Difficult symptoms in specialized atmospheric and vacuum distillation units

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This year is the starting year of the **12th Five-Year Plan for the economy**. The first thing we need to do is fundamentally change the approach of relying on resources to sustain GDP growth. Efforts must be made to reduce energy consumption and waste, turning the resources saved into benefits for the listed company, so as to better reward its shareholders. To this end, **months ago I analyzed the production problems of the atmospheric and vacuum distillation units at a certain plant, ﹙the original text is abridged here﹚. At present, there are various problems in the production of newly commissioned and some existing similar facilities in the country; some of these issues stem from equipment problems, others from human factors, and still others from a lack of technology. From a design perspective, atmospheric and vacuum distillation has a history of a hundred years, and its design has become highly refined; any technical modifications carry risks, as they may lead to a decrease in the yield of light oil or total distillates, as well as an increase in energy consumption. So I can help in accurately determining which step in the production process is problematic. If you need help, it’s best to provide the DCS operation screen, as well as the operation data. Send it to my email address mpcchuang@163.com; I will prepare an analysis and summary for you, along with relevant experience materials. Hello, it took me two hours on ** evening to finally go through the main images. The problem with your production lies in issues with the production system; the key issue is that the proper techniques were not followed during the steam-starting process. In fact, we have also experienced two instances of misguided instructions here, which led to failures. From the first email you sent me, I concluded that the atmospheric pressure furnace was overloaded; later, I suspected that the steam-starting process failed, and as a result, production became passive. What needs to be done now is to try to turn this passive situation into an active one, but it’s quite difficult. As can be seen from the *** footage, severe flow imbalances occur during the first, second, and third heat exchange stages of the crude oil processing; this is caused by an unreasonable distribution of heat sources. The main reason is that insufficient heat is carried away by the atmospheric pressure side lines and the two intermediate reflux streams, resulting in a material imbalance. The ***screen shows that the liquid levels in all the vacuum side lines are excessively high; too much heat is carried away by these side lines, especially in the first vacuum line. This indicates that your yield of light oil is not high, and there is still a long way to go before crude oil can be properly evaluated. At present, the production bottleneck in the entire plant lies in the atmospheric distillation column, and resolving the issues related to this column solves half of the problems. Based on an analysis of the *** data, the pressure at the top of your atmospheric tower is too high, which results in reduced evaporation of oil and gas. This leads to a lower temperature in the gas phase throughout the tower, causing an imbalance between the gas and liquid phases. As a result, the reflux flow in the middle section is weak, and increasing the temperature at the outlet of the atmospheric furnace does not help resolve this issue. Based on the various temperature parameters shown in the display at that time, it is sufficient to control the outlet temperature of the atmospheric pressure furnace at ***°C. An increase in the outlet temperature of the atmospheric pressure furnace is another cause of overload in this furnace, and it also leads to an increase in the top pressure. The number of trays in your column is *** layers in the vaporization section, and there should be *** layers in the stripping section below it; in total, there are *** layers. The extraction ports in your Changyi and Erzhong plants are higher than ours, making them suitable for processing light crude oil. Now we need to try to increase the distillation temperature of the side streams and the mid-stage refluxes in the atmospheric pressure tower; this distillation temperature is essentially the gas phase temperature. Method 1: First, reduce the tower pressure; it is currently at ***, with a reduction of *** per day. Pay close attention to the temperature of the constant-top cooler to prevent vapor blockage and high pressure at the inlet of the constant-circulation pump, thus avoiding vacuum conditions. Observe the changes in quality and gradually increase the cold reflux. The cold reflux at the top should not be too small; especially when there is no diameter reduction in the upper part of your tower, it must not be low. Now, some factories propose eliminating the initial and normal cold return flows, which is nonsense; the ratio of normal cold return flow should be **:**** or ***:* – this should be mentioned in your design specifications. Increasing the cold reflux can improve distillation accuracy, increase the draw rate of each side stream, and facilitate thermal balance throughout the column. In Method 2, if the tower pressure cannot be reduced, then reduce the normal top circulation reflux and increase the amount of top cooling reflux – by *** cubic units per day, until it reaches **** cubic units. At the same time, monitor any changes in the quality of the material at the tower top and adjust the cooling reflux accordingly. Reducing it to *** cubic units per day is done to rebalance the internal recirculation within the tower; this internal recirculation can adjust itself within a certain range, possessing its own balancing mechanism. If that doesn’t work, then the normal circulation volume is reduced to **** cubic units. The purpose of doing this is to use the cold recirculation flow to suppress the rising gases. Also known as cooling, this process reduces the gas-phase load at the upper part of the tower, thereby facilitating more gas to move upward along the tower and be extracted from the normal line. Currently, the flow rate on the main line is only *** cubic units, while it should be around *** cubic units under normal conditions. Only when the traffic on the constant line increases does the load of the constant term decrease. Due to the extensive involvement of the constant top cold reflux, the gas phase in the upper part of the tower is continuously condensed into a liquid phase. This creates sufficient space for the gas phase to rise upward and facilitates better gas-liquid exchange, allowing more of the gas phase to move upward through each tray in the tower. Gradually, the distillation temperatures of the side streams and the mid-section reflux will increase, and the quality of the products from these side streams will become worse. This is because the internal reflux of the liquid phase in the tower decreases and the liquid layer on the trays becomes thinner. At this point, it is necessary to increase the flow rate of the reflux streams from stages 1 and 2; this serves two purposes: firstly, to increase the internal reflux in the lower parts of the tower, and secondly, to remove excess heat from the tower and maintain thermal balance throughout the tower. Treatment Method 3: If the above two methods do not work, reduce the treatment volume to ***% and then apply Method 1 or 2 mentioned above. Only by resolving the issues associated with the atmospheric pressure tower, increasing the yield of light oil, and raising the mid-stage reflux can the material flow within the entire plant be rebalanced. This also helps to address the problem of crude oil skewing during the three-stage heat exchange process. The pressure of the crude oil before and after electrodesalination should also increase slightly, which in turn raises the temperature at the heat exchange ends. At the same time, the outlet temperature of the heater can be reduced, thereby solving the problem of overloading the heater. However, this is merely a process of shifting in a positive direction; it cannot yet be considered as an optimization of the operations. After all, the consequences of a failed engine start cannot be completely eliminated.