Visbreaker performance improvement by application of chemical

46th International Conference on Petroleum Processing, June 7, 2013, Bratislava, Slovak Republic
Improvement of the LUKOIL Neftohim Burgas Visbreaker unit performance by
optimisation of process conditions and application of chemical additive treatment
programme
A.Nedelchev, D. Stratiev, G. Stoilov, R. Dinkov, K. Lepidis, R. Sharpe, C. A. Russell, N.
Petkova, P. Petkov
LUKOIL Neftohim Burgas JSC, Bulgaria, Burgas 8104,
e-mail: [email protected]
e-mail: Stratiev. Dicho @neftochim.bg
1. Abstract
Nalco’s visbreaker optimisation program (Conversion Plus™ II) was implemented on the
visbreaker unit at Lukoil Neftochim Burgas refinery, Bulgaria in 2011. The treatment package
was specifically designed by coupling unique laboratory processability characteristics of
visbreaker feed with actual unit operational observations. Laboratory processing of visbreaker
feeds revealed unique profiles of intrinsic stability degradation and surface fouling with
increasing severity. The technique also produced evidence for the effectiveness of chemical
additive treatment to decrease surface coke formation, and therefore, increasing removal
efficiency. Transferal of intrinsic stability measurement to the real unit resulted in a dramatic
improvement to process monitoring, allowing for a more sensitive measure of severity and
therefore a relatively more rapid reaction to operational and feed changes. As a result, the
bottleneck for unit throughput was then identified to be the feed inlet zone to the main
fractionator, more specifically, the poor design of main distillation trays. The problem was
somewhat alleviated by adding a refinery stream to improve the overall asphaltene solvency
power of the feed, and by reducing the feed inlet temperature from 380 to 360 °C, with the
benefits of the former demonstrated under laboratory conditions. The overall efficiency of the
unit increased dramatically as a result of Nalco’s treatment program and the subsequent
engineering alterations, with average annual conversion increasing from 14.8 to 16.2 %
(products boiling below 360 °C), and unit turnaround time decreasing by nine days. The
reduction in cleaning time for both reactor internals and heat exchangers are a direct parallel
to laboratory observations when applying the feed specific chemical additive treatment. It is
anticipated that a change of distillation tray design from valve to sieve type will further
increase the performance of the visbreaker at Lukoil Neftochim Burgas refinery, Bulgaria.
Key words: visbreaking, residue stability, heavy oil conversion, fouling
2. Introduction
The trend of worsening quality of crude oil (increasing sulphur, density, and residue content)
supplied around the world [1], along with a decline in fuel oil demand [2], makes refiners
look for feasible ways to increase residual oil conversion. Visbreaking, practiced since the
1930s, [3] is a long-time workhorse with regard to bottom-of-the-barrel upgradation.
Approximately 33% of the total petroleum residue processing capacity is met via visbreaking
[4]. Visbreaking is a mild liquid-phase thermal cracking process, which reduces the viscosity
and pour point of the residue so that, with little or no addition of lighter, valuable cutter stock,
it can meet fuel oil quality [5]. Worldwide, about 200 visbreaking units [6] are under
operation, and Europe alone accounts for about 55% of the total visbreaking capacity [4]. The
main objective of the visbreaking unit (VBU) operation is to achieve maximum conversion
while retaining stability of the visbroken residue (VBR) at a level corresponding to the
requirements in the fuel oil product specification, which in the case of the LNB VBU is 0.1
wt. %. Increasing conversion of the VBU destabilizes the residual oil. This destabilization
may cause fouling phenomena, due to precipitation of asphaltenes and conversion of some of
the asphaltenes to generate coke particles at cracking temperatures above 400°C. These
phenomena can also limit unit run lengths. Both the lower conversion and the shorter unit run
length negatively affect VBU performance. However, the application of improved monitoring
techniques and advanced chemical additive technologies for prevention of sludge and coke
deposition may allow for an increase in unit conversion and run length, and therefore
improving the VBU performance [9,10,11]. The aim of this work is to discuss the results
obtained at the Lukoil Neftochim Burgas, Bulgaria (LNB) VBU during application of the
Nalco Conversion Plus II Program.
3. Experimental
3.1. Samples and ASTM Test Procedures
The main feedstock for the LNB VBU is a vacuum residue (VR) distilled from Ural crude oil.
