Today we are going to discuss the factors affecting residual solvent in meal during routine operations.
Temperature is one of the most intuitive key factors. Solvent removal from wet meal is generally achieved by heating to vaporize the solvent and separate it from the meal. Industrial hexane, commonly-used in extraction processes, has a boiling point ranging from 66 ℃ to 69 ℃. Therefore, the minimum temperature for pure solvent vaporization is approximately 70 ℃. However, this only applies to pure industrial hexane, not the solvent bound to meal.
Solvent exists in wet meal mainly in three forms: chemical bonding, physicochemical bonding and mechanical entrapment. Mechanically-entrapped solvent accounts for the largest proportion. It exhibits the normal boiling point of the solvent and can be readily removed by heating. Chemically- and physicochemically-bound solvent occurs in small quantities. Its content is related to solvent composition, material properties and residual oil content of meal. Such bound solvent does not show the normal boiling point and is difficult to eliminate, requiring higher temperature and longer holding time.
In soybean and rapeseed processing, the desolventizing temperature of the desolventizer-toaster (DT) fluctuates with raw-material varieties and even different batches of the same material. Normally, the temperature of the live-steam layer ranges from 105 ℃ to 110 ℃, and the vapor-phase temperature is between 75 ℃ and 80 ℃.
2.Time
Desolventizing time is another important factor. During desolventization, solvent evaporates first on the outer surface of materials, and the evaporation front then moves inward, building up a solvent-concentration gradient across the meal particle. Driven by this gradient, solvent transfers from particle interior toward the outer surface. Accordingly, wet-meal desolventization consists of two stages: surface vaporization and internal diffusion. Surface vaporization removes weakly-bound solvent from particle surfaces, whereas internal diffusion eliminates strongly-bound solvent from particle interiors. Sufficient time is therefore required to maximize solvent removal inside particles.
With unchanged equipment, two common methods can extend desolventizing time: reducing production capacity and increasing material bed height. The actual choice depends on on-site conditions. Increasing bed height is generally preferred. Nevertheless, bed height cannot be raised arbitrarily; it is restricted not only by the desolventizer-toaster itself but also by the current of its stirring motor.
3.Live-steam dosage and vacuum
Live-steam dosage is the third key factor. Live steam acts as an efficient heat carrier in desolventization. It contacts materials directly and rapidly heats them up to target temperatures. Furthermore, live steam lowers solvent-vapor concentration on material surfaces, accelerating solvent evaporation and mass transfer to improve desolventizing efficiency. Live steam is injected into the material bed through orifices in the live-steam layer, and its dosage shall be closely monitored in daily operation. Excessive live steam raises material temperature, causes excessive protein denaturation and degrades the feeding value of meal. It may also shift vapor-phase pressure to positive and trigger severe solvent leakage across the production system. Insufficient live steam leads to inadequate desolventizing temperature and high residual solvent in meal; incomplete protein denaturation may also occur, which likewise impairs the feeding value of meal.
Vacuum inside the desolventizer-toaster maintains negative pressure of the production system to prevent solvent leakage. It also reduces the partial pressure of solvent vapor on material surfaces, speeds up mass-transfer of solvent from particle interiors to surfaces and enhances desolventizing performance. However, excessively low vacuum is not desirable.
Particle size of extracted materials influences solvent-binding patterns in wet meal. Fine powder reduces bed permeability during extraction and raises solvent retention of wet meal. Over-large particles increase residual oil and solvent content in post-extraction meal, strengthening solvent-residual-oil binding and making desolventization harder. More thorough rupture of oil-bearing cells brings higher bed porosity. More solvent exists in the mechanically-entrapped form while less exists in physicochemically-bound form, which facilitates wet-meal desolventization.
In practical production, meal produced from expanded materials usually has lower residual solvent than meal from raw flakes. Expansion thoroughly ruptures oil-bearing cells and provides smoother channels for oil outflow, hence solvent removal from wet meal becomes easier.
5.Solvent content of wet meal
Poor extractor permeability carries excessive solvent into the desolventizer-toaster, causing sharp fluctuations in its vapor-phase temperature and pressure. Vapor-phase pressure may instantly switch from negative to positive and disrupt negative pressure of the whole extraction system. To restore negative pressure in the desolventizer-toaster and extraction system, operators have to cut down live-steam dosage. This inevitably weakens desolventizing performance and finally results in higher residual solvent in meal.
