Control structure selection for four-product Kaibel column

Dividing wall column configurations have a large savings potential in terms of capital and energy. This paper uses dynamic simulation to investigate three alternative control structures for one of these configurations, namely the Kaibel column. Four components, here selected as methanol, ethanol, n-propanol and n-butanol, are separated into pure products within a single column shell. Control structure 1 (CS1) uses only temperature controllers and is therefore particularly interesting from an industrial point of view. Since the control objective is to control the four product compositions, the two other control structures use also composition controllers. Surprisingly, for composition control, the simple temperature control scheme (CS1) is almost as good at steady-state and much better from a dynamic point of view than the two other more complex control structures.     

A generalized corresponding states method for predicting the limits of superheat of mixtures: Application to the normal alcohols

The limits of superheat of the normal alcohols from methanol to octanol and of ethanol/n-propanol, n-propanol/n-butanol, and n-butanol/n-pentanol mixture were measured at atmospheric pressure. The results were predicted using a new method based on the generalized corresponding states principle (GCSP) in which properties of two reference substances were used to predict the superheat limits of the fluids studied. Ethanol and n-butanol, and the two components of each mixture studied, were used as reference fluids for predicting the superheat limits of the pure alcohols and mixtures respectively.Results showed that it is possible to predict superheat limits to well within the accuracy of measured values ( < 1%). The method requires only accurate vapor pressure correlations and acentric factors of the reference fluids, and an accurate method for predicting the variation of the true critical mixture temperature with composition. It is shown that the GCSP reduces to an expression for the superheat limit of a mixture which is a mole fraction weighted average of the limits of superheat of the individual components in solution when the mixture and its components satisfies certain conditions. In the general case, both linear and nonlinear variations of mixture superheat limit with mole fraction can be predicted with the proposed method.Considerable simpiflication using the GCSP over the approach based on classical homogeneous nucleation theory is derived from the fact that no mixture surface tension or bubble point pressure data are required.     

New predictive correlation for mass transfer coefficient in structured packed extraction columns

Developments in the area of packed columns, particularly structured packed columns, are ongoing, specifically in the area of liquid–liquid extractions in different industries. In the present study, mass transfer coefficients have been obtained experimentally in a structured packed extraction column to develop a new correlation for prediction of continuous phase Sherwood number. The experiments were carried out for toluene/acetic acid/water and n-butyl acetate/acetic acid/water systems with counter current flow in different heights of column. A new dimensionless parameter, d₃₂/h, is introduced in proposed equation. This number considers the effect of column height (h) and mean drop diameter (d₃₂) jointly. The main advantage of this approach is that the principal effect of column height is considered in correlation without which the experimental data could not be fitted with a acceptable accuracy.     

Experimental evaluation of a modified fully thermally coupled distillation column

A modified fully thermally coupled distillation column replacing the conventional distillation system is introduced, and its performance is experimentally evaluated for the stable operation of the column. The existing distillation system is modified to an energy-efficient distillation column for the reduction of investment cost and energy requirement. The experiment is conducted for the separation of methanol, ethanol and n-propanol mixture using a 4-in sieve tray column. The temperatures at seven different locations of the column are measured to monitor the column operation, and the measurements indicate that the column is stably operable. The stable operation proves that neither compressor nor pump is necessary for the vapor or liquid flow between tray sections. The experimental result is compared with that of the HYSYS simulation to show how satisfactory the separation is. The modeling of temperature variation provides a successful prediction of the temperature variation.     

Enthalpies of mixing and heat capacities of mixtures containing alcohols and <Emphasis Type="Italic">n</Emphasis>-alkanes with corn oil at 298.15 K

Enthalpies of mixing for mixtures containing alcohols (methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-butanol) plus corn oil or n-alkanes (n-hexane, n-heptane, and n-octane) plus corn oil were measured at 298.15 K and atmospheric conditions. The enthalpy was measured in the range of concentrations in which alcohols were miscible with the vegetable oil. Results were correlated by the Redlich-Kister equation. The mixing of alcohols with corn oil was strongly endothermic, whereas values obtained for the binaries containing n-alkanes were slightly endothermic. Thus, heat capacities were calculated for all the systems studied.     

