The Hydrogen Trains were designed to generate large quantities of hydrogen for use in hydrocracking operations. The Hydrogen has a purity of 97%. 1200 mt/day of food-grade carbon dioxide is also produced from the palant.
The two trains share a natural gas feed system which includes two feed compressors, one recycle compressor, a feed pre-heater furnace, and a hydro-desulfurizer vessel. The Hydrogen Production Trains are nearly identical and are completely independent downstream of the common feed system.
Turn-down is incredibly low – the entire complex can be run at 40 MMSCFD of hydrogen (one train down and one running at 40% capacity). Hydrogen purity is improved to 99% when running at low capacity.
Process control system is a Honeywell Experion DCS. Software/database has been backed up and is ready to go with plant.
New tubes, Incoloy pigtails, and Incoloy collection headers were installed recently in both A-Train and B-Train. With conservative reformer temperatures, the tubes could still have over 10 years of life. The reformer catalyst also has significant life left.
The A-Train reformer has a selective catalytic reduction (SCR) system with upgraded honeycomb catalyst for controlling reformer burner emissions while the B-Train reformer uses Low-NOx burners to accomplish the same.
This plant originally ran with naphtha feed to the reformer. Most of the necessary additional equipment is still in the plant.
Below is a list of major equipment that was replaced in the last 10 years. A trainF-305 1Q2013 SD – Replaced tubes and outlet headers EWO-CB201-E1 and EWO-CB201-E?E-335 1Q2013 SD- Shell and shell cover upgraded to SS EWO-CB411-E2E-325 2Q2016 SD- Replaced shell and shell coverE-342 4Q2016 SD- Replaced complete exchanger
B trainF-355 1Q2014 SD – Replaced tubes and outlet headers EWO-CC201-E1 and EWO-CC201-E3E-375 1Q2017 SD- Replaced shell, shell cover, channel and channel cover EWO CCE375-E1E-384A 1Q2017 SD- Replaced shell and shell cover EWO CCE384A-E1E-384B 1Q2017 SD- Replaced shell and shell cover EWO CCE384B-E1E-392 1Q2017 SD- Replaced channel, channel cover and bundle EWO CCE392-E1
This section briefly outlines the six major hydrogen manufacturing steps:
• Desulfurization• Reforming• Steam Generation• Shift Conversion• Carbon dioxide removal• Methanation
Sulfur poisons the catalysts used in the Hydrogen Manufacturing Plant reforming furnaces and shift converters. These catalysts cannot continue to function after continuous exposure to even a small amount of sulfur. There is no control over the sulfur content in the natural gas feed from PG&E, so it is necessary to have plant equipment that can remove sulfur compounds from the feed gas. The RPG4 component of the feed gas contains approximately 10 ppm of H2S.
The Hydro-Desulfurization section (HDS) reduces sulfur in the feed gas to less than 0.2 ppm by weight. This is achieved by hydrotreating the sulfur compounds in the feed. Zinc oxide beds adsorb the resultant H2S.
A small amount of product hydrogen is recycled back to the HDS from the first stage of the hydrogen booster compressors. This hydrogen allows for hydro-treating the sulfur compounds in the feed. Under normal circumstances, most recycled hydrogen passes through the HDS without reacting and circulates through the rest of the Hydrogen Manufacturing Units. The RPG4 component of the feed gas contains approximately 10 ppm of H2S.
The HDS is located within an area of the unit commonly referred to as the SNR. SNR originally stood for Steam-Naphtha Reforming, but the term is now commonly used to refer to the area that removes sulfur from the feed gas for both Hydrogen Trains.
Reforming is the basic step in hydrogen production. Desulfurized feed and 450 psig steam are mixed together and passed through a reforming furnace. In the reforming furnace, the feed gas and steam react together over the reforming catalyst contained in the furnace tubes.
The reforming reaction products are hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4). Excess steam also appears in the Reformer effluent along with reforming reaction products.
The reforming reaction is an endothermic reaction. It requires a large amount of heat and takes place only at high temperatures (1525F). Without heat input from burning fuel gas in the reforming furnace, the reacting gases will cool down and the reforming reaction will come to an end.
Generating high pressure (450 psig) steam is an important part of operating the Hydrogen Manufacturing Plants. The Hydrogen Trains use the heat from flue gas and reformer effluent to generate 450 psig steam. Most of this 450 psig steam is fed to the reforming furnaces along with feed gas. Any remaining 450 psig steam is exported to the Isomax 450 psig steam system.
The steam that was added to the feed gas may be converted in either the reforming or shift conversion reactions. Any steam that remains in the Low Temperature Shift Converter effluent is condensed and removed. The resultant condensate is reused in the steam generation system.
Carbon monoxide and steam react in the High Temperature (R-315 and R-365) and Low Temperature Shift Converters (R-320 and R-370) to form more hydrogen and carbon dioxide. These reactors are also known as the Primary and Secondary Shift Converters.
The shift conversion reaction serves two purposes. First, it makes more hydrogen. Second, it converts CO, which is difficult and expensive to separate from hydrogen, into CO2 that can easily be separated from hydrogen.
By using different catalysts in the High and Low Temperature Shift Converters, it is possible to reduce CO concentrations to a very low level. Effluent gas from the Secondary Shift Converter contains mostly H2, CO2, a small amount of CH4, and a trace amount of CO.
The Secondary Shift effluent also contains a large amount of excess steam. Nearly all of this steam is condensed and separated from the gas before the gas enters the CO2 removal section.
Carbon Dioxide removal:
Carbon dioxide removal is the major step in purifying the hydrogen. The gas from the Shift Converters enters the CO2 Absorber (C-330 or C-380) column. The CO2 Absorber absorbs CO2 using a Monoethanolamine (MEA) solution.
MEA does not absorb hydrogen, carbon monoxide, or methane. These gases pass through the CO2 Absorber and go on to the Methanator.
Carbon dioxide absorbed into the MEA solution is removed from solution in a CO2 Stripper. The resulting CO2 product has the following uses:• Sale to AIRCO (an outside company)• Inert gas for tank blanketing• Excess CO2 can be vented to the atmosphere
The hydrogen exiting the CO2 Absorber contains less than 1% CO and CO2. This is still too high a level for the catalysts used in North Isomax and RLOP. Methanation almost completely converts the carbon oxides to methane.
Methanation reverses the reforming reaction; hydrogen reacts with CO and CO2 to produce methane and steam. The methanation reaction takes place at much lower temperatures than the reforming reaction.
By design, the gas exiting the Methanator consists of 96.5% hydrogen, 3.4% methane, trace amounts of nitrogen, and a combined total of less than 10 ppm of CO and CO2. From the Methanator the hydrogen is routed to the Hydrogen Booster Compressors which compress this gas and deliver it to the various hydrogen consuming plants.
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