Q & A > Question Details
I am in Diesel Hydrotreater unit. Our feed contains maximum of 20% unsaturates (cycle oils). In the startup procedure, we were told that feed should be cut in at a reactor bed temperature of 260 C. But our current catalyst supplier has suggested that you can cut in feed even at 320 - 330 C. I just want to know what will be process implications if we cut in feed at 220C or 260C or 320C. Our diesel feed API is 33, feed sulfur is 1.2% wt, IBP 147 C and FBP 424 C.
One more thing: what will be effect if we run the high pressure separator at lower and higher temperatures. (It happens some times because of Fin-Fan cooler problems and climatic conditions).
 
Answers
20/11/2010 A: Satish Angadi, Haldor topsoe, satish.angadi@gmail.com
I will try to answer this question in parts:
1) feed should be cut at 260 or 320 or 330: This is specific to catalyst you may be having. Usually at T above 275-300°C the HDS reaction will start.
Each catalyst supplier will indicate how long catalyst can see high Temperature with hydrogen alone, otherwise catalyst will get reduce to metals and damage will be irreversible.
Liquid Feed cut in at low temperature say 200+ ensure that the liquid will give the sulfur required to keep catalyst in sulfided form.
In case of restart of plant after trip, it can be done as per each operating plants comfort (charge heater outlet T- control).
2) varying HP separator temperatures: This will affect the hydrogen loss to stripper(and the recycle gas purity insignificantly). (I assume the temperature is varying by 5-10 deg C).
The stripper will see more or less flashing of HP separator liquid if it is going directly to stripper, but it should not be cause of concern.
16/09/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Mr Ragsdale's answers get an A+ vote from me. I add only--too low a temperature on the HP separator can lead to emulsions and water not settling out. Maintaining good stable control of this temperature is critical to maintaining stable reactor conditions. Temperature fluctuations will change recycle gas compositions and H2/feed ratios which change conversion which changes temperature rise across reactor which changes need for quench ............and so forth. Fix the damn TC and air cooler controls. Install variable speed drive (more stable than blade pitch control) HP temp control is important to operating stability, which is CRITICAL for achieving maximum catalyst life.
15/09/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
It should be noted that, if presulfiding is done by using the sulfur in the feed, rather than by a specific injection step, the feed is normally introduced at reduced temperature for approximately four hours before proceeding at normal SOR temperature.
15/09/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Feed cut-in temperature in that range is not critical. For example, after an upset when all feed was stopped and hydrogen is circulating, it is not necessary to cool to 260ºC before cutting in feed.

Separator temperature affects H2/H.C. ratio at reactor inlet for a given recycle compressor capacity. Reduced hydrogen requires a higher temperature for a given target percent sulfur removal, which translates to a shorter run. Too low a separator temperature can cause a centrifugal recycle compressor to go into surge. To avoid that condition, gas can be bled into the compressor suction to increase the molecular weight and avoid surging during low separator temperature. Reciprocating compressor drivers can be overloaded by high molecular weight fluid, but not low M.W., so high separator temperature can limit the capacity of the recip recycle compressor driver.