Simdean Envirotec Ltd  

Pollution solutions through Engineering Excellence

Foundry Core Shop Installation

The use of the Multivent scrubber has been extremely successful in the removal of Amine odours from the exhaust ventilation systems of  Foundry Core Shops. Emissions to the atmosphere can be reduced to less than 1 ppm TEA or DMEA by the operation of the Multivent scrubber. Due to the non-clogging nature of the scrubber bed, entrained particulates such as foundry sand do not create a problem for the scrubber.

       Gassing Station Extraction Canopy

Process of Amine Absorption

With the use of "cold-box" processes to produce casting cores and moulds an environmental problem arose from the odourous gases which are used to catalyse the resins used in the sand cores.

The catalyst gases used are triethylamine (TEA) and dimethylethylamine (DMEA) and may be present in the exhaust gases from the core moulders in normal concentrations of up to 75 ppm or 300 mg/m3. In order to remove the odourous gases from the exhaust gas stream an examination of the absorption of amines by Sulphuric acid was undertaken ¹. The results of this examination have led to the widespread use of wet scrubbers for the removal of TEA and DMEA from the exhaust gas of core moulders.

Chemistry of Amines

Amines, or ammonia bases, are derivatives of ammonia in which one or more of the hydrogen atoms has been replaced by a hydrocarbon radical. They are classified as primary, secondary or tertiary according to whether one, two or three of the hydrogen atoms have been replaced by alkyl or aryl groups

R. NH2             Primary Amine

R.R’.NH           Secondary Amine

R.R’.R’’. N       Tertiary Amine

Where R, R’ and R’’ represent identical or different hydrocarbon radicals.

The chemical properties of these three classes of amine compound differ considerably, and their properties also differ to some extent according to whether the substituting radicals are (1) all aliphatic, (2) all aromatic or (3) a combination of aliphatic and aromatic.

TEA and DMEA are both aliphatic tertiary amines where R, R’ and R’’ are all ethyl radicals for triethylamine TEA and R, R’ are methyl radicals and R’’ the ethyl radical for dimethylethylamine DMEA

In addition to the primary, secondary and tertiary amines, a fourth type of amine compound is known, namely the fully alkylated ammonium derivative (R₄N)⁺X^- where the (R₄N)⁺ carries a positive charge like the cation (NH₄)⁺ in the ammonium salts. X^- is an anion such as chloride, sulphate or hydroxyl ion.

Such substances are called quaternary ammonium compounds. The salts of amines are similarly constituted e.g. (RNH₃)⁺ X^-, (RR’NH₂)⁺ X^- and (RR’R’’NH)⁺ X^-. It is this last type of salt, which is important in the absorption of TEA and DMEA.

General Properties of Amines

The amines of the aliphatic series are volatile flammable substances, the lower members of the series being gases or liquids with low boiling points, soluble in water and alkaline to litmus. The density of the liquid members is about 0.75 and increase slowly with increasing molecular weight. They have an ammoniacal or "fishy" odour, the smell, flammability boiling point and solubility in water decrease with increasing molecular weight.

The alkaline reaction noted when the amine is dissolved in water is due to excessive hydroxide ion concentration following withdrawal of hydrogen ions.

R₃N + H⁺ OH^- = R3NH⁺ + OH^-

(Tertiary amine) (Water) (Hydroxide)

The amine salts may also be present partly as the unstable hydrate R3NHOH. The basic dissociation constant for the amine is expressed as the product of the concentrations of ammonium and hydroxide ions divided by the total concentrations of un-ionized material. For typical aliphatic amines the value of this constant indicates that they are stronger bases than ammonia.

Salts with mineral acids (e.g. H2SO4 (aq)) are formed in analogous fashion to ammonium salts. Amine salts, owing to their ionic character, contrast in their physical properties with the derivative amines. They are all odourless, non-volatile solids even though the amines they were derived from were odiferous gases or liquids, and they are readily soluble in water where they exist in an ionized condition. The solubility in water of amine salts is decreased by addition of excess of the appropriate mineral acid.

An amine salt can be recognised by the addition of sodium hydroxide to an aqueous solution of it. The alkali is a much stronger base than the amine and liberation of the amine is evident from the smell or from the separation of oil, or both.

