Case Studies

To illustrate the general principles four cases will be discussed. These cases show the variety of possible applications of additives and the problems sometimes associated with the presence of impurities in the mother liquor. Scaling inhibition is demonstrated using barium sulfate as an example, diesel fuel additives illustrate how the crystal size distribution can be changed, crystallization of a-lactose is a good example of self-poisoning, and the template directed crystallization of calcium carbonate is also discussed.

Scaling problems are often encountered in oil production. The two major scaling problems involve gas hydrates and mineral scaling. Gas hydrates occur when methane and water crystallize under high pressure to form clathrate structures; this can happen in gas pipes on the bottom of the sea. These crystals can scale and block the pipes. Nowadays much research effort is put into the design of additives to prevent these clathrate crystals from blocking the pipes.

Mineral scaling occurs in secondary oil recovery. When the immediate vicinity of a borehole becomes depleted in oil, water is injected in the surrounding rock strata to push the remaining oil from the pores towards the borehole. The pores also contain water that at these deep levels contains a high concentration of barium. When seawater (which contains large amounts of sulfate) is injected, barium sulfate precipitates and the pores silt up. In the worst case the pores are plugged and a new hole must be drilled, which is a very costly operation. Scale formation is inhibited by the addition of low amounts (ppm levels) of poly-electrolytes, for instance polycarboxylates and poly-sulfonates. Experiments have shown that the molecular structure of the polyelectrolyte determines the effectiveness of the additive. Polyelectrolytes containing sulfonate or phosphonate groups, which closely resemble the sulfate groups in the crystal lattice, are more effective than polyelectrolytes containing only carboxylate groups. Also the way the sulfonate groups are attached to the backbone is important: molecules with the sulfonate group directly attached are more effective than molecules with the sulfonate indirectly attached. The length of the backbone allows each sulfonate group to replace a sulfate in the lattice, thus binding the additive very strongly to the crystal surface, in a zipper-like manner, and inhibiting growth. For instance, 0.1 ppm of a polymaleic acid (polyvinyl sulfonic acid at a pH of 3.8) decreases the growth rate of barium sulfate by nearly two orders of magnitude (depending on the supersaturation).

Waxes in diesel oil can crystallize at low temperatures. The crystals clog up the fuel filters in cars and prevent the motor from starting. To prevent this problem either wax crystallization should be prevented or only very small crystals should be allowed to form, which can pass through the fuel filter without any problem. As the former option is not very practical (it requires either removal of the waxes or storage of the fuel at higher temperatures) the only option is to add additives that block crystal growth. The design of an additive for such a purpose has been carried out using xylene containing n-C32H66 as a model fuel. A copolymer of fumarate vinyl acetate with different alkyl side chain lengths was used as an additive. The effective concentration at which the additive started to influence the growth of n-C32H66 was determined at a fixed supersaturation as a function of the alkyl side chain length. A clear relationship between the side chain length and the growth inhibition was shown, with a maximum effectiveness when the chain length was very close to the chain length of the solute. This shows that the structural match between the additive and the solute is critical for the effectiveness of the additive.

a-Lactose, a milk sugar, is a disaccharide containing a galactose and a glucose ring. Under influence of an acid, the a-glucose ring can be opened and closed again either in the a or the ft form. In the latter form it is a tailor-made additive. The galactose moiety resembles lactose, but the different position of the hydroxyl group (ft instead of a) hinders subsequent growth. The 'a-side' of the crystal can therefore be poisoned so the ft-lactose molecules cannot attach to the crystal, and only one side is blocked. This results in the typical tomahawk shape of the crystals (Figure 8).

Figure 8 The a-lactose hydrate crystal grown in the presence of ^-lactose. The (0 1 0) surface on the right is blocked because ^-lactose can adsorb on the surface, causing a blocking impurity as shown in Figure 6C, while on the left the (0 T 0) surface is not blocked because the ^-lactose can not adsorb on the surface.

Figure 8 The a-lactose hydrate crystal grown in the presence of ^-lactose. The (0 1 0) surface on the right is blocked because ^-lactose can adsorb on the surface, causing a blocking impurity as shown in Figure 6C, while on the left the (0 T 0) surface is not blocked because the ^-lactose can not adsorb on the surface.

A substance that is capable of crystallizing into structurally different but chemically identical crystalline forms exhibits polymorphism. A very common compound that can form polymorphs is water. Under atmospheric pressure and slightly below 0°C ice is formed with a density lower than that of water, so the ice floats on the water. Under higher pressure the ice can crystallize into polymorphs with higher densities that sink. Different polymorphs crystallize under different conditions. However, these conditions may be almost equal for rather complex organic compounds such as pharmaceuticals. In the pharmaceutical industry a strong desire exists to control the crystallization of polymorphs for a number of reasons. The polymorphic form of a pharmaceutical influences its effect and its lethal dosage. The crystallization processes of particular polymorphs are therefore protected by patents.

One way of controlling the crystallization of the correct polymorph is by means of template-directed nucleation of the polymorph. The template mimics a crystal face of the desired polymorph and nuclei of that polymorph can form onto the template. Up to now attention has mainly focused on templates consisting of Langmuir-Blodgett (LB) layers.

Calcite and vaterite are polymorphs of the compound calcium carbonate (calcite is the polymorph that forms under ambient conditions). Stearic acid spreads at the air-water interface, which when compressed as a LB monolayer is highly structured. When the acid group is ionized at higher pH the monolayer surface facing the solution consists of carboxylates that can bind calcium ions from the solution. At higher calcium concentrations calcite grows from the (1 1 0) face under the monolayer, while at lower calcium concentrations vaterite grows from the (0 0 1) face under the monolayer. It is suggested that the surface concentration of calcium under the monolayer is important: at high concentrations the calcium ions bonded to the monolayer mimic the (1 1 0) face of calcite while at lower concentrations they mimic the (0 0 1) face of vaterite, which is less densely packed with calcium ions. Many examples of template-induced growth are found in nature, as mentioned before and only recently has this technique for controlling polymorphs been exploited.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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