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Needle Coke Quality - Effect Of Operating Paral,1leters

D. Tandon, P. Mondal, G. S. Dang#,Sanat Kumar, Y. K. Sharma, M. O. Garg

Indian Institute of Petroleum Dehradun

Corresponding Author's e-mail id:gdang@iip.res.inTel: 0135-2660146 Fax: 0135-2660202

ABSTRACT

Needle coke is a high value product and is obtained through delayed coking process. In India atpresent the; delayed coking units are producing only sponge! fuel coke and all the indigenousrequirement of needle coke is being met through imports. Premium grade needle coke productiontechnology is a closely guarded secret and not easily available from licensors.

Needle coke, which is an essential precursor for the ultra high power graphite electrodes forelectric arc furnaces, is produced by delayed coking. Characteristics like low coefficient of thermalexpansion, high density, high electrical conductivity and low puffing are essentially required inquality needle coke. Such qualifying characteristics have been recognized to be stronglyinfluenced by the natute of feedstock and operating! carbonization conditions. Operatingparameters also play an important role in establishing the crystalline structure of needle coke.Needle coke manufacture is also considered more an art than a science. Efforts are thereforebeing made at liP to develop the technology indigenously.

In this paper an attempt has been made to study the effect of variation of process conditions suchas furnace outlet temperature, coke chamber pressure etc on the quality of coke produced duringthe delayed coking of an identified feedstock, derived from imported crude oil. The delayedcoking experiments have been conducted at different furnace outlet temperature and coke.chamber pressure keeping other operating parameters constant. During the studies it has beenfound that as furnace outlet temperature increases the gas and liquid yield increases whereascoke yield decreases. The raw petcoke produced under each experimental condition has beensubjected to qualitative analysis including XRD technique to observe the crystalline structure ofpetcoke. Encouraging results have been obtained with the given feedstock under tried operatingconditions. The quality of needle coke obtained under relatively higher operating pressure as wellas using recycle streams is found to be reasonably good.

KEYWORDS: Delayed coking, needle coke

INTRODUCTION

In India, the refineries having delayed coking unit typically produce sponge coke (Singh, 1999),which after calcination is used mostly to produce electrodes for the metallurgical (mainlyaluminum) and other industries. However, high quality petroleum coke is needed for themanufacture of ultra high power grade graphite electrodes (Acciarri and Stockmann, 1989),required for the production of steel. Such type of graphite electrodes are produced from calcinedneedle coke, which is a high quality petroleum coke having high electrical conductivity and lowcoefficient of thermal expansion (CTE). Special characteristics of needle coke such as lowcoefficient of thermal expansion, high density, high electric conductivity and low puffing etc. havebeen recognized to be strongly influenced by the nature of feedstock and operating!

carbonization conditions (Mochida et aI., 1986). Aromatic rich feedstocks (BMCI value more than100) are normally considered as good feedstocks for needle coke production (Tandon et aI.,2003). Moreover, manufacture of needle coke has been considered more an art than a science(Mochida, et aI., 1988, Reintuch, et aI., 1988, Stokes and Guerico, 1985). So it emerges thatoperating conditions play an important role in establishing the crystalline structure of needle coke,which is responsible for its special! desired characteristics.

At present the total indigenous requirement of needle coke (-0.05 MMTPA) is being met throughimports from countries like USA,Austria, France, Germany, Italy, Spain, China and Belgium (Weblink -1) . Needle coke price is high (- Rs. 50,000-60,000 per ton) and it has also good demand ininternational market (Web link -2). Considering great potential for needle coke in the country andabroad, it is worthwhile to assess the possibility of production high quality needle coke in thecountry.

In this paper an attempt has been made to study the effect of different process parameters likecoke drum pressure, temperature, recycle ratio etc. on the quality of coke produced from aromaticrich stream(s) of imported crude origin.

EXPERIMENTAL

MaterialHeavy aromatic rich stream, used in the study, has been procured from a refinery. Anotherfeedstock is prepared atllP by blending the above refinery stream with recycle stock (370 °C +material) generated during delayed coking experiments. Characteristics of the two feedstocks aregiveninTable1. .

