Sanjay M. Mahajani

CFD analysis of single-phase flows inside helically coiled tubes

It has been well established that heat transfer in a helical coil is higher than that in a corresponding straight pipe. However, the detailed characteristics of fluid flow and heat transfer inside helical coil is not available from the present literature. This paper brings out clearly the variation of local Nusselt number along the length and circumference at the wall of a helical pipe. Movement of fluid particles in a helical pipe has been traced. CFD simulations are carried out for vertically oriented helical coils by varying coil parameters such as (i) pitch circle diameter, (ii) tube pitch and (iii) pipe diameter and their influence on heat transfer has been studied. After establishing influence of these parameters, correlations for prediction of Nusselt number has been developed. A correlation to predict the local values of Nusselt number as a function of angular location of the point is also presented.

Recovery of dilute acetic acid through esterification in a reactive distillation column

Abstract The recovery of acetic acid from its dilute aqueous solutions is a major problem in both petrochemical and fine chemical industries. The conventional methods of recovery are azeotropic distillation, simple distillation and liquid–liquid extraction. Physical separations such as distillation and extraction suffer from several drawbacks. The esterification of an aqueous solution (30%) of acetic acid with n-butanol/iso-amyl alcohol is a reversible reaction. As excess of water is present in the reaction mixture, the conversion is greatly restricted by the equilibrium limitations. The esters of acetic acid, namely, n-butyl acetate and iso-amyl acetate, have a wide range of applications. In view of the appreciable value of these esters, the present work was directed towards recovery of 30% acetic acid by reaction with n-butanol and iso-amyl alcohol in a reactive distillation column (RDC) using macroporous ion-exchange resin, Indion 130, as a catalyst bed, confined in stainless steel wire cages. Experiments were conducted in order to achieve an optimum column configuration for the synthesis of n-butyl acetate/iso-amyl acetate in an RDC. The effect of various parameters, e.g. total feed flowrate, length of catalytic section, reflux ratio, mole ratio of the reactants, location of feed points and effect of recycle of water were studied.

Industrial Applications of Reactive Distillation: Recent Trends

Last few years have seen a dramatic rise in the number of applications of reactive distillation (RD). This useful technology is now being applied for any scale of operation- from manufacture of fine chemicals to that of bulk chemicals. This article reviews the very recent applications and serves as a supplement for the exhaustive review on this subject by Sharma and Mahajani (2003). It not only furnishes the information on the ongoing research in the existing applications but also elaborates the newly discovered applications such as manufacture of phenol, linear alkyl benzene, carbonates, chlorosilane derivatives and chiral chemicals. Multiple reactions in RD, failure of RD in some cases and new RD configurations to increase the overall yields are some of the additional aspects being covered.

Fischer-Tropsch synthesis using bio-syngas and CO2

While Fischer-Tropsch synthesis (FTS) using coal and natural gas in conventional reactors is an almost well-established technology, the production of liquid hydrocarbons from syngas obtained from biomass is in its preliminary stages of commercialization in countries like Germany. With concerns about global warming and ways of disposing of CO 2 being searched for, CO 2 hydrogenation using FTS to liquid hydrocarbons can act as a CO 2 sink. A brief review of FTS using CO 2 -rich syngas is given in this paper, looking at FTS as a technology that can help reduce global warming and as a process integration alternative. The reverse water gas shift (r-WGS) reaction is vital for CO 2 hydrogenation. We have studied the effect of this using an FT kinetic model and have proposed a new flow sheet alternative for FTS using CO 2 -rich syngas. Simulations suggested that this new process gives better conversion of CO 2 . The product selectivity and yields from an FT plant are vital to make the process viable economically.

Reactor Model for the Underground Coal Gasification (UCG) Channel

Underground Coal Gasification (UCG) is the process of in-situ conversion of coal into combustible products (syngas) which can be used either as fuel or as a chemical feedstock. In this study, the gasification channel is viewed as a one-dimensional packed bed reactor. The packed bed reactor model is solved incorporating chemical reactions and mass transfer effects. A pseudo-transient model is simulated for temperature and composition profiles of the gas and solid phases. The movements of the pyrolysis and the reaction front are obtained. The model results are in qualitative agreement with literature. The effects of various operating parameters are studied in detail. Steam/O2 ratio, inlet O2 and total pressure determine the solid temperature profile and hence the outlet gas composition. The simulations are performed for two sets of kinetics parameters. The solid temperature profile and outlet gas compositions change significantly with a change in kinetics parameters. The main motivation behind this study is to provide a theoretical base for understanding the critical aspects of UCG and to provide a tool which coupled with experiments will help in determining the commercial feasibility of the UCG process.

