ISSN 0104-6632 Printed in Brazil

www.abeq.org.br/bjche

Vol. 33, No. 04, pp. 1063 - 1071, October - December, 2016 dx.doi.org/10.1590/0104-6632.20160334s20150103

*To whom correspondence should be addressed

Brazilian Journal of Chemical Engineering

CHARACTERIZATION AND EVALUATION OF WAXY CRUDE OIL FLOW

G. B. Tarantino, L. C. Vieira, S. B. Pinheiro, S. Mattedi, L. C. L. Santos,

C. A. M. Pires, L. M. N. Góis* and P. C. S. Santos

Universidade Federal da Bahia, Escola Politécnica, Departamento de Engenharia Química. Rua Aristides Novis, 02, 3° Andar, Federação, CEP 40210-630, Salvador - BA, Brazil.

Phone: + 55 71 3283-9464 E-mail: lmario@ufba.br

(Submitted: February 22, 2012 ; Revised: May 28, 2015 ; Accepted: July 16, 2015)

Abstract - Part of the oil found in the Brazilian subsoil has a high wax content, which makes its flow process difficult at low temperatures because of the increase in the viscosity of the fluid. This paper studied the flow behavior of waxy crude oil under variation in the temperature of the external environment of the flow, the volumetric flow rate of the oil and the emulsified water content of the oil. The results were compared with those obtained for a non-waxy crude oil that had similar rheological properties at temperatures above the wax appearance temperature (WAT). The proposed tests were based on the experimental design technique, and the behavior of the fluids was evaluated based on the pressure variation generated by the flow. Keywords: Waxy crude oil; Pressure variation; Crystallization.

INTRODUCTION

Paraffinization is one of the main problems in oil production and causes considerable losses to the oil industry. The wax precipitation phenomenon associ-ated with paraffin deposition can result in unsched-uled production shutdowns and promote operational risk conditions. Moreover, it can cause production losses and irreparable damage to equipment (Pauly et al., 2004).

In the Bahian Recôncavo region, the produced crude oil exhibits a density of approximately 30° API, almost no sulfur and high concentrations of dis-solved waxes. Although these properties are great for the manufacture of lubricant oils and yield high added value, the presence of wax adds many com-plications to production, transportation and storage by hindering the flow in pipes (Thomas, 2004; Novaes, 2009).

Paraffins are both linear (n-paraffins) and branched (iso-paraffins) chain alkanes, and they have low re-activity with most compounds. Their chains can have a high carbon number, which implies a higher wax appearance temperature. The low-molecular-weight paraffins are the main components of natural gas, and the medium- and high-molecular-weight ones are found in crude oil (Farayola et al., 2010; Gao, 2008; Jamaluddin et al., 2001).

Paraffins are in equilibrium with other crude oil components, and any change in pressure, tempera-ture and even composition can affect the equilibrium, thereby influencing the formation of precipitate. According to Santos (1994), the greater the crude oil wax content, the greater the precipitation rate and, therefore, the amount of precipitated wax. The light oil components keep the waxes soluble. The high pres-sure of the reservoirs maintains the light compounds solubilized in the crude oil, which favors the solubili-

1064 G. B. Tarantino, L. C. Vieira, S. B. Pinheiro, S. Mattedi e Silva, L. C. L. Santos, C. A. M. Pires, L. M. N. Góes and P. C. S. Santos

Brazilian Journal of Chemical Engineering

zation of waxes in the fluid (Tinsley; Prud’Homme, 2010). This condition ensures low viscosity and Newtonian behavior of the crude oil (Azevedo, 2003).

Temperature also influences the solubility of waxes in crude oil. When approximately 5% of the waxes crystallize because of oil cooling, a crystal lat-tice appears and traps some of the oil inside; this process is called “gelling” and hinders the fluid flow. Thus, the crude oil flow rate also interferes with wax solubility. The lower the oil flow rate, the longer it stays inside the piping, which favors heat exchange with the external environment (Vieira, 2008).

Once production starts, the oil flows through the pipelines, losing heat to the external environment, with consequent temperature decreases and reduced soluble light oil fractions. Such production condi-tions cause the oil viscosity to increase, which leads to production problems due to the precipitation of waxes (Venkatesan et al., 2005; Gao, 2008).

According to Vieira (2008), the first paraffin crys-tals start to form at a specific temperature, which is called the wax appearance temperature (WAT) and varies depending on the origin of the crude oil. Crys-tallization occurs in three steps:

Nucleation – formation of small particles of crystallized material from which the first paraffin crystals will grow.

