Fouling Phenomena in Phenolic Sour Water Stripping

HOLA15 - 128

Fouling Phenomena in Phenolic Sour Water Stripping Units during Heavy Oil Upgrading Operations: A Spectroscopic Characterization Approach Miguel Orea, ChemiConsult, C.A; Jenny Bruzual, ChemiConsult, C.A; Anix Diaz, ChemiConsult, C.A; Mario Lattanzio, ChemiConsult, Lola De Lima, Facultad de Ciencias, Universidad Central de Venezuela This paper has been selected for presentation and/or publication in the proceedings for the 2015 Heavy Oil Latin America Conference & Exhibition. The authors of this material have been cleared by all interested companies/employers/clients to authorize dmg::events (Canada) inc., the congress producer, to make this material available to the attendees of HOLA2015 and other relevant industry personnel. ABSTRACT

INTRODUCTION

In thermal heavy oil upgrading, phenolic sour water is mainly produced by delayed coking and hydrocracking units. This acidic stream is very corrosive and shows a marked tendency to form ammonium bisulfide precipitates during hydrogen sulfide and ammonia stripping operations. For this reason, it is very common to relate ammonium bisulfide with fouling problems in this kind of facilities. In this work we report a particular case of fouling where the building-up of an obstructing solid deposit has nothing to do with ammonium bisulfide precipitation, but with the formation of a black, solid-like, organic material. A spectroscopic approach based on FTIR, XRF, XRD, GC/MS, and Ion Chromatography techniques was used to characterize the solid deposit. Results revealed that this material comprised a mixture of organic and inorganic components. The inorganic portion (mainly metallic oxides) derived from corrosion reactions that took place inside the stripping tower; while the organic portion was highly soluble in toluene and chloroform, but insoluble in n-heptane. Even though this solubility behavior resembles the one showed by asphaltenes, spectroscopic results indicated that the organic portion was not asphaltenic in nature, but phenolic with a high polymerization degree. To the best of our knowledge, phenol polymerization has not been reported as a fouling promoter in sour water stripping units, so this might be the first documented case.

In thermal heavy oil upgrading, phenolic sour waters are mainly related to delayed coking and hydrocracking units. These water streams show an imbalance between the molar concentration of ammonia and hydrogen sulfide ([H2 S]>[NH3 ]) and significant amounts of soluble phenolic compounds (Armstrong et al., 1996). Important levels of chloride (Cl-), cyanide (CN-), and carboxylic acids inions, as well as minor quantities of dissolved hydrocarbons, suspended solids, and dissolved carbon dioxide (CO2 ) can also be present (Armstrong et al., 1996; Meyers, 2004) .

KEY WORDS

The first fouling event occurred after four weeks from the start-up of the sour water treatment unit of a heavy oil upgrading facility located at eastern Venezuela. The stripping tower was made of carbon steel and provided with 316stainless-steel internals, a carbon-steel reboiler, and a pump-

Heavy oil upgrading, sour water, phenolic sour water, phenol polymerization, corrosion, stripping, fouling.

Heavy Oil Latin America 2015

Phenolic sour waters have a corrosive nature and show a marked tendency to form ammonium bisulfide precipitates during hydrogen sulfide and ammonia s tripping operations (Toba et al., 2005; Horvath et al., 2006; Weiland et al., 2013; Hatcher et al., 2014). It is very common to face fouling problems in sour water stripping (SWS) units as a consequence of the accumulation of these undesirable compounds. Nevertheless, in this paper we report a particular case of fouling where the building-up of the obstructing solid does not have to do with the phenomenon of ammonium bisulfide precipitation but with the formation of a black, solid like, organic material.

DATA AND OBSERVATIONS

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around cooling system. When the solid material appeared for the first time, the tower was being fed with a water stream composed of 75% (v/v) of phenolic sour water from the delayed coking unit and 25% (v/v) of sour water from the naphtha hydrotreating unit. The solid deposit was a compact mass of a black and brittle organic material (Figure 1). A portion of this material was collected at the pump-around cooling system, as well as a sample of the sour water in contact with it. The solid sample was identified as S1, while the sour water sample was identified as W1.

organic, black-solid material with an asphaltene-like appearance was obtained from chloroform extracts after solvent removal. The organic solid was quantified gravimetrically and then rated as portion O1 and O2 as derived from samples S1 and S2 respectively. Inorganic solid residues at the sohxlet thimble were also recovered, oven dried (60 °C and 60 mm Hg under helium atmosphere) and quantified. These fractions were identified as I1 and I2. ii)

