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Mechanism and Effect Factors of Spontaneous

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Mechanism and Effect Factors of Spontaneous

    Mechanism and Influencing Factors of Spontaneous

    Combustion of Oil Tank Containing Sulfur

    ZHAO Xue’e, JIANG Juncheng & MENG Yifei

    (Institute of Safety Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu, China)

Abstract: Iron sulfur was produced from chemical corrosion and electrochemical corrosion, which could react with oxygen in air at normal temperature and

    then lots of heat was liberated. Oxidation experiment was made on iron sulfide by installation of oxidation experiment under natural condition. The

    spontaneous combustion tendency and oxidation process of iron sulfide in natural environment was explored. The results showed that iron sulfide had

    greater activity and natural oxidation tendency. Four factors significantly affected the oxidation process of iron sulfide. Air speed could either promote or retard oxidation process of iron sulfide. The liability of spontaneous combustion of iron sulfide was increased with decreasing particle size, increasing

    environment temperature and increasing exposed area.

    Keywords: natural environment; oil tank; corrosion; iron sulfide; natural oxidation; spontaneous combustion; influencing factors

    1 Introduction [1, 2]Many accidental explosions and fires in oil tanks occurred in China and overseas. People thought these accidents were the results of spontaneous combustion involving iron sulfide. Iron sulfide was a corrosion product of oil tanks, which consisted of

    several chemical materials, such as FeS, FeS and FeS etc. The basic process taking place was the exothermic reaction of iron 223

    sulfide with oxygen that caused an increase in temperature, with a resultant increase in the reaction rate. Afterwards combustibles

    materials self- ignition and fire accident happened when there was oil gas or oil.

    Spontaneous combustion of oil tank not only damaged equipment, wasted energy resources, but also produced SO poisonous 2

    gas, which polluted the environment, hurt the body of operators. Therefore, spontaneous combustion mechanism of iron sulfide

    has been the subject of a number of experimental studies.

    Some results have been obtained from the research on iron sulfide self-ignition mechanism. Nanjing University of [3-6] Technology explored the principle of spontaneous combustion by ferrous sulfide and made some achievements. Wang Zhirong[4] conducted experiment on the erosion of tanks containing coking gasoline. The surface of corrosion product was analyzed by [6]SEM and the composition and constitution of sulfur corrosion product was analyzed by XRD. Lu Pengyu studied how

    meteorological condition affected the spontaneous combustion procedure of FeSwhich was produced in oil refining plants. [7] Researchers from Liaoning University of Petroleum and Chemical Technologyhave performed the adiabatic tracing tests of analytical FeS oxidation through the application of the self ignition adiabatic testing system. The dynamic characteristics of the

    temperature in the adiabatic oxidation of the FeS was investigated and the impact of water content in the test sample, air flow rate

    and particle distribution of iron sulfide, monocrystal sulfur and particle sizes of FeS on the temperature rise in the oxidation were

    analyzed. They also studied the rate of the oxidation reaction and other influencing factors for oxidation of FeS which was placed [8]in a closed tube during the process of experiment. [9] Daqing Petroleum Institutesystematically studied the formation of sulfide and chemical mechanism of spontaneous

    combustion in oil tank referring to sulfide chemistry, electrochemistry and thermochemistry, discussed measures of controlling the

    spontaneous combustion.

    However, most researches mentioned above were done under adiabatic or closed condition. Data about iron sulfide

    self-ignition in natural environment was rare. This study was focused on oxidation process of iron sulfide under natural condition

    or open system. Oxidation reaction speed was characterized by temperature change, and the kinetic alteration curve was got at last.

    Natural FeS, FeS and market sold FeS can not self-ignite furiously at normal temperature, and they are not covered in this 2

    paper.

    2 Formation of Dangerous Materials in Oil Tank Containing Sulfur

    Generally speaking, spontaneous combustion accidents dont happen to new built tank easily. As for as old tank is concerned, anticorrosion layers applied in oil tanks can be deteriorated in certain small areas because of long time use and action of corrosion.

