Air separation oxygen production area: [Weekly Topic] The last issue of 2010 – Adjustments to operating conditions for internal compression versus external compression

First, let me make one thing clear: I’ve never done external compression. I’ve only been working in air separation for a short time, and my role involves internal compression. And we don’t have high- and low-pressure heat exchangers; we only have one heat exchanger. But our air pressure is 28 bar at medium pressure. Below, based on your question, I provide the answer as I understand it; I’m not sure if it’s accurate. Please understand. 1. The temperature difference across the plates isn’t very large; I have no idea what it should be. We have 4 degrees; the plate-type air (coming from the pressurization section at 24 degrees) and the dirty nitrogen is at 22 degrees. Low-pressure nitrogen at 20 degrees. Medium-pressure nitrogen and oxygen are also around this temperature. 20 degrees at the molecular sieve outlet. {These are the parameters from some time ago.} 2-1: Our air compressor is not operating at full capacity yet, so the pressure has not risen. So generally, when we increase the load, we are raising the outlet pressure of the air compressor. It is sufficient to set the desired value for the outlet pressure of the molecular sieve. (This automatically controls the opening angle of the pressure control vanes.) 2-2: One method is to increase the outlet pressure of the air compressor, and the other is to increase the degree of expansion in the expander. 2-3: We are a heat exchanger, so I don’t know how to answer this question. We generally control the outlet pressure of the air compressor; high-pressure throttling is usually used to control the temperature before the expander. The smaller, the better for us. 2-4: We must charge the pressure in the lower tower before we can start the expander; there is a pressure interlock in place. I’d like to ask the OP this question: For someone like you, does this have no effect on the compressor at all? With so little air being inhaled, isn’t the temperature of the refluxing gas going to be high? (I’m not quite sure; please forgive me.) 2-5: We first turn everything on to pressurize the lower tower; once the expander starts running, we reduce the pressure according to the situation. During pressurization, set it to 100 2-6: Our current operating condition lies between fully liquid and semi-gaseous states. We have oxygen for venting, so we use that oxygen along with the extracted liquid nitrogen to adjust the argon fraction. Overall, it’s quite easy to operate. In a fully gas-based operating mode, only liquid nitrogen extracted can be used for adjustment. 2-7: We also experience this phenomenon. We use imported valves, and after one adjustment, it’s fine. Our carbon steel pipes are protected against cold embrittlement. Depending on the level, the temperature varies: for contaminated nitrogen it’s -2, for low-pressure nitrogen it’s -20, for high-pressure nitrogen, high-pressure oxygen, and high-pressure argon as well it’s -20. As for exactly how much pressure can be tolerated, we haven’t tried it nor have we dared to try it; the lowest temperature we’ve reached was -16. The above information is based on the operating conditions of our equipment, and it may differ from what others experience. If there are any mistakes, I hope everyone can point them out so that I can learn from them. Thank you

“At startup, we first open the low-pressure plate-type air valves fully to pressurize the lower tower; once the expander starts running, we reduce the valve opening according to the situation. Set it to 100 when pressurizing. ”--May I ask: What situations or when do you mean by \"turning it down depending on the circumstances\"?

