Mass transfer investigation and operational sensitivity analysis of amine-based industrial CO2 capture plant

In this article, the industrial process of CO₂ capture using monoethanolamine as an aqueous solvent was probed carefully from the mass transfer viewpoint. The simulation of this process was done using Rate-Base model, based on two-film theory. The results were validated against real plant data. Compared to the operational unit, the error of calculating absorption percentage and CO₂ loading was estimated around 2%. The liquid temperature profiles calculated by the model agree well with the real temperature along the absorption tower, emphasizing the accuracy of this model. Operational sensitivity analysis of absorption tower was also done with the aim of determining sensitive parameters for the optimized design of absorption tower and optimized operational conditions. Hence, the sensitivity analysis was done for the flow rate of gas, the flow rate of solvent, flue gas temperature, inlet solvent temperature, CO₂ concentration in the flue gas, loading of inlet solvent, and MEA concentration in the solvent. CO₂ absorption percentage, the profile of loading, liquid temperature profile and finally profile of CO₂ mole fraction in gas phase along the absorption tower were studied. To elaborate mass transfer phenomena, enhancement factor, interfacial area, molar flux and liquid hold up were probed. The results show that regarding the CO₂ absorption, the most important parameter was the gas flow rate. Comparing liquid temperature profiles showed that the most important parameter affecting the temperature of the rich solvent was MEA concentration.     

Correlation of the Rates of Solvolysis of Two Arenesulfonyl Chlorides and of trans-��-Styrenesulfonyl Chloride �C Precursors in the Development of New Pharmaceuticals

Additional specific rates of solvolysis have been determined, mainly in fluoroalcohol containing solvents, for benzenesulfonyl chloride (1) and p-nitrobenzene-sulfonyl chloride (2). For trans-β-styrenesulfonyl chloride (3), a study has been carried out in 43 pure and binary solvents, covering a wide range of hyroxylic solvent systems. For the specific rates of solvolyses of each of the three substrates, a good correlation was obtained over the full range of solvents when the extended Grunwald-Winstein equation was applied. The sensitivities to changes in solvent nucleophilicity and solvent ionizing power are similar to values determined earlier and an S_N2 process is proposed. For 3, kinetic solvent isotope effects of 1.46 for k_H2O/k_D2O and 1.76 for k_MeOH/k_MeOD were determined. These are also compared to literature values for other sulfonyl chlorides.     

Concentration dependence of diffusion coefficient at various molecular weights and temperatures

The effect of temperature on the concentration dependence of the diffusion coefficient D(c) is investigated experimentally by light scattering using polystyrene with M_w = 179 000 and 900 000 in toluene and cyclohexane at several temperatures. It is found that D(c) decreases with concentration under the theta condition, and increases in good solvents. The continuous transition from theta (θ) to good solvent behaviour is explored, and the results are compared with the existing theoretical predictions.     

Use of Empirical Correlations to Determine Solvent Effects in the Solvolysis of S-Methyl Chlorothioformate

The specific rates of solvolysis of S-methyl chlorothioformate (MeSCOCl) are analyzed in 20 solvents of widely varying nucleophilicity and ionizing power at 25.0 °C using the extended Grunwald-Winstein Equation. A stepwise S_N1 (D_N + A_N) mechanism is proposed in the more ionizing solvents including six aqueous fluoroalcohols. In these solvents, a large sensitivity value of 0.79 towards changes in solvent nucleophilicity (l) is indicative of profound rearside nucleophilic solvation of the developing carbocation. In twelve of the more nucleophilic pure alchohols and aqueous solutions, the sensitivities obtained for solvent nucleophilicity (l) and solvent ionizing power (m) are similar to those found in acyl chlorides where an association-dissociation (A_N + D_N) mechanism is believed to be operative.     

