For such chemical plants, is it necessary to install a continuous nitrogen supply system for air separation?
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A factory plans to install 6 GSP powder gasification units with a capacity of 2,000 tons per day, to produce methanol. 6 sets of domestic air separation units with a capacity of 48,200 cubic meters each are provided. The design institute believes that during normal operation, CO2 is used for both the gasification coal feeding hoppers and the coal supply system, while high and medium pressure nitrogen is only utilized during startup and shutdown periods. Therefore, the design institute has decided not to equip the air separation unit with a continuous nitrogen supply system such as a nitrogen booster or liquid nitrogen pump; instead, nitrogen is regenerated within the cold box and supplied continuously to the nitrogen storage tank or pipeline network. Only one liquid nitrogen tank is installed, along with 2 liquid nitrogen pumps capable of handling 20,000 cubic meters per hour (when operating simultaneously). Nitrogen is supplied to the pipeline network through vaporization by heating; a storage tank shared with CO2 is provided in the vaporization unit as a backup system. I would appreciate it if experts could help analyze whether such a nitrogen configuration can meet the safety requirements, as well as the needs related to startup and shutdown, of the gasification unit and subsequent equipment
1) If the CO2 compressor shuts down suddenly, how quickly can the nitrogen system be activated to maintain the pressure and flow rate in the common CO2 storage tank? At the very least, the stability and reliability of the CO2 system must be carefully considered. The issues related to water absorption by CO2 and pipeline insulation can be completely resolved. Install the drying system, install a spare CO2 compressor – two in use and one as backup? 2) How is the large amount of liquid nitrogen produced by air separation handled during normal driving? Sell it? 3) Additionally, for the security N2 system, aside from the coal feeding system, there are probably other areas where there is a need to supply nitrogen urgently as well, right? I’m not very familiar with the site; this is for reference only.
For a plant with 6 gasification units, it is acceptable to install two CO2 compressors. There should be no issue with water content in the CO2, as the CO2 used comes from the CO2 regeneration tower of the low-temperature methanol washing process; it has been exposed to temperatures below -56 degrees Celsius, and after subsequent heat recovery, its temperature is raised to 100-120 degrees Celsius. Therefore, its dew point should be below -50 degrees Celsius. The compressors only need to raise the pressure to 6.5 MPa, while the buffer tank should maintain a pressure of over 6.0 MPa, since the pressure in the lock-hopper system is only 4.7 MPa. As for the liquid nitrogen system, there are some challenges: it is necessary to maintain a high liquid level in the liquid nitrogen storage tank, with at least one liquid nitrogen pump in hot standby mode, and the heating vaporizer also in hot standby condition. The pressure in the pipeline network should be kept above 7.0 MPa. This system is interlocked with the liquid nitrogen pumps and the heating vaporizer, as well as with the pressure of the CO2 buffer tank. As for the liquid nitrogen in the storage tank on a regular basis, it doesn’t need to be delivered; of course, it can be done if there is a market demand for it. It should be easy to manage 6 air separation units that produce liquid nitrogen or do not produce it. I’m worried whether the liquid nitrogen pump will be able to start instantly when needed and provide a continuous supply for an extended period of time, for example, in case of a malfunction that requires repair of the CO2 compressor ； Also, can the powder gasification island truly operate without the continuous use of high-pressure nitrogen during normal production? When starting up the original gasification unit without CO2, will 20,000 cubic meters of nitrogen be sufficient for startup? Can it only be that one gasifier is started and brought to proper operation with satisfactory H2S levels in the methanol wash, before the second gasifier is started? Isn’t that waiting for over ten hours? It doesn’t seem very reasonable; I would appreciate it if professionals with experience in powder gasification of methanol could provide some clarification!
I suddenly remembered that one of the technical staff at our former company had the idea of building another N2 storage tank, a high-pressure one, to be connected to the CO2 buffer tank. After being produced, liquid nitrogen is stored in the form of high-pressure N2, rather than in its liquid form. In this way, when the CO2 pressure is low or when there is a sudden stop downstream, the N2 in the high-pressure N2 tank is released in a sequential manner into the CO2 buffer tank; this process occurs much more quickly than when liquid nitrogen and a vaporizer are used for such purposes. Furthermore, the time gained allows other liquid nitrogen tanks and vaporizers to be activated to continue supplying N2, preventing a shutdown due to vaporization.
Regarding the determination of technical solutions, the company’s owners should hold a special meeting with the designers, each party should present their views, reach a consensus, sign off on it in the form of meeting minutes, and then file it for record-keeping. Extremely important matters should be determined through communication between the chief engineer and the design project manager.
There’s no need to worry about this. Air separation units usually have two or more liquid nitrogen storage tanks: one is used to store liquid nitrogen for sale, while the other stores liquid nitrogen that is sent to the vaporization unit via a liquid nitrogen pump. The pressure in this case is medium-pressure nitrogen, which can be used for purging and cooling in vaporization furnaces as well as as a shielding gas. It is also used together with low-pressure nitrogen; as long as the blind flanges are switched correctly, there will be no problems. Continuous supply results in high energy consumption and does not meet the process requirements, in my personal opinion.
: This is a commonly used accident nitrogen storage tank, provided there is continuous nitrogen to maintain pressure ? ? It seems I didn’t explain myself clearly; maybe you didn’t catch the main point of what I said. I feel like you didn’t read what I wrote carefully. The premise you mentioned (continuous nitrogen pressure maintenance) requires a liquid nitrogen pump and an evaporator to be operating continuously to supply the necessary nitrogen. In the event of an accident, if the pressure or supply of N2 is insufficient, it is these liquid nitrogen pump and evaporator that are needed to take action. If there is only a liquid nitrogen pump, the pump needs to be started and the heating vaporization process is required. If the accident N2 high-pressure tank is included, the reaction mechanisms corresponding to these two scenarios become different. I hope I made it clear.
In the nitrogen system, if there is an interruption, the system is immediately isolated to maintain pressure, so as to prevent any issues arising from a nitrogen supply disruption. It’s not as concerning as you might think. There are hundreds of cubic meters of liquid nitrogen stored there; is there a risk that the liquid level in the nitrogen storage tanks will be too low? This liquid nitrogen pump also comes with a backup pump, and it can generally operate continuously for a day without any problems. The nitrogen produced by air separation is mainly used for purging and cooling in vaporization furnaces, as well as as a shielding gas; without such a shielding gas, the system would have been isolated long ago
The poster is also concerned about whether, in the absence of CO2 downstream, the CO2 used for coal powder transportation can be replenished in a timely manner using a liquid nitrogen system. You have only improved the security aspect; the issue of coal powder transportation still needs to be addressed.
Oh, I don’t understand this. Hehe, I need to study it
Our factory is also worried about starting up the nitrogen compressor; however, it seems you don’t have any problems of that kind. The main issue lies in the fluctuations in the temperature of the gasification furnace when switching between nitrogen and carbon dioxide during normal operation, and these fluctuations are inevitable