Experimental Unit

A continuous bench scale delayed coking unit is used in the present investigation. Schematicdiagram of this unit is shown in Figure1. Main components of the unit are feed section, pre-heaterand reactor coils, coke chamber and flash chamber for separating gas and liquid products. .Electrically heated molten salt baths are used for heating pre-heater and reactor coils.Temperatures of two salt baths and coke chamber are controlled by PID controllers whereastemperatures at other places of the reactor circuit are controlled by the energy input regulators tothe heaters. To measure the liquid temperatures preciously, a number of sheathedthermocouples are placed at various positions along the length of the coil and in the coke~m~ .

Delayed coking runs

Melted feedstcok from the feed tank is pumped at prefixed'rate (approximately 4 IiUhr) through apre-heated coil, dipped in hot molten salt bath maintained at around 400 °c fluid temperature.The pre-heated feed then passes through the main reactor coil and attains the desired cokingtemperature. From reactor coil the cracked feed enters into the coke chamber maintained at aprefixed temperature. After building required pressure in the coke chamber the outlet valve isslowly throttled to allow vapours to escape to the flash chamber. For a delayed coking run,duration of the feed pumping is about 3 hours with the coke-maturing time of around 2 hours atthe same temperature and pressure conditions of coke chamber as maintained during thefeeding. Vapours from the coke chamber enter the flash chamber, where heavy hydrocarbons getcondensed and the lighters pass on to double jacketed metallic vessel followed by glasscondenser flask. The non-condensable gases are metered off. The operating conditions forvarious delayed coking runs as well as the yield of various products are shown in Table 2. .

Product fractionation and characterization

Liquid products obtained at the end of each run (carried out in duplicate) are mixed together andare dehydrated followed by fractionation through batch distillation into light distillates, middledistillates and vacuum gas oil. Various fractions, as stated above, are characterized for importantphysico-chemical characteristics using IP/ASTM standard test methods. Gas samples arecharacterized by GC and produced coke samples as well as commercial needle coke sampleshave been characterized by XRD. The comparison amongst the XRD patterns of various cokesamples produced in the present study as well as of the commercial needle coke sample isshown in Figure 2, whereas intensities and crystallite size of the coke samples are provided inTable 3.

RESUL 1S & DISCUSSION

Feedstock characteristics (Table-1) particularly high BMCI value indicates that the feedstocks,used in the present study, are of highly aromatic in nature.

Data on delayed coking runs (Table 2) show that under increasing coke drum pressure (keepingreactor outlet temperature same) coke yield increases. This is attributed to the fact that morecondensation and polymerization reactions take place at higher pressure. With the increase ofreactor outlet temperature as well as coke drum temperature coke yields decreases, whichindicates more cracking takes place at higher temperature. It is also evident from Table-2 thatunder similar operating conditions, coke yield decreases when blended feedstock is used, as themixing of 370 °C+ stream (refractory stock) with feedstock decreases the overall reactivity of theblended feedstock.

The X-ray diffractograms (Figure 2) of all the coke samples including commercial needle cokeshow peaks at about 26 degrees 28, which can be attributed to the (002) plane of the crystallitesof petroleum cokes. The commercial sample of needle coke (NC) shows the maximum intensitywith a low background indicating a highly crystalline structure. The coke samples produced in thepresent work show moderate peak intensities with higher background. This indicates that these.samples of cokes have similar structure as the reference needle coke. However, these haverelatively lesser crystallinity compared to commercial needle coke. The mean crystallite diametersof coke samples form runs C1 and C2 are 27.9 and 28.0 AOrespectively, which are higher thanthat of C3 (26.4 AO).Although the crystallite size of the coke sample obtained from run CR4 ishigher (29.0 AO)than those obtained through runs C1, C2 and C3, it is still much lower than thecrystallitesizeof commercialneedlecoke(39.4AO). .

Thus from the XRD analysis of produced coke samples it seems that the coke sample producedat higher pressure is nearer to the commercial needle coke sample than that of the coke sampleproduced at lower pressure, with other conditions constant. Amongst the coke samples producedin the present study the XRD pattern of CR4is nearest to the commercial needle coke. However,it is still away from the actual premium grade needle coke. From Figure 2 it is also clear thatmixing of 370 °C+ stream with feedstock helps the formation of better quality needle coke.

CONCLUSION

The studies carried out on the selected feedstocks under the given coking conditions reveal that asemi finished needle coke product is obtained, which requires upgradation through fine-tuning ofoperating conditions. The study further indicates that by increasing the coke drum pressure andrecycle stock there is a possibility of getting improved quality needle coke. Reduction inasphaltene content is also expected to improve needle coke quality.

