Surveying structural change: seminal contributions and a bibliometric account

The main aim of this paper is to provide a comprehensive survey of the economic literature on structural change. Along with an insightful literature review of the seminal contributions in the field, we perform a quantitative analysis that takes into account all the theoretical and empirical articles on the subject that were published from 1969 onwards. This effort to gather the relevant documentation is based on a review of 910 abstracts from articles published in all the economic journals found in the Econlit database over the past forty years. According to our quantitative analysis, structural change analysis received a major boost in the 1990s, with a considerable increase in the number of articles published on the matter. The marked rise in interest seems to be primarily related to the growing concern to find explanations for the process of technological change and its effects on the economy. In the first half of the current decade technological change comprises a predominant area of research, accounting for about one quarter of the articles published. Despite the increased formalism observed in the 1990s, our results further highlight that the bulk of the research in this field remains empirically led. Furthermore, and quite surprisingly, discussions concerning ICTs do not seem to have been translated so far into a substantial increase in research on structural change-related issues.Structural Change; Bibliometrics; Econlit

Does structure influence growth? A panel data econometric assessment of ‘relatively less developed’ countries, 1979-2003

Neo-Schumpeterian streams of research emphasize the close relationship between changes in economic structure in favour of high-skill and high-tech branches and rapid economic growth. They identify the emergence of a new technological paradigm, strongly based on the application of information and communication technologies (ICTs), in the 1970s, arguing that in such periods of transition and emergence of new techno-economic paradigms the relatively less developed countries have higher opportunities to catch-up. Although this debate is theoretically well documented, the empirics seem to lag behind the theory. In this paper, we contribute to this literature by adding illuminating evidence on the issue. More precisely, we relate the growth experiences of countries which had relatively similar economic structures in the late 1970s, with changes occurring in these countries’ structures between 1979 and 2003. The results reveal a robust relationship between structure and (labour) productivity growth, and lend support to the view that producing (though not user) ICT-related industries are strategic branches of economic activity.Structural change, Economic growth, Technical change

Octane upgrading of C-5/C-6 light naphtha by layered pressure swing adsorption

The performance of a layered pressure swing adsorption (PSA) process for the separation of high research octane number (HRON) paraffins from a C-5/C-6 light naphtha fraction is simulated with a detailed, adiabatic single column PSA model. A zeolite 5A layer is used for selective adsorption of the low RON linear paraffins, while a zeolite beta-layer is used to separate the intermediate RON 3MP from the H RON fraction. The effects of various independent process variables (zeolite 5A to zeolite beta ratio, purge to feed ratio, cycle time, operating temperature, and depressurization mode) on the key dependent process variables (product RON, H RON species recovery, HRON purity, and adsorbent productivity) are evaluated. It is demonstrated that an optimal zeolite 5A to zeolite beta ratio can improve the product average RON up to 1.0 point as compared to existing processes using zeolite 5A only. Moreover, process simulations demonstrated that increasing the operating temperature from 523 to 543 K results in an octane gain of 0.2 RON

An analytical solution for the analysis of zero-length-column experiments with heat effects

An analytical solution for the analysis of zero-length-column (ZLC) experiments with heat effects is developed. The model is an extension of the original one developed by Eic and Ruthven with the inclusion of the energy balance. Two additional parameters are obtained, beta = (DeltaH/C-p) (partial derivativeq/partial derivativeT)\ (c0),(T0) and alpha = (ha/C-p)(r(c)(2)/D-c). A criterion for negligible heat effects, 3L beta/alpha < 0.1, is derived from the analytical solution based on ZLC operating parameters. ZLC desorption curves in nonisothermal operation are discussed. The model reduces to the original solution of isothermal operation developed by Eic and Ruthven when heat effects are negligible. ZLC experiments with heat effects are analyzed, and trends are in good agreement with theory. Because of its simplicity, the model is a valuable tool for the analysis of ZLC experiments with heat effects

Separation of branched hexane isomers using zeolite BEA for the octane improvement of gasoline pool

A sorption study of single, binary, ternary and quaternary mixtures of hexane (C6) isomers nhexane (nHEX), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB) and 2,2- dimethylbutane (22DMB) was performed in commercial pellets of zeolite BETA (BEA structure), covering the temperature range between 423 K and 523 K and partial pressures up to 0.3 bar. From these data, single and multicomponent adsorption equilibrium isotherms were collected. An extended tri-site Langmuir model (TSL) was developed to interpret accurately the equilibrium data, and a dynamic adsorption model was developed and tested predicting with a good accuracy the behaviour of multicomponent fixed bed experiments. At the partial pressures studied, the sorption hierarchy in the zeolite BETA is: nHEX>3MP>23DMB>22DMB. BEA structure demonstrates a significant selectivity between C6 isomers, especially at low coverage, giving a good perspective regarding future workJosé A. C. Silva acknowledges financial support from Fundação para a Ciência e Tecnologia under project EQU/60828/2004. Patrick S. Bárcia acknowledges a FCT grant (SFRH/BD/30994/2006)

