unit 5 hydrogen generation

7/29/2019 Unit 5 Hydrogen Generation

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UNITS IN THIS COURSE

UNIT 1 DESALINATION

UNIT 2 WATER TREATMENT

UNIT 3 STEAM GENERATION BOILERS

UNIT 4 NITROGEN GENERATION

UNIT 5 HYDROGEN GENERATION

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TABLE OF CONTENTS

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5.0 OBJECTIVE 3

5.1 USES OF HYDROGEN GAS IN THE REFINERY 4

5.2 HOW THE HYDROGEN GAS IS PRODUCED 4

5.2.1 REFORMER FURNACE 4

5.2 2 REFORMER CHEMICAL REACTIONS 6

5.2.3 SHIFT CONVERTOR REACTOR 6

5.2.4 PRESSURE SWING ABSORPTORS (PSA'S) 7

5.2.4.1 PSA REGENERATION 7

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5.0 OBJECTIVE

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As a plant operator, you should have a general knowledge and understanding of how your Company operates their hydrogen generation equipment.

This unit will explain the following things:

Why oil processing plants (refineries) need hydrogen gas

How the hydrogen gas is produced.

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5.1 USES OF HYDROGEN GAS IN THE REFINERY

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Pure hydrogen gas is needed in the following processes.

• The hydro-cracker

• The reformer

• The hydro-desulphuriser (hydro-kimmer desulphuriser)

How these processes work will be explained in other modules.

5.2 HOW THE HYDROGEN GAS IS PRODUCED

The process changes natural gas into hydrogen and carbon dioxide gas.

This is done by mixing the natural gas with superheated steam at very hightemperature in the presence of a catalyst.

The natural gas that feeds the process (the feedstock), contains methane, (CH 4),ethane, (C2H6) and some propane, (C3H8).

The feedstock also contains Impurities that have to be removed.

These impurities are mercaptans (RSH), carbon dioxide (CO2), and hydrogensulphide (H2S).

The feedstock is pre-treated to remove these impurities.

The pre-treatment is done by passing the feedstock through a hydrogenator reactor and then through desulphuriser adsorptors.

Recall that the hydrogen unit operates at high temperature.

The feedstock is preheated by a preheat heat exchanger.

If this heat exchanger is out of service we can pass the feedstock through a gasfired heater.

This heater is normally on standby.

The main part of the process takes place in the reformer furnace.

5.2.1 REFORMER FURNACE

Extra heat energy is given to the incoming superheated steam.

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This is done by passing the steam delivery tubes through the hot exhaust gases in

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the reformer furnace.

The very hot, super-heated steam is then mixed with the clean hot feedstock gas.

The mixture of super heated steam and feedstock gas then passes through the tubebundles inside the reformer furnace.

The tube bundles contain a solid catalyst.

The catalyst used is a nickel alumina based catalyst.

The feedstock gas is reformed in the furnace tube bundles in an endothermicchemical reaction.

Recall that enclothermic means that the reaction takes in heat energy in order tohappen.

The reformed gas contains the following products.

• Hydrogen (H2)

• Carbon monoxide (CO)

• Carbon dioxide (CO2)

• Methane (CH4)

• Steam (H2O)

The chemical reactions that produce these products are given below.

You will see that there are two reactions.

The second reaction converts the carbon monoxide produced in the first reaction tocarbon dioxide.

Note -that some of the carbon monoxide is not converted to carbon dioxide.

That is why we have carbon monoxide in the produced gas exiting the reformer furnace.

Some of the methane also passes through the reformer unchanged.

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5.2.2 REFORMER CHEMICAL REACTIONS

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1 st Reaction 2nd Reaction Final Product

Methane: CH4 + H2O → 3H2 + CO } 4H2 +CO2

CO + H2O → H2 + CO2}

Ethane: C2H6 + H2O → 4H2 + 2CO} 6H2 + 2CO2

2CO + 2H2O → 2H2 + 2CO2}

Propane: C3H8 + 3H2O → 7H2 + 3CO} 10H2 + 3CO2

3CO + 3H2O → 3H2 + 3CO2}

This hot raw hydrogen product is then passed through a heat exchanger. There,some of its heat energy is used to generate steam.

This cools the raw hydrogen product.

The cooled hydrogen product is then passed through a shift convertor reactor vessel.

5.2.3 SHIFT CONVERTOR REACTOR

In the shift convertor any carbon monoxide present in the hydrogen product reactswith the steam to give more hydrogen and carbon dioxide.

CO + H2O --> H2 + CO2

We now have a raw hydrogen product containing a lot of carbon dioxide gas.

Recall the hydrogen product also contains some methane, some carbon monoxideand, of course, some water (steam).

These product impurities must be removed.

This is done in special vessels called pressure swing adsorptors. (PSA's)

Before passing through the pressure swing adsorptors the raw hydrogen product iscooled in a cooler.

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5.2.4 PRESSURE SWING ABSORPTORS (PSA'S)

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The PSA's remove any CO, CO2, CH4, and water.

The PSA's contain molecular sieves.

Pure hydrogen product leaving the PSA's is 99.9% pure.

5.2.4.1.PSA REGENERATION

How the PSA's are regenerated is clever, but simple.

Recall that fluids always flow from high pressure to low pressure.

The pressure is reduced in the PSA which needs to be regenerated.

This is done by closing the inlet valve and the product outlet valve.

The purge valve is then opened.

When the pressure inside the PSA is lowered to a set point the product outlet valveis reopened.

Pure hydrogen product then backflows through the PSA and out of the purge valve.

This purge gas removes the adsorpted impurities. It is then used as fuel gas for thereformer furnaces.

When the regeneration is complete the purge valve is closed.

Pure product hydrogen continues to flow into the PSA until the pressure in the PSAis the same as the pressure in the product header.

The inlet valve is then opened and the PSA is back on stream.

This whole process is controlled by a computer programme.

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Hydrogen Generation Unit - Simplified Flow Diagram

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Figure 5.1

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unit 5 hydrogen generation

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