A study of liquid-liquid extraction in a sieve plate column

The literature relating to sieve plate liquid extraction columns and relevant hydrodynamic phenomena have been surveyed. Mass transfer characteristics during drop formation, rise and coalescence, and related models were also reviewed. Important design parameters i.e. flooding, dispersed phase hold-up, drop size distribution, mean drop size, coalescence/flocculation zone height beneath a plate and jetting phenomena were investigated under non-mass transfer and mass transfer conditions in a 0.45m diameter, 2.3m high sieve plate column. This column had provision for four different plate designs, and variable plate spacing and downcomer heights, and the system used was Clairsol `350' (dispersed) - acetone - deionised water (continuous) with either direction of mass transfer. Drop size distributions were best described by the functions proposed by Gal-or, and then Mugele-Evans. Using data from this study and the literature, correlations were developed for dispersed phase hold-up, mean drop size in the preferred jetting regime and in the non-jetting regime, and coalescence zone height. A method to calculate the theoretical overall mass transfer coefficient allowing for the range of drop sizes encountered in the column gave the best fit to experimental data. This applied the drop size distribution diagram to estimate the volume percentage of stagnant, circulating and oscillating drops in the drop population. The overall coefficient Kcal was then calculated as the fractional sum of the predicted individual single drop coefficients and their proportion in the drop population. In a comparison between the experimental and calculated overall mass transfer coefficients for cases in which all the drops were in the oscillating regime (i.e. 6.35mm hole size plate), and for transfer from the dispersed(d) to continuous(c) phase, the film coefficient kd predicted from the Rose-Kintner correlation together with kc from that of Garner-Tayeban gave the best representation. Droplets from the 3.175mm hole size plate, were of a size to be mainly circulating and oscillating; a combination of kd from the Kronig-Brink (circulating) and Rose-Kintner (oscillating) correlations with the respective kc gave the best agreement. The optimum operating conditions for the SPC were identified and a procedure proposed for design from basic single drop data

Another Approach to Evaluating the Productivity of Value Added Tax in Nigeria

This paper reviewed and assessed the productivity of value added tax [VAT] on total consumption expenditure [TCE] and on gross domestic product [GDP] which determines the level of economic growth in Nigeria. Productivity was expressed as effectiveness/efficiency. While effectiveness is the ability to be actively functional in producing expected results, efficiency is the ability to use available resources optimally, assuring that the cost of resources is controlled to the lowest possible level. Therefore the effectiveness and efficiency of VAT were calculated on GDP and adjusted GDP. The non vatable components were net off from GDP resulting in the adjusted GDP. Also VAT effectiveness and efficiency were calculated on TCE and adjusted TCE. The adjusted TCE represents the private consumption expenditure.  Findings revealed that VAT was not effective on GDP but effective on adjusted GDP.  VAT was neither efficient on TCE nor on adjusted TCE.  VAT was not productive at the cross-elasticity of TCE/GDP but was productive at the cross-elasticity of Adjusted TCE/Adjusted GDP.  Part of the recommendations was that government should review VAT threshold and the reduction of exemptions because exemptions are fundamentally inconsistent with the revenue generation objectives of VAT. This logic is preferred to increasing the present five percent VAT-rate Keywords: Productivity, Value Added Tax, Gross Domestic Product,   Total Consumption Expenditure, Effectiveness, Efficiency