Requirements for adopting fully low-variation technology

The requirements for process components for full low conversion are generally: 1. H2S in gas: Early full-low conversion requires the H2S content in the intake air to be above 150mg. When the conversion rate is not high and the amount of steam added is not large, normal operation can be performed under 100mg. ； 2. Oil content in gas: When the oil content in the gas is high, it will deposit on the surface of the catalyst, destroying the pores and causing catalyst deactivation. ； 3. O2 content: Generally, O2 is required to be less than 0.3. High O2 content can easily cause the catalyst to overheat, thus affecting the crystal form and pore structure of the catalyst components, resulting in a decrease in catalyst activity. ； Second, it is easy to oxidize H2S in the semi-water gas to form sulfate, which blocks the pores. The sulfate reacts with the potassium ions in the catalyst to form potassium sulfate, causing the catalyst activity to quickly lose. 4. Cold shock water quality between sections: To use deep desalted water, the conductivity is generally required to be less than 0.5. The main drawbacks in use are: 1. Gas composition requirements are strict. The above H2S, oil pollution, and O2 contents must be controlled according to the indicators. ； 2. Anti-vulcanization may occur: Especially in a high-temperature zone, the hot spot temperature should be around 400 degrees, and the trend of reverse vulcanization increases, so the amount of steam added needs to be strictly controlled. ； 3. Prevent liquid water from entering the catalyst layer: Water in the bed will cause the loss of alkali metals in the catalyst, cause the catalyst to agglomerate, and increase resistance. ； 4. The operation requirements are strict, and the control of various indicators must be stable to prevent large fluctuations.

I think the efficiency of the full low-change catalyst is higher, but the low-change catalyst is more delicate and not as strong and sulfur-resistant as the medium-change catalyst, so our factory uses the medium-low-low process.

