Catalytic cracking instrument backblow air

The backwashing medium used in our company’s catalytic units is purified air. It’s the first time I’ve heard of using gas as a backwashing medium. I believe that when using gas as such a medium, the following points should be taken into account: 1. Its use should occur after switching to the steam seal system. 2. The medium used at the pressure measurement points in the settler and those in the regenerator should be different from each other. 3. The gas should be desulfurized first. 4. The gas must have a pressure stabilization system. This post was last edited by zhenghemqx on 2008-1-9 at 09:23

I’ve heard of it for the first time; safety should be the top concern, namely on the reactor side. The regenerator should also be non-purified air. The reaction should also be started after the feed is introduced.

Fresh. Gas can only be used in settlers; one must pay attention to pressure, ensure there is no liquid present, and consider the timing of switching.

But after all, the gas system contains sulfur, so the entire gas system is quite dirty. It is relatively easy for the backflow port to get clogged.

We use purified air. I think using gas is also aimed at saving energy and reducing consumption. Currently, most of the gas produced by refineries is used as fuel for various heating furnaces, while the excess amount is burned off in flares, which is a great waste. Using gas instead of backdraft air reduces the amount of air needed for purification.

I’m hearing about it for the first time. It has been considered whether it is reasonable to use purified air for instrument air in reactors and sedimentators. In my opinion, Qingdao’s instrument reverse blowing system should have two sets: one for clean air and the other for gas. Clean air is used when the two devices are being heated and fluidized, while gas is switched in once a vapor seal is established. However, when using gas as the backflow air, there are issues related to purification and pressure stabilization.

I’ve also heard that some systems are equipped with a backup nitrogen source for backflow ventilation, which is activated immediately if the backflow air supply is interrupted. Do your devices have such a design?

I believe that when using gas as a pressure measurement point for backwashing the medium, the following points should be noted: 1. The gas must have a pressure stabilization system, and the pressure should not fall below 0.2 MPa. 2. It should be put into use after switching the steam seal. 3. It can be used at the pressure measurement points of the settler. 4. Use in the regenerator is strictly prohibited. 5. The gas should be desulfurized. 6. It is very important to regularly check for leaks in the pipelines.

You are correct; a reactor cannot be operated using purge air! It can easily cause explosion accidents!

The backwashing medium we use is purified air or steam; gas is used as the pressure measurement point. When using gas as the backwashing medium, the following points should be noted: 1. It should be used after switching the steam seal. 2. In case of an emergency, make sure to cut off the gas supply. 3. The gas must be desulfurized and its pressure stabilized

Steam, purified air, gas, and nitrogen can all be used for instrument backflow ventilation, but from the perspectives of overall operation, safety, and energy consumption, it is more reasonable to use purified air. The reason is that both steam and nitrogen require more energy compared to purified air; using gas will undoubtedly increase the power consumption of the compressor. Although it can raise the inlet pressure of the compressor, it actually leads to higher energy consumption, and it also involves difficulties such as leaks and cross-connections in operation. Using purified air makes it easy, safe, and energy-efficient. After all, an orifice plate or throttle valve is used to control the air flow, and the volume of air is very low, so it cannot cause any safety incidents. Of course, if there is a prolonged shutdown, then it is a problem related to operation.

This is the format we use in our devices! Great! Energy saving!

It’s probably to prevent water from being carried by the steam, causing sludge formation and blockages in the pressure guide tubes

The backwashing system for the instruments in the settler uses gas instead of instrument air; this practice has been common in the catalytic units designed by SEI over the past decade or so. I believe the intention behind this is not to save energy, but rather to increase the partial pressure of gas in the reaction gases, thereby steering the cracking reaction in a direction that results in less gas production – and that’s one reason for it； Secondly, it is obvious that the instrument air contains 21% oxygen. Oxygen reacts naturally with hydrocarbons at 500 degrees Celsius, and the reaction products—CO2 and H2O—accumulate over time, and that amount is by no means small. I disagree with the claim mentioned above regarding increasing the inlet pressure of the steam compressor: whether it is gas or instrument air, as long as it is injected into the settler at the same volume flow rate, it will enter the steam compressor at the same volume flow rate. (Here, we haven’t taken into account the decrease in volume caused by the condensation of water vapor generated by the reaction between oxygen and hydrogen in the overhead cooling system of the distillation tower.) I really don’t understand the rationale behind increasing the inlet pressure of the steam compressor; could someone please explain? As for the reasons for energy savings, my analysis suggests that it lies in the fact that when air of the same volume is compressed by a compressor, its enthalpy increase is higher than that of gas of the same volume (the specific heat of air is 1.03 kcal/kg·°C, while that of gas is 0.55 kcal/kg·°C). Compressing gas of the same volume requires less energy compared to compressing air of the same volume. However, another important factor must be taken into account: in terms of catalytic units alone, the energy consumption coefficient for gas (950 kilograms of standard oil per ton) is much higher than that for instrument air (0.04 kilograms of standard oil per ton). By saving instrument air, the consumption of fuel gas for the unit increases. It is clear then which option results in higher overall energy consumption – in this case, using gas that has been desulfurized externally for the unit. What if 1.0MPa pre-desorption gas is produced using the device itself? Pipeline corrosion is the most difficult issue to address, as instrument systems typically use DN20 carbon steel pipelines and needle valves; even a small amount of rust can cause pipeline blockages and lead to malfunction of the instruments. Another troublesome issue is the switching problem; once the gas pressure drops rapidly in an emergency situation, it is necessary to switch to instrument air. In a rush, it’s easy to forget this step, which only adds to the complications. There is another even more serious issue: what are the consequences if internal leakage in the switching valve causes gas and instrument air to mix? Hehe, no need to discuss it anymore. Given the above circumstances, I know very little about how to make proper use of this system; however, I am aware that some devices have already had their gas lines removed. I’m not sure which one is better; could you share some experience? A suggestion: set up a target; criticism is welcome.

The backflow medium at the instrument pressure measurement point can be instrument air, nitrogen, gas, etc., whether in a backflush or regeneration system, but steam cannot be used. The diameter of the blowout holes at the pressure measurement points on the instruments is generally 0.8 to 1.2 mm; for the loose parts of the inclined tubes, the above three types of media as well as steam can be used, and the diameter of those holes is usually 3 to 5 mm. The main purpose of using gas instead of instrument air is to reduce the non-hydrocarbon content in the gas, rather than for energy savings. The SEI design features a switching system that allows for switching as needed.