4.1 Equipments Detail
4.1.1 Natural Gas Desulphuriser
Purpose:
To remove sulfur compounds, which are poisonous for the catalysts, from LNG with activated carbon by absorption.
Operating conditions:
LNG contains about 0.5 to 3ppm sulfur. Sulfur is absorbed by activated carbon. The outlet gas should contain 0.2ppm sulfur.
Description:
The feed gas contains about 0.5 to 3ppm sulfur compounds in the form of sulphides, disulphides, thiophenes etc, and these should be removed as they are poisonous for the process to follow. There are two desulpharisers out of which one is online while the other
is a stand by which is used when the first one is regenerations.
The process employed here is adsorption. For desulphurization activated carbon catalyst is used. There are two beds of catalyst-at the top and bottom. It adsorbs the sulphur compound. RLNG enters the desulpharisers at the top and is removed from the bottom with a sulphar impurity of about 0.2ppm. The gas then enters the Knock-out drum in which the liquid hydrocarbons, if any are knocked out.
4.2 Primary Reformer
Purpose:
To react liquefied natural gas with steam in presence of nickel catalyst to get H2 for
synthesis of NH3. Pressure: 31.3 kg Temperature:818 de
Fig. 4.1Primary Reformer
Description:
Reforming is an initial process of forming Hydrogen and Nitrogen in ammonia plant after the process of desulphurization. The primary reforming is carried out in the primary reforming(furnace). The gas from the desulphuriser is first preheated in the feed preheated in the convection section. The inlet and outlet temperatures are 93°C and
232°C respectively.
The above natural gas is mixed with super- heated steam to get a steam to carbon ratio of 3.5:1. The mixer at 295°C is then passed through the second feed pre heater coil in the convection section to achieve a temperature up to 525°C. The final mixer goes to the primary reformer.
The primary reformer is rectangular in shape. Outside is consisting of S.S body and there are insulator bricks inside. It consists of tubes packed with nickel which is used as catalyst. These tubes are vertically with the help of spring suspension and between the vertical rows of these tubes the burners are installed which use natural gas and associated gas as a fuel for heating the tubes. The spring suspensions are made provide for the
expansions and contractions of the tube material. There are in total 336 tubes and 126 burners.
Inside primary reformer endothermic reaction takes place at about 818°C. The product each raw of tubes is connected at the bottom. The product from each of 42 tubes meets at the centre and forms the single line called „Transfer line‟ which rises upward. The temperature of the gases inside the tube is 852°C. The „Transfer Line‟ directs the flow to the secondary reformer.
4.3 Secondary Reformer:
Purpose:
To complete the reforming of methane, which comes out from primary reformer.
Also nitrogen is introduced in the process by burning a part of reformed gas with oxygen contained in the air at the top section of secondary reformer.
Pressure: 31.0 kg
Fig. 4.2 Secondary Reformer
Description:
The partially reformed gas from primary reformer entered the secondary reformer at a temperature of 820°C. The flow is downward a centrally located air inlet pipe, air supplied by process. Air compressor is preheated with steam. A small quantity of steam is mixed at air inlet to the air pre heater coil to ensure continuous flow into the secondary reformer
The process is namely exothermic and the temperature in the combustion zone is about 995°C. In the secondary reformer the working pressure is about 31 kgf/cm2. The purpose of the secondary reformer is to complete the reforming of methane which comes out from
the primary reformer. Also nitrogen is introduced in the process by mixing a part of reformed gas with oxygen contained in air the section of secondary reformer.
The hot gases pass through the bed of nickel catalyst and their temperature is around 992°C. The heat is recovered from the reformed gas by use of waste heat boiler and the steam produced is again used in the other operations.
4.4 Primary Waste Heat Boiler:
Purpose:
Steam generation from secondary reformer effluent gases. Pressure: 30.9 kg
Temperature: 996.7 deg C
4.5 Secondary Waste Heat Boiler:
The reformed gas from primary reformers enters the top of the secondary reformer,
which is mixed with air by “John Zive Air Mixer”.
