Hydrocarbon Processing - August 2021 - 47

Process Optimization
The relief load calculated by the steady-state UBH method
from major sources of the units during global power failure are
represented in TABLE 1. Six relief valves were considered for the
case study (Units A-E), contributing to GEPF load (FIG. 2).
The total calculated relief load for GEPF was 1,200 t/hr.
Approach 2: Energy balancing considering the heat
integration network. For this case study, two heat integration
networks of the complex were considered: one associated with
the overhead circuit of the column of Unit A (dark solid blue line
in FIG. 2) and another associated with the side draw circuit of the
same column (ochre line in FIG. 2). Another HEN exists inside
Unit B, shown in FIG. 3. A steady-state unbalanced energy calculation
was performed for each HEN, and the resulting relief
loads are presented in TABLE 2.
This resulted in a ~25% reduction of GEPF relief load from the
complex with respect to the relief load evaluated by Approach 1.
In the next approach, it is shown that rather than analyzing
the complex heat integration networks, the relief load can
be evaluated and optimized by a simplified overall mass and
energy balance approach for the entire complex (FIG. 3), as described
in Approach 3.
Approach 3: Overall complex enthalpy balancing method.
In this approach, the steady-state unbalanced mass and enthalpy
balance method was applied for the overall complex (FIG. 4)
during a global power failure.The unbalanced heat for the entire
complex and resulting relief load is presented in TABLE 3. The
latent heat of vaporization of the column associated with PSV1
was used to evaluate the relief load.
This results in a much lower GEPF relief load from the complex
with respect to the relief load evaluated by Approach 2.
Results and takeaway.
1. Expected % reduction-Aproach 3 is quite logical,
but optimistic, as the calculated GEPF load is less
than the largest individual unit GEPF load. However,
TABLE 2. Relief load from individual HEN during GEPF
Unit name
Unit A (paraxylene
separation block)
Unit B (xylene fractionation unit)
Unit C (benzene toluene
fractionation block)
Unit D (solvent extraction block)
Unit E (Trans-alkylation block)
Total GEPF relief load
Relief valve tag
PSV 1
PSV 2
PSV 3
PSV 4
PSV 5
PSV 6
Normalized
GEPF load for
publication
700 t/hr
0 t/hr
200 t/hr*
0 t/hr
0 t/hr
0 t/hr
900 t/hr
*This relief load is generated from the internal HEN of unit B and PSVB in FIG. 3.
TABLE 3. Relief load from overall complex enthalpy balance
during GEPF
Unit name
Overall complex
PSVO
25 MW
Total GEPF relief load
Roth chemical processing pumps include a
standard chemical duty, low NPSH, seal less
magnetic drive, and low NPSH multistage pump
options to pump an extensive array of chemicals
including liquefied gases.
Approach 2 is a very plausible scenario where the
contribution of the integrated HEN networks are
ignored to avoid double counting, and only the
contributions from independent enthalpy sources
(e.g., fired heaters) are considered. Approach 2 offers
signifcant savings without any additional investment
in a high-integrity pressure protection system (HIPS)
or additional layer of instrumented protection.
X
B
Integrated enthalpy in (overhead)
Integrated enthalpy out (overhead)
Fired heat in
PSV
FIG. 3. Heat integration network inside Unit B.
The Low NPSH Advantage
Roth Low NPSH Pumps
provide full curve
performance with only 1ft
(0.3m) of liquid required.
Relief valve tag Unbalanced heat GEPF load
500 t/hr
500 t/hr
1-888-444-ROTH * www.rothpump.com
Hydrocarbon Processing | AUGUST 2021 47
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Hydrocarbon Processing - August 2021

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