The process steps required to produce high purity
gaseous hydrogen from a natural gas stream are summarized as follows:
- Natural Gas Feed Desulfurization
- Steam-Hydrocarbon Reforming
- Water-Gas Shift Conversion
- Hydrogen Purification
- Waste Heat Recovery and Steam Generation
Natural Gas Feed Desulfurization
Natural gas is compressed at 265 psig. A portion of the
gas flows through another pressure reducing valve, a flow control valve, a
series of safety shut off valves, and is burned in the Reformer (RF-200) burner
providing a portion of the required heat input. The burner is equipped with a
continuous gas pilot.
The balance of the natural gas flows through a flow
control valve and into the Feed Heater (HX-200) where the gas is heated to 750
°F by heat exchange with process gas.
Natural gas contains various sulfur compounds (organic,
H2S, mercaptan, COS) that act as poisons to the Reformer catalyst.
Therefore, to remove sulfur compounds, gas from the Feed Heater flows into the
Desulfurizer (CR-201).
The Desulfurizer contains a bed of zinc oxide on alumina
catalyst. As the feed gas flows down through the bed, sulfur compounds react
with the zinc forming zinc sulfide. The feed gas exiting the Desulfurizer will
contain less than 0.2 ppm (v) sulfur. The zinc oxide will absorb (react) up to
20 % (w) sulfur. At this point, the catalyst is fully loaded and must be
replaced. Zinc sulfide is not hazardous. Therefore, after purging the
Desulfurizer with nitrogen, the catalyst can be removed and disposed of as
landfill.
Steam-Hydrocarbon Reforming
Sulfur free natural gas from the Desulfurizer is mixed
with a measured flow of steam according to a fixed steam to carbon ratio. The
gas then flows into the Mixed Feed Heater (WH-300) where its temperature is
increased to 750 °F and into the Reformer catalyst tubes (RF-301).
In the catalyst tubes, in the presence of a nickel on
alumina catalyst and high temperature, hydrocarbons react with steam forming
hydrogen and carbon monoxide. Simultaneously, the partial water shift reaction
occurs as follows:
CnHm + nH2O « nCO + [(2n + m)/2] H2 (1)
CO + H2O « CO2 + H2 (2)
Reaction (1) is endothermic and reaction (2) is
exothermic. The combined reactions are net endothermic requiring heat input.
Heat required for the reaction is provided by burning Hydrogen Purification
Unit waste gas supplemented by natural gas.
The reactions are equilibrium limited. The reformed gas
exiting the catalyst tubes will consist of H2, CO, CO2,
CH4, H2O, and inert (if present in the feedstock). The
percentage composition will approach equilibrium at the design outlet
conditions.
Water-Gas Shift Conversion
Process gas from each catalyst tube is collected in a
circular off take header and then flows into the Reformer Effluent Boiler
(WH-200) where it is cooled to approximately 690 °F by generating steam. The
Reformer Effluent Boiler is fitted with a center bypass tube and two internal
butterfly type flow control valves. The exit temperature is automatically
maintained regardless of the plant capacity by allowing a portion of the inlet
process gas to bypass the boiler tubes.
From the Reformer Effluent Boiler, process gas flows
into the High Temperature Shift Converter (CR-202). In the Shift Converter, in
the presence of chromium promoted iron on alumina catalyst, carbon monoxide
will react with water forming carbon dioxide and hydrogen as depicted in
equation (2) above.
The reaction is equilibrium limited. The reaction is
exothermic, resulting in a temperature rise across the catalyst bed.
Hydrogen Purification
Process gas from the Shift Converter at 790 °F and 230
psig flows through the tube side of the Feed Heater (HX-200) where it is cooled
to 665 °F. The process gas then flows through the Shift Effluent Boiler
(WH-302) where it is cooled to 420 °F by generating steam and flows through the
Boiler Water Heater (HX-300) where it is cooled to 325 °F and some steam is
condensed. From the Boiler Water Heater, the process gas flows through the
shell side of the Process Gas Cooler (HX-401) where the gas is cooled to 100 °F
and water is condensed by exchange with circulating cooling tower water. From
the Process Gas Cooler, the stream flows through the Cold Condensate Separator
(SP-401) where condensed water is removed. The vapor flows through a Demister®
(located in the separator) to eliminate entrained water and then into the
Hydrogen Purification Unit (HPU).
The HPU consists of four adsorber vessels (V-500 A, B,
C, D), a Waste Gas Surge Tank (V-501), switching valves, and a purge/repressure
valve. The HPU operation is controlled using a PLC based sequencing system.
Each adsorber vessel contains separate layers of adsorbents. The system
operates on repeated cycles of impurity adsorption and adsorbent regeneration.
Adsorption takes place at elevated pressures and regeneration (de-sorption)
occurs at low pressure.
Crude hydrogen flows up through one adsorber where all
impurities are selectively removed by the various adsorbents and exits the
vessel as ultra pure hydrogen. Activated alumina removes bulk water. Activated
carbon removes trace water, all carbon dioxide, methane and partial carbon
monoxide. Molecular sieve removes trace methane and the balance of carbon
monoxide. If nitrogen is present, it is adsorbed in the molecular sieve. If
helium or argon is present, they will not be adsorbed and will exit with the
pure hydrogen.