Growth – mass transport of the solution to-wards the nuclei formed during the nucleation stage.

Agglomeration – when the growing crystals are joined together, thereby yielding larger crystals.

With the nuclei already formed, there is incorpora-tion of new paraffin molecules at the growth sites, and additional molecules of other species are grouped at these sites and become part of the structure. The nu-clei form an ordered lamellar-structure arrangement.

After crystallization starts in a medium that con-tains water as an emulsion, the crystal lattice for-mation phenomenon occurs in a different manner. When the emulsion is of the water-in-oil type, the oil is waxy and the fluid temperature is below the WAT, the precipitated waxes are deposited onto the surface of the water drops, thereby contributing to the growth of the formed precipitate (Oliveira et al., 2010). When a large crystal lattice is in the vicinity of the water drops, a structure is formed; this struc-ture percolates the drops into the lattice and captures them. According to Visitin (2008), this structure also provides mechanical resistance to the flow, thereby resulting in an increase in the viscosity and pour point of the oil.

The present study aims to evaluate and compare the flow of two types of crude oil, waxy and non-waxy, by measuring the pressure variation of the

system under the influence of the flow rate, tempera-ture and content of emulsified water.

MATERIALS AND METHODS Crude Oil

The characteristics that influenced the choice of oils used in this study were obtained from the rheo-logical behavior of the samples. Although the availa-ble oils had different wax contents, WATs and com-positions, for a comparative study of the influence of the content of emulsified water, temperature and oil flow rate in the context of loss of flow, it was neces-sary for the oils to be rheologically similar such that any differences originated exclusively from phenom-ena that characterize the increase in fluid viscosity and its implications for the flow.

After a series of comparative tests to search for a non-waxy oil with rheological behavior similar to that of the waxy oil above the WAT, a non-waxy oil was defined as the reference for comparison in the study.

The WAT of each oil was determined through dif-ferential scanning microcalorimetry, µDSC. The analysis was made using a DSC-VII microcalorime-ter, Setaram, with a 500 µL stainless steel pressure cell and the data acquisition and analysis was made through the Setsoft 2000 software. The procedure realized in the tests consists of heating the sample to 80 °C during one hour and a sample of known weight is placed in the cell and then in the equipment. The analysis is made by cooling the sample from 80 °C to 0 °C at a rate of 0.8 °C/min. Microcalorimetry measures any release or absorption of heat by the sample while it cools. The evaluated temperature range consisted in the cooling of the oils from 80 °C to 0 °C. The only possible exothermic event in this temperature range is the release of heat related to the crystallization of waxy species present in the sample. The greater the crystallization peak area, the greater the amount of wax present in the sample, and the higher the temperature at which crystallization oc-curs, the greater the length of the carbon chain of the crystallized paraffins. Figure 1 shows the microcalo-rimetry curve of the waxy oil, which has a WAT of 310.5 K.

Figure 2 shows the microcalorimetry curve of the low-paraffin oil. This sample exhibits two points at which the line tangent to the crystallization curve crosses the abscissa, thereby generating a WAT at 316.4 K and a second crystallization event at 290.7 K. This effect occurs because the long-chain n-alkanes,

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Figure 2: Mirude oil.

The rheoletermined brom Thermoimilar to thoonsisted of clong a linearate of 0.5 K/

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Brazilian Jou

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Three variabnt of emulsifmperature (Tes influencedas quantifiedwo levels weres of the rangted in Table

able 1: Extrece the flow.

Independentvariable A (%) T (K) Q (mL/min)

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able 2: Tests

Tests

1 2 3 4 5 6 7 8 9 10 11

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Characterization and Evaluation of Waxy Crude Oil Flow 1067

Brazilian Journal of Chemical Engineering Vol. 33, No. 04, pp. 1063 - 1071, October - December, 2016

The highest levels of the variable “water cut“ were set to be 35% because, when this level was greater than 40%, the viscosity increased, which yielded difficulties in pumping the sample.

The influence of the temperature on the crystal-lization of paraffins dissolved in crude oil, and con-sequently its viscosity, is of paramount importance to this work. Preliminary tests demonstrated that, below 291.15 K, the waxy crude oil used as a sample did not flow properly because of its high viscosity. For this reason, 293.15 K was set as the lowest tempera-ture level used in this study.

The highest temperature level was defined based on the need to expose the waxy crude oil to condi-tions in which crystallization of solubilized paraffins occurs, i.e., at temperatures close to its WAT. There-fore, the highest level was set to 298.15 K.

