Coking

INDIAN INSTITUTE OF SPACE SCIENCE AND TECHNOLOGY, THIRUVANANTHAPURAM

A STUDY REPORT ON COKING OF KEROSENE Submitted by Rahul Anand [email protected] In 4th Semester, BTECH-AEROSPACE ENGINEERING In Liquid Propulsion System Center (LPSC) Trivandrum

December-January, 2009

ABSTRACT Kerosene is used as a fuel in semi cryogenic engines which also serves the purpose of coolant in regenerative passage. At high temperatures, kerosene gives carbonaceous deposits commonly known as coke. Coking reduces the heat transfer across the chamber wall of the engine as it sticks to the inner walls of the passage, creating an insulating layer between the coolant and the chamber wall. Studies on coking of hydrocarbon rocket fuel have been carried out worldwide but the mechanism of its formation is still uncertain. In this report, we discuss the various ways in which coking can occur and the measures which can be taken to suppress coking.

NOMENCLATURE SEM- Scanning Electron Microscopy

proportionally higher heat fluxes to the chamber walls. As the coolant travels along the regenerative passage, its temperature increases. At elevated temperatures, hydrocarbon fuels

GC- Gas Chromatography MS- Mass Spectroscopy PAH- polyaromatic hydrocarbons

can decompose and leave behind solid deposits on the wetted surfaces in a process called ‘coking’.

These

effectiveness

of

deposits cooling

decrease by

forming

the an

insulating layer over the inner wall of the pipe.

Isp- Specific Impulse

They act as a thermal barrier, which increases the wall temperature of the combustion chamber and can eventually cause material

INTRODUCTION

failure. Coking is a major challenge associated with the use of kerosene as fuel in aerospace

Semi cryogenic liquid rocket engines use

applications, and we will discuss its formation

kerosene as the fuel along with liquid oxygen as

and ways to reduce it in this paper.

oxidizer. They have the advantage of being relatively low cost, creating low pollution, reusable and having high performance. Safety and cost factors associated with the storage and handling of kerosene also gives it an edge over liquid hydrogen.

Coke is a carbonaceous substance formed from kerosene in the flow passage when the temperature goes beyond a certain limit, known as the ‘coking limit’. This temperature rise has to be controlled so as to eliminate the coke formation in the regenerative passage. The

Kerosene is used as regenerative coolant in the

thermal stability of kerosene depends upon

semi cryogenic engine to keep the chamber wall

many parameters like its chemical composition,

material at safe operating temperatures. Prior to

amount of non hydrocarbon compounds like

combustion, fuel is circulated through channels

sulphur, nitrogen and oxygen present in

in the chamber wall to carry the heat away.

kerosene,

Engine performance increases by operating at

conditions and residence time of the kerosene in

high chamber pressures, which results in

the passage. The deposition starts at around

material

of

the

piping,

flow

100°C and continues to increase as the

degrade with the deposition. This is

temperature increases as a result of pyrolytic

because of the reaction with the sulphur

reactions.

present in the kerosene, which leads to the formation of brittle copper sulphide.

The sulphur and the oxygen molecules present in the fuel can facilitate coking. Therefore, studying the mechanism of formation of coke becomes essential to deduce techniques for suppressing it. There are many models for the coking mechanism but it varies with the composition of the kerosene and system parameters.

3. Kerosene is a mixture of more than 100 hydrocarbons, with n-dodecane and its derivatives

being

the

prominent

components. Sulphur is present in trace amounts

(<30ppm)

aromatic(5%),

along

olefins,

dienes

with and

naphthenes. Coke can be formed in different

ways

from

all

these

components at different temperatures. 4. At low temperatures (~200°C), coking

OBSERVATIONS ON COKE

occurs mainly due to rearrangement

FORMATION

and condensation. At high temperatures (>350°C), Coke formation involves

1. Depositions

from

fuels

at

high

temperatures is the agglutination of carbonaceous solid pellets cohered by colloid material [1]. It occurs mainly due to the agglomeration of oxidative non hydrocarbon chemicals in the kerosene. This is inferred by studying the deposits, which show that sulphur, nitrogen and oxygen content of the deposits is always higher than the original fuel. These impurities can react with thermally generated free radicals during the course of the reaction to form stable solids. Studies conducted in this field have shown that removing these impurities improve the thermal stability of kerosene. 2. It has been found that the rate of deposition increases if copper is used as the wall material. The walls start to