Influence of the bed geometry on the kinetics of the extraction of clove bud oil with supercritical CO2

There is a need for scientific research that evaluates the influence of important process variables on the scale up of supercritical technology. For supercritical fluid extraction (SFE), one of these variables is the extractor's bed geometry, which can be defined by the ratio of the bed height (H_B) to the bed diameter (D_B). A systematic study is needed to select suitable criteria that can be used to obtain similar extraction curves among beds with different geometries. In this study, maintaining a constant ratio of solvent mass to feed mass for two beds with 1-L volumes but different geometries (E-1: H_B/D_B = 7.1; E-2: H_B/D_B = 2.7) was confirmed as a successful scale up criterion. For constant values of the temperature, pressure and bed porosity, there is experimental evidence that the mass transfer rate is equal in the two beds when the solvent flow rate is high. When 0.6 kg of clove buds was packed in the beds, the extraction rates were 2.10 ± 0.08 and 2.3 ± 0.1 g extract/min for beds E-1 and E-2, respectively. However, when the solvent flow rate was lower, the extraction rates were 0.93 ± 0.06 and 1.12 ± 0.02 g extract/min for beds E-1 and E-2, respectively. This difference in behavior between the extraction beds is associated with the axial dispersion of the fluid, which is more pronounced when the H_B/D_B ratio is increased. Thin particles tend to compact in the beds with high H_B/D_B ratios, which shorten the solvent passage. Non-isothermal profiles and differences in chemical composition of the extracts were also observed: 17% more α-humulene and 9% more eugenol were extracted in E-1 and E-2, respectively.     

Pilot plant study of post-combustion carbon dioxide capture by reactive absorption: Methodology, comparison of different structured packings, and comprehensive results for monoethanolamine

Post-combustion carbon capture (PCC) from fossil fuel power plants by reactive absorption can substantially contribute to reduce emissions of the greenhouse gas CO₂. To test new solvents for this purpose small pilot plants are used. The present paper describes results of comprehensive studies of the standard PCC solvent MEA (0.3 g/g monoethanolamine in water) in a pilot plant in which the closed cycle of absorption/desorption process is continuously operated (column diameters: 0.125 m, absorber/desorber packing height: 4.25/2.55 m, packing type: Sulzer BX 500, flue gas flow: 30-110 kg/h, CO₂ partial pressure: 35-135 mbar). The data establish a base line for comparisons with new solvents tested in the pilot plant and can be used for a validation of models of the PCC process with MEA. The ratio of the solvent to the flue gas mass flow is systematically varied at constant CO₂ removal rate, and CO₂ partial pressure in the flue gas. Optimal operating points are determined. In the present study the structured packing Sulzer BX 500 is used. The experiments with the removal rate variation are carried out so that the results can directly be compared to those from a previous study in the same plant that was carried out using Sulzer Mellapak 250.Y. A strategy for identifying the influence of absorption kinetics on the results is proposed, which is based on a variation of the gas load at a constant L/G ratio and provides valuable insight on the transferability of pilot plant results.     

A study of sorption/desorption and diffusion of substituted aromatic probe molecules into semi interpenetrating polymer network of polyurethane/polymethyl methacrylate

The sorption-desorption-resorption-redesorption experiments have been conducted to evaluate the transport behaviour of polyethylene glycol (PEG) based polyurethane/polymethyl methacrylate (PU/PMMA, 50/50) semi interpenetrating polymer network (SIPN) with aromatic probe molecules. Molecular migration depends on the nature of the organic solvent, membrane-solvent interaction, temperature, solubility parameter, molecular volume and free volume available within the polymer matrix. The transport parameters like sorption (S), diffusion (D) and permeation (P) coefficients have been calculated for IPN-aromatic probe molecule systems. Sorption and diffusion results measured at different temperatures viz. 20, 40 and 60 °C are found to follow Fickian mechanism. The liquid concentration profiles in the membranes during sorption and resorption processes have been simulated by solving Fick's diffusion equations and by using numerical method. Equilibrium swelling results have been used to measure the molecular mass between cross links, degree of cross linking and cross link density using Flory-Rehner theory. It is found that the mass uptake values decreased with increase in cross link density during polymer-solvent interactions. The estimated Arrhenius activation energy for diffusion (E_D) and permeation (E_P) are indicative of nature of liquid and their interaction with membrane. Thermodynamic parameters like change in entropy (ΔS) and enthalpy (ΔH) have been calculated using van't Hoffs equation. The rate of evaporation of liquids have been calculated for desorption and redesorption runs, and these results depend on volatility of solvents.     

