Chemical & Process Technology

Operating pressure is commonly written as gauge pressure e.g. kPag, barg, etc in engineering whilst absolute pressure e.g kPaa, bara, etc. Absolute pressure equal to gauge pressure plus atmospheric pressure.

Example :
5 bar abs = 5 barg + 1.01325 bar = 6.01325 bara

The standard atmosphere pressure is pressure defined as being equal to 101,325 Pa or 101.325 kPa and normally refer to mean sea level (h=0 m). Atmospheric pressure is decreased with altitude (elevation from mean sea level or earth surface) with following relation.

where
P_b = Static pressure (Pa)
T_b = Standard temperature (K)
L_b = Standard temperature lapse rate -0.0065 (K/m) in ISA
h = Height above sea level (meters)
h_b = Height at bottom of layer b (meters; e.g., h1 = 11,000 meters)
R = Universal gas constant for air: 8.31432 N.m /(mol.K)
g₀ = Gravitational acceleration (9.80665 m/s2)
M = Molar mass of Earth's air (0.0289644 kg/mol)

Altitude = 0 – 11000m, T_b = 288.15 K, L_b = -0.0065 K/m, P_b = 101325 Pa
Altitude =  11000 – 20000m, T_b = 216.65 K, L_b = -1 x 10E-30 K/m, P_b = 22632.1 Pa
Altitude =  20000 – 32000m, T_b = 216.65 K, L_b = 0.001 K/m, P_b = 5474.89 Pa

By inclusion of specific parameters, for altitude = 0 – 11000m, atmospheric pressure is

Example
At altitude of 500m above mean sea level, atmospheric pressure is approximately 95460.84 Pa.
At altitude of 1000m above mean sea level, atmospheric pressure is approximately 89874.57 Pa.

How altitude impacting LNG production rate ?
Let compare LNG production rate change with a plant built at seaside (h = 0) and another LNG plant built at a site with altitude of 1000m. For both plant, LNG store at same gauge pressure (e.g. 50 mbarg), same LNG rundown temperature (e.g. -163 degC), same LNG tank dimension with same in-leak heat (normally higher altitude is with lower ambient temperature, lower in-leak heat is expected. However, the impact is negligible).

LNG from Main Cryogenic Heat Exchanger (MCHE) outlet is set at 50 barg and negative 164.8 degC. A JT valve is letting down pressure to LNG storage tank operating pressure of 50 mbarg. MCHE outlet mass flow set at 50000 kg/h. LNG is pure Methane (C1). Assumed same in-leak heat of 300 kW.

Case : Seaside
Altitude, h = 0m
Atmospheric pressure = 101.325 kPa abs
LNG operating pressure = 50 mbarg = 5 + 101.325 kPa abs = 106.325 kPa abs
From simulation (see below image), BOG flow = 1374 kg/h,
LNG production rate = 48626 kg/h

Case : Inland
Altitude, h = 1000m
Atmospheric pressure = 101325 (1 - 2.25577E-05 x 1000)^5.25588 = 89.8746 kPa abs
LNG operating pressure = 50 mbarg = 5 + 89.8746 kPa abs = 94.8746 kPa abs
From simulation (see below image), BOG flow = 1863 kg/h,
LNG production rate = 48137 kg/h

Same facilities and operating condition, the LNG production in Inland (at 1000m) reduced by 1%.

How altitude impacting Air Compressor / Blower power requirement ?


Concluding Remark
Atmospheric pressure change with altitude and this will have impacts to facilities design. Do not under-estimate this impact.