An application of the Error-in-Variables Model—parameter estimation from Van Ness-type vapour-liquid equilibrium experiments

The Error-in-Variables Model (EVM) provides a means for estimating parameter values in mathematical models where there is error in every measured variable. This is a distinct improvement over the Method of Least Squares in most situations because the latter requires that there be error in measuring only one of the variables. As an example of the use of EVM, a method is presented for estimating Wilson equation parameters in binary and ternary vapour-liquid equilibrium where the data are obtained using the Van Ness experimental technique. The application presents unusual difficulties in that the compositions of the phases are not measured directly and the total quantity of one component is only measured as the sum of a series of increments. Data for the ternary system n-heptane/n-propanol/1-chlorobutane and the binary system n-heptane/1-chlorobutane are analysed by the proposed method, and the results are compared with the empirical results of Sayegh et al. (1979). The results of the EVM analysis are considerably more accurate. Analysis of the residuals obtained by the EVM procedure suggest serious deficiencies in the thermodynamic model.     

Distillation at near terminal compositions: wetting effects at atmospheric and reduced pressures

Contact angles are reported using n-propanol—water, ethanol—water and cyclohexane—benzene mixtures at concentrations near their terminal compositions, measured under distillation conditions at atmospheric and reduced pressures.A literature survey shows that efficiencies of columns incorporating film contacting devices often decrease very significantly in the low composition range. By comparing the wetting data obtained for the system n-propanol—water with reported performance data using a disc column it is suggested that this behaviour is caused by wetting changes occurring in this region.     

Simulations of chemical absorption in pilot-scale and industrial-scale packed columns by computational mass transfer

A complex computational mass transfer model (CMT) is proposed for modeling the chemical absorption process with heat effect in packed columns. The feature of the proposed model is able to predict the concentration and temperature as well as the velocity distributions at once along the column without assuming the turbulent Schmidt number, or using the experimentally measured turbulent mass transfer diffusivity. The present model consists of the differential mass transfer equation with its auxiliary closing equations and the accompanied formulations of computational fluid dynamics (CFD) and computational heat transfer (CHT). In the mathematical expression for the accompanied CFD and CHT, the conventional methods of k-ε and are used for closing the momentum and heat transfer equations. While for the mass transfer equation, the recently developed concentration variance and its dissipation rate ε_c equations (Liu, 2003) are adopted for its closure. To test the validity of the present model, simulations were made for a pilot-scale randomly packed chemical absorption column of 0.1 m ID and 7 m high, packed with 1/2^″ ceramic Berl saddles for CO₂ removal from gas mixture by aqueous monoethanolamine (MEA) solutions (Tontiwachwuthikul et al., 1992 ) and an industrial-scale randomly packed chemical absorption column of 1.9 m ID and 26.6 m high, packed with 2^″ stainless steel Pall rings for CO₂ removal from natural gas by aqueous MEA solutions (Pintola et al., 1993). The simulated results were compared with the published experimental data and satisfactory agreement was found between them in both concentration and temperature distributions. Furthermore, the result of computation also reveals that the turbulent mass transfer diffusivity D_tvaries along axial and radial directions. Thus the common viewpoint of assuming constant D_t throughout the whole column is questionable, even for the small size packed column. Finally, the analogy between mass transfer and heat transfer in chemical absorption is demonstrated by the similarity of their diffusivity profiles.     

Fast Esterification of Acetic Acid with Short Chain Alcohols in Microchannel Reactor

Microchannel reactor was used for the fast synthesis of acetic acid esters, including methyl acetate, ethyl acetate, n-propyl acetate and n-butyl acetate. Effects of the inner diameter of microchannel reactors, dosage of catalyst, residence time, reaction temperature and molar ratio of alcohol to acetic acid on yields of esters were studied in the p-toluene sulfonic acid-catalyzed homogeneous esterification of acetic acid with alcohols. Yields of acetic acid ester reached 74.0, 70.1, 97.2 and 92.2% when methanol, ethanol, n-propanol and n-butanol were used as the alcohols, respectively and the residence time was of 14.7 min.     

Measurement and correlation for solubility of diosgenin in some mixed solvents简

The solubility data of diosgenin in mixed systems of ethanol ＋ 1-propanol （1 ： 1）, ethanol ＋ 1-butanol （1 ： 1）, ethanol ＋ isobutyl alcohol （1 ： 1）, methanol ＋ isobutyl alcohol （1 ： 1）, methanol ＋ isobutyl alcohol （1 ： 4）, ethanol ＋ 1-pentanol （1 ： 1） and carbon tetrachloride were measured over the temperature range from 289.15 K to 334.15 K by a laser monitoring observation technique at atmospheric pressure, with all mixtures mixed by volume ratio. The Apelblat equation, the ideal solution model, and the 2h equation are used to correlate the solubility data. The results show that the three models agree well with the experimental data, providing essential support for industrial design and further theoretical study.     

