Effects of structural parameters on water film properties of transpiring wall reactor

Reactor corrosion and salt deposition problems severely restrict the industrialization of supercritical water oxidation. Transpiring wall reactor can effectively weaken these two problems through a protective water film formed on its internal surface. In this work, the effects of key structural parameters on water film properties of transpiring wall reactor were explored by numerical simulation, and established models were validated by comparing simulation and experimental values. The results show that transpiration water layer, transpiring wall porosity and inner diameter hardly affected organic matter degradation. Increasing transpiration water layer and transpiring wall porosity reduced reactor center temperatures in the middle and lower zones of the reactor. Increasing transpiration water layer, transpiring wall porosity and inner diameter decreased water film temperatures but increased water film coverage rates. Increasing reactor length affected slightly on the volume of the upper supercritical oxidation zone but enlarged the subcritical zone.

Mechanistic modeling and numerical simulation of axial flow catalytic reactor for naphtha reforming unit

Naphtha catalytic reforming (NCR) process has been of tremendous attention all over the world owing to the significant requirement for high-quality gasoline. Industrialized naphtha reforming unit at oil refineries applies a series of fixed bed reactors (FBRs) to improve the quality of the low-octane hydrocarbons and convert them to more valuable products. The prominent purpose of this research is to understand the catalytic reactor of naphtha reforming unit. For this aim, an appropriate mechanistic modeling and its related CFD-based computational simulation is presented to predict the behavior of the system when the reactors are of the axial flow type. Also, the triangular meshing technique (TMT) is performed in this paper due to its brilliant ability to analyze the results of model’s predictions along with improving the computational accuracy. Additionally, mesh independence analysis is done to find the optimum number of meshes needed for reaching the results convergence. Moreover, suitable kinetic and thermodynamic equations are derived based on Smith model to describe the NCR process. The results proved that the proceeding of NCR process inside the reactor significantly increased the concentration amount of aromatic materials, lighter ends and hydrogen, while deteriorated the concentration amount of naphthene and paraffin. Moreover, the pressure drop along the reactor length was achieved very low, which can be considered as one of the momentous advantages of NCR process.

Entropy Generation Rate Minimization for Methanol Synthesis via a CO2 Hydrogenation Reactor

The methanol synthesis via CO2 hydrogenation (MSCH) reaction is a useful CO2 utilization strategy, and this synthesis path has also been widely applied commercially for many years. In this work the performance of a MSCH reactor with the minimum entropy generation rate (EGR) as the objective function is optimized by using finite time thermodynamic and optimal control theory. The exterior wall temperature (EWR) is taken as the control variable, and the fixed methanol yield and conservation equations are taken as the constraints in the optimization problem. Compared with the reference reactor with a constant EWR, the total EGR of the optimal reactor decreases by 20.5%, and the EGR caused by the heat transfer decreases by 68.8%. In the optimal reactor, the total EGRs mainly distribute in the first 30% reactor length, and the EGRs caused by the chemical reaction accounts for more than 84% of the total EGRs. The selectivity of CH3OH can be enhanced by increasing the inlet molar flow rate of CO, and the CO2 conversion rate can be enhanced by removing H2O from the reaction system. The results obtained herein are in favor of optimal designs of practical tubular MSCH reactors.

Effects of Raschig Ring Packing Patterns on Pressure Drop, Heat Transfer, Methane Conversion, and Coke Deposition on a Semi-pilot-scale Packed Bed Reformer

The effects of Raschig ring packing patterns on the efficiency of dry methane reforming reactions were investigated using computational fluid dynamics (CFD). The present study aims to understand the behavior of fluid flow in packed bed reactors, especially under low reactor-to-ring ratios between 4 and 8. Three packing patterns were studied: vertical staggered (VS), chessboard staggered (CS), and reciprocal staggered (RS). It was determined that packing pattern notably affected pressure drop across the reactor length. The VS pattern produced the lowest pressure drop of 223 mPa, while the CS and RS patterns produced pressure drops of 228 mPa and 308 mPa, respectively. The values of methane conversion can be increased by ca. 2 % by selecting a more suitable packing pattern (i.e., 76 % for the VS pattern and 74 % for the CS and RS patterns).

