Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining

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10.7. What is the maximum yield of a pentane-free alkylate that can be

produced from 80,000 BPD of isobutane and 2000 BPD propylene,

4000 BPD of butylene and 2000 BPD amylene?

10.8. Make UNISIM simulation as in the case of example E10.4, assuming

the following olefin composition as mol%.

5405023

REFERENCES

Amico, V. D., Gieseman, J., Brockhoven, E., Rooijen, E., and Nousiainen, H. (2006). The

AlkyClean alkylation process – New technology eliminates liquid acids. NPRA Pub-

lications #AM-06–41, 104th NPRA Annual Meeting, Salt Lake City, Utah, USA.

Edgar, T. F., and Himmelblau, D. M. (1988). ‘‘Optimization of Chemical Processes.’’

McGraw Hill, New York.

Gary, J. H., and Handwerk, G. E. (1994). ‘‘Petroleum Refining Technology and Econom-

ics.’’ Marcel Dekker, New York.

Kranz, K., and Graves, D. (1988). Olefin interaction in sulfuric acid catalyzed alkylation.

215th National Meeting, American Chemical Society (Division of Petroleum Chemis-

try), Dallas, TX.

Rase, H. F. (1977). ‘‘Chemical Reactor Design for Process Plants,’’ vol. 1. Wiley-Interscience,

New York.

UNISIM Design Suite R370, Honeywell Process Solutions, Calgary, Alberta, Canada.

Zhorov, Yu. M. (1987). ‘‘Thermodynamics of Chemical Processes.’’ MIR publisher,

Moscow.

Alkylation 283

CHAPTER ELEVEN

Hydrogen Production

11.1. Introduction

The increasing demand for clean fuels will be a strong incentive to

build new refineries with greater conversion and treating capacity. Hydro-

gen is needed for the conversion processing of heavy petroleum fractions

into lighter products and for removing sulphur, nitrogen and metals from

many petroleum fractions. The demand for hydrogen in refineries will also

depend on the quality of the processed crude oil. Heavier crude oils will

necessitate more demand for hydrogen. The stringent specifications of

product quality will also increase hydrogen demand. Synthesis gas, as a

mixture of hydrogen, carbon oxides and nitrogen, is also produced in

large quantities from the manufacture of ammonia or methanol.

11.2. Hydrogen Requirements in Modern

Refineries

Hydrogen consumption is the result of the chemical reaction of

hydrogen with the petroleum fractions, ranging from naphtha to vacuum

residue in the following processes (Bourbonneux, 2001):



Hydrotreating of the various cuts ranging from naphtha to heavy

vacuum gas oil, to remove sulphur, nitrogen and metals. Consumption

of hydrogen ranges from 0.6 kg (1.32 lb) H

per ton (2204 lb) of light

distillates to 10 kg (22 lb) H

per ton for vacuum distillates.



Hydrocracking and hydroconversion of gas oil and heavier feedstocks

to produce light products. The consumption depends on the quality of

the feed and the severity of the process and ranges between 15 and

35 kg (33 and 77 lb) of hydrogen per ton of feed.

It is important to note that these processes require hydrogen of high purity

(over 99% purity) and at high pressure to meet process and economic

requirements.

Fundamentals of Petroleum Refining

2010 Elsevier B.V.

285

11.3. Steam Reforming

11.3.1. Flow Process

The production of hydrogen from light hydrocarbons can be done through

two process schemes. The old scheme involves the following stages:



Steam reforming to produce synthesis gas, a mixture of hydrogen and

carbon monoxide.



Conversion of CO to CO

with steam. This is done in two stages, a

high-temperature shift converter (HTSC) and a low-temperature shift

converter (LTSC).



The hydrogen rich gas is purified by a CO

removal unit where a hot

potassium carbonate or amine solution absorbs the CO



A methanator converts the remaining CO and CO

into methane and

water.

