Reactor cooling During refinery shutdowns one of the most time consuming operations is the cooldown of reactors. Normal cooldowns circulate process gas, initially to cool the system to around 120°C, then nitrogen is introduced and circulated, then purged to vent until the required temperature is achieved. Although the initial cooling is fairly rapid, as the temperature falls so does the rate of cooling, due to the diminished temperature difference between the cooling gas and the system. The recycle compressor, particularly when circulating nitrogen, adds heat of compression which must also be removed. Under normal cooldown conditions, hydrogen is circulated through the recycle compressor system to cool the reactor circuit from 450°C operating temperature. As the temperature falls, the rate of cooling decreases until the equilibrium temperature is reached (usually between 100°C-200°C), i.e. heat input from the compressor is equal to the heat output from the fin fans and other heat exchangers. At this stage the system is purged then cooled using plant nitrogen which is vented to flare. This 2nd stage cooling is historically the most time consuming. Nicool® is a liquid nitrogen injection system, which utilises the recycle compressor and cools the gas by injecting a carefully controlled stream of liquid nitrogen (LN2) into it. The benefit of this system, compared with other cooldown-systems is that the high mass flow from the recycle compressor is utilised for cooling. This combination of high mass flow and cool gas results in a more rapid cooldown, i.e. rate similar to the initial phase of cooling. Gas that would normally enter the reactor at 110°C is cooled to approx. 10°C by spraying LN2 into the gas stream. Therefore, the LN2 injection typically starts when the rate of cooling slows down and average temperatures approach the outlet temperature of the compressor. This can be anywhere between 200°C-100°C, depending on the type and size of the reactor(s). By cooling the gas temperature entering the reactor(s) to approximately 10°C, reactor cooldown time is greatly reduced and reactor skin temperatures of <40°C and even lower bed temperatures are easily achieved. Additional time is also saved due to the faster and easier catalyst handling and reactor work in the cooler vessel. Scope of Work, Execution, Methodology statement The use of liquid nitrogen to reduce reactor cooldown can sometimes introduce the risk of exceeding vessel and pipework temperature limits and also catalyst cooldown rates. Over the years, A. Hak Industrial Services has demonstrated that these risks can be adequately controlled by : detailed planning careful selection of injection points correct injector design accurate control of liquid nitrogen injection constant monitoring of temperatures constant communication during operations The normal flow of liquid nitrogen is from the nitrogen pump unit through the Nicool® skid, high pressure stainless steel field piping and into the injection point (sparger). The Nicool® skid is equipped with a Temperature Control System in order to prevent damage to the process equipment resulting from extremely cold temperatures. At the injection point, thermocouples are attached to the pipe. These thermocouples are attached to the temperature controllers in the Nicool® skid which independently monitor upstream (recycle gas) and downstream (reactor inlet) temperatures. Furthermore, additional thermocouples may be installed at any point where excessively low temperatures would be critical (i.e. elbows near injection point). The controllers can be used in an ‘indicating only’ or ‘controlling’ mode. Each controller has two set points, a low and a low-low. When the low set point is reached, the alarm beacon will light. When the low-low temperature is reached, an air operated dump valve will open and divert the LN2-flow from the process to a safe dump area. Prior to carrying out each Nicool® operation, the equipment is checked and the instrumentation is calibrated. Independent calibration certificates can be issued when required.