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Modified maleic anhydride-co-octadecene copolymers as flow improver for waxy Egyptian crude oil
Author's Accepted Manuscript
Modified maleic anhydride-co-octadecene Copolymers as flow Improver for Waxy Egyptian Crude Oil Rasha A. El-Ghazawy, Ayman M. Atta, Khalid I. Kabel
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PII: DOI: Reference:
S0920-4105(14)00239-3 http://dx.doi.org/10.1016/j.petrol.2014.07.040 PETROL2752
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Journal of Petroleum Science and Engineering
Received date: 7 April 2014 Accepted date: 31 July 2014 Cite this article as: Rasha A. El-Ghazawy, Ayman M. Atta, Khalid I. Kabel, Modified maleic anhydride-co-octadecene Copolymers as flow Improver for Waxy Egyptian Crude Oil, Journal of Petroleum Science and Engineering, http://dx. doi.org/10.1016/j.petrol.2014.07.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Abstract
2
Polymeric flow improvers represent one of the most important chemicals used in oil industry
3
especially with waxy crudes. The chemical structure of such chemicals is particularly important.
4
In this study, we prepared a family of additives by chemical modification of octadecene-co-
5
maleic anhydride copolymers (OM). The copolymerization of octadecene (OD) with maleic
6
anhydride (MA) was performed at variable ratios of OD: MA (1:1, 1:2 and 2:1). OM copolymers
7
were chemically modified by grafting three different alcohols dodecanol, hexadecanol or
8
docosanol. Grafts were characterized through molecular weight using GPC, melting temperature
9
and enthalpy of melting by DSC and chemical structure through 1HNMR. The comb like
10
copolymers were assessed as flow improver for an Egyptian waxy crude oil by measuring their
11
performance as pour point depressant in addition to their assessment as viscosity improver
12
through rheological behavior. The data indicate that, the additive must retain a hydrophobic-
13
lypophobic balance, low crystallinity and matching melting temperature with that of crude wax
14
in order to inhibit the paraffin and wax crystal depositions.
15
Modified Maleic Anhydride-co-Octadecene Copolymers as Flow
16
Improver for Waxy Egyptian Crude Oil
17
Rasha A. El-Ghazawya,*, Ayman M. Attaa, b and Khalid I. Kabela
18 19 20
a
Department of Petroleum Applications, Egyptian Petroleum Research Institute, Nasr city, P.O.
11727, Cairo, Egypt b
Surfactants research chair, Chemistry Department, College of Science, King Saud University,
21
P.O. 2455, Riyadh - 11451, Saudi Arabia
22
*Corresponding author: Tel.: +20 1 227284228; fax: +20 2 22747433.
E-mail address: [email protected]
23 24
Abstract
25
Polymeric flow improvers represent one of the most important chemicals used in oil industry
26
especially with waxy crudes. The chemical structure of such chemicals is particularly important.
27
In this study, we prepared a family of additives by chemical modification of octadecene-co-
28
maleic anhydride copolymers (OM). The copolymerization of octadecene (OD) with maleic
29
anhydride (MA) was performed at variable ratios of OD: MA (1:1, 1:2 and 2:1). OM copolymers
30
were chemically modified by grafting three different alcohols dodecanol, hexadecanol or
31
docosanol. Grafts were characterized through molecular weight using GPC, melting temperature
32
and enthalpy of melting by DSC and chemical structure through 1HNMR. The comb like
33
copolymers were assessed as flow improver for an Egyptian waxy crude oil by measuring their
34
performance as pour point depressant in addition to their assessment as viscosity improver
35
through rheological behavior. The data indicate that, the additive must retain a hydrophobic-
36
lypophobic balance, low crystallinity and matching melting temperature with that of crude wax
37
in order to inhibit the paraffin and wax crystal depositions.
38 39
Keywords: Flow improver, waxy crude oil, rheology, octadecene‐co‐maleic anhydride, pour point
40
depressants.
1. Introduction
41
From the major problems arising through production, transportation and storage of petroleum
42
crude oil are wax and paraffin depositions, especially when the crude oil has a high content of n-
43
aliphatic hydrocarbon chains that induce, by temperature change, crystallization of n-paraffins
44
(Paso and Fogler, 2004; Wu et al., 2002). When the absorption of both wax and paraffin particles
45
is enough to constitute a stable three-dimensional network within the crude oil system, crude oil
46
reaches its pour point, the minimum temperature at which the oil flows under the influence of
47
gravity. Several kinds of treatments (mechanical, physical and chemical) have been applied to
48
control paraffin separation and its adverse consequences (Ribeiro et al., 1997). Chemical
49
treatments by flow improvers, crystal modifiers or pour point depressant, are used to reduce
50
apparent viscosity, yield point and pour point of crude oil (Dong et al., 2001; Pedersen and
51
Ronningsen, 2003).
52
Two main architectural copolymer classes are commonly used as pour point depressants (PPDs)
53
in industrial formulations: (1) linear ethylene copolymers and (2) comb-like copolymers. In this
54
respect, polymers with linear crystalline/amorphous diblock structures, such as ethylene-vinyl
55
acetate copolymer (Qian et al., 1996; Taraneh et al., 2008), poly (ethylene–butene) (PEB)
56
(Ashbaugh et al. 2002), and comb-type such as poly(maleic anhydride-co-α-olefin) esters (El-
57
Gamal et al., 1992) and amides (Xu et al., 2009) have been used to improve the cold flowing
58
ability of waxy oils by modifying paraffin crystallization.
59
The commercial grades of ethylene/vinyl acetate (EVA) co-polymers have found application as
60
“pour point” depressants in refined fuels. Ashbaugh et al. 2005 focused on EVA behavior as
61
crude oil additives (using model crude oils consisted of 4 wt % wax in decane), to reduce yield
62
stress of the gels that can form when the oil exits the reservoir. They found that EVA dosage
63
levels of ~200 ppm, the reduction in yield stress is 3 orders of magnitude for the C36 wax,
64
whereas the reduction is 1 order of magnitude for C32 and only 3-fold for the C28 wax. This
65
decrease in efficiency with decreasing wax carbon number indicates that the EVA materials
66
would not provide an adequate reduction in yield stress to ensure against gelation in pipeline
67
transport. Taraneh et al. 2008 studied the influence of EVA with different ranges of molecular
68
weight as flow improver for five Iranian waxy crude oils. Results show good depression of pour
69
point but weak efficiency in reducing viscosity and yield values. Performance evaluation of EVA
70
graft copolymers with long side chains (comb-like copolymers) and nitrogen polar groups was
71
demonstrated as pour point depressants for Daqing waxy crude oil (Li-juan et al. 2008). The
72
modified EVAs was found to be better than EVA in pour point depression.
73
The advantages of using of comb-type copolymers as flow improver for crude oil are to control
74
the crystallization of paraffins and prevent asphaltenes from aggregation based on the relation
75
between nonpolar side alkyl chain and polar group (such as carboxylic, ester, amide groups
76
etc.,). Their nonpolar long side alkyl chains can self-assemble to nucleate the long-chain paraffin
77
crystallization or co-crystallize with paraffins to modulate the wax crystals. Moreover, different
78
factors may affect the performance of additives in wax dispersion including the solubility of
79
polymeric additives in different solvents (Mansur et al., 2006; Qian et al., 1996), their thermal
80
behavior and molecular weight (Borthakur et al., 1996). Evaluating rheology of treated waxy
81
crude oil samples is a complementary study that attracts particular interest (Barbato et al., 2014;
82
Marchesini et al., 2012; Teng and Zhang, 2013; Visintin et al., 2005; Zhang and Liu, 2008). The
83
disadvantages of comb-like copolymers may be an increase in the cost of the final product and
84
those dependent on maleic anhydride or acid functionalities may create a potential for a corrosive
85
environment. Eventually, the choice of pour point depressant type is mainly controlled by
86
availability of starting materials and techniques where linear ethylene copolymers require special
87
complicated techniques.
88
Crude oils of western desert in Egypt are mainly wax rich crudes. Treating of such crudes with
89
flow improvers attracts attention of scientific research. El-Gamal and Al-Sabbagh 1996 have
90
prepared comb-like copolymers based on α-olefin with different alkyl chains and maleic
91
anhydride (OMACs). Their esters with NAFOL 20+ and propoxylated tetraethylene pentamine
92
adduct and their amides with tetraethylene pentamine were tested as flow improvers for Egyptian
93
waxy crude oil (Umbaraka) and gas oil blend. A distinguishable depression of pour point was
94
achieved reaching 39
95
copolymers derived from styrene- maleic anhydride for improving flow ability of Qarun waxy
96
crude oil (western desert – Egypt). Improved pour point and rheology were achieved but with
97
high dose of 10,000 ppm. El-Ghazawy and Farag 2010 studied the effect of (docosanyl acrylate-
98
co-(octadecyl or hexadecyl acrylate)-co-maleic anhydride) terpolymers as flow improvers for
99
two Egyptian waxy crude oils. Data show good ability of some terpolymers to disperse wax
100
crystals and improve flow behaviors of the tested crude oils even at low temperatures below their
101
pour points. Comb-like phthalimide acrylate and succinimide acrylate copolymers with vinyl
102
acetate, styrene or methyl methacrylate were evaluated as flow improver for Egyptian waxy
103
crude oil (Al-Sabagh et al. 2013). Remarkable depression of pour point was observed besides
104
improved rheological properties at 1000 ppm.
