PTQ Q1 2025 Issue

feed gas composition is the same for both the base case (air only operation) and the revamp case (oxygen enrichment) operations. The only differential between the two cases for the feed gas is quantity. Table 2 provides a summary of the flow rates for the var- ious streams marked in the simplified flow diagram shown in Figure 1 for the two operating cases: base case – air only operation; revamp case – oxygen enrichment. As Table 2 clearly shows, when the unit operates with oxygen enrichment and processes 150% of the base plant capacity, the process gas flow from the first sulphur con- denser is still less than the base case. Hence, there is no impact on the hydraulic load of the unit downstream of the first sulphur condenser even while it processes more acid gas. The primary drawback of implementing high-level oxy- gen enrichment is that with so little nitrogen diluent in the furnace, the bulk gas temperature achieved via combustion exceeds the allowable limits for any available refractory. Hence, special design modifications are needed to provide adequate temperature control in the furnace. Typically, the major equipment that is either replaced or newly installed in the revamp of an existing SRU includes the COPEII ejector, the COPEII burner, the AAG preheater (if the existing one is insufficient), and the first sulphur con- denser. For this particular revamp, additional updates were required to the existing equipment and piping to accommo- date increased sulphur production capacity. The specially designed COPE burner allows for the safe and effective processing of separate feed streams, includ- ing air, high-purity oxygen, acid gases, start-up fuel gas and, when necessary, recycle gas. The COPEII ejector is a key component in the system that moderates the furnace temperature. It sends a recy- cle stream back to the reaction furnace so that, as more high-purity oxygen is added, the operating temperature does not rise above the design limit of the furnace refrac- tory. The recycle gas is taken from the outlet of the first sulphur condenser. The flow of recycle gas is controlled to maintain the desired temperature in the reaction furnace. The following section provides a detailed description of the process and the design modifications implemented in the existing unit. The associated design for this case study has been defined in Figure 2 . This shows the simplified sketch of the front-end section of the SRU through the first sulphur condenser. The items highlighted in red are the equipment or equipment sections that are part of the revamp (addition of new equipment and replacement or modification of existing equipment).

Feed gas composition

ARU acid gas

SWSU acid gas

Component

Mol% 93.5%

Mol% 38.0% 38.0% 22.0%

H₂S NH₃ H₂O

0.0% 4.5% 2.0%

Balance

0.0%

Total

100.0%

Table 1

enrichment is classified as low-level, mid-level, and high- level. This corresponds to the amount of pure oxygen mixed into the air-oxygen mixture sent to the furnace. Low-level oxygen enrichment typically limits the overall oxygen con- centration to 28 vol% O₂ in the final mixture of air and oxygen, whereas high-level oxygen enrichment involves a concentration greater than 45 vol% O₂. The in-between concentration is categorised as mid-level enrichment. Case study 1: Capacity enhancement The first case study involves a sulphur plant consisting of three identical trains designed to process refinery acid gas equivalent to a total of 450 MTPD (3 x 150 MTPD) of sul- phur production. The existing SRU consisted of the follow- ing systems:  Three identical Claus sulphur recovery sections, includ- ing the thermal and catalytic stages. v Dedicated sulphur pit and ejector for each SRU train. w A common tail-gas incinerator for all three trains. The refinery needed to increase its sulphur handling capa- bility to meet the present and future crude operating trends, as well as to increase its overall flexibility of operation. The owner proceeded to implement Fluor’s proprietary Claus Oxygen-based Process Expansion (COPEII) high-level oxy- gen enrichment to increase the sulphur handling capability by 50%. This technology would increase the amount of acid gas equivalent each Claus train can process to 225 MTPD of sulphur production while maintaining the same sulphur recovery efficiency. Oxygen enrichment reduces the vol- umetric flow of process gas and tail gas by reducing the quantity of nitrogen that enters with the combustion air. This reduction in volumetric flow rate allows for a corre- sponding increase in SRU acid gas feed rate and a subse- quent increase in sulphur production with the same main equipment in the SRU downstream of the Claus furnace. The typical composition of the feed gases to the Claus section for the sulphur plant is summarised in Table 1 . The

Simplified material balance table

Flow rate in kmol/h

Amine

SWS

Oxygen

Process

Claus

Recycle

Process gas from 1st cond.

acid gas

air

furnace outlet

gas

Stream No. Base case Revamp case

3 0

6 0

183 286

67 67

558 137

764 800

742 648

136

152

Table 2

PTQ Q1 2025