The LNB VBU has also processed VR from Kirkuk, Basra, Caspian Pipeline Consortium
(CPC) and Varandey crude oils. Properties of the five VR used in this study are summarized
in Table 1, and were obtained by TBP distillation of parent crude oils using the EURO DIST
SYSTEM TBP column according to АSTM D-2892 (IBP – 360 °C). The vacuum analysis
(above 360 °C) according to АSTM-D 5236 was done using a EURO DIST SYSTEM
POTSTILL apparatus. The TBP distillation was performed in the EURO DIST SYSTEM
TBP column at pressure drops ranging from 760 to 2 mm Hg and in the EURO DIST
SYSTEM POTSTILL column from 1 to 0.2 mm Hg.
The sediments level in the VBR and finished fuel oil was measured in accordance with the
ASTM D4870 (Hot filtration test). The VBR viscosity has been measured in Engler specific
viscosity in accordance with the ASTM D 1665.
3.2. Residual Stability Analyser (RSA)
Small aliquots of residue samples were made up into two toluene dilutions (75 % vol. and 25
% vol.) before being titrated with i-octane. The optically detected flocculation point (decrease
in transmittance) was then used to calculate stability values for the sample. In essence, the
method is a modification to ASTM D 7157-05 [13], as only two dilutions are required for
determination of stability values, which are calculated as:
S – Intrinsic stability (S-value) of the oil. This parameter is an indication of the stability or
available solvency power of oil with respect to the precipitation of asphaltenes;
Sa – The peptisability or ability of the asphaltenes to remain in colloidal dispersion. Sa is
related to the solubility of the asphaltenes, the length and number of the aromatic chains;
So – The peptising power of oil is the “aromatic” equivalent of the oil; it is a measure of the
solvency power of the oil with respect to asphaltene solubility.
3.3. Laboratory Refinery Process Simulation Unit
The pyrolysis apparatus used in this study is a modified high temperature – high pressure
batch autoclave reactor. The main features include constant pressure operation, distillation of
cracked products, additive injection into reactor during pyrolysis, and quantification of
surface deposit via steel micro-reactor inserts. Full details of the equipment and procedure
may be found in Russell et al. (2010) [12].
3.5. Visbreaker Unit Description and Chemical Additive Application Procedure
The LNB visbreaker unit is a soaker visbreaker with a capacity of 1.5 MMTPA. It consists of
three furnaces and three soakers. Two of the furnaces and soakers are in operation and one
furnace and one soaker stands by (figure 1). Implementation of the Nalco Conversion Plus II
program was initiated after a planned shutdown of the LNB VBU in February 2011. The basis
of the program was grounded on chemical treatment with two agents. The first one was
injected into the suction side of the feeding pump. The main purpose of this chemical additive
was both metal surface passivation and mitigation of the coke deposition in the coils of the
furnaces and associated soaker internals. The start-up operations of the unit were
accompanied with passivation procedure which consisted of injecting the additive chemical
for about 72 hours in order to suppress the active metal centres, as during start up, the coking
rates may be relatively high due to clean metal surface of the tubes catalysing coking. After
completion of the passivation procedure the concentration of the chemical agent was reduced
in order to continue its anticoke-antifoulant purpose via constant injection during normal
operation.
The purpose of the second chemical was to prevent agglomeration of the asphaltenes and their
consequent deposition on process surfaces of the stripping section of the main fractionator and
downstream heat exchangers. The chemical additive was injected in two points: the first one
was in the discharged side of the pump in the quenching loop and the second one was to the
outlet quenching collector in order to prevent deposition in the striping section of the main
fractionator. Figure 1 presents principle process flow diagram of the VBU with indicated
injection points of both chemicals.
Implementation of Conversion Plus II Program was monitored using the Residual Stability
Analyser described above. An S-Value of 1.43 was chosen as the baseline or reference point
for unit severity operation.
