The recycle hydrogen desulfurization tower in hydrocracking often foams
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The unit uses an MDEA solution to absorb H2S from the recycled hydrogen; the concentration of H2S is 0.5% (V) before purification and 0.05% (V) after purification, with the specified limit being ≤0.1% (V) after purification. The flow rate of the MDEA solution is designed to be 87 m3/h, while the flow rate of recycled hydrogen is 317,000 Nm3/h. 10-layer double-overflow high-efficiency floating valve trays are used. Currently, the flow rate of the MDEA solution is 68 m3/h, and the flow rate of recycled hydrogen is 305,000 Nm3/h. Foaming often occurs, causing liquid to reach the liquid separation tank at the inlet of the recycled hydrogen compressor, which poses a serious risk. The MEDA flow rate has also been adjusted repeatedly, the liquid separation tank at the inlet of the desulfurization tower has had its liquid drained, oil has been removed from inside the tower on a daily basis; the temperature difference is normal and the quality is also acceptable. All the standard methods have been tried, but there has been no improvement. It is now suspected that the amount of recycled hydrogen is too high, resulting in an excessive load. However, the catalysts were replaced just last year, and the consumption of cold hydrogen cannot be reduced; at the same time, it is necessary to maintain the proper hydrogen-to-oil ratio. Today, a bypass around the recycled hydrogen desulfurization tower was slightly opened to see if this will have any effect. Will the defoamer fail to carry away the deposits inside the tower? Because the wastewater discharged from the level gauge is quite dirty. Do any of you have relevant experience? Could you give some guidance?
What’s the reason? There are many reasons for this; it may be due to process pressure and uneven temperatures, which result in bubbles forming between the particles on the tower trays. It could also be caused by deposits within the tower that create cavities, leading to foaming. Lastly, gas leaks of corrosive substances or demulsifiers within the tower might also cause foaming. -
Check whether there are any changes in the composition of the recycled hydrogen, as well as in the sulfur content of the lean liquid. Also verify if the temperatures of the lean liquid and the recycled hydrogen have decreased; if so, raising these temperatures could help
The composition of recycled hydrogen remains largely unchanged. The temperature is affected by the temperature difference between day and night, but the temperature difference between the amine solution and the circulating hydrogen remains stable: the temperature of the amine solution is 48, while that of the circulating hydrogen is 42
It is difficult to analyze the flow pattern in the tray. But it is possible that the defoamer is condensing on the tray; however, no foaming occurred at the beginning of operation. Foaming started to appear after half a year and intensified over time. It is initially suspected that the oil layer inside the tower and the large volume of circulating hydrogen are the causes, so oil skimming needs to be increased
We can try turning off the desulfurization tower bypass and gradually reduce the amount of circulating hydrogen to see if it has an effect. Given the worsening foaming situation, considering switching to a different defoamer and adjusting the temperature of the desulfurization tower could also help reduce foaming. -
It is possible to start by considering the quality of the amine solution. Is the inlet separation tank a centrifugal separator?
On February 24, the speed of the hydrogen circulation pump was increased by 100 rpm due to a low hydrogen-to-oil ratio; on February 25 and 26, foaming was carried out twice a day. On the 27th, work was carried out to open the bypass of the cyclic hydrogen desulfurization tower by 5%, and by the 28th, no foaming had occurred for 24 hours. It is temporarily suspected that the amount of circulating hydrogen has reached the upper limit for treatment in the desulfurization tower
The load on the desulfurization tower is low; increase the opening rate
Have you tried not mentioning the amount of lean liquid?
There are several main reasons for foaming: 1 is oil contamination in the amine solution, and 2 is a high level of impurities in the amine solution. To prevent the amine solution from containing oil, it is essential to properly control the levels in the high-pressure separator and the liquid separation tank at the inlet of the desulfurization tower; by keeping the temperature in the inlet liquid separation tank at ≤40°C, oil will basically not be present in the solution ； If impurities are present, it becomes problematic; it is necessary to filter out these impurities outside the container, and many manufacturers are capable of doing this