Environmental Engineering Reference
In-Depth Information
tional and less expensive catalyst. The modified ring catalysts can furnish
higher activity for about the same pressure drop.
The reformer tubes typically operate at maximum temperatures of
1,600°F to 1,700°F and are designed for a minimum stress-to-rupture life
of 100,000 operating hours. A 35/25 Ni/Cr alloy is used that is modified
with niobium and microalloyed with trace elements such as titanium and
zirconium. Smaller tube diameters provide better heat transfer and cooler
walls. This reduces tube and fuel costs and increases tube life. But more
tubes increases the pressure drop. The optimum inside tube diameter is 4
to 5 in. The wall thickness may be as low as 0.25 inch with a length of 40 to
45 ft. The lane spacing between tube rows must be enough to avoid flame
impingement from the burners. Typical spacing is 6 to 8 feet.
The burners are located between tube rows. A larger number of burn-
ers reduces the heat release per burner and allows a smaller flame diameter
and a reduced lane spacing. A ratio of one burner for every 2 to 2.5 tubes
provides a uniform heat release. Most burners are a dual-fired design, firing
both PSA offgas and supplemental makeup gas. Low NO x burners are used
to meet environmental requirements. Makeup gas can be used to induce
flue gas into the flame, reducing the flame temperature and NO x level. In a
well functioning unit NO x levels as low as 0.03 lb/MMBtu are possible.
The piping design limits variations in gas flow to the tubes and burn-
ers to ±2.5% to keep tubewall temperatures uniform. The PSA offgas flow is
available to the burners at only about 3 psig. If preheated combustion air is
used, the differential air pressure across each burner is typically less than 2
inches of water. The distribution is aided with symmetrical piping.
The flue gas tunnels are rectangular fire-brick structures at the reform-
er's bottom. They act as horizontal ducts for flue gas removal. The flue gas
exits at 1,800°F to 1,900°F. A heat recovery unit is provided to recover heat
from this gas. This unit contains a reformer feed preheat coil, steam super-
heat coil, steam generation coil and boiler feed water preheat coil.
When combustion air preheat is used, the air preheat unit may re-
place the boiler feed water coil. Flue gas exits this unit at about 300 de-
grees F. This provides a typical heat loss of 3% of the overall reformer ef-
ficiency. Steam is also made in a process steam generator which extracts
heat from the reformer outlet process gas. The heat recovery unit and pro-
cess steam generator normally have a common steam drum.
The steam generation pressure must be high enough to produce
steam. The minimum pressure is 100 to 150 psig above the hydrogen prod-
uct pressure, depending on the plant pressure drop. Considerably high-
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