Infrared Spectroscopy Analysis (FTIR). The Organic portions O1 and O2 were analyzed as solid films on potassium bromide (KBr) windows. The film was prepared by placing 100 µL of a 0.1 wt. % of the organic portion dissolved in chloroform on KBr windows. Inorganic portions I1 and I2 were analyzed as compressed KBr dis cs at a concentration of 1.0 wt. %. Infrared spectra were acquired in a Nicolet instrument model Magna 750 Series II, operated at Fourier transformed mode. A spectral interval of 4000-400 cm-1 was used with a resolution of 4 cm-1 and a scan average of 64 scans.

iii)

Phenolic compounds extraction. Weighed portions of O1 and O2 were dissolved in 100 mL of chloroform. The solutions were transferred to a 250 mL extraction funnels and extracted with a 10% NaOH solution (20×4). The extracts were further neutralized with a 10 % HCl solution and extracted with chloroform. The chloroform extracts were dried with anhydride sodium sulfate salt and further evaporated to yield an organic material that was analyzed by GC/MS.

iv)

GC/MS Analysis. The analyses of the phenolic extracts were performed in a gas chromatograph connected to a mass detector (GC/MS CHEMStation Hewlett-Packard model 6890/5973). 1,0 μL of the sample dissolved in chloroform at a concentration of 0.1 wt. % was injected keeping the injection port at 300°C. Helium was used as carrier gas at a flow of 1,0 mL/min. A 60-meter- DB5-capillary column with a stationary phase film thickness of 0.25 μm and 0.25 mm of internal diameter was used. The temperature program was as follow: 50°C for 5 min., then 5°C/min., until reaching the final temperature of 280°C that stayed the same for 15 min. The total ionic chromatogram corresponding to each sample was obtained in continuous way SCAN in the 41-to500-Dalton-interval.

v)

X-Ray Fluorescence (XRF) analysis. Qualitative elemental composition of inorganic portions I1 and I2 were measured in a Philips PW 1480 X-ray fluorescence (XRF) spectrometer. The specimens were prepared by mixing 0.5 g of each sample with 5

Figure 1. Photographic detail of solid deposit S1. As the solid formation caused the reduction of the liquid flow at the pump-around cooling system, the cleaning protocols were immediately initiated in order to reestablish normal operations. After one week from the reestablishment of operations, the addition of a 20% NaOH solution was started with the purpose of accomplishing NH3 specification in the gas effluent of the sour stripping tower. Unfortunately, during the injection of the alkali solution, another fouling event took place at the caustic injection tray (the first tray of the tower) so, the phenolic SWS plant had to be shut down once again for cleaning operations. Apparently, the use of the alkaline solution rather than improving ammonia release in the stripping tower brought about the massive build-up of another black solid material with asphaltenic-like appearance. This solid formed a compacted block at the alkali injection tray that left no room for the flowing of liquid. A portion of this solid sample, identified as S2, was collected for further characterization. The corresponding sour water sample (W2) had to be taken at the second tray of the stripping tower since the precipitation of the solid caused the total displacement of the sour water contained in the first tray. The characterization protocol followed for the analysis of the solid samples included: i)

Fractionation. Weighed portions of the dried samples were extracted with chloroform under helium atmosphere in a Soxhlet apparatus. An


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g of 110 Spectroflux (Johnson & Mathey). The mixtures were heated at 1000 °C and the melt was casted into a 4 mm thick, 32mm diameter glass disc. vi)

X-Ray Diffraction (XRD) Analysis. The qualitative mineral composition of inorganic portions I1 and I2 was determined from the x-ray diffraction pattern of the crystalline components. The analyses were performed in a Philips PW 1700 X-ray diffractometer using the dust-sample-method. Experimental measurement conditions were: 20 mA and 40 kV, using a Copper anode as excitation source. Samples were continuously scanned over an angular range of Sin2 10°-90° by using a step size of 0.02° and a step time of 2 second per step. Phase identification was carried out using the Philips PC-IDENTIFY software and the ICDD (Newtown Square, PA, USA) Powder Diffraction File database.

Water samples W1 and W2 were assessed in terms of pH, dissolved oxygen, hydrocarbon content, phenolic compounds, ammonia content, dissolved metals (Fe and Mn) and anionic species (cyanide, sulfide, sulfate, thiosulfate chloride, nitrate, formate, acetate, and oxalate ions) as indicated by APHA’s Standard Methods for the Examination of Water and Wastewater (SMWW).

