Ammonia destruction in the reaction furnace
A study based on reaction fundamentals aids a comparison of the advantages of one-zone and two-zone SRU reaction furnace design in ammonia destruction.
SIMON WEILAND, NATHAN HATCHER, CLAYTON JONES and RALPH WEILAND
Optimized Gas Treating Inc.
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Ammonia typically evolves from nitrogen-containing components in processing crude oil. Processing and handling sour gas and sour water containing ammonia in an environmentally acceptable manner has always been a challenge in refineries and is the focal point for many designs and patents. There are two main objectives that these designs and patents attempt to achieve: purifying ammonia-bearing sour water to prepare the water for further processing or discharge; and disposing of the ammonia once it has been removed from the sour water. Options for disposing of ammonia include destroying it, or selling it as a product if it meets purity standards. This article focuses on a common method for disposing of the ammonia: destroying it in the reaction furnace of a sulphur recovery unit (SRU). When directed to an SRU, nearly complete destruction of ammonia is needed to prevent plugging the SRU with ammonium salts.
The effectiveness of ammonia destruction is strongly influenced by the configuration and operation of the reaction furnace. The two hardware configurations considered here are one-zone and two-zone designs. The two-zone design that will be considered has one burner at the front of the first reaction chamber. These are two of the most common reaction furnace configurations used in SRUs. In this article, a brief introduction to each reaction furnace design is followed by a discussion of ammonia destruction chemistry, and concludes with a case study illustrating how design and â€¨operating parameters affect ammonia destruction.
One-zone reaction furnace
The one-zone furnace, also referred to as a straight-through design, operates with all the amine acid gas (AAG) and sour water acid gas (SWAG) feed being mixed together, then fed to the furnace via a single burner (see Figure 1). This design is generally applied when the feed gas contains no ammonia, or small concentrations of ammonia, but with the addition of preheat and incrementally longer residence time it can also be used successfully when ammonia is present in higher concentrations.
A benefit of the one-zone furnace is that all the acid gas passes through the burner flame, thereby providing the maximum available heat for the destruction of contaminants such as hydrocarbons and ammonia. Insufficient destruction of hydrocarbons in the reaction furnace can have disastrous effects on the converter beds downstream, especially if they contain aromatics such as BTEX. Another benefit is a more straightforward control scheme than in a two-zone furnace because the one-zone furnace controls do not have to address splitting gas between zones. As with any design, however, there are also some disadvantages.
Depending on feed gas composition, one-zone furnaces may experience problems with flame stability. Flame stability can be improved in a one-zone furnace by increasing the flame temperature by preheating the feed streams, including the combustion air. With very low quality, lean H2S feeds, natural gas spiking is sometimes employed to maintain flame stability. In a one-zone furnace, there is no acid gas bypass available to increase the flame temperature. Lower flame temperatures can leave one-zone furnaces more susceptible to flame-out in the event of feed composition changes and can also lead to multiple operating and reliability issues caused by incomplete destruction of contaminants. By placing preheaters in the feed gas and air streams, this operating instability can be mitigated; however, it comes at the cost of adding capex for the equipment as well as opex for maintaining and operating that equipment.
Two-zone reaction furnace
An alternative furnace design is the two-zone one-burner furnace (see Figure 2), also referred to as a front side split. This design is generally applied when the ammonia content in the acid gas exceeds the nominal 2 mol% limit, but it can also be used in cases with less ammonia. The general layout directs all the ammonia-bearing SWAG to the front zone where it passes through the burner. The front zone operates at a higher temperature to encourage adequate ammonia and contaminant destruction. The AAG, which is rich in H2S but contains relatively little ammonia, comes from upstream acid gas removal units. It is split and directed to both the front-end and back-end zones. The relative fractional split between the zones is a primary unit control parameter. The Claus process only requires that one-third of the total H2S be oxidised to SO2, which leaves the remaining two-thirds to act as a heat sink in the flame. Bypassing clean AAG around the front zone burner flame increases the flame temperature by reducing the amount of non-combusting gas in the flame that is acting as a heat sink.
The percentage of AAG being bypassed to the second chamber depends on the desired flame temperature as well as the concentration of H2S in the gas. The temperature of the front zone should be kept high enough to ensure adequate ammonia destruction, generally in the range 2300–2700°F (1260-1480°C). The maximum temperature is governed by the refractory lining’s upper temperature limit. Generally, no more than 60% of the total H2S in the SRU feed should be bypassed to ensure the atmosphere in the front zone does not operate in the oxidising region and cause NOx levels to increase. If the atmosphere in the furnace operates in this oxidising region, substantial amounts of NOx and SOx can be produced which, under the right conditions, can form hot aqua regia, an extremely corrosive material capable of dissolving even gold and platinum. The controls for the amount of acid gas bypass are generally based on the flame temperature in the front zone as well as the concentration of H2S in the AAG feed.
One of the benefits of a well-â€¨designed two-zone furnace is improved ammonia destruction over the one-zone furnace. To a large extent, greater ammonia destruction results from higher flame temperatures. This temperature control advantage in the front zone comes from the ability to bypass some of the amine acid gas around the front burner, relieving some of the cold acid gas load on the flame. When the heat sink of cold uncombusted acid gas is removed from the flame by bypassing it to the back zone, the flame temperature can be greatly increased. High temperatures are known to benefit ammonia destruction. The benefit derives from improved kinetics for the complex and somewhat counter-intuitive ammonia destruction mechanism described in the next section.
One of the negative aspects of the two-zone design is that the fraction of AAG that is being bypassed to the second zone does not pass through the burner flame at all. Thus, any contaminants that may be present are not subjected to the high temperatures of the flame. This can pose a problem if the contaminant levels in the clean AAG become excessive. Another negative is that the portion of the acid gas that is bypassed has a much shorter residence time, making it harder to achieve equilibrium of the Claus reaction in the furnace. Finally, since the two-zone design has an additional control point for splitting the clean acid gas flow between the zones, the control system will be inherently more complex. There are also safety concerns associated with bypassing acid gas around the front zone. If too much is bypassed, the temperature of the front zone can exceed the refractory thermal limits and compromise the integrity of the reaction furnace lining, and therefore the furnace wall itself.
Two-zone furnaces also require a relatively large nozzle on the side of the reaction furnace vessel to allow the bypassed acid gas to enter the back zone. This nozzle has accompanying mechanical challenges associated with construction and maintenance of the refractory lining, which are avoided in the one-zone furnace design.
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