4. Results and Discussion
4.1. Laboratory Characterisation of Visbreaker Feed
Nalco’s standard test procedure for the characterisation of visbreaker feed, or any heavy oil
feed for high temperature refinery processing, involves pyrolysis experiments of increasing
time at temperature, typically 410 °C. The resulting pyrolysis residue series yields detailed
information on the compositional changes that occur with increasing processing severity. One
of the key parameters important to the optimisation and monitoring of the visbreaking process
is intrinsic stability. Measured by Nalco’s RSA, there is a systematic decrease in S-Value with
increasing pyrolysis severity, or conversion. Such a degradation in stability directly relates to
the cracking reaction progress of the residue, as coke precursor moieties become less and less
soluble, eventually overwhelming the solvency power of the surrounding maltene like
medium. The example displayed in figure 2 represents the characterisation of a Urals Blend
vacuum residue visbreaker feed, which had an initial stability of 2.82, a value that is
significantly lower than the stability of the Urals Blend processed on the LNB VBU (c.f. table
1). The characteristic degradation curve is similar for most feeds processed in the laboratory,
with a relatively rapid decrease in S-value over the initial severities, followed by relatively
less change as the stability nears the theoretical threshold of 1. It is also the experience of the
laboratory that asphaltene like material insolubility (1-Sa) increases with conversion
following a linear trend. The solvency strength with respect to asphaltene like material of the
surrounding maltene medium (So) does not change in a predictable way, reflecting the
complex cracking reactions that progress with severity, producing high solvency power
aromatic species, and low solvency power aliphatic hydrocarbon species. A detailed
examination of these trends is beyond the scope of the present study, although descriptions are
available elsewhere [12].
It has already been mentioned in section 1 that the process severity of the LNB VBU was
previously controlled by monitoring toluene insoluble sediment in the VBR, with a limitation
of 0.1 wt. % according to fuel oil specifications. The bulk oil sediment level was measured for
the pyrolysis severity series, and the profile is displayed in figure 3. Such measurements tend
to have a relatively low reproducibility, however, a systematic increase with severity is
observed. Interestingly, at the limiting value of 0.1 wt. %, the conversion is relatively low,
perhaps a consequence of the high surface area to volume ratio in the batch pyrolysis reactor.
Nevertheless, the results are valid, and a similar trend is observed from the LNB VBU (figure
8), which is discussed in section 4.2.
One of the more novel aspects of the laboratory pyrolysis procedure concerns the accurate
measurement of surface deposit, or fouling, generated from pyrolysis, using micro-fine mesh
reactor inserts. Essentially, the inserts are sequentially washed in solvents to reveal a deposit
stratigraphy. Here, the toluene insoluble portion of the deposit is considered. However, it is
important to mention that the other layers soluble in solvents less severe than toluene may
reveal important information, particularly for antifoulant additive performance assessment.
The surface fouling profile for the Urals Blend feed is displayed in figure 4. There is a
systematic increase in surface deposit with increasing pyrolysis severity. Furthermore, the
data reveals relatively more reproducibility and sensitivity than the bulk toluene insoluble
sediment measurement, which is important for discerning small but important differences due
to differing feed blends and additive treatment.
The characteristics of the profile displayed in figure 4 are common to all feeds processed on
the pyrolysis rig. Detailed analysis of the curve involves the identification and quantification
of the coke induction period, and subsequent rate of deposit formation. Furthermore, the
assessment of antifoulant additive performance suggested that surface deposit may be reduced
by as much as 40 %.
It is evident from figures 2 – 4 that the profiles generated from a pyrolysis series are unique
for each feed, which therefore has the potential to reveal certain relationships, and groups of
feed types with similar processability, and therefore perhaps, processing behaviour
predictability. One such scenario is the examination of different blend ratios. The following
example is taken from a VBU that processes different blends of VR and deasphalted oil
(DAO) under three modes: 100 % VR, 100 % DAO, and 65 % DAO 35 % VR. The stability
profile for each of these feed blends is displayed in figure 5. The VB feed composed of
entirely DAO reveals the most stable characteristics over the pyrolysis severities examined,
with a high initial stability of 3.93. The VB feed composed of entirely VR has the lowest
stability profile, with a starting S-Value of 3.08, and final value of 1.07. The data
demonstrates a clear relationship with blend composition, with aspects of the characteristic
curves and initial stability permitting the prediction of stability profiles for any VR and DAO
blend composition.
A similar set of data is apparent for the surface fouling profiles (figure 6). Here, the 100 %
VR reveals the highest fouling tendency, followed by 65 % DAO and 100 % DAO. The trend
is directly comparable to stability differences, suggesting that for this particular feed blend
system, stability may be directly related to fouling potential. Once again, the curves may be
described by a suitable equation providing rate parameters that coupled with induction period
may adequately describe the fouling potential.
The initial feed stability (S-Value) and degradation rate provides parameters for a stability
index. Similarly, induction period and fouling rate provides parameters for a surface fouling
index. Figure 7 explores the relationship between these parameters for the feed blends
discussed above, and for many other feeds in Nalco’s heavy residue database. As is evident
from the data above, the 100 % DAO feed has a relatively high stability index and a relatively
low surface fouling index. The trend through 65 % DAO to 100 % VR reveals a gradual
reduction in stability index with an increase in surface fouling index. The relationship reveals
excellent linear correlation for these feed blends.
