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Ammonia Destruction in the Reaction Furnace

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Ammonia Destruction in the Reaction Furnace 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. mmonia typically evolves from nitro- destruction chemistry, and concludes with a gen-containing components in processing case study illustrating how design and operat- Acrude oil. Processing and handling sour ing parameters affect ammonia destruction. gas and sour water containing ammonia in an environmentally acceptable manner has always One-zone reaction furnace been a challenge in refineries and is the focal The one-zone furnace, also referred to as a point for many designs and patents. There are straight-through design, operates with all the two main objectives that these designs and amine acid gas (AAG) and sour water acid gas patents attempt to achieve: purifying ammo- (SWAG) feed being mixed together, then fed to nia-bearing sour water to prepare the water for the furnace via a single burner (see Figure 1). further processing or discharge; and disposing of This design is generally applied when the feed the ammonia once it has been removed from the gas contains no ammonia, or small concentra- sour water. Options for disposing of ammonia tions of ammonia, but with the addition of include destroying it, or selling it as a product if preheat and incrementally longer residence time it meets purity standards. This article focuses on it can also be used successfully when ammonia is a common method for disposing of the ammo- present in higher concentrations. nia: destroying it in the reaction furnace of a A benefit of the one-zone furnace is that all the sulphur recovery unit (SRU). When directed to acid gas passes through the burner flame, an SRU, nearly complete destruction of ammo- thereby providing the maximum available heat nia is needed to prevent plugging the SRU with for the destruction of contaminants such as ammonium salts. hydrocarbons and ammonia. Insufficient The effectiveness of ammonia destruction is destruction of hydrocarbons in the reaction strongly influenced by the configuration and furnace can have disastrous effects on the operation of the reaction furnace. The two converter beds downstream, especially if they hardware configurations considered here are contain aromatics such as BTEX. Another bene- one-zone and two-zone designs. The two-zone design that will be considered has one SWS acid gas Amine acid gas burner at the front of the first reaction chamber. These are two of the most common reac- Air To waste heat boiler tion furnace configurations used in SRUs. In this article, a Burner brief introduction to each reac- Reaction furnace tion furnace design is followed by a discussion of ammonia Figure 1 General layout of a one-zone reaction furnace www.digitalrefining.com/article/1001297 PTQ Q4 2016 1 nia, comes from upstream acid SWS acid gas Amine acid gas gas removal units. It is split and directed to both the front-end and back-end zones. The rela- tive fractional split between the zones is a primary unit control Air To waste heat boiler parameter. The Claus process Zone 1 Zone 2 only requires that one-third of Burner the total H S be oxidised to SO , Reaction furnace 2 2 which leaves the remaining Figure 2 General layout of a two-zone reaction furnace4 two-thirds to act as a heat sink in the flame. Bypassing clean fit is a more straightforward control scheme than AAG around the front zone burner flame in a two-zone furnace because the one-zone increases the flame temperature by reducing the furnace controls do not have to address splitting amount of non-combusting gas in the flame that gas between zones. As with any design, however, is acting as a heat sink. there are also some disadvantages. The percentage of AAG being bypassed to the Depending on feed gas composition, one-zone second chamber depends on the desired flame furnaces may experience problems with flame temperature as well as the concentration of H2S stability. Flame stability can be improved in a in the gas. The temperature of the front zone one-zone furnace by increasing the flame should be kept high enough to ensure adequate temperature by preheating the feed streams, ammonia destruction, generally in the range including the combustion air. With very low 2300–2700°F (1260-1480°C). The maximum quality, lean H2S feeds, natural gas spiking is temperature is governed by the refractory sometimes employed to maintain flame stability. lining’s upper temperature limit. Generally, no In a one-zone furnace, there is no acid gas more than 60% of the total H2S in the SRU feed bypass available to increase the flame tempera- should be bypassed to ensure the atmosphere in