sunfur recovery process.pdf

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284 Petroleum Refining Technologyand Economics

TREATED

GAS

CW

C

W

ACID

GAS

TOSULFUR

PLAN

T

O

FUEL

SOURGAS

STEA

M

ABSORBER

FLASH

TANK

STILL

FIGURE

13.5

Aminetreatingunit.


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2005U.S.GulfCoastcost,

MM$

10080

60

40

20

108

6

4

2

1

0.8

0.6

0.4

0.2

0.1

2 4 6 8 20 40 60 80 200 400 600 8001 10 100 1000

Amine Solution CirculationU.S. GPM

FIGURE 13.6 Amine gas treating unit investment cost: 2005 U.S. Gulf Coast (see Figure 13.5).

Present refinery practice generally provides for removal of hydrogen sulfide from

refinery gas streams by solvent absorption, as discussed in the previous section. The

acid gas stream recovered from these treating processes contains some carbon dioxide

and minor amounts of hydrocarbons, but in most cases, the hydrogen sulfide content

is over 50%. Therefore, the once-through Claus process is used in most sour crude

refineries to convert the hydrogen sulfide to elemental sulfur.

In the partial combustion (once-through) process, the hydrogen sulfide-rich gas

stream is burned with one third the stoichiometric quantity of air, and the hot gases

are passed over an alumina catalyst to react sulfur dioxide with unburned hydrogensulfide to free sulfur. The reactions are

Burner: 2H2S + 3O2 2H2O + 2SO2

Reactor: 2H2S + SO2 2H2O + 3S (13.4)


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TABLE 13.3

Amine Gas Treating Unit Cost Data

Costs included

1. Conventional, single flow, MEA, or DEA treating system

2. Electric motor-driven pumps

3. Steam-heated reboiler

4. Water-cooled reflux condenser and solution cooler

Costs not included

1. Acid gas disposal

2. Cooling water supply

3. Steam (or hot oil) supply for regenerator reboiler

Utility data

Power,a kWh/gal solution circulated 0.01kWh/M3 2.64

Fuel,b BTU/gal solution circulated 1,000.00MJ/M3 280.00

Cooling water,c gal/gal (m3/m3) solution circulated 4.40Amine makeup,d lb/MMscf inlet gas 2.50

kg/MMNm3 0.04

a Assumes amine pumps driven by electric motors and cooling done with water.b Reboiler heat usually supplied as 60 psig steam.c 30F (17C) rise.d In actual practice, amine solution circulation varies in the range of 0.15 to 0.40 gal

per scf of acid gas (H2S plus CO2) removed. For preliminary estimates, a value of 0.30 gal

of solution circulation per scf of acid gas can be assumed.

Note: See Figure 13.5.

The burner is located in a reaction chamber, which may be either a separate

vessel or a part of the waste heat boiler. The purpose of the reaction chamber is to

allow sufficient time for the combustion reaction to be completed before the gastemperature is reduced in the waste heat boiler.

Ammonia frequently is present in the Claus unit feed streams and must be

completely destroyed in the reaction furnace to avoid plugging of equipment with

ammonium salts. Specially designed burners and combustion zones have been devel-

oped for this purpose [13].

The waste heat boiler removes most of the exothermic reaction heat from gases

by steam generation. Many types of waste heat boilers are in use. Usually they are

arranged so that the gas flows through several tube passes in series with chambers,

or channels, where a portion of the gases many be withdrawn at elevated temper-atures to use for reheating the main gas flow stream prior to the catalytic converters.

Some elemental sulfur is often condensed and removed from the gas in the waste

heat boiler. In some plants, a separate condenser is used after the waste heat boiler.

The gas temperature entering the first catalytic converter is controlled at about 425 to

475F (218 to 246C), which is necessary to maintain the catalyst bed above sulfur

dewpoint in order to avoid saturating the catalyst with sulfur and thereby deactivating


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Supporting

Processes

287

REHEAT GAS

GAS 7 -10PSIG

STM

450F

560F

BFW

BFW

350F

400F

440F

BFW

BFW

250F

TAIL

AIR S S S

BURNER WASTE HEAT FIRST FIRST SECOND SECONDAND BOILER CONVERTER CONDENSER CONVERTER CONDENSER

REACTION

NOTE:

BFW = BOILER FEED WATER

FIGURE 13.7 Once-through Claus sulfur process.


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Supporting Processes 289

2005U.S.GulfCoastcost,

MM$

100

80

60

40

20

Basis is 93% sulfur recovery by Clausunit from refinery gas.

For 98% recover multiply cost by a factor 2.0

10

8

6

4

2

1

2 4 6 8 20 40 60 80 200 400 600 800

Sulfur Production, Long Tons/Day

FIGURE 13.8 Claus sulfur plant investment cost: 2005 U.S. Gulf Coast (see Table 13.4).

the catalyst. The reaction between hydrogen sulfide and sulfur dioxide in the con-

verter is also exothermic. Gases from the converter are cooled in the following

condenser for removal of most of the elemental sulfur as liquid.

The condenser outlet temperatures must be maintained above about 275F

(135C) to avoid solidifying the sulfur. Two converters and condensers in series are

indicated in Figure 13.7, but most plants have three converters.

Modifications of the once-through process include various reheat methods for

the converter feed temperature control, such as heat exchange with converter outlet

gases, in-line burners, and fired reheaters. Overall recovery is usually not over 96%

and is limited by thermodynamic considerations, as described in the literature [11].

When refineries process crude oils with sulfur contents higher than design, it is

necessary to recover more sulfur in the Claus unit. An economical method for increas-

ing the capacity of Claus units is the substitution of oxygen for a portion of the

combustion air needed in the reaction furnace. This modification can increase the

capacity for sulfur production by 50% or more at a relatively small capital cost [14].


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Supporting Processes 289

TABLE 13.4

Claus Sulfur Recovery Unit Cost Data

Costs included

1. Once-through modified Claus unit designed for 94 to 96% recovery

2. Three converters (reactors) with initial charge of catalyst

3. Incinerator and 150-ft (46-m)-tall stack

4. Sulfur receiving tank and loading pump

5. Waste heat boiler

Costs not included

1. Boiler feed water treating

2. Boiler blowdown disposal

3. Solid sulfur storage or reclaiming

4. Sulfur loading facilities (except for loading pump)

5. Supply of power and water

6. Tail gas clean-up process unit

Utility data

Power, kWh/long ton of sulfur 100

kWh/MT 98

Boiler feed water, gal/long ton of sulfur 820

M3/MT 3

Waste heat steam production at 250 psia (1740 kPa)

lb/long ton sulfur 6500

kg/MT 2900

Fuel None

Cooling water None

Note: See Figure 13.8

13.4.2 CARBON-SULFUR COMPOUNDS

Carbonyl sulfide (COS) and carbon disulfide (CS2) have presented problems in many

Claus plant operations due to the fact that they cannot be converted completely to

elemental sulfur and carbon dioxide. These compounds may be formed in the

combustion step by reaction of hydrocarbons and carbon dioxide, as shown below:

CH4 + SO2 COS + H2O + H2

CO2 + H2 COS + H2O

CH4 + 2S2 CS2 + 2H2S (13.5)

Many more complex reactions are possible. These compounds, if unconverted,

represent a loss of recoverable sulfur and an increase in sulfur emission to the

atmosphere. Special alumina catalyst is significantly more effective in converting

both COS and CS2 to elemental sulfur than the conventional bromide catalyst [6].

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