The residence time in the test tube is also a factor that influences the crude oil flow. The greater the residence time, the greater the heat exchange with the external environment, which results in a wider variation of fluid viscosity. Based on this infor-mation, it was assumed that 150 mL/min would be adequate for the minimum flow rate level.

The maximum flow rate level was set to 200 mL/min, which is close to its maximum operating condition, 275 mL/min. Experimental Methodology

One liter of the sample with a determined water cut was initially heated to 333.15 K in a storage ves-sel such that the solubility of all paraffins in the oil was ensured. At the same time, the oil was circulated through the system’s bypass until its temperature was stabilized. At the end of this step, the sample flowed through the cooling system.

During the test, the oil that fed the unit was cooled to 328.15 K. After the cooling that occurred throughout the test tube, the oil returned to the begin-ning of the process to be reheated to make the pre-cipitated wax soluble.

The temperature and pressure values at the inlet and outlet of the test tube were recorded during the tests every 10 seconds. The pressure difference be-tween the inlet and outlet of the test tube measured over time was the response obtained for each test. In most of the tests, the maximum pressure variation was achieved in less than two hours of operation. The results indicated that the best strategy to com-pose the experimental design was acquisition of pres-sure change data at a fixed time. The shortest time at which the maximum pressure was attained was used as the time to measure the differential pressure value for all tests.

RESULTS AND DISCUSSION Waxy Crude Oil

The tests were conducted according to the distri-bution in Table 2 and the pressure differential oc-curred due to the wax precipitation. The pressure difference was calculated from the pressure variation between two manometers, the first one located at the beginning of the flow and the second one at the end of the flow; these manometers present 99 percent accuracy. In most cases, the maximum pressure vari-ation of the system occurred at different times. Be-cause the goal of the research is to study the pressure variation based on the influence of the variables (the emulsified water in the crude oil (A), temperature (T) and flow rate (Q)), the differential pressure values considered as test responses were those attributed to the lowest operating time that reached the maximum pressure variation of the system. The worst flow con-dition was achieved in test 1, with a temperature of 293.15 K, flow rate of 150 mL/min and water cut of 5%; in this test a pressure difference of 8.03 bar was achieved in 36 minutes. Thus, the reference operat-ing time was 36 minutes. The pressure variation responses are found in Table 3. Table 3: Pressure differential of the samples pre-pared with waxy crude oil.

Tests Temperature (K)

Flow rate (mL/min)

Water cut (%)

Pressure differential

(bar)1 293.15 150 5 8.032 298.15 150 5 1.073 293.15 200 5 4.924 298.15 200 5 0.715 293.15 150 35 5.676 298.15 150 35 3.067 293.15 200 35 3.77

The data in Table 3 were processed with the aid

of parametric statistics. The empirical models pre-sented in Figures 6 and 7 were evaluated for signifi-cance through analysis of variance based on the Pa-reto diagram shown in Figure 5.

It can be observed from Figure 5 that the varia-bles that significantly influenced the flow process were the temperature, oil flow rate, the interaction between the temperature and oil flow rate and the interaction between the temperature and water cut. The significance of the interactions between the two variables leads to the conclusion that it is not possi-ble to analyze the behavior of the system based on only one variable while keeping the other variables fixed. The experimental data were properly fitted to a linear plane model with a regression coefficient (R²)

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when a higher

Figure 6: Resp

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Brazilian Jou

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Figure 9 shlow rate on te observed thhe water cut evel. Howev

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Chara

Journal of Chemic

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Evaluation of Wa

Vol. 33, No. 04,

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pp. 1063 - 1071,

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Although there very simiidenced by Faxy crude oile waxy crudorking rangeoved to be dd interactionte and water

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EDGMENT

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for Scientifinselho Nacio

S. Santos

es of both oive the WAT, ior of the nont from that ratures. In thaxy crude ote, temperatus of the flo

nificant for thwhich was iin. Because nt in the crudlowering th

fin crystallizesure change significant ansure gain. Thates caused e line, therebntly decrease

ed water in thgel formatio

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uenced by thn of these twof wax presee that was th

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- no-en-

Characterization and Evaluation of Waxy Crude Oil Flow 1071

Brazilian Journal of Chemical Engineering Vol. 33, No. 04, pp. 1063 - 1071, October - December, 2016

volvimento Científico e Tecnológico - CNPQ), and Petrobras (Petróleo Brasileiro S.A).

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