This post was last edited by 525h on 2010-11-7 at 22:16. 1. Is the temperature difference at the hot side larger in both high-pressure and low-pressure plate types? (Specific numerical values would be appreciated.)； Compared to external compression! There is a temperature difference of about 10 degrees between us. 2. Methods for adjusting the load: First, increase the outlet pressure of the air compressor (compared to what is the outlet pressure of the air compressor itself?) Specific figures? ) It can be improved by increasing the main cooling and heating load. Second, what is the most common method for increasing the air volume? Increase the extraction rate and the degree of expansion; increase the load on the tower. Thirdly, when the air separation unit is not operating at full capacity, how should the load be distributed between the high-pressure plate type and the low-pressure plate type? The amount of air supplied can be adjusted by using a high-pressure air throttle valve and the outlet pressure of the air compressor. Which method do you usually prefer, and under what circumstances is which method used (or which one is used first)? For the distribution of cooling capacity between high and low modes, it can be handled through the control of those few valves. Fourthly, during startup, can the expander be started first after the lower tower has been pressurized, with the high-pressure plate heat exchanger being put into use at that time? ； Then open the air valve on the low-pressure side; (in my opinion, this is theoretically feasible.) We can first switch to low-pressure air supply, then to high-pressure supply, or we can use air supply at one pressure level first and then start the expander after achieving a balance between the two pressures. Fifthly, the control opening degree of the high-pressure air throttle valve during startup ； When does it open? How big? This valve is opened only during high-level temperature exchange; it should be opened slowly according to the high-exchange temperature, as well as the expander temperature. Sixth, how is the argon fraction adjusted in the internal compression process? Personally, I find it easier to control without external compression ； Let’s talk about everyone’s opinions! There is a valve that can be used to adjust the oxygen content in the argon fraction. As for the issue of cooling of the plate heat exchanger after shutdown, this is caused by internal leakage in the valves used to supply nitrogen and contaminated nitrogen, which results in a very low temperature at the hot side of the plate heat exchanger after shutdown; what is the lowest temperature that carbon steel pipes can withstand? ) Just run it then? Feed it into the plate exchanger’s drain line to release the pressure; or use a handwheel to hold it in place. I’ve seen situations at -100 degrees with no problems

The original poster is absolutely right; we are indeed things related to Air Liquide. For preventing cold brittleness specifically, we set up temperature interlocks. Generally, there are 3 temperature points, located right next to each other at the outlet of the heat exchanger. For interlock control, it’s a “3 out of 2” setup. Once the interlock temperature is reached, the valves are directly closed via interlock, or the pump is stopped. Overall, Air Liquide has quite high safety requirements.

This post was last edited by Hacker Empire on 2010-11-16 at 18:47. 1. The temperature difference at the hot side of plate heat exchangers with internal compression should be treated differently: the temperature difference in low-pressure plate heat exchangers should be larger, typically around 4-5 degrees, while in high-pressure plate heat exchangers it is generally around 2 degrees; the temperature difference at the hot side of plate heat exchangers with external compression is usually within 2 degrees. It’s not the case that the temperature difference at the hot end should be as small as possible; if this difference is too small, the temperature difference at the cold end will become excessive. In this regard, the situations for external compression and internal compression are the same. 2. Methods for adjusting the load: First, increase the outlet pressure of the air compressor (compared to what is the outlet pressure of the air compressor itself?) What is the specific operating pressure? ) It is not recommended to increase the outlet pressure of the air compressor, because an increase in load means an increase in the amount of air to be processed; if the air compressor is controlled at a constant pressure, its guide vanes will automatically open wider. Second, what is the most common method for increasing the amount of air? If the air compressor operates under constant-pressure control, the way to increase the amount of air is to raise the extraction rate of the product gas. Of course, to achieve this, corresponding adjustments must also be made to the expander, the liquid nitrogen throttle valve, and other components. Third, when air separation is not operating at full capacity, how is the load distributed between the high-pressure plate type and the low-pressure plate type? That is, does the load favor low-voltage plate type or high-voltage plate type? Simply put, it’s sufficient to keep the temperature difference at the hot end of the high-pressure plate heat exchanger within 4 to 5 degrees. Efforts should be made to keep the temperature difference on the hot side of low-pressure plate heat exchangers from becoming too large. Since low-pressure plate heat exchangers handle a relatively large volume of gas, the amount of cold loss resulting from a one-degree increase in temperature differs between low-pressure and high-pressure plate heat exchangers. Specifically, the cold loss associated with low-pressure heat exchangers is considerably greater. Therefore, it’s important to maintain a minimal temperature difference in low-pressure heat exchangers. Fourth, during startup, can the expander be started first after pressurization in the lower tower is completed, with the high-pressure plate heat exchanger being put into use? ； And then open the low-pressure plate-type air valve? During stamping, if the valve for air entering the tower has a bypass, the bypass can be opened first; once the pressure is balanced, the valve allowing air to enter the lower tower can be opened fully. At this point, not all of the air will enter the lower tower, as the valve for liquid air to enter the upper tower is not open, nor is the valve for liquid nitrogen to enter the upper tower. For the high-pressure plate type, there is no need to rush when it is at cold temperature; as the main cooling liquid level rises steadily, the pressure can be increased gradually. Then the product pump is started to feed liquid into the high-pressure plate type, and once the liquid level stabilizes, a product gas vent is established. Fifth, the control opening degree of the high-pressure air throttle valve at startup ； When does it open? How big? That is, during the cooling phase? Fluid accumulation stage? Should we wait until the temperature before throttling drops to the design value before turning it on? At startup, it is recommended to open the high-pressure air throttle valve to 50%. It should be opened before the compressor takes in air. It is suggested that this valve be controlled based on the compressor’s flow rate or the liquid air level in the high-pressure heat exchanger. Sixth, how should the argon fraction be adjusted in the internal compression process? Personally, I find it easier to control without external compression ； Let’s talk about everyone’s opinions! In the internal compression process, the adjustment of the argon distillate is no different from that in external compression. The key to adjusting the argon distillate lies in maintaining the stability of the main column; the timing of initiating the crude argon column cycle is also crucial. When the oxygen purity in the main column stabilizes at 99%, the crude argon column should be put into operation promptly; otherwise, this argon will have a negative impact on the main column. If it is not desired to activate the argon column immediately, the flow rate of impure nitrogen exiting the column should be increased, thereby venting this argon away. Seventh, how to address the issue of cooling in the plates after shutdown? Specifically, internal leakage in the outlet valves for nitrogen and impure nitrogen causes the temperature at the hot end of the plates to become very low after shutdown. Introducing a certain amount of dry air or nitrogen can equalize the pressures on both sides of the leaking valves. Of course, this is on the condition that there is no liquid in the plate heat exchanger. Carbon steel can withstand certain low temperatures; however, at excessively low temperatures, it becomes brittle. If the pressure inside the pipeline is too high under such conditions, the pipeline’s pressure-bearing capacity diminishes, potentially leading to a physical explosion.