The final hydrogen purity ranges between 95 and 98%. Figure 11.1 shows

the process flow diagram.

Heat

Integration

Steam

Methane

Reformer

Fuel

Methanation

Steam

Natural Gas

Desulfurization

Steam

Generation

HTSC

LTS C

Fired Heater

Hydrogen

Removal

Reformer

High

Temperature

shift

Low

Temperature

shift

Wash

System

Methanator

Feed

Steam

Fuel

Hydrogen

98 vol%

Figure 11.1 Steam reforming with CO

absorber and methanator

286 Chapter 11

The newer process scheme differs from the old scheme in CO

removal

technology. The steps involved in this scheme are as follows:



Steam reforming to produce synthesis gas, a mixture of hydrogen and

carbon monoxide.



Conversion of CO to CO

with steam. This is done in one-stage shift

converter.



A pressure swing adsorption (PSA) unit is used to selectively separate

through membranes, thus purifying the hydrogen rich product

gas stream.

The final product gas is typically 99.9% hydrogen. The higher hydro-

gen purity is beneficial to the downstream hydrotreating units since it

increases the hydrogen partial pressure, lowers the recycle flow, lowers

compression costs and increases catalyst life. Figure 11.2 shows the process

flow diagram.

PSA

Unit

Steam

Methane

Reformer

Fuel

Steam

Natural Gas

Desulfurization

Steam

Generation

HTSC

Hydrogen

Heat

Integration

Reformer

High

Temperature

shift

PSA

Unit

Feed

Steam

Fuel

Hydrogen

99.9 vol%

Figure 11.2 Steam reforming with pressure swing ads orber

Hydrogen Production 287

Details regarding these process schemes will be presented. In any case,

since the unit uses catalysts, the feed may contain compounds which can act

as poisons to these catalysts. A feed purification step is necessary.

11.3.2. Feed Preparation

The feed to the steam reforming unit is light hydrocarbon streams from the

various refinery processes (C

to C

). These streams may contain poisons to

the nickel catalyst. Poisons are sulphur compounds such as hydrogen sul-

phide and mercaptans, and halogenated compounds such as chlorides.

Therefore, feed preparation involves the hydrogenation of organic sulphur

and chloride into H

S and HCl respectively, in the presence of a Co–Mo

hydrotreating catalyst at 350–400



C (662–752



F). The hydrogen sulphide

is then adsorbed in a ZnO bed. The treated feed should contain 0.1 ppm

sulphur or less, and the chloride content should be limited to 0.5 ppm.

11.3.3. Steam Reforming Reactions

This process produces hydrogen by the chemical reaction of methane

and other light hydrocarbons with steam at temperatures of 820–880



(1508–1616



F) and pressures of 20–25 bar (294–368 psi). The catalyst is

nickel on alumina support. The feed of light hydrocarbons, mainly methane,

is fed to the steam reforming reactor which is essentially tubes filled with a

nickel catalyst passing through a furnace. The feed is heated to 540–580



(1004–1076



F) by passing it through the convection section of the furnace.

The feed is converted to synthesis gas which is a mixture of H

, CO,

,CH

and H

O by reaction with excess steam in the radiation section

of the furnace. The steam to carbon molar ratio is between 2.5 and 5

(Bourbonneux, 2001).

The reforming reactions are essentially equilibrium reactions meaning

that the conversion with the aid of the nickel catalyst approaches the

maximum conversion that can be achieved at the reaction temperature

and pressure. Therefore, in addition to the catalytic activity of the catalyst,

the reaction temperature and pressure, as well as the amount of steam used,

will have a great effect on the quality of the hydrogen stream produced from

the unit. The main equilibrium reactions are:

þ H

CO þ 3H

;

DH ¼þ206 kJ=mol; DG

298

¼58:096 kJ=mol

ð11:1Þ

CO þ H

þ H

;

