure.

Elements constituent for the design of a riser system in areas deep water and extreme deep water applied for offshore drilling

Riser systems are integral components of the offshore developments used to recover oil and gas stored in the reservoirs below the earth’s oceans and seas. These riser systems are used in all facets of the development offshore process including exploration and exploitation wells completion/intervention, and production of the hydrocarbons. Their primary function is to facilitate the safe transportation of material, oil and gases between the seafloor oceans and seas and the marine platform. As the water depth increases, the working conditions of this system becomes challenging due to the complex forces and extreme environmental conditions which are impacting the operational mode as well as the stability. In this paper several aspects concerning riser mechanics and the behaviour of the riser column will be evaluated against different operational situations.

Dynamic risk assessment approach of riser recoil control failure during production test of marine natural gas hydrate

AbstractThe marine natural gas hydrate is considered to have huge resource potential in the South China Sea, and a sixty‐day production test has been successfully conducted in 2017. As the crucial and most vulnerable connection of floating platform and subsea wellhead, the risers are of paramount importance in safe production test of the hydrate. Due to harsh environment and the possible drive off and drift off events induced by failure of dynamic positioning system of the platform, the unplanned emergency disconnect of risers is an integral preventive measure to avoid accidents and secure the well. Once the disconnect occurs, the huge potential energy stored in the riser column will release and consequently cause violent axial recoil, which will cause serious consequences if the recoil control fails. This paper presented a quantitative risk assessment approach on the basis of dynamic Bayesian networks to dynamically predict the risk of riser recoil control failure during production test of hydrate. By using the proposed method, the dynamic failure risk of the riser recoil control was figured out, and the most contributory factors were identified. The practical application of the proposed methodology was demonstrated through a case study. Results indicated that the probability of riser recoil control failure can be significantly decreased within one year through equipment repair, and the failure and repair rates of shutoff valves and main air isolation valves are the most contributory factors. Some corresponding safety control measures were proposed to further mitigate the risk of riser recoil control failure during the production test of hydrate.

Combining self-lift and gas-lift: A new approach to slug mitigation in deepwater pipeline-riser systems

Recent slugging studies have identified the need to develop new and optimal slug mitigation strategies as the key gap in slugging studies. This paper focused on combining the self-lift and gas-lift slug mitigation strategies for slugging mitigation in deepwater scenarios.The Self-Lift Slug mitigation Strategy is a novel approach that involves tapping off gas via a by-pass pipe along the pipeline upstream of the riser. The gas is then re-introduced into the riser column via an injection point on the riser, to break the liquid slugs within the riser and mitigate severe slugging.This research investigated the performance of the Self-Lift Strategy when adapted to a deepwater oil field case scenario via OLGA simulation. An alternative scenario of combining self-lift and gas lift was also investigated.The approach for this study involved a robust methodology of firstly validating the field data by comparing pressure field data with OLGA simulation of the input data from the field. The field case consists of a 2712 m pipeline section and a 1513 m riser section. In this study, the severe slugging scenario was first run at the following flow conditions: well X1 was set up at 72.2 °C temperature and mass flow of 3.25 kg/s, well X2 was set up at 70.5 °C and 12.13 kg/s mass flow rate and the outlet section set up at 65.5 °C and pressure 20 bara.Self-lift was able to reduce holdup within the riser column, however pressure behaviour at the riser column was observed to be over 20 bara which is the design pressure for the inlets of the separator. Further studies involving a combination of Self-lift and gas lift at 8 kg/s and 2 inches by-pass internal diameter, indicated that the pressure at the riser column was reduced to 20 bara, allowing for stable flow into the separator.

