Apr-2010

Processing NH3 acid gas in a sulphur recovery unit

The feasability and economics of a two-stage sour water stripper with an SRU for NH3 contents of 25% and higher are discussed

Michael Quinlan and Ashok Hati
KBR

Article Summary

Today’s refineries are processing crude slates with higher sulphur and nitrogen contents. Some of these crude slates have such high nitrogen contents that the feed to the sulphur recovery unit (SRU) contains a far higher sour water stripper (SWS) acid gas to amine acid gas ratio than has been typical for existing ammonia (NH3)-burning SRUs. In the industry’s experience with NH3-burning SRUs, the SWS acid gas is processed in the SRU along with the amine acid gas for recovery of elemental sulphur from hydrogen sulphide (H2S), and the NH3 content of the aggregate acid gas has been low enough that the increase in SRU equipment sizes to process the NH3 and the loss in Claus sulphur recovery can be tolerated.

For these low NH3 content acid gas feeds, the furnace temperature is, or can be designed, high enough to ensure complete NH3 destruction, and it is less expensive to process the NH3 in the SRU than to employ a two-stage SWS that would strip the H2S and NH3 in separate towers, enabling the NH3 to bypass the SRU.

With higher NH3 content in the SRU feed, the refiner must decide if a NH3-burning SRU is still the optimum choice. The refiner will recognise that even higher temperatures are needed for NH3 destruction, that the incremental mass flow through the SRU to process the NH3 will be proportionately larger, that the loss in sulphur recovery from the additional N2 and H2O will be more severe, and that there is little operating experience with high NH3 content SRU feeds.

Despite these disadvantages, it might still be economical to select a NH3-burning SRU, leading to assessment of technical risk vs economics. This article discusses the issues with high content NH3-burning SRUs, and looks at the technical and economic issues that the refiner must address in choosing between a single-stage SWS with a NH3-burning SRU compared with a two-stage SWS with a SRU that does not process the NH3.

SRU feed in a refinery
The crude feed to a refinery contains both sulphur (S) and nitrogen (N) compounds. These compounds are converted to H2S and NH3 as the crude is refined into finished products, such as fuel gas/LPG, gasoline, diesel and coke. As shown in Figure 1, amine and sour water remove the H2S and NH3 to meet finished product specifications. The amine regeneration units (ARU) produce an acid gas containing H2S with traces of NH3. The SWS may employ single- or two-stage strippers. In a single-stage SWS, H2S and NH3 are stripped from sour water in a single column, whereas a two-stage or double stripper has the H2S and NH3 strippers as two columns in series. All of the amine acid gas is processed in the SRU together with SWS acid gas (containing both NH3 and H2S if a single-stage SWS, or H2S only if the SWS is a two-stage stripper).

As the SWS acid gas increases as a proportion of the total acid gas feed, or as the NH3 content of the acid gas feed increases as a result of processing crudes with higher N/S content ratios, there may be technical and/or economical reasons why the NH3 component of the SWS acid gas should bypass the SRU.

The technical reasons why higher NH3 content feeds may not be a good fit for the SRU are incomplete NH3 destruction, increased NOx formation and inadequate burner/furnace designs. The economical reasons are the increased size of SRU equipment and the additional SRU equipment that may be needed to compensate for the loss in sulphur recovery.

NH3 destruction in a SRU
In a Claus SRU, about a third of the H2S is burned to SO2 using air. The produced SO2 then reacts with uncombusted H2S to form elemental sulphur. The reactions are shown as Equations 1 and 2 below. The overall reaction is shown as Equation 3:

H2S + 3/2 O2  → SO2 + H2O    (1)

2 H2S + SO2  → 3 S + 2 H2O    (2)

3 H2S + 3/2 O2 → 3 S + 3 H2O  (3)

While the H2S is only partially oxidised in Equation 1, the NH3 is completely combusted to nitrogen and water, as shown by Equation 4:

2 NH3 + 3/2 O2 → N2 + 3 H2O   (4)

It is vital to destroy the NH3, meaning the residual NH3 leaving the furnace should be 30 ppmv or lower. If the NH3  is not sufficiently destroyed, NH3 -H2S salts can form at the cold spots of the Claus (for instance, in the final sulphur condenser) and can plug the SRU. The reactions that occur in the Claus furnace are complex and not fully understood, and the destruction of NH3  is governed by kinetics rather than equilibrium. For NH3-burning SRUs, the three Ts — turbulence, temperature and time — are the key to ensuring that the NH3 is sufficiently destroyed.

A number of different approaches are available to destroy the NH3 in NH3-burning SRUs. To achieve the required turbulence, a high-intensity burner is recommended. If the burner is a high-intensity type (for instance, Duiker or HEC), all of the amine acid gas may be combined with the SWS acid gas and the combined stream sent to the burner and a single-zone combustion chamber, as shown in Figure 2a. For low content NH3 feeds, a good mixing of the high-intensity burner plus a minimum combustion chamber temperature of 2250°F (1230°C) has been deemed adequate by the industry to destroy the NH3. If the combined acid gas stream is not sufficiently rich in combustibles, such that 2250°F is not attainable, air and/or acid gas preheat may be used to achieve the minimum temperature for NH3 destruction. Even when the combustibles are rich enough in the combined acid gas stream, it is often considered a good idea to preheat the air and/or acid gas anyway, as this ensures better NH3 destruction.