Page 178 - Gas Purification 5E

Alkanolamines jor Hydrogen Surfde and Carbon Dioxide Removal 165

LPG Treater Operating Conditions

The operating temperatures and pressures of amine-type LPG treaters must be maintained
within narrow limits to maintain the hydrocarbon in the liquid state, minimize hydrocarbon
and amine entrainment, and optimize amine-hydrocarbon separation. Veldman (1989) rec-
ommends that the lean amine temperature be controlled so that the viscosity is around 2 cen-
tipoise at the aminelLPG interface to assure effective phase separation. The relationship
between amine solution temperature and viscosity for several amines is illustrzted in the pre-
vious section entitled “Physical Properties.” As the data indicate, operating temperatures
must be greater than about 100°F for typical MEA: DEA, and MDEA solutions to meet the 2
centipoise. viscosity requirement. Changela and Root (1986) report that low amine operating
temperatures (60-70°F versus a design of 110°F) increased the viscosity of the lean amine
solution at one plant to the point that a significant amount of LPG was entrained in the rich
amine. This problem was corrected by increasing the lean amine temperature above 110°F.
A similar problem was reported by DuPart and Marchant (1989) when low temperatures led
to excessive amine entrainment in the LPG product. In this latter case, improved control of
both the amine and LPG inlet temperatures reduced amine losses.
As previously noted, the operating temperature must be below the LPG vaporization tem-
perature throughout the treating system. The designer must carefully evaluate the pressure
profile of the entire LPG treating system to ensure that an adequate safety margin is main-
tained. Maximum and minimum LPG and amine operating temperatures should be consid-
ered in making this evaluation and it may be desirable to prode some means of temperature
control to ensure that neither the amine nor the LPG are too hot or too cold. The effect of the
heat of reaction on the LPG bubblepoint should also be considered-particularly when treat-
ing a very sour LPG. Typical design margins between the LPG bubblepoint pressure and the
LPG treater operating pressure are often 100 psi or greater, and the difference between the
LPG bubblepoint pressure and the minimum operating pressure in the LPG treating system is
set by most designers at 50 psi or more. For example, Changela and Root (1986) describe a
gas plant liquids treater where bubblepoint hydrocarbon liquids from a surge drum with a
design operating pressure of 330 psig were treated at 500 psig. In this case the margin
between the operating and bubblepoint pressures in the LPG treater was about 170 psi.

Amine Solution Flow Rates and Composition

Rigorous calculation of the minimum required amine flow rate, the maximum product
purity attainable, and the relationship between amine flow rate and number of theoretical
trays requires accurate liquid-liquid equilibrium data. As previously discussed, these data
can be estimated by the approximate method of Honerkamp (1975) or the more precise
method of Holmes et al. (1984).
The maximum LE purity possible represents equilibrium with the lean amine solution.
Since equilibrium can only be approached in actual equipment, it is necessary to regenerate
the amine to acid gas concentrations below the levels which would be in equilibrium with the
desired LPG product. For the case of CQ removal from NGL with MEA, Honerkamp (1975)
recommends that the lean amine be regenerated to less than 0.1 mole CQ/mole amine.
The theoretical minimum amine flow rate occurs when the rich amine leaving the contactor
is in equilibrium with the entering LPG. However, to provide an adequate driving force for
mass transfer over the entire column, it is necessary to use an actual flow rate well above the
minimum. As a rcsult, the rich solution loading is always below the equilibrium value. Typi-