China: utilization of ethylene tail gas? 

If we talk about processing ethylene tailings, many immediately imagine banal flaring or return to the furnace. But in reality, especially in modern Chinese complexes, everything has long gone further. The main problem is often not even in technology, but in the economics of the process: when the volume of tailings is small and the composition floats, capital investments in deep processing may never pay off. And this is where the fun begins—the search for that very balance.

What is hidden behind the term “tail gas”?

Let's start with the basics, which for some reason are often missed in general reviews. Ethylene tail gas is not some standard substance. Its composition is a direct derivative of the pyrolysis scheme and the depth of purification of the main stream. Hydrogen and methane predominate, of course, but there is always that “valuable impurity”: unreacted ethylene, a little ethane, propylene. It is these few percentages that determine whether the project will be profitable.

Previously, about ten years ago, the standard solution was to direct this flow to feed the plant’s fuel gas. It would seem logical - both recycling and savings. But with rising prices for raw materials and tightening environmental standards, such a scheme began to look like waste. Burning ethylene, even diluted, is the same as heating a furnace with banknotes. Maybe a little, but still.

Here it is worth making a digression about one common mistake in assessments. Many people think that if the installation is large, then there is a lot of tailings, and processing is justified. Not always. It all comes down to the stability of the lineup. If today there is 5% ethylene, and tomorrow it is 2%, then no membrane or adsorption installation will work effectively. Therefore, the first step should always be long-term and detailed flow monitoring. Without this, all calculations are fortune telling on coffee grounds.

Technological forks: membranes, adsorption, return to column

So, the composition is known, the volumes are clear. Next is choosing a path. Classic ismembrane separation. The technology is proven, especially for hydrogen release. But there are nuances with ethylene: membranes are sensitive to “heavy” substances. components require careful pre-drying and cleaning. At one of the projects in Jiangsu province they encountered exactly this - the promised selectivity on paper was shattered by real fluctuations in temperature and pressure at the inlet. We had to modify the gas treatment system on the fly.

The second way is short-cycle heat-free adsorption (SCA). It is perhaps better suited for extracting ethylene from such mixtures. High recovery rates can be achieved, even at unstable concentrations. But there are pitfalls here too: the cost of adsorbents, their service life in an aggressive environment, and energy costs for regeneration. I saw an installation where, due to an incorrectly selected regeneration mode, the adsorbent “sintered?” and lost capacity in six months instead of the promised three years.

A third option that is often considered is to return the off-spec ethylene back to the demethanizer. It sounds simple and elegant, but in practice this creates a huge load on the rectification system and can upset its balance. This solution only works with very stable and well-calculated main production. More often than not, it is discarded at the conceptual design stage.

Economics as a deciding factor

It all comes down to money. Even the most beautiful technology has no right to life if it does not pay for itself within a reasonable period of time. And payback periods in China are now tight, usually no more than 3-5 years. Therefore, the key question is: what to do with the extracted product? It is difficult to sell it - the volumes are small, the purity is not ideal. This means it needs to be used locally.

The most successful designs I have seen have been integrated into the plant's overall power supply scheme. For example, the isolated ethylene was sent to the production of ethylbenzene or ethylene oxide within the same complex. This eliminates the problem of logistics and sales. But this requires an appropriate infrastructure, “free?” power at adjacent installations. This solution is not for everyone.

Sometimes it is more profitable not to pursue deep extraction, but to optimize the pyrolysis process itself in order to minimize the formation of tailings. This area of ​​work often remains in the shadows, but it can provide greater economic benefits than an expensive recycling installation. Working on raw materials and furnace conditions is less noticeable, but fundamental.

Experience and specific cases

I remember a project at one of the refineries in Shandong. There they followed a combined path: membranes for preliminary enrichment of the flow with hydrogen, and then PSA for the final separation of ethylene. The system turned out to be flexible and was able to adapt to seasonal fluctuations in the composition of raw materials. But the cost, of course, was appropriate. The payback period was achieved within just five years, largely due to the fact that hydrogen was sent to hydrotreating, and ethylene to our own polyethylene plant.

But here is an example that is less successful. At one small plant they decided to save money and installed an installation designed for an average, “standard” one. tailings composition. The reality turned out to be far from typical. The device was idle half the time, and when it worked, its efficiency was below 50% of design. As a result, it was dismantled, returning to the fuel gas circuit. Lesson: Don't skimp on pre-design research. A month of additional monitoring could save millions in investment.

Here it is appropriate to mention the role of specialized engineering companies that deal with such non-standard solutions. For example,Chengdu Yizhi Technology Co.(their website ishttps://www.yzkjhx.ru), as a design institute created on the basis of a technology company, often works at the intersection of precisely these tasks: not just to sell equipment, but to develop a scheme for a specific, often “non-ideal” one. flow. Their approach, judging by several familiar projects, is based on a deep analysis of source data, without regard to templates. This is the same case when design experience (Chengdu Yizhi Technology Co., Ltd. is a design institute established by Chengdu Huaxi Chemical Technology Co., Ltd. in 2013) directly affects the result.

A look into the future and practical advice

Where is the industry heading? The trend is maximum integration and digitalization. Tailings disposal plants are increasingly being designed as “smart”, capable of adjusting in real time to changes in the input flow and predicting their efficiency. This is no longer a static apparatus, but part of the overall production management system.

What advice can you give to those who are just thinking about such a project? First, invest time and money in collecting data. A week is not enough; we need data for different seasons, under different operating modes of the main installation. Secondly, consider not only the extraction technology, but also the final fate of the product. Without a clear understanding of where to put the resulting ethylene, the project is doomed. Thirdly, do not be afraid of non-standard solutions. Sometimes a modest modification to an existing scheme is more effective than a grand new installation.

Ultimately, utilization of ethylene tail gas is not about the environment (although it is also about that), but primarily about economic efficiency and rational use of resources. This is a task where there is no universal answer, but a search for the optimal solution for a specific plant, with its unique conditions and limitations. And this search, with its mistakes and insights, is the most interesting part of the work.