In-loop oxygen reduction in HLM thermal-hydraulic facility NACIE-UP

NACIE-UP loop located at the ENEA Brasimone Research Centre is an experimental thermal-hydraulic facility working with liquid lead-bismuth eutectic (LBE). The facility was designed and constructed with the purpose to support the research activity on future nuclear reactors cooled by HLMs (Heavy Liquid Metals).A major problem in the operation of experimental HLM facilities concerns the chemical control of the coolant and, in particular, of the dissolved oxygen. Indeed, a reduction of the oxygen concentration in the HLM must be performed before each experimental thermal-hydraulic campaign in order to prevent the formation of PbO, whose deposition above piping and components may affect the experimental results. The present work describes the conditioning to low oxygen of the LBE inside NACIE-UP facility. The oxygen concentration reduction was performed by injecting Ar-3%H2 gas mixture in the riser column of the loop for 650 h and by varying the LBE temperature in the range 230–400 °C. The oxygen concentration in the LBE was monitored in the expansion vessel of the loop using a potentiometric Cu/Cu2O oxygen sensor.The sensor showed that a deep oxygen reduction in the LBE was successfully obtained, even if the optimal working concentration for the operation of a HLM nuclear system was not achieved. The oxygen concentration was globally reduced from 10−6 to 10−12% in weight at around 250 °C. Furthermore, the sensor revealed the establishment of M/MxOy equilibria in the expansion vessel, indicating that metal impurities dissolving from loop steel walls were here collected and influenced the electric potential of the sensor.

Investigation of slug mitigation: self-lifting approach in a deepwater oil field

Slug flow is a flow assurance issue that staggers production and, in some cases, 'kills the flow' of the well. Severe slugging, a type of slugging which usually occurs at the base of the riser column, causes large amplitudes in the fluctuation of pressure within the riser column and consequently damages equipment placed topside. An adaptation of a novel concept to slug mitigation: the self-lifting model, is presented. This model presents variations to the internal diameter of the self-lift bypass to produce effective mitigation to severe slugging.

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.

Experimental and Simulation Study of Fluidization Behavior of Palm Biomass in a Circulating Fluidized Bed Riser

Circulating fluidized bed combustion (CFBC) technology is being used for few decades to burn biomass wastes while meeting the stringent emission standards; however, much is needed to be understood about the complex flow patterns that are encountered. This work is aimed at understanding the major fluidization characteristics in a CFB loop for palm shell waste powders. Effects of inlet air on the fluidization behavior were experimentally investigated. The voidage of the bed material was found to be a function of axial distance above the distributor. The CFD modeling has also been done to quantify the riser exit effects. The flow was simulated using the “algebraic slip mixture (ASM)” model. In the simulation results the deviation of velocity contours owing to variation in riser exit geometries is discussed. It was found that the influence is found to be significant in the upper region of the riser column and the velocity contours are also swayed by the exit geometry.

Study of Hydrodynamics and Dry Beneficiation Characteristics of Coal in the Riser of a Circulating Fluidized Bed Reactor

Dry beneficiation is an effective way of reducing the ash content of coal especially in the water scarce region. Fast fluidization region was used to carry out some exploratory investigations with raw coal to reduce the ash below a 12% level from a feed ash of 43%. As the coal was fluidized in the riser of a fast fluidized bed, the heavier and coarse particles generally settled at the bottom in a dense zone of the riser while the finer and lighter particles were entrained to the top dilute zone and, using a cyclone, were fed back to the bottom of the circulating fluidized bed (CFB) riser column. Most of the low-ash fine-size coals being lighter moved up to the riser, where most of the beneficiation occurred.

Process Simulation of Using Coal Pyrolysis Gas to Control NO and N2O Emissions during Coal Decoupling Combustion in a Circulating Fluidized Bed Combustor Based on Aspen Plus

A process simulation model of CFB coal decoupling combustion process has been developed based on Aspen Plus and verified by comparing with the reported NO and N2O emissions from experiments. The detailed information on formation and decomposition of NO and N2O along the height of CFB riser column can be quantitatively simulated by the developed process simulation model. The simulated results show that about 99.9% of the emitted NO in flue gas is controlled by fuel-N combustion; however, the introduced NH3 in coal pyrolysis gaseous products as an NO precursor can play an important role on NO formation from about 39.7% to 97.6% in the dense phase region at the lower CFB riser column. About 98.9%-99.8% of NO decomposition along the height of CFB riser column is dominated by char particles reduction. About 91.8%-95.8% of the emitted N2O in flue gas is controlled by fuel-N combustion; however, the introduced HCN in coal pyrolysis gaseous products as an N2O precursor has an obvious effect on N2O formation from 74.2% to 94.6% in the dense phase region at the lower CFB riser column. The contribution of char particles reduction on N2O decomposition can be found as 1.8%-3.6% along the height of CFB riser column. A competitive relationship between O-2 oxidization and CO reduction on N2O decomposition along the height of CFB riser column during the CFB coal decoupling combustion process has been revealed.