Physical and Chemical Properties of Triethylamine (TEA)²

Primary Name        N,N diethylethanamine ( CAS Index Name)

Synonym Name      Triethylamine

Molecular Formula  C₆H₁₅N

Molecular Weight   101.19

Melting Point         -114.7 degC

Boiling Point           89 degC

 

Density                 0.7275 kg/m3 @ 20 degC

Refractive Index    1.4010 @ 20 degC

Solubility

Water                    Soluble

Ethanol                   Soluble

Ethyl ether              Soluble

Acetone                  Very Soluble

Chemistry of Absorption

The reaction that occurs between triethylamine (TEA) and Sulphuric acid can be written as follows: -

2(C₆H₁₅N) + H₂SO₄ +H₂0 = (C₆H₁₅NH)₂SO₄ +H₂0

From the above equation it can be seen that the stoichiometry of the reaction involves the consumption of 2 moles of TEA by one mole of Sulphuric Acid.

When the pH of a solution of the above chemicals is controlled to less than pH 4 the equilibrium established between the amine in its free and protonated form is such that the free amine is negligible and the non volatile protonated form or amine salt is formed. As long as acid is kept in excess of that required, the pH will remain low and the odour of the amine is unnoticeable.

Two acids can be used to produce the reaction as detailed above; these are Phosphoric acid (H₃PO₄) and Sulphuric acid (H₂SO₄). Phosphoric acid has three acidic hydrogens available for reaction with the amine, however, only one, the most acidic, will react at pH levels low enough to keep the solution effective. Thus one mole of Phosphoric acid reacts with one mole of amine.

Sulphuric acid has two acidic hydrogens both of which are acidic enough to react with the amine at low pH levels. Thus one mole of sulphuric acid will react with two moles of amine. For this reason the Multivent scrubber utilises an aqueous solution of sulphuric acid as the scrubbing medium. In addition sulphuric is cheaper than phosphoric acid, but is more corrosive and care must be taken in the handling of the acid.

Although the stoichiometry of the reaction predicts two moles of TEA reacting with one mole of Sulphuric Acid, what is more relevant, in terms of the scrubber function, is the mass of TEA which will react with a given mass of Sulphuric Acid.

The relative molecular weights of TEA and Sulphuric Acid are 101 and 98 respectively, thus: -

As one mole of H₂SO₄ reacts with two moles of TEA then

1kg of H₂SO₄ reacts with (2*101/98) kg of TEA

i.e. 1 kg of H₂SO₄ will "neutralise" 2.06 kg of TEA

The Sulphuric acid used within the scrubber is not normally supplied as 100 % w/w, but at the commercial strength of 77% w/w. The reason why Sulphuric acid is supplied at such a strength are due to the freezing point and considerations of external storage (See section on Sulphuric Acid Handling and Storage)

Assuming the use of 77% Sulphuric acid it is possible to estimate the rate of usage of "commercial acid" per kg of TEA reacted. The calculation is shown below:-

Density of 77% H₂SO₄ (aq)                           1.7 kg/l @ 15 degC

Weight of H₂SO₄ present                                   1.309 kg/l

Usage of "commercial acid"/ kg TEA                    0.37 l / kg

Thus it is possible to approximately estimate the usage of "commercial acid" for a given usage of TEA on the Core moulders. As the TEA is a pure catalyst then essentially all of the TEA used on the core moulders is exhausted to atmosphere. However, there is always a small residual amount left trapped within the cores after gassing at the core moulder station, thus any calculation of acid usage can only be approximate.

Regulatory Requirements

The Environmental Protection Act 1990 requires that the emission to atmosphere for Amines from a foundry process should be less than 5 ppm v/v. Simdean Envirotec Ltd can guarantee that the emissions from the Multivent scrubber will not exceed 2 ppm v/v provided that the unit is operated in accordance with our operating instructions.

References

1. The Chemistry of absorption of Amines by Sulphuric Acid P.B. Smith BCIRA Report 1298 March 1978 BCIRA Alvechurch, Birmingham B48 7QB

2. CRC Handbook of Chemistry and Physics 77^th Edition 1196-7 ISBN 0-8493-0477-6 CRC Press Inc., 2000 Corporate Blvd. N.W., Boca Raton, Florida 33431