    Bare iron reacts repeatedly with oxygen in air. A series of reactions can be showed as follows:

    Fe+O?FeO (1) 2

    FeO+O?FeO+541.8 kJ (2) 223

    FeO+3HO?2Fe (OH) (3) 232 3

    Fe+2HO?Fe(OH)+H (4) 222

    3Fe(OH)?FeO+H+2HO (5) 23422Fe and Oxides generated from Equs.(1)-(5) can react with HS escaped from oil at normal temperature. Equations are as 2follows:

    Fe+HS?FeS +H(6) 22

    FeO+3HS?2FeS+3HO+S (7) 2322

    2Fe(OH)+3HS?2FeS+6HO+S (8) 322

    Fe(OH)+3HS?FeS+6HO (9) 32232

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    O+4HS?3FeS+4HO+S (10) Fe3422

    When floating plate falls down, fresh air with high oxygen content supplied from riser vent react with base metal of oil tank to form iron scale, as can be seen from equations (1)-(5).

    When floating plate ascends, HS will escape from oil and dissolve in water film on tank wall to generate hydrosulphuric acid. 2

    Hydrosulphuric acid can react with Fe and iron scale, as can be seen from equations (6)-(10). General expression of corrosion products can be formulated as FeS. FeS with loose structure is not protective and it can accelerate corrosion. mnmn

    In conclusion, corrosion products in the tank wall result from chemical corrosion combined with electrochemical corrosion. It is the mixture of the oxidant, hydroxide, sulfide of the iron, among which sulfide of the iron has great spontaneous combustion [5]tendency. So iron sulfide makes up the main components of dangerous material in oil tank leading to spontaneous combustion and fire hazards.

    3 Experiments

    Iron sulfide specimen obtained from production field has great difference from corrosion product on the oil tank inwall because it has been oxidized already. In order to truly investigate spontaneous combustion liability of iron sulfide, according to

    equations (6)-(9), the author prepared iron sulfide specimen by equipment for sulfuration using Fe (OH), FeO and FeO as 32334

    well as HS to make corresponding experiment. 2

    3.1 Testing Apparatus

    (1) Equipment for sulfuration

    Equipment for sulfuration was shown in Fig.1, including gas feeding system, sample preparing system and waste gas absorbing system. In gas feeding system, critical devices included hydrogen sulfide cylinder and relief pressure valve. Relief

    pressure valve should be adjusted carefully to stabilize air flow. In sample preparing systemthe sample should be as loose as

    possible to make it easy for HS to diffuse. Waste gas should be absorbed by NaOH solution. Fe (OH), FeO, FeO and NaOH 2 32334

    are all analytical reagent.

    (2) Installation of oxidation experiment under natural condition

    Installation of oxidation experiment under natural condition consisted of natural oxidation system, data collecting system and data processing system. The natural oxidizing system was made up of supporter, asbestos net or other fire proofing material. Temperature was measured with chromel-alumeltype Kthermocouple. A multi-channel chart recorder was used for continuously

    monitoring of the thermocouple outputs. The data collector frequency is 1000 ps and the precision is 0.00002%0.0002%.

    3.2 Experiment and Procedure

    Air tightness of equipment for sulfuration must be checked before sample was placed in the quarts tube. Then air in the tube was driven with nitrogen. After the air was driven out completely, HS would be introduced into the tube to start the sulfuration 2

    reaction at normal temperature. After sulfuration reaction finished, N would be introduced into the tube till reaction products were 2

    cooled to ambient temperature. Then reaction products would be transferred as soon as quickly to asbestos net. Data collector was

    turned on to test and record temperature rises continuously. Influencing factors tests on self-ignition including environment temperature and air speed were made in air tunnel where temperature and air speed could be controlled automatically. The rest tests in natural environment were made in ventilating cabinet with the temperature about 8 ? inside.

    All tests were replicated three times under the same conditions.

