Hydrocarbon Processing - February 2021 - GP-19

SPECIAL FOCUS: TREATING TECHNOLOGY

Technical and economic impacts of
piperazine content in MDEA solvents

S. WILLIAMS, C. RIDGE and D. ENGEL, Nexo Solutions, Amine Optimization Division, The Woodlands, Texas

Amine units used in gas and liquid hydrocarbon treating utilize various solvent
formulations. Some are amine-based, and
some are not. These blended formulations can be open-knowledge or proprietary formulas from a number of different
amine suppliers. Usually, these formulations are selected based on a review of the
plant design, feed acid gas content (CO2
and H2S) and treated gas requirements.
In the authors' experience, a considerable
number of amine units over-treat or, conversely, are strained to meet specification
with the solvent in place. Further evaluation of these units has revealed that modifications to the solvent formulation may
result in more ideal treating conditions.
This article discusses the effects of
non-ideal amine formulations and the
opportunities for reduced costs and improved treating conditions made by reformulating the solvent. A particular case
is studied where a formulated methyl
diethanolamine (MDEA) solvent was
over-treating. An evaluation of several
formulations with reduced piperazine
concentration was performed to identify
formulations that would potentially reduce solvent cost and rich CO2 loading.

The plant was processing up to 90
MMsft3d of natural gas, but generally bypassing up to 20 MMsft3d of gas to reduce
operational costs while still meeting a 2.5%
specification for CO2 content in the treated gas. The plant normally operates with
an amine circulation rate of 190 gal/min
(GPM); the design circulation rate is 250
GPM. The operator wanted to decrease
piperazine concentration in the amine solvent to reduce both solvent costs and rich
loading while staying just under the 2.5%
specification. A conditional evaluation
was performed to understand if a potential
reformulation was feasible and, if so, what
conditions must be modified.
The process was evaluated under several sets of conditions with varying MDEA/
piperazine formulations to determine the
lowest piperazine concentration required
to meet specifications. This article summarizes the evaluation work performed,
the results and interpretations, and the recommendations for solvent reformulation.
Process simulation overview. A rate-

based simulator packagea was used to

evaluate the amine system because of its
ability to accurately predict the actual
operating conditions and performance of
amine gas treating systems. The software
package uses a mass transfer rate-based
model for column calculations in the
amine contactor and regenerator, as opposed to equilibrium stage models used
in other simulators. The rate-based model
delivers a more accurate prediction of system operation and performance because it
takes into account the actual design of the
absorber column, as well as its internals.
Equilibrium stage models, by contrast,
do not incorporate column design details
into the simulation but instead rely on
empirical data to estimate reaction kinetics. Equilibrium stage models use ideal
stages for column calculations, which do
not account for actual tray or packedbed designs and do not provide a predictive output. The assumptions made for
column designs in an equilibrium stage
model will be accurate only with empirical data provided for that case, so any
modification to the model input will not
be simulated accurately without empirical

Amine unit background. The amine

solvent at a gas plant amine unit (FIG. 1)
comprised 50% water, 47% MDEA and
3% piperazine. Piperazine is a cyclic diamine used to improve CO2 removal in
MDEA-based solvent formulations. The
plant had little trouble meeting specifications for CO2 content within the range of
conditions observed and the formulation
in use, but the rich CO2 loading estimated
in the system was above recommended
guidelines. The plant engineers wanted to
explore a potential reduction in piperazine
content in future solvents to reduce amine
solvent costs and rich amine CO2 loading.

FIG. 1. Amine system evaluated for solvent reformulation.
Gas Processing & LNG | JANUARY/FEBRUARY 2021 19

Hydrocarbon Processing - February 2021

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