Hydrocarbon Processing - July 2022 - 53

Process Optimization
based on the original design feed or with a
change in the catalyst bed operations. Also,
the study evaluated the product properties
for these cases to quantify any revamp
scope. Furthermore, it also explored the
use of additives [CFPP, lubricity, antistatic
additive (ASA) and WASA], including
the quantity required for the changed
mode of operation. This also necessitated
laboratory requirements for diesel specification
based on new analytical methods.
All units are single-stage catalytic dewaxing
DHT units. The hydrotreating
process scheme is an integrated stripper
concept with a vacuum dryer. To limit the
diesel yield loss, the CFP units have been
designed for a 6-pt CP upgrade.
The hydrotreater dewaxing (catalyst
bed) scheme impacts product properties-some
properties are intentionally
changed, and others can be corrected
due to the stripper and/or dryer design.
Conversely, some properties are changed
(improved or worsened) because of hydrotreating
reactions, while some are feedrelated
properties that are determined by
the inside cleanliness of the unit. However,
most important are the properties
that affect fluidity, like CP, which relates to
catalyst bed operations and specification,
including CFPP.
For most of the units, the feed is a mix
of raw diesel and cracked and atmospheric
residue desulfurization (ARD) units'
distillate. TABLE 4 summarizes the main
parameters of the feed composition. The
original blended design feed had a CP of
-6°C. The product received a ∆ improvement
of 6°C for the winter cases. Without
additives, the CFPP product in the winter
case equaled -14°C. Therefore, the
original CFPP specification of -20°C can
only be met if CFPP additives are applied.
The DHT and gasoil desulfurization feed
properties are detailed in TABLE 5.
CFPP can be achieved by dewaxing
bed operations and partly by CFPP additives.
Furthermore, a lubricity additive,
WASA and ASA must be added to achieve
product specifications. The unit should
be revamped to meet specifications, as
the dewaxing capabilities are utilized to
reach the CFPP specification for the base
and the stringent specifications. However,
the lubricity additive, WASA and ASA are
still required to meet the specification.
To increase catalytic dewaxing capabilities,
the weighted average bed temperature
(WABT) in the dewaxing bed must
be increased compared to the original design.
For a single-stage catalytic dewaxer,
this means that the hydrogen desulfurization
catalyst must be operated at higher
WABTs, as well. There will be product
giveaway with respect to sulfur. The product
stripper overhead air cooler is too small
in these cases. In addition, the reactor effluent
trim cooler is too small, so the cold
low-pressure separator offgas cooler in the
hydrogen sulfide removal (HSR) unit,
which is dedicated to the DHT unit offgas
stream, was proposed to be replaced.
Due to the higher production of naphtha
in these cases, the increase in vapor
and liquid traffic in the product stripper
might cause flooding. Currently, the
product stripper is designed with conventional
trays. The advice was to re-tray
the stripper using proprietary trays. Some
nozzles (naphtha streams) exceeded
the maximum impulse momentum. A
risk-based inspection was deemed an effective
barrier to mitigate this. The gas
load (impulse momentum) on the highpressure
absorber increased by approximately
22%. The high-pressure amine
absorber is fitted with conventional trays.
GOD
CDU/eocene
CDU/eocene Lt. diesel
GOD diesel (10 ppm sulfur)
HCR
ARD - 1
ARD distillate - 1
DHT
ARD distillate - 2
ARD - 2
DCU
FIG. 1. Diesel configuration at the MAA refinery.
TABLE 6. DHT mode of operations
Property
Unit feed rate, bpd
CFPP, °C
Naphtha, bpd
Diesel, bpd
Diesel yield, vol%
DHT diesel
(10 ppm sulfur)
Coker
diesel
Lubricity/
WAFI
injection
ASA/lubricity/
WAFI injection
HCR diesel (10 ppm sulfur)
Diesel finished
product
It was advised to re-tray the column with
high-capacity trays. The product stripper
offgas is treated in the HSR unit. The
low-pressure absorber for the HSR unit
is likely to be replaced. Vacuum dryer
packing should be increased without revamping
the dryer. This will generate deteriorated
slops quality. This is indicated
by the fact that ASTM D86 T98 changes
from 172°C to 226°C. The specific gravity
changes slightly. The quantity will
slightly increase (2.18 × the normal design
rate), which will affect the crude distillation
unit (CDU) if not designed for
this function. Otherwise, the dryer slops
can be safely routed to a slops tank, as the
dryer slops have been stabilized. Vessels
handling more naphtha will see lower
holdup times as the capacity increases.
The recycle gas compressor is a turbinedriven
centrifugal compressor. As the
megawatts (MW) increase by 22%, the
compressor should be evaluated by the
vendor if it cannot perform its duty.
Calculations show that increasing the
dewaxing in the unit to meet the CFPP of
-20°C or -22°C results in yield losses vs.
the base case, as shown in TABLE 6.
Lubricity/
WAFI
injection
Base Case
45,000
Winter
45,000
-14
3,540
42,488
94.4
-20
5,308
40,445
89.9
Stringent
45,000
-22
7,641
37,966
84.4
Hydrocarbon Processing | JULY 2022 53
Hydrocarbon Processing - July 2022

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