2.

3 Process Concept

This sub-chapter will discuss the concept of the natural gas steam reforming
process in terms of the basic reaction, operating conditions, reaction mechanism,
thermodynamics and kinetics that occur.
2.3.1 Basic Reaction

The process of producing hydrogen gas with the steam methane reforming
process goes through three stages of the process, namely the process of forming
hydrogen gas in the steam reformer, the process of forming additional hydrogen
gas in the water gas shift reactor, and the process of purifying residual gas (CO and
CO2) in the methanator.

The reactions that occur in the three stages of the process are as follows:

a. Hydrogen forming in steam reformer

The reaction that occurs in the steam reformer is :

CH4 + H2O CO + 3H2 ΔH298 = +206 kJ/mol…..(2.1)

CH4 + 2H2O CO2 + 4H2 ΔH298 = +165 kJ/mol..…(2.2)

b. Hydrogen forming in water gas-shift reactor (WGSR)
The reaction that occurs in water gas shift reactor is:
CO(g) + H2O(g) CO2(g) + H2(g) ΔH298 = -41 kJ/mol (2.3)
c. The methanation reaction in the methanator as the removal of CO2 content
and residual CO
The reaction that occurs in methanator is:
CO+3H2 CH4 + H2O ΔH298 = -206,1 kJ/mol...........(2.4)
CO2+4H2 CH4 + 2 H2O ΔH298 = -165 kJ/mol..............(2.5)
CO+ H2O CO2 + H2 ΔH298 = 41,15 kJ/mol.............(2.6)
2.3.2 Reaction Mechanism

Reaction Mechanism consist of three phase :

1. Cracking phase, in which the natural gas phase which is a hydrocarbon
undergoes cracking at high temperatures and is formed into lighter
compounds.
2. Reaction phase, in which the reaction of hydrocarbons with oxygen is
exothermic.
3. Soaked phase, in this phase the reaction temperature is very high and
CO and H2 are formed.
(Higman, 2008)
While the reaction mechanism in WGSR :
1. The adsorption stage of CO and H2O on the surface of the catalyst
2. The reaction stage of CO and H2O forms CO2 and H2 on the surface of the
catalyst
3. Decorption stage of CO2 dan H2
The reaction is:
CO (g) → CO (ads) (2.7)

H2O (g) → H2O (ads) (2.8)

CO (ads) +H2O (ads) → CO2 (ads) + H2 (ads) (2.9)

CO2 (ads) → CO2 (g) (2.10)

H2 (ads) → H2 (g) (2.11)

(Smith, 2010)
2.3.3 Thermodynamics Review

The thermodynamic view is used to determine characteristic of the

reaction (endothermic/exothermic) and the direction of reaction
(reversible/irreversible). It is possible to determine whether the reaction is
exothermic or endothermic by calculating the standard heat of formation (∆Hf°) at
P = 1 atm and T = 298 K.
Change of enthalpy (∆H) can be determined by the equation:
(∆H)°reaction = (∆H)°f product - (∆H)°f reactant
Reaction:
 CH4 + H2O CO2 + 3H2 ΔH298 = +165 kJ/mol
Table 2.x Standard enthalpy of formation
Component ΔHfo (kJ/mol) ΔGfo (kJ/mol)
Methane -74.52 -50.49
Water -241.81 -228.59
Carbon Dioxide -393.51 -394.37
Hydrogen 0 0
(Green and Perry, 2008)
∆HR° Calculation
∆HR°= ∆Hf°product - ∆HR° reactant
∆HR°= (∆Hf°carbon dioxide + ∆Hf° Hydrogen) – (∆Hf°methane - ∆Hf°water)
∆HR°= (-393.51 – 0) – ( -74.52 – (3) 241.81)
∆HR°= -1044.42 kJ/mol
Based on the calculations, it is obtained that ∆HR° is negative, so the reaction
for producing hydrogen by the steam reforming process is exothermic.

To find out whether the reaction is reversible or irreversible, it can be

known by calculating the equilibrium value (K) according to the Gibbs energy
calculation or the Van't Hoff equation.
∆G° = - RT ln K (Smith Van Ness, 1987)
298
Where:
∆G
298
°
= Standard Gibbs free energy of formation at 298 K ( kJ/mol )
R = Gas constant ( R = 8.314. kJ/kmol.K )
T = Temperature ( K )
K = Balance constant

∆G°298= ∆G°product - ∆G° reactant

∆G°298= (G°carbon dioxide + ∆G° Hydrogen) – (∆G°methane - ∆G°water)
∆G°298= (-394.37 – 0) – ( -50.49 – (3) 228.59)
∆G°298°= -1029.65 kJ/mol

∆G°298= - RT ln K298
-1044.42 kJ/mol = -8.314 kJ/k mol.K × 298 K × ln K298
Ln K298 = 0.4215
Value of K at operation temperature 450°C or 723.15 K
 K 723.15 ∆ H R °298 1 1
ln = × −
K 298 R T T 298
102965 1 1
ln K 723.15−ln K 298= ×( − )
8.314 723.15 298
102965 1 1
ln K 723.15−(0.4215)= ×( − )
8.314 723.15 298
ln K 723.15=−24.433
K 723.15=2.448 ×10−11
The reaction for formation of hydrogen has a very small value of the equilibrium
constant for the reaction K (K << 1), at temperature 450ºC. Thus the reaction is
reversible. In order for the reaction not to shift to the left (towards the reactants),
there are two ways to do it, namely one of the reactants must be in excess and the
reaction products must be quickly separated.

