Proefschrift Peter van Hoof

Promotor Prof.Dr. P.Bennema

Copromotor Dr. W.J.P. van Enckevort

Summary

The crystallisation of n-paraffins in crude oil and its products is well known, but undesired in the petroleum industry. Crude oil and petrochemical products consist of a significant amount of n-paraffins which, upon cooling crystallise, starting with the longer chain homologous. This results in the blockage of filters and tubes, amongst others in diesel powered engines, and the formation of slack-wax during the production of lubricates. The present study aims to investigate the growth habit and different growth characteristics of n-paraffins when crystallising from apolar (paraffinic hydrocarbons) and more polar (containing media. This is done by experiments, e.g. growing and characterisation of crystals, Monte Carlo simulations and theoretically.

First, a morphology study has been performed. In chapter 2 a complete connected net analysis of orthorhombic n-paraffins, which are the subject of interest in this thesis, have been performed. It shows that 71 different connected nets, corresponding with 16 crystal faces can be identified. Some multiple connected nets result in symmetry roughening and some probably result in pseudo-symmetry roughening. This will cause their absence on the growth form or decrease their morphological importance. Finally the growth form of constructed which will be bounded by the {001}, {110}, {111} and {010} faces. Especially the occurrence of the {111} faces is striking and this has not found by other studies.

To be able to compare these results with experiments, C₂₃ crystals have been grown from different media, e.g. vapour, melt and solution. Chapter 3 discusses the results of these experiments. It shows that the growth morphology of the crystals which have been grown from the vapour is very similar to the predicted morphology. All the predicted faces, including the {111} faces, have been observed. The crystals which were grown from solutions never showed facetted {010} faces and it was not possible to distinguish between the {110} and the {111} faces. The aspect ratio of these crystals is much larger crystals grown from the vapour, because they are much thinner.

Chapter 4 is based on calculations of Ising temperatures of (pseudo) hexagonal lattices. Many faces of organic compounds have a pseudo-hexagonal symmetry and the study presented in this chapter helps to understand the stability of these faces. It shows that if one out of three bonds is left out, the faces still have a very high Ising temperature. Furthermore it is shown that rather anisotropic faces still maintain relative high Ising temperatures. Finally, it is stated that the roughening temperature is much more dependent on the total energy of the bonds in the face than of the anisotropy in those bonds.

The first step in studying flow impovers is to study the effect of homologous impurities on the crystallisation of n-paraffins. Chapter 5 presents a study on the effect of three different homologous impurities on the morphology and growth of C₂₃ and C₂₅ crystals. Homologous impurities with a longer chain tend to reduce the thickness of the crystals to less than one micrometer. This induces new effects in the growth of the crystals. One was that the side faces were obstructed by dust particles, inducing "particle induced rough growth". Second, a Gibbs-Thompson effect was encountered. Both phenomena have been observed in-situ and were measured quantitatively.

The observations of very thin crystals prompted us to start Monte Carlo simulations to study the effect of crystal thickness on the crystal growth parameters (chapter 6). Two important effects have been observed. First, the Gibbs-Thompson effect, which could be measured, both by simulation and by experiment. This effect is introduced by the surface tension and the finite size of the crystal systems. Second, a shift in roughening temperature of narrow crystal faces. For both phenomena analytical models have been developed.

Other Monte Carlo simulations have been applied to study the kinetic roughening of crystal faces (chapter 7). Simulations on inclined (001) Kossel faces have been performed to obtain growth rates, from which kinetic Wulff plots were constructed. To do so, analytical expressions were derived, which contain two important parameters: e (a roughening parameter) and v_step (the velocity of the steps on the surface). Different criteria to characterise the on-set of kinetic roughening are compared with each other and with the roughening parameter e . It shows that the transition from a flat to a rounded morphology of a crystal face is quite gradual. The different criteria used to identify kinetic roughening yield a broad range of transition points in the supersaturation-bond strength space.

Chapter 8 describes the thermal roughening of the side faces of crystals grown from a apolar solvent (n-hexane) and a slightly polar solvent (toluene). The roughening transition of the side faces of crystals grown from n-hexane is a first order phase transition, whereas that of the crystals grown from toluene solutions is of infinite order. A possible explanation is that the n-hexane molecules, which are alike the molecules, induce a solid phase transition at the surface of the side faces. This phase transition results in a sudden drop of the roughening temperature and in a first order roughening transition.

In chapter 9 this solvent effect has been investigated in more detail. The roughening and wetting of crystals grown from mixtures of solvents have been studied. First, it is shown that mixtures of n-hexane and toluene behave as a regular solution. Second, the difference in wetting at the crystal-solution interface in different solvent mixtures, seems to be due to the different solute-solvent bond strength in the bulk of the solution and less due to a difference in bond strength at the interface. When the roughening temperature is compared to the bulk properties of the liquid it seems to be rather independent of the solvent composition. Thirdly, the addition of 25% of toluene to a n-hexane solution is enough to change the roughening transition from a first order to a higher, probably infinite, order phase transition.

Ex-situ AFM measurements on the {001} surfaces of and crystals have been presented in chapter 10. In this chapter it is shown that the AFM tip can induce growth and etching of the top faces of  crystals just as they were growing and etching in an in-situ environment. Several phenomena, such as spiral growth, two-dimensional nucleation and the pinning and bunching of steps, have been observed and discussed. In contrast, the {001} faces of the  crystals did not show any growth or etching when scanned by the AFM. The steps observed on these faces were very straight and sometimes crossed each other. It is suggested that the latter occurred after crystal growth, due to the movement of screw dislocations in the crystals.

The crystallisation of n-paraffins in crude oil and its products is well known, but undesired in the petroleum industry. Crude oil and petrochemical products consist of a significant amount of n-paraffins which, upon cooling crystallise, starting with the longer chain homologous. This results in the blockage of filters and tubes, amongst others in diesel powered engines, and the formation of slack-wax during the production of lubricates. The present study aims to investigate the growth habit and different growth characteristics of n-paraffins when crystallising from apolar (paraffinic hydrocarbons) and more polar media. This is done by experiments, e.g. growing and characterisation of crystals, Monte Carlo simulations and theoretically.