RESULTS Yields on a dry base of organic and inorganic portion s from S1 (O1 and I1 respectively) and from S2 (O2 and I2) are listed in Table 1. The relative distributions of organic and inorganic portions in solid deposits are useful for sample classification in terms of the organic or the inorganic character. In this regard, we have introduced the mass relationship between organic and inorganic portions as a classification criterion, according to the following equation: MR = (xo )/(xi )

(1)

Where: MR = mass relationship; xo = mass fraction of the organic portion, and xi = mass fraction of the inorganic portion. MR values higher than 2.33 mean that the solid deposit has an important contribution of organic constituents (xo ≥ 0.70); so it is considered to be organic in nature. MR values lower than 0.43 are indicative of a high inorganic character (xi ≥ 0.70); whereas solid deposits with MR between 0.43 and 2.33 are considered to be mixed solid deposits.


Table 1. Chemical composition of solid deposits found in a phenolic SWS tower S1

S2

Sampling Location

Pump-Around Cooling System

Caustic Injection T ray

Appearance

Solid with asphalteniclike appearance

Solid with asphalteniclike appearance

Organic portion (O), wt%

15.6

88.2

Inorganic portion (I), wt%

84.4

11.8

Mass relationship, M R

0.185

7.475

Calculation of MR for solid S1 yielded the value MR-1 = 0.185, but in the case of solid S2, the result was quite different: MR-2 = 7.475. Interpretation of these results indicates that S1 has a higher inorganic character than S2, which means that its origin is related mainly to the formation and/or accumulation of inorganic particles. Conversely, the origin of S2 is certainly related to the build-up of organic material. Characterization of organic portions. The first step to elucidate the origin of solid depositions consisted in showing whether accumulation of asphaltenic material had been the main cause of the obstructions inside the SWS tower. In this regard, O1 was analyzed by IR spectroscopy and then compared with a sample of n-heptane-asphaltenes obtained from the cracked organic material recovered from the delay cocking drums. The FTIR spectrum of the asphaltene sample is showed in Figure 2. This spectrum is very similar to IR spectra of thermally treated asphaltenes reported in the literature (Buenrostro-González et al., 2002). As main features, it shows a smooth and broad band at 3361 cm-1 , which is assigned to N-H and O-H stretching bands of amine and hydroxyl groups. Asymmetrical and symmetrical stretching modes of methyl and methylene groups of alkyl chains appear at 2923 and 2853 cm-1 respectively. The small signals at 1740 and 1690 cm-1 correspond to the stretching of C=O in carbonyl groups. The former carbonyl signal is related to ester or carboxylic acid functionalities and the later one to amide moieties in asphaltenic structures (Buenrostro-González et al., 2002).

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Figure 2. FTIR spectrum of asphaltenes isolated from the organic material obtained from one of the drums of the delay cocking unit. The band at 1602 cm-1 corresponds to the stretching frequency of C=C in aromatic systems. Signals at 1456 and 1376 cm-1 are assigned to the in-plane and out-of-plane bending vibrations of methyl and methylene groups. Th e signal observed at 1301 cm-1 is related to the stretching of C-O bond in ester groups, whereas weak bands at 1230 and 1151 cm-1 correspond to the stretching vibration of -C-O-C- in aromatic and aliphatic ether structures respectively. The sulfoxide vibration band is at ~1032 cm-1 and the C-H out-of-plane bending vibrations of aromatic structures (overtone bands) are between 866-746 cm-1 . IR spectrum of the organic portion O1 is showed in Figure 3. In this case the spectrum displays an IR pattern completely different from that of asphaltenes. For instance, at 3238 cm-1 appears an intense and broad band related to O-H stretching of hydroxyl groups. Signals observed between 2956 and 2855 cm-1 are assigned to the asymmetrical and symmetrical stretching vibrations of methyl and methylene groups of alkyl chains.

