Jun-2021

Protecting your hydroprocessing reactor

Owners and operators respect a hydroprocessing reactor’s minimum pressurisation temperature but can fail to understand factors other than the temperature.

ERIC LIN and RICHARD TODD
Norton Engineering Consultants

Article Summary

When an owner or operator in charge of a hydroprocessing unit is asked what a reactor minimum pressurisation temperature (MPT) is, they may correctly respond that it is the lowest temperature at which a reactor can start pressurising above a predetermined limiting value after it has already seen hydrogen service. Some may even understand that the purpose of respecting the MPT is to protect the reactor against brittle fracture caused by diffused hydrogen. However, far fewer understand MPT implications beyond those two critical points.

Most hydroprocessing reactors are susceptible to failure mechanisms caused by temper embrittlement as well as hydrogen embrittlement. Whether it be concluding turnaround activities or overcoming an upset condition, respecting the MPT is important to ensure reliable and safe operation of the reactor vessel. This article provides insights and understanding around MPT that can impact hydroprocessing reactor integrity along with other equipment in the high pressure loop that is exposed to hydrogen.

Many papers and industry standards have been published that address concerns surrounding MPT. A recent series of articles by Pillot et al. focused on the effect of temper embrittlement and hydrogen embrittlement on a material’s mechanical properties,¹ along with a methodology to determine MPT for reactors that are already in service.² These two articles provide a framework to further examine MPT for a hydroprocessing unit.

Temper embrittlement
API RP 571 describes temper embrittlement as “the reduction in toughness due to a metallurgical change that can occur in some low-alloy steels as a result of long-term exposure in the temperature range of about 650°F to 1070°F (343°C to 577°C). This change causes an upward shift in the ductile-to-brittle transition temperature as measured by Charpy impact testing. Although the loss of toughness is not evident at operating temperature, equipment that is temper embrittled may be susceptible to brittle fracture during start-up and shutdown.”³

API 571 best practice recommends limiting the amount of tramp elements such as manganese, silicon, phosphorus, tin, antimony, and arsenic in the base metal and welding consumables to avoid the effects of temper embrittlement. When the steel contains vanadium in the base material, it is less sensitive to temper embrittlement than when the steel does not contain vanadium.1 It has become standard practice in recent years to use a low alloy steel that contains vanadium with tight control on the tramp elements to fabricate the steel walls of hydroprocessing reactors.

Hydrogen embrittlement
The National Association of Corrosion Engineers (NACE) describes hydrogen embrittlement as “the ingress of hydrogen into a component, an event that can seriously reduce the ductility and load-bearing capacity, cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials.”⁴

At elevated temperature and pressure, molecular hydrogen partially dissociates to form atomic hydrogen, H2 ↔ 2 H, which is a reversible, equilibrium-limited reaction. Atomic hydrogen is soluble in steel and will enter the lattice structure of the walls. The inner surface of the steel becomes saturated and atomic hydrogen starts diffusing towards the outer surface. If a discontinuity or defect is present, then atomic hydrogen that is diffusing through the steel can reversibly form molecular hydrogen in the void, which becomes trapped and starts to accumulate. These trapped pockets of hydrogen create fissures that lead to intergranular cracking. Figure 1 depicts atomic hydrogen entering the wall and accumulating in the grain boundaries, weakening the material and leading to a loss of ductility and strength. A reactor wall that has become compromised is not immediately obvious without the assistance of tests and scans. Eventually, microfractures become large cracks that can lead to complete loss of integrity and failure.

As stated previously, MPT is a minimum temperature that must be obtained during start-up before a reactor can be pressurised beyond 25% of its design pressure. Per API RP 571, this 25% point is critical to prevent brittle fracture. The MPT tries to re-establish the equilibrium profile for atomic hydrogen through the steel to minimise the potential for molecular hydrogen being present at the grain boundaries that could result in rapid brittle failure. During unit start-up, this is normally accomplished by heating up a recycle gas stream that passes through the heat transfer coil of a heater before entering the reactor. Once the minimum temperature has been achieved, it is safe to start increasing pressure in the system up to the normal operating point.

Factors such as climate, heater, and compressor size in the hydroprocessing unit will also play an important role in determining the time required to heat up the reactor. It is far quicker to heat up the steel walls of a reactor in a warm climate during the summer months than it is to achieve the same MPT in a cold climate during the winter months.

Determining MPT
With an understanding of the mechanisms that lead to brittle failure in a hydroprocessing reactor, how is MPT determined? MPT is a function of fracture mechanics and/or Charpy V-Notch (CVN) toughness calculations. It will not be surprising then that the MPT is normally calculated by the reactor fabricator based on the base material selected for the reactor wall. Common base materials for hydroprocessing reactors include 2¼ Cr—1 Mo and 2¼ Cr—1 Mo—V. The addition of vanadium allows a reactor of the same size to be lighter in weight. Another advantage with the addition of vanadium to the base metal is the difference in atomic hydrogen content between the cold start-up condition and the hot, steady state condition during normal operation. Figure 2 shows an example profile of the hydrogen content through the steel wall between the hot and cold condition. Although hydrogen content is higher in vanadium-enhanced steel, the more important feature is that the difference between the hot and cold condition is small, making this a desirable material of construction to use. (There is much less hydrogen diffusing in and out of the wall during a cooldown and heat-up cycle when vanadium is present in the steel.)

Additionally, the base material is usually followed by a stainless steel weld overlay such as 347, which protects the low alloy steel from corrosion and degradation. The addition of this overlay, though necessary, increases the potential for premature failure of the reactor due to a phenomenon known as hydrogen induced disbonding (HID), which is characterised by a crack propagating at the interface between the base material and the austenitic stainless steel weld overlay/cladding.5 The difference in hydrogen solubility between the base material and the austenitic stainless steel overlay creates a region where hydrogen content goes through a step change (see Figure 3). During reactor cooldown, the interface becomes a point where hydrogen content increases, which will accumulate if a defect is present. Coudreuse et al. provide more guidance that identifies how parameters such as operating temperature, hydrogen partial pressure, cooldown rate, material thickness, hydrogen diffusion coefficient and time can influence hydrogen content at the interface.⁵ Anecdotal evidence from an API survey suggests that nearly 30% of reactors suffered from HID. ASTM G146 - 01(2018) is the latest industry standard that evaluates disbonding in high temperature, high pressure hydrogen service.