    1 relief pressure valve; 2 tee fitting; 3 N cylinder; 4 HS cylinder; 5 supporter; 6 quartz tube 22

    Fig.1 Sulfuration apparatus

4 Results and Discussion

    4.1 Oxidization Process of Iron Sulfide in Natural Environment

    In this test iron sulfide specimen was prepared from mixture of Fe (OH), FeO and FeO after 30 minutes sulfuration with 32334HS. 2

    As can be seen from Fig.2, iron sulfide reacting with oxygen in air is an exothermal reaction. Oxidation process can be divided into three stages: OA, AB, and BC. In different stage temperature variation tendency is different. OA: self-heating stage of the superficial layer

    In this stage, iron sulfide on superficial layer reacts with oxygen quickly, and then heat was released. Because quantity of iron

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sulfide on superficial layer is limited, heat released is relative few. When temperature reaches to certain level, oxidization process

    will enter the stage of incubation period.

    AB: incubation period

    Slow variation in temperature significantly characterizes this stage. In this period lots of oxygen is absorbed on the surface of

    iron sulfide particle. Whole range of temperature is little .Temperature rising rate is 4.2 ?/min in average in this period.

    BC: deeply oxidizing period

    In this period, oxygen molecule is adsorbed by active center on iron sulfide surface. Concentration of oxygen on the active surface increasing rapidly, meanwhile, oxygen molecule is activate by the heat released from adsorption. Then activated oxygen reacts drastically with active iron sulfide and lots of heat is released. Acceleration oxidization lasts 30min and temperature reaches

    to 640 ?. In this period temperature rising speed is 21.3 ?/min in average.

    Consequently, iron sulfide with high activity also has high spontaneous combustion tendency. Speed of oxidation reaction with air is basically controlled by diffusion speed and adsorption speed of oxygen.

    When the experiment terminated, there are no black iron sulfide found in the specimen. It illustrates that the oxidation reaction is almost completely and the diffusion of oxygen is not rejected.

    4.2 Research on Influencing Factors

    O after 20 minutes sulfuration with HS. In this research iron sulfide was obtained from 30 g of Fe232

    4.2.1 Effect of exposed area

    Samples piled in open container are called partly open system, while the samples piled on the asbestors are called open system in this paper. In half-open system oxygen diffusion was rejected, so incubation period had been prolonged, as can be seen

    from Fig.3. In open system exposed area was bigger than that in half-open system, therefore, more oxygen was brought to the ferric sulfide and the chance for oxygen to meet ferric sulfide was much greater, which led to the reaction speed rising obviously.

    Fig.2 Oxidization process of iron sulfide in natural environment

    800

    open system 700partly open system 600

    500

    400?T/

    300

    200

    100

    0

    1471013161922252831343740434649t/min

    Fig.3 Oxidation process of iron sulfide with different exposed area

4.2.2 Effect of air speed

     Oxygen is one of the most important materials that can result in oxidation reaction of iron sulfide. It means that only iron

    sulfide contacting with oxygen could physical adsorption, chemical adsorption and chemical reaction happen. Oxygen concentration relates to air speed directly. As can be seen from Fig 4, iron sulfide could react with oxygen in natural environment when air speed was 1m/s, 1.5 m/s, 2 m/s, 2.5 m/s respectively. Air speed had double effect on self-ignition of iron sulfide. On one

    hand, in the same period oxygen concentration through the iron sulfide surface at the larger air speed was high, which could accelerate the speed of oxidation reaction. On the other hand, when air speed was excessive larger, heat was removed rapidly by

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    air because of higher convective heat transport. When air speed is 1.5 m/s, heat liberation intensity reached to the maximum, and iron sulfide had the strongest self-ignition tendency. 4.2.3 Effect of environment temperature

    Environment temperature has no significant effect on process of temperature rising for iron sulfide. As can be seen from Fig.5, in the former 8 minutes of oxidation reaction, the temperature rising rate was almost the same at 20 ?, 30 ? and 40 ?. Analyzing from heat transfer, heat can’t diffuse easily outwards to the environment media at high temperature, so the rate of heat dissipation is relative low. Compared to 20 ? and 30 ?, when environment is 40 ?, natural oxidation system of iron sulfide

    could maintain high temperature, which enhanced self-ignition tendency of iron sulfide.