The weak signal at 1654 cm-1 could be attributed to the stretching vibration of the C=O group in Quinone structures (Coates, 2000). Bands at 1594 and 1512 cm-1 correspond to the stretching frequency of C=C in aromatic systems. The in plane and out-of-plane bending vibrations of methyl and methylene groups in alkyl chains appear at 1459 and 1378 cm-1 respectively. The small peak that looks like a shoulder at 1254 cm-1 is related to the C-OH stretching vibration in phenolic structures; whereas signals at 1231 and 1177 cm-1 are ascribed to -C-O-C- vibration bands in phenylene oxide (PhO-Ph) moieties (Buenrostro-González et al., 2002). The band at 1110 cm-1 corresponds to vibration mode of -C-O-C- in methylene oxide (-CH2 -O-CH2 -) linkages (Nishioka et al., 2001) and at lower frequencies (833 and 754 cm-1 ) are observed the overtone bands of aromatic rings. Results show a noticeable structural difference among asphaltenes form cracked material and the organic portion O1. Moreover, these results strongly suggest the existence of a complex phenolic network in O1 formed mainly by aromatic and aliphatic ether linkages. Similar IR patterns have been observed in natural and synthetic polyphenols and in some complex quinone derivatives as the thelephoric acid (Read & Vining , 1959; Komarova et al., 1967; Hiroshi et al., 1995; Nishioka et al., 2001). Further attempts to obtain a more detailed characterization of O1 by 1 H and 13 C NMR were unfruitful due to the appearance of broaden and poorly-resolved spectral signals that were useless for spectroscopic interpretation. This behavior could be related to the existence of paramagnetic organometallic complexes in O1 that could interfere with the NMR analysis. As an alternative to support FTIR results, O1 was dissolved in chloroform and extracted with 10% wt NaOH solution in order to isolate simpler compounds that could give more insights about the phenolic nature of the solid deposit. The alkaline extract was neutralized with 10% HCl and then, extracted with chloroform in order to be analyzed by GC-MS. Results obtained from this technique (Figure 4) revealed high concentrations of propyl and ethyl phenols and lower quantities of cresols and phenol as main phenolic structures. Apparently these compounds were trapped in the tridimensional poly-phenylene oxide network that forms the organic portion, and then releas ed after its dissolution in chloroform. The most important findings from this particular experiment are that (i) the nature of O1 is indeed polyphenolic rather than asphaltenic and (ii) substituted alkyl phenol compounds with attached alkyl chain lengths between C1 and C3 could be the starting material of polymerization.

Figure 3. FTIR spectrum of the organic portion O1 obtained from the solid deposit S1.


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Figure 4. GC-MS chromatograms of phenolic compounds extracted from O-1.

Figure 5. FTIR spectrum of the organic portion O2 obtained from the solid deposit S2.

IR spectrum of O1 can give additional insights about the preferential arrangements of phenolic moieties in the polymeric structure. In this regard, Nishioka et al. (2001) have pointed out that the IR absorption band located at 1231 cm-1 corresponds to the asymmetric stretching mode of phenylene oxide moieties spatially arranged in such a way that both phenylene rings are located on the same plane. This spatial arrangement favors the maximum conjugation of the system through the oxygen atom, so that a double bond character is provided to the C-O bond. Conversely, the IR band located at 1177 cm-1 , which is also ascribed to Ph-O-Ph vibration mode, relates to a spatial conformation where each phenylene ring is fixed on individual planes perpendicularly arranged. In this spatial disposition, the disruption of electronic conjugation through the oxygen atom occurs and the C-O bond acquires a single bond character that makes the corresponding IR vibration band shift to lower frequencies. Integration of the two IR vibration bands of phenylene oxide showed that c.a. 87 % of Ph-O-Ph linkages in O1 adopted the spatial configuration of maximu m conjugation.

was difficult to achieve, but we believed that they are associated to paramagnetic organometallic complexes in the organic portion, since attempts to obtain a more d etailed characterization of O2 by 1 H and 13 C NMR also failed due to broad and poorly-resolved signals showed by the spectra.

In the case of the organic portion obtained from sample S2, the analysis of FTIR (Figure 5) afforded a similar polyphenolic network. The IR spectrum of O2 shows a strong band at 1228 cm-1 corresponding to the asymmetric stretching mode of phenylene oxide moieties . It also shows a band at 1061 cm-1 , which is ascribed to methylene oxide structures and the sharp peak at 1004 cm-1 that is associated to C-OH stretching vibration of methylol groups (-CH2 -OH) (Komarova et al., 1967; Nishioka et al., 2001). The spectrum also shows a weak signal at 1725 cm-1 attributed to carboxylic acid functionalities and vibration bands concerning aromatic structures (C=C stretching at 1638 cm-1 and C-H out-of-plane deformation at 883-775 cm-1 ). Methyl and methylene vibration modes in alkyl chains appear as very weak peaks at 2960, 2914, 2847 and 1459 cm-1 . Proper assignation of the last two bands located at 588 and 442 cm-1


