New measurement approaches for film thickness and wall temperature in falling film heat exchangers

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.Falling film evaporation is used in various fields, e.g. food and pulp & paper industry. Evaporation is very energy intense and relatively small efficiency improvements to the techniques can lead to large savings in absolute numbers. Falling film evaporation is affected by the wave dynamics; hence further knowledge of the behaviour of the liquid film flow can promote efficiency improvements. In this work, two new measurement approaches have been investigated. The first approach is to use a laser triangulation scanner combined with a high-speed camera where the laser scanner continuously measures the film thickness along a vertical line, resolving the flow pattern in high detail. The second approach is to measure local wall temperatures, enabling calculation of local heat transfer coefficients at any desired location. These methods has been tested and evaluated in a falling film test facility. Both approaches have proven to give valuable insights into the process and the results are in good agreement with literature data.am201

Experimental study of the heat transfer in a falling film evaporator: influence of the co-flowing vapor

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.A large number of industrial processes are based on the concentration of liquid products by means of falling-film evaporation. In the dehydration of dairy products, concentrating a fluid by evaporating represents one of the most important steps of the whole drying process. Among the advantages of this technology is that it is possible to operate within small temperature differences which results in low heat consumption. In this sense, it is necessary to increase the amount of removed water during the falling-film evaporation to reduce the energy cost of the overall process. However, reducing the fraction of the solvent leads to an increase in viscosity of the product which can show non-Newtonian features. This aspect significantly affects the heat transfer, that is to say, the higher the solid content, the lower the heat transfer coefficient. One of the possible solutions to this drawback consists in drawing maximum benefit from the interaction between the fluid film and the co-flow of the gaseous phase resulting from the evaporation process. Unfortunately, accurate studies of the effect of co-flow on evaporative falling films are very rare and difficult to perform because of the high costs of the implementation of a suitable experimental apparatus. In this work, the experimental study of the influence of the co-flow on the heat transfer coefficient is presented as a function of both the solid content and the mass flow rate of the feed. The experimental set-up, consisting in a unique industrial pilot scale evaporator, provides the possibility to obtain results useful for realistic industrial conditions. Tests were conducted with varying dry solid content from 10 to 50%. The results show that the co-flow has the effect to decrease the potential for fouling/poor wetting. Above this, the influence on the heat transfer is not as large as expected because of the dominant influence of the viscosity.cf201

Hydrothermal liquefaction of kraft lignin in sub-critical water: the influence of the sodium and potassium fraction

As a part of developing a hydrothermal liquefaction (HTL) process to valorise lignin, it is important to consider integration possibilities with existing infrastructures in order to obtain an overall positive economic impact. One obvious example is to integrate the HTL process with the kraft pulp mill: transport and storage costs is reduced, the temperature levels on process streams can be matched (energy integration) and the recovery/use of alkali can be made efficient. In this study, softwood kraft lignin was depolymerised using sub-critical water (623\ua0K; 25\ua0MPa) in a continuous, small pilot unit with a flow rate of 2\ua0kg/h. ZrO2, K2CO3/KOH and Na2CO3/NaOH were used as catalytic system, and phenol as the capping agent. The influence of the ratio between sodium and potassium in the feed on the yield and composition of the product stream was investigated. The results showed that bio-oil, water-soluble organics (WSO) and char yields were not remarkably influenced by shifting the catalytic system from potassium to sodium. Moreover, the yields of most phenolic compounds did not change significantly when the sodium fraction was varied in the feed. The amounts of suspended solids in the bio-oil produced showed, however, a diminishing trend, (decrease from 10.8 to 3.8%) when the sodium fraction was increased in the feed, whilst the opposite trend was observed for the heavy oil, which increased from 24.6 to 37.6%

An alginate-layer technique for culture of Brassica oleracea L. protoplasts

Ten accessions belonging to the Brassica oleracea subspecies alba and rubra, and to B. oleracea var. sabauda were used in this study. Protoplasts were isolated from leaves and hypocotyls of in vitro grown plants. The influence of selected factors on the yield, viability, and mitotic activity of protoplasts immobilized in calcium alginate layers was investigated. The efficiency of protoplast isolation from hypocotyls was lower (0.7 ± 0.1 × 106 ml−1) than for protoplasts isolated from leaf mesophyll tissue (2 ± 0.1 × 106 ml−1). High (70–90%) viabilities of immobilized protoplasts were recorded, independent of the explant sources. The highest proportion of protoplasts undergoing divisions was noted for cv. Reball F1, both from mesophyll (29.8 ± 2.2%) and hypocotyl (17.5 ± 0.3%) tissues. Developed colonies of callus tissue were subjected to regeneration and as a result plants from six accessions were obtained

