A brief analysis of the purification processes for several feed gases in ammonia synthesis

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A Brief Analysis of the Purification Processes for Several Feed Gases in Ammonia Synthesis Zhou Daming, Li Menglu (Tianji Jincheng Chemical Co., Ltd.) Abstract: This paper briefly describes several methods for purifying feed gases, such as copper washing, hydroformylation, dimethyl process, and alcohol-to-hydrocarbon conversion, along with their characteristics. Keywords: Synthetic ammonia; Feed gas; Purification The purification of feed gas for synthetic ammonia is a crucial process in production; excessive levels of trace gases (CO+CO2) in the feed gas can cause poisoning of the ammonia synthesis catalyst, rendering it unable to function. At present, most small and medium-sized nitrogen fertilizer plants in China that use coal as a raw material and employ fixed-bed gasification rely on the copper washing method to remove these trace gases (CO+CO2). The traditional copper washing method is a relatively outdated purification process with disadvantages such as numerous equipment requirements, complex procedures, cumbersome operation, high material consumption, and being a major source of environmental pollution in production. With the development of sulfur-resistant low-temperature shift catalysts and advanced desulfurization technologies, small and medium-sized nitrogen fertilizer plants have recently begun to use processes such as methanol production, methanation, dimethyl synthesis, and alcohol-oil conversion at various pressures for the purification of feed gas, as alternatives to copper washing. These feed gas purification methods are gradually becoming more refined and mature, and they offer clear advantages over the traditional copper washing approach. 1. Shortcomings of the copper washing purification process The traditional copper washing purification process has been in use in small and medium-sized nitrogen fertilizer plants for decades, and it is still employed in most of these plants today. Although copper washing purification yields varying results across different plants, it generally has many shortcomings. 1.1 Copper washing is a process prone to accidents in ammonia synthesis production. Due to the long process flow, numerous equipment involved, and the fact that the composition of the copper melt is affected by various factors, accidents of varying severity have occurred in copper washing operations at various plants. Many plants have encountered problems such as excessively high levels of trace (CO+CO2), liquid in the copper towers, blockages in the equipment packing, fluctuations in the composition of the copper melt, and difficulties in adjusting the copper ratio; these are processes prone to accidents. 1.2 High material consumption in the copper washing purification method: During gas purification, the copper solution requires the addition of ammonia, copper, and acid. It absorbs trace substances at low temperatures and releases them back at high temperatures, which results in both heat loss and cooling energy consumption (in the form of steam and electricity). The copper solution absorbs (CO+CO2) during the purification process, and it also dissolves useful gases such as H2; even with systems in place for regeneration and recovery, gas losses still occur. The operating costs for copper washing vary from plant to plant, with differences in material consumption; however, they are generally not less than 50 yuan per ton of NH3, and can even reach over 100 yuan per ton of NH3. 1.3 Copper washing is a process that takes place in an environment with poor conditions and high levels of pollution at the production site. Leaks and spills at the copper washing site represent a challenge in terms of management. Copper melt leakage and the emission of recycled gases contaminate water bodies and the atmosphere, hindering the improvement of enterprises’ environmental protection efforts. 2. Co-production of methanol reduces the production load associated with copper purification. Currently, a considerable number of small and medium-sized nitrogen fertilizer plants have introduced methanol co-production processes before the copper purification step. This not only increases the range of chemical products produced by these plants, but more importantly, it reduces the burden on copper purification. The processes of conversion and decarburization also become less demanding as a result, resulting in clear overall benefits. · 2.1 Process for producing methanol jointly With the synthesis of ammonia, the methanol production reaction is as follows: CO + 2H2 → CH3OH; CO2 + 3H2 → 2CH2OH + H2O. In many plants, methanol production is added after the ammonia synthesis unit comes online. Given the production processes of small and medium-sized nitrogen fertilizer plants as well as the characteristics of their compressors, methanol production is usually carried out before the copper washing stage, at the same pressure level of 10–13 MPa; this is known as medium-pressure methanol production. Process flow diagram for ligand alcohol production 1: http://www.anchun.com/images/pic/q4.JPG Figure 1: Block diagram of the ligand alcohol production process. Note: Depending on the internal components of the synthesis tower, an external heat exchanger can be installed in front of the tower to raise the temperature of the material entering the tower and lower the temperature of the water flowing into the cooling system; a waste heat recovery device can also be employed. 2.2 Production of Unicellulose Alcohol The production technology for unicellulose alcohol is highly mature; the internals used in the synthesis towers are mostly of the uniform temperature type, while the catalysts primarily consist of C207, C301, and Wc-I(Ⅱ). For the temperature-induced reduction of catalysts, refined gas is commonly used; the NH3 content and sulfur content in this gas must be strictly controlled, with H2 accounting for 70%-75%. During the temperature-raising reduction phase, it is advisable to maintain low temperature and pressure conditions when discharging water in order to enhance catalyst activity. A key aspect in ensuring stable production of lysoleucine is to properly purify the gases entering and leaving the process. The shift gas must undergo wet desulfurization, while the decarburized gas requires further precise desulfurization; the total sulfur content in the gas fed into the lysoleucine production process