Several issues with the transformation section?
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To my colleagues in the ammonia synthesis industry: Our company is a small nitrogen fertilizer manufacturer with an annual production capacity of 40,000 tons. The shift converter section uses a combination of medium-pressure and low-pressure converters; the catalysts in the low-pressure converter were replaced just one year ago, while those in the medium-pressure converter were replaced during equipment maintenance before last New Year’s Day. A small amount of filtered used catalyst has been used in the first stage of the medium-pressure converter, and at present the plant operates at only 75% of its normal capacity. The temperature in the first stage of the medium-pressure converter has dropped to 310–330 degrees Celsius, forcing us to use electric heaters to maintain the necessary temperature for production. I would like to ask whether it is better to use the filtered used catalyst in the first or third stage of the medium-pressure converter – and what are the reasons behind this??? Since the low-pressure converter started operating too early during the heating and reduction process of the medium-pressure catalyst, excess steam ended up reaching the low-pressure converter, resulting in poor catalyst activity (before shutdown, the CO level at the outlet of the medium-pressure converter was 8–10%, while the temperature was around 250 degrees Celsius; now it’s necessary to reduce this CO level to 3–4%, with a temperature of around 210 degrees Celsius). The catalyst should exhibit good activity when the temperature is above 180 degrees Celsius. Therefore, a large amount of steam is required at present. The measure we have taken is to reduce the CO level at the outlet of the low-pressure converter in order to improve its catalyst activity. Do you have any other suggestions???
Place the old catalyst in section 3; this way, the temperatures of sections 1 and 2 remain unaffected. The temperature in section 3 can be maintained by using less cold water – a steam pipeline can be installed in the water heating unit. Additionally, increasing the oxygen level can effectively raise the temperature.
It is rare for the temperature to drop to such a level. It is highly likely that the regulation is inadequate, including imported hydrogen sulfide, cold shocks, heat recovery, etc.
The guy on the second floor is really ruthless! I don’t know how many years he’s worked in the chemical industry. It’s not possible to increase the oxygen level – if there’s too much oxygen, it becomes uncontrollable; in mild cases this leads to the shutdown of the system, while in severe cases it can cause catastrophic failures. I suggest that old catalysts should be kept on the first floor, so that even if they stop reacting, those on the second floor can still function. You can reduce the water vapor ratio appropriately; if this ratio is too high, it’s possible to lower the temperature of the bed layer
The medium-pressure converter in our facility uses a catalyst from 2006, the low-pressure converter No. 2 uses a catalyst from 2005, and the low-pressure converter No. 1 uses an older catalyst. If you want to control the temperature of a certain section, it’s better to control the intermediate inlet temperature. If the value is 310-330, to increase it, adjustments can be made by using a gas double line, a low-pressure conversion double line, changing the flow rate of the circulating water, or adjusting the opening degree of the double line. The second stage uses cooling water for regulation. Are all of them possible?
I think it’s still better to use three stages; in these three stages, one can make use of the temperature ranges from stages one and two, thereby reducing the need for cold shocks or even eliminating them altogether, while still ensuring the appropriate temperature in stage three. As you said, you’ll need to replace the catalyst there
The purpose of installing a catalyst is not to regulate temperature but for the reaction! ! ! The old catalyst is installed in Section 1; there, the temperature is high and the CO concentration is high, so its lower activity has little impact on the reaction. Moreover, the activity of Section 1 declines more rapidly anyway, which also serves to protect the subsequent sections. The new catalyst is installed at the back; it has a long lifespan and high activity, which ensures a better conversion rate. In the situation described by the poster, it should be parked and then vulcanized again at a lower temperature.
Personal opinion: 1. The old catalyst should be placed in the highest temperature zone of the medium-temperature reactor, specifically in the lower part of it; since the low-temperature activity of the old catalyst is no longer sufficient, only its high-temperature activity can be utilized. 2. The poor activity of low-volatility catalysts, which is caused by an excessive amount of this catalyst, can be temporarily addressed by raising the temperature; it is recommended to carry out sulfiding again during the next shutdown.
If the quality of the gas used for gas production is good, the old catalyst can be placed on the surface layer of the middle conversion section; if the gas quality is poor, it should be placed at the bottom of the first section to make the most of it. Since the old catalyst is not sensitive to oxygen, the reaction lag causes the new catalyst to overheat frequently; we have had precedents for this! As you said, I think it’s either an imbalance in the system’s heat, or the misty water layer in the saturation tower makes it difficult to maintain the temperature in the middle and upper sections, which causes the reaction zone to shift backward; or there could be a problem with the instruments. In the previous years, when the temperature near us was as high as nearly 50°C, we also carried out production using electric furnaces with insulation.
In my opinion, it might be a problem with the operation – specifically, too much steam was added, which leads to several consequences: 1. It’s difficult to maintain an appropriate intermediate temperature; in severe cases, the furnace temperature has to be maintained using an electric heater； 2. A large amount of steam can also cause desulfurization of the low-temperature conversion catalyst, reducing its activity and leading to higher conversion rates. When operators see these high values, they increase the amount of steam used, creating a vicious cycle; this can also result in high sulfur content in subsequent processing stages ； 3. High steam consumption, which is not conducive to energy savings and cost reduction. The specific consumption of synthetic ammonia has increased.
Put the old catalyst in section 3? ? It should start with the lower temperature areas, and the 3 stages from lowest to highest temperature should use new ones! Section 1 requires a sulfur-resistant catalyst. There are many reasons for the deactivation of medium-shift catalysts; one of them is the sulfur content in the feed to the shift unit. Is the catalyst you purchased sulfur-resistant? Is the total solid content at saturation high? Yes, it is; fine particles are carried by the gas into the catalyst layer, blocking its pores. Is the oxygen content exceeding the limit again? What is the quality of the soft water? Does the heat exchange equipment leak internally? Wait, catalyst deactivation may seem like a simple issue, but it’s actually not easy to determine accurately. The low-temperature catalyst becomes deactivated; if the rate of deactivation is fast (within 1 year), it is likely due to oxygen poisoning of the catalyst. Check whether there are any internal leaks in the heat exchanger – it’s possible that gas has leaked into the medium-temperature gas stream, resulting in a high oxygen content that causes deactivation. Last edited by zhuoyue on 2009-3-3 17:52