Effect of the Particle Packing Configuration on Fixed Bed Performance

Fixed-bed reactors are generally considered the optimal choice for numerous multi-phase catalytic reactions due to their excellent performance and stability.However,conventional fixed beds often encounter challenges related to inadequate mass transfer and a high pressure drop caused by the non-uniform void fraction distribution.To enhance the overall performance of fixed beds,the impact of different packing configurations on performance was investigated.Experimental and simulation methods were used to investigate the fluid flow and mass transfer performances of various packed beds under different flow rates.It was found that structured beds exhibited a significantly lower pressure drop per unit length than conventional packed beds.Furthermore,the packing configurations had a critical role in improving the overall performance of fixed beds.Specifically,structured packed beds,particularly the H-2 packing configuration,effectively reduced the pressure drop per unit length and improved the mass transfer efficiency.The H-2 packing configuration consisted of two parallel strips of particles in each layer,with strips arranged perpendicularly between adjacent layers,and the spacing between the strips varied from layer to layer.

Simultaneous Adsorption of Aqueous Pb2+, Cu2+, Zn2+, and Cd2+ Using Surfactant-Modified and Unmodified Activated Carbon in a Fixed Bed Column

The goal of this work is to improve the simultaneous removal of Pb2+, Cu2+, Zn2+, and Cd2+ ions from synthetic wastewater in a fixed bed column by incorporating sodium dodecyl sulfate (SDS) onto the surface of activated carbon made from coconut shells. The activated carbons were characterized using Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy-energy dispersive x-ray (SEM-EDX). The adsorption column dynamics were studied by varying the flow rates (5, 10 and 15 mL/min), bed heights (10, 15 and 20 cm), and initial concentrations (50, 150, and 250 mg/L). The activated carbon has a pore volume of 0.715 cm3/g and a BET-specific surface area of 1410 m2/g. Sodium dodecyl sulfate (SDS) surfactant incorporation onto the surface of the activated carbon enhances its capacity for simultaneous adsorption of Pb2+, Cu2+, Zn2+, and Cd2+ from the aqueous medium. The affinity of the heavy metals to both unmodified (AC) and modified (AC-SDS) activated carbons followed the order of Pb2+ > Cu2+ > Zn2+ > Cd2+. The dynamic adsorption of the column depends on the flow rate, bed height, initial metal concentration, and SDS surface modification. With a 5 mL/min flow rate, a 20 cm bed height, and a 50 mg/L initial metal concentration, a maximum break-through time of 150 minutes for the unmodified activated carbon (AC) and 180 minutes for the SDS-modified activated carbon (AC-SDS) was reached.

Adsorption dynamics of ethane from air in structured fixed beds with different microfibrous composites

Adsorption dynamics of ethane in two granular fixed beds and structured fixed beds with microfibrous composites was studied.5A zeolite membrane 5A/PSSF(paper-like sintered stainless steel fiber)and microfibrous entrapped activated carbon(MEAC)composites were prepared by wet layup papermaking/sintering technique and in-situ hydrothermal method.Microfibrous composites were characterized by X-ray diffraction,scanning electron microscopy and N2 adsorption/desorption.Structured fixed beds were designed by filling granular adsorbents(5A zeolite or activated carbon)and microfibrous composites at the inlet and outlet of the beds,respectively.Effects of flow rate,bed height and structure on the breakthrough curves were investigated.The length of unused bed(LUB)was determined,and Yoon–Nelson model was used to fit the breakthrough curves.The experimental results showed ethane was effectively adsorbed on the granular adsorbents and microfibrous composites.Both composites could decrease the LUB values and enhance bed utilization.All breakthrough curves fitted well to Yoon–Nelson model,with correlation coefficient exceeding 0.89.The adsorption rate of ethane could be improved in the structured fixed beds,which showed an enhanced mass transfer efficiency for ethane adsorption.LUB values of structured fixed beds with 5A/PSSF composites were larger,the bed utilization values were lower,and the adsorption rate constants were higher than those with MEAC composites under the same conditions.