105
At present, more and more researchers are dedicated to explore effective polymer additives for
106
preservation and transportation of crude oils (Atta et al., 2008a; Borthakur et al., 1996; Castro et
107
al., 2011; Deshmukh and Bharambe, 2014; El-Gamal et al., 1997, 1994; Soni et al., 2008;
108
Taraneh et al., 2008).
109
In this work, we synthesized different copolymer compositions of modified comb-type poly (α-
110
octadecene–co-alkyl maleate esters) with varied pendant alkyl chain lengths viz. 12, 16 or 22.
111
The effect of copolymer composition, molecular weight, melting point and length of alkyl
112
substituents on the crystallization of long-chain n-paraffins upon cooling model waxy crude oil
113
were observed with regard to pour point depression and rheology.
114
C at 1000 ppm dose. Al-Sabagh et al. 2009 prepared comb-like
115
2. Experimental
116
2.1. Materials
117
Maleic anhydride (MA), α-octadecene (OD), azobisisobutyronitrile (AIBN), 1-dodecanol, 1-
118
hexadecanol, 1-docosanol and p-toluene sulfonic acid (PTSA) were purchased as analytical
119
grade from Aldrich Chemicals Co. (Germany). Methanol, tetrahydrofuran, n-hexane and toluene
120
were obtained from Merck.
121
Egyptian Waxy crude oil produced from Norpetco (Egypt, eastern desert) was delivered without
122
treatment from Fardos field. The physicochemical characteristics and composition of Fardos
123
mixed crude oils are listed in Table 1.
124
2.2. Copolymerization of α-octadecene and maleic anhydride
125
α-Octadecene - maleic anhydride copolymers were prepared by copolymerizing OD and MA in
126
different molar feed ratios, OD:MA (1:1), (1:2), and (2:1). Copolymerization was performed
127
according to procedures described by (Davies et al., 2002). Three different copolymers of
128
octadecyl-co-maleic anhydride were designated as OM, O2M and OM2 for OD: MA molar ratio
129
of 1:1, 2:1 and 1:2, respectively.
130
2.3. Esterification of the prepared copolymers with different alcohols
131
The esterification reaction of the prepared copolymers was carried out using 0.01 mol of the
132
copolymer solution in toluene, with one of the previously described ratios, and 0.02 mol of
133
alcohol (1-dodecanol, 1-hexadecanol or 1-docosanol) refluxed in presence of 0.1 (wt%) PTSA
134
catalyst. The reaction was carried out at the refluxing temperature until the theoretical amount of
135
water was collected azeotropically. The resulting esters were washed out with water to remove
136
the catalyst. Purification of the raw products was carried out by pouring in an excess of
137
methanol, followed by filtration, vacuum drying, and washing three times with hot water.
138
Esterified samples were coded as OMn, OM2n and O2Mn, where n represents the number of
139
carbons of the alkyl group for the used alcohol (n = 12, 16 or 22).
2.4. Characterization of the prepared additives and crude oil
140 141
1
HNMR analysis was carried out on Varian NMR 300 MHz spectrometer using deutrated DMSO
142
for determining copolymers’ compositions for different feed ratios. Comparing the integrals of
143
maleic anhydride moiety (m1) and α-octadecene (m2) methylene group regions in the spectra of
144
the copolymer was done according to the Eq. (1).
145
m1 / m2 = n2. Am1 / n1. Am2
(1)
146
where Am1 and Am2 are the normalized areas per H from the corresponding functional groups of
147
the monomer unit regions in 1HNMR spectra and n1 and n2 are integers of proton(s) in the
148
corresponding functional group. The molecular weight of the prepared additives were
149
characterized (in terms of Mw and Mn) and polydispersity index using Shimadzu's gel permeation
150
chromatograph (GPC) equipped with refractive index detector and polydivinylbenzene mix gel-
151
D column using tetrahydrofuran as an eluent and polystyrene as a standard.
152
The specific gravity and kinematic viscosity of crude oil were determined according to ASTM-D
153
287 and ASTM-D 445 methods, respectively. Wax and asphaltene contents were determined
154
according to the UOP-46 norm and the ASTM-D 2007 method, respectively. The carbon
155
distribution number of separated wax was determined using GC-Mass spectrometer.
156
The treated and untreated crude oil samples were evaluated with regard to their pour points
157
according to the ASTM-D 97 using Seta Cloud and Pour Point Cryostat, Stanhope-Seta, U.K.
158
method without reheating to 46
159
2000, 3000 and 4000 ppm). In addition, viscosity and flow curves (Rheogram) for treated and
C at different concentrations of the prepared additives (1000,
160
blank crude oil samples were measured using a Brookfield Viscometer equipped with
161
thermostated cooling system. Rheological measurements were performed at different
162
temperatures (9, 15 and 21 C) according to Norpetco Co. recommendation to mime winter
163
transportation and storage conditions.
164
3. Results and discussions
165
3.1. Characterization of the prepared additives
166
Comb like poly (α-octadecene-co-maleate esters) were prepared through this work with different
167
maleate contents and varied grafted alkyl chain lengths to study the effect of polar carboxylate
168
groups’ inclusion and pendant chain lengths on flowability of high waxy crude oil. 1HNMR
169
spectroscopic analysis has been established as a powerful tool for the determination of
170
copolymer compositions because of its simplicity, rapidity, and sensitivity (Dincer et al., 2002;
171
Kesim et al., 2003; Nicolescu et al., 2009). Here, the copolymer composition of OM, O2M and
172
OM2 was confirmed by 1HNMR. Comparing peak integral at 3.3 ppm assigned for methine
173
group of succinic anahydride to that at 1.18 ppm ascribed to pendant methylene groups of
174
α-octadecene moiety (using Eq. (1)) confirms the efficient copolymerization of OD and MA
175
with the stated feed ratios (see Figure 1 a-c). In addition, vanishing peak integrals at about 7
176
ppm (assigned to MA protons) and 4.9 and 5.8 ppm (assigned to double bond protons of OD)
177
confirm efficient copolymerization.
178
The number average molecular weights (Mn), weight average molecular weights (Mw) and
179
polydispersity (PD= Mw/Mn) for OM, OM2 and O2M were determined by GPC and listed in
180
Table 2. The data indicate that the molecular weight decreases with increasing MA or OD
181
contents and the PD decreases with increasing MA content. Such low molecular weights and
182
high PD of OM, O2M and OM2 suggest the suitability of the corresponding esters to be applied
183
as pour point depressant additives (Castro et al., 2011; El-Gamal et al., 1997, 1994). Literature
184
survey proved that the molecular weight of polymeric additives affects their performance as flow
185
improver and it is reported that the optimum number average molecular weight for polymeric
186
additives ranges from 20,000 -100,000 g/mol.
187
Esterification % was calculated experimentally through evaluating the amount of collected water
188
during esterification and it ranges from 85 – 93%. The data indicate that the esterification %
189
increases in the order OM > O2M > OM2. This may be rationalized by the large difference in
190
polarity and compatibility between reactants which is a factor that may affect the reactivity of the
191
functional groups (Atta et al., 2008a, b; Wesslen B. and Wesslen K.B., 1989).
192
3.2. Isolation and characterization of paraffin from crude
193
Wax crude oils have a complex composition, comprising wax, resins and asphaltenes that
194
drastically affect their cold flow properties. Hence these components were quantified in the
195
investigated Fardos crude oil. Melting point of the isolated wax was measured as 48-50 °C. It
196
was then subjected to gas chromatographic analysis to determine the distribution of n-paraffins
197
by carbon number and the average carbon number. The results are given in Table 3 and
198
illustrated in Figure 2. The data indicate that the average carbon number distribution is 40.9 with
199
concentration of about 50 wt % of the n-paraffin content having a broad distribution in crude oil
200
expressed in Wh/2 (see Figure 3). Two types of signals, Figure 2, can be distinguished: sharp
201
peaks, attributed to the n- and iso-paraffins, and a hump attributed to elution of cyclic paraffins.
202
Inspecting the overlaid chromatograms, it is clear that there is a retention time shift in the cyclo-
203
and iso-paraffins fraction where one or more compounds from these classes are eluted between
204
two intense peaks corresponding to the n-paraffins present in the whole saturates fraction. Such
205
paraffins tend to precipitate suddenly in the form of a solid at a fairly high temperature above the
206
pour point. Moreover, they have the ability to rapidly construct a massive interlocking network
207
that would hinder the response of the crude to additive at a preceding stage of formation of fine
208
crystals.
209
3.3. Effect of additives on pour point of crude oil
210
Crude oil additives are chemical compounds added to base oils to impart specific properties to
211
the oils. Pour point depressants may act as anti-settling and improves the flow ability for crude
212
oil simultaneously. Although crude oil shows high specificity with respect to flow improvers,
213
there are some common structural features among these polymeric additives (Borthakur et al.,
214
1996). Our previous work (Al-Sabagh et al., 2013; Atta et al., 2013; El-Ghazawy and Farag,
215
2010) and literature (Deshmukh and Bharambe, 2008; Feng et al., 2014; Han et al., 2010;
216
Kuzmic et al., 2008; Soni et al., 2010; Zhang et al., 2014) suggest that comb like polymers with
217
side arms favoring interaction with paraffin wax of crude oil besides polar groups are capable of
218
dispersing crude wax have large potential worldwide. Matching of such side arms with paraffin
219
composition is also essential. This creates a problem for especially long-chained waxes, for
220
which it would be difficult to introduce a comb polymer (or ethylene copolymer) of sufficient
221
length to provide efficient inhibition, and also makes it important to have a range of comb
222
polymers available for treating different crudes.
