The 14thIranian National Chemical Engineering Congress (IChEC 2012) Sharif University of Technology, Tehran, Iran, 16-18 October, 2012 Article code:07210

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Effect of dilution water salinity on required dilution water in a crude oil desalting plant

NedaNeisi1*, Salem Banooni2

1 NISOC (National Iranian South Oil Company) 2Department of Mechanical Engineering, Shahid Chamran university, Ahwaz, Iran

*Neda Neisi : (neisineda@yahoo.com)

Abstract In most of wells, produced oil contain water and dissolved salts, mainly NaCl, CaCl2 and MgCl2, which cause several problems in the refining processes. Maximum allowable water content in the outlet crude oil has been limited up to 0.1% volume and salt content up to 10 pounds in thousand barrels, So dilution water mixes with crude oil to reduce salinity of crude oil. At first, dilution water mixes with formation water, then it removes from crude oil. The injection rate of dilution water is usually between 3 to 7 percent of the inlet crude oil. In this research, a two stages desalting plant is modelled and reveals, by 100 ppm increasing salinity of the dilution water (from 700 to 1500 ppm), the amount of dilution water rises 25.39 barrel per day which means by reducing salinity of dilution water, the required dilution water decreased. Keywords: crude oil desalting, two stage desalting, dilution water, salinity. Research Highlights Illustrating a model for two stage crude oil desalting validated by a real plant. Evaluating effect of dilution water salinity on dilution water consumption for a plant. Reducing dilution water consumption by decreasing dilution water salinity.

1. Introduction Oil produced in most of wells contain water and dissolved salts, specialy NaCl, CaCl2 and MgCl2, which can cause many difficulties in downstream processes. In a desalting process, dilution water mixes with the formation water, then water eliminates from crude oil. Settling coalescence, chemical coalescence, heat coalescence, mechanical coalescence, electrical coalescence or combination of above methods used for crude oil desalting [1, 2]. The dilution water which is usually between 3 to 7 % of the inlet oil flow should be injected with the same droplet size as the formation water [3,4]. Maximum allowable water content has been limited up to 0.1% volume and salt content up to 10 pound per thousand. The most common method of mixing is to inject the dilution water into the crude oil and then flow it through a mixing valve [3]. The pressure drop across mixing valve supplies the energy to shear the dilution water droplets to small size. Fortuny et al developed a new procedure to determine salt content of crude oil in the laboratory scale. Their method was able to provide reliable information about the salt content on Brazilian crude oil[5]. Alves and Olivera investigated

Effect of dilution water salinity on …… Article code:07210

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different samples of Brazilian crude oil by experimental method. They developed a mathematical model for crude oil desalting plant [6]. Mahdi et al had a research on effect of mixing time, settling time, dilution water ratio, temperature, and demulsifier on dehydration and desalting. They calculated desalting efficiency and dehydration efficiency [7]. In this research, by using a computer language program a two stages desalting plant has been modelled. The model had a good agreement by a real plant in south west of Iran. The object of this work is to investigate effect of dilution water salinity on dilution water consumption. 2. Theory Figure 1 is a schematic of a two stage desalting with a recycle system. It is awkward to solve the material balance for this system. The best method for determining the dilution water rate is trial and error [3]. The code (Fortran language) helps to calculate the required dilution water flow rate to meet the specified outlet salt content of the crude oil. In equation (1), the water cut at outlet of first stage, determines the water flow rates at outlet of first stage.

(1) bXbXQ

B

1

0

By assuming, a value for salinity of Kb and Ks, the required dilution water rate could be obtained from equation (2). Mixing efficiency (E2) defines as the fraction of the dilution water mixed with crude oil and usually considered 80%.

