Why is steam dilution chosen instead of a vacuum reaction for the dehydrogenation of ethylbenzene to produce styrene?

Diluting with water vapor serves three purposes: providing heat, causing charring, and preventing coking

The reason I read in some books is that operating under vacuum at high temperatures is unsafe; I have never been able to understand why this is the case.

Are you looking at this book? ? It is the school teachers who write textbooks or PPTs, not the engineers in factories; as a result, the textbooks are not updated, and many of them still contain outdated content. It’s normal that they fail to keep up with actual technological advancements. Otherwise, how can we say that it’s better not to rely entirely on books? Pay attention to the red line; regardless of safety considerations for now, adding water vapor is a means to achieve the goal of reducing the ethylbenzene partial pressure. The key to reducing the ethylbenzene partial pressure lies in that red line, which helps to increase the equilibrium conversion rate. As for vacuum high-temperature operations, are there any safety concerns? Of course there are such cases, but it’s not absolute. In actual industrial facilities, especially in the fields of fine chemicals, pharmaceutical chemicals, and the production of intermediates, high-vacuum distillation is quite common. For example, in a pharmaceutical intermediate production process I was involved with, operations had to take place at 190°C and a absolute pressure of 1 mmHg. Given that these are organic substances, isn’t that dangerous? Is it safe or not? High-vacuum systems are vulnerable to air leakage; if the leakage rate is too high, the oxygen that enters the system can indeed be dangerous. However, with the appropriate technical measures in place, it is possible to ensure safety. The American space shuttles – 5 were built, 3 of which crashed, but 2 remained functional. Even though the overall system failed, it cannot be said that its basic design was a “complete failure.” The same is true here: ensuring safety in a high-vacuum environment (in other words, preventing leakage or keeping the leakage rate below the allowable limit) is not particularly difficult from a technical standpoint. As early as the 1960s and 1970s, China’s vacuum technology experts managed to solve the problem of leakage rates in high-vacuum conditions. In other words, there is only one reason why steam dilution is used in the ethylbenzene dehydrogenation process: it is beneficial for the process (by increasing the equilibrium conversion rate) as well as for achieving low costs. Reactions at normal pressure and high temperature are definitely cheaper than those in a vacuum at high temperatures; it’s all about costs. Using a vacuum is a costly and inefficient process – as the boss, which process would you choose? Money is sometimes much more important than technology. . .

There’s a joke related to this. A few days ago, I was chatting with some friends on WeChat about how the former Soviet Union collapsed within just over 70 years. There are many reasons for this, but in the eyes of these technical experts, it’s simple: during the Cold War, both the United States and the Soviet Union were willing to spend large amounts of money on aerospace technology. The Americans had the money and were willing to spend it, while the Soviets didn’t have as much money, but they were still willing to invest. However, the Soviets had one extremely problematic trait – they not only spent money recklessly but also didn’t value it at all. The construction of numerous nuclear submarines is proof of this; such submarines require a significant amount of funding, yet the Soviets’ technical level was far inferior to that of the Americans, which resulted in low operational efficiency of their nuclear submarines. Out of 10 submarines built, only 2 were functional, while the Americans could get 8 out of 10 to function properly. It’s no wonder this led to their downfall. . . Die from this root cause of not valuing money – no work, no death. With the scale of the former Soviet Union, as long as its economy held strong, who could defeat it? It’s all money that causes the problems. . .

A high vacuum of 1,190℃ can solve the problem, but can vacuum leakage at a reaction temperature of 600℃ resolve the issue? 2. On the one hand, the higher equipment costs associated with high vacuum are a concern; but at normal pressure, a large amount of water is introduced, and the waste resulting from this water is wastewater, which also increases costs How should we consider it?

For current ethylbenzene dehydrogenation catalysts, under negative pressure and adiabatic conditions, an appropriate amount of dilution steam can help reduce the hydrocarbon partial pressure, minimize carbon deposition on the catalyst surface, and supply heat for the endothermic dehydrogenation reaction. In the absence of dilution steam, carbon deposition on the catalyst becomes severe, significantly shortening its service life.

From this perspective, it must be diluted by water vapor.

Vacuum operation is not a problem; in some cases, vacuum is indeed used, such as in atmospheric and vacuum distillation. Deciding whether vacuum operation or the addition of steam is more appropriate is the result of a comprehensive consideration of factors such as economy, safety, and long-term operational efficiency

Among all available processes, those that are widely adopted and still in use are the best from a comprehensive perspective considering factors such as energy consumption, production volume, and safety! Personal opinions are for reference only and do not represent anyone!