亚临界水萃取植物精油的模型

Mechanisms that control the extraction rate of essential oil from Zataria multiflora Boiss. (Z. multiflora) with subcritical water (SW) were studied. The extraction curves at different solvent flow rates were used to determine whether the extractions were limited primarily by the near equilibrium partitioning of the analyte between the matrix and solvent (i.e. partitioning thermodynamics) or by the rates of analyte desorption from the matrix (i.e. ki-netics). Four simple models have been applied to describe the extraction profiles obtained with SW: (1) a model based solely on the thermodynamic distribution coefficient KD, which assumes that analyte desorption from the ma-trix is rapid compared to elution; (2) one-site kinetic model, which assumes that the extraction rate is limited by the analyte desorption rate from the matrix, and is not limited by the thermodynamic (KD) partitioning that occurs dur-ing elution; (3) two-site kinetic model and (4) external mass transfer resistance model. For SW extraction, the thermodynamic elution of analytes from the matrix was the prevailing mechanism as evidenced by the fact that ex-traction rates increased proportionally with the SW flow rate. This was also confirmed by the fact that simple re-moval calculations based on determined KD (for major essential oil compounds) gave good fits to experimental data for flow rates from 1 to 4 ml•min－1. The results suggested that the overall extraction mechanism was influenced by solute partitioning equilibrium with external mass transfer through liquid film.     

Determination of the Henry's constant and the mass transfer rate of VOCs in solvents

Absorption of hydrophobic volatile organic compounds (VOCs): dimethylsulfide (DMS), dimethyldisulfide (DMDS) and toluene, in organic solvents: di-(2-ethyl)hexyladipate (DEHA), n-hexadecane, oleyl alcohol and PEG 400, was studied. In order to characterise the absorption capacity of various VOC/solvent systems, the Henry's constant (H) was determined. DMS was found to be the least absorbable in all the selected solvents. Amongst these solvents, DEHA was found to be the most efficient to absorb the considered VOCs. The effect of water addition to the considered solvents (emulsions) on the Henry's constants was examined and confirmed a decreasing VOC absorption for an increasing amount of water in solvent. Finally, to quantify the process rapidity, the absorption rate (N) and the overall liquid mass transfer coefficient (K_La) were measured for some selected couples VOC/solvent and revealed a superior efficiency of DEHA compared to other solvents in trapping DMS, DMDS and toluene.     

Peak parking-moment analysis: A strategy for the measurement of molecular diffusivity in liquid phase

The peak-parking (PP) method permits the measurement of molecular diffusivities (D_m) in solutions. D_m is first measured for benzene in pure methanol and acetonitrile (ACN), using an empty open tube. This yields an effective axial diffusion coefficient (D_ax,m) equal to D_m because there is no tortuosity nor constriction in the flow channel. The same measurements made for the same combinations of solute and solvents, using a column packed with non-porous silica particles provides the obstructive factor (Y_m), defined as the ratio D_ax,m/D_m, which accounts for the influence of tortuosity and constriction of the interparticulate space in packed columns on axial molecular dispersion. The value obtained for Y_m, 0.74–0.75, is constant, irrespective of the solvent. Then, PP experiments were made with the same apparatus and column to measure D_m for benzene, toluene, and ethylbenzene in aqueous solutions of methanol and ACN. The values of D_m obtained by correcting the experimental D_ax,m values with Y_m were compared with values previously reported and those estimated by several literature correlations. They were in good agreement with each other. The average relative error is estimated at 4.5–10%, demonstrating that the PP method is practically effective for experimental measurements of D_m.     