ture. Lower flame temperatures can leave the front zone does not operate in the oxidising one-zone furnaces more susceptible to flame-out region and cause NOx levels to increase. If the in the event of feed composition changes and atmosphere in the furnace operates in this can also lead to multiple operating and reliabil- oxidising region, substantial amounts of NOx ity issues caused by incomplete destruction of and SOx can be produced which, under the right contaminants. By placing preheaters in the feed conditions, can form hot aqua regia, an gas and air streams, this operating instability extremely corrosive material capable of dissolv- can be mitigated; however, it comes at the cost ing even gold and platinum. The controls for the of adding capex for the equipment as well as amount of acid gas bypass are generally based opex for maintaining and operating that on the flame temperature in the front zone as equipment. well as the concentration of H2S in the AAG feed. One of the benefits of a well-designed two-zone Two-zone reaction furnace furnace is improved ammonia destruction over An alternative furnace design is the two-zone the one-zone furnace. To a large extent, greater one-burner furnace (see Figure 2), also referred ammonia destruction results from higher flame to as a front side split. This design is generally temperatures. This temperature control advan- applied when the ammonia content in the acid tage in the front zone comes from the ability to gas exceeds the nominal 2 mol% limit, but it can bypass some of the amine acid gas around the also be used in cases with less ammonia. The front burner, relieving some of the cold acid gas general layout directs all the ammonia-bearing load on the flame. When the heat sink of cold SWAG to the front zone where it passes through uncombusted acid gas is removed from the flame the burner. The front zone operates at a higher by bypassing it to the back zone, the flame temperature to encourage adequate ammonia temperature can be greatly increased. High and contaminant destruction. The AAG, which is temperatures are known to benefit ammonia rich in H2S but contains relatively little ammo- destruction. The benefit derives from improved 2 PTQ Q4 2016 www.digitalrefining.com/article/1001297 kinetics for the complex and somewhat coun- of ammonia to free nitrogen and water. The ter-intuitive ammonia destruction mechanism second (Reaction 2) is thermal cracking to free described in the next section. nitrogen and hydrogen. Both pathways are well One of the negative aspects of the two-zone documented and both occur in the reaction design is that the fraction of AAG that is being furnace. However, there is another pathway that bypassed to the second zone does not pass has been reported by Alberta Sulphur Research through the burner flame at all. Thus, any Limited6 (ASRL), which usually dominates in an contaminants that may be present are not operating plant. subjected to the high temperatures of the flame. The influence of hydrocarbons and other This can pose a problem if the contaminant contaminants in sulphur plant feeds will be set levels in the clean AAG become excessive. aside so the main focus can be placed on the Another negative is that the portion of the acid reactions involving H2S and ammonia. It is well gas that is bypassed has a much shorter resi- established that H2S is present in significant dence time, making it harder to achieve quantities in the front zone where ammonia equilibrium of the Claus reaction in the furnace. destruction takes place and where H2S oxidises Finally, since the two-zone design has an addi- to SO2. The kinetics of H2S oxidation have been tional control point for splitting the clean acid shown by ASRL3, 6 to be much faster than the gas flow between the zones, the control system reaction between ammonia and oxygen. Most, if will be inherently more complex. There are also not all of the oxygen present in the flame envi- safety concerns associated with bypassing acid ronment is consumed by H2S long before gas around the front zone. If too much is ammonia even has a chance to react. It being the bypassed, the temperature of the front zone can case that oxygen is rapidly consumed predomi- exceed the refractory thermal limits and compro- nantly by H2S, one must ask the question as to mise the integrity of the reaction furnace lining, what oxidises ammonia? ASRL has shown that a and therefore the furnace wall itself. pathway for this oxidative reaction begins with Two-zone furnaces also require a relatively H2S oxidation, then proceeds to ammonia oxida- large nozzle on the side of the reaction furnace tion as follows: vessel to allow the bypassed acid gas to enter the back zone.
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