The Urals Blend is also plotted on figure 7. Compared to the VR and DAO blends, the
stability profile is relatively worse than the VR, and the surface fouling appears very much
higher in actual quantity. Therefore, on figure 7, the stability index is lower and the surface
fouling index higher. The relationship of overall stability with fouling appears to hold true for
these feeds. There are nine other feeds that also hold the same systematic relationship between
stability and surface fouling, and these are labelled Group 1. However, four samples do not
appear to fit with this relationship, and these are labelled as Group 2. These samples have a
much higher surface fouling index, or potential, for a given stability index. Therefore,
processing of these feed types is relatively more dangerous to operating efficiency as they
have a high surface fouling affinity for a given stability or conversion. The testing apparatus
described here offers a means to identify these feeds prior to processing, and potentially may
permit operational and antifoulant additive adjustments to be implemented before a reduction
in processing efficiency occurs.
For the present study, the initial laboratory data demonstrated the sensitivity of the RSA
technique, the potential fouling problems associated with processing a VBU feed containing
Urals residue, and how the impact of blending can be accurately monitored. Furthermore,
laboratory investigations demonstrated the compatibility and identity of an appropriate
antifoulant additive for Urals Blends.
4.2 Visbreaker Unit Operation and Performance
As discussed previously, the conversion of the LNB VBU was governed by sediment content
of the VBR, which was deemed to be 0.1 wt.%. Figure 8 displays toluene insoluble sediment
data from the LNB VBU over a range of severities, or conversion [7]. A similar trend is
observed as in figure 3 for the laboratory data. Furthermore, the actual volumes are not that
different considering the process volumes are many orders of magnitude apart. Therefore, the
data provides credibility to the laboratory pyrolysis characterisation process.
It has been demonstrated in the laboratory that during the thermal cracking of vacuum residue,
stability decreases. Figure 9 presents data of stability of the LNB VBU feedstock (VR) and
the VBR. It is evident from these data that the VB feedstock stability varied between 2.4 and
3.7 while that of the VBR varied between 1.4 and 1.6. It was established earlier that if a
longer unit run length is desired the stability of VBR should not drop below 1.43. For that
reason during application of the Nalco Conversion Plus II Program the conversion of the LNB
VBU was controlled by this VBR stability value. When the stability value of the VBR was
found to be below 1.43, the severity of the LNB VBU was reduced. Factors that may
contribute to a reduction in stability may include higher reaction temperatures, and lower
liquid hourly space velocity (LHSV) at constant feed quality. When the stability value of the
VBR was greater than 1.43, the severity of the LNB VBU was increased.
Before application of the Nalco Conversion Plus II Program, conversion in the LNB VBU has
been controlled by sediments level in the VBR. If the sediment level was below 0.1 %, the
severity of the LNB VBU was increased. In cases when sediment levels were above 0.1 %,
the severity of the LNB VBU was reduced. Data in figure 9 suggests that there may have been
ample opportunity to increase severity, as the sediment level did not approach 0.1 %.
However, the S-Value data shows otherwise, and clearly demonstrates the heightened
monitoring sensitivity.
The conversion term relating to the LNB VBU considers conversion of the VR (VBU
feedstock) to products boiling below 360 ºC. The conversion has been calculated by following
expression:
Conversion % 
VR  VBR360
VR.100
Where, VR = Vacuum Residue feeding the LNB VBU in ton / hour; VBR360 = Visbroken
Residue product boiling above 360°C in ton / hour
In order to determine the VBR fraction boiling above 360 ºC and estimate conversion,
distillation data of the VBR product are required. However distillation of the LNB VBR is not
performed on a regular basis. The VBR viscosity, which is measured twice per day, has been
shown to be dependent on VBR distillation characteristics [13]. Therefore, we investigated
the dependence of VBR viscosity on content of fraction boiling below 360 ºC by performing
distillation analysis according to ASTM D-1160 of several samples of LNB VBR having
different viscosities. Figure 10 presents the relationship between the LNB VBR viscosity and
the content of the fraction boiling below 360 ºC. This relationship has been established during
processing VR from Ural crude oil in the LNB VBU. Based on data of the VBR flow rate in
ton / hour, VBR viscosity and the relation shown on Figure 10, and data of density of the
fractions boiling below (d420= 0.87 [14]) and above 360ºC (d420= 1.015 [14]), the VBR
product boiling above 360ºC in ton/hour can be determined.
