Hydrocarbon Processing - July 2021 - 19

Project Optimization
light gases, such as liquefied petroleum gas (LPG) and propylene.
Any light hydrocarbon released by the primary absorber is
generally sent to the fuel gas pool or to flare. In this
example, 964 kg/hr of propylene were lost; at $870/t,
this translated into $6.71 MM/yr sent to flare.
A key to increased propylene recovery was the
improved efficiency of the primary absorber, operated
with a mix of un-stabilized naphtha from the
M.F. condenser and stabilized naphtha from the
debutanizer bottom. The recovery yield performed
was 95% based on a net propylene flowrate to the
gas plant. The mix of naphtha supplied to the absorber
included 1 wt% of propylene, and the offgas
to the sponge absorber was at 51°C.
The sensitivity analysis conducted on all candidate waterleaving
a margin of $3.58 MM/yr before rental costs.
Several criteria can be applied to guide a future project
Hiring engineered temporary cooling
solutions allows harvesting new profitability
during short-term market opportunities
at reduced financial and technological
risk without committing CAPEX.
cooled exchangers-the M.F. condenser, WGC intercooler,
debutanizer trim cooler, stripper water-cooled condenser and
sponge oil cooler-showed maximum propylene recovery
when targeting the stabilized naphtha recycled to the primary
absorber. The same analysis should include the primary absorber
water-cooled condenser as well as the top and bottom
pumparounds, not present in this case study.
It was determined to apply external refrigeration to the stabilized
naphtha fed to the primary absorber top tray. The following
changes were considered:
* Operating the debutanizer bottom pump at maximum
capacity to improve the propylene concentration
in the scrubbing naphtha mix to 0.78 wt%.
* Installation of a high-efficiency shell and plate exchanger
on the stabilized naphtha line to the primary absorber
to perform 1.5-MW refrigeration. This decreased the
primary absorber top temperature down to 36°C and the
H.P. receiver temperature down to 40°C.
This addition reduced the propylene lost to flare from
964 kg/hr to 440 kg/hr-at $870/t, this translates to
$3.65 MM/yr. Considering a chillers' COP=5, 0.30 MW of
electrical power had to be supplied from the refinery network.
At a cost of $30/MWh, the energy cost totalled $72,000/yr,
WGC intercooler
CW
2,430 kW
348,540*
21,774
M.F. OVHD
M.F. condenser
147,241*
2,028
Reflux to M.F.
0C
Flow kg/h
C3= kg/h
122,743
Light naphtha
CW
Debutanizer
Trim cooler
FIG. 2. Gas plant at improved conditions.
Hydrocarbon Processing | JULY 2021 19
270,518
32,493
41,445 kW
CW
15,297 kW
1,741 kW
L.P.
receiver
1st stage
WGC
1,808
79,793
18,360
2nd stage
WGC
1,209
50,000
Aggreko
exchanger
100,590
1,386
Stripper
condenser
4,182 kW
CW
40
3,072 kW
150.590
1,386
29,748
7,744
Absorber
170,022
8,690
H.P.
receiver
Stripper
218,919
19,306
239,618
24,749
172,743
13,074 kW
Debutanizer
36
45
40
46,176
19,306
C3= separation
(FIG. 2): maximization of the benefits/costs ratio, maximization
of propylene recovery yield, minimization of a plant's
piping modification costs or required power, as well as maximization
of the temporary refrigeration solution's reliability,
as low probability of refrigeration downtime will reduce costs
of flared propylene.
Takeaway. When facing limited opportunities to improve
overall refinery profitability, engineered temporary cooling solutions
for lease offer a strong alternative to capital investment,
with low financial and technological risk. The case study presented
here allowed new profitability on a lucrative short-term
opportunity; benefits were collected as soon as costs started.
Additionally, any cost can be terminated as the benefits decline,
so no unproductive capital assets are left behind.
MASSIMO CAPRA is the manager of Aggreko Process Services in Europe,
supporting the Russian and Caspian areas, as well as the Middle East's refining
and petrochemical sectors. He has 31 yr of experience in the refining and chemical
process industries as a process and project engineering manager for companies
that include AGIP, Foster Wheeler Italiana, Tecnimont, Aramco Overseas Co., and
as a freelance consultant. He worked as a Process Technologist and Team Leader
at OPCW, conducting more than 150 process assessments in chemical facilities.
Mr. Capra has earned an MS degree in chemical engineering from Politecnico di
Torino (IT), a PGC in project management from the University of Wales (UK),
an MBA from Robert Kennedy College (CH), Lean 6 Sigma Black Belt from
BMGI (USA) and ISO9001 Lead Assessor certification from SGS (NL).
10,289
440
20,699
5,443
Sponge
absorber
Hydrocarbon Processing - July 2021

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