2.3.4 Kinetics Review

Reaction for the formation of hydrogen from natural gas and steam with a
nickel catalyst is a homogeneous gas reaction that occurs two directions and the
reaction rate depends on temperature according to the Arrhenius equation. The
reactions that occur in the reactor are:

CH4 + H2O CO2 + 3H2

To review the effect of temperature from a kinetic perspective, the Arrhenius equation is used:

k = A e-Ea/RT
Where: k = rate of reaction (M/s)
A= frequency factor
Ea= activated energy (kJ/mol)
R= gas constant (8,314 J/mol K)
T= temperature (T)
The values of A, E and R from the equation above are constant, so the value of k is
only affected by temperature. The higher the temperature, the greater the value of k,
in other words, the faster the reaction. So that temperature control must be
considered in the design of the reactor.

2.3.5 Operating Condition

 The operating conditions selected in the steam reforming process are steam with a
temperature of around 400 - 450ºC and a pressure of 32 kg/cm2g, then a mixture of natural
gas and steam is heated to a temperature of 538 ºC, the output product of the reformer has a
temperature of 849 ºC. Temperatures above 750 ºC with more than 8 hours duration can
cause a spinel formation reaction on the nickel catalyst so it must be avoided. Nickel on
Alumina has Bulk density + 850 kg/m3 on Potash act catalyst, whether the Nickel on Alumina
has Bulk density + 800 kg/m3 on Non-potash act. Potash ash accelerates the rate of carbon
gasification because there is an increase in adsorption of H2O & CO2 on surface area.

The higher the carbon chain, the easier it is for the reforming reaction to occur, but the higher
the potential for carbon formation. With an increase in temperature, the Reformer outlet will
produce a lower Methane Slip at the same pressure. Lower methane slip is achieved at a
lower system pressure. Increasing the Steam-carbon ratio will result in a lower Methane Slip
at the same pressure.

2.3.6 Process Flow

2.3.6.1 Material Preparation

1. Desulphurization Unit
At the beginning of the process, natural gas with a temperature between 20 – 37°C
and a pressure of 42 kg/cm 2a from the Gas Compression Station (GSC) is separated
from heavy hydrocarbons through a Knock Out Drum. The heavy hydrocarbon-free
process gas stream containing organic sulfur is converted to inorganic sulfur in the
desulfurizer unit. Heavy hydrocarbons and condensate are burned in the burn pit so
they don't clog the pipes and interfere with the process. Prior to desulphurization, the
process gas is first heated to a temperature of 300 – 400°C in the feed gas preheat coil
in the convection section of the reformer unit by utilizing heat from the reformer
primary flue gas. The process gas then flows into a desulfurizer with operating
conditions of 350 – 400°C which contains Cobalt Molybdenum (CoMo) catalyst at
the top and ZnO adsorbent at the bottom. The CoMo catalyst serves to accelerate the
hydrogenation reaction of organic sulfur to become inorganic, while the ZnO
adsorbent functions to adsorb inorganic sulfur. It is expected that the process gas that
comes out of the desulfurizer does not contain sulfur of more than 0.05 ppm by
volume (even trace). if the sulfur content is still large it indicates the catalyst and
adsorbent are saturated. Sulfur compounds will cause hotspots on the primary
 reformer tube. Sulfur compounds contained in natural gas consist of 2 types, namely
organic sulfur and inorganic sulfur. ZnO adsorbent is only able to adsorb inorganic
sulfur. Therefore all organic sulfur must be converted to inorganic sulfur through the
Hydrogenation process so that it can be separated from the process gas stream.
Desulphurization Reaction :
RSH(g) + H2(g) → RH(g) + H2S.......................................(2.x)
R1SSR2 + 3H2 → R1H + R2H + 2H2S.........................(2.x)
R1SR2 + 2H2 → R1H + R2H + H2S...........................(2.x)
(CH)4S + 4H2 → C4H10 + H2S.......................................(2.x)
COS + H2 → CO + H2S.......................................(2.x)
The H2S formed will be adsorbed by ZnO through these 3 reaction :
H2S + ZnO → ZnS + H2O......................................(2.x)

2.3.6.2 Main Process

After the stage of removing impurities in the process gas, the next step is the stage of
making synthesis gas, namely H2. To get the gas, the reforming process is carried out in
the Steam Methane reformer. The gas output from the reformer unit is then channeled to
the shift converter unit to convert CO gas, which is a by-product of the reformer, into CO2.

