DESORPTION WITH CHEMICAL REACTION IN CONTACTING DEVICES

Chemical processes where a gas is absorbed into a liquid and reacts to give a product that desorbs back into the gas are quite common in the industry. In this study, the reaction between the absorbed gaseous reactant and a nonvolatile reactant is considered to be slow, fast or instantaneous, while the volatile product may react further in a slow reaction. Effects of various operating parameters such as the liquid and gas flow rates, the gas-liquid interfacial area and the coefficients for mass transfer are examined. Situations can arise where the absorbing gas reacts but the product diffuses completely to the bulk of the liquid, and the rate of desorption is null. The expressions presented here define conditions to avoid this situation and allow the calculation of the optimal design and operation of the gas-liquid contacting device     

Selective Removal of Hydrogen Sulfide from Gases Containing Hydrogen Sulfide and Carbon Dioxide Using Diethanolamine

Abstract

It is sometimes necessary to selectively remove hydrogen sulfide from gases containing carbon dioxide. This may be the case for example in the production of sulfur using the Claus process. When two gases are simultaneously absorbed into a solution containing a reactant with which each gas can react, the rate of absorption of each component is affected by the presence of the other gas. For the absorption of hydrogen sulfide into primary and secondary amines, the reaction which occurs can usually be considered to be instantaneous. An instantaneous reaction is diffusion-limited since the reaction occurs so rapidly that the liquid phase reactant and the absorbed gas cannot coexist in the same region of the liquid. For primary and secondary amines used for gas treatment, the reaction with carbon dioxide is much slower than for hydrogen sulfide and can often be considered to be second order.

In this work the simultaneous absorption of two gases into a liquid containing a reactant with which both gases can react is modeled using penetration theory. It is assumed that one gas reacts instantaneously and the other gas undergoes a second order reaction. Parameters used in the calculations are those available in the literature corresponding to the absorption of hydrogen sulfide and carbon dioxide in diethanolamine.     

Gas-liquid reactions in packed beds: The effectiveness of static hold-up in presence of gas side resistance

The interfacial area in a packed bed may be divided into two parts, i.e. the interfacial area corresponding to the dynamic hold-up and that corresponding to the static hold-up. The effectiveness of the latter for gas absorption with a chemical reaction can become quite low due to the depletion of the liquid phase reactant in the static hold-up. As a result, the effective interfacial area may become substantially smaller. This aspect has been investigated earlier for liquid phase controlled absorption with a chemical reaction. In this paper, a procedure is developed for calculating the effectiveness of the interfacial area of the static hold-up for the case of significant gas-side mass transfer resistance for a second-order chemical reaction. It is shown that the gas-side mass transfer resistance increases the effectiveness of the static hold-up. The results are presented in a convenient graphical form. It is shown that under certain conditions this effectiveness is quite sensitive to the magnitude of the gas-side mass transfer resistance. The analysis presented here is based on the film theory.     

PERTURBATION ANALYSIS OF GAS ABSORPTION WITH CHEMICAL REACTION: A NONVOLATILE LIQUID REACTANT

Enhancement factors for gas absorption in a liquid containing a nonvolatile reactant are calculated using a regular perturbation technique. The perturbation solutions are found to be quite accurate in the slow reaction regime but lead to erroneous results for moderate and large reaction rates. To extend these solutions to the intermediate and fast reaction regimes, we have used the method of Padé approximants and Shanks transformations. The results obtained by these acceleration techniques are shown to be in excellent agreement with the direct numerical solutions for the enhancement factors in the intermediate and fast reaction regimes.     

有机相的加入对气液体系流动及传质特性的影响

以难溶气体的吸收作为研究的基本过程,纯CO2为气体溶质,0.5MK2CO3／0.5MKHCO3为连续相,异戊醇、苯、正己烷分别作为有机相,探索有机相对气液传质的增强作用,实验探求从不同有机相以不同油分率、表观气速对三相传质系数、增强因子等的影响。     

伴有二级不可逆反应的非牛顿幂律流体薄膜流中化学吸收的研究

本文对伴有二级不可逆反应的非牛顿幂律流体降膜流中的吸收过程进行了研究,提出扩散反应方程和它的解,并得到实验验证.     