Figure 11 presents data of conversion for 12 months of operation of the LNB VBU before
applying the Nalco Conversion Plus II Program in 2010. It is evident from the data that the
average conversion for 2010 was 14.8 %. It also can be seen from the data on Figure 11 that
total sediment level varied between 0.01 and 0.23 %. The primary operating philosophy was
to maintain sediment levels as close as possible to 0.1 %. The variation of sediment level was
due to VBU severity change, or due to feed quality change. In 2010 the main VBU feedstock
was Urals VR. For short periods of time blends of Urals VR with VR from the crudes CPC,
and Kirkuk were processed. The highest value of sediments (HFT = 0.23%) was obtained
during processing of a blend of 2/3 Urals VR and 1/3 Kirkuk VR. On occasion, sediment
increased even during processing of 100% Urals VR at constant severity. One possible
explanation for this phenomenon is linked to the process scheme of the LNB refinery. After
transportation of the finished fuel oil product from the refinery to the marine terminal, the
pipeline used for fuel oil transportation is flushed with crude oil. During the flush some
quantity of the fuel oil is mixed with crude oil, which is later processed in the refinery. The
processing of such a blend of crude oil with fuel oil may create problems in the crude
distillation unit (CDU) because the fuel oil may stabilise water-oil emulsions and therefore
deteriorate desalter operation. Moreover, the heavy content of the original fuel oil is processed
in the VBU, and may deteriorate VBU feed quality in terms of colloidal stability. In these
cases, the LNB operators are enforced to reduce the VBU severity, and as a consequence, a
reduction in the VBU conversion is obtained.
As discussed previously, the visbreaker severity was controlled by total sediments level, with
0.1 % as the target. By practising this way of controlling the LNB visbreaker unit, the unit had
to stop operation twice per year for cleaning of equipment. The first down time duration was 7
days after 5.5 months of operation and the second unit down time lasted 6 days after 5 months
of operation. The main problem with fouling of equipment that made the unit stop operation
was a severe fouling of the main fractionator. During the cleaning process it was observed
that the main fractionator feed zone and the column bottom were the most coked column
areas. The feed trays were covered by a thick layer of coke, and the column bottom was also
lined with a thick layer of coke. Having cleaned the main fractionator feed zone trays and the
bottom of the main fractionator internals, fouling occurred after around 6 months of operation.
The column coking was detected by quality of VB diesel which got black and/or by the
limitation of the unit to operate at a maximum capacity. The level of liquid in the main
fractionator bottom started to increase at the same capacity when the column internals got
coked.
Figure 12 presents data of conversion for 12 months of operation of the LNB VBU after the
start of application of the Nalco Conversion Plus II Program in March 2011. It is interesting to
note here that regardless of application of the chemical treatment program of Nalco no
improvement of conversion was registered during the first 5.5 months. Its level remained at
14.8%. Moreover after 5.5 months of LNB visbreaker operation the unit had to be stopped
again for cleaning of the main fractionator. However the downtime for cleaning was 4 days
instead of 7 days registered in 2010 when chemical treatment program was not applied. The
heat exchangers were also cleaned with a reduced frequency after the Nalco Conversion Plus
II Program had been applied in the LNB VBU.
It became clear that the bottleneck of the LNB VBU main fractionator fouling could not be
alleviated if the process philosophy is not changed regardless of application of the chemical
treatment program. After discussion of the possible ways to mitigate the main fractionator
fouling a decision was taken to reduce the main fractionator feed temperature from the
typically applied 380ºC down to 360ºC. Addition of 1.5 – 2% of main fractionator feed fluid
catalytic cracking heavy cycle oil (FCC HCO) and an increase of the rate of the chemical
injected in the main fractionator feed were also applied as measures to reduce the LNB VBU
main fractionator fouling. After implementation of these process conditions, changes to the
conversion were observed, with an increase from 14.8 to 16.0%, and the average conversion
for the period March 2011 – February 2012 became 15.5%, that is 0.7% higher than that
obtained in the previous cycle (March 2010 February 2011) when no chemical treatment
program was applied. A reduction of total unit down time for cleaning of 9 days was
registered after implementation of the Nalco Conversion Plus II Program.