Nine key factors for the normal operation of the full low-change process. l Full-low-change series catalysts For the full-low-change process of oxygen-containing gas, oxygen scavengers are a necessary and sufficient condition for the successful application of the full-low-change process. In other words, for fertilizer manufacturers that use intermittent gas generation, oxygen scavengers must be used when using the full-low-change process, and the amount must be sufficient, otherwise there will be failure. There are many commercial names for oxygen scavengers, and there are also many commercial promotions, but there is only one essence, and that is to remove the oxygen in semi-water gas to less than 0.2% to ensure the normal operation of the full low-change process. Currently, only a few domestic manufacturers have mastered this technology. Fully low-variation catalysts also have their own particularities. They mainly have two performance indicators. First, the active component content must reach a certain value. Long-term operation cannot be guaranteed if it does not meet the ministerial standards. Second, the carrier, especially the first low-variation catalyst carrier, must choose a carrier with good oxygen resistance. The higher the pseudo-thin water content in the carrier raw material, the more oxygen-resistant it is, but the cost of the carrier is also higher. 2 Sulfurization of high-quality sulfur-tolerant shift catalysts does not necessarily mean good shift activity. The key lies in the sulfurization of the catalyst. Production practice shows that although CS2 is flammable and has certain toxicity, it is the best vulcanizing agent currently discovered. There are three necessary conditions for successful vulcanization. First, there must be a high enough vulcanization temperature, generally above 4000C; secondly, there must be sufficient forced vulcanization time. If it exceeds 4h, it is best to have a furnace of more than 2h; thirdly, during forced vulcanization, the higher the hydrogen sulfide in the raw gas, the better. It is generally not less than 15g/m3. When vulcanizing, the catalyst must be prevented from overheating. Exceeding 5500C will cause harm to the catalyst, but short-term overheating has little effect on the catalyst activity. For non-original start-up sulfidation catalysts, it is best to add some new shift catalysts or Υ-AI2O3 balls at the reactor inlet, otherwise the CS2 sulfidation will not have a significant resurrection effect, because the CS2 hydrolysis and hydrogenation activity of the oxygen scavenger or sulfur-tolerant shift catalyst will be significantly lost after being used for a period of time. The CS2 added during sulfidation cannot be converted into H2S to sulfide the catalyst, but is adsorbed on the catalyst or brought into the back system to cause corrosion of the equipment. 3. Impact of impurities on the catalyst. Oxygen in the semi-water gas causes sulfation of the low-variation catalyst and rapid loss of activity. Compressor oil and tar are adsorbed on the catalyst at low temperatures, blocking the pores and causing the catalyst to lose its specific surface. At high temperatures, partial catalytic dehydrogenation turns into coke, which deposits on the catalyst, greatly reducing the catalyst activity. Arsenic is highly toxic to low-variability catalysts, and Shaanxi Weihe Fertilizer Factory suffered greatly from it. Calcium and magnesium ions in the steam and saturation tower will cause the low-temperature activity of the low-variation catalyst to be lost, and the content above 0·5% will have a significant impact. Phosphorus in the steam and feed gas will cause rapid deactivation of the low-shift catalyst. When the content is above 0.3%, the shift activity will decrease rapidly. NH3 in the feed gas has no obvious effect on the activity of the low-shift catalyst. However, at higher pressures such as greater than 2.0MPa, it will accelerate the transformation of the Υ--AI2O3 carrier into AIOOH, thereby damaging the catalyst strength and reducing the specific surface of the catalyst. The SO42- entrained in the shift gas causes great damage to the low-shift catalyst and reacts with the K in the catalyst to form K2SO4, which quickly deactivates the catalyst. Chlorine and chloride are not poisons to the low-variation catalyst, and HCN has no effect on the activity of the catalyst. The light-based iron and carbon-based nickel generated in the feed gas will decompose on the oxygen scavenger, causing the oxygen scavenger to block the pores and become inactive, but it will not affect the activity of the low-variation catalyst. 4 The atomized water in the raw gas causes irreversible deactivation of the low-variation catalyst and cannot be regenerated. This phenomenon must be avoided whether the catalyst is being sulfurized or in normal production. Possible factors that produce atomized water include water in the steam, water in the saturated tower, liquid in the spray water, condensation liquid and sulfide water due to improper design of the temperature control water. The deactivation mechanism may be that the atomized water causes the potassium in the low-variation catalyst to migrate from the inside of the catalyst to the surface, thus blocking the surface pores of the catalyst. As a result, the reaction gas cannot enter the catalyst, making the catalyst inactive and unable to be sulfurized and regenerated. SEM, TEM and EDAX analyzes have confirmed the above results. 5. Total sulfur in semi-water gas The sulfur problem in the full low-change process is mainly considered from two aspects. One is the desulfurization of the low-change catalyst, which determines the lower limit of total sulfur; the other is the corrosion of equipment, which determines the upper limit of sulfur. The desulfidation of low-change catalysts is determined by the following four factors: temperature, total sulfur concentration, H2O/gas and catalyst. High temperature and high H, H2O/gas are conducive to the desulfurization of low-variation catalysts. Different low-variation catalysts have different desulfurization trends under the same process conditions. The lower the total sulfur concentration, the easier it is for low-variation catalysts to be desulfurized. The industrial experience value is that the total sulfur in the semi-water gas coming out of the saturated tower should not be less than 100 mg/m3. High sulfur in the raw gas can easily cause serious corrosion of equipment. Saturated hot water towers, heat