Lumped-parameter model for the retorting of a large block of oil shale with an internal temperature gradient

A lumped-parameter model which combines the internal and external heat transfer resistances was proposed for the retorting of a large block of oil shale. In this model the temperature of oil-shale block is obtained by solving the ordinary differential equation of energy balance using an overall heat transfer coefficient U, where U is equal to h/(1 + Bi/5). The results of this model were compared with those of the uniform-temperature model which uses the external heat transfer coefficient h, instead of U, in the energy balance equation and of the rigorous model which calculates the block temperature profile more rigorously by a partial differential equation. The comparison showed that the lumped-parameter model is much better than the uniform-temperature model and is a useful alternative to the rigorous model for modelling the retorting of a large block of oil shale with an internal temperature gradient.     

The effect of sorbed penetrants on the aging of previously dilated glassy polymer powders. I. Lower alcohol and water sorption in poly(methyl methacrylate)

Sorption kinetics and equilibria for methanol, ethanol, and n-propanol in 0.544 μm diameter poly(methyl methacrylate) microspheres were determined at 35°C over a wide range of relative pressures. Sorption isotherms were concave to the pressure axis at low relative pressures and convex to the pressure axis at higher relative pressures. These results, considered in the context of recently reported data for high pressure sorption of gases in polymeric glasses, suggest that the S-shaped isotherms reported here are examples of a generalized isotherm which describes sorption behavior of all penetrants in glassy polymers if an appropriate range of concentration is traversed by the experimental protocol. The effects of dialating the microspheres by preswelling with methanol were studied by subsequent low pressure sorption of water, methanol, ethanol, and n-propanol at 35°C. The preswollen microspheres exhibited initially higher sorption capacities than the as-Received samples, but tended to consolidate with time following the preswelling treatment. The aging process, monitored by periodic short-term sorption with the various penetrant probes, was arrested by contacting the microspheres with an activity of n-propanol sufficient to maintain a sorbed concentration of approximately 1 wt %. The aging was significantly retarded by the presence of low concentrations of water and ethanol. Conversely, the aging process appeared to be essentially unaffected by the presence of correspondingly low concentrations of methanol. The complex kinetics describing the sorption of the various penetrants ranged from Fickian diffusion to polymer relaxation-controlled absorption, depending upon penetrant, relative pressure, and prior exposure history. The low temperature preswelling of the microspheres markedly increased the rate of sorption as well as the respective apparent equilibrium sorption.     

Control structure comparison for three-product Petlyuk column

The focus of this paper is to investigate different control structures (single-loop PI control) for a dividing wall (Petlyuk) column for separating ethanol, n-propanol and n-butanol. Four control structures are studied. All the results are simulations based on Aspen Plus. Control structure 1 (CS1) is stabilizing control structure with only temperature controllers. CS2, CS3 and CS4, containing also composition controllers, are introduced to reduce the steady state composition deviations. CS2 adds a distillate composition controller (CCDB) on top of CS1. CS3 is much more complicated with three temperature-composition cascade controllers and in addition a selector to the reboiler duty to control the maximum controller output of light impurity composition control in side stream and bottom impurity control in the prefractionator. CS4 adds another high selector to control the light impurity in the sidestream. Surprisingly, when considering the dynamic and even steady state performance of the proposed control structures, CS1 proves to be the best control structure to handle feed disturbances inserted into the three-product Petlyuk column.     

Electrocatalytic oxidation of aliphatic alcohols: Application to the direct alcohol fuel cell (DAFC)

The electrooxidation of some low molecular weight alcohols, such as ethanol, ethylene glycol and n-propanol, is discussed in terms of reaction mechanisms and catalytic activity of the anode material. Some examples of a single cell, using a proton exchange membrane (PEM) as electrolyte, are given to illustrate interesting results, particularly for the direct electrooxidation of ethanol. This alcohol may replace methanol in a direct alcohol fuel cell.     

Sorption of vapors in cellulose film

The rates of sorption of water vapor and of methanol vapor in cellulose film were examined by stepwise increments of vapor pressure. In addition, the rates of sorption of the series of vapors water, methanol, ethanol, n-propanol, and n-butanol were studied at pressures just below the respective saturated vapor pressures. The results obtained from both parts of the study are explained in terms of a bimodal diffusion mechanism. This consists of the simultaneous diffusion of vapor into the microporous structure of the cellulose film and diffusion from the micropores into the polymer matrix.     

