Is Your High-Pressure Heat Exchanger Always Failing? These 4 Common Faults You Must Know

The magnitude of pressure drop directly affects the energy consumption of the unit
In hydrocracking units, most high-pressure heat exchangers are used in the recycle hydrogen circuit, where the pressure drop directly impacts the energy consumption of the recycle hydrogen compressor. For once-through hydrocracking units, the energy consumption of the recycle hydrogen compressor accounts for approximately 15%–30% of the total unit energy consumption. Therefore, the pressure drop across the high-pressure heat exchanger significantly influences the unit's overall energy consumption, and a lower pressure drop helps reduce operating costs.

Heat exchangers operate under severe conditions
Hydrocracking units operate under high-pressure, hydrogen-rich environments, imposing high requirements on equipment and materials. In some emergency situations, the reaction system must be depressurized at a rate of 0.7 MPa/min or 2.1 MPa/min. During such rapid depressurization, the pressure in the high-pressure heat exchanger drops quickly while the temperature rises rapidly, making leaks and fires more likely.

Larger scale increases manufacturing difficulty
With the rapid development of larger-scale units in recent years, high-pressure heat exchangers have grown in size, increasing manufacturing complexity. For thread-locking ring type heat exchangers, units with a diameter greater than 1600 mm are considered large-scale, presenting greater processing challenges. The tube sheet is prone to deformation, requires strict flatness, and is more susceptible to internal leakage. In the past two years, thread-locking ring type heat exchangers with a diameter of φ1800 mm have emerged, but their manufacturing difficulty is even higher, and the risk of internal leakage is greater.

High content of nitrogen, sulfur, and other impurities leads to corrosion and coking
The nitrogen content in feedstock for hydrocracking units is mostly in the range of 500–2000 μg/g. Ammonia present in the reactor effluent combines with hydrogen sulfide or trace amounts of hydrogen chloride to form ammonium salts. The ammonium salt crystallization temperature in hydrocracking units is mainly between 160°C and 210°C. The higher the ammonia content in the effluent, the higher the crystallization temperature. Moreover, ammonium chloride crystallizes more easily than ammonium bisulfide.

Intermittent and continuous water injection is required to dissolve ammonium salts and prevent under-deposit corrosion and erosion corrosion that can lead to internal leakage or tube perforation in heat exchangers. Feedstocks for hydrocracking units may include deasphalted oil, FCC diesel, coker diesel/wax oil, straight-run diesel/wax oil, etc. The operating temperature of feed-effluent heat exchangers is typically between 190°C and 440°C. Aromatics, resins, and asphaltenes in the feedstock are highly prone to coking in high-pressure heat exchangers—the higher the impurity content, the more likely coking occurs. Coking reduces heat transfer efficiency and increases pressure drop; in severe cases, it can force the unit to shut down.