ISSN 0104-6632 Printed in Brazil
www.abeq.org.br/bjche
Vol. 33, No. 04, pp. 1063 - 1071, October - December, 2016 dx.doi.org/10.1590/0104-6632.20160334s20150103
*To whom correspondence should be addressed
Brazilian Journal of Chemical Engineering
CHARACTERIZATION AND EVALUATION OF WAXY CRUDE OIL FLOW
G. B. Tarantino, L. C. Vieira, S. B. Pinheiro, S. Mattedi, L. C. L. Santos,
C. A. M. Pires, L. M. N. Góis* and P. C. S. Santos
Universidade Federal da Bahia, Escola Politécnica, Departamento de Engenharia Química. Rua Aristides Novis, 02, 3° Andar, Federação, CEP 40210-630, Salvador - BA, Brazil.
Phone: + 55 71 3283-9464 E-mail: [email protected]
(Submitted: February 22, 2012 ; Revised: May 28, 2015 ; Accepted: July 16, 2015)
Abstract - Part of the oil found in the Brazilian subsoil has a high wax content, which makes its flow process difficult at low temperatures because of the increase in the viscosity of the fluid. This paper studied the flow behavior of waxy crude oil under variation in the temperature of the external environment of the flow, the volumetric flow rate of the oil and the emulsified water content of the oil. The results were compared with those obtained for a non-waxy crude oil that had similar rheological properties at temperatures above the wax appearance temperature (WAT). The proposed tests were based on the experimental design technique, and the behavior of the fluids was evaluated based on the pressure variation generated by the flow. Keywords: Waxy crude oil; Pressure variation; Crystallization.
INTRODUCTION
Paraffinization is one of the main problems in oil production and causes considerable losses to the oil industry. The wax precipitation phenomenon associ-ated with paraffin deposition can result in unsched-uled production shutdowns and promote operational risk conditions. Moreover, it can cause production losses and irreparable damage to equipment (Pauly et al., 2004).
In the Bahian Recôncavo region, the produced crude oil exhibits a density of approximately 30° API, almost no sulfur and high concentrations of dis-solved waxes. Although these properties are great for the manufacture of lubricant oils and yield high added value, the presence of wax adds many com-plications to production, transportation and storage by hindering the flow in pipes (Thomas, 2004; Novaes, 2009).
Paraffins are both linear (n-paraffins) and branched (iso-paraffins) chain alkanes, and they have low re-activity with most compounds. Their chains can have a high carbon number, which implies a higher wax appearance temperature. The low-molecular-weight paraffins are the main components of natural gas, and the medium- and high-molecular-weight ones are found in crude oil (Farayola et al., 2010; Gao, 2008; Jamaluddin et al., 2001).
Paraffins are in equilibrium with other crude oil components, and any change in pressure, tempera-ture and even composition can affect the equilibrium, thereby influencing the formation of precipitate. According to Santos (1994), the greater the crude oil wax content, the greater the precipitation rate and, therefore, the amount of precipitated wax. The light oil components keep the waxes soluble. The high pres-sure of the reservoirs maintains the light compounds solubilized in the crude oil, which favors the solubili-
1064 G. B. Tarantino, L. C. Vieira, S. B. Pinheiro, S. Mattedi e Silva, L. C. L. Santos, C. A. M. Pires, L. M. N. Góes and P. C. S. Santos
Brazilian Journal of Chemical Engineering
zation of waxes in the fluid (Tinsley; Prud’Homme, 2010). This condition ensures low viscosity and Newtonian behavior of the crude oil (Azevedo, 2003).
Temperature also influences the solubility of waxes in crude oil. When approximately 5% of the waxes crystallize because of oil cooling, a crystal lat-tice appears and traps some of the oil inside; this process is called “gelling” and hinders the fluid flow. Thus, the crude oil flow rate also interferes with wax solubility. The lower the oil flow rate, the longer it stays inside the piping, which favors heat exchange with the external environment (Vieira, 2008).
Once production starts, the oil flows through the pipelines, losing heat to the external environment, with consequent temperature decreases and reduced soluble light oil fractions. Such production condi-tions cause the oil viscosity to increase, which leads to production problems due to the precipitation of waxes (Venkatesan et al., 2005; Gao, 2008).
