Hydrate Crystal Growth Processes

Hydrate crystal growth, unlike hydrate nucleation, is not a stochastic process. Hence, the intrinsic kinetics of hydrate growth can be studied experimentally and modeled. The intrinsic rate constant obtained for this process can be independent of the equipment used for its determination, if determined carefully with proper experimental design, and thus can be used for industrial or other applications. 

When the water is added to the final dry cement material, the hydration of the cement begins immediately. The water is combined chemically with the cement material to eventually form a new immobile solid. As the cement hydrates, it will bond to the surrounding surfaces. This cement bonding is complex and depends on the type of surface to be cemented. Cement bonds to rock by a process of crystal growth. Cement bonds to the outside of a casing by filling in the pit spaces in the casing body [163]. 

There have been many instances of examination of the effect of additive product on the initiation of nucleation and growth processes. In early work on the dehydration of crystalline hydrates, reaction was initiated on all surfaces by rubbing with the anhydrous material [400]. An interesting application of the opposite effect was used by Franklin and Flanagan [62] to inhibit reaction at selected crystal faces of uranyl nitrate hexa-hydrate by coating with an impermeable material. In other reactions, the product does not so readily interact with reactant surfaces, e.g. nickel metal (having oxidized boundaries) does not detectably catalyze the decomposition of nickel formate [222],... 

Nucleation and growth of gas hydrate crystals have been investigated with optical methods under different pressures and temperatures. The particle sizes measured during gas hydrate nucleation ranged from 2 to 80 imi [1334,1335]. The nucleation process is nondeterministic, because of a probabilistic element within the nucleation mechanism [1393]. 

The hypothesis was extended to nucleation of hydrates from liquid water. An alternative hypothesis was proposed by Rodger [1516]. The main difference between these two sets of theories is that Rodger s hypothesis relates the initial formation process to the surface of the water, whereas the theory of Sloan and coworkers considers clusters related to soluted hydrate formers in liquid water as the primary start for joining, agglomeration, and crystal growth. The theories of Sloan and coworkers have been discussed and related to elements of the hypothesis proposed by Rodger [1043]. 

Uchida, T. Ebinuma, T. Kawabata, J. Narita, H. (1999b). Microscopic observations of formation processes of clathrate-hydrate films at an interface between water and carbon dioxide. J. Crystal Growth, 204 (3), 348-356. 

Physical chemists established a process called self-organization in which water-insoluble amphiphiles firstly form a molecular brush on the water surface and then assemble to spherical droplets or bladders in bulk water if a threshold concentration (cmc, critical micellar concentration) is surpassed. It was also shown that the self-organization of molecular mono- and bilayers is commonly not followed by crystal growth which would normally be favoured as it diminishes surface energies. Repulsive hydration and undulation effects were held responsible for preventing the growth of the delicate bilayer structures to 3D crystals. 

Another possible mechanism of trehalose molecules as a kinetic inhibitor is mentioned below. In the growth process of CO2 hydrate, trehalose may work as the kinetic inhibitor that prevents the rate-determining process of the crystal formation at the reaction site which might have small dependence on AT. Trehalose has been observed to prevent ice-crystal growth with the reduction of the free-water providing because trehalose strongly hydrated in the solution. This effect is found apparently when the trehalose concentration increases beyond the intrinsic hydration number of trehalose molecules. It is thus reasonable that the kinetic effect of trehalose on the inhibition of the hydrate formation would be resulted from the smaller supplement of free water from the solution of higher trehalose concentrations. 

Hydrate crystal decomposition, like the hydrate growth, is a deterministic process. Hence, the process is amenable to study experimentally and modeling. Although some studies have been undertaken at the molecular level, most of them on the hydrate decomposition kinetics are based on a macroscopic approach. The hydrate decomposition is a heterogeneous process where liquid water and gas are released as the solid... 

Needle-like and plate-like crystals create additional process complications. For example, these crystals generally have higher filtration resistance and poorer solid flow characteristics for formulation than cube-1 ike crystals. Therefore, it is highly desirable to grow thicker crystals. To grow thick crystals, experimentally, we should try to find the best solvent which favors the formation of such crystals. Meanwhile, solvates and hydrates may form in different solvent environments. Chemical forms, such as salt, free base, and free acid, can also be evaluated. Also, control of release of supersaturation and selection of crystallization conditions to enhance crystal growth over nucleation, which are addressed in the later chapters, would be very helpful. 

The crystal growth of amino acids is puzzling. As already mentioned, they commonly grow as concomitant polymorphs. An altered rate of crystallization resuits readily in the formation of a hydrate instead of an anhydrous form. For example, attempts to grow L-serine by slow evaporation of its aqueous solutions yield a monohydrate, unstable in air [81, 169]. If a powder sample of L-serine is added to the solution (ethanol and H2O, 1 1), large single crystals of the stable anhydrous L-serine precipitate immediately [81]. The latter process is not observed when D-serine is taken instead of L-serine, likely because of the presence of other impurities in the o-serine powder [170]. The dissolution of OL-serine powder produces... 

Many properties other than those presented will determine the eventual worth of an agent in a desalting process. Among these are the stability in water, the solubility in water and salt solutions, the rates of nucleation and growth and the nature of the hydrate crystals, and the cost of the agent. 

The hydration of calcium aluminate is an excellent example of the Le Chatelier model that is the through solution reactioa Obviously, as the results of other authors have shown [6,123,124], the process is composed of dissolution, followed by nucleation and crystal growth of new phase. This can be well followed on the example of CA hydration. 

The mechanism of CAand CJ2A2 hydration consists in the congruent dissolution, nucleation of hydrates and further crystals growth. The hydration process can be easily followed taking as an example the reaction of monocalcium aluminate with water, without admixture of CJ2A2 or other phases with C/A>1. Due to the low nucleation rate in this condition, in the liquid phase a high concentration of calcium and aluminium ions is maintained for a long time (Fig. 9.2). 

---