Many natural gas liquefaction plants with production
capacities ranging from 0.5 MMtpy to 8 MMtpy use mixed refrigerants to cool and
liquefy the feed. The mixed refrigerants used comprise a mixture of light
hydrocarbon components and nitrogen for LNG subcooling. The composition of the
refrigerant is optimized for each application to maximize performance.
When operating a mixed refrigerant loop, it is important
that the vaporized refrigerant returning to the compressor is superheated to
avoid liquid from accumulating in the bottom of the exchanger or the compressor
suction drum or entering the compressor. It is also important for optimal
performance that the vaporized refrigerant is not superheated too much. Too
much superheating can increase pressure drop through the exchanger and result
in inefficient use of the heat exchanger surface area due to large internal
temperature differences.
Plant design heat and material balances serve as the basis
for plant operations; however, LNG plants are often operated at conditions that
are significantly different than the design. Changes in the mixed refrigerant’s
composition, relative flowrates, LNG production, ambient temperature and
refrigerant pressure can all result in too little or too much superheating of
the vaporized refrigerant leaving the main exchanger. While process simulation
software can be used by plant engineers to calculate the dewpoint temperature
of the mixed refrigerant, this is of limited use to the plant operators at the
control panel in maintaining an appropriate amount of mixed-refrigerant
superheat as they adjust the plant.
To address these challenges, the authors’ company has
developed a simplified method of calculating mixed-refrigerant superheat that
can be programmed in the distributed control system (DCS) and made available to
panel operators.
Calculation
method. The method uses Raoult’s law, along with a slight
modification to Dalton’s law, by introducing a fitted constant z to
account for liquid and vapor non-ideality (Eq. 1):
zyi P = xi pisat (1)
Where z is a single constant used for all
compositions and pressures.
The saturation vapor pressure for each component is
calculated using the Antoine equation (Eq. 2):
log(pisat) = A + (B / T + C) (2)
The dewpoint temperature of a mixture is when the vapor
phase reaches the saturation point. At that point, the liquid phase
composition must equal 1. This is shown by Eq. 3, which combines Eqs. 1 and 2,
where yi is the mixture composition, and the
requirement that the liquid phase mole fractions sum to 1.
Σi xi = z Σi (yi P / pisat) = 1 (3)
At the dewpoint temperature, Eq. 3 is correct. If the temperature used to compute Psat is too low or high, the sum of xi will not equal 1. The dewpoint temperature can be found using a damped successive substitution, with the temperature on iteration n+1 computed from the guessed temperature and xi values on iteration n (Eq. 4):
Tn + 1 = Tn Σi xi,n0.1 (4)
For typical mixed-refrigerant LNG liquefaction systems, the initial guess for the dewpoint (T1) can be assumed to be 10°C lower than the operating temperature.
FIG. 1 describes the convergence method. It is found that
this method reliably converges within 6 or 7 iterations on temperature. TABLE 1 shows
Antoine equation coefficients for typical refrigerant components from
literature1.
Accuracy of the
method. To determine z, a least squares data fit was
used. The authors’ company has evaluated this method of calculating dewpoint
over a wide range of pressures and compositions that could be encountered while
operating their proprietary technology liquefaction unit(s)a. The
range of compositions is listed in TABLE 2, and the pressure varies from 2.5
bara–10.5 bara.
The data fit shows that using z=0.914 over the
range of compositions and pressures, the average deviation from the authors’
company’s proprietary equation of state was 0.4°C, and the maximum deviation
was less than 1.5°C. This is well within the accuracy needed for plant
operations and control. FIG. 2 shows a parity plot comparing the dew point
temperature predicted by this simplified method to the proprietary equation of
state used by the authors’ company to design and rate LNG liquefaction units.
Takeaway. This article provided a
quick and accurate method that can be used within a DCS or other plant control
software to compute the dewpoint of the mixed refrigerant leaving the main
exchanger. Knowing this dewpoint allows the process to be optimized in real time.
If desired, rather than using a tolerance test to stop the successive
substitutions, as described in FIG. 1, simply repeating the substitution
6 to 8 times and stopping without checking will give sufficiently accurate
results. HP
NOTE
a AP-C3MR™, AP-DMR™,
AP-SMR™ or AP‑X™ liquefaction unit
LITERATURE CITED
Mark Julian Roberts joined Air
Products in 1996 and has 32 years’ experience developing cryogenic cycles for
gas separation and liquefaction in the Air Separation, Hydrocarbon Processing
and LNG industries. He has over 40 US and international patents issued in his
name, including the patents for the AP-X™ liquefaction process used for the six
mega-trains in Qatar, the patent for the AP-N™ liquefaction process deployed in
the first offshore project, Petronas FLNG Satu, and the patents for the AP-DMR™
dual mixed refrigerant liquefaction process used on the Coral South FLNG vessel
currently under construction.
William P. Schmidt is the Technology
Manager for LNG Process Engineering. This includes developing and maintaining
the process design methods, project and field support, technical risk
management, and troubleshooting. Mr. Schmidt has 43 years of process
engineering experience, including air separation, non-cryogenic gas
separations, computer model development, and the past 14 years have been in
LNG. He is co-inventor on 11 US patents and has 44 external publications. He
received Chemical Engineering degrees from Case Western Reserve University
(B.S.) and Lehigh (M.S.).
Sophia Gripp was an intern for LNG Process Engineering. As an intern, Ms. Gripp supported several LNG process engineering projects focused on process improvement. She has had three internships at Air Products, including roles in EH&S and HyCO Customer Service Engineering. She received her B.S. in Chemical Engineering from Purdue University in December of 2022. She is currently pursuing a career in pharmaceutical process engineering.