Heat Recovery in Sulfur Plants

A sulfur plant forms an important part of a gas processing system in a refinery or gas processing plants. Sulfur is present in natural gas as hydrogen sulfide (H 2 S); it is the by-product of processing natural gas and refining high-sulfur crude oils. For process and combustion applications, the sulfur in the natural gas has to be removed. Sulfur recovery refers to the conversion of hydrogen sulfide to elemental sulfur. The most common process for sulfur

removal is the Claus process, which recovers about 95%–97% of the H 2 S in the feed stream (Figure 4.13). Waste heat boilers are an important part of this process (Figure 4.14a). Due to low gas inlet temperature in sulfur condenser, a single-shell fire tube boiler is often used.

The Claus process used today is a modification of a process first used in 1883, in which

H 2 S was reacted over a catalyst with air to form elemental sulfur and water. The reaction is expressed as follows:

HS 2 + 12 / O 2 → S + HO 2

Control of this exothermic reaction was difficult, and sulfur recovery efficiency was low. Modifications later included burning a third of the H 2 S to produce SO 2 , which is reacted with the remaining H 2 S to produce elemental sulfur; this process consists of multistage

catalytic oxidation of H 2 S according to the reactions:

2 HS 2 + 3 O 2 → 2 SO 2 + 2 2 H O heat +

2 HSO 2 + 2 → 2 S + 2 HO 2

Each catalytic stage consists of a gas reheater, a catalyst chamber, and a sulfur condenser as shown in Figure 4.14. In addition to the oxidation of H 2 S to SO 2 and the reaction of SO 2 with

H 2 S in the reaction furnace, many other side reactions occur to yield gas stream containing

CO 2 ,H 2 S, SO 2 ,H 2 , CH 4, and H 2 O in addition to various species of sulfur. The duty of the boiler behind the sulfur combustor includes both sensible heat from cooling of gas stream from 1426°C (2600°F) to 343°C (650°F) and the duty associated with the transformation of

Converter 2 HP steam

Acid gas

WHB Tail gas

Air

LP steam Air

LP steam

BFW

WHB Acid gas

WHB

Ko drum

Sulfur pit

FIGURE 4.13

Claus process for sulfur recovery.

Waste Heat Boilers 179

(a)

Multipass tube-side arrangement

Sulfur condenser

(b)

FIGURE 4.14

(a) Two-pass fire tube waste heat boiler for Claus plant. (b) Sulfur condenser with multistreams from reaction generating low-pressure steam.

various species of sulfur. The reaction furnace operates at 982°C–1537°C (1800°F–2800°F), and the flue gas is passed through a waste heat boiler in which saturated steam at about 40 barg is generated. This is typically a two–gas pass design, though single-pass designs are available. The gas is cooled to 650°C (1200°F) in the first pass and finally to 343°C in the second pass.

Figure 4.12 shows the boiler for a large sulfur recovery plant, which consists of two separate shells one for each pass connected to a common steam drum. The external down- comer–riser system ensures adequate circulation and cooling of the tube sheet, which is refractory lined. Ceramic ferrules are used to protect the tube-to-tube sheet joint, as shown in Figure 4.15. The inlet gas chamber is also refractory lined. The casing is kept between 175°C and 205°C (350°F–400°F) through a combination of internal and external insulation

180 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Insulation

Tube

Tube sheet

Gas in Ferrule

(a) Tube to tube sheet joint. (b) Picture of tube sheet with refractory and ferrules. (Courtesy of Cleaver Brooks, Thomasville, GA.)

to minimize concerns regarding acid dew point corrosion. The exit chamber is externally insulated. About 65%–70% of the sulfur is removed in the boiler as liquid sulfur by using heated drains. Figure 4.14b shows a single-pass design for the same application. The pur- pose of using ferrules is explained later. Refractory on tube sheet and ferrules reduce the heat flux through the tube sheet and hence keep the tube sheet cool. Calculations for tube sheet temperature are shown later with and without tube sheet refractory and ferrules.

Waste Heat Boilers 181

Though the boiler operates above the sulfur dew point, some sulfur may condense at partial loads and during transient start-up or shutdown mode. The cooled gases exiting the exchanger are reheated to maintain acceptable reaction rates and to ensure that process gases remain above the sulfur dew point and are sent to the catalyst beds for further conversion as shown in Figure 4.13. The catalytic reactors using alumina or bauxite cata- lysts operate at lower temperatures ranging from 200°C to 315°C. Because this reaction represents an equilibrium chemical reaction, it is not possible for a Claus plant to convert all of the incoming sulfur to elemental sulfur. Therefore, two or more stages are used. Each catalytic stage can recover one half to two-thirds of the incoming sulfur. Acid gas is also introduced at each catalyst stage as shown. The gas stream from each stage is cooled in another low-pressure boiler at 3–5 barg called the sulfur condenser, which condenses some of the sulfur. If the flue gas quantity is small, a single-shell fire tube boiler handles all the streams from the reactors. Each stage has its own gas inlet and exit connections. The outlet gas temperatures of these exchangers are about 175°C. From the condenser of the final catalytic stage, the process stream passes on to tail gas treatment process. Carbon steel tubes are adequate for the boiler for both the low-pressure sulfur condenser and Claus plant heat high-pressure heat recovery boiler.

Heat Recovery in Sulfur Plants