Non-equilibrium stage modeling and non-linear dynamic effects in the synthesis of TAME by reactive distillation

Abstract Tertiary-amyl methyl ether (TAME) is a potential gasoline additive that can be advantageously synthesized using the reactive distillation (RD) technology. This work emphasizes on non-linear effects in dynamic simulations of reactive distillation column. For certain configurations, dynamic simulation with equilibrium stage (EQ) model leads to sustained oscillations (limit cycles) which have been reported in our earlier work [Katariya, A. M., Moudgalya, K. M., & Mahajani, S. M. (2006). Nonlinear dynamic effects in reactive distillation for synthesis of TAME. Industrial and Engineering Chemistry Research , 45 (12), 4233–4242]. Feed condition and Damkohler number are the important parameters that influence the existence of these effects. To confirm the authenticity of the observed non-linear behaviors, a more realistic and rigorous dynamic NEQ model for a packed column is developed which uses a consistent hardware design. The steady state behavior of the NEQ model is examined by varying the number of segments and the column height. The dynamic simulation and the bifurcation study with stability analysis indicate that the parameter space, in which oscillations may be observed, is shifted in the case of NEQ model.

Extractive reactions with cationic exchange resins as catalysts (acetalization of aldehydes with alcohols)

Abstract The acid catalysed acetalization of aldehydes such as formaldehyde, acetaldehyde, glyoxal and glyoxylic acid, with alcohols are reversible reactions. In order to enhance the conversion of aldehydes, extractive reactions were carried out with the alcohols (e.g. 2-ethyl hexanol and n -butanol) having very low to low solubility in water. In such cases alcohols, in addition to being reactants act as extractive solvents for the acetals formed in the reaction. An experimental investigation was carried out using cation-exchange resins (Indion-130, Amberlist-15 and Amberlite IR-120) as catalysts and significant conversion levels were realised for the reactions of formaldehyde and acetaldehyde; results were not encouraging for the extractive reactions of glyoxal and glyoxylic acid. The effect of different parameters, such as catalyst loading, temperature, mole ratio and aqueous phase concentration of aldehyde on the rate of reaction was studied.

Reactive Chromatography for the Synthesis of 2-Ethylhexyl Acetate

Abstract Reactive chromatography (RC) combines reaction and chromatography in a single unit. Due to simultaneous separation of products it can be advantageously used for reversible reactions to obtain the enhanced performance. It is a promising alternative to reactive distillation when the components are temperature sensitive and less volatile. In the present work, we study the application of RC for the synthesis of 2-ethylhexyl acetate through esterification of acetic acid with 2-ethylhexanol. Amberlyst-15 has been used as a catalyst/adsorbent. Several aspects such as reaction kinetics, multicomponent adsorption equilibria, and reaction-separation study using fixed bed chromatographic reactor are covered. The kinetics was studied in a stirred batch reactor. The effects of various parameters such as speed of agitation, particle size, temperature, mole ratio of reactants, and catalyst loading have been investigated. Kinetic modeling was performed using modified activity based Langmuir-Hinshelwood-Hougen-Watson (LHHW) model. Non-reactive binary adsorption experiments were performed to determine the parameters of Langmuir adsorption isotherm by minimizing the errors between the experimental and predicted breakthrough curves. Further, the reactive chromatography experiments were performed to obtain the reactive breakthrough curves which indicate a clear potential for the use of reactive chromatography for this reaction. The obtained data has been compared with the predictions of mathematical model of the chromatographic reactor.

Direct hydration of propylene in liquid phase and under supercritical conditions in the presence of solid acid catalysts

Abstract Hydration of propylene has been studied in both liquid phase conditions and above the critical temperature and pressure of propylene. There is a significant rise in initial rate while switching over from sub-critical to supercritical conditions, mainly because of the increase in solubility of propylene in water. Substantial amount of isopropanol exists in the supercritical phase. This suggests the possibility of the enhancement in equilibrium conversion under supercritical conditions. The reaction pressure has negligible effect on reaction kinetics under these conditions (40– 80 atm ; temperature 373– 393 K ).

Extractive hydration of n-butene with solid acid catalysts in the liquid phase and under supercritical conditions

The hydration of butene-1 is investigatedin the presence of di+erent solidacidcatalysts. The reacting conditions were variedsuch that butene was present either as a liquidphase or as a supercritical ,uidin the reaction mixture. The d esiredprod uct sec-butanol is extractedout from the reacting aqueous phase d uring the course of the reaction andsuch a transfer helps in increasing the conversion andreaction rates. The hydration is accompaniedby isomerization, oligomerization andetheri.cation reactions. Zeolite ZSM-5 was foundto be a more active catalyst than ion exchange resins anda SiO 2=Al2O3 ratio between 30 and50 was foundto be most e+ective. However, the zeolites rapidly get deactivated during the course of the reaction. The rate of deactivation increases with temperature and hence the reaction under supercritical conditions did not provide much advantage when zeolite catalysts were used. The reactions in solid–liquid–liquid mode, on the other hand, o+er similar conversion level with milder reaction condition andmay be a promising alternative. ? 2001 Elsevier Science Ltd. All rights reserved.