Foam control methods in delayed cokers

REFINING
Injection of high viscosity PDMS fluids for controlling the buildup of foam in
delayed cokers reduces costs while at the same time increasing capacity and
maintaining downstream quality control
LawrenceN Kremer BakerPetrolite
Timothy G Hueston Dow Corning
have been used for decades to control
foam in refinery delayed coking units.
such as asphaltenes and resins, Since PDMSacts as a defoamer (to knock
are a prevalent cause of foaming in down existing foam) and as an antifoam
delayed cokers. If foam is allowed to (to prevent the build up of new foam) in
build to a point where it is carried over the coke drum, both terms can be
with the vapour takeoff, it can cause applied to the silicone oil used in the
severeand costly downstream fouling or coking process. For simplicity, we will
refer to the PDMS as a defoamer.
even necessitatea shut down to unplug
Over the years, it has been observed
the vapour line. Therefore, it is critical
to keep this carryover from occurring. in the field that increasing PDMSmolecular weight (as indicated by higher visTo prevent carryover, refiners will limit
the level to which foam is allowed to cosities) results in lower dose rates;
build in the coke drum. Once this level Initially 12500 centistoke (cSt) fluid was
is reached, feed will be switched out of the viscosity of choice, but that has
the drum to prevent further level gradually evolved to where 60000cSt is
now more commonly used. In the past
increase.
There are typically two ways to reduce few years, there has been a movement
the overall foam height. The first is to toward even higher viscosities, such as
change the coker operation to give the 100 OOOcStand 600 OOOcSt,and even
foam head time to break before it reach- 1 million cSt is now being considered.
es the top of the drum by either lower- All other things being equal, such as
ing the feed rate or lowering the target adequate material handling capabilities,
capacity for the drum. Needless to say, higher viscosity PDMS appears to be
these operational adjustments can rep- more efficient for delayed coking.
A negative aspect of the use of siliresent a significant economic penalty.
The second method is to use a chemical cone defoamers in the coker is
defoamer or antifoam. to control the hydrotreater catalyst poisoning by silicon species. The coking process profoam head.
Given that this second method allows duces products high in unsaturation
greater capital utilisation, especially for and sulphur. For this reason, it is comexisting assets,and is usually quite inex- mon practice to hydrotreat coker naphtha and coker gasoil. It has been
pensive from a material standpoint,
chemical foam control is the most pop- observed in many refineries that the
hydrotreater catalyst is slowly poisoned
ular method of the two.
Standard foam control theory says by silicon containing species, causing
that a good foam control additive will be the activity to decline until the expenmore surfaceactive than the foam stabil- sive catalyst must be replaced. The
ising surfactants present in the system, source of this silicon contamination has
be chemically inert in th~ system,be dis- always been assumedto be the defoamer
persible, but not soluble, and of course, used in the coke drum.
There are methods to remove the silibe cost effective as compared to both
con-containing
species from
the
mechanical and chemical alternatives.
Some silicon-based materials, particu- hydrotreater feed stream. Perhaps the
larly polydimethylsiloxanes [Me3SiO- easiestof these is to run the hydrotreater
(MezSiO)x-SiMe3,
where x = 0 to >5000], feed through a guard bed containing a
are very effective foam control additives relatively inexpensive sacrificial catalyst
(or a wide range of chemical processes to adsorb/react the silicon speciesbefore
they reach the actual hydrotreater cata[Clarson S] and Semlyn] A, Siloxane Polymers;
lyst. The sacrificial catalyst can then be
Prentice Hall Polymer Science and Technolochanged out periodically. However, the
gy series, 1993].
Polydimethylsiloxane (PDMS) fluids labour and expense of this method usuV
apour
presence
generation
of
natural
and
the
surfactants,
65
PTQ SUMMER 2002
www.eptq.com
ally makes it less attractive than just
dealing with the fouling of the
hydro treater catalyst.
Since physical removal of the silicon
speciesfrom the coker products is usually not attractive, there have been many
attempts to eliminate or reduce the silicone defoamer usage in the coke drum.
While chemical alternatives to PDMS
have not been very successful,there are
methods to minimise the defoamer consumption, including' proper addition
methods and using the higher molecular weight PDMS as the active ingredient
of the coker defoamer.
Addition methods
Several addition strategies have been
tried for the defoamers to minimise the
use of PDMS, maintain good control of
foam, and produce the desired coke
product. Severalpoints of addition have
been used, including into the hydrocarbon feed to the drum, through nozzles
on the side of the drum, and through
nozzles at the top of the drum. Most
refineries have now settled on using
nozzles at the top of the drum.
