Today, we will focus on the causes and solutions for high solvent content in leaching tail gas. First, let us understand the sources of tail gas. In the leaching production process, tail gas mainly originates from the water-separating tank, underground solvent storage tank, wastewater tank, leacher, desolventizer-toaster and evaporation system. Solvent-laden vapor volatilized from these equipment first passes through respective condensers for condensation. Condensed liquid flows into the water-separating tank for water separation and is recycled. Uncondensed vapor proceeds to the condenser of the tail-gas treatment system for secondary condensation. Condensed liquid again flows into the water-separating tank for water separation and recycling, while uncondensed vapor enters the mineral-oil absorption tower to be absorbed by mineral oil.
As can be seen from the tail-gas treatment process, high solvent content in tail gas occurs when the solvent vapor entering the absorption tower exceeds the absorption capacity of the mineral oil. Unabsorbed solvent vapor is extracted and vented by the tail-gas fan. Three factors contribute to high solvent content in tail gas: first, excessive solvent-vapor inflow to the absorption tower; second, low absorption capacity of the mineral oil in the absorption tower; third, excessive extraction capacity of the tail-gas fan, which causes overly fast flow of solvent vapor inside the absorption tower and reduces contact opportunities between solvent vapor and mineral oil.
We will analyze these three factors one by one.
- Possible causes of excessive solvent-vapor inflow to the absorption tower: ① Excessively high cooling-water temperature degrades condenser performance, which frequently occurs in hot summer. ② The volume of solvent vapor to be condensed exceeds the condenser load and impairs condensation efficiency. This often happens when the leaching workshop expands production capacity without replacing condensers, or under abnormal production conditions such as excessively high solvent content in wet meal or excessively low miscella concentration. Massive stripped solvent vapor overloads the condenser.Solutions: Regardless of the root cause of excessive solvent-vapor inflow to the absorption tower, reduce condenser load and improve condensation efficiency.
- Possible causes of low absorption capacity of mineral oil in the absorption tower: ① Insufficient mineral-oil circulation flow leads to inadequate solvent-vapor absorption capacity. ② High mineral-oil temperature raises the risk of solvent vapor escaping. ③ Improper mineral-oil selection, resulting in limited solubility of solvent vapor in the mineral oil.Solutions: ① Increase the circulation flow rate of mineral oil. ② Lower mineral-oil temperature. Three options are available: increase cooling-water flow rate, reduce cooling-water temperature, or enlarge the heat-exchange area of the heat exchanger. Select the most feasible option. ③ Re-select mineral oil or increase mineral-oil flow rate.
- Possible causes of excessive extraction capacity of the tail-gas fan: ① Excessively large damper opening. ② Improper tail-gas-fan selection: high negative pressure can be generated even with a slightly opened damper. ③ Poor equipment sealing: large volumes of air infiltrate the production system, requiring higher fan extraction capacity to maintain system negative pressure.Solutions: ① Turn down the damper. Maintain reasonable negative pressure for the production system; excessively high negative pressure is unnecessary. ② Re-select the tail-gas fan. ③ Inspect the sealing performance of the production system to minimize air ingress.