Volatile fatty acids build-up and its effect on E. coli inactivation during excreta stabilisation in single-stage and two-stage systems

Digestion and co-digestion of faecal matter collected from urine diverting dehydrating toilet faeces (UDDT-F) and mixed organic market waste (OMW) was studied in single stage pilot scale mesophilic plug-flow anaerobic reactors at UDDT-F:OMW ratios 4:1 and 1:0. Escherichia coli inactivation and volatile fatty acids (VFA) build-up was monitored at sampling points located along the reactor profile. When applying UDDT-F:OMW ratio of 4:1 at 12% total solids (TS), E. coli inactivation achieved was 2.3 log times higher than that achieved in UDDT-F:OMW ratio of 1:0. In subsequent trials, a two-stage reactor was researched, applying a UDDT-F:OMW ratio of 4:1 and 10 or 12% TS slurry concentrations. Highest VFA concentrations of 16.3 ± 1.3 g/L were obtained at a pH of 4.9 in the hydrolysis/acidogenesis reactor, applying a UDDT-F:OMW ratio of 4:1 and 12% TS, corresponding to a non-dissociated (ND)-VFA concentration of 6.9 ± 2.0 g/L. The corresponding decay rate reached a value of 1.6 per day. In the subsequent methanogenic plug-flow reactor, a decay rate of 1.1 per day was attained within the first third part of the reactor length, which declined to 0.6 per day within the last third part of the reactor length. Results show that a two-stage system is an efficient way to enhance pathogen inactivation during anaerobic digestion.

Analysis of the dynamics of the system of nonlinear differential equations describing a tubular reactor

A characterization of the solution(s) of nonlinear boundary value problems (BVPs) arising from a class of chemical reactions occurring in a adiabatic tubular reactor when the mass and thermal Peclet numbers are different is performed. Results show that for large Peclet numbers and activation energy, and for sufficiently small Damkohler number and reactor length, the solution to the BVP is unique. While for small Peclet numbers and activation energy, and for large Damkohler number and reactor length, there exist at least three solutions to the BVP. The conclusion is that the number of solution (s) for the BVP depends on the choice of parameter values. Likewise, the first set of parameter values listed above models the adiabatic plug flow reactor, while the other parameter set models the adiabatic continuous stirred tank reactor.

Designing a high-throughput viscous heater to process feces: heater geometry

Viscous heating technology can destroy disease-causing microorganisms with no additional heat input. A laboratory-scale unit was constructed and tested with a simulant, and viscous heating achieved temperatures as high as 190°C. This study discusses additional variables – length and spacing – that are important to process design and optimization. The viscosity (μ) was described as a function of shear rate (γ̇); μ = 140 Pa s for t = 0 s and μ = 32*(γ̇)−0.6 Pa s for t > 0 s. The advantages of viscous heating to sanitize fecal mass are presented. The results show temperature gradient is more sensitive to changes in gap spacing than reactor length. For high throughput, the viscous heater length must be increased to provide fluid sufficient residence time to achieve the desired effluent temperature.

Reactivity of Olefins and Thiophenes in Hydrodesulfurization of FCC Gasoline

The reactivity of olefins and S-compounds and their distributions in different catalyst-bed lengths were experimentally evaluated with a FCC gasoline in a high-pressure fixed-bed continuous flow pilot unit over the CoMoS/γ-Al2O3 catalyst. The evaluation results demonstrated that the increased steric hindrances around the double bond (C=C) and that to the thiophene molecules could suppress the hydrogenation of olefins and hydrodesulfurization (HDS) of S-compounds, respectively. Meanwhile, the reaction temperatures could influence the acidic property of the CoMoS active phase confirmed by FT-IR analysis, and thus induced the different reactions. It was found that the isomerization of terminal olefins to internal olefins was promoted by the Brønsted acid sites (-SH) at low temperatures, as well as the skeletal isomerization by the strong Lewis acid sites occurred to a minor extent at high temperatures. Besides, the distributions of olefins and S-compounds in different catalyst-bed lengths showed that the removal of S-compounds reached 80% of its maximum conversion at the first 40% of the reactor length, however, the saturation of olefins increased linearly as the reactor length increased. Therefore, a new catalyst-loading method was developed, i.e., the upper 40% of the reactor length filling with catalyst of high HDS activity and the bottom 60% with catalyst of low olefin saturation activity, respectively. The evaluation results showed that the graded catalyst loading process showed higher selectivity in HDS of FCC gasoline.