Hydrogen Migration and Instrumentation in a Hydroprocessing Unit 1

[Yordif]

This is kind of an instrumentation / metaturgy / process questions but I think refining may be the best place to ask it.

Hydrogen migration is a normal occurance in hydrocrackers. The explanation that I have heard is that elemental hydrogen can pass through a metal structure where it may bond with with something or meet up with another hydrogen atom and from gaseous hydrogen. In an FCC this goes hand in hand with Ferric Cyanide formation and leads to metal blistering. In the old hydrocrackers, your temperature measurement system was multipoint TC assembly in a pipewell. The pipewells over time could pressure up to say 3 or 4 hundred PSI and then usually had a system for venting.

The new measurment systems invovled heavy walled TC's that are out in the active catalyst. Is there any way to quantify if there could be some hydrogen migration and how much? There is proabably about 15% or so void space in the TC with the rest taken up with magnesium oxide or the TC conductors. Does anyone know any industry experts that I might get in touch with that could help define the phenomeon? I certainly would be happy to talk with anyone who might have experience or has written a related paper.

Also is hydrogen migration, say through stainless steel, a function of hydrogen partial pressure and/or temperature?

Thanks for your help.

 

[danw2]

I never had to be concerned with it, but I recalled a Honeywell application note from a decade ago on hydrogen ion migration that might shed some light.

Honeywell 1996
Application Bulletin: Hydrogen Migration

Problem:
Pressure and differential pressure transmitters using Stainless Steel diaphragms along with close coupled zinc or cadmium plated components having water as the process, are commonly susceptible to Hydrogen migration. Simply put, monatomic Hydrogen migrates through the Stainless Steel diaphragm, primarily due to it’s Nickel component, and combines to form Hydrogen (H2) gas inside the cell. Gas trapped inside a transmitter causes measurement inaccuracies over time as the trapped gas increases. If the migration continues long enough, permanent distortion of the diaphragm will take place. This distortion will be most evident once the static or operating pressure is removed from the transmitter with the trapped (H2) still at static pressure inside the cell; the diaphragm ‘bulges’ or ‘blows-out’.

Factors Involved:
The source of the Hydrogen gas (H2) has a significant influence on the way migration affects a transmitter. The worst possible case is where (H2) is cathodically generated on the face of the diaphragm. All it takes is a weak electrolyte (water serves very well) coupled with zinc or cadmium plated transmitter flanges or galvanized pipe or fittings near the Stainless Steel diaphragm, and a galvanic cell is created.

Zinc or Cadmium plating serve as limited but significant types of corrosion protection when the base metal can not provide the needed protection. For applications that do not require maximum protection, zinc or cadmium offer an inexpensive solution. Due to environmental protection limitations, Cadmium is no longer offered and zinc is now mainly used.

Zinc is applied as a thin coating sufficient to withstand normal atmospheric corrosion. However, its resistance to corrosion by most chemicals is low. Zinc acts as a sacrificial anode. This means the underlying metal is protected at the expense of the zinc plating - even when the zinc plating is scratched or nicked, exposing the metal substrate.

A potential difference results when the electrically connected zinc plated heads or galvanized piping (anode) and the positive Stainless Steel diaphragm (cathode) are separated in a conductive medium (water). This potential difference causes positively charged particles to flow from the anode to the cathode through the conductive medium. To complete the circuit, the negatively charged electrons flow from the anode to the cathode through the metal-to-metal contact between the heads and diaphragm.

The loss of electrons by the zinc plating is called oxidation, and it causes the metal to become positively charged. The positively charged ions on the surface (Zn++ ) attract negative ions found in the water to form new compounds. This new compound no longer has its former metallic characteristic, but rather takes on a new form, such as Zinc Oxide (ZnO2). The gain of electrons at the Stainless Steel diaphragm is referred to as reduction and allows the metal to retain its metallic properties while liberating monatomic Hydrogen (H-) and Oxygen (O) in the process. Some of the monatomic Hydrogen (H-) migrates through the Stainless Steel diaphragm because of the nickel content; the remainder combines to form Hydrogen gas (H2), which bubbles away harmlessly.

Recommendations:
Zinc plated heads or galvanized piping material coupled with Stainless Steel diaphragms in a water solution will cathodically generate monatomic Hydrogen (H-) which will migrate through the diaphragm and must be avoided. Stainless Steel flanges, manifolds and impulse piping should be used instead.