D. Barba

Kinetic analysis of Kluyveromyces lactis fermentation on whey: batch and fed-batch operations

A kinetic analysis of Kluyveromyces lactis fermentation on whey is reported. Batch and fed-batch operations have been considered in 10, 100 and 1000 l fermentors. A simple kinetic model for cell growth during batch and fed-batch operation was used. As expected, the specific growth rate was well represented by the Monod equation. Parameters have been estimated by fitting the model to experimental results achieved in the batch step. Results obtained in the fed-batch operation showed the ability of the model to predict K. lactis fermentation courses on whey in different scales.

A thermodynamic model of CaSO4 solubility in multicomponent aqueous solutions

A thermodynamic model is here developed to describe the solubility of calcium sulphate in salt water. A system of equations based on the ionic atmosphere theory of Debye and Huckel, Born model contribution, and local compositions of the nonrandom two-liquid (NRTL) model is used to describe isothermal activity coefficients for aqueous multicomponent solutions of partially or completely dissociated electrolytes. This approach is an extension of the model of Cruz and Renon for binary electrolyte solutions. n nUnlike previously published models, where the solubility behaviour of CaSO4 depends only on the total dissolved solid (TDS) content, our model takes into account the nature and the amount of each ion which is present in the aqueous solution. n nUsing only binary parameters, good agreement is obtained between the experimental and predicted values of calcium sulphate solubility in sea water and in natural and synthetic water (brackish brines), at 25 °C. n nBy letting a limited set of binary parameters to be adjusted in the multicomponent case, we are able to calculate more accurately than previously the solubility behaviour of CaSO4 over a wider range of salt compositions, including sea-water brines with magnesium augmentation.

A method based on equilibrium theory for a correct choice of a cationic resin in sea water softening

Abstract Increase of the performance ratio in evaporative desalination processes is connected with the increase of top brine temperature, i.e. with removal of calcium sulphate from sea water. The softening of sea water by cationic resins and the regeneration of the bed by blowdown from the evaporator is a feasible process. Its economical convenience is dependent on a correct selection of the resin that takes into account both a high selectivity Ca++/Mg++ and a low concentration factor of the blowdown. In fact a high concentration factor is a favorable parameter for the ionic pretreatment but, since it increases the boiling point elevation of the brine, its influence on the evaporation process is negative. For processes at high performance ratios (14–18) a concentration factor ranging from 2 to 4 causes a reduction of logarithmic mean temperature difference of 50–100% and consequently an increase of heat transfer area of 50–100%. A general criterion based on equilibrium theory useful for a correct selection of the resin to be coupled with evaporative process is shown here. The method, that requires only few experimental determinations, has been utilized to compare various commercial resins tested in the laboratory that do not appear completely satisfactory. A sample of a family of “tailor-made” resins, modified according to the indications deriving from the method, is then presented.

High temperature distillation process with sea water feed decalcification pretreatment

Abstract The performance ratio of a desalination plant can be increased to reach 16–18 by coupling it with a decalcification pretreatment by ion exchange of the sea-water make-up, so making MSF comparable with RO from energetic point of view. A new strong cationic resin DECAL has been tailored; its main features are high selectivity towards the calcium ion and low concentration of the regenerating brine. These characteristics make it compatible with the normal operating range of multiflash plants and very suitable for the above service, when compared with other resins. The optimum coupling conditions of the whole plant, as has been pointed out, lie between 140°C and 150 °C for the top brine temperature and 2 to 2.5 for the brine concentration factor. In these conditions it is possible to operate with sea water-resin bed ratio higher than 20.