Liquid phase axial mixing in solid–liquid circulating multistage fluidized bed: CFD modeling and RTD measurements

Liquid phase residence time distribution (RTD) studies have been performed in conventional solid–liquid fluidized bed (SLFB) and solid–liquid circulating multistage fluidized bed (SLCMFB). The riser column was made up of 50mm i.d. and 2m long glass pipe while the multistage down comer column (glass) consisted of seven stages of 100mm i.d. and 100mm long sections each having perforated plate as a distributor (having 480 holes of 2mm diameter). RTD experiments for SLFB were carried out in the column having the same diameter as the downcomer of SLCMFB. RTD has been estimated for both the riser column and the multistage column of SLCMFB. Computational fluid dynamic (CFD) simulations of SLFB and riser section of SLCMFB have been performed to predict the RTD. In all the above cases good agreement was found between the CFD predictions and the experimental measurements. Ion exchange resins and glass beads were used as a solid phase and water as a fluidizing medium. The dispersion characteristics of SLFB and SLCMFB have been investigated for resin particles with size range of 0.36–0.72mm and glass beads with size range of 0.1–0.7mm. It was observed that the liquid phase axial dispersion coefficient depends strongly on superficial liquid velocity, particle size and particle density. Based on the experimental data, empirical correlations have been proposed for liquid phase axial dispersion coefficient and have been found to be applicable to all the available data in the published literature.

Drilling From an FDPSO: A Two-Stack Approach

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 20491, ’Drilling From an FDPSO: A Two-Stack Approach,’ by Kenneth C. Hampshire, SPE, and J. Greg Noles, Murphy West Ltd., and Albert Kachich, Doris Inc., originally prepared for the 2010 Offshore Technology Conference, Houston, 3-6 May. The paper has not been peer reviewed. One reason for the success of the Azurite floating drilling, production, storage, and offloading (FDPSO) vessel was the innovative approach in specifying the equipment to fulfill the drilling requirements. This resulted in the specification and deployment of many unique drilling components that combine to make up the drilling structure that extended from the seafloor to the surface. Introduction The Azurite field is in 4,600-ft water depth in the Mer Profonde Sud Block offshore Republic of Congo, a part of the world that is without significant infrastructure to transport or store hydrocarbons. As a result, a requirement for developing the Azurite field was to find a way to combine drilling activity and oil storage economically. To do this, a drilling rig was installed on a floating production, storage, and offloading (FPSO) vessel—making it an FDPSO. This unique and innovative combination presented a variety of challenges. To address these challenges, one solution was to use two blowout-preventer (BOP) stacks while drilling, to ensure well control and reduce drilling risk. Two-Stack Rationale Controlling wellbore pressure is paramount to safety in drilling operations. Conventional deepwater-drilling operations use a subsea stack of BOPs to achieve this. Many combinations of closure scenarios are available with a properly specified stack to reduce or remove internal pressure in the riser column above the stack. Well control requires frequent actuation of BOP functions. Functionality and ease of maintenance would be enhanced greatly if the stack could be installed above the water line. Additionally, the size and weight of the surface stack would be significantly smaller than a subsea stack. The riser also would be simplified in that it would not need to carry hydraulic control lines down to the stack, and the riser would not need to have a choke or kill line. The idea of having a wellhead connected to a stack mounted above the water line with a high-pressure riser system was appealing, but having no protection at the seabed was not. The Azurite project team recognized that if the integrity of the riser was compromised for any reason and drilling or reservoir fluids escaped, the environmental consequences would be unacceptable. To address this concern, another stack, customized to safeguard against such an occurrence, was specified that would be a standalone module that would not require the rigorous functionality of the surface stack or require control lines running down to it from the surface. This package was called the subsea isolation device (SID). Fig. 1 shows the surface stack (left) and the SID.