According to Vieira (2008), the first paraffin crys-tals start to form at a specific temperature, which is called the wax appearance temperature (WAT) and varies depending on the origin of the crude oil. Crys-tallization occurs in three steps:
Nucleation – formation of small particles of crystallized material from which the first paraffin crystals will grow.
Growth – mass transport of the solution to-wards the nuclei formed during the nucleation stage.
Agglomeration – when the growing crystals are joined together, thereby yielding larger crystals.
With the nuclei already formed, there is incorpora-tion of new paraffin molecules at the growth sites, and additional molecules of other species are grouped at these sites and become part of the structure. The nu-clei form an ordered lamellar-structure arrangement.
After crystallization starts in a medium that con-tains water as an emulsion, the crystal lattice for-mation phenomenon occurs in a different manner. When the emulsion is of the water-in-oil type, the oil is waxy and the fluid temperature is below the WAT, the precipitated waxes are deposited onto the surface of the water drops, thereby contributing to the growth of the formed precipitate (Oliveira et al., 2010). When a large crystal lattice is in the vicinity of the water drops, a structure is formed; this struc-ture percolates the drops into the lattice and captures them. According to Visitin (2008), this structure also provides mechanical resistance to the flow, thereby resulting in an increase in the viscosity and pour point of the oil.
The present study aims to evaluate and compare the flow of two types of crude oil, waxy and non-waxy, by measuring the pressure variation of the
system under the influence of the flow rate, tempera-ture and content of emulsified water.
MATERIALS AND METHODS Crude Oil
The characteristics that influenced the choice of oils used in this study were obtained from the rheo-logical behavior of the samples. Although the availa-ble oils had different wax contents, WATs and com-positions, for a comparative study of the influence of the content of emulsified water, temperature and oil flow rate in the context of loss of flow, it was neces-sary for the oils to be rheologically similar such that any differences originated exclusively from phenom-ena that characterize the increase in fluid viscosity and its implications for the flow.
After a series of comparative tests to search for a non-waxy oil with rheological behavior similar to that of the waxy oil above the WAT, a non-waxy oil was defined as the reference for comparison in the study.
The WAT of each oil was determined through dif-ferential scanning microcalorimetry, µDSC. The analysis was made using a DSC-VII microcalorime-ter, Setaram, with a 500 µL stainless steel pressure cell and the data acquisition and analysis was made through the Setsoft 2000 software. The procedure realized in the tests consists of heating the sample to 80 °C during one hour and a sample of known weight is placed in the cell and then in the equipment. The analysis is made by cooling the sample from 80 °C to 0 °C at a rate of 0.8 °C/min. Microcalorimetry measures any release or absorption of heat by the sample while it cools. The evaluated temperature range consisted in the cooling of the oils from 80 °C to 0 °C. The only possible exothermic event in this temperature range is the release of heat related to the crystallization of waxy species present in the sample. The greater the crystallization peak area, the greater the amount of wax present in the sample, and the higher the temperature at which crystallization oc-curs, the greater the length of the carbon chain of the crystallized paraffins. Figure 1 shows the microcalo-rimetry curve of the waxy oil, which has a WAT of 310.5 K.
Figure 2 shows the microcalorimetry curve of the low-paraffin oil. This sample exhibits two points at which the line tangent to the crystallization curve crosses the abscissa, thereby generating a WAT at 316.4 K and a second crystallization event at 290.7 K. This effect occurs because the long-chain n-alkanes,
wnth(S
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Figure 1: Mrude oil.
Figure 2: Mirude oil.
The rheoletermined brom Thermoimilar to thoonsisted of clong a linearate of 0.5 K/
The rheologic
Brazilian J
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to co-crystahereby form
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Vol. 33, No. 04,
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pp. 1063 - 1071,
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The tempend output ofpot meters, nce betweenver time, wa
Due to thressure, the ure of 9 bar,ity. This limalues with aests relating o 8 bar, wheific operatioeen reached,f flow. The il were accoue to a small
Figure 4 sh
Figure 4: Scnit.
B. Tarantino, L. C
olled using ahereas the tempiping was a heating b
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were conducted out with th
within 2 hourss. uipment sche
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inheiro, S. Matted
Brazilian Jou
ate placed unf the oil passusing a systl flow rate whereas the inmeasured us
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L. Santos, C. A. M
al Engineering
xperimental
Three variabnt of emulsifmperature (Tes influencedas quantifiedwo levels weres of the rangted in Table
able 1: Extrece the flow.
