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Caustic Scrubber Designs for H_2S Removal from Refinery Gas Streams

机译：苛性洗涤塔设计，用于从精炼气中脱除H_2S

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Caustic scrubbing is a technology that has been used for the removal of H_2S and other acid species from gases in different industries for many years. Improvements in design and control strategies, favorable prices for caustic compared with other non-regenerable H_2S scavenging chemicals, and the potential for sales value for the NaHS (sodium bisulfide) product have made caustic scrubbing an economic and viable option for some refinery streams. Caustic scrubber designs are available to treat sour refinery fuel gas both with and without CO_2, sour water stripper (SWS) gas, acid gas from amine units, and other refinery gas streams. This paper will review caustic scrubber designs applicable to these applications. The presence or absence of significant CO_2 is an important consideration with caustic scrubbers. If little or no CO_2 is present, caustic scrubbing is traditionally done by contacting the gas with recirculated caustic solution in a packed tower. More complicated designs, such as the dual-loop system in which distinct high strength and partially spent caustic recirculation loops are maintained in a two-section column, are available which can allow for the removal of H_2S in the gas to low levels while producing a high quality NaHS product. Caustic scrubbing of gases containing high CO_2 levels is particularly problematic because the CO_2 can also react with caustic, causing unwanted caustic consumption and the possibility of sodium carbonate solids precipitation. Special designs have been developed that allow the H_2S to be scrubbed preferentially while slipping the majority of the CO_2 in the gas; these designs take advantage of the fact that (similar to alkanolamines) the absorption of CO_2 in caustic is significantly slower than that of H_2S. Other designs are available to treat sour water stripper off-gases containing ammonia by caustic scrubbing the H_2S followed by steam stripping the NH_3 absorbed in the spent caustic product. Caustic scrubbers have even been used as backup devices to Claus units and other processes during shutdowns. There are many factors to consider when determining which caustic scrubber design is most appropriate for certain applications, such as the treated gas H_2S removal specification, the total quantity of H_2S to be removed, whether other components are in the gas that may be absorbed, how often the unit will be used, if the spent caustic will be sold or disposed of, and the cost of the feed caustic. This paper discusses the importance of each of these factors, and presents several caustic scrubber designs used at refineries and the rationale for implementing the different designs. Some of the important characteristics that must be considered during the detailed design of each of the scrubber systems are also presented (e.g., materials of construction, solubility, heat of reaction, and operating temperature and equilibrium limits). An important aspect in the effective design of caustic scrubber systems is the accuracy of the equilibrium data (e.g., pK_(a2) value) for H_2S removal and spent caustic composition, and the kinetic predictions for H_2S and CO_2 selectivity. Finally, the tools - simulation and other calculations - that can be used to predict that performance of caustic scrubbers are discussed, along with their limitations.
机译：苛性碱洗涤技术是一种用于去除不同行业的气体中的H_2S和其他酸类物质的技术，已有多年的历史。设计和控制策略的改进，与其他不可再生H_2S清除剂相比苛性碱的价格优惠以及NaHS（二硫化钠）产品的潜在销售价值，使得苛性碱洗涤成为某些精炼厂物流的经济可行的选择。苛性洗涤塔设计可用于处理含或不含CO_2的含硫精炼燃料气，含硫汽提塔（SWS）气体，来自胺装置的酸性气体以及其他精炼气物流。本文将回顾适用于这些应用的苛性碱洗涤塔的设计。对于苛性洗涤塔，重要的CO_2的存在与否是重要的考虑因素。如果几乎没有CO_2，则传统上是通过使气体与填料塔中的再循环苛性溶液接触来进行苛性碱洗涤的。可以使用更复杂的设计，例如双回路系统，在该系统中，在两段塔中保持独特的高强度和部分用尽的苛性再循环回路，这样可以将气体中的H_2S去除至低水平，同时生成高质量的NaHS产品。含CO_2含量高的气体的苛性洗涤特别成问题，因为CO_2还会与苛性碱反应，导致不希望的苛性碱消耗和碳酸钠固体沉淀的可能性。已经开发出特殊的设计，允许优先洗涤H_2S，同时使气体中的大部分CO_2逸出。这些设计利用了以下事实：（类似于链烷醇胺）苛性碱中CO_2的吸收明显慢于H_2S。还可以通过碱洗H_2S，然后水蒸气汽提吸收废碱产品中吸收的NH_3的其他设计来处理含氨的酸水汽提塔尾气。在关闭期间，苛性碱洗涤器甚至已用作Claus装置和其他过程的备用设备。在确定哪种苛性洗涤塔设计最适合某些应用时，有许多因素需要考虑，例如处理过的气体H_2S的去除规格，要去除的H_2S的总量，气体中是否可能吸收其他成分，如何去除。如果要出售或处置用过的苛性碱，通常会使用该装置，以及饲料苛性碱的成本。本文讨论了每个因素的重要性，并提出了炼油厂使用的几种苛性洗涤塔设计以及实施不同设计的原理。还提出了在每个洗涤塔系统的详细设计期间必须考虑的一些重要特征（例如，结构材料，溶解度，反应热以及操作温度和平衡极限）。有效设计苛性洗涤塔系统的一个重要方面是去除H_2S和废苛性碱成分的平衡数据（例如pK_（a2）值）的准确性以及H_2S和CO_2选择性的动力学预测。最后，讨论了可用于预测苛性洗涤塔性能的工具-模拟和其他计算-以及它们的局限性。

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