Hydrocarbon Processing - September 2021 - 68

Process Optimization
their own caustic treatment sections, and the rest send their
products to the Merox units. In the LPG pool, if a unit has an
LPG product with high mercaptan or H2
S content, then its
product is routed to the Merox units to treat the product further.
This problem may arise from bad caustic quality or high
caustic spent rate of these units.
While the product is routed to the Merox units, necessary actions
are taken in those units to achieve required product quality.
An alternative solution is routing the product to off-spec tanks
and then processing in Merox units with desired flowrates. This
solution can be applied if the Merox unit feedrate is near the
design limit, and it is advantageous because the extra flowrate,
rather than the entire stream, can be processed in a Merox unit.
In Merox units, high sulfur in the LPG product should be
analyzed from different perspectives. Caustic quality, operating
parameters and feed properties affect the efficiency of Merox
units. First, the sulfur type should be examined as a key for how
to solve the problem. Mercaptans, disulfides, H2
S and others
that may come from crude oil or cracking units (e.g., thiosulfate)
are common examples. Ethyl and methyl mercaptans can
be treated more easily in Merox units. When the chain becomes
longer, treatment becomes more difficult. If the cause of high
sulfur is dimethyl sulfide, then the caustic circulation rate should
be checked. A high circulation rate may increase " re-entry sulfur, "
which causes high sulfur in the product. Naphtha or kerosene
wash systems may be implemented to treat re-entry sulfur.
Another common issue is caustic quality. Caustic quality and
the spent rate should be checked, and if these parameters are over
the limits, then the caustic should be replenished. Prewash and
regenerated caustic inventory have different percentage spent
limits. Among the operating parameters, feed temperature and
caustic temperature affect extraction efficiency heavily, while air
injection rate and catalyst quality and amount affect the oxidation
process. Air injection may cause corrosion problems in the
lines and equipment, so it should be observed closely.
In a large LPG pool with many sources, even if the sulfur
content of some LPG products increases, it does not necessarily
put the pool in jeopardy. This effect may be diluted with the
help of the entire LPG pool. Even if an LPG tank is out of specification
due to a high sulfur parameter, blending with other
LPG tanks may be enough to overcome the issue, depending
on the sulfur level.
On the other hand, if high-sulfur-containing LPG is routed
to the SHU, then the catalyst may be affected badly. Its efficiency
may decrease, and recovery of the catalyst may take some time,
depending on the sulfur level it is exposed to and the capability
of the catalyst against sulfur. This may lead to another problem:
butadiene. To avoid this problem, if an LPG stream that is being
treated by the SHU experiences a high-sulfur situation, then it is
removed from the SHU, and the unit may need to be taken into
circulation in case of low throughput.
Butadiene. The FCCU and DCU are the sources of butadiene
in the İzmit refinery LPG pool. If all the units are operating
near design values, then this butadiene content is diluted in the
pool and remains within limits. If the pool becomes too narrow
due to upsets or planned shutdowns, then the butadiene content
may go off-spec.
Normally, the SHU combined with a C3
/C4
splitter is used to
decrease butadiene content by changing dienes into butenes. If a
68 SEPTEMBER 2021 | HydrocarbonProcessing.com
performance issue is experienced in the SHU or if the catalyst is
in an end-of-run state, then removing some of the C4
LPG pool may be necessary. By removing C4
decreased. This C4
s from the
stream is taken from the splitter C4
and used in the gasoline pool. The amount of C4
from the system causes the C3
, butadiene is also
outlet
is limited by
ratio to inthe
control valve range, as well as line and tank metallurgy, as
removing so much C4
crease, and vapor pressure approaches design limits. In addition,
it is not possible to add C4
to dry vapor pressure equivalent (DVPE) specifications.
If a market option is available, then selling these C4
s to the gasoline pool in summer due
s may also
be evaluated. A second option is to route the LPG product with
high butadiene content to the evaporator. In this way, high-butadiene-containing
LPG is evaporated and used in the fuel gas
system. This method is useful when natural gas prices are high,
as the fuel gas system is normally supported by natural gas.
Another option is to increase the amount of LPG that does not
contain butadiene in the pool as a way of diluting butadiene. For
this purpose, the offgas compressor may be used and the net gas
of the isomerization unit may be routed to the Platformer unit to
gain LPG. A final option is to adjust the DCU and FCCU parameters
to decrease their LPG product or their butadiene content.
The decreasing riser temperature of the FCCU also decreases the
amount of LPG product. However, low riser temperature has the
disadvantage of octane loss. Also, decreasing wash oil (naphtha)
circulation in the absorbers of the DCU gas plant decreases butadiene
content by leaving some C4
s to the fuel gas stream.
Purity. Some units have high purity in their LPG products
like the FCCU, DCU or new HCU, which may go as high as
100%, while some old units like the CDU or the diesel desulfurization
unit (MQD)/Platformer have a purity of 95% or lower.
If high-purity LPG is lost due to an upset or planned shutdown,
then the pool purity may be in jeopardy.
In such a case, the first option is to check all the debutanizer
columns and make necessary parameter changes to increase purity.
This may take some time, as it may be necessary to perform
purity tests to determine the problematic debutanizer column,
if the temperature and pressure profiles do not provide enough
information to troubleshoot. In addition, it is necessary to determine
if the cause of low purity is C2
- or C5+. Once determined,
the debutanizer top temperature, pressure or reboiler
duty are adjusted accordingly.
A second option is to activate the deethanizer column. This
column increases the purity by removing the C2
- content of the
product. The İzmit refinery has one deethanizer column, which
has a capacity sufficient for only the LPG products of the old
HCU and the MQD.
A third option is to open the pressure control valves in the
Merox units' LPG feed drums to shift some of the C2
- content
to the fuel gas system. This action has a limited effect and has
the disadvantage of increasing H2
tem, since LPG Merox feed may contain some amount of H2
A final option is to increase the high-purity LPG and/or
S content in the fuel gas sysS.
to
decrease the low-purity LPG in the system. Decreasing the
low-purity LPG may be achieved by routing this stream to the
evaporator; the pros and cons of this action was previously discussed.
The net gas stream may be routed to the fuel gas system
from the Platformer debutanizer column, which produces relatively
low-purity LPG. The deethanizer and debutanizer modes
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