Maintenance issues are an important
factor for injection nozzle location and
design. While side of the drum injection
nozzles can efficiently
spray the
defoamer on the surface of the foam
head, they tend frequently to become
plugged with coke. Since top of the
drum nozzles are least likely to become
plugged in this way, they are the most
common system. Sophisticated nozzles
have been tried, but they can easily be
fouled or plugged in the coking environment. Many refineries have settled on
injectors that are straight pipe cut 90° at
the end. Injection quills that extend into
the drum have been rejected since they
can be bent during the cutting process.
The purpose of the injection system
is to get the PDMS molecules to the
foam front as efficiently as possible, so
that no defoamer is wasted. Defoamer
that is entrained into the vapours leaving the drum will never reach the foam
front to affect the foam in the drum,
REFINING
but be carried over to the fractionator.
Likewise, defoamer that is on the walls
of the drum will not efficiently defoam.
Therefore, the injector is typically located as close to 1800 as possible from the
overhead vapour line to minimise
entrainment of defoamer in the overhead gasses.
The injector should also be at least
O.S-lm _(1.S-3ft) from the vertical walls
of the drum to prevent the defoamer
from running down the wall of the
drum. Figure 1 depicts two typical injection systems.
The defoamerspurchased by refineries
are typically dilutions of the PDMS in
hydrocarbon solvent. Even the 60 OOOcSt
silicone oil is so viscous that it can only
be pumped using expensive specialised
handling equipment. The defoamers
basedon diluted PDMShave much lower
viscosities «1000cSt.), so they can be
easily handled using standard equipment available in the refinery. Many
refiners have found it helpful to further
dilute the as-receiveddefoamer to obtain
optimum
performance.
. Dilution
can help in severalways.For
example, increased flow through the
injection quill increases the velocity,
which better sprays the defoamer onto
the foam front. Also, dilution of the
defoamer enables the molecules to better spread across the foam front. As an
added benefit, the coker naphtha and
coker gasoil commonly used as diluents
appear to provide some additional
knock down capability in some cokers.
While coker naphtha is sometimes used
as the diluent, coker diesel or coker
gasoil are often preferred because the
higher boiling range solvents are less
likely to flash off when injected into the
hot coke drum.
Two basic types of.defoamer dilution
systemsexist. Many of the early designs
employed a day tank system as illustrated on the left side of Figure 1. The
defoamer and diluent are added to the
day tank. The tank is mixed either with
an impeller or air sparge.A 10:1 diluent
to defoamer ratio is common. The diluted contents are then pumped to the
injection quill at the top of the drum by
means of a metering pump.
In some cases the metering pump
can be varied manually or electronically through the control system.
Other systems rely on a small pump
and a big pump in parallel. In any
case, there is usually some flexibility
in the pump system for the operators
to adjust the injection rate of the
diluted defoamer for variations in the
foaming conditions.
While this system is straightforward,
there are many disadvantages.Since the
make down system is typically a manual
batch process, it requires significant
In line
Day tank
Figure 1 Injection systemsused to meter defoamer into coke drums: automated inline
injectionof defoamergivesmoreflexibilitythan day tank system
operator involvement. Some systems
require the operator to open the day tank
and pour in defoamer, thus exposing the
operator to hydrocarbon fumes. It is
common for the dilution ratio to vary
from batch to batch.
Once the day tank has been filled, the
only way to change the amount of
defoamer injected into the coke drum is
to change the metering pump settings. If
the operator is distracted by other duties,
it is possible for the tank to run dry. For
these reasons, many refineries have
designed and installed automated inline
injection systems.
The inline system is shown at the
right of the figure. There is a continuous
slipstream of diluent, typically coker
gasoil. The defoamer product is metered
into this slipstream with a variable
injection pump. In this way the dilution
ratio can be easily changed. It is also
possible to easily increasethe amounfof
defoamer injected into the drum. This
automated system also frees the operators for other duties.
Refineries have optimised
the
defoamer injection using the systems
previously described. In order for
refineries to further reduce silicone carryover it is now necessary to optimise
the defoamer product as much as possible. The rest of this discussion describes
how the viscosity (or molecular weight)
of the silicone oil affects defoamer usage
and silicone carryover.
Theory
The theoretical explanation why bigger
silicone molecules can be better begins
with the recognition that the operating
temperatures of delayed cokers (800+oF)
is well above the initial thermal decomposition temperature of PDMS (about
600°F).Fortunately, the polymers do not
just {'blow apart" once this temperature
is reached, but slo"'ly degrade at a rate
dependent on temperature.