DH ¼41 kJ=mol; DG

298

¼28:606 kJ=mol

ð11:2Þ

þ 2H

þ 4H

;

DH ¼þ165 kJ=mol; DG

298

¼ 113:298 kJ=mol

ð11:3Þ

288 Chapter 11

The first reaction is highly endothermic (DH

∘

298

¼ 206 kJ= mol) whereas

the carbon monoxide conversion is moderately exothermic (DH

∘

298

41 kJ=mol). For light hydrocarbons other than methane, the reforming

reaction becomes:

þ nH

nCO þ

m þ 2n



ð11:4Þ

This reaction occurs through the conversion of the light hydrocarbons to

methane followed by the methane reforming reaction. Overall, the reform-

ing reaction (11.4) is also endothermic.

11.3.4. Thermodynamics of Steam Reforming

The steam reforming of methane consists of two reversible reactions: the

strongly endothermic reforming reaction (11.1) and the moderately exo-

thermic water-gas shift reaction (11.2). Due to its endothermic character,

reforming is favoured by high temperature. Also, because reforming is

accompanied by a volume expansion, it is favoured by low pressure. In

contrast, the exothermic shift reaction is favoured by low temperature,

while unaffected by changes in pressure.

Increasing the amount of steam will enhance the CH

conversion, but

requires an additional amount of energy to produce the steam. In practice,

steam to carbon ratios (S/C) around 2.5–5 are applied. This value for S/C

will also suppress coke formation during the reaction.

In calculating the thermodynamic yields, we can apply the standard

chemical reaction equilibrium methods which involve calculating the equi-

librium constant in the gas phase (K

) and then solving to obtain the

hydrogen yield. Since reforming involves multi-reactions, the method of

minimization of the total Gibbs energy at equilibrium is appropriate in this

case. In this method, we only specify the species involved in the reactions

(reactants and products) and their initial or inlet amounts without necessar-

ily specifying the actual reactions. The total Gibbs energy of the reaction

mixture is given by (Elliot and Lira, 1999):

G ¼

and

þ ln y

þ ln P ð11:5Þ

The standard Gibbs energy G

is given by Gibbs energy of formation at the

reaction temperature and at the standard pressure of 1 bar (14.7 psia).

f ;i

ð11:6Þ

The standard Gibbs energy is obtained for each compound from DG

∘

f ;298

and corrected for the temperature of the reaction.

Hydrogen Production 289

The minimization of the total Gibbs energy of the reaction system is

performed, subject to certain constraints which must be satisfied. These

constrains are the atomic balances. This means that the number of moles of

each atom in the feed must equal the number of moles of that atom in all

species at equilibrium. The atomic balances are:

O-balance: 2n

þ n

¼ n

feed

ð11:7Þ

H-balance: 4n

þ 2n

¼ 4n

feed

þ 2n

feed

ð11:8Þ

C-balance: n

þ n

¼ n

feed

ð11:9Þ

The minimization process is performed using Solver in the Excel spread-

sheet. The temperature and pressure of the reaction as well as the

steam-to-methane ratio are entered. A typical example of Gibbs minimiza-

tion is shown in Example E11.1.

Example E11.1

Feed containing 1 mol CH

and 3 mol of H

O enters a steam reformer at 40 bar

and 1073 K. Calculate the composition of the product on a dry basis.

The standard Gibbs free energies of the components are given in the table

below

Component G

/RT

3.193

44.375

CO 23.164

0.0

O 21.146

Solution:

Based on the steam-to-methane ratio, the moles of O, H and C atoms are

calculated from equations (11.7)–(11.9). Initial guess is inserted for the number

of moles of each component in the product stream. Based on this guess, compo-

sition is calculated and the Gibbs free energy is calculated from equation (11.5).

The total Gibbs free energy is then summed up. The Excel Solver is used to

minimize the total Gibbs free energy cell by changing the number of moles

which was initially guessed with the constraints of:

O-bal: n

¼ 3

H-bal: 4n

þ 2n

¼ 4ð1Þþ2ð3Þ¼10

C-bal: n

¼ 1

Figure E11.1 shows the results.

