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The refinery I work in has a 40 year old hydrotreating reactor with a very high design minimum pressurization temperature (MPT) of 120degC. it always takes ages to clear the MPT before we can proceed to the next step in the unit startup. We run our 2x recycle gas compressors which are fixed volume reciprocating compressors, loaded to 200%. The question I would like to ask is: is it better to have higher purity H2 for heating up? Higher H2 purity would mean less mass flow at constant volume. However, H2 thermal conductivity is way higher than other gases such as N2 or CH4, C2H6. The other school of thought is to bring in less pure H2 source to boost the mass circulation.
This is probably a heat transfer question also. Would like to hear if anyone has done any research on this.
 
Answers
20/07/2020 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Assuming the heater is not the limiting component--one must observe the reciprocating recycle and make-up compressors outlet temperatures when heavily loading them. Hydrogen can have high discharge temperatures which can damage the cylinder discharge valves if too hot. One can easily calculate the total heat capacity potential of the two recycle gas cases. The Objective is of course maximum heat input to the reactor, which depends on temperature and heat capacities and mass flows. Some catalysts have temperature limits at hydrogen partial pressures which must not be exceeded to prevent loss of active catalyst sites. The Thermal Conductivity of either case is immaterial, the gas exiting the reactor will surely be identical to the reactor and catalyst at the exit. The important characteristic is Heat Capacity (BTU/#-deg.F) at the furnace outlet. Hydrogen has higher heat CAPACITY per pound than other gases. The maximum sensible heat input to the reactor will likely be limited by the volumetric capacity of the recycle compressors when running high hydrogen purity, and recycle gas compressor driver horsepower when running a higher molecular weight gas.

One MUST consider and evaluate the actual reactor metal condition when approaching the minimum pressurization temperatures. Hydrogen embrittlement is a common factor in evaluating reactor safety and the condition of the old reactor should be evaluated.
06/07/2020 A: Ed Ouwerkerk, OptiTreat Hydrotreating consultancy, ced.ouwerkerk@outlook.com
Contrary to cooling down a HT, its heatup rate is normally limited by the furnace capacity. If you think its duty cannot be transferred into the RG-loop, why don't you start the feed pump (on a light, low-visco feed), to increase the mass flow at, say 60 C, and simultaneously perform catalyst wetting?
03/07/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
Higher molecular weight gas (i.e. low purity hydrogen) will assist the heat-up rate.

There are other procedural changes that will improve the heat-up rate.
• Firstly, introducing oil to the unit as early as possible in the heat-up. Exactly when this happens will depend on various unit-specific factors.
• Secondly, many units with high MPT use an MPT curve rather than treating the MPT as a step-change. For a typical unit, this might allow a slight pressure increase at about 50°C below MPT, following an increasing curve until reaching full pressure at MPT.

Units which suffer with slow heat-up because of MPT usually suffer with slow cooling when they shut down. Again, there are procedural changes (and engineering changes) which can improve the cool-down rate.

Please get in touch if you’d like help reviewing and optimising your start-up and shut-down procedures.