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19/01/2017
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Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Injecting hydrogen upstream of the preheat train lowers the partial pressure of the diesel stream and helps it to vaporize in the preheat train, which improves the heat transfer efficiency of the system and reduces furnace size. The two phase flow does increases DP in an existing system or requires larger exchangers keep DP from increasing.
Hydrogen injection location can also influence metallurgy selection. Downstream of the hydrogen injection point naphthenic acid corrosion risk is greatly reduced, but traditional low chrome materials like 5 & 9Cr do not provide much benefit above CS for protection against high temperature sulfidation. Thus, people are forced to go to 300 series stainless steels for corrosion resistance. In lower sulfur systems that would not require 300 series SS, the risk of high temperature hydrogen attack is introduced (HTHA) downstream of the hydrogen injection point.
Besides exchanger vibration, combined hydrogen and diesel flow can also cause vibration issues in the furnace tubes. Fouling risk is reduced downstream of the hydrogen injection point, especially in systems processing cracked feedstocks. As you can see a number of factors come into play in the decision, which is why there is not a one size fits all solution.
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18/01/2017
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Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
Optimum solution may be to have hydrogen injection capability at both locations. Hydrogen injection upstream of the CFE can help mitigate fouling and polymerisation of CFE through increased velocity and turbulence and can help assure CFE integrity by maintaining gas low on both sides of the CFE during abnormal conditions such as startup, trip or shutdown. It also facilitates oil-freeing of CFE bundles for maintenance. However too much hydrogen injection at this location can result in risk of vibration problems and/or larger diameter (more expensive) CFEs. Hydrogen injection downstream of the CFE provides an additional means of managing high system pressure drop at end-of-run (EOR) conditions and of accelerating cooldown and final temperature at shutdown by preventing reactor effluent transferring heat back to reactor feed during final reactor cooldown on gas circulation.
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16/01/2017
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Jesus Perez, Alfa Laval Packinox, jesus.perez@alfalaval.com
DHT units with Packinox exchangers use the hydrogen mixing point before the Packinox CFE.
The recycle gas is used to pressurize the shell side before the mixing point. The shell metallurgy is designed for hydrogen service following the Nelson curves. The internal bundle metallurgy is austenitic stainless steel.
These exchangers are installed vertically. Alfa Laval Packinox has wide experience of achieving proper liquid lift therefore, even flow distribution is achieved which reduces fouling in channels.
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15/01/2017
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Ganesh Maturu, Self, maturu.ganesh@gmail.com
When you are mixing H2 with HC at the upstream of exchanger, most of the times exchanger metallurgy in cold side (Feed and recycle gas mix) comes out to be higher metallurgy like 1 1/4 Cr or 2 1/4 Cr as per Nelson Curve. Whereas if we heat only feed without mixing RG and providing feed in shell side of the exchanger, metallurgy will be CS and hence is cheaper. If we heat only feed above 680-700 deg F, there might be a possibility of polymerization/cracking and fouling of exchanger and hence suggested to provide small amount of H2 (Soak Gas) along with feed to shift the reaction. Typically if the throughput is higher like more than 50000 KBPSD, heating feed and RG separately and mixing just upstream of the heater is economical and also can recover maximum amount of heat from reactor effluent circuit.
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13/01/2017
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karthik ramesh, indian oil corporation, rameshkarthik810@gmail.com
It mostly depends on licensor and client where to mix hydrogen with the feed. in one our diesel hydrotreating units hydrogen was mixed before the feed vs effluent exchangers and there was major problem of vibration due to two phase flow in the exchangers. exchanger design has to be good taking care of all the problems.
In our another VGO hydrotreater H2 was mixed at the end there was separate heater for heating the H2. This eliminates uncertainties in exchanger design and the client will also be happy because there will be no interruptions in the plant due to exchangers problem.
Regarding fouling antifoulants are used in our DHDT plant. usually exotherm takes care of heat gain by the exchangers when you process high olefinic feed stuff.
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13/01/2017
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NS Murthy, Suez, murthy.ns.ext@suez.com
Couple of thoughts on this. If your feed does not have any cracked components, you can skip the hydrogen injection after the CFE, which also gives an advantage of HP loop pr. drop. Carry out a fouling test of the DHT feed and there are options like use of anti-foulant to overcome fouling issues as well. Additionally, even if cracked components are present in DHT feed, one can mitigate fouling by smart design of exchanger to eliminate 'dead' zones and higher LMTD which propagate fouling.
Good luck. GE Water and Process Technology can assist in offering suitable anti-foulant for your need.
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12/01/2017
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Keng Chung, Well Resources Inc., kengchung@hotmail.com
Put a small amount of hydrogen before the CFE, which results in two-phase flow to reduce or eliminate boundary layer causing fouling.
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12/01/2017
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keith bowers, B and B Consulting, kebowers47@gmail.com
Long experience in the industry shows it better to introduce at least some of the hydrogen circulation into the mixed feed prior to any significant heating. This has been effective innings reducing exchanger fouling.
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