Comparison of Heat Transfer for R22 and Some Alternatives in a 25 MW Shell-and-Tube Condenser

Calculations have been carried out for a 25 MW horizontal shell-and-tube condenser with shell-side condensation, part of a heat pump for district heating. Due to a coming ban on refilling R22, which is used in some district heating heat pumps, calculations of heat transfer were carried out for R22, R134a and four zeotropic refrigerant mixtures, to see how well they perform. It was found that some of the refrigerant mixtures are competitive to R134a as replacement for R22. Two comparisons were made, one for fixed duty and one for a fixed heat pump system. In both comparisons some of the mixtures seem to have better performance than R134a when considering duty and heat transfer whilst maintaining lower flow rates

Evaporation of Na2CO3-Na2SO4 solutions: A method to evaluate the distribution between bulk and surface crystallization

Precipitation of sodium salts in black liquor evaporators causes problems by forming scales on the evaporator surface, reducing heat transfer and cleaning intervals. Most problems are connected with the crystallization of sodium carbonate and sodium sulfate. As the solubility of these salts is exceeded, a crystal mass must form somewhere. Crystallization can occur either in the bulk solution, on the heat transfer surface, or on other surfaces. It is always desirable to create bulk crystals. If crystals form and remain on the surfaces, a layer of scales will build up with time. A method for estimating the distribution of crystal masses between the bulk and on surfaces has been developed in this work. The method is primarily based on inline density measurements combined with inline measurements of the system\u27s water mass. It has been applied to aqueous solution of sodium carbonate and sodium sulfate in a research black liquor falling film evaporator. Experiments have proven that the method gives valuable information on the crystallization process. It shows where crystals are formed during primary nucleation, as well as during the subsequent continuous crystallization. In an industrial black liquor evaporator, the metastable limit can be passed if it is operated under non-steady-state conditions. During evaporation, upon passing the metastable limit, the experiments showed that the surface crystallization is as high as or higher than the bulk crystallization. During the subsequent crystallization process, when concentration is further increased, the crystallization rate is higher in the bulk solution than on the surfaces

Evaporation of Na2CO3-Na2SO4 solutions: A method to evaluate the distribution between bulk and surface crystallization

Precipitation of sodium salts in black liquor evaporators causes problems by forming scales on the evaporator surface, reducing heat transfer and cleaning intervals. Most problems are connected with the crystallization of sodium carbonate and sodium sulfate. As the solubility of these salts is exceeded, a crystal mass must form somewhere. Crystallization can occur either in the bulk solution, on the heat transfer surface, or on other surfaces. It is always desirable to create bulk crystals. If crystals form and remain on the surfaces, a layer of scales will build up with time. A method for estimating the distribution of crystal masses between the bulk and on surfaces has been developed in this work. The method is primarily based on inline density measurements combined with inline measurements of the system's water mass. It has been applied to aqueous solution of sodium carbonate and sodium sulfate in a research black liquor falling film evaporator. Experiments have proven that the method gives valuable information on the crystallization process. It shows where crystals are formed during primary nucleation, as well as during the subsequent continuous crystallization. In an industrial black liquor evaporator, the metastable limit can be passed if it is operated under non-steady-state conditions. During evaporation, upon passing the metastable limit, the experiments showed that the surface crystallization is as high as or higher than the bulk crystallization. During the subsequent crystallization process, when concentration is further increased, the crystallization rate is higher in the bulk solution than on the surfaces

Crystallization in a pilot evaporator - Comparison between black liquor and black liquor model substances

Crystallization measurements during semi-batch operation of a pilot evaporator have been performed. The experimental analysis is based on changes in heat transfer coefficients and crystal masses. With this method of operation, the total crystal mass as well as the distribution of the crystal mass between the circulating solution and on surfaces can be measured continuously during the evaporation. Experiments with black liquor and different black liquor model substances, all with equal carbonate-to-sulfate ratio, crystallizing burkeite have been evaluated. The results are compared to each other and to two aqueous reference solutions crystallizing dicarbonate and sodium carbonate. The results presented include solubilities and metastable limits as well as different crystallization rates and fouling rates. Generally, the supersaturation at the metastable limit observed for the different liquors was high (14-25%). All the different liquors crystallizing burkeite were seen to have significantly lower fouling rates than the reference solutions crystallizing dicarbonate and sodium carbonate. The burkeite liquors showed no specific attraction to crystallize on the heat transfer surface as the reference solutions did