Gas absorption with photochemical reaction

Gas-liquid reactions can be activated by high energy radiations (photons). When gas absorption is accompanied by pseudo-first order reaction further enhancement in the specific rate of absorption can be realised for situations where either the reactive species in the liquid phase is activated or the dissolved solute gas is activated.     

氯与植物纤维原料中木素的反应速度及反应性质的探讨

<正> 在氯化法制造纸浆的过程中,氯气与经过碱液处理的植物纤维原料作用,使其中的木素成为氯化木素,以便用稀碱除去.本工作的目的是寻求在氯化法的条件下氯与木素的反应速度,并试图明确该反应的性质.     

Enhancement Factor for Gas Absorption in a Finite Liquid Layer. Part 4: Influence of Gas‐Phase Mass Transfer during a Second‐Order Reaction in a Liquid in Laminar Flow

Numerical analysis is presented for gas absorption accompanied by a second‐order reaction into a liquid layer of finite thickness in laminar flow, with the gas‐phase mass transfer resistance and the axial decrease of the gas‐phase solute concentration due to absorption being taken into account. Both cocurrent and countercurrent flow modes are analyzed, where the presence of significant resistance or axial decrease of the solute concentration in the gas phase can lead to substantially lower rates of gas absorption than those found when the influence of gas‐phase mass transfer is not considered. Approximate expressions describing the exact numerical solution to the enhancement factor in the cocurrent flow mode are developed and can be extrapolated for estimating the enhancement factor in a more general case of a (1, n)‐th‐order reaction in which the influence of gas‐phase mass transfer cannot be neglected.     

PENETRATION THEORY ANALYSIS FOR NONISOTHERMAL GAS ABSORPTION ACCOMPANIED BY A SECOND-ORDER CHEMICAL REACTION

The penetration theory equations for nonisothermal gas absorption with second-order reaction were solved numerically using the Saul'yev method. Results for the enhancement factor and interfacial temperature rise were obtained for Hatta numbers between 1 and 10, It was found that the enhancement factor and interfacial temperature rise depend on four parameters; (i) the effective activation energy, ε_eff = (ε_R + ε_DA)/2 - ε_S, where ε_R, ε_DA, and ε_S are the activation energies for the reaction, diffusion, and solubility of the solute gas, respectively; (ii) the effective heat of generation, β_eff = (β_R + β_s)√Le, where β_R and β_s are the dimensionless heat of solution and reaction, respectively, and Le is the Lewis number; (iii) the product √rq where r is the ratio of the diffusivity of the liquid reactant to that of the solute gas and q is the ratio of the concentration of the liquid reactant in the bulk solution to that of the solute gas at the interface at the bulk temperature of the solution; and (iv) the activation energy for diffusion of the liquid reactant, ε_DB. This effectively reduces the number of parameters required to give a complete picture of the absorption from nine to four. The reduction is important since it reduces the number of van Krevelen-Hoftijzer plots and interfacial temperature-Hatta number plots required for the design of gas-liquid reactors.     

ENHANCEMENT FACTOR OF GAS ABSORPTION ACCOMPANIED BY REVERSIBLE CYCLIC CHEMICAL REACTIONS

Theoretical effect of mass transfer by reversible cyclic chemical reactions is studied in this paper. On the basis of the film theory, an analytical equation has been derived to express the enhancement factor of gas absorption with first-order reversible cyclic chemical reactions. The finite difference method has been used to study the enhancement of gas absorption with second-order reversible cyclic reactions. The results indicate that the gas absorption rate is enhanced by the forward reaction rate, while is suppressed by the backward reaction. Although the diffusivity has some effects on enhancement factor, the influence is not as much as that of reaction rate. The theoretical equations are used to predict the enhancement factors for absorptions of carbon dioxide in acidic solution and l-butene in liquid phosphoric acid.     

