(406c) Novel Colloid Chemistry Approach to Resolving Emulsions for Fouling Control in Steam Crackers

Steam cracking is the principal technology used to produce olefins such as ethylene, propylene and butadiene. Cracked gas from the furnaces is cooled in a quench water tower where steam and pyrolysis gasoline condense. Water is an effective heat transfer medium, but mixing of hydrocarbons and water can lead to the formation of stable emulsions. If the subsequent gasoline-water separation is incomplete hydrocarbons entrained in the water phase make their way into downstream units. This potentially leads to severe fouling issues, notably in the dilution steam system or saturators where the steam used in the cracking process is generated.

The study described in this paper is unique in that it addresses emulsion formation in quench water towers in steam crackers from a different angle, probing the emulsifying properties of different interfacially active fractions, and analyzing the correlation between emulsion stability and the physico-chemical properties of the interfacial films formed by these fractions.

Freeze-drying according to a protocol developed in-house was used to extract potentially emulsion stabilizing species present in quench water sampled from a steam cracker. Such species were subsequently fractionated into three solubility classes: water soluble, soluble in polar organic solvents and soluble in apolar organic solvents. Liquid chromatography/mass spectrometry analyses indicated differences in the chemical composition of these fractions, with oligomeric series and species having a high oxygen content detected solely in the fractions soluble in organic solvents.

Bottle test results demonstrated that the interfacial material soluble in polar organic solvents was the most effective in stabilizing water-in-gasoline (W/O) and gasoline-in-water (O/W) emulsions. The type of emulsion formed with the fraction soluble in organic solvents was a function of the energy imparted to the system. O/W emulsions required lower energy to form, but also destabilized more rapidly, compared to W/O type emulsions. These results suggest that water droplets in gasoline were stabilized by relatively large molecules that did not promptly desorb from the interface after sorption had taken place. Conversely, gasoline droplets in water were likely stabilized by molecules that required limited energy to adsorb to and desorb from the gasoline-water interface. Further investigation of these aspects will be the object of future work.

The water soluble fraction decreased the stability of emulsions stabilized by the fraction soluble in organic solvents, and its demulsifying properties increased from pH 8.5 to pH 5.5. In the absence of the water soluble fraction emulsion stability was largely independent of pH, suggesting that electrostatic forces played a marginal role when emulsions were stabilized by the fraction soluble in organic solvents.

Scanning Electron Microscopy (SEM)/Energy Dispersive X-Ray Spectroscopy (EDX) was used to probe the morphology and elemental composition of the films extracted from the gasoline-water interface and deposited onto silicon wafers using a modified Langmuir-Blodgett method. SEM images highlighted the presence of particles at the gasoline-water interface, suggesting Pickering stabilization mechanisms. SEM/EDX data reveal a correlation between the composition of interfacial films and emulsion stability. Interfacial films contained a high density of particles containing carbon, sulfur and/or iron when emulsions were most stable, and a low density of such particles otherwise. It is speculated that sulfur or iron particles, rendered amphiphilic due to contact with organic material, were particularly effective in stabilizing the interface. The hypothesis that sulfur or iron compounds acted as aggregation nuclei for organic species with emulsifying properties cannot be discounted. Additional efforts will made to analyze sulfur and iron species as part of our future research.

Interfacial tension measurements were conducted using the pendant drop method. The interfacial tension decrease with time for gasoline-water interfaces was relatively low and similar for all fractions, in agreement with the hypothesis that the material adsorbing at the gasoline-water interface was particle-like.

The compressional visco-elastic moduli of interfacial films and the force required to break them were determined using a pulsating drop rheometer and a modified Wilhelmy plate method, respectively. Measurements were conducted at pH 5.5 and 8.5, with either one or all solubility fractions added to the system. In pulsating drop measurements a sessile gasoline droplet was formed in water and its volume was varied sinusoidally, while continuously monitoring the change in the interfacial tension caused by the droplet deformation. In the modified Wilhelmy plate method, films were aged for 30 minutes, after which they were stretched in a controlled manner using a Wilhelmy plate till film rupture occurred, while continuously monitoring the force exerted on the plate. The compressional visco-elastic moduli and the force required to break the films were similar with all fractions, and do not account for the differences in their stabilizing properties.

The zeta potential of gasoline droplets dispersed in water was also found to be similar for all fractions, and does not explain their effect on emulsion stability. Zeta potential measurements further revealed that gasoline droplets in water were neutrally charged at pH 5.5, at which emulsions were stable with the fraction soluble in organic solvents. This result supports the hypothesis that electrostatic repulsion was not a relevant mechanism in stabilizing emulsions in the presence of the fraction soluble in organic solvents.

In summary, this study shows that interfacially active species in a stable gasoline-water emulsion from a steam cracker are comprised of three major fractions: 1) two fractions soluble in organic solvents (polar and apolar), which contain stabilizing carbon, iron and sulfur compounds, and 2) a water soluble fraction with pH-dependent demulsifying properties. The stabilizing fractions soluble in organic solvents are complex mixtures of compounds, which play different roles in stabilizing W/O and O/W emulsions.

It is believed that the ongoing pioneering work presented in this paper will ultimately open up novel avenues for chemical resolution of problematic emulsions in steam crackers.