Solid−Liquid Circulating Multistage Fluidized Bed: Hydrodynamic Study

The solid-liquid circulating multistage fluidized bed (SLCMFB) was constructed with the principal components being the riser column and the multistage column. The riser column was made up of 50 mm i.d. and 2 m long glass pipe while the multistage column was made up of seven glass stages of 100 mm i.d. and 100 mm length. The ion-exchange resin was used as solid phase and water as fluidizing medium. The flow characteristics of SLCMFB were investigated for 0.365, 0.605, and 0.725 mm particle sizes (dry basis). The voidage in the riser and multistage column was measured using γ-ray tomography (GRT). In the riser column, the voidage was found to be maximum at the center and minimum near the wall. However, in the multistage column, voidage was found to be uniform over cross section of the column. The solid particle velocity was measured using ultrasonic velocity profiler (UVP) in the riser column. The particle velocity profiles show similar trend as the voidage, confirming the existence of the radial non-uniformity in the riser column. Further, the non-uniformity was analyzed using the drift flux model (DFM) and the effect of particle size and superficial liquid velocity on the non-uniformity was also investigated.

Numerical modeling of axial bed-to-wall heat transfer in a circulating fluidized bed combustor

The water-wall surfaces located above the secondary air inlet within the circulating fluidized bed (CFB) combustor are exposed to the axial bed-to-wall heat transfer process. In the current work, the axial bed-to-wall heat transfer coefficients are estimated for three different axial voidage profiles (covering three widely occurring average particle concentrations) in order to investigate the effect of voidage, time, initial and fixed temperature of the bed and annulus, and gas gap between wall and solid particles; on the axial heat transfer process. A 2D thermal energy balance model is developed to estimate the axial heat transfer values for the gas–solid suspension along the height of the riser column with horizontally changing mass distribution. The gas–solid mass distribution is fixed with time thus providing a spectrum of changes in axial bed-to-wall heat transfer profile with time. The current work provides an opportunity to understand the axial heat transfer relationship with particle concentration and instantaneous behaviour. The results from the work show that: (i) first few seconds of the suspension temperature near the wall has maximum energy thus providing a small time frame to transfer more heat to the surface (CFB wall); (ii) both axial and horizontal particle concentrations (influenced by the operating conditions) affect the axial heat transfer locally; (iii) initial temperature of the bed between average and maximum values provide end limits for the axial heat transfer; (iv) annulus region has higher thermal energy than the core due to increased particle presence; and (v) a particle-free zone near the wall (gas gap) having a maximum thickness of 1 mm, tends to reduce up to 25% of axial heat transfer value. The model trends have close agreement with experimental trends from published literature; but the model values differ when correlating with real values due to inconsistencies in riser diameter and nature of variation in parameters.

Flow boiling heat transfer in circulating fluidized bed

In order to enhance heat transfer and mitigate contamination in the boiling processes, a new type of vapor-liquid-solid (3-phase) circulating fluidized bed boiling system has been designed, combining a circulating fluidized bed with boiling heat transfer. Experimental results show an enhancement of the boiling curve. Flow visualization studies concerning flow hydrodynamics within the riser column are also conducted whose results are presented and discussed.

Modeling of axial bed-to-wall heat transfer in a CFB combustor with abrupt riser exit geometry

The shape of the riser exit geometry in a circulating fluidized bed (CFB) combustor influences the hydrodynamics of the riser column in the exit region and into the riser column to a considerable length. In the present work, the change in axial hydrodynamics for two different riser exit shapes under different operating conditions is examined to predict the corresponding axial heat transfer coefficients. The influence of the two riser exit shapes (smooth and abrupt) on the axial voidage profile is estimated using the core-annulus mass flux balance model and the corresponding axial bed-to-wall heat transfer coefficient is estimated using the cluster renewal mechanistic model. The results are reported for different hydrodynamic relations and operating parameters. The analysis provides fundamental understanding on the influence of the riser exit geometry on the axial heat transfer characteristics of the CFB combustor.