Liquid-phase esterification of methacrylic acid with methanol catalyzed by cation-exchange resin in a fixed bed reactor:Experimental and kinetic studies

The kinetic behavior of esterification between methacrylic acid and methanol catalyzed by NKC-9 resin was studied in a fixed bed reactor.The reaction was conducted in the temperature range of 323.15 to 368.15 K with the molar ratio of reactants from 0.8 to 1.4 under certain pressure.The measurement data were regression with the pseudo-homogeneous(P-H),Eley-Rideal(E-R),and Langmuir-Hinshelwood(L-H)heterogeneous kinetic models.Independent adsorption experiments were implemented to gain the adsorption equilibrium constants of four components.Among the above three models,the L-H model exhibited the best fitting results.The stability of NKC-9 was evaluated by long-term running with the yield of methyl methacrylate no decrease during 3000 h operation.The structure and physicochemical properties of the new and used catalyst were performed by several characterizations including thermogravimetric analysis(TG),scanning electron microscope(SEM),X-ray diffraction(XRD)and Fourier transform infrared spectroscopy(FT-IR)and so on.

Oxidative coupling of methane in a fixed bed reactor over perovskite catalyst:A simulation study using experimental kinetic model

The oxidative coupling of methane （OCM） to ethylene over a perovskite titanate catalyst in a fixed bed reactor was studied experimentally and numerically. The two-dimensional steady state model accounted for separate energy equations for the gas and solid phases coupled with an experimental kinetic model. A lumped kinetic model containing four main species CH4, O2, COx （CO2, CO）, and C2 （C2H4 and C2H6） was used with a plug flow reactor model as well. The results from the model agreed with the experimental data. The model was used to analyze the influence of temperature and feed gas composition on the conversion and selectivity of the reactor performance. The analytical results indicate that the conversion decreases, whereas, C2 selectivity increases by increasing gas hourly space velocity （GHSV） and the methane conversion also decreases by increasing the methane to oxygen ratio.

Improved Generalized Predictive Control Algorithm with Offline and Online Identification and Its Application to Fixed Bed Reactor

An improved generalized predictive control algorithm is presented in thispaper by incorporating offline identification into online identification. Unlike the existinggeneralized predictive control algorithms, the proposed approach divides parameters of a predictivemodel into the time invariant and time-varying ones, which are treated respectively by offline andonline identification algorithms. Therefore, both the reliability and accuracy of the predictivemodel are improved. Two simulation examples of control of a fixed bed reactor show that this newalgorithm is not only reliable and stable in the case of uncertainties and abnormal disturbances,but also adaptable to slow time varying processes.

Modelling and Fixed Bed Column Adsorption of Cr(Ⅵ) onto Orthophosphoric Acid-activated Lignin

The advantage of using an available and abundant residual biomass, such as lignin, as a raw material for activated carbons is that it provides additional economical interest to the technical studies. In the current investigation, a more complete understanding of adsorption of Cr（VI） from aqueous systems onto HaPO4-acid activated lignin has been achieved via microcolumns, which were operated under various process conditions. The practice of using microcolumn is appropriate for defining the adsorption parameters and for screening a large number of poten- tial adsorbents. The effects of solution pH （2-8）, initial metal ion concentration （0.483-1.981 mmol.L-1）, flow rate （1.0-3.1 cm3-min-1）, ionic strength （0.01-0.30 mmol-L-1） and adsorbent mass （0.11 0.465 g） on Cr（VI） adsorption were studied by assessing the microcolmnn breakthrough curve. The microcolumn data were fitted by the Thomas model, the modified Dose model and the BDST model. As expected, the adsorption capacity increased with initial Cr（VI） concentration. High linear flow rates, pH values and ionic strength led to early breakthrough of Cr（VI）. The model constants obtained in this study can be used for the design of pilot scale adsorption process.

Kinetic Model of Fixed Bed Reactor with Immobilized Microorganisms for Removing Low-Concentration SO_2

On the basis of the analysis of the process of treating low concentrations of sulfur dioxide （SO2） gas in a fixed bed reactor, a kinetic model is proposed for this process after taking into consideration the effects of internal diffusion, cell concentration, and production yield of microorganisms but ignoring the effect of external diffusion. The results obtained from the model simulation show that this model can indicate the influence of the process factors, Cin, η μmax, Cx, A, h, Kin, and Q, on the removal of SO2 and that the prediction of the results by this model is also satisfactory. This kinetic model can also provide some very important indications regarding the preparation of immobilized microorganisms, selection and domestication of proper species of microorganisms, as well as the design of bioreactors.

