China: ethylene gas utilization, technology? 

When people talk about the utilization of ethylene-containing gases in China, many immediately imagine giant pyrolysis plants at new petrochemical complexes. But the reality one encounters in practice is often more complex. The main challenge is not so much in scale as in the diversity of sources and composition. These are not only gases from pyrolysis plants, where the concentration of ethylene is high, but also side streams from catalytic cracking, waste gases from various processes where ethylene can be diluted with an ethane-propylene fraction, methane, and hydrogen. And this is where the fun begins, and sometimes the headache: how to extract value from such a “cocktail?” economically justified.

From theory to “field conditions”: what is often overlooked

In the textbooks everything looks harmonious: select, burn, transform. In fact, when you arrive at the site, say, at one of the many modernized refineries, the first thing you are faced with is the issue of profitability. There may be technology, but its implementation depends on issues of energy efficiency and the final cost of the product. For example, classical low-temperature fractionation to separate pure ethylene from dilute streams is an energy-intensive process. If the volumes of gas are not very large, and the infrastructure for collecting it is scattered, the project may never pay off.

Hence the active development of technologies for the selective hydrogenation of acetylene and MAPD (methyl acetylene and propadiene) in such streams. The task is not just to obtain ethylene, but to obtain it of the required purity, suitable for further synthesis of polyethylene, for example. Chinese engineering companies such asChengdu Yizhi Technology Co., they are actively working here, adapting catalysts and schemes for specific compositions of raw materials. On their websiteyzkjhx.ruyou can see that they position themselves as a design institute created on the basis of chemical technologies. This is an important nuance - it is the project approach, and not just the sale of equipment, that allows you to choose a “non-standard” solution. gas.

A common mistake is to try to apply the same process flow diagram for all types of gases. I have seen projects where they tried to install standard absorption columns for gases with high inert content. The result is low recovery and constant tuning problems. I had to revise the scheme and add preliminary membrane or adsorption purification. This will increase the cost, but without this, it’s a waste of time.

Technological puzzle: from absorption to membranes

So what options are even on the table? If we talk about the release of ethylene, then in addition to deep cooling, there are adsorption methods, for example, using zeolites or MOFs (metal-organic frameworks). The latter is a promising area that is actively being worked on in China. But again, on an industrial scale, everything depends on the stability of the adsorbent in the presence of impurities such as hydrogen sulfide or water. Laboratory successes and work on a real installation are two very different things.

Another way is not to select it, but to use it directly. Catalytic oligomerization of ethylene into gasoline fractions or dimers. The technology, in principle, is not new, but its use for the utilization of ethylene side streams is an interesting task. The problem is with the catalyst: it must be resistant to poisoning and operate under variable loads, because the flow is a side stream and its volume is not constant. I heard about attempts to use such solutions at some chemical enterprises in Sichuan province. The results were ambiguous: the yield of target fractions fluctuated, and sometimes selectivity dropped. But the very fact of attempts speaks of the search for flexible solutions.

Membrane separation is a trendy topic. For preliminary enrichment of the flow with ethylene before the main installation - sometimes it works very well. But the key word is ?sometimes?. Membranes are sensitive to pressure, temperature and, again, impurities. If the gas is not pre-prepared, the membrane will quickly fail. Therefore, it is often only one step in the chain. I saw a project where they combined membrane separation with short-cycle heatless adsorption (SCA). It turned out to be compact and quite effective for a flow with a moderate concentration of ethylene.

Practical case and pitfalls

I’ll tell you about one specific case, without naming the plant. The task was to utilize waste gas from a plant where ethylene was mixed with nitrogen and methane. Ethylene concentration is about 15%. The option of allocating it for sale was out of the question immediately: it was expensive. We considered the option of sending it to our own furnace to generate heat, but the calorie content of the gas was rather low.

In the end, you settled on a scheme with catalytic oxidation in a reactor with treated zeolite to produce ethylene oxide? No, it would be too complicated for such a flow. We decided to follow the path of selective catalytic combustion of impurities to increase the concentration of ethylene, and then supply it to the existing network as a higher quality fuel gas. It would seem simple. But at the start-up stage, it turned out that traces of organochlorine compounds from another installation periodically appeared in the gas. The catalyst began to deactivate faster than expected. We had to urgently install an additional carbon filter at the inlet, which changed the hydraulics and the adsorbent replacement schedule. Trifle? No, this is a typical “pitfall” that is not always written about in feasibility studies.

It is in such situations that the experience of a design institute, which has seen different scenarios, is valuable. CompanyChengdu Yizhi Technology Co., Ltd., as a design institute with a registered capital of 120 million yuan, created by Huaxi Technology, usually approaches the issue systematically: not just “let’s deliver the plant?”, but first conducts a detailed analysis of raw materials, looks at the entire logistics of flows in the enterprise, and assesses the possible risks of changing the composition. This is the very “design” that distinguishes just a contractor from a technology partner.

Economics as a deciding factor

Ultimately, the choice of ethylene recycling technology comes down to money. Not only in capital expenditures (CAPEX), but also in operating expenses (OPEX). The same adsorption requires costs for regeneration of the adsorbent, membranes for maintaining pressure, and deep cooling for electricity. Therefore, now they often consider not just “how much ethylene will we save?”, but “what added value will we get throughout the entire cycle?”

An interesting trend is the integration of by-product gas recovery plants into the general “circular” scheme. enterprise economics. That is, the ethylene stream is not considered as a waste, but as a raw material for another process at the same site. For example, for the synthesis of ethylbenzene or oxyethylation, if there are appropriate production facilities. This reduces logistics costs and increases overall margins. But this requires a competent plant-wide scheme, and designing such a scheme is precisely the task for strong engineering companies.

It also happens that with current prices for energy resources and polymers, the most economical option is to send gas for combustion in power plants with heat recovery. And this is not a defeat, but a balanced business decision. The recycling technology must be adequate to economic conditions. Chasing cutting-edge solutions that won’t pay for themselves in a reasonable period of time is a mistake.

Looking to the future: flexibility and digital

Where is everything going? In my opinion, the key words are flexibility and adaptability. The flows of by-product gases are not constant and their composition may change. Future installations are likely to be more modular, allowing parameters or even the process chain to be quickly reconfigured depending on the quality of incoming raw materials. Perhaps hybrid schemes combining, say, membranes and adsorption will become more widespread.

Digitalization also plays a role. The implementation of APC (advanced process control) systems in such plants allows them to optimize their operation in real time, adapting to changes. This is no longer fantasy, but real projects. Sensors for online analysis of the gas composition at the inlet, associated with a control algorithm that adjusts temperatures, pressures, and flow rates - this seriously increases the efficiency and stability of operation.

And of course, work continues on catalysts that are more selective, stable and cheaper. Especially for processes that directly convert dilute ethylene into valuable products. There is room for both scientific institutes and applied engineering centers to develop here. The main thing is that the connection between them is strong, so that laboratory developments quickly progress to pilot industrial tests. In this regard, a structure similar toChengdu Yizhi Technology Co., which is a project part of a larger technology company (Huaxi Technology), seems logical - theoretical developments can quickly find a path to practical implementation.

In general, the topic of utilization of ethylene-containing gases in China is far from closed. This is not about simply “getting rid of waste”, but about extracting maximum value from every cubic meter of gas within an ever-changing economic and environmental framework. And here it is not the most complex technology that wins, but the most intelligent and adapted to real life on the factory floor.