| American Fuel & Petrochemical Manufacturers

Question 36: What are the best practices for maximizing catalyst run length in NHT units that are limited by reactor pressure drop?

Marathon’s experience with NHT fouling has been primarily corrosion products from upstream units and oxygen related polymerization of the naphtha. The use of feed filters can help minimize the effects of corrosion products. Oxygen related polymerization has impacted our NHT units from air leakage across intermediate naphtha storage tank seals and in purchased naphtha.

Question 35: What experiences do the panelists have with naphtha hydrotreater combined feed exchanger fouling? How do you monitor fouling in this exchanger service?

Feed side fouling of the NHT feed/effluent exchanger can be attributable to polymerization of olefins or diolefins. Storage of unsaturated materials from the Coker/FCC/Visbreaker may exacerbate the problem with the introduction of oxygen as a free radical that can promote polymerization.

Question 34: What has been the experience of refiners operating selective hydrotreating of FCC naphtha regarding gum formation potential of the low sulfur gasoline? Is gum inhibitor addition a recommended practice?

Gum potential exists with unsaturated hydrocarbons (olefins or diolefins) stored in refinery operations with oxygen as a free radical. Organic sulfur and nitrogen compounds remaining in FCC gasoline product can form gums in the presence of olefins and oxygen.

Question 33: How do refiners avoid De-isobutanizer (DIB) column/reboiler fouling in sulfuric acid alkylation? What process conditions on the column do you use to detect this fouling? What process modifications do you take to minimize the impact of this fouling?

Fouling in the DIB column is almost always caused by salt deposits. These salts are typically sodium sulfate and sodium sulfite but can also contain calcium or magnesium if the effluent treating water is not demineralized. If these water-soluble salts are present in the DIB feed, the water will evaporate once inside the column leaving the solids behind.

Question 32: In your experience, what contributes to Monel denickelification in the HF Acid Regenerator circuit? What are the potential problems associated with this?

Oxygen is a major cause of monel denickelfication. Oxygen can enter the circuit during loading operations. Care should be taken to avoid pressuring air contained within loading pipes/hoses into the unit.

Question 31: In your experience, when sampling the HF Alky iso-recycle stream, how and where is the sample neutralized prior to analysis? Can this approach be used for online GC analysis as well?

The isobutane recycle sample can be neutralized at the sample location using a chamber filled with alumina or KOH pellets. If using a KOH chamber, it is best to add a filter downstream to filter out any fines.

Question 30: What process parameters can affect alkylate T90? What are the critical variables you monitor in both sulfuric and HF units? Discuss processing schemes, feed impacts and operating variables.

The type of feed is very significant for T90. Amylenes make alkylate with higher T90 in both sulfuric and HF units. Propylene generally makes lower T90 than butylene in HF units. However, with sulfuric-catalyzed technologies, propylene can increase T90 as discussed below.

Question 29: What technologies do you use for treating or recovering VOCs from small-scale truck loading terminals? Discuss the merits associated with each?

The three main VOC treating systems for small scale truck loading are vapor combustion system, flare gas recovery unit, and an adsorption/absorption vapor recovery system.

Question 28: The Clean Air Act required refineries to develop and implement a Leak Detection and Repair (LDAR) program to control fugitive emissions. What is the current status of this implementation and who is responsible for it in a typical refinery management structure: production, maintenance or EHS?

Since the inception of the Clean Air Act of 1955 and multiple amendments through 1990, Leak Detection and Repair or LDAR regulations have been a part of air pollution control. Today’s LDAR programs are governed by Federal and State regulations and agreed orders (consent decrees) that provide the control of fugitive emission leaks from process equipment by requiring equipment inspections and leaking equipment repair.

Question 27: It has become increasingly common to chemically neutralize / passivate refinery towers and vessels prior to entry. What are the recommended practices for implementing these tasks? In your experience, what conditions trigger the need for chemical treatment?

Although this list is not “all-inclusive”, here are some general recommended practices when chemical cleaning and/or neutralizing towers and vessels. First, there needs to be a single point of contact for the chemical cleaning vendor. This person is responsible for the planning, preparation and execution of the chemical cleaning process. Prior to cleaning, P&ID’s need to be marked up to identify all injection points, steam and chemical flows and even line ups for the chemical cleaning.