exchangers and temperature regulating water heaters are all equipment that are prone to corrosion, especially saturated hot water towers. High concentrations of sulfides can easily form a large amount of SO42-, causing an increase in total solids. SO42- is brought into the conversion furnace and can easily deactivate the deoxidizer. For low-shift catalysts, the impact of sulfide on their activity can be divided into two stages. Experimental results show that the higher the sulfide concentration in the feed gas, the higher the activity of the low-shift catalyst. Below a certain sulfide concentration, the activity of the low-shift catalyst is reduced due to desulfidation, and above this concentration, the activity of the low-shift catalyst is reduced due to the sulfide concentration. The activity of the sulfide agent changes, and this sulfide concentration changes with the low-variation catalyst. Therefore, based on production practice, the appropriate sulfide concentration for the full low-variation process of intermittent coal gas production is recommended to be 13Omg/m3-2g/m3. Within this range, the sulfide concentration should be increased as much as possible to facilitate the activity of the low-variation catalyst. 6. Hot spot temperature of the full low-change stage. Under normal circumstances, the hot spot temperature of the full low-change stage catalyst is the highest temperature point in the entire system. It is determined by seven factors, namely: the oxygen content in the feed gas, CO concentration, sulfide concentration, H, H2O/gas, low-change catalyst type, catalyst loading and the height-to-diameter ratio of the reactor. The higher the oxygen content in the semi-water gas, the higher the hot spot temperature. The higher the CO concentration, the higher the hot spot temperature. High sulfide concentration can increase the hot spot temperature. H2O/gas can increase the hot spot temperature within a certain range. The more catalysts installed, the more conducive to the increase of the hot spot temperature. The higher the reactor height-to-diameter ratio, the higher the reaction hot spot. High hot spot temperature can easily cause desulfidation and sintering of low-variation catalysts, reducing service life. Industrial practice shows that under general process conditions, the hot spot temperature should not exceed 4000C, and is strictly prohibited from exceeding 4300C. For normal operation, 360 to 3800C is appropriate. When changing the composition of raw materials, such as changing coal to coke, when increasing production without changing the reactor, or when starting up a new device for the first time, all low-variation process manufacturers should seriously consider this issue. 7 Resistance From the perspective of production practice, the main reasons for the increase in resistance in the full low variable process are nothing more than two categories. One is caused by equipment. For example, baffle heat exchangers can easily cause resistance to increase, plate heat exchangers can easily cause resistance to increase, equipment size is too small, etc. The particularly unobtrusive wire mesh on the grate plate can also easily cause resistance to rise. Due to the scouring effect of gas, catalyst dust will be washed downward. If the mesh number of the wire mesh is too large or there are too many layers, dust will accumulate here, causing resistance to rise. The other type is caused by the catalyst and filler, especially the oxygen scavenger. Due to the current technical level, the strength and wear of the oxygen scavenger are not satisfactory, and it is easy to pulverize during use, causing the resistance to rise. Since the low-variation catalyst contains about 13% K2CO3, when it encounters atomized water, the migration of potassium can easily cause agglomeration, causing the resistance to rise. Due to poor purification of semi-aqueous gas, a large amount of tar enters the saturated tower and is adsorbed on the packing in the saturated tower, which can easily cause resistance to rise. 8. Bias flow in the fully low-variable system is extremely harmful to normal production. It usually causes local overheating of the bed and partial penetration of the catalyst bed, causing shutdown and maintenance. Especially biased flow in the oxygen scavenger bed will cause rapid deactivation of the first-stage fully low-variable catalyst. The main reasons are as follows: ① The catalyst is unevenly loaded and is not loaded as required. People cannot step directly on the catalyst and must place a wooden board. ② The design of the gas distributor is unreasonable. Some manufacturers have an air inlet pipe extending directly above the bed, causing the surface of the catalyst foam layer to deform during operation, forming a pot-bottom-shaped or mushroom-shaped bias flow. ③ The quality of the insulation layer is unqualified, cracks and separation from the wall appear, and gas flows from the cracks along the reactor wall. ④The typical symptom of bias flow caused by agglomeration of low-variation catalysts is that the bed resistance increases and the outlet CO concentration increases. 9 Corrosion The corrosion of equipment in the full low-variation process is relatively serious. There are three types of corrosion that have been found: first, corrosion caused by potassium enrichment CI-; second, corrosion caused by SO42-; third, dew point corrosion. Corrosion caused by potassium enrichment CI- has been reported in large nitrogen fertilizer Wu Petrochemical and Ningxia Chemical Plants, and similar reports have been reported in a methanol plant in the United States. CI- mainly comes from water, steam and low-variation catalysts. Qilu Institute has detected that CI- is added to the low-variation catalysts of some manufacturers. Potassium mainly comes from low-variation catalysts, whose K2CO3 content is about 13%. The corrosion caused by SO42- has been confirmed by SEM-EDAX. SO42- mainly comes from saturated towers, warmers and hot water towers, and is produced by the oxidation of H2S. Dew point corrosion is unavoidable, and there are many prevention and control measures. However, one method is inappropriate, which is to use scrap iron filings as a pre-corrosion measure to protect the main heat exchanger. This method is actually not worth the gain. Not only can it not solve the dew point corrosion, but a large amount of rust blocks the main heat exchanger and the first low-variation catalyst bed, causing the system resistance to rise.