Independentvariable A (%) T (K) Q (mL/min)
For the testr three varianter point, as
able 2: Tests
Tests
1 2 3 4 5 6 7 8 9 10 11
The factorsr each level aLaboratory
ater in the waude oil, theseometric titratitentiometric
ith the titratioental design e same for bo
on-waxy crudopriate becaul properties othe waxy c
ucted under hmponents, w
ns, thereby chherefore, the
M. Pires, L. M. N.
Design
bles were deffied water in T) and the fld the flow o
d in terms of re defined foge to be anal1.
eme values o
t Lowest(-5
29315
ts, a full facables (2³), ws described in
conducted a
A X1 - + - + - + - + 0 0 0
involved inare described
tests indicaaxy crude oil e analyzes wion with Kartitrator mod
on unit modecreated requoth oils. It wade oil from 0use this proceof the samplcrude oil froeating, result
which transfohanging the minimum lev
. Góes and P. C.
fined for the the crude oi
low rate (Q)of waxy cru
f the pressureor each varialyzed), and th
of the variab
t value -)
H
5 .15
50
ctorial designwith three rep
n Table 2.
according to
T X2 - - + + - - + + 0 0 0
n choosing thd below. ated 5.12% l and 0.3% in
were made usrl-Fischer rea
del Titrando 8el Dosino 80uired the leve
was found tha0.3% to 5% ess preservele. In contrasom 5.12% tted in volatilormed solvenoil’s rheolog
evel was set t
S. Santos
tests: the coil (A), the ba). These variude oil, whice change (∆Pable (the extrheir values a
bles that infl
Highest value (+) 35
298.15 200
n was applieplicates at th
the 2³ desig
Q X3 - - - - + + + + 0 0 0
he values use
of emulsifien the non-waxsing the poteagent with th852, Metrohm00. The experel values to bt hydrating thwas more as the rheologst, dehydratioto 0.3%, coization of lignts into paragical behavioto 5%.
on-ath ia-ch P). re-are
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Characterization and Evaluation of Waxy Crude Oil Flow 1067
Brazilian Journal of Chemical Engineering Vol. 33, No. 04, pp. 1063 - 1071, October - December, 2016
The highest levels of the variable “water cut“ were set to be 35% because, when this level was greater than 40%, the viscosity increased, which yielded difficulties in pumping the sample.
The influence of the temperature on the crystal-lization of paraffins dissolved in crude oil, and con-sequently its viscosity, is of paramount importance to this work. Preliminary tests demonstrated that, below 291.15 K, the waxy crude oil used as a sample did not flow properly because of its high viscosity. For this reason, 293.15 K was set as the lowest tempera-ture level used in this study.
The highest temperature level was defined based on the need to expose the waxy crude oil to condi-tions in which crystallization of solubilized paraffins occurs, i.e., at temperatures close to its WAT. There-fore, the highest level was set to 298.15 K.
The residence time in the test tube is also a factor that influences the crude oil flow. The greater the residence time, the greater the heat exchange with the external environment, which results in a wider variation of fluid viscosity. Based on this infor-mation, it was assumed that 150 mL/min would be adequate for the minimum flow rate level.
The maximum flow rate level was set to 200 mL/min, which is close to its maximum operating condition, 275 mL/min. Experimental Methodology
One liter of the sample with a determined water cut was initially heated to 333.15 K in a storage ves-sel such that the solubility of all paraffins in the oil was ensured. At the same time, the oil was circulated through the system’s bypass until its temperature was stabilized. At the end of this step, the sample flowed through the cooling system.
During the test, the oil that fed the unit was cooled to 328.15 K. After the cooling that occurred throughout the test tube, the oil returned to the begin-ning of the process to be reheated to make the pre-cipitated wax soluble.
The temperature and pressure values at the inlet and outlet of the test tube were recorded during the tests every 10 seconds. The pressure difference be-tween the inlet and outlet of the test tube measured over time was the response obtained for each test. In most of the tests, the maximum pressure variation was achieved in less than two hours of operation. The results indicated that the best strategy to com-pose the experimental design was acquisition of pres-sure change data at a fixed time. The shortest time at which the maximum pressure was attained was used as the time to measure the differential pressure value for all tests.
