Synthetic ammonia – full low-temperature shift process

This post was last edited by 654262293 on 2012-4-10 at 13:49. The full low-temperature shift process with a saturated tower has low steam consumption; however, it is inevitable that a small amount of water will be carried away in the form of mist, and this water may enter one of the catalyst beds, causing caking and scaling of the catalyst, and in severe cases leading to catalyst channeling. In addition, it increases power consumption (for the circulating hot water pump) and the consumption of desalinated water (for regular replenishment of circulating hot water). The full low-temperature shift process without a saturated tower does not require hot water circulation, which saves electricity consumption; the consumption of desalinated water is also reduced, although steam consumption increases. It is simpler and more convenient to operate. Furthermore, the capital cost of a process without a saturation tower is slightly lower.

As the primary device in conversion systems for recovering the waste heat (sensible and latent heat) generated by the conversion reaction, saturated hot water towers have seen their role diminish over time as Co–Mo low-temperature catalysts were developed and conversion processes were improved; the amount of waste heat from such reactions has become increasingly small. Moreover, saturated hot water towers used for heat recovery come with many drawbacks. Primarily, O2 and sulfides in the gas produce corrosive sulfate compounds in the circulating water of the saturated hot water tower, which have an adverse effect on both the catalysts and the equipment. The reasons for eliminating the saturated hot water tower are sufficient, and there are also successful examples. However, when deciding whether to eliminate the saturated hot water tower in the conversion system, a detailed analysis of the advantages and disadvantages must be conducted, and the decision should be taken carefully. For this renovation of the 120,000 tons/year synthetic ammonia full low-temperature shift unit, it was still decided to retain the saturated hot water tower, for the following main reasons. ①For the fully low-temperature conversion process, the temperature of the converted gas in the final bed is 220–230°C; the temperature of the circulating hot water, which is heated indirectly by a single-stage heater, can reach 105–115°C. Through heat and mass transfer in the saturation tower, the gas outlet ratio of vapor to gas is generally around 0.15, resulting in an additional moisture addition of 400 Kg/tNH3. If the saturated hot water tower is removed (retaining only the hot water tower) and the waste heat from the converted gas is used for copper washing, so that copper melt regeneration can be achieved without the need for external steam, the steam consumption remains comparable in both cases. However, the quality of the steam used for conversion is high, and the value of waste heat recovery from the conversion system for internal use is greater than that when it is used in other processes. ②Although saturated hot water towers have drawbacks, improving the quality of the water used for replenishment can reduce their negative effects ； The circulation of saturated hot water in a hot water tower has both advantages and disadvantages; the advantage is that the circulating water can wash away the oil contaminants present in the gas, thereby reducing damage to the catalysts. ③Canceling the saturated hot water tower is a new technology; corresponding technical adjustments must be made to the transformation process and related processes in order to achieve good results from the renovation. In terms of conversion processes, the full low-conversion process can be eliminated or retained; a decision should be made by weighing the pros and cons based on specific circumstances. It is best to retain the medium-low-low process, as medium-temperature Fe—Cr catalysts require a steam/gas ratio of ≥0.35 in the furnace gas. If the saturation tower is removed and reliance is placed solely on external steam, approximately 1000 Kg/t of steam must be supplied, which represents a huge difference compared to the steam consumption of the medium-low-low process; even with full utilization of waste heat recovery, this gap cannot be bridged. Therefore, the saturated hot water tower was retained in this renovation as well.

In short, the process with a saturated hot water tower presents equipment corrosion issues, while the process without a saturated hot water tower has higher steam consumption; otherwise, there are no fundamental differences.