This article reviews the application of piperazine as a reaction promoter for enhancing carbon dioxide (CO2) removal rates from syngas preparatory to its conversion to ammonia and from natural gas prior to liquefaction. Apart from a few tangential references, the focus is on piperazine-promoted N-methyldiethanolamine (MDEA), this being the most increasingly used solvent today in most CO2-removal applications. After addressing CO2-piperazine chemistry and piperazine degradation products, consideration is given to CO2 vapour-liquid equilibria in aqueous piperazine and MDEA-piperazine composite solvents. Reaction kinetics between CO2 and piperazine are critical to its successful application, both in amine systems and in hybrids such as Shell’s Sulfinol-M® process.
CO2 reactions with piperazine-MDEA solvents
CO2 absorption by tertiary amines is fundamentally different from absorption by primary and secondary amines in that the latter react rapidly and exothermically with CO2 to form carbamates, whereas the former do not. Thus, regeneration of primary and secondary amines is comparatively more energy intensive, making a tertiary amine like MDEA preferable from an energy standpoint. Its disadvantage is that it does not react directly with CO2, so absorption rates are not enhanced by reaction with the amine. However, reactivity can be gained in a tertiary amine system if the solvent is spiked with a relatively small amount of a very fast-reacting amine. This produces a blended solvent just as alkaline as before (and therefore with the same high CO2 capacity), but now with high reactivity and therefore able to absorb CO2 much more rapidly.
Deep CO2 removal is achieved by using a relatively small concentration of a highly reactive amine such as piperazine. Piperazine reacts very quickly with CO2 which greatly enhances absorption rates, and then the piperazine carbamate gradually decomposes and releases the CO2 back into solution as bicarbonate. The hydrogen ion (acid) produced in the original piperazine reaction is mopped up by MDEA (base) which, therefore, really acts just as a sink for hydrogen ions:
HNRNH + CO2 ? HNRNCOO– + H+ (1)
HNRNCOO– + H2O ? HNRNH + HCO–3 (2)
MDEA + H+ ? MDEAH+ (3)
There is a little more to the chemistry than this, however, because piperazine is a diamine. Thus, piperazine monocarbamate can react with another CO2 molecule to form the dicarbamate, and the dicarbamate can hydrolyse to reform the monocarbamate and release a bicarbonate ion:
HNRNC00– + CO2 ? COO–NRNCOO– + H+ (4)
COO–NRNCOO– + H2O ? HNRNCOO– + HCO–3 (5)
Piperazine is a very effective activator, although it is not the only one used commercially. Regardless of which promoter is employed, when using an activated MDEA solvent, one is dealing with a highly reactive system in which a substantial amount of heat is released by:
- The physical dissolution of CO2 into the liquid solvent.
- Its subsequent reaction with a low concentration of the reactive amine (small heat effect).
- The titration reaction of tertiary amine with released hydrogen ions (substantial heat generated).
Enjoyed what you've read so far? Read the full article and the rest of the May issue of LNG Industry by registering today for free!