Hydrocarbon Processing - June 2022 - 30

Process Optimization
TABLE 2. Static gas head at diff erent temperature of PDT piping
Assumed T in PDT connecting piping, °C
Gas density of N2 in the piping at 555 kPa, kg/m3
Static head in PDT connecting piping, kPa
Predicted PDT value, kPa
-80
9.81
1.24
2.72
-60
8.85
1.12
2.85
-40
8.07
1.02
2.95
-20
7.41
0.94
3.03
6.86
0.87
3.1
20
6.38
0.81
3.16
30
6.17
0.78
3.18
temperature of the piping based on the measured pressure drop.
A large temperature gradient between the column and the
wall of the cold box was expected, as the temperature inside the
HP column was close to -180°C (-292°F), in contrast with the
27°C (81°F) ambient temperature. By varying the temperature,
the gas density of a pure N2
at 5.51 bar was calculated and is
h1
h
tabulated in TABLE 2. As shown, the gas density changes with
temperature, and its values in the temperature range investigated
are at least two times smaller than the gas density in the HP
column. Similarly, the density of the O2
-enriched gas is also calculated
at the corresponding temperature. The static pressure
drop in the piping is calculated based on Eq. 3:
∆Pstatic pressure drop in the tubing = ρg1 × g × h1 + ρg2 × g × h2 (3)
PDT
h2
where,
ρg1
and ρg2
refer to gas density in the upper and lower piping,
respectively. Then, the predicted pressure drop by the
PDT is obtained by correcting the dynamic pressure
drop (Eq. 4):
PDT = ∆Ppacking dynamic pressure drop
∆Pstatic pressure drop in the piping
FIG. 3. The simplified HP column and PDT piping.
+ ∆Pstatic pressure drop in HP column
-
(4)
When the piping temperature is assumed as -40°C (-40°F)
and 20°C (68°F) for Units 1 and 2, respectively, the calculated
PDT values match the site PDT readings at a unit capacity of
20 kNm3
/hr.
It is worth mentioning that in the two identical units, the
main equipment and their space arrangements are exactly the
same, but the space arrangements of instrument piping are not.
FIG. 4 exemplifies the impact of the temperature of PDT
piping on different types of pressure drop. It can be seen that
the predicted PDT reading rises with increasing temperature
in the PDT piping, and the slope is steep at lower temperatures.
It is also apparent that the piping for Unit 2 is closer to
the wall of the cold box. The calculations at other loads deduced
the same temperature for each unit, confirming the
plant data acquired are consistent and of quality.
Assuming a uniform piping temperature in the calculations
FIG. 4. Pressure drop of the HP column against the temperature of
PDT piping at a capacity of 20 kNm3
/hr.
detailed pressure piping design may vary among licensors.
Unfortunately, like in many other plants, the PDT piping
local temperature was not available despite the fact that the
temperature of the cold box foundation is closely monitored.
A different approach was taken in corrections: match back the
30 JUNE 2022 | HydrocarbonProcessing.com
above is correct for qualitative analysis. However, considering
the isometric of the pressure piping in the cold box and
its variations in different projects, precise quantification of
the gas static head in the piping requires temperature gradient
along the piping.
In addition to the variation associated with the temperature
gradient along the piping, the location of the transmitter outside
the cold box-which is also project dependent-affects
the gas static head in the piping to a small extent. Specifically,
in the case investigated here, the gas in the upper piping is nearly
pure N2
with trace of oxygen (O2
gas in the lower piping consists of 37.8% O2
) and argon (Ar), and the
and 1.6% Ar. As
the gas density is a function of its composition, if the location
Cold box wall
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