China: utilization of ethylene tail gas? 

When it comes to recycling ethylene tail gas in China, many people immediately imagine large-scale hydrogenation plants or expensive projects to return it to pyrolysis. But the reality on the ground is often much more prosaic and complex. The main problem is not the lack of technology, but its economic feasibility for a particular plant, especially when it comes to relatively small or old production facilities. I often see engineers trying to apply “book” ideas. decisions without taking into account the actual composition of the gas, load fluctuations or a simple lack of space for equipment.

What's really hidden in the "tails"?

Let's start with the basics, which for some reason are often overlooked. Ethylene tail gas is not just a standard mixture. Its composition is the fingerprint of a particular pyrolysis production. Besides ethylene and propylene, there could be anything from hydrogen and methane to acetylene, MAPD (methyl acetylene and propadiene) and even traces of aromatics. In one project I came across about five years ago, the main problem was not even ethylene, but the high hydrogen content, which was “confusing?” standard hydrogenation schemes and required an additional separation step.

That is why the first step should always be not choosing a technology from a catalog, but a lengthy and detailed analysis of the gas composition. Not one-time, but over time, taking into account all operating modes of the furnaces. It has happened that an installation designed for “average” composition simply could not cope with peak loads or, conversely, became unprofitable with minimal productivity. This is the very point where theory diverges from practice.

Here it is worth mentioning the approach of some design institutes that specialize in such non-standard solutions. For example, Chengdu Yizhi Technology Co., which is the design and research division of Huaxi Technology. Their work often begins with an in-depth audit and modeling, and not with the sale of a finished device. This is the right way, although not the fastest.

Hydrogenation: not a panacea, but a tool

When it comes to recycling, the first thought is catalytic hydrogenation to recover ethylene and propylene. The technology is proven, but... The key word is catalyst. Selecting a catalyst is an art. It must be selective specifically for our target components (say, acetylene and MAPD), but at the same time not affect ethylene and not cause overheating. At one of the installations, we struggled for a long time with the rapid coking of the catalyst. It turned out that the culprit was traces of higher olefins, which were simply not “caught” in the original analysis.

Another nuance is heat removal. The hydrogenation reaction is highly exothermic. If you do not ensure effective heat removal, you can get not selective hydrogenation, but complete hydrogenation of everything, with a sharp increase in temperature and a risk for the reactor. It was necessary to design multi-stage systems with intermediate cooling, which, of course, increased the cost of the project.

And of course, a source of hydrogen. It is ideal if you have your own cheap hydrogen from reforming plants. If not, you have to consider either purchasing it or alternative schemes. This immediately hits the economy. Sometimes it turns out to be easier and cheaper to direct the tail gas for combustion in furnaces, although from the point of view of resource efficiency this is, of course, a defeat.

Alternatives: when hydrogenation is not beneficial

For small streams or gases with a special composition, classical hydrogenation can be like shooting sparrows from a cannon. What then? One of the options we considered for the plant in Shandong province was the separation of individual valuable components. For example, concentrating ethylene by adsorption or membranes for use in other processes on the same site (for example, in the production of ethylene oxide or styrene).

Another way is energy recycling. But not just flaring, but using it as fuel gas for process furnaces or for steam generation. The problem here is the unstable calorific value. In order for a boiler or furnace to operate stably, a mixing or adjustment system is needed. We implemented such a scheme, and the main headache was associated with the automation, which had to respond in real time to changes in the composition of the tail gas.

There have also been more exotic attempts, such as the use of ethylene-enriched gas in the production of linear alpha olefins or as a feedstock for high-density polyethylene. But here you come up against logistics, the purity of raw materials and competition with the main flow of polymer ethylene. Most often, such projects remained on paper.

Experience and pitfalls: an inside look

I would like to share one specific case that well illustrates all the difficulties. This was a project to modernize an old ethylene complex. The task is to utilize tail gas from the treatment plant. The technical specifications were initially made for hydrogenation. But when we (I then worked with a team from Chengdu Yizhi Technology Co.) conducted a detailed analysis, it turned out that the gas contains an indecent amount of inerts (nitrogen, methane). Hydrogenating such a composition means driving huge ballast through the reactor and compressors, wasting energy.

As a result, they proposed a hybrid scheme. First, membrane separation for pre-concentration of ethylene and propylene. Then - a compact unit for selective hydrogenation of this concentrate. The remaining lean gas was used for fuel. Company websiteyzkjhx.rudescribes similar integrated approaches, and in this case it worked. The economic effect was achieved by reducing capital costs for compression and reactor size, as well as by saving fuel gas.

The main lesson from this project: there is no one-size-fits-all solution. Each case requires its own technological audit and, often, a combination of methods. The biggest pitfall is the desire of plant management to get a quick and cheap solution. But in this area, fast and cheap usually means ?ineffective? or ?doesn't work in the long run?.

The future and economics of the process

Where is the industry heading? The trend is maximum integration and digitalization. We are talking about systems that optimize the recycling regime in real time depending on the composition of raw materials and the market value of the products. If the price of polymer ethylene is high, the system will seek to maximize its recovery. If it is more profitable to produce electricity, it will switch to the priority of energy utilization.

The second trend is miniaturization and modularization of installations. This is especially true for medium and small manufacturers. Instead of giant capital structures, there are compact, almost container solutions that can be quickly deployed and adapted. By the way, Chengdu Yizhi Technology Co., Ltd. is actively working on such developments, using the experience of its parent institute Huaxi Technology in the field of catalysis and separation processes.

Ultimately, the issue of utilization of ethylene tail gas does not depend on technology, but on economics and ecology. Pressure from regulators to reduce emissions and carbon footprints is growing. It's just getting more expensive to burn because of the fees. Therefore, investing in competent recycling systems is no longer a matter of premium, but a matter of business survival in the medium term. But investments must be smart, based on deep analysis, not fancy brochures. And here the experience of practitioners who have gone through dozens of such projects turns out to be invaluable.