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American Journal of Scientific ResearchISSN 2301-2005 Issue 71 (2012), pp. 135-142© EuroJournals Publishing, Inc. 2012http://www.eurojournals.com/ajsr.htm

Natural Gas Sweetening by using Iron Sponge Process: A Case

Study of Hamilton North Gas Field

Ribwar Kermanj Abdulrahman

School of Chemical and Petroleum Engineering

Koya University, Kurdistan Region-Iraq

E-mail: [email protected]: +964(0)7702134369

Immanuel Sebastine

School of Science & Engineering

Teesside University, United Kingdom

E-mail: [email protected]: +4401642 738523

Abstract

Natural gas is considered the most important and popular fuels in the current eraand future as well. Moreover, the demand of natural gas in recent decade has beendramatic. However, because the natural gas is existed in deep underground reservoirs so itmay contain several non-hydrocarbon components for example, hydrogen sulphide andcarbon dioxide. Moreover, these impurities are undesirable compounds and cause severaltechnical problems for example, corrosion and environment pollution. In fact, there are

several methods and processes for gas sweetening for example, chemical absorption, solid bed method and physical absorption. Indeed, Hamilton North Field is located in the EastIrish Sea-UK (Gluyas and Hichens, 2003). According to raw natural gas streamcompositions, the Hamilton North gas field is contained small amount of Hydrogensulphide (H2S) gas about 30 ppm. Moreover, this H2S amount may consider small and itcould be treated by adopting iron sponge process. Thus, this study aims to design ironsponge process for Hamilton north field. Moreover, design calculations covered several process design points for example, vessel diameter which it calculated (76 cm) and gascycle time (30 days).Furthermore, the iron sponge cost is also calculated in this study.

Keywords: Iron sponge, Gas sweetening, Process design, hydrogen sulphide

Some nomenclatureRMM Relative Molecular Weight CO2 Carbon dioxideH2S Hydrogen sulfide SG Specific gravity

1. Introduction Natural gas considers the most important and popular fuels in the current era and future as well.Moreover, the demand of natural gas in recent decade has been dramatic. In fact, natural gas poses ahuge role in the recent world economy and development. Furthermore, Natural gas has various usingfor instance, household energy, industrial energy, electricity generation and used as main raw material



Natural Gas Sweetening by using Iron SpongeProcess: A Case Study of Hamilton North Gas Field 136

for chemical and petrochemical industries. However, because natural gas is located in undergroundreservoirs so it usually contains some impurities for example, H2S and CO2. Indeed, the presence ofthese impurities could cause several problems for instance, corrosion and environment pollution.Hence, raw natural gas should be cleaned from acid gases. Moreover, gas sweetening is also achievedin order to meet gas pipelines specifications and gas sale contracts that it referred to 4 ppm for H 2S and2% for CO2 in natural gas stream. In addition, there are several natural gas sweetening method and

processes for example, chemical absorption, solid bed method and physical absorption. However, eachof these method has unique advantages and disadvantages for specific gas conditions and processselection may be depended on several parameters for example, cost, gas stream compositions andtechnology available. Indeed, Indeed, Hamilton North Fields are located in the East Irish Sea-UK(Gluyas and Hichens, 2003). According to raw natural gas stream compositions, the Hamilton Northgas field is contained small amount of Hydrogen sulfide(H2S) gas about 30 ppm or (0.003%).Moreover, this H2S amount may consider small and it could be treated by adopting iron sponge process. As Stewart and Arnold (2011) notes that “Iron sponge process economically applied to gascontaining small amount of H2S”. However, this work is just an academic study to investigate andapply iron sponge gas sweetening process to treating this gas.

2. Solid Bed Sweetening MethodSolid bed sweetening method may consider one of important Natural gas sweetening methods andoldest method as well. Furthermore, this method consists of several processes that it has the sameconcept for instance, iron sponge process and zinc oxide proses. Infect, it achieves by using solidsweetening agents. Furthermore, solid bed sweetening method bases on adsorption principles.Moreover; it is used fixed bed of solid particulars which it can remove the acid gases from the naturalgas. As Stewart, M. and Arnold, K. (2011) notated that “Fixed bed of solid particles be used to removeacid gas either through chemical reactions or through ionic bonding”. Indeed, the modus operandi ofthis method is achieved by passing the sour gas through a fixed bed of solid particles that adsorbs theacid gases and removes it from the Natural gas. Therefore, the bed should be regenerated time to time

to remove the impurities and make sure it at high purity to achieve maximum sweetening efficiency. Infact, the most common processes of this method could be showed in the following:

•

Iron sponge process

• Molecular sive process

• Sulfa-Treat process

• Zinc oxide processIron sponge process is a good example of solid bed method. Iron sponge process may consider

the most common process in solid bed method. Furthermore, it considers the oldest process that used in Natural gas sweetening. However, it may have limited application in time being because it mayconsider suitable to remove small amount of Hydrogen sulfide and it needs handle care as well.



