【Haichuan’s Recommended Articles】An Overview of the Technical Development of Quench Heat Exchangers in the Ethylene Cracking Process
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In the ethylene production process, in order to prevent a decrease in the yields of key components such as ethylene and propylene due to secondary reactions, it is necessary to rapidly cool the high-temperature pyrolysis gas produced by the pyrolyzer (with a temperature range of around 750°C) to a temperature range close to that at which the pyrolysis reaction comes to an end (around 500°C). There are various methods to achieve this rapid cooling process; in the early days, oil quenching or water quenching was primarily used. However, both of these methods have significant drawbacks. For example, they dilute the pyrolysis gas, increasing separation costs ； At the same time, too fast coking speed can interfere with deep cracking. Furthermore, oil quenching may increase the operational risks of the equipment, while water quenching presents problems related to high energy consumption and environmental pollution, as the cooling water is often discharged directly as wastewater.
In December 2024, the 1.6 million-ton ethylene plant of ExxonMobil’s Huizhou Ethylene Project Phase I was successfully commissioned, marking another important milestone for this project (Image source: ExxonMobil China). Since the 1960s, various quench boilers, namely quench heat exchangers, have been widely used in foreign ethylene production to efficiently cool the pyrolysis gas indirectly by absorbing heat through the evaporation of high-pressure water. This type of heat exchanger plays a crucial role in ethylene plants; it has high process efficiency, which is why it is called \"quenching\". The time it takes for the pyrolysis gas to pass through the heat exchanger must be strictly controlled to ensure effective cooling and the safe, stable operation of the equipment. within .s, to prevent secondary reactions and ensure that the total recombined components and secondary reactants do not deposit on the tube walls. In ethylene cracking units, the quench heat exchanger serves a dual purpose: it must rapidly cool the cracking gas, which is at around 800°C, below the temperature required for the secondary reaction, and it must also recover the high thermal energy contained in the cracking gas to generate high-pressure steam at approximately 12.0 MPa. Such operating requirements pose severe challenges to the mechanical structure of the heat exchanger.
Comparison between USX-type quench heat exchangers and Schmidt-type quench heat exchangers. The German company Schmidt (Frima Schmidt ScheHeissdampf-Gaselleechaft, abbreviated as SHG) has been working on the design of innovative quench heat exchangers since 1959. This type of heat exchanger replaces the traditional tube sheet design with an elliptical collector tube, using batch welding of these elliptical collector tubes to form the upper and lower tube sheets. Its uniqueness lies in the fact that two sleeves are welded to either side of the oval collector tube, thereby forming what is known as a \"double-sleeve type\" or \"Schmidt type\" (SHG type) structure. In such heat exchangers, the high-temperature pyrolysis gas flows from bottom to top through the inner tube, while the cooling water enters the lower elliptical collector tube from the upper drum via downcomers, and is heated from bottom to top along the annular space between the inner and outer tubes. The resulting high-pressure steam-water mixture then enters the upper elliptical collector tube and returns to the steam drum via the rising pipe. Compared to traditional shell-and-tube heat exchangers, this design has the advantage that the oval-shaped collector tube can effectively absorb the temperature difference stresses between the inner and outer tubes, ensuring the stable operation of the heat exchanger. At the same time, this structure also avoids the problems associated with thick tube sheets, making the heat exchanger more compact and efficient. “\"Schmidt-type\" heat exchangers are widely used due to their excellent performance; many companies, including Yanshan and Jinshan Petrochemical in China, have adopted and successfully utilized this type of product.
Innovations in SHG-type quench heat exchangers and “bathtub-type” quench heat exchangers: To address the issues related to flow distribution in traditional double-tube quench heat exchangers, Schmidt Company, in collaboration with Lummus Company, developed the “bathtub-type” quench heat exchanger. What sets this new type of heat exchanger apart is its inlet head design, which resembles a bathtub, hence the name. Although its heat exchange unit remains the same as that of conventional double-tube quenching heat exchangers, significant improvements have been made at the pyrolysis gas inlet. Specifically, they abandoned the traditional single-feed method in favor of a multi-feed design, with each bath-type quenching heat exchanger typically equipped with 2 or 4 pyrolysis gas inlets. In addition, devices such as diffusers, distribution spaces, and spreaders have been added to achieve a more even flow distribution.
Maoming Gravity Builds First Batch of Quench Heat Exchangers for 1.5 Million Tons/Year Ethylene Production Using LUMMUS Process in Fujian’s Gulei https://bbs.hcbbs.com/thread-5692252-1-1.html (Source: Haichuan Chemical Industry Forum)
Furthermore, the Borsig-type rapid cooling heat exchanger is also worth mentioning. This type of heat exchanger also exhibits innovation in its flow channel design. Its uniqueness lies in the fact that by optimizing the flow channel structure, it achieves more efficient heat transfer and a more uniform flow distribution. This design not only improves the performance of the heat exchanger but also extends its service life, bringing significant benefits to industrial production. In 1965, the German company Borsig broke with tradition by innovatively designing a new type of quenching heat exchanger, namely the Tunneflow-type quenching heat exchanger. Its uniqueness lies in the design of the tube sheet, which is carefully made thin (only about 15–20 mm) in order to reduce the thermal stress on the tube sheet at the inlet end. At the same time, reinforcing ribs are cleverly arranged behind the thin tube sheet and connected to the thick tube sheet, thereby forming a tube sheet assembly structure that combines thin and thick sections. Such a design not only meets the different requirements of high-temperature gas and high-pressure water on the tube sheet, but also significantly improves the performance of the heat exchanger. However, the manufacturing process for such heat exchangers is relatively complex, presenting certain technical challenges. Furthermore, during operation, coking cannot be directly removed on the pyrolysis gas side; regular mechanical coking removal using high-pressure water is required.