RESULTS AND DISCUSSION Waxy Crude Oil
The tests were conducted according to the distri-bution in Table 2 and the pressure differential oc-curred due to the wax precipitation. The pressure difference was calculated from the pressure variation between two manometers, the first one located at the beginning of the flow and the second one at the end of the flow; these manometers present 99 percent accuracy. In most cases, the maximum pressure vari-ation of the system occurred at different times. Be-cause the goal of the research is to study the pressure variation based on the influence of the variables (the emulsified water in the crude oil (A), temperature (T) and flow rate (Q)), the differential pressure values considered as test responses were those attributed to the lowest operating time that reached the maximum pressure variation of the system. The worst flow con-dition was achieved in test 1, with a temperature of 293.15 K, flow rate of 150 mL/min and water cut of 5%; in this test a pressure difference of 8.03 bar was achieved in 36 minutes. Thus, the reference operat-ing time was 36 minutes. The pressure variation responses are found in Table 3. Table 3: Pressure differential of the samples pre-pared with waxy crude oil.
Tests Temperature (K)
Flow rate (mL/min)
Water cut (%)
Pressure differential
(bar)1 293.15 150 5 8.032 298.15 150 5 1.073 293.15 200 5 4.924 298.15 200 5 0.715 293.15 150 35 5.676 298.15 150 35 3.067 293.15 200 35 3.77
The data in Table 3 were processed with the aid
of parametric statistics. The empirical models pre-sented in Figures 6 and 7 were evaluated for signifi-cance through analysis of variance based on the Pa-reto diagram shown in Figure 5.
It can be observed from Figure 5 that the varia-bles that significantly influenced the flow process were the temperature, oil flow rate, the interaction between the temperature and oil flow rate and the interaction between the temperature and water cut. The significance of the interactions between the two variables leads to the conclusion that it is not possi-ble to analyze the behavior of the system based on only one variable while keeping the other variables fixed. The experimental data were properly fitted to a linear plane model with a regression coefficient (R²)
10
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Figure 5: Paresults.
Figure 6 serature and ressure variaressure dropnt temperatuange. The higst ambient tressure drop
when a higher
Figure 6: Resp
The lowesosed an averhat was beloion of large ncreasing therom Figure 3est tube diret lost heat to mmersed oveower the oil
B. Tarantino, L. C
ch implies thg the behavio
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shows the beflow rate as
ation. It can p occurred ature for all flghest pressuremperature
p was indeper ambient tem
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C. Vieira, S. B. Pi
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endent of themperature wa
e - flow rate a
mperature leature of the (37.4 °C). Thf waxes occf the oil, as c
dence time oced its tempement in whictime period.
n the system
inheiro, S. Matted
Brazilian Jou
l is appropridied system.
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L. Santos, C. A. M
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M. Pires, L. M. N.
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axy crude oiethodology a
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1 22 23 24 25 26 27 28 29 2
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model properltudy.
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Figure 8: Parlow results.
Figure 9 shlow rate on te observed thhe water cut evel. Howev
when the wat
Brazilian J
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293.15 298.15 293.15 298.15 293.15 298.15 293.15 298.15 295.65 295.65 295.65
n Table 4 wernce values foriagram in Fects of the opon, with a c6%. This resuly represente
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Chara
Journal of Chemic
pressure difysis.
