Solids-stabilized emulsions with interfacial mass transfer and aggregate formation

This study was conducted in order to address two main problems of the halobutyl rubber production process of ExxonMobil at Fawley refinery plant: a) the interfacial mass transfer of acids (by-products) from the organic to the aqueous phase in order to be neutralised by alkaline environment and b) the understanding of the formation of the final rubber particles through rapid emulsion evaporation in a flash drum.

The first aim of this research was to investigate the effect of parameters, such as organic phase viscosity and presence of calcium stearate solids, that could affect the interfacial mass transfer of a solute, which is hydrogen bromine or hydrogen chloride in the industrial process. In addition, the size and stability of the emulsion in the presence calcium stearate as an emulsifier was investigated. In the lab, initially the interfacial mass transfer was investigated using metals (copper and chromium) as the solute species. There were interactions of each system with the calcium stearate, which is added in the industrial process as stabiliser, and so a weak acid was chosen as a more appropriate solute. The investigation of the above parameters effect on the interfacial mass transfer took place in a Lewis-cell device.

The viscosity of the organic phase was modified by the addition of butyl rubber and it was increased exponentially with the increase of the butyl rubber content, especially above 100 g/L. In the Lewis-cell, the slightest increase of the organic phase viscosity (50 g/L BR) resulted in sharp decrease of the overall mass transfer coefficient. Further increase of the butyl rubber content had negligible effect. The effect of calcium stearate on the interfacial mass transfer rate of the acetic acid was also investigated in the Lewis-cell. A slight addition of calcium stearate (5 g/L) increased the interfacial mass transfer rate. However, further addition of calcium stearate resulted in reduction of the rate due to the formation of stearic acid, which acted as a barrier to the interfacial transfer.

Furthermore, the effects of the organic phase viscosity and the calcium stearate concentration on the stability of the emulsion were examined. Calcium stearate is a common industrial emulsifier which is added in the halobutyl rubber production process as a stabiliser. The solid emulsifier, calcium stearate, formed Pickering water-in-oil emulsions. Concentration of butyl rubber above 100 g/L resulted in relatively stable emulsion, under gentle stirring, with emulsion size of approximately 20 μm. Also, concentration of calcium stearate above 5 g/L resulted in the same result when 100 g/L of butyl rubber was present in the organic phase. Lack of calcium stearate had as a result unstable emulsion after the 10th minute of gentle mixing, even the butyl rubber concertation was 100 g/L.

In addition to the above, the effect of acids (hydrogen chloride or acetic acid) on the calcium stearate was investigated. Both acids were used in excess resulting in a maximum amount of calcium stearate able to react, due to the barrier that formed on the calcium stearate particle from the stearic acid. Also, the morphology of the calcium stearate was tested in the SEM and its shape was plate-like. As calcium stearate encounters alkaline environment and high temperature in the industrial scale, their effects on the calcium stearate were tested in the lab. No reaction took place between the calcium stearate and the sodium hydroxide and the temperature had no effect on the calcium stearate up to 70 oC.

The second aim was to understand how the rubber particles were formed inside the industrial flash drum. An initial experiment took place to understand the effect of the temperature on the emulsion evaporation. A W/O emulsion drop, containing butyl rubber in the continuous phase, was exposed at high temperatures, up to 130 oC, in a controlled environment. It was shown that the higher the temperature, the faster the evaporation. Due to the presence of butyl rubber in the continuous phase, the external organic solvent was initially evaporated, and vapours were trapped inside, resulting in expansion of the drop. Also, it was concluded that calcium stearate solids did not have any effect on the evaporation mechanism.

In addition, an experimental configuration was built in the lab to mimic the industrial flash drum in order to investigate the rubber particles formation. The emulsion was formed in an emulsification vessel at 50 °C and, with the use of a peristaltic pump, it was discharged inside the “flash drum”, where there was boiling water. Limited experiments took place and it was concluded that, in this configuration, the emulsion was partially evaporated before hitting the water surface. For better heat transfer rate and full emulsion evaporation, modifications were required in lab scale. On the other hand, in the industrial scale, where the volume of the emulsion stream entering the flash drum is higher (lower surface to volume ratio), it would be required very high amount of heat transfer and it is considered possible fraction of the organic solvent to evaporate inside the boiling water.