223
In this study, pour point temperatures (PPT) of blank Fardos waxy crude oil and those treated
224
with prepared additives were measured and listed in Table 4. The reported data in Table 4
225
indicate that the esterified poly (α-octadecene-co-dodecyl maleates) (OM12, O2M12 and OM212)
226
have no pronounced effect on pour point of treated crude oil. This can be ascribed to the
227
structure of these additives which contains branches of 18 and 12 C atoms. Although 18 C atoms
228
in side branches may be able to co-crystallize with paraffin molecules in crude oil but these
229
branches are spaced out by shorter branches (12 C atoms) which is not large enough to induce
230
accountable spatial hindrance for paraffin molecules. This is evident through pour point
231
decreasing inability of OM212 and OM12 even at high concentrations as their structures contain
232
high ratio of dodecyl side chains. In addition, O2M12 show somehow effect on pour point at high
233
concentration (4000 ppm), a result that may be ascribed to higher content of octadecyl side
234
branches. However, increasing additive concentration than 4000 ppm is not commercially
235
acceptable and thus not considered. This runs in harmony with that reported by (Chen et al.,
236
2010) that during solidification of polymeric additive structures with pendent moieties less than
237
C16 in length, no crystalline packing of the polymers occurs that matches the crystalline packing
238
of waxes during growth, and so no interaction takes place.
239
Results in Table 4 show that the higher the length of side branches found in poly (α-octadecene-
240
co-hexadecyl maleates) and poly (α-octadecene-co-docosanyl maleates), the better the pour point
241
depression. This assumes that longer alkyl side chains become able to co-crystallize with paraffin
242
molecules of crude oil. Poly (α-octadecene-co-hexadecyl maleates) show reduction of PPT in the
243
order of OM16 > O2M16 > OM216 while that of poly (α-octadecene-co-docosanyl maleates) is
244
OM22 > OM222 > O2M22 (see Table 4). This aforementioned order of efficiency can be
245
correlated to another factor. Even though, the presence of ester group can boost the polarity of
246
additive -responsible for wax dispersion-, the balance between such groups and paraffin like
247
groups -have the ability to solubilize paraffin molecules of crude oil- is important. In other
248
words, although OM216 and OM222 possess high content of polar ester groups, they are less
249
efficient than OM16 and OM22. Therefore, it is believed that additives with higher MA content
250
(OM2) may show poor solubility in media unless high nonpolar long chains (C22) counterpart this
251
effect in OM222.
252
It should be also noted that besides length of carbon number of pendant chain and the balance
253
between hydrophobic content and polar groups of the additives, other factors can critically
254
impact PPD efficiency including melting temperature of the prepared additives and copolymer
255
molecular weight. In this context, melting point (Tm) of OM16, O2M16, OM22 and OM222
256
(additives showing best performance as PPD) were evaluated through DSC. Thermograms of the
257
selected polymers are presented in Figure 4 whereas Tm and enthalpy of melting (ΔHm) are
258
summarized in Table 5. DSC data indicate that the prepared copolymers show endothermic
259
peaks of melting temperature between 43 and 49 oC. Regarding the effectiveness of the
260
polymeric structures, it is obvious that higher pour point depression is observed for additives
261
with melting points matched well with that of the wax constituent (m.p. 48-50 oC). OM16 e.g.
262
(m.p. 48.7 oC) shows ΔPP = 24 oC at 4000 ppm, whereas O2M16 (m.p. = 43.05 °C) shows
263
significant decrease in efficiency (ΔPP = 15°C at 4000 ppm) where the m.p. of the additive
264
differs greatly from that of wax. This may be attributed to the fact that the interaction of an
265
additive with wax (adsorption or co-crystallization) takes place in the solid phase; consequently,
266
for complete interaction, their melting points must be close to each other (El-Gamal et al., 1997).
267
It should be noted that crystallinity of the additive can also affect its behavior. More crystalline
268
additive may impede co-crystallization with wax paraffin molecules. In this framework, the
269
effect of crystallinity of OM16, OM22 and OM222 (the best PPDs) on their performance is
270
regarded. Crystallinity percent (Xc) can be calculated according to equation [2]:
271
% Xc = ΔHm / ΔHm° x 100
[2]
272
The term ΔHm° is a reference value and represents the enthalpy of melting if the polymers were
273
100% crystalline and ΔHm is heat of fusion of samples. Knowing the values of ΔHm will be useful
274
as an indication for crystallinity, where the higher the value of it the higher the crystallinity. ΔHm
275
was determined by integrating the areas (Jg-1) under the endothermic peak. Melting enthalpies
276
for OM16, OM22 and OM222 were calculated from the DSC traces and shown in Table 5.
277
The effectiveness of the OM copolymers as an inhibitor is influenced greatly by the percentage
278
of α-octadecene and alkyl chains in the copolymer. Maleic anhydride content aids dispersion of
279
aliphatic long chain and lowers crystallinity, whereas the α-octadecene besides long chain grafts
280
are necessary for co-crystallization with structurally similar wax and boost crystallinity. OM16
281
and OM22 result in better PPT depression compared to O2M16 and OM222, a result that could be
282
ascribed to higher crystallinity and good packing of additives rich in α-octadecene thus poor co-
283
crystallization with crude wax. Finally, it is noteworthy that OM16 gives the best performance of
284
PPT depression among those tested samples at 2000 ppm.
285
It is worth to mention that crude oil wax content and average carbon number of wax play an
286
important role in pour point depression. Low crude oil PPT and high wax content reflect low
287
average carbon number of wax and vice versa. Comb-like NAFOL 1822 acrylate homopolymer
288
and NAFOL 2022 methacrylate-vinyl acetate copolymer suggested by El-Gamal et al. 1994
289
show good PPT depression for two Egyptian waxy crudes (PPT = 14 and 21, wax content = 21
290
and 18.7%, respectively). NAFOL 20+ ester of tetradecene – maleic anhydride copolymer
291
proposed by El-Gamal and Al-Sabbagh 1996, shows remarkable depression of PPT (>39 C at
292
1000 ppm) for Umbaraka Egyptian waxy crude oil. This notable efficiency can be attributed to
293
comparatively low average carbon number of the tested crude oil (PPT = 28 C, wax content
294
20.5 %). 10,000 ppm of styrene-octadecene maleate copolymer (Al-Sabagh et al. 2009) show 30
295
C depression in PPT of Egyptian waxy crude oil containing 20.5% wax with average carbon
296
number of 29. This extra high additive concentration cannot be used practically, thus it is
297
considered as inefficient PPD. The proposed comb-like (docosanyl acrylate-co-(octadecyl or
298
hexadecyl acrylate)-co-maleic anhydride) terpolymers with low maleic anhydride content (El-
299
Ghazawy and Farag 2010) show good efficiency as PPDs for Khalda waxy crude oil - Egypt
300
(wax content = 12%, average carbon number = 10-30) using 250 ppm. Lately, Al-Sabagh et al.
301
2013 proposed comb-like copolymers PPDs that efficiently depress PPT of Egyptian waxy crude
302
oil (wax content = 11.9% and average carbon number = 21.7). In this respect, the prepared PPDs
303
through this article can be considered as efficient PPDs where they efficiently affect PPT of a
304
waxy crude oil having high average carbon number = 40.9 and wax content = 8.4% at applicable
305
concentration of 2000 ppm .
306
307
3.4. Rheological measurements
308
Pour point does not completely describe crude oil flow properties but rheology should also be
309
considered. There is a considerable interest in developing an improved understanding of the
310
rheology of waxy crude oils (Lin et al., 2011, Livescu, 2012, Martinez-Palou et al., 2011). Under
311
low temperatures, the tendency of waxy crude oils to form a percolated gel phase may result in
312
the blocking of a flow-line that can cause a halt in production. After hydrate precipitates, wax
313
precipitates are the second most common cause for blocked flowlines in oil production scenarios.
314
The formation of these precipitates affects the flow of the resulting multiphase system. Several
315
approaches have been taken towards understanding the impact of wax precipitates on the
316
rheology of these fluids and the practical applications thereof. Rheometric studies of waxy crude
317
oils and waxy crude oil emulsions below their wax appearance temperature have been carried out
318
by (Visintin et al., 2008; 2005). The authors demonstrated that waxy crude oils exhibit a strongly
319
temperature-dependent yield stress when they are below their wax appearance temperature.
320
Other workers have shown that thermal and shear history can have a significant effect on the
321
strength of the gelled crude oil (Rønningsen, 1992; Venkatesan et al., 2005). On the other hand,
322
pretreatment of waxy crude oils with PPDs show -in most cases- an enhancement of flow
323
properties at low temperatures. Chemical structure, molecular weight and polarity of PPDs are
324
important factors that control their performance.
325
Complex rheological relationships are usually characterized by a shear rate/shear stress
326
rheogram. Rheological measurements can provide a direct account of additive performance in
327
production, transportation, and storage of oil. This in particular refers to viscosity, which
328
controls the pressure necessary to restart a pipeline or a well and corresponds to the minimum
329
shear stress (yield value, yield point). Yield value is obtained by extrapolating the shear stress to
330
zero shear rate on a linear plot. In this study, rheological behavior of untreated and additive
331
treated crude oil was determined by measuring shear stress and viscosity at varied shear rates.
