Hydrocarbon Processing - June 2022 - 38

Catalysts
yields (FIG. 5, top). The product fractionator separates products
from the unconverted oil (UCO) in the commercial unit,
then recycles UCO back to the reactor for product yield adjustment.
Consequently, UCO gets a second chance to be hydrogenated
and converted into products, which usually means
product quality improvements.
Unfortunately, most independent laboratories cannot perform
a true recycle operation. Instead, a simulated recycle operation
is usually achieved by feeding a UCO sample from the
actual unit to the pilot-scale reactor and adjusting the single-pass
conversion across the pilot-scale reactor to match the reality
(FIG. 5, bottom).
Although this simulated recycle operation seems fair on the
surface, as every catalyst vendor is under the same assumption,
it can still somehow create biases between each catalyst loading
scheme. For example, the catalyst system with a better hydrogenation
function than the incumbent one (where the UCO sample
is taken from) will experience product quality disadvantages, and
vice versa for the loading scheme with a poorer hydrogenation
function (e.g., free aromatics saturation from the existing unit).
This can also distort the product qualities for different combined
feed ratios (CFRs) and overall conversions to certain
degrees. CFR is a ratio between the total reactor feed and fresh
Commercial operation
Recycle gas
Feed
Commercialscale
hydrocracking
reactor
Offgas
Separator
Recycle
oil
Laboratory experiment
(UCO sample from the refinery)
Simulated recycle gas (Pure H2)
Simulated recycle oil
LaboratoryFeed
scale
hydrocracking
reactor
Products (recycle gas, offgas/
naphtha/kerosene/diesel)
Simulated recycle oil
(fractionated in laboratory)
FIG. 5. A single-stage hydrocracker with recycle, commercial operation
vs. laboratory.
Baseprediction+1.0
Product
fractionator
Naphtha
Kerosene
Diesel
UCO
feed. A higher CFR means more UCO is recycled back to the
reactor inlet. In reality, the smoke point and cetane number usually
improve when the overall conversion increases (e.g., from
better aromatics
saturation/ring-opening). Nonetheless, the
laboratory experiment gave the opposite results for the cetane
number, as seen in FIG. 6. This discrepancy is from the fact that a
higher overall conversion operation (98%) has a higher percentage
of UCO, and this simulated recycle stream does not have a
second chance to be hydrogenated/ring-opened, but is already
converted to products during the first pass.
Moreover, the RCP between diesel and UCO is higher in
laboratory-scale experiments, with much sharper fractionation
(e.g., much less overlap between diesel and UCO). In
many commercial units, the separation between diesel and
UCO is poor by nature from lower internal reflux at the point
of separation. Moreover, the maximum heater duty can also
limit the actual RCP.
From FIG. 7, VGO and UCO from the refinery contain
around 10%-16% of diesel (defined as a portion with the boiling
point below 370°C), while UCO from the pilot plant only
has 4%-5% of diesel. This fact implies that the true conversion
of a laboratory-scale experiment is lower than that of a commercial-scale
operation. In a sense, hydrocracking catalysts are under
less conversion stress in a laboratory-scale experiment.
RCP, 370 °C
Baseprediction
250
300
350
400
450
500
550
600
Baselab
Baselab-0.6
10
20
30
40
50
wt%
FIG. 7. UCO and VGO distillation by simulated distillation analysis
(ASTM D2887), refinery vs. laboratory.
TABLE 4. An example of product cutpoints from ITB
Product
80% conversion (CFR=1.06)
Prediction
98% conversion (CFR=1.41)
Laboratory experiment
FIG. 6. Cetane number for different overall conversions (and CFRs),
prediction vs. laboratory.
38 JUNE 2022 | HydrocarbonProcessing.com
Light naphtha
Heavy naphtha
Kerosene
Diesel
UCO
C5
90-145
145-210
210-370
370-Endpoint
60
70
UCO (laboratory)
UCO (refinery)
VGO (refinery)
80
90
100
Boiling range, °C
-90
Cetane number
Temperature,°C
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