The roles of adsorption in hydrate precipitation

It has been well established that, in Bayer process liquor, organic compounds with adjacent hydroxyl groups can inhibit gibbsite precipitation by acting as seed poisons. The degree of inhibition is a function of the number and stereochemistry of the hydroxyl groups and the presence of electron withdrawing groups. Seed poisons generally adsorb strongly onto hydrate surfaces, implying that surface coverage is the mechanism for yield inhibition. However, there are examples of organics that strongly adsorb but do not lead to yield inhibition. The current work investigates the relationship between adsorption and hydrate precipitation with a series of aromatic compounds. At the same level of adsorption, straight chain alcohols have a greater yield inhibition effect than aromatic alcohols. Aromatic carboyxlates have been found to adsorb onto gibbsite and promote hydrate precipitation through secondary nucleation. This result suggests that carboxylic groups facilitate adsorption and adjacent hydroxyl groups are responsible for yield inhibition.