332
Additives OM16, OM22 and OM222 were evaluated for their chemical structure influence on the
333
rheological behavior of Fardos crude oil at a temperature lower than blank pour point (< 27°C).
334
This selection is based on their satisfactory influence on the pour point. Shear stress-shear rate
335
data for doped crude samples with 3000 ppm OM16 at different temperatures (9, 15 & 21°C) are
336
shown in Figure 5a. It can be seen that shear stress increases sharply with increasing shear rate
337
at all tested temperatures in such a way that the cold flow pattern follows a non-Newtonian
338
pseudoplastic rheological behavior. Figure 5b presents shear rate – shear stress plots of blank
339
and treated crude oil samples with different concentrations of OM16 (1000, 3000 and 4000
340
ppm). The influence of the additives on the rheology of the crude oil is shown by the marked
341
decrease in shear stress obtained. Data of yield shear stress values were determined for blank,
342
OM16, OM22 and OM222 treated crude oils and tabulated in Table 6. The yield value of the
343
untreated crude oil was much higher than treated samples. Favorable influence on the flow
344
properties is in the order OM16 > OM22 > OM222. On the other hand, apparent viscosities
345
(mPa.S) of treated crude oils with 3000 ppm of OM16 were determined at different shear rate at
346
temperatures 9, 15 and 21 oC and presented in Figure 6a. A decrease in apparent viscosity was
347
observed with increasing temperature conveying non-Newtonian behavbior of treated crude oil
348
samples. The decrease in viscosity, when the shear rate is increased, is more evident at low
349
temperatures because of the larger polymer molecular orientation, resulting from larger shear
350
forces acting on the polymer and the crude oil under these conditions. At high shear rates the
351
apparent viscosity reached a constant value at which the equilibrium steady state was attained.
352
The effect of increasing OM16 concentration (1000, 3000 and 4000 ppm) on apparent viscosity
353
is shown in Figure 6b. High reduction in apparent viscosity was observed upon increasing
354
OM16 concentration from 1000 ppm to 3000 ppm while insignificant decrease was observed
355
when the concentration was raised to 4000 ppm. Thus a concentration of 3000 ppm was selected
356
for comparing rheological properties. Plastic viscosity data are presented in Table 6. The high
357
plastic viscosity of blank crude oil was effectively ceased upon treating with 3000 ppm of
358
OM16, OM22 and OM222. OM16 was the most effective rheologically.
359
It is also observed that there is a correlation between the effect of the additives on pour point and
360
the two rheological parameters (see Tables 4 & 6). Additives that promote pour point depression
361
also produce the highest reduction in viscosity even at low temperatures. This is clearly
362
associated with the additive’s ability to provide formation of small wax crystals dispersed in the
363
solvent medium. This behavior can be also correlated to crystallinity of PPDs through enthalpy
364
of melting (ΔHm). A strong composition-dependence of crystallinity is clearly observed for
365
varied MA content in the copolymers. Melting enthalpies for OM16, OM22 and OM222 are
366
37.48, 43.26 and 80 Jg-1, respectively. This high ΔHm exhibited by OM222 reflects its high
367
crystallinity with respect to OM16 and OM22. Highly crystalline polymers can form perfect
368
crystals that hinders the conformational freedom of the backbone. On the other hand, lower
369
crystalline OM16 and OM22 can disperse polar asphaltenes in crude oil through polar carboxyl
370
and ester groups and prevent them from aggregation and increase the possibility of co-
371
crystallization with crude oil wax. This in turn explains less efficiency of OM222 as flow
372
improver with regard to OM16 and OM22.
373 374
4. Conclusions
375
The following conclusions can be represented from this work:
376
1. Three comb like copolymers of octadecene -co- maleic anhydride (OM, OM2 and O2M) were
377
chemically modified by inserting different lengths of long hydrocarbon chains to produce
378
structures with different molecular weights and melting temperatures.
379
2. The efficiency of flow improvers with structures that matching with wax structure are
380
critically affected by number of carbon atoms in the pendant chain, balance between
381
hydrophobic content and polar groups of the additive, copolymer molecular weight and
382
composition, melting temperature of the prepared additive and crystallinity.
383 384 385
3. The prepared PPD exhibit dual function both as pour point depressant and flow improvers simultaneously at applicable dose (2000 ppm).
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Table 1: The Physicochemical properties of Fardos crude oil Test
Method
Value
API gravity at 60 oF
ASTMD-1298
41.1
SPECIFIC GRAVITY @ 60/60 oF
ASTM D-1298
0.820
Wax content (Wt. %)
UOP 46/64
8.4
Asphaltene content, (Wt. %)
IP 143/84
3
Water content (vol. %)
IP 74/70
0.23
KINEMATIC VISCOSITY (cSt)@
ASTM D-445
Pour Point oC 387 388
50oC
7
60 oC
4.3 ASTM-D97
30
389
Table 2: Molecular weight data of OM based on GPC analysis GPC data Sample Mn, (g/mol)
Mw, (g/mol)
PD
OM
49710
117762
2.369
O2M
37954
72287
1.905
OM2
35096
91556
2.610
390 391
Table 3: Carbon number distribution of n-Paraffin fraction separated from Fardos crude No. of Moles Carbon No. Wt% Mol Wt.
No. of Moles Carbon No. Wt%
Mol Wt.
-4
x10-4
x10 23
0.10
324
3.08
43
6.20
604
102.6
25
0.50
352
14.01
44
0.04
618
0.647
27
0.05
380
1.32
45
5.06
632
80.06
28
1.07
394
27.01
46
3.23
646
50.00
30
1.40
422
33.17
47
0.08
660
1.212
31
0.10
436
2.29
48
2.20
674
32.64
32
1.64
450
36.40
49
1.50
688
21.80
33
0.11
464
2.37
50
1.14
702
16.24
34
1.74
478
36.40
51
0.80
716
11.173
35
0.02
492
0.406
52
0.76
730
10.41
36
2.17
506
42.88
53
0.61
744
8.199
37
0.06
520
1.15
54
0.36
758
4.74
38
3.60
534
67.41
56
0.32
786
4.07
40
4.73
562
84.16
57
0.020
800
0.25
41
6.40
576
111.1
59
0.014
828
0.17
392
Total wt% = 46.33, Total no. of moles = 807.367 x 104
393
Average molecular weight = 46.33 /807 x 104 = 574= CnH2n+2, i.e. 12n + 2n + 2 =574; hence
394
average carbon number (n) =40.9
395
Table 4: Pour point temperatures (PPT) of treated Fardos crude oil at different concentrations of esterified poly (α-octadecene –co- maleate esters)
396
OM esters
397 398 399
O2M esters
OM2 esters
Conc. (ppm)
1000 2000 3000 4000 1000 2000 3000 4000 1000
2000 3000 4000
OM12
27
27
27
27
27
27
24
21
27
27
27
27
OM16
15
6
6
3
21
15
15
12
27
24
24
21
OM22
15
12
12
9
21
18
18
15
18
15
12
12
PPT for blank Fardos crude oil = 27 °C
400
Table 5: Melting temperatures and melting enthalpies for OM16, OM216, OM22 and
401
OM222
Sample
Tm (⁰C)
ΔHm (Jg-1)
OM16
49
37.48
O2M16
43.55
35.88
OM22
50.51
80
OM222
51.31
43.26
402 403
Table 6: Yield values of untreated and treated crude oil with 3000 ppm of selected additives Oil sample
T⁰C
Yield value, (mPa)
Untreated
Correlation
(mPa.S)
Coefficient
12
8489
2940
0.923
15
7393
1955
0.902
21
4897
1337
0.911
9
1.62
65.2
0.989
15
1.05
60.3
0.991
21
0.622
50.4
0.992
OM22
9
3.83
116.23
0.993
OM222
9
8.62
122.4
0.995
OM16
404
Plastic viscosity,
405 406 407 408 409
Highlights: • • •
Comb like copolymers have dual function: flow improver and pour point depressant. Crystallinity and melting point of flow improver are controlling factors. Balance in nonpolar-polar content show significant effect on pour point depression.
410 411
Figure 1:1HNMR for different compositions of poly (α-octadecene-co-maleic anhydride) (a)
412
OM, (b) OM2 and (c) O2M
(a)
413
(b)
414
(c)
415 416 417 418 419 420 421 422
Figure 2: Chromatogram of paraffins extracted from Fardos Crude oil.
Peak area
423
424 Time (min.)
425 426
Figure 3: Relation of n-paraffin carbon number distribution versus weight percentages of
427
separated wax of Fardos crude oil.