ENHANCEMENT FACTOR OF GAS ABSORPTION ACCOMPANIED BY REVERSIBLE CYCLIC CHEMICAL REACTIONS

Theoretical effect of mass transfer by reversible cyclic chemical reactions is studied in this paper. On the basis of the film theory, an analytical equation has been derived to express the enhancement factor of gas absorption with first-order reversible cyclic chemical reactions. The finite difference method has been used to study the enhancement of gas absorption with second-order reversible cyclic reactions. The results indicate that the gas absorption rate is enhanced by the forward reaction rate, while is suppressed by the backward reaction. Although the diffusivity has some effects on enhancement factor, the influence is not as much as that of reaction rate. The theoretical equations are used to predict the enhancement factors for absorptions of carbon dioxide in acidic solution and l-butene in liquid phosphoric acid.     

Absorption of Carbon Dioxide into Aqueous Solution of Sodium Glycinate

Abstract

Carbon dioxide was absorbed into aqueous solution of sodium glycinate (SG) at different SG concentrations, CO₂ partial pressures, and temperatures in the range of 0.5–3.0 kmol/m³, 25–101.3 kPa, and 298–318 K, respectively, using a stirred semi-batch vessel with a planar gas-liquid interface. Both the reaction order and rate constant are determined from gas absorption rates under the fast reaction regime. The reaction was found to be first order with respect to both CO₂ and SG. The activation energy for the CO₂-SG reaction has been found to be 59.8 kJ/mol. The second-order reaction rate constants were used to obtain the theoretical values of absorption rate based on the film theory.     

GAS ABSORPTION WITH CHEMICAL REACTION INVOLVING A VOLATILE LIQUID REACTANT: PENETRATION THEORY SOLUTION

A penetration theory analysis is presented for the problem of gas absorption, with chemical reaction involving a volatile liquid reactant. Model equations are derived and solution techniques are presented. It is shown that as long as the diffusivities of two reactants are equal, the enhancement factors predicted by film and penetration theories only mildly differ. Except for slow and instantaneous reaction regimes, the liquid reactant volatility can be markedly detrimental to enhancement of absorption rate by chemical reaction.     

Experimental study on the ozone absorption accompanied by instantaneous chemical reaction

This work deals with gas absorption accompanied by chemical reaction in a liquid phase. Ozone absorption in potassium indigotrisulfonate solution was investigated in a batch bubble column. Enhancement factor for absorption accompanied by instantaneous chemical reaction in the liquid phase was experimentally determined, as a ratio of the volumetric mass transfer coefficient for the absorption accompanied by reaction to that for pure physical absorption. The influence of (a) the initial concentration of the solute from liquid phase and (b) the ozone concentration in gas phase on the enhancement factor were experimentally examined. The absorption accompanied by instantaneous chemical reaction is a diffusion-controlled process, whose rate depends upon the diffusivities of the absorbing gas and the solute in liquid phase. The influence of these diffusivities was found to be more significant for lower values of the enhancement factor. The rate of ozone absorption was followed by the time change of the solution color, using new method based on the computer program SigmaScan Pro 5 (Systat Software, Inc., San Jose, CA, USA). This investigation is a contribution to the prediction of the ozone consumption in wastewater treatment, in cases when ozone instantaneously reacts with substances present in water.     

GAS ABSORPTION WITH INSTANTANEOUS CHEMICAL REACTION IN A FALLING FILM FOR PARALLEL AND COUNTERCURRENT FLOWS

A theoretical analysis is presented for the absorption of a gas accompanied by instantaneous chemical reaction with a dissolved reagent in a falling liquid film of liquid. The reaction occurs on a moving front inside the liquid film separating the zones containing either of the reactants. The governing equations are solved by using a coordinate transformation to immobilize the reaction front. The effects of the different system parameters on the reaction front profile, absorption rate and enhancement factor are presented for both cocurrent and countercurrent flow of the gas. In the former case the reaction front profile shows a maximum, but in the later case it is monotonic in the axial distance along the film. The enhancement factor plots exhibit maxima in both the cases.     