dehydrogenation,

cyclisation,

isomerization and hydrogen transfer in addition to condensation. 5. The mechanism for coke formation is a free

radical

consecutive

reaction:

aliphatic-polycyclic-resins-asphaltenescoke. Isolation of the intermediate has shown a continuous increase in the molecular weight, degree of aromaticity and C:H ratio. 6. Asphaltenes are complex molecules, believed to consist of associated system of polyaromatic sheets, bearing alkyl side chains. They are highly polar and surface active. Preheating of fuel prior to

their

burning

encourages

precipitation of asphaltenes, which ultimately break down to give coke. Asphaltene

deposition

is

the

consequence of the thermal instability

of kerosene. Kerosene is thought to be a colloidal system and asphaltenes are the dispersed phase. They are stabilized by resins, formed as intermediates during the course of reaction. Fig: butadiene undergoing Diels-Alder reaction with ethane.

RECOMMENDED ACTIONS TO CONTROL COKING

Fig: Coke obtained from asphaltene on heating

There are a few measures which can be adopted to suppress coking. They are as

kerosene.

follows-: 7. The SEM results of the copper sample after coking showed heterogeneous

(i) By plating the inner surface of the

deposition. The maximum deposits

regenerative coolant passage with some

were found at the middle portion, which is the hottest part. This can be explained by the adsorption properties of the copper which acts as a catalyst. The coke formed in this region is a mixture of pyrolytic and asphaltic coke. 8. Formation of such huge rings from

inert material would affect the rate of coking, as copper acts as a catalyst for pyrolysis of the hydrocarbons, which ultimately lead to the formation of coke. Some eligible materials are Nickel, Gold, Silver, Zirconium or some other noble metals.

condensation of long chain paraffins

(ii) By increasing the flow rate, the

and aromatics can be explained by the

residence

Diels- Alder reaction, shown by the

regenerative passage would decrease and

dienes present in the kerosene.

[9]

These

time

of

the

fuel

in

the

hence, the deposition may decrease.

dienes on cycloadditions can increase the size of the rings considerably. It

(iii) By distillation of the kerosene fuel by

may be noted that the rate of reaction

special methods like Deer Distillation[1] to

increases with temperature.

remove suspended impurities. The oxidative coke, formed during the initial stages of heating is mainly due to the presence of oxygen, sulphur and nitrogen atoms in the fuel.

(iv) Precooling the fuel before it is sent to

a long chain polymer. This would increase

the regenerative passage, will increase the

the thermal stability of the hydrocarbons

margin of the coking limit.

present in kerosene, thereby reducing coking. The other advantages of using

(v) By hydrogenating the fuel before it is kept in the storage tank, we can suppress coking to a large extent as the ring propagation is believed to be caused due to the presence of dienes.

[9]

gelled propellants are safer handling, reduced slosh, reduced the O/F ratio leading to lighter exhaust gases, less leakage and greater Isp than normal liquid propellants.

Hydrogenation

prior to heating would reduce the amount of

(viii) The use of nanometal additives or

unsaturation compounds, thus preventing

nanocomposites is another alternative. Once

coking and would also result in an increase

the precursors to the coking reactions are

in the calorific value of the fuel.

known completely, we can design a nanocomposite can be designed which

(vi)

By

adding

metal

particles

like

Aluminum to the fuel would increase the Isp of the fuel. If we add a composite of boron (or boron hydride) and a suitable transition metal, it could reduce coking by forming organometallic complexes with the active compounds like asphaltene, formed

would increase the Isp as smaller particles undergo more efficient combustion. It would also trap the coking process by reacting with the precursor element. As of now,

availability

and

the

cost

of

nanoparticles is an issue so it can be pursued later.