Figure 13 presents data of conversion for 9 months of operation of the LNB VBU after
applying the Nalco Conversion Plus II Program in 2012. It is evident that in 2012 when the
chemical treatment program application continued the average conversion reached the value
of 16.2%. It should be noted here that during this period of time, together with Urals vacuum
resid, blends containing vacuum resids from crudes Basrah and Varandey were processed. As
can be seen from Table 1, the stability of VR from these crudes is lower than that of the Urals
VR. As a consequence the conversion registered during the periods of processing blends of
Urals and Basra and Varandey was lower because of decreasing the unit severity. We knew
that the Basrah VR had lower stability and in this respect a reduction of the LNB VBU
severity was preliminary implemented in order to prevent accelerated fouling of equipment, a
feature alluded to from the laboratory characterisation database. As a result of this action a
conversion decrease of 4% was observed. However the visbreaker residue stability value
during processing of Basrah VR (S=1.55) was much higher than the reference value of 1.43
and a lower reduction in conversion could be obtained if this reference value was kept. In
other words, the average conversion for the whole 2012 would have been higher than the
registered 16.2% if the reference value of 1.43 was kept during the whole period of VBU run
in 2012.
It should be noted that in the first 15 days of the VBU cycle in 2012 the conversion was about
5% lower than 16.0% because of technical problems in the unit. This fact of course negatively
impacted average conversion in 2012. Also observed in 2012, was a reduction of VBU
conversion of about 3% when a blend of Urals VR with Varandey VR was processed. An
estimation of average conversion for 2012, if only Urals VR was processed and the VBR S
value was about 1.43, showed the value of 16.5%, i.e. a conversion increase of 1.7% can be
achieved as a result of application of the chemical treatment program and optimization of
process conditions in the VBU main fractionator. In 2012 the VBU stopped once for 4 days
for cleaning of the main fractionator after 6 months of operation. Obviously at present the
LNB VBU main fractionator is the main bottle neck in the attempts to prolong cycle run
between two consecutive cleanings. Figures 14-15 show pictures of two types of trays used in
the LNB VBU main fractionator feed zone. It is evident that the valve trays (Figure 14) are
not suitable for service in visbreaker main fractionator feed zone. They are severely coked
while the sieve trays (Figure 15) are much cleaner. It may be concluded that the use of valve
trays in the feed zone of the main fractionator in the LNB VBU is the main reason for
accelerated coking of the column. Therefore, the current design of internals of the main
fractionator limited any potential increase in VBU cycle length between two consecutive
cleanings. One may suggest that change of valve trays to sieve trays in the main fractionator
feed zone could be the successful solution for improving fouling rate in the main fractionator.
Figure 16 presents a graph of relationship of the LNB VBU conversion and VBR stability (Svalue). These data confirm the findings established in the Nalco laboratory visbreaking unit [8
and 12] (c.f. figures 2 and 5) indicating that the increase of conversion leads to a decrease of
VBR stability. Therefore by improving the VBR stability an increase of the VBU conversion
can be achieved while keeping the fouling rate of the unit in a reasonable limit. The
application of Nalco Conversion Plus II program and optimization of the operating conditions
in the LNB VBU main fractionator allowed increasing stability of the VBR which resulted in
1.4% higher conversion.
In order to understand the impact of addition of FCC HCO on VBR stability an experiment
was carried out in the LNB Research Laboratory. Figure 17 presents a graph of dependence of
VBR stability (S- value) on FCC HCO concentration in the blend VBR – FCC HCO. The data
show that in general the addition of FCC HCO to the VBR improves VBR stability, much like
Nalco’s laboratory study summarised in figure 5 . However this positive effect is pronounced
when the concentration of FCC HCO in the blend is not lower than 8%. Based on this finding
one may conclude that the addition of FCC HCO in amount of about 1.5% to the VBU main
fractionator feed has no noticeable effect on the registered improvement in the VBR stability.
It seems that the decrease of the VBU main fractionator feed temperature from 380ºC to
360ºC may have the biggest impact on the registered increase of VBR stability and
consequently on conversion. If the injection of the chemical had impact on VBR stability a
conversion increase would have been registered during the first 5 months of applying the
chemical treatment program in the LNB VBU in 2011. Most probably in the commercial
VBU the increased conversion has been obtained by combining the three factors:1).
Decreasing the main fractionator feed temperature from 380ºC to 360ºC; 2) Addition of FCC
HCO to the main fractionator feed; 3) Injection of dispersant to the main fractionator feed.
The first factor seems to have the greatest impact among the other 2 factors.
5. Conclusions and Future Work
Laboratory characterisation of various heavy residue feeds has revealed some intriguing
relationships between process severity, stability and surface fouling, which may be used to
examine the effect of antifoulant additive performance, and different blend compositions.
