Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining

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Heating is necessary to prevent sulphur condensation in the catalyst bed

which can lead to catalyst fouling.

The catalytic conversion is maximized at lower temperatures, but care

must be taken to ensure operation above the dew point of sulphur.

The condensation heat is used to generate steam at the shell side of the

condenser. Before storage, the liquid sulphur stream is degassed to remove any

dissolved gases.

If the acid gas feed contains COS and/or CS

they are hydrolyzed in the

catalytic reactor in the following manner:

High temperature helps to hydrolyze COS and CS

COS þ H

S þ CO

ð15:24Þ

þ 2H

S þ CO

ð15:25Þ

The tail gas from the Claus process still containing combustible components

and sulphur compounds (H

S, H

and CO) is either burned in an incineration

unit or further desulphurized in a downstream tail gas clean-up unit (TGCU).

A typical Claus process with two catalytic stages yields 97% of the

sulphur in the input stream. Over 2.6 tons of steam will be generated for

each ton of sulphur yield.

Example E15.11

An acid gas has the following composition: 40.2 mol% H

S, 54.0 mol% CO

1.35 mol% CH

, 0.15 mol% C

and 4.3 mol% water. It is fed to a thermal

stage (combustion reactor) of a Claus Process Sulphur Recovery Unit (SRU) at



C, 179 kPa and 5000 kmol/h. The air feed to the SRU is at 118



179 kPa. The product stream is fed to the waste heat boiler. The product stream

is then separated into sulphur and other product streams. The remaining product

enters the catalytic step made of three reactors as shown in Figure E15.11. Each

reactor operates at 240



C; therefore, a heater is placed before each reactor.

Feed

OutR3

Air

Water

Steam

Sulphur

Separator

Out R1 Out R2

S/R1 S/R2 S/R3

Boiler

Combustion

Reactor

To boiler

Figure E15.11 C laus process flow chart

Acid Gas Processing and Mercaptans Removal 395

Table E15.11 Summary results (all components in kmol/h)

Feed Air To boiler Water Steam Out R

S/R

Out R

S/R

Out R

S/R

S 2010 0 1205.9 241.2 24 0

2700 0 2700 2700 2700 2700

67.5 0 67.5 67.5 67.5 67.5

O 215 0 1019 5505 5505 1984 1694.3 1718.4

7.5 0 7.5 7.5 7.5 7.5

0 0 603 120.65 12.1 0

0 1005 0 0 0 0

0 3781 3781 3781 3781 3781

S 0 0 201 1447 832.2 36.17

T (



C) 40 118 833 25 100 171 171 70 212

P (kPa) 179 179 179 101 101 154 154 153 154

Total sulphur produced ¼ 2516.4 kmol/h.

All H

S and SO

are consumed.

396 Chapter 15

Simulate the process on the UNISIM software and calculate material balances.

Use conversion reactors for thermal and catalytic stages.

Solution:

Reactions (15.21) and (15.22) occur in the combustion reactor. Reaction

(15.23) occurs in the three reactors for sulphur recovery. Table E15.11 sum-

marizes the UNISIM results.

15.3.2. Tail Gas Clean Up

Incinerating the residual H

S after sulphur recovery produces SO

. There-

fore, further sulphur recovery is done for the tail gases. Typical tail gas

clean-up (TGCU) process can reduce SO

to 0.15 vol% and H

Sto

0.3 vol%. This process contains a Claus catalytic reaction followed by H

and SO

recovery section. Figure 15.8 shows a tail gas clean up scheme.

15.4. Mercaptans Removal

The predominant sulphur compounds in refinery products that usually

have an unpleasant smell are mercaptans. They are corrosive and disturb the

fuel stability due to gum formation. The principle of mercaptans removal is

oxidation. The mercaptan oxidation is called MEROX process. The role of

MEROX in the refinery is shown in Figure 15.9.

Claus

Adsorption

Incineration

Tail

Gas

Sulphur

Flue gases

Claus

Hydrogenation

Incineration

Tail

Gas

Sulphur

Solvent

Scrubbing

Claus

Hydrogenation

Incineration

Tail

Gas

Sulphur

Direct

Oxidation

Figure 15.8 TypicalTail g as clean-up scheme (Legrand and Castel,2001)

Acid Gas Processing and Mercaptans Removal 397

Gas Extraction

Merox

LPG Extraction

Merox

LSR Gasoline

Merox

Jet Fuel

Merox

LPG Extraction

Merox

Cracked Naphtha

Merox

FCC Gasoline

Merox

Alkylation

Isomerization

Gas Con.

FCC

Visbreaker

or Coker

Vacuum

Tow er

Feed

Crude

Tow er

Refinery

Gas

LPG

Gasoline

Jet Fuel

Refinery

Gas

Figure 15.9 Role of MEROX in a refinery

398 Chapter 15

MEROX sweetening involves the catalytic oxidation of mercaptans to

disulphides in the presence of oxygen and alkalinity. Air provides the

oxygen, while caustic soda provides the alkalinity. Oxygen reacts with

mercaptans through the following reaction:

4RSH þ O

! 2RSSR þ 2H

O ð15:26Þ

Removal of mercaptans by extraction starts with dissolving them in caustic

soda based on the following reaction:

RSH þ NaOH

NaSR þ H

O ð15:27Þ

The equilibrium occurs between the RSH oily phase and the RSH that

dissolves in the aqueous phase.