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ow rate
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(%
150 5150 5200 5200 5150 3150 3200 3200 3175 2175 2175 2
re fitted withr the variabligure 8. Figperational vaoefficient ofult indicates
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h a linear modes are shown
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les that signion in the nrate, water c
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-waxy crude
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Evaluation of Wa
Vol. 33, No. 04,
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were
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axy Crude Oil Flo
pp. 1063 - 1071,
he water inflme manner texhibited a
owing with a e continuouscurred, this
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gure 9: Resp
It is notewooplets in the
mply wax depmposed on teased the prow rate exhibbserved for taxy crude oile pressure vacrease in the
Figure 8 shmperature wown in Figurnge was veryce of the tem
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ot significant
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ponse surface
orthy that, wflow, the inc
position but these dropleressure variabited behaviothe waxy crl a decrease ariation, in thflow rate inc
hows that theas not signifre 10. Becauy small, frommperature onsmall, which
mperature lev temperaturethe non-waxre 2. alorimetry ce the second affin crystallned soluble, in Figure 2. let temperatuf the system lization evenin the define
ember, 2016
non-waxy crenced the waar to that oincreasing th
cause no waxn appeared tf those that i
e – water cut
with the presncrease in flo
rather increaets and conation. For thor that was orude oil. Win the flow
he non-waxycreased the pe variation oficant, and thuse the studiem 20 °C to 25n the increaseh justifies itvels set for e of the secoxy crude oil
curves indicacrystallizatio
lization, and as indicatedIn the tests a
ure was 55 °m remained w
nt; thus, thised level rang
10
rude oil in thaxy oil, that if small solihe viscosity x precipitatioto be the mointerfered wi
and flow rate
sence of watow rate did nased the she
nsequently ihis reason, thopposite to th
Whereas in thrate increase
y crude oil thpressure gainof the ambiehis fact is aled temperatu5 °C, the infle in oil viscots non-signifthe tests wend crystalliz(17.6 °C) o
ate that at temon event, thed most of thd by the smaanalyzed herC, the averag
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Figure 10: Rerature.
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As can be urred in triplate phenome
Figures 11riplicate witht the middleressure, the redibility to t
Figure 11: T
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B. Tarantino, L. C
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differential the test and i
Triplicate tes
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Brazilian Jou
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waxy oil.
di e Silva, L. C. L
urnal of Chemica
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L. Santos, C. A. M
al Engineering
Although there very simiidenced by Faxy crude oile waxy crudorking rangeoved to be dd interactionte and water
The variablaxy crude oirsely propore large amoul, once the mperature, ad contributee system. Thas inversely pfect occurrenger residencusing furthermperature. Tow, with waxrough the inax precipitateore flexible,sence of prelit because ohaved like creasing its v
The non-water cut, flowfects. Becausthe non-wax
ost significanrtional to theemulsified wcause the ber to that of ad, unlike thoportional toow rate, the gcrease in asso
A
To Brazilianvaluation of perfeiçoamenAPES), Natiogical Develo
M. Pires, L. M. N.
CONCLU
he rheologicilar at tempeFigure 3, the l was complede oil at loe used, the ependent on n between ccut.
le that was thil was the tetional to the
unt of wax thaWAT was alarge amou
d to increasihe flow rateproportional d because lce time for tr heat loss anhe presence x precipitationteraction ofe. This intera
thereby facecipitate, theof the shear a solid disp
viscosity. axy crude ow rate and cse of the smaxy crude oil,nt was the we pressure gawater, the grehavior of thea solid. The
he waxy cruo the pressugreater the shociated press
CKNOWLE
n Federal AgGraduate Ed
nto de Pessonal Council opment (Con
. Góes and P. C.
USIONS
cal propertieeratures above flow behavietely differen
ower temperflow of wathe flow rat
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he most signemperature, w pressure ga
hat was preseachieved by unt of paraffing the presse was also sl to the presslow flow rathe oil in thend consequeof emulsifie
on, induced f water dropaction made cilitating its
e water dropimposed by
persed in th
oil was influcombinationall amounts o, the variablwater cut, wain. The greareater the sye water dropflow rate w
ude oil, its ure gain. Thhear and, consure variation
EDGMENT
gency for thducation (Cosoal de Nív
for Scientifinselho Nacio
S. Santos
es of both oive the WAT, ior of the nont from that ratures. In thaxy crude ote, temperatus of the flo
nificant for thwhich was iin. Because nt in the crudlowering th
fin crystallizesure change significant ansure gain. Thates caused e line, therebntly decrease
ed water in thgel formatio
plets with ththe precipita
s flow. In thplets tendedy the flow ane oil, thereb
uenced by thn of these twof wax presee that was th
which was prater the amoustem viscosi
plets was simwas significainfluence whe greater thnsequently, thn.
TS
he Support anoordenação del Superioric and Technnal de Dese
ils as
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- no-en-
Characterization and Evaluation of Waxy Crude Oil Flow 1071
Brazilian Journal of Chemical Engineering Vol. 33, No. 04, pp. 1063 - 1071, October - December, 2016
volvimento Científico e Tecnológico - CNPQ), and Petrobras (Petróleo Brasileiro S.A).
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