There are several different thermal
degradation mechanisms in the absence
of oxygen, primarily molecular bond
interchange and chain-end cyclisation
and cleavage.Work over the past decade
points to the chain ends as being very
important ~o siloxane polymer stability,
so it is thought that the chain end cyclisation and cleavagethermal degradation
mechanism will be dominant in this
application. In this mechanism, the
chain end curls back on the rest of the
polymer forming a small loop.
The bonds then rearrange to form a
cyclic group and a correspondingly
shorter linear chain. The cyclic formed
is usually a small group like D3 (hexamethylcyclotrisiloxane), D4 (octamethylcyclotetrasiloxane), or DS (decamethylcyclopentasiloxane), which will then
become volatile at coker temperatures.
The boiling points of these compounds
are listed in Table 1.
Several authors have observed that
there is not one single breakdown product
Breakdown products and boiling points
Breakdown species
Cyclic D3
Cyclic D4
Cyclic D5
Cyclic D6
Cyclic D7
Cyclic D8
Formula
[(CH3)2SiOb
[(CH3)2SiO]4
[(CH3)2SiO]5
[(CH3)2SiO]6
[(CH3)2SiO]7
[(CH3)2SiO]s
Table 1
66
PTQ SUMMER 2002
Bp @ 760mm ("C)
134
175
210
245
Bp @ 20mm (OC)
154
175
Bp @ 760mm rF)
273
347
410
473
Bp @ 20mm (OF)
309
347
REFINING
but rather a range of several small
molecules with 03, 04, and OS cyclic
oligomers as the most common breakdown products [see references at end].
The new chain end is still subject to further cyclisation and cleaving, so this
will continue until the remaining linear
species is small enough to be volatile
(containing seven or fewer silicon
atoms at coker temperatures), or until
the remaining material is formed into
the coke.
This creates an "unzippingll of the
polymer chain. Since the reaction will
occur on both ends of the polymer
chain, for a given weight of material,
fewer polymer chain ends means an
overall slower rate of degradation. This
means that longer initial polymer
chains (higher viscosities for rOMS)
should lead to greater material life in the
coke drum, which will translate into
lower use levels.
Also, recall that for a defoamer to
work, it must be insoluble (but dispersible) in the system. The smaller the
chain size, the more soluble rOMS
becomes in oil. Therefore, once the
polymer falls below some critical minimum chain size (this size will vary with
the specific composition of the oil), it
will no longer be an effective defoamer.
Again, if chain end degradation mechanism holds, longer initial chains are
beneficial so that this critical minimum
size is reached more slowly.
Lab experimental results
To support the theoretical degradation
hypothesis, TGA and GPC were used to
compare the effect of thermal' degradation on two commonly used PDMS viscosities (60000 and 600000cSt) when
exposed to a temperature of 400°C
(750°F) for two hours at atmospheric
pressure under a nitrogen blanket. This
is reasonably close to coker conditions
except for the lack of oil contact (the
coker feed oil would be likely to accelerate the degradation due to some minor
catalytic effects, but should not change
the mechanism).
The PDMS monomer unit is MezSiO,
which has a molecular weight of 74.
Therefore, the calculated starting average chain lengths are about 1100 for the
600000cSt (60M) and about 2100 for
Figure 2 Monthly antifoam usage changing to high viscosity antifoam (600 OOOcSt)
the 600M (end group effects on this calculation are minimal). Fora given mass
of sample, that means that there are
almost twice as many chains, and chain
ends, in the lower viscosity material.
As previously discussed, chain end
cyclisation and cleavageis thought to be
the dominant polymer thermal degradation mechanism in this case.We would
therefore expect the 60M cSt material to
degrade at almost twice the rate of the
600M material, which is exactly what
we see in the weight loss numbers (the
primary reversion products are volatile
at these temperatures). The total weight
losseswere 33.7 per cent for the 60M
polymer and 18.3 per cent for the 600M
polymer during the test (Table 2). This
strongly supports the chain end degradation theory.
In summary, the weight loss and
decreased Mn after heat ageing shows
that degradation occurs in both materials, but it occurs more slowly in the
higher viscosity material. Also, it is
important to note that the molecular
weight of the 600 OOOcStis still higher
after the thermal exposure than the
starting 60000cSt values. This is a good
indication that the higher initial molecular weight materials will stay above the
critical minimum size for foam control
longer, as well as offering lower overall
weight loss and bottom line greater effi-
Table 2
68
PTQ SUMMER 2002
ciency. The main question, of course, is
do these theoretical and laboratory
results translate into the field?
Field results
In working with seveial refineries over
the past two decades to control coker
antifoam usage, Baker Petrolite has
switched to higher viscosity silicone oils
in the defoamer formulations. Two
examples of this field experience will be
given to illustrate the points raised by
the thermal stability tests previously
described.