Extraction equilibrium is favoured by lower molecular weight mercap-

tans and lower temperatures. The rich caustic soda containing the extracted

mercaptans in the form of mercaptides is regenerated as shown in the

equation given below:

4NaSR þ O

þ 2H

2RSSR þ 4NaOH ð15:28Þ

Prior to flowing to the reactor, the feedstock is passed through a caustic pre-

wash to reduce the acid. The MEROX unit consists of a fixed-bed reactor

followed by a caustic settler. Air, the source of oxygen, is injected into the

feedstock upstream of the reactor. The operating pressure is chosen to assure

that the air required for sweetening is completely dissolved at the operating

temperature. The sweetened stream exits the reactor and flows to the

reactor caustic settler.

The caustic soda settler contains a reservoir of caustic soda for use in

keeping the MEROX catalyst alkaline. The solvent product leaving the

water wash flows to a sand filter containing a simple bed of coarse sand that

is used to remove free water and a portion of the dissolved water from the

product (Figure 15.10). The regenerated caustic soda is recycled to the

MEROX reactor.

15.4.1. Gasoline MEROX

MEROX as discussed above treats mercaptans (RSH) in gasoline to meet the

desired specifications. The alkyl group (R) could be aliphatic, aromatic or

cyclic, and saturated or unsaturated.

A dilute caustic soda solution is continuously injected into the gasoline feed

prior to the addition of air. The combined stream passes through a fixed bed

of MEROX catalyst where the mercaptans are oxidized to disulphides and

the dilute caustic soda is coalesced. The sweetened hydrocarbon stream is

essentially free of entrained caustic soda and requires no further separation.

Coalesced spent caustic soda is collected and sent to the refinery’s sewer, since

it is minute in volume, low in NaOH concentration and partially neutralized.

Acid Gas Processing and Mercaptans Removal 399

15.4.2. Kerosene MEROX

The MEROX process for kerosene/jet fuel sweetening is one of the

MEROX process applications developed for the control of mercaptans.

The conventional version of this process uses air and caustic soda (NaOH)

to sweeten kerosene feedstock. Pre- and post-treatment sections are

included to ensure that jet fuel specifications are met.

Prior to flowing to the reactor, kerosene is passed through a caustic soda

pre-wash to reduce the naphthenic acids. The reactor section of the kero-

sene/jet fuel MEROX unit consists of a fixed-bed reactor followed by a

caustic settler. Air is injected into the feedstock upstream of the reactor. The

operating pressure is chosen to assure that the air required for sweetening

will be completely dissolved at the operating temperature. The kerosene

leaving the caustic soda settler passes through a water wash, which removes

trace quantities of caustic soda as well as water soluble surfactant.

Question and Problems

15.1. Absorption is to be used to recover H

S from a gas mixture. Readily

available pure amine will be used as the solvent. The inlet gas contains

10 mol% H

S and 90 mol% methane. Determine the exit gas mole

fraction if 95% of the H

S is recovered in the liquid phase.

Pre

Washer

MEROX

Reactor

Caustic

Settler

Water

Washer

Sand

Filter

Air

Untreated

feed

Dilute

caustic

soda

Treated

feed

Figure 15.10 Typical MEROX flowsheet

400 Chapter 15

15.2. Estimate the required absorber column diameter for the given data:



Entering gas flow is 100 kmol/h, 10 mol% CO

and 20 mol% H

and the balance is CH

, The process occurs at 66



C and atmo-

spheric pressure.



Entering liquid absorbent: 200 kmol/h pure MEA (20 wt% in

solution).



Required recovery 97% of H



Assume a tray spacing of 24 in., working at 80% of flooding case,

and a surface tension of 70 dynes/cm.

15.3. Acid gas flowing at 100 MMSCFD at 100 psia and 100



F contains

5 mol% CO

and 8 mol% H

S, and the balance is CH

. Calculate the

flow rate of DEA required, then calculate the column diameter.

15.4. A tower is to be designed to absorb SO

from gas by using pure water at

293 K and 101.3 kPa absolute pressure. The entering gas contains

20 mol% SO

and the leaving gas is 5 mol%. The tower cross-sectional

area is 0.0929 m

,andtheheightoftransferunitH

is 0.825 m. Calcu-

late the SO

concentration in the exit liquid stream and the tower height.

Equilibrium data can be represented by y ¼ 20x.AssumeL

¼ 30.

15.5. CO

is absorbed in a hot carbonate process at 240



F and 1000 psia. The

feed gas stream is 100 MMSCFD and contains 10 mol% CO

.Itis

required to release the gas stream with 2 mol% CO

. Assume the

circulation rate is 3.5 ft

/gal. Calculate the flow rate of the hot carbonate.

15.6. Perform Example E15.6 on UNISIM simulator for a feed rate of 30

MMSCFD with the same feed composition and operation conditions.

