Problems during the commissioning of MTO product gas compressors
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Hello everyone. I’m still not clear about the operation procedures for MTO. I would appreciate some guidance from experienced users. During initial operation of MTO, the fourth stage of the product gas compressor is located between the high-pressure depropanizer and the demethanizer; for nitrogen-based startup, it is used to purge the reactor. 1. Is nitrogen startup to pass through sections 1-4? If it is not continuous, and sections 1-3 are vented from the dryer, then what should be done with section 4? Nitrogen-purged full reflux? 2. When propylene is fed in later, do the high-pressure tower and demethanizer need to be filled with propylene?
So many DMTO units use the CB&I Rums process; I would appreciate any advice from those who have experience operating them
Personal opinion: Operate the reactor by passing nitrogen through sections 1–4 in full reflux mode; after the reactor is started, replace the nitrogen with process gas, which is then discharged from the outlet of section 4. Gradually reduce the reflux rate as the reactor load increases, increase the pressure at the outlet, and feed it to the demethanization tower. There is one thing that concerns me during this process: when nitrogen is displaced by process gas, the compressor load does not increase, and the outlet pressures at each stage do not reach the design values. As a result, the dehydration effect is poor, and I am worried whether the dryer can handle the conditions; I am also concerned that a shortened saturation cycle might affect the switching process. I would also appreciate the guidance from the experts
We also use Rums technology for separation; with nitrogen as the driving gas, there is little need to worry about the dryer. However, when nitrogen is used in full reflux, it is important to note that due to the poor compression ratio of nitrogen, excessive temperature and pressure can occur in various sections of the system. As for the dryer, it’s no problem, because the regeneration of the dryer itself also uses nitrogen, and there is a backup unit available during operation. Now, once the compressor is started, the pressures in each stage generally do not change significantly. The liquid phase dryer will not contain any liquid when nitrogen is used to start the system; liquid will only begin to appear gradually over time, and the amount of liquid will not be large. It should be noted that when the MTO unit is started, the amount of water in the product gas can be much higher than under normal operating conditions, so it is necessary to remove this water promptly from the third-stage discharge tank to prevent excessive amounts of water from entering the liquid phase dryer and causing damage to it. Once driving is operating normally, the backup unit can be switched on to ensure a dry environment.
When starting up with nitrogen, it is necessary to adjust the opening of the return line in a timely manner, and carry out air prevention during the fourth-stage discharge. Nitrogen reflux will cause the temperatures in each section to rise progressively; it is necessary to adjust the water injection volume and the outlet circulating water promptly to ensure that the discharge temperature of the unit does not become too high, nor does the pressure. If the pressure is high, it should be released to prevent the compressor from stalling.
During startup with nitrogen, the nitrogen from the MTO is directly drawn into the compression unit, undergoing a four-stage integrated cycle before being discharged in the fourth stage. When propylene is used in operation, in order to reduce the startup time, the high and low pressure propane removal columns can be operated in full reflux mode with propylene. The advantage of full reflux is that it reduces material waste; otherwise, due to excessive levels of C4 components, the fourth stage would have to remain under vacuum. Starting the deethanizer and propylene distillation column in the distillation system earlier can reduce startup time significantly.
When propylene is present, if full reflux of propylene is used in both the high-pressure and low-pressure columns, and the outlet of the dryer is closed, the inlet to stage 4 of the compressor is at a pressure of around 1.4 MPa, corresponding to the pressure in the high-pressure column. Stages 1–3 of the compressor are filled with nitrogen at a pressure of 0.2 MPa; the compressor is started under these conditions. Full reflux of nitrogen occurs in stages 1–3, while full reflux of propylene occurs in stage 4. After the reactor starts operating, nitrogen is displaced starting from stage 1, and the gas from stage 3 is discharged. Once there is no more nitrogen in the system, the valve of the dryer is opened to allow flow into the high-pressure column. Is my understanding correct?