The first question is: Does use of the
higher molecular weight silicone result
in less silicone u~agein the coke drum?
The graph in Figure 2 shows the actual
monthly usage of silicone defoamer on
the coker for a North American refinery
producing fuel coke.
The silicone usage is expressedas the
averagenumber of pounds of silicon (Si)
added per 1000 barrels of feed to the
coker. Each point is a monthly average
calculated from actual defoamer inventory reduction during the month and
the coker production for the month.
The refinery switched from defoamer
basedon 60 OOOcSt
silicone to one based
on 600000cSt between February and
March of the first year.
The silicone usage dropped dramatically from an average of about 1.4
pounds Si1000bbl to about 0.4 pounds
Sij1000/bbl. This is a reduction of 70 per
cent in the amount of silicone used to
control the foam. Such a reduction has
been seen at several other refineries that
have made such a switch.
The second question is, Does use of
the high molecular weight silicone fluids result in reduced silicon contamination of the coker products? To answer
that question, look at one North American refinery that recently switched from
REFINING
100000cSt based defoamer to a
600 OOOcStbased product. The level of
silicon was measured in the coker products using direct injection ion coupled
plasma.
From previous work at other refineries, Baker Petrolite had determined that
the level of silicon in the products could
vary by as much as two orders of magnitude, depending on time in the coking
cycle that the samples are collected. For
this reason, samples were taken just
before the drum switch on the last six
filling cycles using the 100000cSt based
antifoam and the first six filling cycles
using the 600 OOOcStbased antifoam.
Table 3 is a summary of the trial results.
It should be emphasised that the silicon levels determined during the time
in the cycle with peak silicone injection
are expected to be an order of magnitude greater than the averagevalue over
the entire cycle. The highest levels of silicon were seenin the heavy coker naphtha (HCN) but substantial silicon was
also found in the light coker gasoil
(LCGO) and the heavy coker gasoil
(HCGO). This can be explained by the
fact that polydimethylsiloxane has multiple breakdown products, primarily the
small cyclics resulting from the breakdown as previously described, and each
has a different boiling point. The boiling
points of several of these breakdown
products measured by Patnode are listed
in Table 1.
Since these boiling points fall
throughout the range of typical coker
products, any attempt to adjust the endpoints of the coker products in order to
produce a silicon-free product cut would
not be practical.
The second conclusion that jumps
out from the data is that the use of higher viscosity silicones in the coke drum
can result in dramatic reduction in the
silicon contamination of the coker products. Samples taken during other times
in the coking cycle were orders of magnitude smaller but also showed similar
reductions in the silicon contamination
of the coker products.
The switch from 100000cSt to
600000cSt silicone fluid was made without any other mechanical or operational
changes to the coking process.The time
at which the defoamer pumps were started and stopped during the cycle
remained the same through the 12 test
drums. Thus the silicon reduction can be
attributed completely to the change in
molecular weight of the coker defoamer.
Table 3
degradation studies show that higher
viscosity (higher molecular weight)
PDMS decomposes more slowly on a
weight basis compared to lower viscosity
silicone fluids. They are thus expected to
retain their foam control characteristics
longer than the lower viscosity material,
which would translate into a lower dose
rate to maintain foam control.
Field experience confirms that less
high viscosity silicone is required to
control the foam in the coke drum and
that this results in lower silicon contamination of the coker products and lower
overall cost in use, when including the
replacement cost of the catalyst.
LawrenceN Kremerhasworkedwith
BakerPetrolitefor 79 years,and his
currrent researchinterestsinclude emulsion
technolog)l;defoamersand other aspects
of surfacechemistry.He earneda BSin
chemistryfrom the Universityof Texasat
Austin and a PhD in physical chemistry
from the Universityof Utah, followedby a
post doctorate at YaleUniversitjt:
Timothy G Hueston is senioroil and gas
applicationsspecialistfor Dow Corning
Corporation,having workedin various
functions for the past 18 years.He is
currently focusedon the use of
silicon-basedmaterialsin oil and gas
applications.He holds a BSin chemical
engineeringfrom the Massachusetts
Institute of Technology.
R~ferences
Patnode Wand Wilcock D Fi J Am Chem Soc,
68, pp358-363, 1946.
GrassieN and MacFarlane I Gi EuropeanPolymerJournal, 14, pp875-884, 1978.
Kremer L N, Silicone contamination of coker
products; 5th International Conference on
Refinery Processing,AIChE Spring National
Meeting, 11-14 March 2002.
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69
PTQ SUMMER 2002