This invention provides a composition comprising I. at least one reaction product between a nitrogen-free monohydric alcohol and an aldehyde or ketone, and II. at least one reaction product between a nitrogen-free sugar alcohol and an aldehyde or ketone, and optionally III. at least one reaction product from III.a) formaldehyde, and III.b) an amine, selected from the group consisting of primary alkyl amines having 1 to 4 carbon atoms, and primary hydroxy alkyl amines having 2 to 4 carbon atoms, and optionally IV. at least one solid suppression agent selected from the group consisting of IV(a). alkali or alkaline earth metal hydroxides IV(b). mono-, di- or tri-hydroxy alkyl, aryl or alkylaryl amines, IV(c). mono-, di- or tri-alkyl, aryl or alkylaryl primary, secondary and tertiary amines or IV(d). multifunctional amines and IV(e). mixtures of compounds of groups IV(a) to IV(c). wherein alkyl is C1 to C15, aryl is C6 to C15 and alkylaryl is C7 to C15.