Carbon dioxide systems disadvantages

With the conductivity cell as described by Ehrhardt (1976), the change in conductivity of a stream of dilute sodium hydroxide solution is related to the mass of carbon dioxide absorbed. Disadvantages to this method include the temperature dependence of electrical conductivity and the potential for interference from compounds other than carbon dioxide, which must be removed from the gas stream. Flame ionisation detector (FID) systems measure methane, which results from the reduction of the carbon dioxide in the gas stream from the DOC analyser by an in-line reductor. Whilst FID systems have a wide dynamic range, and good sensitivity, additional gases are required in the analysis, and the hydrogen used for the flame is explosive. A colorimetric technique described by Schreurs (1978) relies on the partial neutralisation of an acid solution by the carbon dioxide. The neutralization is followed by the change in absorbance of a coloured phenolphthalein solution. The technique is compatible with autoanalyser systems it is not as sensitive as the other techniques mentioned here. 

Carbon dioxide systems have the following disadvantages ... 

From an industrial point of view, homogeneous catalysis has significant advantages concerning selectivities and due to mild reaction conditions [47]. In fact, there is only a limited munber of processes established in industrial applications because of the disadvantageous separabihty of the catalyst from substrate and product. A possible and convenient solution for this limitation can be the application of supercritical carbon dioxide as part of a reaction system due to the following ... 

There are many factors that influence the outcome of enzymatic reactions in carbon dioxide. These include enzyme activity, enzyme stability, temperature, pH, pressure, diffusional limitations of a two-phase heterogeneous mixture, solubility of enzyme and/or substrates, water content of the reaction system, and flow rate of carbon dioxide (continuous and semibatch reactions). It is important to understand the aspects that control and limit biocatalysis in carbon dioxide if one wants to improve upon the process. This chapter serves as a brief introduction to enzyme chemistry in carbon dioxide. The advantages and disadvantages of running reactions in this medium, as well as the factors that influence reactions, are all presented. Many of the reactions studied in this area are summarized in a manner that is easy to read and referenced in Table 6.1. 

The use of hydroxyl compounds is normally with either MDI or NDI systems. The simplest hydroxyl compound, water, can be used. The disadvantage of this is the evolution of carbon dioxide. Water is normally only used in film-type situations or when the bulk of the gas can be removed. 

The major disadvantage of the single-cycle system to date has been the diflSculty of predicting the cleaning efficiency of the cosol-vent/carbon-dioxide mixture at the various operating conditions. 

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