How to use defoamer in delayed coking?
Why should use defoamer for delayed coking?
Delayed
coking is a process in which heavy cracked oil is heated to undergo deep
cracking and condensation reactions and converted into gas, gasoline, diesel,
wax oil and coke.
During the
reaction process, the coke tower can be divided into three main areas along its
height:
u The
lower part is a coke layer;
u The
middle part is a high-viscosity colloidal asphaltene called a foam layer;
u
The
upper part is a hydrocarbon layer.
The foam layer
will maintain a certain height under a certain temperature and pressure.
The main factors affecting the height of the foam layer are:
the amount of
processing, the nature of the raw materials, the exit temperature of the
heating furnace and the circulation ratio. Trying to reduce the height of the
foam layer is an important measure to increase the throughput. In order to
achieve the purpose of reducing the foam layer, generally adopt methods such as
setting a material level meter, using a foaming agent, and selecting a suitable
furnace outlet temperature. Among them, the use of antifoam injection is
actually a feasible and effective measure. The coke towers in foreign countries
are about 3 meters in height.
In China, in order
to prevent punching towers, they often reach more than 5 meters. The volume
utilization rate of coke towers in foreign countries can generally reach 86℅, and China's only
55℅ ~ 70℅. If the defoamer
technology is adopted, and it is matched with the Chinese character, the coke
tower height is reduced and the coke tower volume utilization rate is
increased, the processing capacity of the delayed coking unit can be increased
by more than 16%.
Application of defoamer for delayed coking
At present,
silicone oil is generally used as a defoaming agent in delayed coking devices,
and silicone oil with high molecular weight and high viscosity has a better
effect. It is also useful as a defoamer made from a combination of propylene
oxide and silicone oil. No matter which silicone oil is used, it must be
diluted with kerosene or aromatic components in advance.
The defoamer
is usually added from the top of the coke tower and is often injected with
light diesel oil. The main purpose of carrying oil is to keep the injection
pipeline clean and help the silicone oil to distribute on the foam. The typical
amount of silicone oil is 2-20 micrograms / gram. There are a series of level
gauges in the coke towers of foreign installations. When the foam layer reaches
the lowest level, it starts to add defoamer and even switches the coke tower.
Due to the
high temperature in the coke tower, a small amount of silicone oil will
decompose into a series of volatile compounds, which will enter the coking
gasoline, kerosene and diesel. Silicon has a toxic effect on hydrorefining
catalysts. Therefore, the use of silica gel will affect the reprocessing of
these distillates. At present, the development direction of delayed coking
defoamers is to develop low-silicon or even silicon-free defoamers. By reducing
the amount of silicone oil added to the coke tower, the silicon carried by
distillate oil can be reduced.
For example,
the processing capacity of the delayed coking unit of Jingmen Petrochemical
Plant is 600,000 tons / year. Due to the large steam speed in the coke tower
and the fractionation tower, the serious carry of coke powder causes the heat
exchanger and air cooler to be blocked, and the excessively low coke powder in
the fractionation tower seriously affects the safe and stable production and
increases the maintenance cost; The catalyst in the hydrogen plant is poisoned
and the life is shortened. Using XP-3 type defoamer (injection amount 55 ～ 80μg / g) produced by
this plant, it can meet the process requirements, and the outlet temperature is
increased from 495 ℃to 500 ℃, which promotes
the cracking reaction and makes the light oil recover. The rate has improved
and it is more economical.
Another
example is the 1.1 million-ton / year delayed coking unit of Zhenhai Refining
and Chemical Co., Ltd., for a period of time, the coke powder was seriously
carried, which caused serious coke accumulation at the bottom of the
fractionation tower. Since the use of the CDF-10 defoamer, the coke powder
carried by coking oil and gas has been greatly reduced, reducing the
possibility of clogging of the coking radiation feed pump, the coking times of
the fractionation tower are significantly reduced, the coking unit is in a
benign circulation state, and the hydrogenation catalyst There was no
recurrence of silicon poisoning.
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