Regeneration of any given adsorber vessel is initiated
prior to the total exhaustion of that vessel’s adsorbent and is accomplished
sequentially by:
1) initial partial
depressurization of the adsorber by partially repressurizing another adsorber
with a portion of the remaining pure hydrogen found in the depressurizing bed,
2) further
depressurization of the adsorber by providing pure hydrogen purge (with most
the remaining pure hydrogen) to another adsorber,
3) final
depressurization to approximately 5 psig by venting into the Waste Gas Surge
Tank, and
4) final purging
with pure hydrogen from the outlet of another depressurizing adsorber (purged
gas continues to flow to the surge tank).
At the completion
of the purge step, the vessel is partially repressurized with pure hydrogen
flowing from the initial depressurization step of the most recent “on-stream”
adsorber, and is then fully repressurized with pure hydrogen from the pure
hydrogen product line.
The system is designed for twenty to sixteen minute
cycles at full plant capacity. System time is a function of the
purge/repressure gas flow rate and the point at which the freshly regenerated
adsorber pressure equals the current “on-stream” adsorber pressure. When the
pressures are near equal, a differential pressure switch will initiate the next
step.
Off-Gas Recovery As Fuel
The off gas from regenerating the
HPU absorbers flows to the HPU Vent Tank from which it is sent on flow control
to the reformer burner to provide a significant portion of the fuel
requirements. Vent tank vessel will be supplied
by Buyer.
Waste Heat Recovery and Steam Generation
Waste heat is recovered from various areas of the plant
and is used to preheat the feed gas, superheat the feed/steam mixture prior to
its entering the Reformer catalyst tubes, preheat the deaerator make up water,
preheat the boiler water, and generate all the steam required by the plant.
Flue gas from the top of the Reformer (RF-200) at
approximately 1850 °F and a slight negative pressure flows through an
internally insulated duct to the Mixed Feed Heater (WH-300) where it is cooled
to 1640 °F by exchange with feed and steam. The flue gas then flows through the
tube side of the Waste Heat Boiler (WH-301) where it is cooled to 520 °F by
generating steam and through the Economizer (WH-303) where it is cooled to 350
°F. Finally, it flows into the Induced Draft Fan (F-300) and is then vented to
the atmosphere.
As an Option, steam is generated from three sources in
the plant, the Reformer Effluent Boiler (WH-200), the Waste Heat Boiler
(WH-301), and the Shift Effluent Boiler (WH-302). The Waste Heat Boiler and the
Shift Effluent Boiler share a common shell. All three boilers are connected to
a single Steam Drum (V-300) by thermo siphon lines designed for circulating
liquid to vapor ratio of 20:1 by weight.
Condensate from the Hot Condensate Separator (SP-400)
flows through the separator level control valve and into the middle of the
Deaerator stripping column (DA-400). Condensate from the Cold Condensate
Separator (SP-401) flows through the separator level control valve, mixes with
make-up water, flows through the Deaerator Water Heater (HX-400) where the
water is heated to 190 °F and into the top of the Deaerator stripping column.
In the stripping column, condensate flowing down through
a packed bed contacts with steam flowing up through the bed, is heated to
saturation at near atmospheric pressure and entrained H2, CO, CO2,
inert and any oxygen that may have been in the make-up water are removed.
The stripped gases and approximately 150 lb/h of excess
stripping steam exits the top of the stripping column and vents to the
atmosphere. Steam to the Deaerator is flow controlled.
Deaerated water at 212 °F and near atmospheric pressure
flows from the Deaerator storage section to the suction of the Boiler Water
Pump (P-400). Water from P-500 discharging at 275 psig, flows through a boiler
level control valve and into the shell side of the Boiler Water Heater (HX-300)
where it is heated to 340 °F by exchange with process gas. From the Boiler
Water Heater, boiler water flows through the Economizer (WH-303) where it is
heated to 390 °F and into the Steam Drum (V-300).
Return risers from each boiler enter the Steam Drum at
one end, steam separates from the excess water at the diffusion plates, and
exits the drum through a Demister®. Internals are provided within the Steam
Drum to limit boiler water carryover to less than 1 ppm. Steam from the Steam
Drum will flow to the steam to catalyst flow control valve and the Deaerator
flow control valve. Excess steam will flow through a steam system back pressure
control valve and into the customer export steam header.
A continuous boiler blow down with valve and distributor
is provided approximately 2 inches below the Steam Drum’s normal liquid level.
Manual, intermittent blow downs are accomplished with block valves and a quick
opening valve at the bottom of the boiler shell. A blow down water sample
cooler (HX-301) is also provided. Chemical feed connections with block valves
and check valves are provided in the boiler shell and Deaerator water storage
section.
For the purpose of a heat and material balance, a 3% of
inlet water flow continuous blow down has been allowed for. Actual continuous
blow down, frequency of intermittent blow down, and treating chemicals employed
is to be determined by the Buyer’s boiler water treating specialist.
Blow down water will flow through the blow down valves
where its pressure is decreased to near atmospheric, partially vaporizing
(reducing its temperature to approximately 212 °F) and flows into the Blow down
Separator (SP-300).
Flash vapor exits the top of the separator and vents to
atmosphere. Condensate from the bottom of the separator flows through a liquid
seal into the customers waste water system.