during the course of the reaction. Boron hydride

addition

would

inhibit

coke

(ix) According to literature, semi cryogenic

formation to some extent. This has to be

technology was used by both the US and

further studied to understand completely. A

USSR. Though the US engines have

similar method has been employed in the

reported coking, the Russians didn’t face

aviation industry and NASA has also used

this problem. It is not surprising since the

aluminium gelled RP-1 for spaceflight.

kerosene used by Russia (RG-1) was

Laboratory experiments and some more

superior in quality compared to RP-1, used

research in this field may help us find a

by US. RG-1 was low in sulphur content

suitable element which would wash away

(<1ppm), aromatic content (approx 5%) and

the coke without affecting the combustion.

low dienes content, as these are thought to be the major facilitations for coking. RG-1

(vii) The use of gelled propellants is another

was 3% denser than the RP-1.

area of research which can lead to a solution to the coking problem. Here, an

(x) Boriding the inner surface of wall of the

external gellant is used which creates a

cooling passage can help in reducing coking

cross linked structure in the liquid fuel, like

as Boron being a trivalent element has

capability to form complexes readily. It can

liquid kerosene, oxidative coke is formed.

form

depositing

At high temperatures, coke is mainly

asphaltene and dissolve away. Boriding the

pyrolytic because of cracking. It is due to

surface will also increase the hardness and

the formation of acetylene, benzene and

smoothness of the surface.

other PAHs. A catalytic coke is formed

complexes

with

the

because the wall material acts as a catalyst.

CONCLUSION, DISCUSSIONS & RECOMMENDATIONS

When the pyrolysed kerosene is cooled down, asphaltic coke can appear. It is

To understand the coking phenomenon, we

because of condensation of PAHs. These

must have the knowledge of the major

different

components of the kerosene before heating

amorphous, tubular or filamentous structure

as well as during the course of heating at

depending upon the temperature at which

different temperatures. The study of the

they are formed.

intermediates can help us to understand the precursors

and

mechanisms

formation.

For

this

of

types

of

coke

may

have

REFERENCES

coke be

[1] Liang, Yang, Zhang, ‘Investigation of

employed to analyze the samples of

heat transfer and coking characteristics of

kerosene as it has been successfully used to

hydrocarbon fuels’, Journal of Propulsion

study volatile mixtures with more than

and Power, Vol.14 No.5, pp. 789-796,

hundred components. X-ray spectroscopy or

September-October 1998.

GC/MS

can

13

C NMR spectroscopy of the coke samples

can help us elucidate on the actual structure of the coke formed at that particular temperature (given in appendix). The

[2]

Giovanetti,

Anthony

J.,

‘Deposit

formation and heat transfer in hydrocarbon rocket fuels’, NASA Report 168277, October 1983.

structure of the coke may help for conducting a retro-organic analysis of the

[3] Wickam D.T., Alptekin G.O., Engel

coking

this

J.R., Karpuk M.E., ‘Additives to reduce

mechanism is essential for the development

coking in endothermic heat exchangers’,

of a semi cryogenic engine indigenously,

35th AIAA/ASME/ASEE Joint Propulsion

and the design of a synthetic kerosene to

Conference and Exhibit, 20-24 June 1999

mechanism.

Study

of

suit the needs of ISRO.

APPENDIX 1

[4] Goodger E.M., Hydrocarbon Fuels, Mcmillan publication, 1960, pp. 483-486.

Coke formed at different temperatures has

[5]

Zhiming

different structure. At low temperatures

‘Formation

(<473K), if some oxygen is present in the

carbonaceous

Fan, and

Watkinson characteristics

deposits

from

Paula, of heavy

hydrocarbon coking vapours’, Industrial & Engineering

Chemistry

Research,

Vol.45,No.19, September 13, 2006, pp6428 to 6435. [6] Evaluation of coking limits of kerosenepreliminary

results,

Report

no-

LPSC/SCED/TR/051/08 [8] Brown Sarah, Frederick Robert A., ‘Laboratory

scale

thermal

stability

experiments on RP-1 and RP-2’, Journal of Propulsion

and

Power,

Vol.5

No.2,

October-November 2007. [9] Wickham D.T., Alptekin G.O.,. Engel J.R and Karpuk M.E., TDA Research Inc, ‘Additives to reduce coking in endothermic heat exchangers’, AIAA 99-2215