Nitrogen is introduced in the process by burning of part of the performed in the air at
HT top section.
The heat of combustion is made available for the endothermic reaction of the methane
reforming at elevated temperature at bottom section. Pressure: 105.4 kg
Temperature: 313.90 deg C
4.6 Shift Convertor:
Purpose:
To convert CO of reformer gas to gain one mole of H2 per mole of CO for ammonia synthesis.
Design And Operating Details:
Table 4.1 High Pressure Section
Design pressure 34.1 kg/cm2 Vessel design temperature 482.2°c
Inlet gas temperature 355°c
Outlet gas temperature 422°c
Table 4.2 Low Pressure Section
Operating pressure 29.1 kg/cm2
Design pressure 34.1 kg/cm2
Vessel design temperature 301.7°c
Inlet gas temperature 204°c
Outlet gas temperature 221°c
Max. allowable pressure drop through beds(total) 2.0
4.7 Ht Shift Conversion:
The reformed gas enters the HT section of the shift convertor at a temperature of
345°C and flows through the catalyst bed. The bypass is provided to control the HT shift converter feed inlet temperature. The catalyst used here is iron oxide.
By the reaction most of the carbon monoxide is converted to carbon dioxide also
gives out heat. Above reaction is reversible and carbon monoxide is converted back into carbon dioxide which is favored at low temperature.
4.8 Lt Shift Conversion:
The gas coming out of the HT shift converter still contains about 2.5% carbon monoxide. The removal of this CO is done in this section. The temperature of the gas in the LT shit converter is about 200°C. The catalyst used here is copper. The heat is recovered from the gas before it is sent to the carbon dioxide removal section at low temperature.
The gas is first cooled by a desulphur heater with a spray of recycled process condensate pumped by pump from the raw gas separator. The quenching is done with the desulphur
heater as the excessive high temp. can cause degradation of a-MDEA(aqueous-Methyl Diethanol Amine)
4.9 CO2 Absorber:
Purpose:
To remove the carbon dioxide contained in the raw synthesis gas by scrubbing it with 30% to 40% MDEA solution.
Pressure: 27.4 kg Temperature: 75 deg C
Description:
In this section the bulk of CO2 in the raw synthesis gas is removed by absorption using 40% MDEA solution at high pressure and low temperature. The absorption of CO2 involves the reaction of dissolved CO2 in water.
With a-MDEA to form a loose chemical compound which can be easily dissolved at high temp. and low pressure. The raw synthesis gas at a pressure of 27.3 kg/cm2 .g and a temp. of 63°C contains about 18% dry volume of CO2 is introduced at the bottom of CO2
absorber.
4.10 CO2 Stripper:
Purpose:
To regenerate rich MDEA solution by steam stripper with the help of reboiler and there by realizing CO2.
Pressure: 0.74 kg Temperature: 75 deg
Description:
In this section the bulk of CO2 in the raw synthesis gas is removed by absorption, using at 40% aqueous methyl ethanol amine solution at relatively high pressure and low temperature.
The CO2 is sent to urea plant , dry ice and balance if any is vented by atmosphere. The lean a-MDEA solution form the bottom of strippers is reused after cooling , for absorption of CO2 in absorber.
4.11 Methanator:
Purpose:
To convert small quantity of CO and CO2 to methane by reacting with hydrogen in presenc4e of highly active nickel catalyst.
Pressure: 26.7 kg
Temperature: 375.6 deg C
4.12 Synthesis Convertor:
Purpose:
In synthesis converter, H2 and N2 contained in the converter feed combine to form ammonia in presence of promoted iron catalyst at the prevailing temperature and pressure of 420°c and 135 kg/cm2 respectively.
Pressure: 146.2 kg
Temperature: top head=283.9 deg C Startup heater outlet=537.8 deg C Others part=146.1 deg C
4.13 Ammonia Separator:
Purpose:
To separate and remove maximum amount from recycle gas and slight traces of water, CO, CO2 contained in the makeup gas which are chilled to 28.9 C.
Table 4.3 Operating Condition
Temperature -28.9c