Solvent effect on the nucleation and growth mechanisms of poly(thiophene)

Applying the step potential method, the effect of parameters such as solvent, potential, electrolyte and monomer concentration on the nucleation and growth processes of poly(thiophene) on Pt electrode in tetrabuthylammonium hexafluorophosphate-acetonitrile or dichloromethane has been studied. The j/t transients were generally fitted by means of a mathematical equation that considers different contributions. In acetonitrile the j/t transient (0<t<30 s) present three contributions corresponding to the following mechanisms: two-dimensional instantaneous nucleation (IN2D), three-dimensional progressive nucleation (PN3D_CT) under charge transfer control and three-dimensional progressive nucleation (PN3D_dif) under diffusion control. Similar results were obtained in dichloromethane, but in this case the 3D_CT nucleus presented an instantaneous nucleation mechanism (IN3D_ct). A second wave has been observed in the j/t transients obtained in CH₃CN at t>30 s, which was fitted by a mathematical equation that included two contributions corresponding to a PN3D_CT and PN3D_dif mechanisms. In general, the charge associated to each contribution depended on the solvent, the monomer and electrolyte concentration and the applied potential. However, the PN3D_CT (CH₃CN) or IN3D_CT (CH₂Cl₂) mechanisms were always the more important contributions. The scanning electron microscopy (SEM) analysis of the deposits morphology are in agreement with the nucleation and growth models that are proposed by this method.     

Fixed-bed ion exchange columns operating under non-equilibrium conditions: Estimation of mass transfer properties via non-equilibrium modeling

The present work describes a new technique for the estimation of particle film mass transfer coefficients, k_f and homogeneous solid diffusivity, D_hs for resins in fixed-bed ion exchange columns under non-linear adsorption isotherm conditions. The experimental set-up is composed of a fixed-bed ion exchange column operating under recycling flow, and a reservoir. The experiments are carried out with custom made para-magnetic composite ion exchange resin filled columns. The single pass non-equilibrium packed-bed adsorption model is adapted to the present recirculation system and the model equations are numerically solved by the implicit schemes of the finite differences technique for predicting the mass transfer parameters. In order to find the k_f values, model predicted reservoir concentration profiles for early-time data are generated, and different k_f values were substituted into the model until the predicted and the experimental reservoir vs. time profiles agree. Once k_f is known, a second version of the model is used by substituting different D_hs values where no time constraint is made. When the model output and the experimental reservoir concentration profiles agree, the corresponding D_hs value is taken as the homogeneous solid diffusion coefficient. The present methodology allows the experimental determination of k_f and D_hs values for fixed-bed ion exchange columns through a simple recirculation system.     

采用泰勒分散法测量蜡分子扩散系数

蜡分子扩散系数是蜡沉积预测模型中非常重要的物性参数。为测量蜡分子（高碳数正构烷烃）在液相体系中的分子扩散系数,建立了基于泰勒分散法的扩散系数测量装置,并对装置操作注意事项进行了详细探讨。采用该装置测量了丙酮水溶液、n-C₆+n-C₇溶液以及甲醇水溶液中溶质在溶剂中的分子扩散系数,以文献值对测量值进行校验,验证了装置的可靠性。分别以n-C₁₈、n-C₂₀、n-C₂₂、n-C₂₄、n-C₂₆为溶质,以n-C₇为溶剂,在不同温度、浓度下测量蜡分子在液相体系中的分子扩散系数,并将实验测量值与Hayduck-Minhas关系式计算值进行比较。结果表明：蜡分子扩散系数随温度升高而线性增大,随溶液中蜡分子摩尔分数的增大而以指数形式减小;在相同条件下,高碳数正构烷烃的分子扩散系数低于低碳数正构烷烃的分子扩散系数。采用Hayduck-Minhas关系式的计算结果比实验测量结果平均小50%,应用于蜡沉积预测时,将低估蜡分子扩散质量流。     

