Shell-And-Tube Heat Exchangers Are Used To Utilize Waste Heat From Chemical Industry Exhaust
Chemical exhaust gas typically has high temperatures (150-800°C), complex composition (including dust and corrosive gases such as SO₂ and HCl), and large flow rate fluctuations. Therefore, the structural design of a shell-and-tube heat exchanger requires targeted adaptation:
Tube-side and shell-side allocation:
Typically, clean heating media (such as boiler feed water, process water, and cooling air) are routed through the tube side (to facilitate cleaning and prevent contamination), while high-temperature exhaust gas is routed through the shell side (to accommodate larger flow rates and dust-laden gases, and the shell side offers ample space for installing baffles to enhance flow). If the exhaust gas contains highly corrosive components (such as from the chlor-alkali industry), the exhaust gas must be routed through the tube side, using corrosion-resistant tubing (such as titanium or Hastelloy), with the heating medium flowing through the shell side. Core Component Design:
Heat exchange tubes: Material selection is based on exhaust gas characteristics (e.g., ordinary carbon steel for flue gas temperatures ≤ 400°C and non-corrosive environments; 316L stainless steel for exhaust gas containing small amounts of sulfur and chlorine; titanium tubes for highly corrosive environments). Finned tubes can be used to enhance heat transfer (increasing the heat transfer area, particularly for low heat transfer coefficients on the exhaust side).
Baffles: Baffles (e.g., arched, disc-shaped, or ring-shaped) are installed on the shell side to force exhaust gas to flow horizontally across the heat exchange tubes, breaking up the boundary layer and improving heat transfer efficiency. They also reduce exhaust gas short-circuiting and ensure sufficient heat recovery.
Tube sheet to shell connection: Welding or flange connections are used, with welding preferred for high-temperature environments to avoid leakage. Thermal stress compensation is also implemented (e.g., a U-shaped tube structure to accommodate thermal expansion caused by temperature differences).
Anti-corrosion Design
Chemical waste gas (such as chemical synthesis and incineration exhaust) often contains acidic gases or condensate (such as SO₂ forming H₂SO₃ when it reacts with water). Corrosion risks can be reduced through material upgrades (such as duplex stainless steel or nickel-based alloys), coatings (such as enamel or non-metallic anti-corrosion coatings), or process optimization (such as controlling the waste gas temperature above the dew point to prevent condensate formation).
Anti-clogging and dust removal design
Dust-laden waste gas (such as coal chemical and kiln exhaust) easily accumulates dust on the surface of heat exchange tubes, reducing heat transfer efficiency. The design requires:
Controlling the shell-side exhaust gas velocity (generally 10-15 m/s) to utilize airflow to reduce dust accumulation;
Reserving dust removal channels (such as steam soot blowers or mechanical vibration devices);
Using large-diameter heat exchange tubes or tubes with special cross-sections (such as elliptical tubes) to reduce the probability of dust adhesion. Thermal Stress Compensation
The temperature difference between the exhaust gas and the medium being heated can reach over 500°C. The thermal expansion differential between the heat exchange tubes and the shell due to material and temperature differences can easily lead to tube sheet deformation or weld cracking. Flexible structures (such as U-tube heat exchangers and floating head heat exchangers) or expansion joints are required to relieve thermal stress.
Safety Redundancy Design
To address the potential presence of flammable and explosive components in the exhaust gas (such as VOCs and incompletely burned combustibles), explosion-proof seals (such as metal bellows) are required, along with temperature and pressure monitoring sensors to automatically shut down the process if pressure exceeds the specified limit, preventing safety incidents.
The heat recovered by shell-and-tube heat exchangers in chemical waste gas waste heat utilization is primarily used in the following scenarios, achieving "waste heat to useful energy" conversion:
Preheating process media:
Recovering heat from the reformer exhaust gas (approximately 600-800°C) in synthetic ammonia and methanol plants to preheat feed gas (such as natural gas or air), reducing fuel consumption in the heating furnace. Hot Water/Steam Generation:
Recycled flue gas from the refinery's catalytic cracking unit (approximately 500-700°C) is used to heat desalinated water, generating low-pressure steam (0.5-1.0 MPa) for process heating or power generation.
Preheating Boiler Feed Water:
Chemical park incinerator exhaust gas (300-500°C) is used to heat boiler feed water through a shell-and-tube heat exchanger, improving boiler efficiency and reducing fuel consumption.
Drying Heat Source:
Heat is recovered from drying exhaust gas (150-300°C) from the pesticide and dye industries to preheat fresh air for use as a heat source in dryers, reducing electricity and steam consumption.