Numerical simulation of low-concentration CO_(2) adsorption on fixed bed using finite element analysis

Accurately predicting distributions of concentration and temperature field in fixed-bed column is essential for designing adsorption processes.In this study,a two-dimensional(2D),axisymmetric,nonisothermal,dynamic adsorption model was established by coupling equations of mass,momentum and energy balance,and solved by finite element analysis.The simulation breakthrough curves fit well with the low-concentration CO_(2) adsorption experimental data,indicating the reliability of the established model.The distributions of concentration and temperature field in the column for CO_(2) adsorption and separation from CO_(2)/N_(2) were obtained.The sensitivity analysis of the adsorption conditions shows that the operation parameters such as feed flow rate,feed concentration,pellet size,and column height-to-diameter ratio produce a significant effect on the dynamic adsorption performance.The multi-physics coupled 2D axisymmetric model could provide a theoretical foundation and guidance for designing CO_(2) fixed-bed adsorption and separation processes,which could be extended to other mixed gases as well.

Study and Commercial Application of Graded Optimization of Catalysts for Fixed Bed Residue Upgrading Unit

The SHIFT-G technology of inverse catalyst loading is used to optimize the catalyst grading in the residue hydrotreating unit. The results, taken from pilot tests and commercial units, have showed that the optimized catalyst grading system can reasonably distribute the reaction load, effectively improve the prop- erties of hydrotreated products, prolong the operating cycle and promote economic benefits.

Numerical simulation of fixed bed reactor for oxidative coupling of methane over monolithic catalyst

A three-dimensional geometric model was set up for the oxidative coupling of methane(OCM) fixed bed reactor loaded with Na_3PO_4-Mn/SiO_2/cordierite monolithic catalyst,and an improved Stansch kinetic model was established to calculate the OCM reactions using the computational fluid dynamics method and Fluent software.The simulation conditions were completely the same with the experimental conditions that the volume velocity of the reactant is 80 ml·min^(-1) under standard state,the CH_4/O_2 ratio is 3 and the temperature and pressure is800 ℃ and 1 atm,respectively.The contour of the characteristic parameters in the catalyst bed was analyzed,such as the species mass fractions,temperature,the heat flux on side wall surface,pressure,fluid density and velocity.The results showed that the calculated values matched well with the experimental values on the conversion of CH4 and the selectivity of products(C_2H_6,C_2H_4,CO,CO_2 and H_2) in the reactor outlet with an error range of±4%.The mass fractions of CH_4 and O_2 decreased from 0.600 and 0.400 at the catalyst bed inlet to 0.445 and0.120 at the outlet,where the mass fractions of C_2H_6,C_2H_4,CO and CO_2 were 0.0245,0.0460,0.0537 and 0.116,respectively.Due to the existence of laminar boundary layer,the mass fraction contours of each species bent upwards in the vicinity of the boundary layer.The volume of OCM reaction was changing with the proceeding of reaction,and the total moles of products were greater than reactants.The flow field in the catalyst bed maintained constant temperature and pressure.The fluid density decreased gradually from 2.28 kg·m^(-3) at the inlet of the catalyst bed to 2.18 kg·m^(-3) at the outlet of the catalyst bed,while the average velocity magnitude increased from 0.108 m·s-1 to 0.120 m·s^(-1).