137 Ribwar Kermanj Abdulrahman and Immanuel Sebastine

Figure 1: Typical iron oxide process flow diagram.

3. Process Design Calculations (Iron Sponge Process)Given Hamilton North field data:

Table 1: Hamilton North Field composition data (Gluyas and Hichens, 2003).

Hamilton North Field Data

Component Mole%

H2S 0.003

CO2 0.4

Field Hamilton North Field N2 7.7Flow rate 2400 std. m3 / hrr CH4 83Pressure 8400 K.Pa C2H6 5

Gas density 0.65 Kg/m3 C3H8 1.8

Gas SG (Air=1.0) 0.6 C4H10 1.3Temperature °C 38 °C C5H12 0.797

3.1. Water Content Calculations

The raw natural gas which been given is analysis in dry based. Therefore, it’s quite important tocalculate the water content in the gas. However, many scientists have mentioned different methods tocalculate the water content in the gas. However, some of it is not quite accurate. Therefore, many

designers and researchers have recommended that to use several methods for given natural gas to eithercalculate or estimate the water content.

1. The Empirical Method

Empircal method may consider the oldest and most pupular method which it gives fastresult.Moreover, by using figure (2) and adopting given gas pressure and temprature water con tant

could be estimated easily. Therfor, for given gas the water contant is about3

800 MMstdm

kgW = .




Figure 2: The McKetta-Wehe Chart for Estimating the Water Content (Carroll, 2009).

2. Using Graph and Equations (Checking for First Method)

As Campbell (1979), notes that this method could be considered more accurate than other methods andit may specialize in calculating the water contain for sour gas.

W =( y*W hc)+( y1 W 1)+( y2 W 2)Where:W= water content into natural gasW hc= water contants of hydrocarbons in part of natural gas (calculate by graph)W 1=water contanys of C 2O (calculate by graph)W 2 = water contents of H 2S (calculate by graph) y = hydrocarbon mole friction in total natural gas. y1=C 2O mole friction y2= H 2S mole frictionFrom table (1) given, y=0.9189 for total HCs in gas feedY1=0.004 CO2, Y2= 0.00003 H2S

By using the, figure (2)3

800 MMstdm

kgW hc =

To calculate W1: Use figure (3),31 1400

MMstdm

kgW = and use figure (4) to calculate

32 2500 MMstdm

kgW =




Table 2: Natural gas compositions and calculations. Done by using MS excel V.2010.

3.2. Design Calculations

Step one: calculation minimum vessel diameter.As Stewart and Arnold (2011) note the following equation to calculate

2/1

minmin : ⎟⎟ ⎠

⎞⎜⎜⎝

⎛ =

PVgmax

QgTZ d d

Where:d min= Minimum internal vessel diameter, cmQg= gas flow rate, std.rm

3/hr T = operating temperature, K°Z= compressibility factor =0.85(GPSA figure 23-8)P = operation pressure, Kpa.Vgmax= maximum gas velocity, m/s, for design v=3 m/s (new book)

cmd min 95.5638400

85.03114200 58.8

2/1

=⎟ ⎠

⎞⎜⎝

⎛

∗

∗∗=

Step two: calculate minimum diameter for deposition2/1

2 4255 ⎟⎟ ⎠

⎞⎜⎜⎝

⎛

/=

o

S H Qgd

X

min

Where:Ø=Rate of deposition g/h, for design = 628 (Stewart and Arnold, 2011).

S H X 2

=Mole friction of hydrogen sulfide

2/1

628

0.00002994200 4255 ⎟

⎠

⎞⎜⎝

⎛ ∗= x

Thus, d min=45.484 cm The above result is Compatible with design standardsStep three: calculate maximum diameter:

2/1

8.58 ⎟⎟ ⎠

⎞⎜⎜⎝

⎛ =

PVgmin

QgTZ d

max

Where:d max= maxmum internal diameter, cm



141 Ribwar Kermanj Abdulrahman and Immanuel Sebastine

Vgmin= Minimum gas velocity, m/s. for design = 0.61 m/s (Stewart and Arnold, 2011).2/1

61.08400

85.03112400 8.58 ⎟

⎠

⎞⎜⎝

⎛

∗

∗∗=maxd

=95.5cmThis value is consistent with the design considerations. As (new) note “any diameter from 43.1

to 95.5 cm is acceptable.