Furthermore, a sub-type of feed class was identified which revealed a relatively greater
affinity for surface fouling for a given process stability / severity, information that may be
very useful for the introduction of new feeds or blends.
The application of Nalco Conversion Plus II monitoring and additive program, along with
decreasing the main fractionator feed inlet temperature from 380ºC to 360ºC and addition of
FCC HCO to the main fractionator feed allowed the LNB VBU to increase conversion to
products boiling below 360ºC from 14.8 to 16.2% and reduce the unit down time for cleaning
by 9 days. The design of main fractionator trays in the column feed zone seems to be the
reason for running the LNB VBU no longer than 6 months because of tray coking. The valve
trays seem to be not suitable for application in VB main fractionator feed zone because of
their inclination to form coke which blocks the valves. The sieve trays tend to form less coke
on them which may make them more suitable for application in VB main fractionator feed
zone in the future. The unit will continue to be closely monitored using Nalco’s Conversion
Plus II program and further refined to increase conversion. Laboratory work will also be
available to characterise new feed blends on the horizon and to screen for most appropriate
antifoulant additives.
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visbreaker conversion. PTQ Q1 2011
[12] Russell, C. A., Crozier, S., and Sharpe, R. Observations from Heavy Residue Pyrolysis:
A Novel Method to Characterize Fouling Potential and Assess Antifoulant Additive
Performance. 2010. Energy Fuels, 24, 5483-5492
[13] ASTM D 7157-05 – Standard Test Method for Determination of Intrinsic Stability of
Asphaltene-Containing Residues, Heavy Fuel Oils, and Crude Oils (n-Heptane Phase
Separation; Optical Detection) – American Society for Testing and Materials.
[14] Stratiev D. Nedelchev A. Bachvarov A. Dinkov R. Investigation on variation of
visbreaking residue viscosity. OGEM March 1/2012; 38: 34-37
[15] Stratiev D. Minkov D. Stratiev G. Exploiting the synergy between fluid catalytic
cracking and visbreaking to increase the high-value product yields. Oil Gas European
Magazine September 2003;3:141-144
Figure 1: Principle process flow diagram of Visbreaker Unit in LUKOIL NEFTOCHIM BURGAS
Figure 2. Degradation of Intrinsic Stability (S-Value) with increasing laboratory pyrolysis
severity
Figure 3. Increase of toluene insoluble bulk oil sediment with increasing laboratory pyrolysis
severity
Figure 4. Increase in surface deposit (toluene insolubles) with increasing laboratory pyrolysis
severity
Figure 5. Degradation of Intrinsic Stability (S-Value) with increasing laboratory pyrolysis
severity for typical vacuum residue mixes: 100 % VR (black), 100 % DAO (green) and 65 %
DAO : 35 % VR (blue)
Figure 6. Surface deposit with increasing laboratory pyrolysis severity for typical vacuum
residue mixes: 100 % VR (black), 100 % DAO (green) and 65 % DAO : 35 % VR (blue)
Figure 7. Relationship between stability degradation and surface fouling
0.4
0.35
Sediments, wt.%
0.3
y = 2E-05e0.3965x
R2 = 0.9265
0.25
0.2
0.15
0.1
0.05
0
17
18
19
20
21
22
23
24
25
26
Conversion to products boiling below 360 oC, wt.%
Figure 8. Dependence of sediments in the visbreaking residue on conversion of vacuum
residue from Ural crude oil to products boiling below 360 °C [ref.7]
2.
6
16 .20
.6 11
30 .20
.6 11
14 .20
.7 11
28 .20
.7 11
11 .20
.8 11
25 .20
.8 11
.2
8. 01
9. 1
22 20
.9 11
6. .20
10 11
20 .2
.1 01
0 1
3. .20
11 11
17 .2
.1 01
1 1
1. .20
12 11
15 .2
.1 01
29 2.2 1
.1 01
2 1
12 .20
.1 11
26 .20
.1 12
.2
9. 01
2. 2
23 20
.2 12
.2
8. 01
3. 2
22 20
.3 12
.2
5. 01
4. 2
19 20
.4 12
.2
3. 01
5. 2
17 20
.5 12
31 .20
.5 12
14 .20
.6 12
28 .20
.6 12
12 .20
.7 12
26 .20
.7 12
.2
9. 01
8. 2
23 20
.8 12
.2
6. 01
9. 2
20 20
.9 12
4. .20
10 12
18 .2
.1 01
0 2
1. .20
11 12
15 .2
.1 01
29 1.2 2
.1 01
13 1.2 2
.1 01
27 2.2 2
.1 01
2. 2
20
12
S-value
VBR
VR
Figure 9: Stability of the LNB VBU feedstock (VR) and the VBR
ref. S-value
3.400
2.600
2.400
2.200
1.400
1.200
0.05
0.03
2.000
1.800
0.01
1.600
-0.01
-0.03
-0.05
HFT, %
3.600
HFT, %
0.15
0.13
3.200
0.11
3.000
2.800
0.09
0.07
Figure 10: Dependence of VBR viscosity on content of diesel fraction
Date
Figure 11: Operation of the LNB VBU before applying the Nalco Conversion Plus II Program in 2010
10
11
11
2.