Octane Upgrading of TIP Processes by Recycling in a Layered Zeolite 5A/BETA PSA

The objective of this work consists in studying the separation mono/dibranched paraffins by cyclic adsorption process using a layered bed of zeolites 5A and Beta (Figure 1). Aspen ADSIM 2006.5 (AspenTech Inc.) was used for numerically solving an adiabatic dynamic model incorporating mass, energy and momentum balance. Model parameters were taken from experimental data reported in the literature 1, 2. Parametric studies were simulated to determine how process performance is affected by purge quantity, 5A-to-Beta ratio, repressurization/blowdown schemes and operating temperature. Figure 2 shows that a combination of zeolites 5A and Beta can produce an octane gain of 1 RON at 523 K comparatively to the conventional TIP3 by reducing the monobranched C6 fraction in the product. Another advantage of this configuration is the possibility to increase the penetration distance because zeolite Beta acts like a “barrier” to the linear alkanes desorbed from zeolite 5A during the cocurrent depressurization step. It was also demonstrated that a slight increase in temperature (20 K) results in a RON benefit of 0.2 points. Several alternatives are provided to improve the performance of the existing TIP processes with this combination of adsorbents

Adsorption equilibrium and kinetics of branched hexane isomers in pellets of BETA zeolite

Sorption equilibrium and kinetics of hexane isomers: n-hexane (nHEX), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB) were studied in commercial pellets of zeolite BETA in the form HBEA with a SUM ratio of 150, between temperatures of 423 and 523 K and partial pressures up to 0.3 bar. Four different models were used to interpret the equilibrium data, named: Langmuir, multi-site Langmuir, dual-site Langmuir and Toth. The affinity to the adsorbent measured by the Henry's constants decreases with the degree of branching, with selectivities that can reach a value of 8.8 between nHEX and 22DMB. The heats of adsorption at zero coverage decrease with the degree of branching, being: 63.4 kJ/mol for nHEX, 59.7 kJ/ mol for 3MP, 57.1 kJ/mol for 23DMB and 53.6 kJ/mol for 22DMB. However, the isosteric heat of sorption changes with coverage with a different behavior for the three isomers. Sorption kinetics studied by the Zero Length Chromatography (ZLC) technique allowed us to find the nature of controlling the diffusion mechanism (macropore or micropore); for nHEX and 3MP macropore diffusion is controlling with activation energies similar to the heats of sorption at zero coverage. For 23DMB and 22DMB, the controlling mechanism changes, being the system governed apparently by both macropore and micropore diffusion. Data from this work are also compared with those reported in literature for both zeolite BETA and silicalite. (c) 2004 Elsevier Inc. All rights reserved

Separation of light naptha by adsorption processes

In this work we studied the separation linear/mono/di-branched paraffins in a cyclic Pressure Swing Adsorption process using a layered bed of zeolites 5A and Beta (Figure 1). Zeolite Beta proved to be an efficient separator of mono-branched from di-branched paraffins [3]. Aspen ADSIM (AspenTech Inc.) was used for numerically solve an adiabatic dynamic model incorporating mass, energy and momentum balances. Model parameters were taken from experimental data obtained in our lab. The studies were performed with the objective to determine how process performance is affected by purge quantity, 5A-to-Beta ratio, repressurization/blowdown schemes and operating temperature which are typical operating parameters of PSA processes. Figure 2 shows the product average RON as a function of the zeolite 5A layer length and purge-to-feed ratio at T = 523 K, where it can be seen that RON is above 90 when the ratio of the layered bed is nearly 0.6. It was also demonstrated that a slight increase in temperature (20 K) results in a RON benefit of 0.2 points. Several alternatives are also provided to improve the performance of the existing TIP processes with this combination of adsorbents [4]

Upgrading of the light naphata fraction with zeolite beta

Multicomponent adsorption of alkanes in the C5-C6 range was investigated in a sample of zeoliteBETA. 6-components breakthrough experiments were performed showing interesting properties forthe separation of the light naphtha fraction. The lower adsorption enthalpy of C5 alkanes was explored by a temperature sensibility analysis demonstrating that at 583 K C5/C6 equimolar mixture can be separated in a single step into low and high octane number (RON) fractions. The separation 23DMB/3MP can not be achieved in the case of mixtures with the typical composition of the isomerization products, but zeolite BETA still selective for 22DMB resulting in a slight increase of the RON comparatively to the final isomerate from the conventional processes

Separation by fixed-bed adsorption of hexane isomers in zeolite BETA pellets

An experimental study of single and binary fixed-bed adsorptions of hexane isomers n-hexane (nHEX), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB) was performed in commercial pellets of zeolite BETA, covering the temperature range between 423 and 523 K and partial pressures up to 0.3 bar. The effect of partial pressure and temperature on the shape of the breakthrough curves was addressed. From these data, single and binary adsorption equilibrium isotherms were collected. On the basis of the analysis of sorption events at the molecular level, two different models were used to interpret, with good accuracy, the equilibrium data: dual-site Langmuir (DSL) for nHEX and 3MP and multisite Langmuir (MSL) for 23DMB and 22DMB. Thereafter, a dynamic adsorption model was developed and tested, predicting with good accuracy the behavior of the fixed-bed experiments. At the partial pressures studied, it was found that the affinity of the isomers to the zeolite is nHEX > 3MP > 23DMB > 22DMB. The selectivity between the isomers is higher at low partial pressures, decreasing as the amount adsorbed increases. The Ideal Adsorbed Solution Theory using the DSL model to describe the pure component equilibrium of nHEX and 3MP and the MSL model for the dibranched isomers 22DMB and 23DMB gives a good prediction of the mixture adsorption data