1. Steam Methane Reformer

Steam Methane reformer functions as a place where the reforming/cracking

reaction stage takes place. Top-fired reformer with a processing furnace of
radiant section and convection section. The reaction occurs in tubes with a
nickel catalyst which is in the radiant section. The heat needed for the
reaction is obtained from the combustion of natural gas with air. In the
convection section there are coils that are used as preheaters by utilizing
flue gas heat from the radiant section, so that maximum thermal efficiency
will be obtained. Process gas from the desulfurizer is mixed with steam at
>400°C with a steam to carbon (S/C) ratio of 3 – 6 (U.S Pat. No. 3,361,534)
This mixture is heated in a mixed feed preheat coil in the convection section
reformer until the temperature reaches a range between 450 – 600°C, then
distributed into tube-packed containing catalyst. The process takes place
according to the reaction equations:
 CH4(g) + H2O(g) CO(g) + 3H2(g)
CO(g) + H2O(g) CO2(g)+ H2(g)

The first reaction is called Steam Methane Reforming while the second
reaction is called the water-gas shift reaction. The reforming reaction is highly
endothermic and occurs at temperatures of about 560°C or above. The heat
requirement for reaction (3.7) is met by burning natural gas in the radiant
furnace reformer. This reaction is not good at high pressure because it will
shift to the left so that the reformer is operated at low pressure

2. Shift Converter
The shift converter functions to increase the H2 content and at the same time
converts CO to CO2 through a shift reaction between CO and H2O. The
reaction that occurs is shown by the reaction equation
CO(g) + H2O(g) ↔ CO2(g) + H2(g)
To maximize H2 conversion, 2 phase shift converters are carried out,
namely the High Temperature Shift converter (HTS) and the Low
Temperature Shift converter (LTS).

a. High Temperature Shift converter (HTS)

A mixture of gas and steam enters the HTS catalyst bed at a temperature
of 350-371°C. The catalyst used in HTS is iron oxide (Fe2O3). Because
the reaction is exothermic, the HTS outlet temperature will rise to 420-
438°C. The temperature entering the HTS bed is higher (350-360°C)
than the temperature entering the LTS bed (200-235°C). The
temperature in HTS is maintained at no more than 360°C, this is because
if it is too high then the reaction will shift to the left, so that no CO and
H2O are converted to H2. After the HTS process gas is cooled in a heat
exchanger with BFW so that the temperature drops. Then it is cooled
again with cooling water media so that the temperature drops again,
because it is expected that the process gas enters the LTS in lower
temperature conditions, to achieve maximum conversion.
 b. Low Temperature Shift converter (LTS)
In HTS, it is operated at high temperatures to catch up on the
reaction rate, while in LTS it is operated at low temperatures to increase
conversion. The type of catalyst used is Copper Oxide (CuO). This
catalyst is very sensitive to sulfur compounds, so this catalyst is
equipped with Zinc Oxide (ZnO).

2.3.7 Pressure Swing Adsorption (PSA)

Process gas that has been cooled and has been separated from its water content is then
passed through a PSA (pressure swing adsorption) device which aims to purify/separate
H2 from impurities, for example H2O, CO, CO2 and CH4 which do not react. The content
of CO and CO2 remaining in H2 is a maximum of 20 ppm while in CH4 it is a maximum
of 50 ppm. During the process in this PSA, it cannot be avoided. The removal or
inclusion of a small amount of H 2 and gases that must be disposed of (separated). The
desired gas from this PSA unit is only Hydrogen. The exhaust gases produced by the
PSA unit will be used as fuel for the Steam Reformer, having previously been stored in a
tank called the Surge Drum.

This PSA system consists of 4 adsorbent vessels A, B, C, D. Each vessel consists of 3

different adsorbent sections:

- At the top, it contains Zeolite which functions to absorb CO2

- In the middle, it contains Activated Carbon which functions to absorb CH4
- At the bottom, it contains Activated Alumina which functions to absorb H2O
There are 4 ways PSA works continuously

1. Adsorption
Process gas enters from the bottom of the vessel so that impurities from process gas
are absorbed by the adsorbent and exit through the top of the vessel. The purity of H 2
that comes out is around 99.99% with impurities of CH4 (<5 ppm) and CO2 (<5
ppm)

2. Regeneration
 The regeneration stage begins with depressurization, during this process the gas
produced by the depressurization process is used to purge other vessels that are
undergoing a regeneration process.

3. Purging

At this stage the gas pressure in the vessel is very low, close to ambient pressure
and this purging process is taken from another gas vessel which is in the
depressurization process. The gas resulting from this process is called purge gas (as
raw material for CO2 plant and fuel for burner reformers).

4. Repressurization

At this stage the previously low vessel pressure is increased until it reaches the
adsorption operating pressure (25 bar).
 Higman, Christopher, 2008, “Gasification”, Elsevier Science, USA

Smith, R J. Byron, 2010, “A Review of theWater Gas Shift Reaction Kinetics”, The
Berkeley Electronic Press, India

Perry, Green, 2008, “Perry’s Chemical Engineers’ handbook”, 8th edition, McGrow
Hill Companies, Inc., United State.