(2) KsKE

KKBYy

bs

2

The water material balance around second stage desalter and second stage mixer gives : (3) CYBV 2

The salt material balance around second stage desalter and second stage mixer gives recycle water salinity: (4) cyb CKYKBKKV 22

(5) 21VEAR

(6) YEBS 2 By salt material balance around first stage mixer make we have Kb, similarly, Salt material balance around second stage gives Kc, Substituting for Kb, Kc in equation(4):

(7)

CSBESRCESRYKCSBAK

KV ya

1

222

By rearranging equation (4), Kb obtained. If the calculated Kb from Equation (7) is not equal to initial Kb, the next Kb should be determined, and returned to equation (2). 3. Results and discussion By developing a code, effect of increasing salinity of dilution water from700 ppm to 1000 ppm is shown in figure 2, 3, 4. Figures depict 100 ppm increasing in salinity of dilution water, causes 25.39 BPD rise in dilution water flow rate, 24.39 BPD augmentation in inlet water flow to first stage and second stage. Salinity of first stage outlet water in oil and salinity of second stage recycled water stream approximately remain constant.

The 14thIranian National Chemical Engineering Congress (IChEC 2012) Sharif University of Technology, Tehran, Iran, 16-18 October, 2012 Article code:07210

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Figure1: schematic of two stage desalting system[8]

Figure2: change of parameter versus salinity of dilution water

Figure 3: salinity of first stage versus salinity of dilution water

Figure4: salinity of recycle water versus salinity of dilution water

76

76.5

77

77.5

78

78.5

1515.5

1616.5

1717.5

1818.5

19

7 8 9 10 11 12 13 14 15

R, B

PD*1

00

Y, S

, BP

D*1

00

salinity of dilution water, ppm * 100

YSR

6666.5

6767.5

6868.5

7 8 9 10 11 12 13 14 15

salin

ity o

f fir

st

stag

e,lb

/bbl

salinity of dilution water, ppm * 100

2.522.527

2.5342.5412.548

2.555

7 8 9 10 11 12 13 14 15

salin

ity o

f rec

ycle

w

ater

,lb/b

bl

salinity of dilution water, ppm * 100

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4. Conclusions To sum up, We should restate that, the less salinity of dilution water , the reduction in amount of dilution water. This sentence also means by ascending the inlet water to desalters the cost of pumping and electrostatic separation will rise. So it is prosper to implement dilution water with less salinity.

Nomenclature Inlet water flow rate to first stage desalter R Inlet water flow rate in oil A Inlet water flow rate to second stage desalter S First stage water outlet flow rate in oil B Specific gravity @ 60 FO SWG flow rate of residual water in the outlet oil C flow rate of disposal water V1 salinity of inlet water Ka

flow rate of recycle water V2 salinity of first stage outlet water in oil Kb inlet water cut Xa Dilution water salinity Ky

first stage water cut Xb salinity of second stage recycled water stream K2

outlet water cut Xc crude oil flow rate Q Dilution water flow rate Y oil salt content Z

References [1] Pak, A., Mohammadi, T., “Wastewater treatment of desalting units”, Desalination, Vol.

222, 2008, pp. 249-251. [2] K.W. Warren, Reduction of Corrosion through Improvements in Desalting, Presented to

Benelux Refinery Symposium, Lanaken, Belgium, 1995. [3] E. Kennetb, A. Skaug, L. Skaug, Desalting, Production Facility Book Series, IHRDC,

Boston, MA 02116 USA,1987 , pp. 13-95. [4] Process Design Of Crude Oil Electrostatic Desalters, Ips-epr-700, original edition, July

1995. [5] Montserrat Fortuny, et al, Measuring Salinity in crude oils: Evaluation of methods and an

improved procedure, Fuel, Vol. 87, 2008, pp. 1241-1248. [6] . R.P. Alves, R.C. Oliveira, Petrobras, How to establish a mathematical model for the

electrostatic desalting process based on pilot plant studies, SPE Annual Technical Conference and Exhibition , SPE 102790, San Antonio, Texas, USA, 24-27 September, 2006.

[7] K. Mahdi, et al, Characterization and modeling of a crude oil desalting plant by a statistically designed approach, Journal of Petroleum Science and Engineering, Vol. 61, 2008, pp. 116-123.

[8] N. Neisi, B.Banooni, Effect of mixing efficiency in dilution water consumption in a crude oil desalting plant, 3rd International Conference on Chemical, Biological and Environmental Engineering (ICBEE 2011), September, Singapore, 2011.