Test of ‘scaling laws’ describing the concentration dependence of osmotic pressure,diffusion and sedimentation in semidilute macromolecular solutions

Data of osmotic pressure, diffusion and sedimentation in semidilute macromolecular solutions are systematized with regard to scaling laws. The observed main features are: (a) the concentration dependence of the osmotic pressure both at good and theta solvent conditions is in excellent agreement with that predicted by the scaling laws. (b) The concentration dependence of the mutual diffusion coefficient D_m could not uniquely be described by a simple scaling law. A difference in concentration dependence between D_m and the cooperative diffusion coefficient was observed for polystyrene under good solvent conditions. (c) The concentration dependence of the permeability coefficient (related to sedimentation) and the sedimentation coefficient were for most of the systems in close agreement with that predicted by the scaling laws.     

REVIEW OF REVERSE OSMOSIS MEMBRANES AND TRANSPORT MODELS

After a brief introduction to membrane processes in general, and the reverse osmosis process in particular, the structure and properties of membranes and membrane transport theory are described. The mechanism of salt rejection and transport properties of membranes are discussed in detail. Solubility, diffusivity, and permeability of membranes to solutes and solvents are reviewed critically and compared with each other. Special attention is given to two particular types of membranes, cellulose acetate (CA) and aromatic polyamide (AP) membranes, which are often used for water desalination.

The major portion of this article is devoted to the review and discussion of membrane transport theory with application to the reverse osmosis and ultrafiltralion processes. It is shown that the solvent flux can be represented reasonably well by linear models such as the solution-diffusion model (Lonsdale, et al., 1965). The contribution of pore flow to the solvent flux is small. The solute flux, however, is not linearly dependent on the driving forces and one has to solve the differential equation of transport within the membrane which results in models such as the Spiegler-Kedem (1966) or the finely-porous (Merten, 1966) models. When the wall Peclet number is small, Pe_w =uτδ/D_sw ?1, (D_sw = bD_e one can linearize the nonlinear models. This requirement is not satisfied in most practical cases. Furthermore, the pore flow has significant effect on the solute flux equation and thus it can not be neglected.

The ambiguities that exist in the literature concerning the types of fluxes are discussed. The fluxes used in models derived from irreversible thermodynamics are purely diffusive (concentration and pressure diffusion) and they do not contain any convective effects; whereas the experimentally observed fluxes are the total fluxes with respect to the membrane which consist of a diffusive flux and a convective flux. A new model, based on irreversible thermodynamics, is derived which includes a convective term.

A membrane model is especially useful when the transport coefficients which define the model are not functions of the driving forces, i.e., pressure and concentration gradients. The coefficients in the solution diffusion and sotution-diffusion-imperfection (Sherwood, et al., 1967) models are functions of both pressure and concentration, while the coefficients in the Kedem-Katchalsky (1958) model are relatively insensitive to pressure and concentration. The nonlinear model of Spiegler-Kedem (1966) further improves the Kedem-Katchalsky model.     

Flow behavior of concentrated solutions of an SBS block copolymer

The non-Newtonian viscosity of concentrated solutions of a styrene-butadiene-styrene, SBS, block copolymer was measured with a novel capillary viscometer. Polymer concentrations ranged from 0.165 to 0.306 g/cc. Apparent shear rates ranged from 1 to 10⁵ sec^?1. Five different solvents were employed. All of the flow curves can be reduced to a single master curve with the same shape exhibited by monodisperse polystyrenes and the Graessley theory. The shift factor for the shear rate axis, τ₀, approximately parallels the Rouse relaxation time, τ_R, but shows a residual concentration and solvent dependence not predicted by the Rouse form. For different solvents at the same concentration, better solvents show a minimum relative zero shear viscosity, η₀/η_s, and a maximum ratio τ_R/τ₀. It is concluded that all solvent effects are not adequately incorporated into the zero shear viscosity for the purposes of constructing master plots; however, the shape of the master plot is not affected by the solvent or the polymer block structure.     