Step four: choosing a cycle Time of one month and optimize the vessel diameter and bedheight.

S H Qg

Hed Fetc

X

2

261048.1 −∗=

Where:tc = cycle time ,daysFe = iron sponge content, kg Fe2O3/m3 e = efficiency (0.65-0.8)H = bed height, md= vessel diameter, cm

Thus, ⎟ ⎠

⎞⎜⎝

⎛

∗∗∗

∗∗

= − 65.01161048.1

0000299.02400306

2

H d

d 2 H =19,291

d= 76 cm could be considered acceptable choice for vessel diameter. Since tc and e arearbitrary and H=3.3 m bed is appropriate with design standards.

The relationship between vessels diameter and bed height could be showed by mathematicalgraph:

Step five: Calculate volume of iron sponge to purchase. Bu=0.0022d

2 H

Bu=0.0022 x 762 x3.3 Bu=42 bushels The above result is Compatible with design standards

4. Results and DiscussionThe process design is done and the whole design results can be summarized in table (3):

Table 3: design calculation results

Minimum

internal vessel

diameter

minimum

diameter for

deposition

maximum

internal

diameter

cycle Timevessel

diameterBed height

volume of

iron sponge

0.5695 m 0.45484 m 0.955 m 30 day 0.76 m 3.3 m 42 bushels




It seems that from results above, the cycle time is 30 days and it proves that this method needslong time to be achieved. Furthermore, the bed of vessel need 42 bushels which about 1.14 tonnes ofiron sponge. The iron sponge price in market today is about $400 per iron sponge tonne (BeijingKaibiyuan Trading, 2012). Thus, the bed cost is about is £294. Infect, this price may consider a quitecheap for gas industry. As a result, it seems that the operation and capital cost for iron sponge ischeaper than other sweetening methods. However, it has several disadvantages for example; need long

time, batch process, needs handle care to change the bed.

5. Conclusion and RecommendationThis study is attempted to describe the solid bed method for natural gas sweeting and it is achieved process design calculations for Hamilton North fields as a case study. Moreover, the iron sponge process design is achieved for several iron sponge vessel parameters for instance, vessel diameter isabout 0.76 m, bed height is about 3.3 m and cycle time is 30 day. It seems that from this studycalculations, the using of iron sponge process for Hamilton North Gas Field is quite possible andeconomical. However, solid bed method could not consider suitable to apply in practical work forseveral reasons for example, batch process, dealing with small quantity of natural gas, need a long timeand handle care. Therefore, it can argue that solid bed method may suitable to apply for natural gasstream with low acid gases content there is no requirement for high daily production of natural gas.

References[1] Gluyas, G. and Hichens, H. (2003) United Kingdom Oil and Gas Fields. Bath: Geological

Society. Mokhatab, S. ,Poe, W. and Speigh, J. (2006) Handbook of natural gas transmission and processing. Texas: gulf publishing co.

[2] Kumar, S. (1987) Gas production engineering. Taxis: gulf publishing company.[3] Smith, R. 1990 Practical natural gas engineering .Oklahoma: pennwell books.[4] Beijing Kaibiyuan Trading (2012) Iron sponge. Available at: http://www.kbytrade.com/

(Accessed: 22 June 2012).[5] Stewart, M. and Arnold, K. (2011) Gas Sweetening and Processing Field Manual. Waltham:

Gulf professional publishing.[6] Maddox, R. (1982) gas and liquid sweetening. Oklahoma: Campbell petroleum co.[7] Carroll, J.(2010) Acid Gas Injection and Carbon Dioxide Sequestration .New jersey: Scrivener

Publishing.[8] Coker, A. (2010) Ludwig's Applied Process Design for Chemical and Petrochemical Plants:

Distillation, Packed Towers, Petroleum Fractionation, Gas Processing and Dehydration vol2.Oxfork: Elsevier.

[9]

Arnold, k. and Stewart, M. (1999) Surface Production Operations: Design of gas- handling

systems and facilities. Oxford: PennWell.[10]

Abdel-Aal, K. and Aggour, M. (2003) Petroleum and Gas Field Processing. New York: CRCPress.

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