HYDROGEN PRODUCTION - STEAM METHANE REFORMING

             PROCESS DESCRIPTION  

            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:

C_nH_m + 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.