A catalytic reactor is used to convert methanol and water into
hydrogen via the following reactions: Methanol decomposition
--Reaction 1
Methanol-Steam Reforming --Water-Gas Shift Reaction CH3OH + Heat
CH3OH + H2O CO + H2O ------- ----- -----
Reaction 2 (Overall Eqn)Reaction 3 CO + 2H2 CO2 + 3H2 CO2 + H2 +
Heat ----(1) ----(2) ----(3)
The combined reaction is net endothermic, requiring the input of
heat.
Water and methanol in the ratio of 1.2 is pumped to 25 bar via
P-101, mixed, vaporised (E-101) and superheated(E-102) to 250 280
oC in 2 heat exchangers in series. The gaseous mixture passes
through a tubular reactor where reaction between methanol and steam
takes place over a metal oxide catalyst. The reformed gas leaving
the reactor at 250 280 oC is cooled down 1st to about 140oC in a
heat exchanger(E-101) used to preheat methanol-water mixture to its
boiling point. The gas is then cooled down to about 40 oC with
cooling water in a second heat exchanger(E-103) where remaining
water vapor and methanol vapor are condensed. The liquid and gas
phases are separated in a condensate separator. The liquid phase
which contains some methanol and dissolved CO2 is recycled to the
feed water tank.
Heat for the reactor is provided by hot thermal fluid (hot oil)
heated up to ~300oC by Burner F-101. There are 2 types of fuel used
by the burner: methanol & off gas.
Methanol is used to light up the burner during start-up. Once
there is off gas produced, the fuel for the burner will be changed
to off gas.The burner pilot flame is fuelled by Liquified Petroleum
Gas (LPG). Methanol is supplied to the burner at 10 bar while LPG
is supplied at around 0.5 - 0.8 bar.
The essential characteristics of the PSA type adsorption can be
summarised as follows: -It consists of removing one or more
components from a gas stream by adsorption under pressure. -The
regeneration of adsorbents is carried out without heating, by
depressurising and purging. -The cycles used are always very short
(in the order of a few minutes).
The cycle of the PSA unit consists of 4 following main
operations: -Adsorption -Depressurisation -regeneration
-Repressurisation During one cycle, each vessel successively
undergoes the three above operations. At any time, when a vessel is
under adsorption, the second vessel is under depressurisation or
regeneration and the third one under repressurisation. This is only
possible if each operation lasts the same length of time.
Regeneration consists in lowering the partial pressure of
impurities present in the adsorbers:
-Depressurisation (lowering of total pressure)-Elution
(scavenging with hydrogen recovered from the regeneration drum)
Depressurisation is carried out by: -pressure equalisation with
another vessel (once the adsorbents of this vessel has been
regenerated) -pressure equalisation with regeneration drum B202
-final depressurisation to the elution pressure. Elution after
depressurisation consists in carrying over the waste impurities in
order to improve regeneration. When elution is completed, the
adsorbents has recovered its adsorption capacity. It is then
necessary to repressurise the vessle for a new cycle. This
repressurisation is carried out in 2 steps: -pressure equalisation
with another vessel to be regenerated. -final repressurisation with
hydrogen withdrawn from production. -a surge drum (B201) is
installed in order to stabilise the flow and composition of the
offgas before sending it to the burner.
R-201
R-202
R-203
B-204
B-202
DI water is used to react with methanol under the chemical
reactions because the reactor catalysts will be poisoned by
chlorine & sulphur. Chlorine is present in raw(portable)
water.
Raw water will be passed via the raw water pump through the
cation/anion beds which will remove all ionic salts in the water
via ion-exchange. The deionised water will be stored in the DI
water tank.The DI water pump will then be started to supply DI
water to the plant (B-102) and to the methanol pump for priming of
the pump. DI water will only be allowed into B-102 when the level
hit low. Otherwise, the level will be make-up by condensate from
B-101.
There are a total of 14 vessels in the plant: 1. Methanol tanks
(underground)(X02 nos) - to store methanol. 2. B-102 Feed mixing
tank to hold DI water and condensate recycled from B-101 3. B-101
Condensate Separator to separate H2 gas from any condensate
produced by the reaction. 4. R-201/202/203 Adsorbers to purify H2
gas via pressure swing adsorption. 5. B-201 Off Gas drum(surge
drum) to stabilize flow and composition of the off gas before
sending to the burner as fuel. 6. B-202 Regenerating Drum to
stabilise the flow of the gas used for regeneration of each
adsorbers. (low pressure) 7. B-204 Repressurisation drum to
stabilise the flow of the gas used to repressurisation of the
adsorbers. (high pressure) 8. H2 buffer tank to stabilise the flow
and pressure the the Product gas before supplying to customer. 9.
