Synthesis Gas Purification Methanation Biology Essay

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After the low temperature water-gas shift, there is still 0.2- 0.5% CO and 0.1-0.2% CO2. This must be further reduced to about 5 ppm in order to use the hydrogen produced in the ammonia synthesis, as CO and CO2 poison the ammonia synthesis catalyst. The remaining CO and CO2 are removed by catalytic reduction to methane with hydrogen i.e. methanation, in a fixed bed reactor. Noble metals catalysts supported on Al2O3 can be used in the methanation. In ammonia production, the Ni catalyst is predominantly used. Magnesium oxide and mixtures of magnesium oxide activated alumna mixtures may also be used as supports (Hiller et al, 2006, pp 85-86). The volume of catalyst required is very small.

The reactions in the methanator are as shown below ( Appl, 1999, pp 56);

ΔH = −206 kJ/mol (reaction)

ΔH = −165 kJ/mol (reaction)

For reactions 1 and 2, the equilibrium constants for are; (Hiller et al, 2006, pp84)

Reaction 1

(equation)

Reaction 2

(equation)

Methanation has the advantages of simplicity and low cost which outweigh the disadvantages of hydrogen consumption and the production of additional inerts in the synthetic gas.(Appl, 1999, )

Methanation is carried out in an adiabatic fixed bed reactor at a pressure of 30 bar with inlet temperatures of 250 - 300 0C and exit temperatures of 315 - 365 0C. As shown by the heats of reaction, the reactions are highly exothermic. If a breakthrough of carbon monoxide from the low-temperature shift or carbon dioxide from the absorption system occurs, the intensely exothermic methanation reaction can reach temperatures exceeding 500 0C very quickly. For example, 1% CO2 breakthrough leads to an adiabatic temperature rise of 60 0C. Controls must therefore be used to avoid the bed temperature exceeding 400 0C, in turn avoiding catalyst sintering and carbon deposition ( Appl, 1999, pp ). When using the Ni catalyst, care must be taken to avoid the formation of the poisonous Ni(CO)6 which takes place at temperatures of about 2000C and partial pressures of CO and CO2­ of greater than 0.2 atm. ( Bartholomew and Farrauto, 2006, pp370-371)

Procedure:

The inlet gas is heated by methanated gas in a heat exchanger. The final inlet temperatures of 300-330 0C can be achieved by a trim heater before being fed into the adiabatic reactor. The adiabatic shaft reactor is filled with the Nickel catalyst where CO and CO2 are converted with H2 to methane and water. Since the temperature gradient is very high, the trim heater can be removed and the heat recovered used instead. (Hiller et al, 2006, pp85-86) Fig() shows a schematic of the methanation process

a) Reactor; b) Heat exchanger; c) Fired trim heater;

d) Cooler

Figure (): Methanation process (Hiller et al, 2006, pp 86)

Mass balance:

The mass balance below is worked out under the assumption that the methanator is 100% efficient since the amount of carbon oxides is so small (of the order of parts per million) that it is negligible.

Methanation

CO + 3H2 = CH4 + H2O

CO2 + 4H2 = CH4 +2 H2O

S12 S13

Dryer

Mol frac comp Mol frac comp

YN2 N2 YN2 N2

YH2 H2 YH2 H2

YCO CO YCH4 CH4

YCO2 CO2 YAr Ar

YH2O H2O YH2O H2O

YCH4 CH4

YAr Ar

Feed

Nitrogen balance

(equation)

Energy balance [6]

The methanation is carried out in an adiabatic fixed bed reactor. The feed is preheated to about 3000C prior to entry into the reactor. ( Hiller et al, 2006, pp85)

(equation)

Since the reactor is adiabatic with no shaft work and w are 0, equation x becomes,

(equation)

Explain where this comes from??

(equation)

Neglecting kinetic and potential energy, equation y becomes,

(equation)

Explain where this comes from??

(equation)

(equation)

where ai and bi are the stoichiometric coefficients of the reactants and products respectively and Cpr and Cpp are the reactant and product heat capacities respectively.