    4.2.4 Effect of particle size

    As can be seen from Fig.6, oxidation rate increased with decreasing particle size. In general, the rate of oxidation must

    depend on the effective surface area per unit volume of the iron sulfide particles. This surface area increased with decreasing particle size. When particle size of iron sulfide increased, the specific area increased accordingly. Increasing contact area of oxygen molecule with iron sulfide particles resulted in oxygen consumption rate increasing and heat liberation accelerating.

    600

    500

    400

    ?/300T

    200

    1001m/s1.5m/s2m/s2.5m/s

    0

    14710131619222528313437404346495255586164 t/min

    Fig.4 Oxidation process of iron sulfide at different air speed

    500

    450

    400

    350

    300

    250?T/200

    150

    20?10030?5040?

    0

    135791113151719212325272931333537394143t/min

    Fig.5 Oxidation process of iron sulfide at different environment temperature

    450

    40#400

    100#350

    300

    250?/T200

    150

    100

    50

    0

    13579111315171921232527 t/min

    Fig.6 Oxidation process of iron sulfide with different particle size

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5 Conclusions

    (1) The corrosion product on the oil tank wall consists of mixture of ferrous oxidant, iron sulfide, in which iron sulfide has

    great spontaneous combustion tendency. It can react with oxygen at normal temperature and then lots of heat is released.

    (2) Natural oxidation process of iron sulfide in natural environment can be divided into three periods, namely, self-heating

    stage of the superficial layer, incubation period and deeply oxidizing period.

    (3) Effects of environment temperature, air speed, particle size and ventilation condition on spontaneous combustion was

    investigated using the same iron sulfide formed from FeO. Four factors mentioned above have significant influence on oxidation 23

    process of iron sulfide. Air speed can either promote or retard oxidation process. Higher environment temperature, smaller particle

    size together with better ventilation condition can accelerate oxidation process.

References

    [1] Jiang Juncheng, Wang Sanming, Wang Zhirong, et al. On reasons and mechanism of spontaneous combustion of petroleum containing in tank. Journal

    of Safety and Environment, 2001, 1(2): 7-10(in Chinese)

    [2] Liu Tonghua.The cause analysis of a spontaneous combustion of ferro sulphide compounds in naphtha tank. Petrochemical Safety Technology, 2002,

    18(4):13-16 (in Chinese)

    [3] Zhao Xuee, Jiang juncheng, Wang Ruojun. Spontaneous combustion mechanism and preventive technology of tank containing sulfur oil. Oil & Gas

    Storage and Transportation, 2006, 25(3):51-54 (in Chinese)

    [4] Wang Zhirong, Jiang Juncheng.Experimental and theoretical study on erosion of tanks containing coking gasoline.Joural of Petrochemical Universities,

    2002, 15(4): 65-69(in Chinese)

    [5] Xing Peiyu, Wen Li, Cui Keqing. Ferrous sulfide self-ignition and its prevention in storage tank of coke gasoline, Coal Chemical Industry, 2001,

    26(4):27-31(in Chinese)

    [6] Lu Pengyu. Prevention and control on fire accidents by FeS in oil refining industry. Ms D thesis.Nanjing: Nanjing University of Technology, 2004(in

    Chinese)

    [7] Li Ping, Zhai Yuchun, Zhao Shanlin. Self-ignition of tank containing sulfur oil. Fire Safety Science, 2004, 13(3): 145-147 (in Chinese)

    [8] Li Ping, Li Jiandong, Zhao Shanlin, et al. Research on the danger of fires in oil tanks with sulfur.Fire Safety Journal,2005,40:331-338(in English)

    [9] Wang Baohui, ChenYin. Chemical mechanism and control technology of sulfide spontaneous combustion in oil tanks.China Safety Science Journal,

    2003, 13(1):23-25 (in Chinese)

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