Bed-to-wall heat transfer modelling in the top region of a CFB riser column with abrupt riser exit geometries

The paper presents a mechanistic model to predict bed-to-wall heat transfer coefficient in the top region of a circulating fluidized bed (CFB) riser column by considering the riser exit geometry effects on bed hydrodynamics. With abrupt riser exit geometry, some solids will reflect back in to the riser column, thereby increasing the solids concentration in the top region of the riser column of a CFB. This in turn results in higher bed-to-wall heat transfer coefficients in the top region. At present, not much information exists in the literature to predict bed-to-wall heat transfer coefficient in the top region of a riser column with riser exit geometry effects. In the present work, a mechanistic model is proposed to estimate bed-to-wall heat transfer coefficient with riser exit geometry configurations. The length of influence of gas–solid flow structure from the riser exit due to various riser exit geometries is also presented. The solids reflux ratio is an important parameter, which influences the heat transfer rate in the top region. For the same operating conditions the bed-to-wall heat transfer coefficient increases with the abrupt riser exit geometry configuration compared to a smooth riser exit in the top region. The proposed model predictions are compared with the published experimental data for right angle exit configuration and a reasonable agreement is observed.

Effect of pressure on thermal aspects in the riser column of a pressurized circulating fluidized bed

In the present paper the effect of pressure on bed-to-wall heat transfer in the riser column of a pressurized circulating fluidized bed (PCFB) unit is estimated through a modified mechanistic model. Gas–solid flow structure and average cross-sectional solids concentration play a dominant role in better understanding of bed-to-wall heat transfer mechanism in the riser column of a PCFB. The effect of pressure on average solids concentration fraction ‘c’ in the riser column is analysed from the experimental investigations. The basic cluster renewal model of an atmospheric circulating fluidized bed has been modified to consider the effect of pressure on different model parameters such as cluster properties, gas layer thickness, cluster, particle, gas phase, radiation and bed-to-wall heat transfer coefficients, respectively. The cluster thermal conductivity increases with system pressure as well as with bed temperature due to higher cluster thermal properties. The increased operating pressure enhances the particle and dispersed phase heat transfer components. The bed-to-wall heat transfer coefficient increases with operating pressure, because of increased particle concentration. The predicted results from the model are compared with the experimentally measured values as well as with the published literature, and a good agreement has been observed. The bed-to-wall heat transfer coefficient variation along the riser height is also reported for different operating pressures. Copyright © 2005 John Wiley & Sons, Ltd.

Effect of dilute and dense phase operating conditions on bed-to-wall heat transfer mechanism in a circulating fluidized bed combustor

In the present paper investigations are conducted on bed-to-wall heat transfer to water-wall surfaces in the upper region of the riser column of a circulating fluidized bed (CFB) combustor under dilute and dense phase conditions. The bed-to-wall heat transfer depends on the contributions of particle convection, gas convection and radiation heat transfer components. The percentage contribution of each of these components depends on the operating conditions i.e., dilute and dense phase bed conditions and bed temperature. The variation in contribution with operating conditions is estimated using the cluster renewal mechanistic model. The present results contribute some fundamental information on the contributions of particle convection, gas convection and radiation contributions in bed-to-wall heat transfer under dilute and dense phase conditions with bed temperature. This leads to better understanding of heat transfer mechanism to water-wall surfaces in the upper region of the riser column under varying load conditions i.e., when the combustor is operated under dilute and dense phase situations. The results will further contribute to understanding of heat transfer mechanism and will aid in the efficient design of heat transfer surfaces in the CFB unit.

04/02025 Industrial waste combustion boiler installed at paper mill - EBARA internally circulating fluidized bed boiler (ICEB) & gasifier (ICFG): Tsukamoto, K. and Miyoshi, N. Kami Pa Gikyoshi, 2003, 57, (5), 645–653. (In Japanese)

In the present paper investigations are conducted on bed-to-wall heat transfer to water-wall surfaces in the upper region of the riser column of a circulating fluidized bed (CFB) combustor under dilute and dense phase conditions. The bed-to-wall heat transfer depends on the contributions of particle convection, gas convection and radiation heat transfer components. The percentage contribution of each of these components depends on the operating conditions i.e., dilute and dense phase bed conditions and bed temperature. The variation in contribution with operating conditions is estimated using the cluster renewal mechanistic model. The present results contribute some fundamental information on the contributions of particle convection, gas convection and radiation contributions in bed-to-wall heat transfer under dilute and dense phase conditions with bed temperature. This leads to better understanding of heat transfer mechanism to water-wall surfaces in the upper region of the riser column under varying load conditions i.e., when the combustor is operated under dilute and dense phase situations. The results will further contribute to understanding of heat transfer mechanism and will aid in the efficient design of heat transfer surfaces in the CFB unit.