428 429
430 431
Figure 4: DSC thermograms of, a) O2M16 and b) OM16 c) OM222, d) OM22 copolymers
a)
c) 432 433 434 435 436
b)
d)
437
Figure 5: The shear stress-shear rate relationship for treated crude oil with OM16 additive
438
at a) different temperatures at 3000 ppm and b) different concentrations and untreated
439
crude at 9 C
a)
440
b)
441 442 443 444 445 446 447 448
449
Figure 6: Viscosity-shear rate relationship for OM16 treated crude oil at a) different
450
temperatures b) different concentrations at temperature of 12 C and for untreated crude
451
oil
a)
452 453
454
Modified maleic anhydride-co-octadecene Copolymers as flow Improver for Waxy Egyptian Crude Oil Rasha A. El-Ghazawy, Ayman M. Atta, Khalid I. Kabel
www.elsevier.com/locate/petrol
PII: DOI: Reference:
S0920-4105(14)00239-3 http://dx.doi.org/10.1016/j.petrol.2014.07.040 PETROL2752
To appear in:
Journal of Petroleum Science and Engineering
Received date: 7 April 2014 Accepted date: 31 July 2014 Cite this article as: Rasha A. El-Ghazawy, Ayman M. Atta, Khalid I. Kabel, Modified maleic anhydride-co-octadecene Copolymers as flow Improver for Waxy Egyptian Crude Oil, Journal of Petroleum Science and Engineering, http://dx. doi.org/10.1016/j.petrol.2014.07.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Abstract
2
Polymeric flow improvers represent one of the most important chemicals used in oil industry
3
especially with waxy crudes. The chemical structure of such chemicals is particularly important.
4
In this study, we prepared a family of additives by chemical modification of octadecene-co-
5
maleic anhydride copolymers (OM). The copolymerization of octadecene (OD) with maleic
6
anhydride (MA) was performed at variable ratios of OD: MA (1:1, 1:2 and 2:1). OM copolymers
7
were chemically modified by grafting three different alcohols dodecanol, hexadecanol or
8
docosanol. Grafts were characterized through molecular weight using GPC, melting temperature
9
and enthalpy of melting by DSC and chemical structure through 1HNMR. The comb like
10
copolymers were assessed as flow improver for an Egyptian waxy crude oil by measuring their
11
performance as pour point depressant in addition to their assessment as viscosity improver
12
through rheological behavior. The data indicate that, the additive must retain a hydrophobic-
13
lypophobic balance, low crystallinity and matching melting temperature with that of crude wax
14
in order to inhibit the paraffin and wax crystal depositions.
15
Modified Maleic Anhydride-co-Octadecene Copolymers as Flow
16
Improver for Waxy Egyptian Crude Oil
17
Rasha A. El-Ghazawya,*, Ayman M. Attaa, b and Khalid I. Kabela
18 19 20
a
Department of Petroleum Applications, Egyptian Petroleum Research Institute, Nasr city, P.O.
11727, Cairo, Egypt b
Surfactants research chair, Chemistry Department, College of Science, King Saud University,
21
P.O. 2455, Riyadh - 11451, Saudi Arabia
22
*Corresponding author: Tel.: +20 1 227284228; fax: +20 2 22747433.
E-mail address: [email protected]
23 24
Abstract
25
Polymeric flow improvers represent one of the most important chemicals used in oil industry
26
especially with waxy crudes. The chemical structure of such chemicals is particularly important.
27
In this study, we prepared a family of additives by chemical modification of octadecene-co-
28
maleic anhydride copolymers (OM). The copolymerization of octadecene (OD) with maleic
29
anhydride (MA) was performed at variable ratios of OD: MA (1:1, 1:2 and 2:1). OM copolymers
30
were chemically modified by grafting three different alcohols dodecanol, hexadecanol or
31
docosanol. Grafts were characterized through molecular weight using GPC, melting temperature
32
and enthalpy of melting by DSC and chemical structure through 1HNMR. The comb like
33
copolymers were assessed as flow improver for an Egyptian waxy crude oil by measuring their
34
performance as pour point depressant in addition to their assessment as viscosity improver
35
through rheological behavior. The data indicate that, the additive must retain a hydrophobic-
36
lypophobic balance, low crystallinity and matching melting temperature with that of crude wax
37
in order to inhibit the paraffin and wax crystal depositions.
38 39
Keywords: Flow improver, waxy crude oil, rheology, octadecene‐co‐maleic anhydride, pour point
40
depressants.
1. Introduction
41
From the major problems arising through production, transportation and storage of petroleum
42
crude oil are wax and paraffin depositions, especially when the crude oil has a high content of n-
43
aliphatic hydrocarbon chains that induce, by temperature change, crystallization of n-paraffins
44
(Paso and Fogler, 2004; Wu et al., 2002). When the absorption of both wax and paraffin particles
45
is enough to constitute a stable three-dimensional network within the crude oil system, crude oil
46
reaches its pour point, the minimum temperature at which the oil flows under the influence of
47
gravity. Several kinds of treatments (mechanical, physical and chemical) have been applied to
48
control paraffin separation and its adverse consequences (Ribeiro et al., 1997). Chemical
49
treatments by flow improvers, crystal modifiers or pour point depressant, are used to reduce
50
apparent viscosity, yield point and pour point of crude oil (Dong et al., 2001; Pedersen and
51
Ronningsen, 2003).
52
Two main architectural copolymer classes are commonly used as pour point depressants (PPDs)
53
in industrial formulations: (1) linear ethylene copolymers and (2) comb-like copolymers. In this
54
respect, polymers with linear crystalline/amorphous diblock structures, such as ethylene-vinyl
55
acetate copolymer (Qian et al., 1996; Taraneh et al., 2008), poly (ethylene–butene) (PEB)
56
(Ashbaugh et al. 2002), and comb-type such as poly(maleic anhydride-co-α-olefin) esters (El-
57
Gamal et al., 1992) and amides (Xu et al., 2009) have been used to improve the cold flowing
58
ability of waxy oils by modifying paraffin crystallization.
59
The commercial grades of ethylene/vinyl acetate (EVA) co-polymers have found application as
60
“pour point” depressants in refined fuels. Ashbaugh et al. 2005 focused on EVA behavior as
61
crude oil additives (using model crude oils consisted of 4 wt % wax in decane), to reduce yield
62
stress of the gels that can form when the oil exits the reservoir. They found that EVA dosage
63
levels of ~200 ppm, the reduction in yield stress is 3 orders of magnitude for the C36 wax,
64
whereas the reduction is 1 order of magnitude for C32 and only 3-fold for the C28 wax. This
65
decrease in efficiency with decreasing wax carbon number indicates that the EVA materials
66
would not provide an adequate reduction in yield stress to ensure against gelation in pipeline
67
transport. Taraneh et al. 2008 studied the influence of EVA with different ranges of molecular
68
weight as flow improver for five Iranian waxy crude oils. Results show good depression of pour
69
point but weak efficiency in reducing viscosity and yield values. Performance evaluation of EVA
70
graft copolymers with long side chains (comb-like copolymers) and nitrogen polar groups was
71
demonstrated as pour point depressants for Daqing waxy crude oil (Li-juan et al. 2008). The
72
modified EVAs was found to be better than EVA in pour point depression.
73
The advantages of using of comb-type copolymers as flow improver for crude oil are to control
74
the crystallization of paraffins and prevent asphaltenes from aggregation based on the relation
75
between nonpolar side alkyl chain and polar group (such as carboxylic, ester, amide groups
76
etc.,). Their nonpolar long side alkyl chains can self-assemble to nucleate the long-chain paraffin
77
crystallization or co-crystallize with paraffins to modulate the wax crystals. Moreover, different
78
factors may affect the performance of additives in wax dispersion including the solubility of
79
polymeric additives in different solvents (Mansur et al., 2006; Qian et al., 1996), their thermal
80
behavior and molecular weight (Borthakur et al., 1996). Evaluating rheology of treated waxy
81
crude oil samples is a complementary study that attracts particular interest (Barbato et al., 2014;
82
Marchesini et al., 2012; Teng and Zhang, 2013; Visintin et al., 2005; Zhang and Liu, 2008). The
83
disadvantages of comb-like copolymers may be an increase in the cost of the final product and
84
those dependent on maleic anhydride or acid functionalities may create a potential for a corrosive
85
environment. Eventually, the choice of pour point depressant type is mainly controlled by
86
availability of starting materials and techniques where linear ethylene copolymers require special
87
complicated techniques.
88
Crude oils of western desert in Egypt are mainly wax rich crudes. Treating of such crudes with
89
flow improvers attracts attention of scientific research. El-Gamal and Al-Sabbagh 1996 have
90
prepared comb-like copolymers based on α-olefin with different alkyl chains and maleic
91
anhydride (OMACs). Their esters with NAFOL 20+ and propoxylated tetraethylene pentamine
92
adduct and their amides with tetraethylene pentamine were tested as flow improvers for Egyptian
93
waxy crude oil (Umbaraka) and gas oil blend. A distinguishable depression of pour point was
94
achieved reaching 39
95
copolymers derived from styrene- maleic anhydride for improving flow ability of Qarun waxy
96
crude oil (western desert – Egypt). Improved pour point and rheology were achieved but with
97
high dose of 10,000 ppm. El-Ghazawy and Farag 2010 studied the effect of (docosanyl acrylate-
98
co-(octadecyl or hexadecyl acrylate)-co-maleic anhydride) terpolymers as flow improvers for
99
two Egyptian waxy crude oils. Data show good ability of some terpolymers to disperse wax
100
crystals and improve flow behaviors of the tested crude oils even at low temperatures below their
101
pour points. Comb-like phthalimide acrylate and succinimide acrylate copolymers with vinyl
102
acetate, styrene or methyl methacrylate were evaluated as flow improver for Egyptian waxy
103
crude oil (Al-Sabagh et al. 2013). Remarkable depression of pour point was observed besides
104
improved rheological properties at 1000 ppm.