Pilot plant study of two new solvents for post combustion carbon dioxide capture by reactive absorption and comparison to monoethanolamine

Application of new solvents will substantially contribute to the reduction of the energy demand for the post combustion capture of CO₂ from power plant flue gases. The present work describes tests of such new solvents in a gas-fired pilot plant, which comprises the complete absorption/desorption process (column diameters 0.125 m, absorber/desorber packing height 4.25/2.55 m, packing type: Sulzer BX 500, flue gas flow 30–100 kg/h, CO₂ partial pressure 35–135 mbar). Two new solvents CESAR1 (0.28 g/g 2-amino-2-methyl-1-propanol+0.17 g/g piperazine+0.55 g/g H₂O) and CESAR2 (0.32 g/g 1, 2-ethanediamine+0.68 g/g H₂O), which were developed in an EU-project, were systematically studied and compared to MEA (0.3 g/g monoethanolamine+0.7 g/g H₂O). The two new solvents and MEA were studied in the same way in the pilot plant and detailed results are reported for all solvents. In the present study the structured packing Sulzer BX 500 is used. The measurements are carried out at a constant CO₂ removal rate of 90% by an adjustment of the regeneration energy in the desorber for systematically varied solvent flow rates. An optimal solvent flow rate leading to a minimum energy requirement is found from these studies. Direct comparisons of such results can be misleading if there are differences in the kinetics of the different solvent systems. The influence of kinetic effects is experimentally studied by varying the flue gas flow rate at a constant ratio of solvent mass flow to flue gas mass flow and constant CO₂ removal rate. Results from these studies indicate similar kinetics for CESAR1, CESAR2 and MEA. The direct comparison of the pilot plant results for these solvents is therefore justified. Both CESAR1 and CESAR2 show improvements compared to MEA. The most promising is CESAR1 with a reduction of about 20% in the regeneration energy and 45% in the solvent flow rate.     

Dynamical adsorption and temperature-programmed desorption of VOCs (toluene, butyl acetate and butanol) on activated carbons

The elimination of solvent vapors from paint shop atmospheres and their recovery for possible re-use is a current environmental requirement. We have studied the dynamical adsorption at 23 °C of typical car paint solvents, i.e. toluene, butylacetate and butanol, on two microporous activated carbons, and the thermal regeneration of these carbons with hot air at 150 °C. A sequence of seven adsorption-desorption cycles with a mixture of these solvents left the carbons with some textural changes but the adsorption capacity remained virtually unaffected. Heavy volatile compounds, proceeding from the possibly unstabilized binder, are eliminated in the course of the first adsorption-desorption runs. Differential scanning calorimetry applied to the desorption in argon of pure solvents showed that the desorption enthalpies ΔH_des are close to, or slightly larger than, the evaporation enthalpies ΔH_vap except for butanol, which exhibited a particularly high ΔH_des value. Temperature-programmed desorption experiments, obtained with carbon samples heated in a helium flow containing oxygen traces, evidenced the desorption of numerous oxidation products. This finding may have consequences for hot air regeneration processes.     

Mechanism of Cf/SiC hole making with high shape precision using abrasive waterjet based on response surface method

This paper aims to explore the accurate control of hole shape for AWJ hole-making of C_f/SiC based on experimental and mathematical analysis methods, and the influence mechanism involved is emphatically analyzed. The results reveal that D_difference is most influenced by the standoff distance, followed by the traverse speed, while it is less influenced by the pressure and the abrasive flow rate. The traverse speed, pressure and abrasive flow rate affect the D_difference by changing the total energy of the jet; while the standoff distance mainly affects the D_difference by changing the effective impact area, which is fundamentally different from other process parameters. In terms of interaction effects, decreasing the traverse speed and increasing the waterjet pressure can amplify the effect of interaction with the involvement of the standoff distance on the D_difference. Based on the established D_difference model and D_offset model, a control method that can gain acquired hole shape is finally obtained for selecting the process parameters of AWJ machining.