Aeration / Stripping of n-Butyl Mercaptan in aqueous environment

Abstract

Μany applications in water quality management have a common key water quality parameter,
 dissolved oxygen, resulting to the critical role of aeration. On the other hand, in municipal and
 industrial wastewater, especially where aeration is applied, the presence of volatile organic
 compounds (VOCs) causes several concerns including a direct threat to humans, partly due
 to their emission from treatment tanks. pH, temperature and Henry’s Law govern VOCs’
 speciation and consequently their emission characteristics. Limited data and simplifications of
 available mass-transfer models pose obstacles to a realistic approach, especially in the
 presence of a chemical equilibrium, for example in the case of mercaptans.
 In the present study the importance of oxygen transfer and stripping of a VOC (n-butyl
 mercaptan) on aeration’s overall effectiveness are examined separately. Clean water
 oxygenation and stripping of mercaptan to an inert gas (nitrogen) were studied aiming to
 consider mass transfer aspects and to investigate the influence of chemical equilibrium
 between ionic and neutral form of the target compound in neutral and alkaline solutions.
 Using appropriate mass transfer relationships (dynamic method), experimental data were
 analyzed for the determination of overall mass transfer coefficient ( kOL,O2α ) of oxygen.
 Correlating kOL,O2 α with the corresponding mass transfer coefficient of n-butyl mercaptan in
 neutral solutions (calculated according the model proposed by Matter-Muller et al. [1]), a
 value of ratio βy of 0.566 is found, close to the reported values of other VOCs with similar
 values of Henry’s constant. At alkaline pH however the conventional simplified model fails to
 predict realistic values of mass-transfer coefficients. A coupled differential algebraic equation
 system, based on mass balances, taking into account dissociation of the compound to be
 stripped and assuming chemical non-equilibrium conditions during stripping, was developed.
 Reaction parameter k2 was calculated with non-linear least-squares analysis. The model
 predicts satisfactorily the experimental data and it provides a useful tool for the semibatch
 stripper design in situations where a reversible reaction is involved. At pH values below 8.5
 mercaptan concentration falls exponentially whereas above 10.5 it tends to linearity. The
 bubble equilibrates and mercaptan transferred depends upon solubility and not diffusivity.
 Especially after depletion of initial neutral compound, transport depends upon neutral/ionic
 form speciation. The effectiveness of stripping n-butyl mercaptan, at a given pH, is mainly
 determined by a proportionality constant considered as “fugacity capacity” (removal effect on
 the process) and by a reversible reaction rate constant k2 (kinetic effect on the process). The
 ‘’fugacity capacity” is determined by hydrophobicity (i.e. low solubility and high limiting activity
 coefficient) rather than pure-component volatility (i.e. vapor pressure or boiling point). High
 limiting activity coefficient promotes mercaptan emission due to established vapor-liquid
 equilibrium, while the low reaction parameter k2, controls neutral compound quantity. At high
 pH, where ionic form predominates, experimental data showed that stripping was almost
 independent of the gas flow rate applied. A strong sensitivity of the model to uncertainty of γ∞
 was found: γ∞ controls emission rate and through this the dynamic variations of neutral/ionic
 concentration profiles whereas reaction rate law parameter k2 controls the neutral/ionic
 transformation and it is the crucial quantity which governs the process at high pH values.