Controllability and energy efficiency of a high-purity divided wall column

The divided wall column system is a promising energy-saving alternative for separating multi-component mixtures. However, high energy efficiency and stable operations can only be achieved with careful design of steady state operation and control scheme. In this study, the effects of liquid split and vapor split ratios on the energy efficiency and controllability of a divided wall column system for separating ethanol, n-propanol, and n-butanol were investigated. A region with high energy efficiency was identified. However, relative gain analysis found that the performance of multi-loops composition control would be very poor. Dynamic tests showed that multi-loop temperature control cannot return the product compositions to the desired values in case of feed composition disturbances. Outside this region, composition control can compensate for external disturbances such as feed flow rate and feed composition changes but not changes in operating region caused by internal variations such as liquid and vapor splits. Offsets in the product purities were found if temperature controls were used and there are disturbances in feed composition or changes in operating region caused by upsets in liquid and vapor splits. There is a trade-off between energy efficiency and controllability. A composition + temperature cascade scheme was proposed to stabilize the operation with high energy efficiency. The proposed scheme was able to maintain high product purity and reject external disturbances in feed flow and composition changes as well as internal disturbances such as changes in liquid and vapor splits.     

Spectroscopic and Kinetic Analysis of a New Low-Temperature Methanol Synthesis Reaction

The spectroscopy and kinetics of a new low-temperature methanol synthesis method were studied by using in situ DRIFTS on Cu/ZnO catalysts from syngas (CO/CO₂/H₂) using alcohol promoters. The adsorbed formate species easily reacted with ethanol or 2-propanol at 443 K and atmospheric pressure, and the reaction rate with 2-propanol was faster than that with ethanol. Alkyl formate was easily reduced to form methanol at 443 K and 1.0 MPa, and the hydrogenation rate of 2-propyl formate was found to be faster than that of ethyl formate. 2-Propanol used as promoter exhibited a higher activity than ethanol in the reaction of the low-temperature methanol synthesis.     

The determination of zeolite crystal diffusivity by gas chromatography—II. Experimental

Experimental results are reported for the adsorption of propane and n-butane in a column packed with Linde 5A zeolite crystals, and methane, propan n- and iso-butane in a column packed with silicalite crystals. The adsorption equilibrium constant K and crystal diffusivity D_c, we obtained at different temperatures in the range 100–300°C, for each sorbate and packing and the heat of adsorption and diffusional activation energ were determined.The results show that in order to separate the effects of K and the pressure drop, the latter must be independently estimated by a suitable correla     

Formation of gas hydrates in the systems methane–water–ROH (ROH=ethanol,n-propanol,i-propanol,i-butanol)

Numerical data on the decomposition curves of gas hydrates formed by methane and aqueous solutions of ethanol (with the concentration up to 24 mol%), isopropanol (concentration: up to 42 mol%), n-propanol (concentration: up to 32 mol%) and isobutanol (up to the saturated aqueous solution) are presented. Investigations were carried out within the pressure range 1–16 MPa. It has been found that hydrate formation occurs in a specific manner for each of these systems and no general regularities are observed. Ethanol acts as an inhibitor of hydrate formation; however, for a given molar concentration the decrease in the temperature of hydrate formation is smaller for ethanol than for methanol. Isopropanol and n-propanol form double hydrates with methane; decomposition temperatures of these hydrates differ insignificantly from each other and from the decomposition temperature of the hydrate of pure methane. Finally, decomposition temperatures of the hydrates formed by aqueous isobutanol solutions are lower in comparison with the hydrate formed by pure methane.     

Exact universal uniqueness criteria for the adiabatic tubular packed bed reactor

Exact, universal a priori bounds and regions of multiplicity for the entire tubular packed bed reactor are developed by application of a technique reported in Chang and Calo, Chem. Engng Sci. 1979 34 285 to a cascade two-phase cell model for an nth order chemical reaction with interphase resistance to mass and heat transport, Le ≠ 1 (or Le = 1) and either lumped parameter catalyst particles or with intraparticle concentration gradients with uniform temperature. Both the more common case of interphase heat transfer greater than interphase mass transfer rate and the inverse case of particle over-temperature have been considered. In all cases it has been shown that the reactor conservation equations can be decoupled at certain points along the bed determined by the Lewis number, and that questions of multiplicity and uniqueness reduce to consideration of a single algebraic equation which is actually a form of the two-phase adiabatic CSTR. Also as for the CSTR, the topology of the adiabatic packed bed reactor is shown to be the simple cusp catastrophe. The application of the resultant criteria is quite simple and represents a practical step in the design procedure for highly exothermic reactions in packed beds. A flow chart of a suggested procedure is included.