IR spectra of organic portions yielded clear information about nature of the organic materials found in the solid deposits. So, no further efforts to establish the polyphenolic character of such materials were necessary. Evidences show that O1 and O2 comprise a complex polyphenolic network mainly formed by alkyl phenol structures linked through phenylene oxide moieties. Consequently, the hypothesis of a possible asphaltene carryover as the main cause of fouling was discarded after all. Characterization of inorganic portions . Inorganic portions I1 and I2 were qualitatively analyzed by X-ray fluorescence (XRF), infrared spectroscopy (FTIR), and X-ray diffraction (XRD). X-ray fluorescence analysis identified elemental Fe, Cr, and Mn as major constituents in I1 and I2 (Table 2). Sulfur was detected as a major constituent in I1, but in trace levels in I2. Elements like Ni, Cu, Zn, and P were also detected in trace levels in the analyzed inorganic portions. Table 2. Elemental composition and crystalline structures found in inorganic portions I1 and I2 I1 Qualitative XRF Analysis Major elements Minor elements Crystalline structures detected by XRD

I2

Fe, Cr, Mn, S P, Si, Ni, Zn, Cu, Se, Mg.

Fe, Cr, Mn Se, Ni, Cu, Zn, Mg, Si, S, P

Fe4 [Fe(CN) 6 ]3; (NH4 ) 2 SO4 ; FeS2 ; Fe3 O4 ; -MnO2

-FeOOH; FeOOH; -Fe2 O3; -MnO2 ; FeS2 ; Fe3 O4

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Elements such as Cr, Mn, Ni, P, S, Cu, and Zn, among others, are commonly used as alloying elements in different formulations of stainless steels. So, the fact of being found along with Fe in the inorganic portion of both solid deposits indicates degradation of the metallurgy of the SWS tower through corrosion mechanisms. Figure 6 displays the IR spectrum of inorganic portion I1. The first striking feature are the bands that appear at 3223 and 1410 cm-1 which coincide with the stretching and deformation vibrations of the NH4 + ion (Miller &Wilkins, 1953; Wan et al., 2003)

Figure 7. FTIR spectrum of the inorganic portion I2 obtained from the solid deposit S2. XRD results included in Table 2 confirmed the presence of iron(III) ferrocyanide (Fe 4 [Fe(CN)6 ]3 ) in amounts higher than 80 wt. % in the inorganic portion I1. Lower quantities of Ammonium Sulfate ((NH4 )2 SO4 ~15 wt %) and Pyrite (FeS2 ~5 wt %)) were also found along with traces of Magnetite (Fe3 O4 ) and Manganese dioxide (-MnO2 ). Figure 6. FTIR spectrum of the inorganic portion I1 obtained from the solid deposit S1. Signals observed at 1100-1020 and 616 cm-1 are assigned to stretching modes of S-O bond in sulfate ion (Miller &Wilkins, 1953; Adler & Kerr, 1965). Bands at 2064 and 592 cm-1 are ascribed to the C≡N stretching and Fe-CN vibration in ironcyanide complex species (Miller &Wilkins, 1953; Xia & McCreery, 1999); while the sharp peak that appears at 426 cm-1 is related to pyrite (FeS2 ) (Lennie &Vaughan, 1992). These results indicate that iron-cyanide complex salts, ammonium sulfate and pyrite apparently are the major components of the inorganic portion obtained from S1. IR spectrum of I2 (Figure 7) shows a simpler pattern than the one observed in I1. The most important signals appear between 1110-400 cm-1 where iron-oxyhydroxides and oxides exhibit their most important IR absorption bands (Cornell & Schwertmann, 2003; Orea et al., 2006). Signals at 1170 and 1021 cm-1 are associated to the Fe-OH inplane and out-of-plane bending vibrations in Lepidocrocite (FeOOH). Vibration bands at 887 and 796 cm-1 are respectively assigned to the same Fe-OH bending vibration modes in Goethite (-FeOOH) (Cornell & Schwertmann, 2003). Signals observed at 560 and 462 cm-1 are ascribed to the symmetric Fe-O stretching vibration modes in Maghemite (-Fe2 O3 ), (Cornell & Schwertmann, 2003; Balasubramaniam & Kumar, 2002; Balasubramaniam et al., 2003); while the vibration band at 426 cm-1 was assigned to FeS2 .


Crystal structures of Lepidocrocite (-FeOOH), Goethite (FeOOH), and Maghemite (-Fe2 O3 ), were confirmed as major components (> 80 wt %) in the inorganic portion I2. -MnO2 was detected in lower quantities (~15 wt %) while FeS2 and Fe3 O4 were present at trace levels (