105
At present, more and more researchers are dedicated to explore effective polymer additives for
106
preservation and transportation of crude oils (Atta et al., 2008a; Borthakur et al., 1996; Castro et
107
al., 2011; Deshmukh and Bharambe, 2014; El-Gamal et al., 1997, 1994; Soni et al., 2008;
108
Taraneh et al., 2008).
109
In this work, we synthesized different copolymer compositions of modified comb-type poly (α-
110
octadecene–co-alkyl maleate esters) with varied pendant alkyl chain lengths viz. 12, 16 or 22.
111
The effect of copolymer composition, molecular weight, melting point and length of alkyl
112
substituents on the crystallization of long-chain n-paraffins upon cooling model waxy crude oil
113
were observed with regard to pour point depression and rheology.
114
C at 1000 ppm dose. Al-Sabagh et al. 2009 prepared comb-like
115
2. Experimental
116
2.1. Materials
117
Maleic anhydride (MA), α-octadecene (OD), azobisisobutyronitrile (AIBN), 1-dodecanol, 1-
118
hexadecanol, 1-docosanol and p-toluene sulfonic acid (PTSA) were purchased as analytical
119
grade from Aldrich Chemicals Co. (Germany). Methanol, tetrahydrofuran, n-hexane and toluene
120
were obtained from Merck.
121
Egyptian Waxy crude oil produced from Norpetco (Egypt, eastern desert) was delivered without
122
treatment from Fardos field. The physicochemical characteristics and composition of Fardos
123
mixed crude oils are listed in Table 1.
124
2.2. Copolymerization of α-octadecene and maleic anhydride
125
α-Octadecene - maleic anhydride copolymers were prepared by copolymerizing OD and MA in
126
different molar feed ratios, OD:MA (1:1), (1:2), and (2:1). Copolymerization was performed
127
according to procedures described by (Davies et al., 2002). Three different copolymers of
128
octadecyl-co-maleic anhydride were designated as OM, O2M and OM2 for OD: MA molar ratio
129
of 1:1, 2:1 and 1:2, respectively.
130
2.3. Esterification of the prepared copolymers with different alcohols
131
The esterification reaction of the prepared copolymers was carried out using 0.01 mol of the
132
copolymer solution in toluene, with one of the previously described ratios, and 0.02 mol of
133
alcohol (1-dodecanol, 1-hexadecanol or 1-docosanol) refluxed in presence of 0.1 (wt%) PTSA
134
catalyst. The reaction was carried out at the refluxing temperature until the theoretical amount of
135
water was collected azeotropically. The resulting esters were washed out with water to remove
136
the catalyst. Purification of the raw products was carried out by pouring in an excess of
137
methanol, followed by filtration, vacuum drying, and washing three times with hot water.
138
Esterified samples were coded as OMn, OM2n and O2Mn, where n represents the number of
139
carbons of the alkyl group for the used alcohol (n = 12, 16 or 22).
2.4. Characterization of the prepared additives and crude oil
140 141
1
HNMR analysis was carried out on Varian NMR 300 MHz spectrometer using deutrated DMSO
142
for determining copolymers’ compositions for different feed ratios. Comparing the integrals of
143
maleic anhydride moiety (m1) and α-octadecene (m2) methylene group regions in the spectra of
144
the copolymer was done according to the Eq. (1).
145
m1 / m2 = n2. Am1 / n1. Am2
(1)
146
where Am1 and Am2 are the normalized areas per H from the corresponding functional groups of
147
the monomer unit regions in 1HNMR spectra and n1 and n2 are integers of proton(s) in the
148
corresponding functional group. The molecular weight of the prepared additives were
149
characterized (in terms of Mw and Mn) and polydispersity index using Shimadzu's gel permeation
150
chromatograph (GPC) equipped with refractive index detector and polydivinylbenzene mix gel-
151
D column using tetrahydrofuran as an eluent and polystyrene as a standard.
152
The specific gravity and kinematic viscosity of crude oil were determined according to ASTM-D
153
287 and ASTM-D 445 methods, respectively. Wax and asphaltene contents were determined
154
according to the UOP-46 norm and the ASTM-D 2007 method, respectively. The carbon
155
distribution number of separated wax was determined using GC-Mass spectrometer.
156
The treated and untreated crude oil samples were evaluated with regard to their pour points
157
according to the ASTM-D 97 using Seta Cloud and Pour Point Cryostat, Stanhope-Seta, U.K.
158
method without reheating to 46
159
2000, 3000 and 4000 ppm). In addition, viscosity and flow curves (Rheogram) for treated and
C at different concentrations of the prepared additives (1000,
160
blank crude oil samples were measured using a Brookfield Viscometer equipped with
161
thermostated cooling system. Rheological measurements were performed at different
162
temperatures (9, 15 and 21 C) according to Norpetco Co. recommendation to mime winter
163
transportation and storage conditions.
164
3. Results and discussions
165
3.1. Characterization of the prepared additives
166
Comb like poly (α-octadecene-co-maleate esters) were prepared through this work with different
167
maleate contents and varied grafted alkyl chain lengths to study the effect of polar carboxylate
168
groups’ inclusion and pendant chain lengths on flowability of high waxy crude oil. 1HNMR
169
spectroscopic analysis has been established as a powerful tool for the determination of
170
copolymer compositions because of its simplicity, rapidity, and sensitivity (Dincer et al., 2002;
171
Kesim et al., 2003; Nicolescu et al., 2009). Here, the copolymer composition of OM, O2M and
172
OM2 was confirmed by 1HNMR. Comparing peak integral at 3.3 ppm assigned for methine
173
group of succinic anahydride to that at 1.18 ppm ascribed to pendant methylene groups of
174
α-octadecene moiety (using Eq. (1)) confirms the efficient copolymerization of OD and MA
175
with the stated feed ratios (see Figure 1 a-c). In addition, vanishing peak integrals at about 7
176
ppm (assigned to MA protons) and 4.9 and 5.8 ppm (assigned to double bond protons of OD)
177
confirm efficient copolymerization.
178
The number average molecular weights (Mn), weight average molecular weights (Mw) and
179
polydispersity (PD= Mw/Mn) for OM, OM2 and O2M were determined by GPC and listed in
180
Table 2. The data indicate that the molecular weight decreases with increasing MA or OD
181
contents and the PD decreases with increasing MA content. Such low molecular weights and
182
high PD of OM, O2M and OM2 suggest the suitability of the corresponding esters to be applied
183
as pour point depressant additives (Castro et al., 2011; El-Gamal et al., 1997, 1994). Literature
184
survey proved that the molecular weight of polymeric additives affects their performance as flow
185
improver and it is reported that the optimum number average molecular weight for polymeric
186
additives ranges from 20,000 -100,000 g/mol.
187
Esterification % was calculated experimentally through evaluating the amount of collected water
188
during esterification and it ranges from 85 – 93%. The data indicate that the esterification %
189
increases in the order OM > O2M > OM2. This may be rationalized by the large difference in
190
polarity and compatibility between reactants which is a factor that may affect the reactivity of the
191
functional groups (Atta et al., 2008a, b; Wesslen B. and Wesslen K.B., 1989).
192
3.2. Isolation and characterization of paraffin from crude
193
Wax crude oils have a complex composition, comprising wax, resins and asphaltenes that
194
drastically affect their cold flow properties. Hence these components were quantified in the
195
investigated Fardos crude oil. Melting point of the isolated wax was measured as 48-50 °C. It
196
was then subjected to gas chromatographic analysis to determine the distribution of n-paraffins
197
by carbon number and the average carbon number. The results are given in Table 3 and
198
illustrated in Figure 2. The data indicate that the average carbon number distribution is 40.9 with
199
concentration of about 50 wt % of the n-paraffin content having a broad distribution in crude oil
200
expressed in Wh/2 (see Figure 3). Two types of signals, Figure 2, can be distinguished: sharp
201
peaks, attributed to the n- and iso-paraffins, and a hump attributed to elution of cyclic paraffins.
202
Inspecting the overlaid chromatograms, it is clear that there is a retention time shift in the cyclo-
203
and iso-paraffins fraction where one or more compounds from these classes are eluted between
204
two intense peaks corresponding to the n-paraffins present in the whole saturates fraction. Such
205
paraffins tend to precipitate suddenly in the form of a solid at a fairly high temperature above the
206
pour point. Moreover, they have the ability to rapidly construct a massive interlocking network
207
that would hinder the response of the crude to additive at a preceding stage of formation of fine
208
crystals.
209
3.3. Effect of additives on pour point of crude oil
210
Crude oil additives are chemical compounds added to base oils to impart specific properties to
211
the oils. Pour point depressants may act as anti-settling and improves the flow ability for crude
212
oil simultaneously. Although crude oil shows high specificity with respect to flow improvers,
213
there are some common structural features among these polymeric additives (Borthakur et al.,
214
1996). Our previous work (Al-Sabagh et al., 2013; Atta et al., 2013; El-Ghazawy and Farag,
215
2010) and literature (Deshmukh and Bharambe, 2008; Feng et al., 2014; Han et al., 2010;
216
Kuzmic et al., 2008; Soni et al., 2010; Zhang et al., 2014) suggest that comb like polymers with
217
side arms favoring interaction with paraffin wax of crude oil besides polar groups are capable of
218
dispersing crude wax have large potential worldwide. Matching of such side arms with paraffin
219
composition is also essential. This creates a problem for especially long-chained waxes, for
220
which it would be difficult to introduce a comb polymer (or ethylene copolymer) of sufficient
221
length to provide efficient inhibition, and also makes it important to have a range of comb
222
polymers available for treating different crudes.
