What are the different causes of pressure drop increment (gradually to maximum allowable limit) in a naphtha hydrotreater?
Virendra Kapoor, Petroleum Refining Consultants, firstname.lastname@example.org
Causes of delta P build up in naphtha hydrotreater (NHDT) are dependent on several factors. Some of them are: (1) Source of naphtha stream. Straight run naphtha (SRN) normally do not lead to pressure problems. However, I have observed acute delta P problem when the SRN from Crude Distillation Unit (CDU) was directed in fixed roof storage tank and from there fed to NHDT. A little value of bromine was reported and delta P build up was appreciable in the feed effluent exchangers too of NHDT. On cutting the pipe appreciable size reduction by deposits was seen. (2) Some designers recommend to use magnetic filter for the SRN feed. (3) Specific designs, operating procedures and catalyst combination beds are used when thermally cracked/catalytically cracked blending streams are used with SRN (4) Existing gum and potential gum contents are to be monitored and kept under control with good design for high value feeds
(5) Trash baskets and gradings are recommended by some designers in the reactor top bed. (5) reactor loading and catalyst mechanical characteristics need to be attended (6) Trash materials in naphtha feed, high chloride content in make up H2, recycle gas and NHDT feed need attention.(7) coke lumps due to recycle failure, hydrogen deficiency, feed problem need investigation.
keith bowers, B and B Consulting, email@example.com
Several different mechanisms are at work when hydrotreater catalyst beds build up pressure drop. They are:
1) Solids in the feedstock--rust, sand, corrosion products
2) Coke formed in the reactor caused by insufficient hydrogenation activity, H2 partial pressure insufficient
3) Alphaltenes becoming insoluble as they are dealkylated and reaction mix increases in paraffinicity.
If only the top bed is developing pressure drop, then # 1 is likely the cause. Coker gas oils can contain dispersed fine coke particles or dissolved coke precursors which quickly 'come out of solution' as soon as the hydrogen is mixed with the feed.
Pressure drop increases below the top bed of a cat feed or coker gas oil hydrotreater is likely #2 if near the top bed, but more likely cause 3 if pressure drop build is in the lower bed, particularly if the reactor is not loaded with enough high activity large molecule hydrotreating catalyst
I suggest you consult with your catalyst supplier and process licensor for help in determining the specific causes and methods of addressing the problem in your specific unit and operating conditions.