Comparison of Concentration Profiles for Boundary Layers in Absorption with Chemical Reaction Processes

An analysis of five different systems of absorption‐with‐chemical‐reaction in gas‐liquid reactors, commonly encountered in various industrial processes, is presented. To analyze the interphase mass transfer from gas to liquid, the rate limiting parameters and the concentrations at the gas‐liquid interface were determined on the basis of pertinent theories. The calculations presented, are based on the Whitman theory for gas and liquid phase mass transfer coefficients and Henry equilibrium constants. The necessary diffusion coefficients were calculated from existing correlations, and the corresponding chemical reaction rate constants were obtained from the literature, assuming pseudo first order chemical reaction. The process parameters required (pressure, temperature, and the gas‐liquid contact time) were within the values that occur in industrial processes. The results presented, are the concentration profiles in the boundary layers for the systems studied, calculated and graphically presented, together with the gas and liquid film thicknesses and Hatta numbers, obtained from calculations for the liquid phase mass transfer. The results may contribute to a better understanding of the absorption‐with‐chemical‐reaction processes in industrial plants, thus lowering the operational costs of these processes and alleviating the ecological problems of existing technologies.     

Measurement of infinite dilution activity coefficients of selected environmentally important volatile organic compounds in polydimethylsiloxane using gas - liquid chromatography

Silicon oil, chemically known as polydimethylsiloxane (PDMS), is a high boiling point solvent highly suitable for volatile organic compounds (VOCs) absorption. To use PDMS as an absorption solvent for a specific waste gas problem, it is important to determine the infinite dilution activity coefficients of the VOCs to be separated with PDMS. This work reports activity coefficients at infinite dilution of 13 VOCs in polydimethysiloxane determined by the dynamic gas liquid chromatographic technique. The measurements were carried out at various temperatures (303.15, 313.15, 323.15., 333.15, 353.15, 373.15, 393.15 and 423.15 K). Four PDMS polymers with average molecular weight ranging from 760 to 13,000 were used as solvents. A control column packed by Perkin Elmer to our specifications was used to validate the coating and packing methods. Flow rate dependence of the elution peaks was also investigated by varying it from 10–50 ml/min. Precision was improved by reproducing the results using columns with different liquid loading, thus also studying the retention mechanism. The results compare well with the data from previous work using simple headspace and UNIFAC predictions and literature values. The successful comparison gives an indication of the GLC as a rapid, simple and accurate method for studying the thermodynamics of the interaction of a volatile solute with a nonvolatile solvent.     

Photogalvanic cells

An analysis of photogalvanic cells is carried out and applied to a specific system, iron-thionine, with only slight simplifications. The criteria for successful cell design are given and discussed. It is concluded that many formidable obstacles to practical application exist. The principal difficulties are the following: (1) Diffusion to the electrodes and reaction there must be sufficiently rapid to make bulk back reactions unimportant. (2) A means for keeping at least one active species from reaching one electrode must be found. (3) A sufficient range of the solar spectrum must be exploited without increasing the rate of photon absorption to the point where bulk back reaction becomes important. These problems are discussed in some detail and quantitative criteria are presented in terms of absorbed photon flux, cell size parameters, chemical rate constants, exchange current densities, overvoltages and diffusion coefficients.     

连续相变的加氢反应器

引　言加氢反应是一类强放热反应 ,在采用滴流床反应器的情况下 ,经常遇到由于液体分布不均而导致的催化剂不完全润湿的现象[1] 。当液体反应物中含有挥发性组分时 ,在一定条件下 ,液相反应物可能会蒸发 ,伴随着反应速度的突然增加[2 ] ,使催化剂局部过热而失活 .另外 ,由于液体在催化剂床层中的不均匀分布而使床层出现未被润湿的干区 ,在强放热反应中 ,会经常观察到出现热点 ,床层的轴向温度出现振荡[3] .两相并流向上的固定床反应器 ,由于床层内的持液量较高 ,液体分布均匀 ,催化剂能很好地润湿 ,因此有可能避免出现热点[4 ] .这种操作形…