223
In this study, pour point temperatures (PPT) of blank Fardos waxy crude oil and those treated
224
with prepared additives were measured and listed in Table 4. The reported data in Table 4
225
indicate that the esterified poly (α-octadecene-co-dodecyl maleates) (OM12, O2M12 and OM212)
226
have no pronounced effect on pour point of treated crude oil. This can be ascribed to the
227
structure of these additives which contains branches of 18 and 12 C atoms. Although 18 C atoms
228
in side branches may be able to co-crystallize with paraffin molecules in crude oil but these
229
branches are spaced out by shorter branches (12 C atoms) which is not large enough to induce
230
accountable spatial hindrance for paraffin molecules. This is evident through pour point
231
decreasing inability of OM212 and OM12 even at high concentrations as their structures contain
232
high ratio of dodecyl side chains. In addition, O2M12 show somehow effect on pour point at high
233
concentration (4000 ppm), a result that may be ascribed to higher content of octadecyl side
234
branches. However, increasing additive concentration than 4000 ppm is not commercially
235
acceptable and thus not considered. This runs in harmony with that reported by (Chen et al.,
236
2010) that during solidification of polymeric additive structures with pendent moieties less than
237
C16 in length, no crystalline packing of the polymers occurs that matches the crystalline packing
238
of waxes during growth, and so no interaction takes place.
239
Results in Table 4 show that the higher the length of side branches found in poly (α-octadecene-
240
co-hexadecyl maleates) and poly (α-octadecene-co-docosanyl maleates), the better the pour point
241
depression. This assumes that longer alkyl side chains become able to co-crystallize with paraffin
242
molecules of crude oil. Poly (α-octadecene-co-hexadecyl maleates) show reduction of PPT in the
243
order of OM16 > O2M16 > OM216 while that of poly (α-octadecene-co-docosanyl maleates) is
244
OM22 > OM222 > O2M22 (see Table 4). This aforementioned order of efficiency can be
245
correlated to another factor. Even though, the presence of ester group can boost the polarity of
246
additive -responsible for wax dispersion-, the balance between such groups and paraffin like
247
groups -have the ability to solubilize paraffin molecules of crude oil- is important. In other
248
words, although OM216 and OM222 possess high content of polar ester groups, they are less
249
efficient than OM16 and OM22. Therefore, it is believed that additives with higher MA content
250
(OM2) may show poor solubility in media unless high nonpolar long chains (C22) counterpart this
251
effect in OM222.
252
It should be also noted that besides length of carbon number of pendant chain and the balance
253
between hydrophobic content and polar groups of the additives, other factors can critically
254
impact PPD efficiency including melting temperature of the prepared additives and copolymer
255
molecular weight. In this context, melting point (Tm) of OM16, O2M16, OM22 and OM222
256
(additives showing best performance as PPD) were evaluated through DSC. Thermograms of the
257
selected polymers are presented in Figure 4 whereas Tm and enthalpy of melting (ΔHm) are
258
summarized in Table 5. DSC data indicate that the prepared copolymers show endothermic
259
peaks of melting temperature between 43 and 49 oC. Regarding the effectiveness of the
260
polymeric structures, it is obvious that higher pour point depression is observed for additives
261
with melting points matched well with that of the wax constituent (m.p. 48-50 oC). OM16 e.g.
262
(m.p. 48.7 oC) shows ΔPP = 24 oC at 4000 ppm, whereas O2M16 (m.p. = 43.05 °C) shows
263
significant decrease in efficiency (ΔPP = 15°C at 4000 ppm) where the m.p. of the additive
264
differs greatly from that of wax. This may be attributed to the fact that the interaction of an
265
additive with wax (adsorption or co-crystallization) takes place in the solid phase; consequently,
266
for complete interaction, their melting points must be close to each other (El-Gamal et al., 1997).
267
It should be noted that crystallinity of the additive can also affect its behavior. More crystalline
268
additive may impede co-crystallization with wax paraffin molecules. In this framework, the
269
effect of crystallinity of OM16, OM22 and OM222 (the best PPDs) on their performance is
270
regarded. Crystallinity percent (Xc) can be calculated according to equation [2]:
271
% Xc = ΔHm / ΔHm° x 100
[2]
272
The term ΔHm° is a reference value and represents the enthalpy of melting if the polymers were
273
100% crystalline and ΔHm is heat of fusion of samples. Knowing the values of ΔHm will be useful
274
as an indication for crystallinity, where the higher the value of it the higher the crystallinity. ΔHm
275
was determined by integrating the areas (Jg-1) under the endothermic peak. Melting enthalpies
276
for OM16, OM22 and OM222 were calculated from the DSC traces and shown in Table 5.
277
The effectiveness of the OM copolymers as an inhibitor is influenced greatly by the percentage
278
of α-octadecene and alkyl chains in the copolymer. Maleic anhydride content aids dispersion of
279
aliphatic long chain and lowers crystallinity, whereas the α-octadecene besides long chain grafts
280
are necessary for co-crystallization with structurally similar wax and boost crystallinity. OM16
281
and OM22 result in better PPT depression compared to O2M16 and OM222, a result that could be
282
ascribed to higher crystallinity and good packing of additives rich in α-octadecene thus poor co-
283
crystallization with crude wax. Finally, it is noteworthy that OM16 gives the best performance of
284
PPT depression among those tested samples at 2000 ppm.
285
It is worth to mention that crude oil wax content and average carbon number of wax play an
286
important role in pour point depression. Low crude oil PPT and high wax content reflect low
287
average carbon number of wax and vice versa. Comb-like NAFOL 1822 acrylate homopolymer
288
and NAFOL 2022 methacrylate-vinyl acetate copolymer suggested by El-Gamal et al. 1994
289
show good PPT depression for two Egyptian waxy crudes (PPT = 14 and 21, wax content = 21
290
and 18.7%, respectively). NAFOL 20+ ester of tetradecene – maleic anhydride copolymer
291
proposed by El-Gamal and Al-Sabbagh 1996, shows remarkable depression of PPT (>39 C at
292
1000 ppm) for Umbaraka Egyptian waxy crude oil. This notable efficiency can be attributed to
293
comparatively low average carbon number of the tested crude oil (PPT = 28 C, wax content
294
20.5 %). 10,000 ppm of styrene-octadecene maleate copolymer (Al-Sabagh et al. 2009) show 30
295
C depression in PPT of Egyptian waxy crude oil containing 20.5% wax with average carbon
296
number of 29. This extra high additive concentration cannot be used practically, thus it is
297
considered as inefficient PPD. The proposed comb-like (docosanyl acrylate-co-(octadecyl or
298
hexadecyl acrylate)-co-maleic anhydride) terpolymers with low maleic anhydride content (El-
299
Ghazawy and Farag 2010) show good efficiency as PPDs for Khalda waxy crude oil - Egypt
300
(wax content = 12%, average carbon number = 10-30) using 250 ppm. Lately, Al-Sabagh et al.
301
2013 proposed comb-like copolymers PPDs that efficiently depress PPT of Egyptian waxy crude
302
oil (wax content = 11.9% and average carbon number = 21.7). In this respect, the prepared PPDs
303
through this article can be considered as efficient PPDs where they efficiently affect PPT of a
304
waxy crude oil having high average carbon number = 40.9 and wax content = 8.4% at applicable
305
concentration of 2000 ppm .
306
307
3.4. Rheological measurements
308
Pour point does not completely describe crude oil flow properties but rheology should also be
309
considered. There is a considerable interest in developing an improved understanding of the
310
rheology of waxy crude oils (Lin et al., 2011, Livescu, 2012, Martinez-Palou et al., 2011). Under
311
low temperatures, the tendency of waxy crude oils to form a percolated gel phase may result in
312
the blocking of a flow-line that can cause a halt in production. After hydrate precipitates, wax
313
precipitates are the second most common cause for blocked flowlines in oil production scenarios.
314
The formation of these precipitates affects the flow of the resulting multiphase system. Several
315
approaches have been taken towards understanding the impact of wax precipitates on the
316
rheology of these fluids and the practical applications thereof. Rheometric studies of waxy crude
317
oils and waxy crude oil emulsions below their wax appearance temperature have been carried out
318
by (Visintin et al., 2008; 2005). The authors demonstrated that waxy crude oils exhibit a strongly
319
temperature-dependent yield stress when they are below their wax appearance temperature.
320
Other workers have shown that thermal and shear history can have a significant effect on the
321
strength of the gelled crude oil (Rønningsen, 1992; Venkatesan et al., 2005). On the other hand,
322
pretreatment of waxy crude oils with PPDs show -in most cases- an enhancement of flow
323
properties at low temperatures. Chemical structure, molecular weight and polarity of PPDs are
324
important factors that control their performance.
325
Complex rheological relationships are usually characterized by a shear rate/shear stress
326
rheogram. Rheological measurements can provide a direct account of additive performance in
327
production, transportation, and storage of oil. This in particular refers to viscosity, which
328
controls the pressure necessary to restart a pipeline or a well and corresponds to the minimum
329
shear stress (yield value, yield point). Yield value is obtained by extrapolating the shear stress to
330
zero shear rate on a linear plot. In this study, rheological behavior of untreated and additive
331
treated crude oil was determined by measuring shear stress and viscosity at varied shear rates.
