Markus Riihimäki

Molecular modeling approach on fouling of the plate heat exchanger : Titanium hydroxyls, silanols, and sulphates on TiO2 surfaces

Molecular modeling is a novel approach in the field of fouling research. A method was used to calculate fouling reactions and molecular level interactions between heat transfer surface and flowing fluid. The focus was on the comparison of the reaction mechanisms of Ti(OH)4 and Si(OH)4 on a rutile (101) surface. The calculated reaction energies indicate strong chemical bonding via condensation reaction of titanium(IV) hydroxyls and weak hydrogen bonding of silanols without a chemical reaction on the surface. The chemical composition and structural properties of fouling layers from a real process were characterized. On the heat transfer surfaces, deposits containing titanium had dense structure and were very difficult to clean while silica was porous and amorphous, causing less severe problems in cleaning. Molecular modeling was found to be an effective tool in predicting reaction mechanisms and interaction forces between the fouling fluid and heat transfer surface at a molecular level.

Density Functional Theory Studies on the Formation of CaCO3 Depositions on Cristobalite, Diamond, and Titanium Carbide Surfaces

Fouling caused by inversely soluble salts, like CaCO3, is a general problem on heat transfer surfaces. Carbonate depositions are typically cleanable with acids, but costs of energy losses, operation, and maintenance are significant. In this study, formation of CaCO3 depositions was investigated on cristobalite, diamond, and titanium carbide surfaces. The aim of the study was to clarify the detailed mechanisms of crystallization fouling during the initiation on crystalline phases existing in materials used as coatings (SiOx, TiCN, diamond-like carbon [DLC]), and to compare the results to the fouling mechanism of stainless steel (Cr2O3). In experimental studies of fouling, detailed mechanisms and description of sterical and electrostatic factors of surfaces are often very much simplified. In this work, molecular modeling was used to describe surface structures and to investigate the effect of process fluid (water) on the structures. The adsorption of water can be molecular or dissociative. During the dissociative adsorption, hydroxylated surface structures are formed. The existence of hydroxyl groups on the surfaces has an effect on the fouling mechanism. First, the dissociation probability of water on different surfaces was determined according to the adsorption mechanism and energies, and then the attachment of CaCO3 onto optimized and hydroxylated surfaces was investigated. As a result, the formation mechanism with detailed intermediate steps of CaCO3 deposition was obtained. The fouling takes place via hydrogen carbonate intermediates, but the final deposition structure was found to vary between surfaces.

Surface Patterning of Stainless Steel in Prevention of Fouling in Heat Transfer Equipment

Fouling of surfaces is a major challenge in design and operation of many industrial heat transfer equipment. Fouling causes significant energy, material and production losses, which increase the environmental impact and decrease economic profitability of processes. Even small improvements in prevention of fouling would lead to significant savings in a wide range of heat transfer applications. In this study, crystallization fouling of aqueous calcium carbonate solutions on a heated stainless steel surface is used to investigate the prevention of fouling in heat transfer equipment by physical surface modifications. Fouling behaviour of different surface patterns are studied experimentally in a laboratory scale fouling test apparatus. CFD modelling is used to study hydrodynamic and thermal conditions near surfaces with different patterns. In addition, the effect of surface pattern on the removal of particles is studied numerically. Surface patterning is found to affect the hydrodynamic and thermal conditions near the wall, and therefore to change the conditions for fouling layer build-up and removal, when compared to a flat heat transfer surface. The most promising surface pattern includes curved shapes, and it seems to create flow conditions in which improved convective heat transfer decreases the driving force for crystallization fouling. In addition, curved surfaces increase the shear forces at the wall, which prevents adhesion of the foulants to the surface and increases resuspension.