(equation)

The values of the coefficients A to G can be found in Yaws (insert year)

(equation)

This section has lost of equations stated but does not say what they do or where they come from etc etc

Dryers:

Water and traces of carbon dioxide are removed from the synthesis gas downstream of methanation. This is done by passing the gas through molecular sieve adsorbers. (Appl, 1999, pp57)

Molecular sieves [5]

The molecular sieves are mainly used to remove water, traces of carbon oxides and traces of ammonia from the synthesis gas. Using the molecular sieves has the advantages of decreasing the synthesis loop pressure drop and compression costs, lowers refrigeration energy requirements, increases production capability and operating flexibility and increases the efficiency of the heat exchanger network.

The use of molecular sieves to purify the make-up gas allows a relatively simple alteration to the process flow scheme which results in significant energy savings. Previously the fresh make-up gas was added to the converter effluent, which was then compressed and cooled. Water and CO2 contained in the make-up were removed in solution with liquefied ammonia. The recycle gas was then reheated and supplied to the converter inlet.

In newer plants, the oxides of carbon are removed prior to entry into the synthesis loop. After the reaction the increased ammonia concentration in product recovery section allows condensation at higher temperatures reducing refrigeration costs.

The molecular sieve adsorbers can be placed either at the suction side or in the intermediate- pressure stage of the synthesis gas compressor. The molecular sieves operate at pressures ranges of 20-35 bar or 55- 69 bar depending on their position relative to the compressor and temperatures of 5-350C and the performance is very good over the whole range. The sieves can be designed to remove all the saturated water and any traces of carbon oxides. (Zeochem adsorbents,)

Mass balance in dryer:

Assuming that the dryer has an efficiency of 100%

Feed from methanator

Dryer S14

S13 H2O

Mol frac comp S15

YN2 N2 Compressor

YH2 H2 Mol frac comp

YCH4 CH4 YN2 N2

YAr Ar YH2 H2

YH2O H2O YCH4 CH4

YAr Ar

Compressors ( Appl, 1999, pp58-62)

The synthesis gas needs to be compressed from pressures of about 30bar at which it exits the methanator, to pressures of 300bar before being fed into the converter. In the past reciprocating compressors were used to compress the synthesis gas. Horizontally balanced compressors in which the cylinders are in parallel configuration on both sides of a common crankshaft are now more commonly used. Since the 1960 centrifugal compressors such as the one shown in figure() have been in use in ammonia synthesis processes.

a) Air cooler; b) Separator; c) Silencer; d) Water cooler

Figure (??): Centrifugal compressor for synthesis gas compression. ( Appl, 1999, pp60)

Centrifugal compressors have the advantages of higher speeds and the use of asynchronous motors is possible. They are cheap to buy and to maintain, have less frequent shutdowns for preventive maintenance, are highly reliable and occupy much less space than reciprocating compressors.

Centrifugal compressors are run by steam turbines which avoid energy losses associated with the generation and transmission of electric power. This makes the centrifugal compressor superior to the reciprocating compressors even though they are less efficient.

Due to the large pressure differences ( 30 - 300 bar), about 20 impellers are required in the centrifugal separator, however only about 9 impellers can be fit into a standard compressor, therefore many compressors are used in series to achieve the required pressure. Two to three compressor casings in series are usually used. ( Appl, 1999, pp58-62)

Energy Balance

Using the first law of thermodynamics

(equation)

(equation)

The work done to operate the compressor is dependent on the volume of the gas being compressed and the pressure difference required in the compression.

(equation)

(equation)

The energy for compression is provided by steam turbines.

(equation)

where hs is the enthalpy of steam

Most of the steam required will be produced using recycled heat from the reactions. The rest will be produced using electricity heaters.

Ref:

[1]

[2] Bartholomew

[3] Max Appl

[4] Gas Production

[5] Molecular sieves

[6] http://www.engin.umich.edu/~cre/course/lectures/eight/index.htm (check 4th Edition Fogler chapter 8)

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