332
Additives OM16, OM22 and OM222 were evaluated for their chemical structure influence on the
333
rheological behavior of Fardos crude oil at a temperature lower than blank pour point (< 27°C).
334
This selection is based on their satisfactory influence on the pour point. Shear stress-shear rate
335
data for doped crude samples with 3000 ppm OM16 at different temperatures (9, 15 & 21°C) are
336
shown in Figure 5a. It can be seen that shear stress increases sharply with increasing shear rate
337
at all tested temperatures in such a way that the cold flow pattern follows a non-Newtonian
338
pseudoplastic rheological behavior. Figure 5b presents shear rate – shear stress plots of blank
339
and treated crude oil samples with different concentrations of OM16 (1000, 3000 and 4000
340
ppm). The influence of the additives on the rheology of the crude oil is shown by the marked
341
decrease in shear stress obtained. Data of yield shear stress values were determined for blank,
342
OM16, OM22 and OM222 treated crude oils and tabulated in Table 6. The yield value of the
343
untreated crude oil was much higher than treated samples. Favorable influence on the flow
344
properties is in the order OM16 > OM22 > OM222. On the other hand, apparent viscosities
345
(mPa.S) of treated crude oils with 3000 ppm of OM16 were determined at different shear rate at
346
temperatures 9, 15 and 21 oC and presented in Figure 6a. A decrease in apparent viscosity was
347
observed with increasing temperature conveying non-Newtonian behavbior of treated crude oil
348
samples. The decrease in viscosity, when the shear rate is increased, is more evident at low
349
temperatures because of the larger polymer molecular orientation, resulting from larger shear
350
forces acting on the polymer and the crude oil under these conditions. At high shear rates the
351
apparent viscosity reached a constant value at which the equilibrium steady state was attained.
352
The effect of increasing OM16 concentration (1000, 3000 and 4000 ppm) on apparent viscosity
353
is shown in Figure 6b. High reduction in apparent viscosity was observed upon increasing
354
OM16 concentration from 1000 ppm to 3000 ppm while insignificant decrease was observed
355
when the concentration was raised to 4000 ppm. Thus a concentration of 3000 ppm was selected
356
for comparing rheological properties. Plastic viscosity data are presented in Table 6. The high
357
plastic viscosity of blank crude oil was effectively ceased upon treating with 3000 ppm of
358
OM16, OM22 and OM222. OM16 was the most effective rheologically.
359
It is also observed that there is a correlation between the effect of the additives on pour point and
360
the two rheological parameters (see Tables 4 & 6). Additives that promote pour point depression
361
also produce the highest reduction in viscosity even at low temperatures. This is clearly
362
associated with the additive’s ability to provide formation of small wax crystals dispersed in the
363
solvent medium. This behavior can be also correlated to crystallinity of PPDs through enthalpy
364
of melting (ΔHm). A strong composition-dependence of crystallinity is clearly observed for
365
varied MA content in the copolymers. Melting enthalpies for OM16, OM22 and OM222 are
366
37.48, 43.26 and 80 Jg-1, respectively. This high ΔHm exhibited by OM222 reflects its high
367
crystallinity with respect to OM16 and OM22. Highly crystalline polymers can form perfect
368
crystals that hinders the conformational freedom of the backbone. On the other hand, lower
369
crystalline OM16 and OM22 can disperse polar asphaltenes in crude oil through polar carboxyl
370
and ester groups and prevent them from aggregation and increase the possibility of co-
371
crystallization with crude oil wax. This in turn explains less efficiency of OM222 as flow
372
improver with regard to OM16 and OM22.
373 374
4. Conclusions
375
The following conclusions can be represented from this work:
376
1. Three comb like copolymers of octadecene -co- maleic anhydride (OM, OM2 and O2M) were
377
chemically modified by inserting different lengths of long hydrocarbon chains to produce
378
structures with different molecular weights and melting temperatures.
379
2. The efficiency of flow improvers with structures that matching with wax structure are
380
critically affected by number of carbon atoms in the pendant chain, balance between
381
hydrophobic content and polar groups of the additive, copolymer molecular weight and
382
composition, melting temperature of the prepared additive and crystallinity.
383 384 385
3. The prepared PPD exhibit dual function both as pour point depressant and flow improvers simultaneously at applicable dose (2000 ppm).
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Table 1: The Physicochemical properties of Fardos crude oil Test
Method
Value
API gravity at 60 oF
ASTMD-1298
41.1
SPECIFIC GRAVITY @ 60/60 oF
ASTM D-1298
0.820
Wax content (Wt. %)
UOP 46/64
8.4
Asphaltene content, (Wt. %)
IP 143/84
3
Water content (vol. %)
IP 74/70
0.23
KINEMATIC VISCOSITY (cSt)@
ASTM D-445
Pour Point oC 387 388
50oC
7
60 oC
4.3 ASTM-D97
30
389
Table 2: Molecular weight data of OM based on GPC analysis GPC data Sample Mn, (g/mol)
Mw, (g/mol)
PD
OM
49710
117762
2.369
O2M
37954
72287
1.905
OM2
35096
91556
2.610
390 391
Table 3: Carbon number distribution of n-Paraffin fraction separated from Fardos crude No. of Moles Carbon No. Wt% Mol Wt.
No. of Moles Carbon No. Wt%
Mol Wt.
-4
x10-4
x10 23
0.10
324
3.08
43
6.20
604
102.6
25
0.50
352
14.01
44
0.04
618
0.647
27
0.05
380
1.32
45
5.06
632
80.06
28
1.07
394
27.01
46
3.23
646
50.00
30
1.40
422
33.17
47
0.08
660
1.212
31
0.10
436
2.29
48
2.20
674
32.64
32
1.64
450
36.40
49
1.50
688
21.80
33
0.11
464
2.37
50
1.14
702
16.24
34
1.74
478
36.40
51
0.80
716
11.173
35
0.02
492
0.406
52
0.76
730
10.41
36
2.17
506
42.88
53
0.61
744
8.199
37
0.06
520
1.15
54
0.36
758
4.74
38
3.60
534
67.41
56
0.32
786
4.07
40
4.73
562
84.16
57
0.020
800
0.25
41
6.40
576
111.1
59
0.014
828
0.17
392
Total wt% = 46.33, Total no. of moles = 807.367 x 104
393
Average molecular weight = 46.33 /807 x 104 = 574= CnH2n+2, i.e. 12n + 2n + 2 =574; hence
394
average carbon number (n) =40.9
395
Table 4: Pour point temperatures (PPT) of treated Fardos crude oil at different concentrations of esterified poly (α-octadecene –co- maleate esters)
396
OM esters
397 398 399
O2M esters
OM2 esters
Conc. (ppm)
1000 2000 3000 4000 1000 2000 3000 4000 1000
2000 3000 4000
OM12
27
27
27
27
27
27
24
21
27
27
27
27
OM16
15
6
6
3
21
15
15
12
27
24
24
21
OM22
15
12
12
9
21
18
18
15
18
15
12
12
PPT for blank Fardos crude oil = 27 °C
400
Table 5: Melting temperatures and melting enthalpies for OM16, OM216, OM22 and
401
OM222
Sample
Tm (⁰C)
ΔHm (Jg-1)
OM16
49
37.48
O2M16
43.55
35.88
OM22
50.51
80
OM222
51.31
43.26
402 403
Table 6: Yield values of untreated and treated crude oil with 3000 ppm of selected additives Oil sample
T⁰C
Yield value, (mPa)
Untreated
Correlation
(mPa.S)
Coefficient
12
8489
2940
0.923
15
7393
1955
0.902
21
4897
1337
0.911
9
1.62
65.2
0.989
15
1.05
60.3
0.991
21
0.622
50.4
0.992
OM22
9
3.83
116.23
0.993
OM222
9
8.62
122.4
0.995
OM16
404
Plastic viscosity,
405 406 407 408 409
Highlights: • • •
Comb like copolymers have dual function: flow improver and pour point depressant. Crystallinity and melting point of flow improver are controlling factors. Balance in nonpolar-polar content show significant effect on pour point depression.
410 411
Figure 1:1HNMR for different compositions of poly (α-octadecene-co-maleic anhydride) (a)
412
OM, (b) OM2 and (c) O2M
(a)
413
(b)
414
(c)
415 416 417 418 419 420 421 422
Figure 2: Chromatogram of paraffins extracted from Fardos Crude oil.
Peak area
423
424 Time (min.)
425 426
Figure 3: Relation of n-paraffin carbon number distribution versus weight percentages of
427
separated wax of Fardos crude oil.
428 429
430 431
Figure 4: DSC thermograms of, a) O2M16 and b) OM16 c) OM222, d) OM22 copolymers
a)
c) 432 433 434 435 436
b)
d)
437
Figure 5: The shear stress-shear rate relationship for treated crude oil with OM16 additive
438
at a) different temperatures at 3000 ppm and b) different concentrations and untreated
439
crude at 9 C
a)
440
b)
441 442 443 444 445 446 447 448
449
Figure 6: Viscosity-shear rate relationship for OM16 treated crude oil at a) different
450
temperatures b) different concentrations at temperature of 12 C and for untreated crude
451
oil
a)
452 453
454