Who can explain the control of liquid level in the plate and frame type? How to control it? What’s an appropriate level of control? What is the purpose of controlling this liquid level? 》

The internal compression process is in contrast to the external compression process. The external compression process involves air separation equipment producing low-pressure oxygen, which is then pressurized to the desired pressure using an oxygen compressor before being supplied to users; this is also known as conventional air separation. The internal compression process involves eliminating the oxygen compressor, and instead producing medium- to high-pressure oxygen directly from the distillation tower of the air separation unit to supply it to users. The main difference between this process and the conventional external compression process is that the oxygen supply pressure for the product oxygen is achieved by pressurizing liquid oxygen in the cryogenic tank using a liquid oxygen pump, followed by vaporization and reheating through heat exchange with high-pressure air. Compared with the external compression process, the main technical changes in the internal compression process lie in two aspects: distillation and heat exchange. In external compression air separation, low-pressure oxygen is generated directly in the distillation tower, and then reheated in the main heat exchanger before being discharged from the cold box ； In internal compression air separation, liquid oxygen is drawn from the main condensation evaporator of the distillation tower and pressurized to the desired pressure using a liquid oxygen pump; thereafter, high-pressure air is used to exchange heat with the liquid oxygen, causing it to vaporize and emerge from the cryogenic tank as the product gas. It can be simply understood that the internal compression process replaces the oxygen compressor in the external compression process with a liquid oxygen pump and an air booster. The application of internal compression process technology is driven by three main factors: (1) safety considerations. The oxygen production industry has very high requirements regarding safety; external compression involves compressing oxygen in gaseous form at normal temperatures, while internal compression involves compressing liquid oxygen in a liquid state at low temperatures. Clearly, internal compression offers higher safety levels compared to the former method ； (2) Oxygen compressors require many safety factors to be taken into account during design and manufacturing, which makes them very expensive; the cost of the liquid oxygen pump and air booster required for an internal compression process is considerably lower ； (3) While providing gas services at their operational sites, gas companies usually also sell a large amount of liquid products; under such circumstances, the internal compression process for air separation offers significant advantages in terms of operating costs.