20
.2
0
11
1.
20
.1
6.
23
9.
10
2.
20
.1
Downtime for cleaning - 6 days
22.0
0.5
20.0
0.45
18.0
0.35
0.3
0.25
0.2
0.15
8.0
0.1
6.0
0.05
4.0
0
Hot Filtration sediments, % wt.
Light Products Recovery, %
26
10
2.
20
.1
12
10
1.
20
.1
28
10
1.
20
.1
14
10
10
0.
20
.1
31
0.
20
.2
0
10
10
10
10
.2
0
.9
9.
20
.1
17
3.
19
5.
.2
0
10.0
.8
12.0
Downtime for cleaning - 7 days
14.0
10
10
Conversion, %
16.0
22
8.
20
.2
0
.7
10
10
.2
0
.7
.2
0
.6
10
10
.2
0
.6
.2
0
.5
10
10
.2
0
.5
8.
25
11
27
13
30
16
5.
20
10
10
10
.2
0
.4
2.
18
4.
20
.2
0
10
3.
20
.3
4.
21
7.
Average Conversion for 2010 - 14.8%
Hot filtration Sediments, %
0.4
Average Conversion for 2011 - 15.5%
Conversion, %
S-value
ref. S-value
22.0
2.30
Average conversion for the period
28.02.2011 ÷ 17.07.2011 - 14.8%
20.0
Average conversion for the period
23.07.2011 ÷ 10.02.2012 - 16%
Processing of
Kirkuk crude
2.25
2.20
2.15
2.10
18.0
14.0
12.0
10.0
2.00
1.95
1.90
1.85
Processing of
Basrah crude
1.80
1.75
1.70
1.65
1.60
1.55
8.0
1.50
1.45
6.0
1.40
1.35
1.30
28
.2
.2
01
1
14
.3
.2
01
28
1
.3
.2
01
11
1
.4
.2
01
25
1
.4
.2
01
1
9.
5.
20
11
23
.5
.2
01
1
6.
6.
20
11
20
.6
.2
01
1
4.
7.
20
11
18
.7
.2
01
1
1.
8.
20
11
15
.8
.2
01
29
1
.8
.2
01
12
1
.9
.2
01
1
26
.9
.2
01
10
1
.1
0.
20
24
11
.1
0.
20
11
7.
11
.2
01
21
1
.1
1.
20
11
5.
12
.2
01
19
1
.1
2.
20
11
2.
1.
20
12
16
.1
.2
01
2
30
.1
.2
01
2
4.0
Date
Figure 12: 12 months operation of the LNB VBU after applying the Nalco Conversion Plus II Program in 2011
Intrinsic Stability - S-value oih
Conversion, %
16.0
Downtime for cleaning - 4 days
2.05
Figure 13: 9 months operation of the LNB VBU after applying the Nalco Conversion Plus II Program in 2012
Figure 14: Valve tray
Figure 15: Sieve tray
Figure 16: Relationship between the LNB VBU conversion to products boiling below 360 °C
and VBR stability (S-value)
Figure 17: Dependence of VBR stability (S value) on FCC HCO concentration in the blend
VBR – FCC HCO
Table 1: Properties of five vacuum residues used in the study
+550°C cut (ASTM
D-5236) derived
from:
URALS
KIRKUK
BASRAH
CPC
VARANDEY
Relative
density
1.0038
1.0447
1.0460
0.9805
0.9874
Relative
viscosity at
120°C
47.5
120.8
127.5
22.5
24.3
CCR,
Asphaltenes
wt. %
18.4
22.3
23.2
15.97
15.12
5.00
14.75
12.30
3.29
4.70
Intrinsic Stability
SSa
So
value
3.749 0.749 0.939
2.495 0.689 0.776
2.967 0.743 0.762
2.387 0.767 0.556
2.724 0.707 0.799