It makes sense. Nowadays, it is also a medium-low and low-voltage process. When the system is new, I want to make it full low-voltage.

The organic sulfur conversion rate of fully low-density conversion is very high, which can basically reach more than 90%.； The conversion principle is the same as that of medium to low.: http://bbs.hcbbs.com/redirect.php?goto=findpost&pid=2390634&ptid=442783

Thank you, you responded very seriously. study * Got it! Are there any special requirements for the content of organic S? In addition, what is the hydrolysis conversion capability of organic S through the full low-change process? Is there a better way to solve the problem of organic S in process gas?

The full low-variation process is now relatively mature. Currently, a number of joint alcohol plants with greater capabilities are transforming the medium-low-low process into a full low-variation process. For the 2.5MPa pressure level, heat recovery can be carried out on a saturated hot water tower or not on a saturated hot water tower. The corrosion of the saturated hot water tower can be effectively controlled. The outlet CO is high and the steam consumption is only about 80Kg/tNH3. The fully low-variable reaction temperature is relatively low, and the conversion rate of organic sulfur is about 90% as mentioned on the 6th floor. The larger the loading volume, the higher. Theoretically, the hydrolysis Kp of COS is much greater than the hydrogenolysis Kp. The hydrolysis furnace can be connected in series with an appropriate temperature control behind the low-variation furnace. However, the current effect of the hydrolysis catalyst is not very good. Our experience is that within the permissible range, when other conditions are similar, low-variation catalysts have better activity and longer service life under working conditions with higher total sulfur. The same is true for low-change catalysts in low- to medium-low processes.

At present, there are three main conversion process technologies in nitrogen fertilizer plants in my country:: Mid-string low, full low conversion and mid-low low conversion processes. The mid-range converter is developed based on the original medium converter process. The converter uses an iron-chromium series catalyst, and the low-converter uses a cobalt-molybdenum series low-converter catalyst. The iron-chromium series catalyst has a high reaction temperature and a large steam-to-gas ratio, but the conversion efficiency is not high, resulting in excessive equipment size and heat exchange area, which will increase energy consumption, operating costs, and investment. In the early 1990s, during the continuous development and application of low-variation catalysts, a full low-variation process was developed. Because the hot spot temperature of the entire system is reduced, the production capacity of the equipment is increased and the steam consumption is also significantly reduced. In the past few years of use, it was discovered that the full low-variation process has weaknesses that are sensitive to oil pollution, oxygen, etc., which often cause large fluctuations in production. In order to solve this problem, the research department adopts the method of antioxidant pre-conversion. At the outlet of the pre-conversion furnace, the gas temperature reaches 400°C. The medium-low-low conversion process uses the medium-change catalyst as an antioxidant and anti-toxic agent to solve the poison of oil pollution, oxygen and other gases on the low-change catalyst. The latter part of the process is the same as that of full low-conversion process, so the medium-low low-conversion process mainly overcomes the disadvantages of the full low-conversion process and has the advantages of full low-conversion process, so it is promoted and adopted. Its energy consumption is slightly higher than that of full low-conversion process.