Magnesium oxide production from concentrated brines

Abstract Magnesium oxide production from concentrated brines wasted in sea water desalting plants has been investigated. Unlike traditional Mg recovery plants from sea water, it is necessary to undertake purification of the precipitated magnesium hydroxide by means of a carbonation-decarbonation section in order to remove calcium sulphate impurities. Such an operation gives flexibility to the whole plant and allows a high purity product to be obtained. This note illustrates all the stages of the proposed process together with the relevant operating conditions. In comparison with the traditional Mg recovery plants from sea water, it has been also found that the capital and operating costs related to the carbonation-decarbonation section are substantially balanced by the elimination of the sea water intake and of the feed pretreatments, as well as by the reduction of the flow rates to be handled in the first plant section.

Numerical simulation of multicomponent ion exchange operations

Abstract A mathematical model of fixed bed ion exchange operations has been developed with reference to multicomponent systems with variable separation factors, such as those involved in the decalcification of multi-ion solutions. The model makes use of the orthogonal collocation method to solve the solid-phase diffusional equations. A preliminary test of the numerical procedure has been made by comparison with results on a binary exchange system, for which the values of the diffusivities inside the resin particle had been already determined. Experimental exhaustion breakthrough curves with sea water of a new tailor-made cationic resin and its regeneration with concentrated brines are predicted successfully by the model.

CaSO4 scale conditions of membranes predicted by a multi-ion thermodynamic model

Abstract The increase of TDS concentration in RO process, due to the permeation and polarization effects, together with particular internal distributions of salts can create the conditions for Gypsum scaling at membrane interfacies. The analysis of such conditions is done by a Multi-ion interaction model that describes the Gypsum solubility taking into account the nature and the amount of each ion present in concentrated brines of sea water or brackish water to be treated. The model for the activity coefficients of salts is based on a theory which considers the excess Gibbs energy of the solution as a sum of three terms: a Debye-Huckel contribution; a Born term as correction factor for the change of the dielectric constant of the solvent due to the presence of ions and other indissociated species; and a short range contribution calculated according to NRTL equation. The parametrization of the model requires only parameters characteristic of the pure components and of the binary systems.

An analytical method for design of electrodialysis stacks operated at high concentrations

Abstract An analytical method analogous to the Kremser-like procedure used in chemical engineering unit operations has been developed for the design and performance evaluation of co-current and counter-current electrodialysis stacks operated at high concentrations. Membrane area or concentrations and flow rates of the outlet streams can be explicitly calculated. When the method is compared with numerical procedures, no relevant discrepancies are found between the two. For its simplicity it is very useful to process engineers for quick initial estimates and plant arrangements.

A general thermodynamic tool for predicting salting coefficients of non-electrolytes in brackish or seawater

Abstract A purely predictive method is proposed for evaluation of the salting coefficient of non-polar and polar organic compounds in aqueous salt solutions. The method is based on a first order perturbation theory and requires only molecular parameters for calculating the salting effect. It appears to be a generally useful procedure for engineeering applications when experimental data on solubility are not available.

Gas - liquid equilibrium of aqueous salted solutions under high pressure and temperature

Abstract In several countries, geothermal resources have great potential for an economic production of energy. Mineral salts and/or desalted water can be valuable by-products in several cases. The knowledge of the thermodynamic behaviour of geothermal fluids, characterised by the presence in solution of salts and gases at high pressure and temperature, can be useful for optimal design , control and optimal ranning of large scale industrial plants. However, because of substantial deviations from ideal behaviour of these two phase multicomponent mixtures, particular problems are encountered in modelling their thermodynamic properties with the required degree of accuracy and generality. Furthermore, since hot geothermal fluids are available at high pressure, we have to account for the influence of the pressure on both the liquid and the vapour phase. We have developed a general analytical procedure based on the SRK equation of state by which it is possible to correlate and predict the thermodynamic behaviour of any kind of natural geothermal fluids under high pressure. Available experimental information for the pure components of natural geothermal fluids as well as for the sub-binaries are used for the parametrization of the model. The comparison between experimental and calculated results for the system water-carbon dioxide-sodium chloride shows that the model has satisfactory correlation and predictive capabilities.