15.7. An acid gas feed flow rate is 100 MMSCFD at pressure 421 psia and at

temperature 81



F with the following composition (mol%): H

S ¼

5%, CO

¼ 15%, and the balance is the carrier gas. This feed is

introduced to a Selexol process to treat the gas to reach 50 ppm H

Calculate the % recovery of sulphur.

15.8. An acid gas contains 30 mol% CO

, 40 mol% H

S and the balance is

the carrier gas. A Morphysorb solvent is used to treat this gas to

50 ppm H

S. Calculate the amount of H

S and CO

absorbed.

15.9. It is desired to capture CO

from a gas stream containing 10 mol% of

via a silicone rubber membrane. The membrane thickness is

1.0 mm and has a surface area of 3000 m

. The applied pressure

gradient is 760 cmHg. Calculate the permeation rate.

REFERENCES

Abdel-Aal, H. K., Aggor, M., and Fahim, M. A. (2003). ‘‘Petroleum and Gas Field Proces-

sing.’’ Marcel Dekker, New York.

Chianelli, R. R., Berhault, G., Raybaud, P., Kasztelan, S., Hafner, J., and Toulhoat, H.

(2002). Periodic trends in hydrodesulfurization: In support of the Sabatier principle.

Applied. Catalysis., A 227, 83–96.

Acid Gas Processing and Mercaptans Removal 401

Geankoplis, J. C. (2003). ‘‘Transport Processes and Separation Process Principles (Includes

Unit Operations)’’, 4th ed. Prentice-Hall, Upper Saddle River, NJ.

Kohl, A., and Nielsen, R. (1997). ‘‘Gas Purification’’, 5th ed. Gulf Publishing Company,

Houston, TX.

Legrand, C. and Castel, J. (2001). ‘‘Acid gas treatment’’, In ‘‘Conversion Processes’’

Petroleum Refining Vol. 3, Leprince, P., ed., TECHNIP, France.

Topse, H., Clausen, B. S., and Massoth, F. E. (1996). ‘‘Hydrotreating Catalysis: Science

and Technology.’’ Springer-Verlag, Berlin, Germany.

UNISIM (2007) Design Suite R370. Honeywell Process Solutions, Calgary, Alberta,

Canada.

Yamaguchi, N. (2003). ‘‘Hydrodesulfurization Technologies and Costs.’’ William and Flora

Hewlett Foundation Sulfur Workshop, Mexico City, 29–30 May 2003. Trans-Energy

Research Associates, Seattle, WA, USA.

402 Chapter 15

CHAPTER SIXTEEN

Refinery Economics

16.1. Introduction

The supply and demand for crude oil and petroleum products are key

factor in determining the status of the world economy. The increase in

demand of crude oil in southeast Asia, for example, in India and China has

been largely responsible for the increase in the price of crude oil to over US$

100 per barrel in 2008. However, as the world economy started to go

through a crisis in the latter half of 2008, demand has started to slow

down, and supply has surpassed demand. This situation has resulted in a

drastic drag of crude oils to less than US$ 40 in 2009. Since the increase in

demand is due to increase in demand for petroleum products, the econom-

ics of the petroleum refining industry will undergo tremendous change. In

general, refining has historically been significantly less profitable than other

petroleum industry segments. Therefore, refiners have to be careful to

control costs to make a profit.

In this chapter, global refining capacity is discussed. Factors affecting the

refinery economics, such as costs, products prices, refinery complexity and

on-stream factors, are covered.

16.2. Refining Capacity

World refining capacity, as measured by crude distillation capacity, has

increased from just over 1 billion tons per year in 1950 to around 4 billion

tons per year or 80 million barrels per day in 1980 (1 barrel per day is

approximately 50 tons per year). Due to energy-saving measures and a

decrease in demand, the capacity decreased to around 73 million barrels

per day in the early 1990s. The worldwide refining capacity has increased in

the new millennium and it reached around 85.46 million barrels per day in

2008 as shown in Figure 16.1 (EIA, 2009). Table 16.1 lists the top refining

countries in 2008.

Fundamentals of Petroleum Refining

2010 Elsevier B.V.

403

16.3. Refining Costs

16.3.1. Capital Costs

The capital investment for a new refinery depends on its throughput,

complexity and location. Added to the capital cost is the lengthy time

needed for the preliminary study, obtaining the necessary permits (environ-

mental and otherwise), designing building and commissioning. The capital

cost involves the cost of the process units, utilities, security and environ-

mental facilities, storage and handling facilities, civil work, buildings and

Million barrels per day

1970

1975 1980 1985 1990 1995 2000 2005

Figure 16.1 World crude oil refining c apacity1970-2008

Table 16.1 Top reﬁning countries in 2008 (EIA, 2009)

Country

Refi ning c apacity

Million barrels per day

United States 17.594

China 6.25

Russia 5.43

Japan 4.65

South Korea 2.58

Germany 2.42

Italy 2.34

Saudi Arabia 2.08

Canada 1.97

France 1.93

Brazil 1.91

United Kingdom 1.86

404 Chapter 16