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REFERENCES

Acciarri, J. A., Stockmann, G. H., Demand for super premium needle cokes on upswing, Oil &Gas journal, Dec 25, 1989, p.118-120.

Mochida, I., Koral, Y., Fei, Y. Q., Oyama, T., Optimum carbonization conditions needed to formneedle coke, Oil and Gas journal, May 2, 1988, p. 73-77.

Mochida, I., Koral, Y., Nesuml, Y., Oyama, T., Carbonization in a tube bomb. 1. Carbonization ofpetroleum residue into a lump of needle coke, Industrial Engineering. Production Research &.Development,25, 1986, p. 198-201.

Reintuch, H M., Bolilla, J. A, Godin, R.L., in Handbook of petroleum refining process, Ed.A.R.Meyers, McGraw-Hili, New York, 1986.

Singh, H, Needle coke - A Technological study, Petrochimie vereinigt mit Brennstoff-Chemie, Bd.44, Heft 2, 1991, p. 67-70.

Stokes, C. A., Guerico, V.J., Feedstocks for carbon black, needle coke and electrode pitch,Petrochimie vereinigt mit Brennstoff-Chemie, Bd. 38, Heft 1, 1985, p. 31-34.

Tandon, D., Kumar, M.M., Ram, Y., Feasibility of needle coke production from lube oil extracts-Operational aspects, Pr:oceedings:Petrotech 2003, Jan 9-12, New Delhi

Web link 1; Sichuan Shuangyou Carbon Industry Co. Ltd. News rhttp://en.shuanQvoucarbon.com/ newEbizll EbizProtalFG/portallhtmlll

Web link 2; Notification, Department of Commerce, Government of India, 9th June1997.[http://commerce.nic.in/writereaddata/traderemedies/adpref-9raphite.J

Table 1: Feedstock characteristics

SI. No. Characteristics Net feedstock Blendedfeedstock

1 Specific gravity 1.1037 1.10472 Pour point, DC + 21 +213 Ash, wt % 0.007 .-4 BMCI 137 1385 Flash point, DC 182 1996 NitroQen,ppm 1600 22287 Kinematic viscosity at 70 D C,cSt 63.29 -

Table 2: Operating conditions and yields

Table 3: Intensities and Crystallite Size of coke samples

. Feedstock

Heavy aromatic rich feedstock Blended

feedstock..

Process conclitions Run number Run number Run number Run number& yields

C1 C2 C3 CR4

Process conditions

1. Reactor outlet temperature, DC 495-497 505-507 505-507 505-507

2. Cokedrumtemperature,D C (Bottom) 460-462 470-472 470-472 470-472

3. Pressure in Coils, kg/em:.! 4 4 4 4

4. Coke drum Pressure, kg/cm2 2 2 3 2

5. Feed Flow rate, I/h 4 4 4 4

6. Water injection, vol % of feed rate 1 1 1 1

Yiels, wt %

Coke 27 25.5 29 23

Gases 9 11 10 9

Liquid products 63 62 58 65'

Loss 2 1.0 3 3

Sample I Run Peak top (FWHM in Crystallite size Lc (A°)

(8 in degrees) degrees)

C1 25.7834 2.888 27.9

C2 25.7880 2.879 28.0

C3 25.8492 3.0544 26.4

CR4 25.7758 2.7794 29.0

NC 25.7432 2.0457 39.4

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~TC

M,.5U.I.

WATER(OPTIONAL)

I. Feed Drum

2. MeasuringCylinder

3. Feed Pump

4. Pre-heater Bath

5. Pre-heater Coil ,"

7. Flash Chamber

8. Heigbt Adjustable

9. Reactor Coil

TC Tbermocouple

([) Pressure Gauge:z:J Needle Valve

TC

6. Reactor Bath

Figure-I: Schematic view of Delayed coking plant

6000

5000

C 4000:::s

8 3000c:.- 2000..J

1000

0

14

CR.

C3

16 18 20 22 24 26 28 30 32 34 36

2 Theta

rt

.1

-. .,ii

--- NC-<>- C1-0- C2 !

-+- CR4 i

C3 '

-. _.. - ------- - _n.__.__-----.

Figure 2: XRD plot of various coke samples