B-105 Hot oil Reservoir to store hot oil. 10. DI water tank - to
store DI water 11. Raw water tank - to raw water for DI water
production.
Start-up 1. Cooling Tower i. Ensure the cooling tower is filled
with water.
ii.iii iv.
Open inlet & outlet valve of cooling water pumps.Open
cooling tower return & supply valve. Start one of the cooling
water pump.
v.
Bleed off any air pockets in the discharge line.
vi. Check the stability of the water level in the cooling tower.
vii. Start cooling tower fan. viii. Check the whole cooling water
circuit for any abnormality.
Start-up 2. Air Compressor i. Check cooling water is available
and circulating.
ii.iii iv. v.
Open inlet & outlet valve to the air compressor.Select
switch to unload position. Start the air compressor. When auto
cut-in, put switch to load position.
vi. Check the cut-in/cut-out pressure @ 6.8 & 7.8 bar. vii.
Check the discharge temperature of the air compressor.
Start-up 3. DI Water System i. Check raw water tank is filled
with water.
ii.
Open inlet valve to mixed bed.
iii. Open inlet valve to raw water pump. iv. v. Open outlet
valve of raw water pump. (inlet DI Water tank) Start raw water
pump.
vi. Check that the conductivity is low. vii. Check that there is
water flow and the DI water tank level is rising. viii. Open inlet
& outlet valve of DI water pump. (to P-101 & methanol pump
skid)
ix. Start DI water pump.x. Check DI water is available at
methanol pump skid.
Start-up 4. Methanol Pump. i. Check methanol tank level
sufficient.
ii.
Check Nitrogen blanket available at 0.5 bar.
iii. Open inlet & outlet valves of the methanol pump. iv.
Check that DI water is available. (for priming the pump, open the
valve for DI water to pump suction when necessary)
v.
Start methanol pump.
vi. Crack open the valve for methanol to return to the methanol
tank if necessary(if the pressure is too high) vii. When the
discharge pressure of the methanol pump stabilised, slowly close
the DI water(to methanol pump suction) valve. viii. Check that the
discharge is purely methanol and not a mixture of methanol and
water.
Start-up 5. Burner i. ii. iii. iv. v. vi. Check hot oil
reservoir (B-105) > 30%. Check that N2 blanket is available at
around 1 bar. Check B-105 bypass valve is closed. Start the
scrubber system (to be discussed in detail) Select Hot oil pump A/B
(P-604A/B) on the pillard panel. Open inlet & outlet valve of
the selected hot oil pump.
vii. Start hot oil pump to circulate hot oil to the circuit.
viii. Start methanol fuel pump (P-606). ix. x. xi. Commission the
LPG cylinders for pilot ignition. Select methanol on the pillard
panel and start burner. Ensure that the burner temperature is
increasing, vent off air pockets in the hot oil circuit by opening
vent valve for top of reactor in necessary(if temperature increase
too fast).
xii. Target temperature for burner : 300oC
Start-up 5. Plant i. ii. iii. iv. v. Open N2 valve to reactor to
pressure up the reactor to around 6 bar. Open HV 110. Start feed
pump P-101. Open PIC217/FIC-398 to 30% to allow feed to flow to the
reactor. Ensure that the reactor temperature do not decrease too
much. PSA will start to pressurise. (to around 20 bar)
vi.
Put analysers on cal mode.
vii. Once PSA is ready to start (all interlocks cleared), press
F4 to start PSA. viii. Ensure that the cycle time is set at 180 to
200 seconds. ix. Increase PIC217/FIC398 gradually to increase flow
to PSA and the product flow.
x.xi.
Set PIC 395-1 to 20.5 bar and PIC 395-2 to 21.5 bar. (product
pressure)Set PIC 226-1 to 0.3 bar and PIC 226-2 to 0.35 bar. (off
gas pressure)
Start-up 5. Plant (contd) xii. Check and ensure that the PSA is
stabilised. xiii. When off gas flow and pressure is stabilised,
stop burner. xiv. Stop the scrubber. (to be discussed in details)
xv. Put the fuel select switch to off gas.
xvi. Start burner.(off gas) xvii. Ensure that the off gas
pressure from PSA does not drop too much. xviii. Ensure that the
PSA and burner is stabilised. xix.Once purity is achieved, ready to
supply, put the analysers off cal mode. xx. Supply gas to
customer.