Although I agree with the theory of common ion effect, I am still in disagreement with the addition of phosphoric acid for a couple of reasons: 1) lowering of available 'free' amine, 2) yes, the addition of phosphoric acid will lower the vaport pressure of H2S, and will contaminate the MDEA by forming heat stable salts. Since production streams are rarely clean, there will be water which can have ions such as calcium and magnesium dissolved into solution. In particular, Calcium forms a calcium phosphate salt, which can be almost insoluble. The added heat stable salts will cause an elevated risk of foaming, which decreases efficiency of adsorption and removal of acid gases. 3) The tolerance of salts will be decreased. With the insoluble nature of phosphate salts, and that phosphates are a common detergent (variations), this tends to promote emulsification and foaming tendencies (through different mechanisms than Heat Stable Salts).
I agree that a mixer-container be placed before the absorber be used due to changes in feeds and concentrations. This will help the system loadings become more predictable, and also provide a reservoir in case some amine fluid is accidently removed from the system. As far as the length is concerned, one length doesn't immediately appear. From a 'surging' perspective, the more fluid reserve you have, the better. I hope this helps.