Effect of Temperature and Pressure
As evidenced by the laboratory separation of asphaltene constituents and the fractionation of feedstocks (Speight, 1994, 2014, 2015), temperature and pressure are both variables because of the solvent power of low-boiling hydrocarbon is approximately proportional to the density of the solvent. That is, decreasing temperature or increasing pressure will increase the average molecular weight of hydrocarbon derivatives soluble in solvent-rich phase. For propane, at temperatures below 80°C (176°F) and a pressure above the vapor pressure of propane, temperature is the most important factor in determining solubility of the feedstock constituents. At temperatures near the critical region, pressure is also an important factor, as properties of the liquid propane approach those of gaseous propane. Thus, higher temperatures typically result in decreased yields of deasphalted oil, although the converse has also been observed (Speight, 1994, 2014). This is accompanied by the decrease in viscosity and molecular weight range of the deasphalted oil.
Thus, during normal operation, when both the solvent composition and the extraction pressure are fixed, the yields and qualities of the various products recovered in the solvent deasphalting unit are controlled by adjusting its operating temperature. Increasing the extraction temperature reduces the solubility of the heavier components of the feedstock, which results in improved deasphalted oil quality but reduced yield of deasphalted oil yield. Subsequent increases in the extractor temperature can further improve the quality of the deasphalted oil by causing further rejection of asphaltene constituents.
Generally, the control of the process may become difficult when rapid changes in temperature occur especially near the critical region because at conditions close to the critical point, the rate of change of solubility is very large. For practical applications, the lower operating temperature is set by the viscosity of the asphaltene phase. The upper limit is to stay below the critical temperature while maintaining the desired yield of deasphalted oil and stable operation. In some cases, a temperature gradient may be maintained along the length of the column with the higher temperature at the top of the column to generate an internal reflux by precipitation of dissolved heavier material - which improves the quality of the deasphalted oil - but a high rate of internal reflux can limit the capacity of the extraction column.
The operating pressure of the extractor is based on the composition of the solvent, which is being used. In the process, sufficient operating pressure must be maintained to ensure the solvent/feed- stock mixture in the extractor is in the liquid state. Although the unit may be designed for a range of operating pressure, once it is in operation, the extractor pressure may not be typically considered a control variable.
Effect of the Solvent-to-Oil Ratio
In general, increasing solvent-to-oil ratio increases the recovery of deasphalted oil with an increase in viscosity. The yield of deasphalted oil can be further adjusted with other variables, such as solvent type or the temperature. At higher ratios of solvent-to-oil, the quality of the deasphalted can be improved by increasing the extraction temperature but with variable decreases in the yield of deasphalted oil. The solvent-to-oil ratio is important from the standpoint of solvent selectivity and the yield advantage at a given product quality at higher solvent ratio varies from feedstock to feedstock and needs to be estimated.