Thermal Decomposition of Potassium Titanium Hexacyanoferrate(Ⅱ) Loaded with Cesium in a Fixed Bed Calciner

The thermal decomposition of potassium titanium hexacyanoferrate（ Ⅱ ） （KTiFC） loaded with cesium （referred to as Used Exchanger,or UE） was-studied at different flow rate of air in a fixed bed calciner. The calcina t ign processconsisted of four stages：ambient temperature- 180℃ （stageⅠ ）, 180-250℃（stage Ⅱ）, 250-400℃ （stage Ⅲ）, and constant 400℃ （stage Ⅳ）.The most intense reaction occurred in stage .Ⅱ. The rate of thermal decomposition was controlled, depending on the O2 flux, by O2 or CN concentration in ditterent stages. Results from differential thermal analysis （DTA） showed that the calcination reaction of the anhydrous UE was exothermic, with an approximate heat output of 4.6kJ·g^-1, which was so large to cause the possible agglomeration of calcined residues. The agglomeration could be avoided by enhancing heat transfer and controlling the O2 flux. It was found that there was no cyanides in the calcined residues and no CN-bearing gases such as HCN and （CN）2 in the off-gas. It seemed that the catalytic oxidation furnace behind the fixed bed calciner could be cancelled.

Variation of toxic pollutants emission during a feeding cycle from an updraft fixed bed gasifier for disposing rural solid waste

The variation of toxic pollutants emission during a feeding cycle was examined by field monitoring from a batch feeding updraft fixed bed gasifier for disposing rural domestic solid waste. Results showed that the content of oxygen in flue gas gradually increased, while SO_2 and HCl in flue gas decreased with time after feeding in a whole feeding cycle. Although large amount of CO was produced during the gasifying, low CO content in flue gas could be obtained after the heat treatment with an electric heating device. The distribution characteristics of dioxin congeners in flue gas indicted the re-synthesis of dioxins after flue gas heating, and the increase of oxygen promoted the synthesis of dioxins. The emission content of dioxins could meet the standard(0.1 ng I-TEQ·m^(-3),GB18458-2014) of China when the oxygen content was controlled below 8.3%. Hence, for a batch feeding gasifier,low oxygen condition should be offered by reducing air intake at the later stage of feeding cycle in order to decrease the re-synthesis of dioxins after the flue gas heating.

Assessment of coal gasification in a pressurized fixed bed gasifier using an ASPEN plus and Euler–Euler model

With the help of Aspen Plus,a two-dimensional unsteady CFD model is developed to simulate the coal gasification process in a fixed bed gasifier.A developed and validated two dimensional CFD model for coal gasification has been used to predict and assess the viability of the syngas generation from coal gasification employing the updraft fixed bed gasifier.The process rate model and the sub-model of gas generation are determined.The particle size variation and char burning during gasification are also taken into account.In order to verify the model and increase the understanding of gasification characteristics,a set of experiments and numerical comparisons have been carried out.The simulated results in the bed are used to predict the composition of syngas and the conversion of carbon.The model proposed in this paper is a promising tool for simulating the coal gasification process in a fixed bed gasifier.

Conversion of Non-a-Tocopherols to a-Tocopherol in a Fixed Bed Reactor

Since a-tocopherol has special biological and nutritional activities, it is important to convert non-a-tocopherols to a-tocopherol. This paper focuses on the effects of residence time, pressure, temperature and the mass ratio of formaldehyde to tocopherols on the content of a-tocopherol, the conversion of non-a-tocopherols, the selectivity and yield of a-tocopherol in a fixed-bed reactor. The reactor is made from φ12 (I.D.)×360mm stainless steel pipe. It is packed with a mixture of 5% Pd/C and cation exchange resin catalysts. Preliminary results indicated the suitable operation conditions are pressure 5.0 MPa, temperature 180℃, and mass ratio of formaldehyde to tocopherols 2.0. A product of more than 50% a-tocopherol content has been obtained by using 88.85% mixed tocopherols as raw material.

Runaway Criterion in Fixed Bed Catalytic Reactors with Radial Temperature Profile

The discrepancy between pseudo-homogeneous one-dimensional model and pseudo-homogeneous two-dimensional model is studied. It is found that there are great differences between two models. This paper compares the maximum and minimum values of the radial temperature in the hot spot in case that a single exothermic reaction is carried out, a correlation is obtained with pseudo-homogeneous one-dimensional model to describe the entire reactor behavior. A new runaway criterion, based on the occurrence of inflection in the hot spot locus, is developed for the case of pseudo-homogeneous two-dimensional model. This criterion predicts the maximum allowable temperature for safe operation and the regions of runaway, respectively. The calculated results show that, compared with the results based on pseudo-homogeneous one-dimensional model, runaway will easily occur when the radial temperature gradient has to be considered.