290 Chapter 11

Example E11.2

For a reformer feed of methane and steam at a ratio of 2.5 mol of H

O to 1 mol

and at 20 bar, study the effect of the reaction temperature on the hydrogen

purity on dry basis in the range 800–900



C. If the reaction temperature is

850



C, study the effect of the steam-to-methane ratio in the range 2–5.

Solution:

Using the Excel spreadsheet, vary the temperature and use the solv er to get the

equilibrium yield. The results are plotted in Figure E11.2.1. As temperature

increases, the hydrogen purity increases.

Figure E11.2.2 shows the effect of the H

O/CH

ratio at 850



C and 20 bar.

As this ratio increases, the equilibrium conversion increases and the hydrogen

purity increases.

Figure E11.1 The reforming reactions spreadsheet

Hydrogen Production 291

123456

Steam to Carbon ratio

Hydrogen mol %

Figure E11.2.2 Effect of feed H

O/CH

ratio on equilibrium hydrogen purity at 850



and 20 bar

750 800 850 900 950

Temperature, ⬚C

Hydrogen mol %

Figure E11.2.1 Effect of reforming reaction temperature on hydrogen purity at 20 bar

and 2.5 H

O/CH

ratio

292 Chapter 11

11.3.5. Operating Variables (Crew and Shumake, 2006)

11.3.5.1. Steam to Carbon Ratio

The steam to carbon ratio is the ratio of moles of steam to moles of carbon in

the reformer feed. It is obtained by dividing the molar flow rates of steam

and feed. The reformer feed must contain sufficient steam to avoid thermal

cracking of the hydrocarbons and coke formation. An excess of steam (over

the stoichiometric ratio) is usually used. The higher the steam to carbon

ratio, the lower the residual methane will be for a given reformer outlet

temperature. Hence, less fuel energy is required in the furnace. The design

steam to carbon is typically 3.0 with a range between 2.5 and 5.0.

11.3.5.2. Reformer Inlet Temperature

Since the reforming reaction is endothermic, it is favoured by high temper-

ature. The reformer catalyst tube inlet temperature is maintained at 540–

580



C (1004–1078



F). The hydrocarbon steam feed is preheated by the

hot flue gas in the waste heat recovery (convection) section of the furnace.

A higher inlet temperature decreases the amount of fuel required to supply

heat to the reaction tubes and decreases the number of tubes and the size of

the furnace. Utilization of the hot flue gas to reheat the feed increases the

energy efficiency of the process and decreases the steam generation in the

waste heat recovery section.

11.3.5.3. Reformer Outlet Temperature

The reformer outlet temperature is the most important process variable that

determines the purity of the hydrogen product. The higher the reformer

outlet temperature the lower will the residual methane be (higher hydrogen

purity) for a given feed rate and steam to carbon ratio. The upper limit of

the outlet temperature is governed by the design maximum tube skin

temperature which is 1093



C (2000



F). High-temperature operation is

not necessarily the most economic method taking into consideration the

amount of fuel to be burned for an increase in purity. The reformer has been

designed for normal operation at outlet temperature in the range of

820–880



C (1508–1616



F). The lower feed gas rate will lower the

required reformer outlet temperature for the same hydrogen purity. Simi-

larly, the higher steam to carbon ratio will lower the required reformer

outlet temperature for the same hydrogen purity.

11.3.5.4. Reaction Pressure

In the reforming reaction, the volume of the products is three times higher

than the volume of reactants. Therefore, at a fixed temperature and steam to

carbon ratio, lower pressure favours the equilibrium of the reaction.

The design outlet pressure of the reformer is in the range 20–25 bar

(294–368 psia). The operating pressure of the heater is not fixed locally.

Hydrogen Production 293