Reuse of Anaerobic Sludge for Treating Synthetic Wastewater Containing Organic and Inorganic Pollutants in Fixed Bed Adsorber

Dried anaerobic granular sludge （DAGS） for removing phenol （organic） and lead （inorganic） pollutants from synthetic wastewater was examined in a fixed bed system. A general rate multi-component model has been built to predict the fixed bed breakthrough curves for single and dual-component system. This model considers both external and internal mass transfer resistances as well as axial dispersion with non-liner multi-component isotherm. The equilibrium isotherm model parameters such as isotherm model constants （qm and b）, pore diffusion coefficients （Dp） were obtained from batch experiments, while the external mass transfer coefficients and axial dispersion （kf, Dz） were calculated from empirical correlations. The results show that the general rate multi-component model was found suitable for describing the biosorption process of the dynamic behavior of the DAGS adsorber column and that DAGS can be used efficiently to remove organic and inorganic pollutants.

Determination of Bed Void Fraction from Fixed Bed Breakthrough Curve

Accurate physical and chemical properties of adsorbents are required for the efficient design of fixed bed adsorbers. The bed void and particle void are significant physical properties. One of the experimentally determination technique for the bed void is a breakthrough curve method. In the present paper, experimental conditions for determining the bed void from breakthrough curve were investigated because complex conditions are required. The values of bed void were determined from theoretical elution curve at the point of c/c0 = 0.5 under the condition of negligible amount adsorbed. The bed void value determined from elution curve was clearly affected by particle void and fluid velocity, and regarded as ＂apparent＂ bed void values. For large porosity particles, it was difficult to determine the true bed void value due to the effect of penetration into macro pores. Then, the bed void fraction can be determined safely from breakthrough curve when particles possessing small particle void are used.

Adsorption Characteristics of 2,4-Dichlorophenoxybutric Acid Using Bamboo Activated Carbon in a Fixed Bed

Adsorption and desorption characteristics of 2,4-dichlorophenoxybutyric acid (2,4-DB) from aqueous solution on bamboo activated carbon (BAC) were studied in a fixed bed adsorber. The adsorption equilibrium capacity of 2,4-DB on BAC increased with decreasing initial pH of the solution and with a maximum adsorption capacity of 1.61 mol/kg. The adsorption rate of 2,4-DB on BAC could be best fitted by the pseudo first-order model. The adsorption model based on the linear driving force approximation (LDFA) was used for simulating the adsorption behavior of the 2,4-DB in a fixed bed. More than 95% desorption of 2,4-DB was obtained using distilled water.

Emission Factors of Particulate Matter Emitted from Co-Firing of Thai Lignite and Agricultural Residues in Fixed Bed Combustor

Emission factors （EFs） of particulate matter （PM） derived from mono and co-firing of Thai lignite and agricultural residues have been investigated. Two sampling methods for PM, total filtration （TF） and electrical low-pressure impactor （ELPI）, were used together. The study is focused on the influence of fuel mass fraction, and of secondary air to total air; SA：TA on EFs of PM. The results have shown that EFs of PM in mass-basis given by TF method are 8.9, 5.3 and 8.1 mg/kgfuel, while 3.3, 2.7 and 3.3 mg/kgfuel when using ELPI, for firing at constant SA：TA （30%） of lignite, rice husk and bagasse, respectively. For co-firing with 30%SA of coal/rice husk, higher EFs of PM is observed. They are 7.17 and 10.9 mg/kgfuel （TF） for 40 and 70% rice husk share, respectively, or 4.18 and 5.19 mg/kgfuel （ELPI）. However, lower PM emission; 1-3.3 mg/kgruel （TF） or 0.72-2.83 mg/kgfuel （ELPI） are obtained during co-firing of coal/rice husk with lower degree of air staging （i.e. 0-10% SA：TA）. For the influence of oxygenation state, increasing of SA： TA leads to a low formation of ultrafine particles （Dp 〈 0.1 μm）. Apart from PM, major gases （CO, NO, SO2） will be documented in this paper.