Summary of Unit Operations
The plant has been designed to feed the natural gas and the ensuing products through the different process steps at the required conditions and to supply the heat and the steam required for the process. The process involves 5 distinct steps:
The equipment and connecting lines are represented on the process flow diagram (available upon request) which also gives the detailed material balance for the plant operating at its nominal capacity.
Detailed Process Description
The feed gas flow controller FIC-3003 on the outlet of desulfurizer determines the plant capacity. Downstream of the control valve the hydrocarbon feed is mixed with superheated steam from exchanger E-10; the steam is also flow controlled FIC-3006. The ratio between the two flows is 3.28 kg steam/kg feed which corresponds to a steam/carbon ratio of 3.0 on molar basis. The mixed feed/steam (340°C, 19.8 Barg) is sent to CRG feed superheater CC-10 to be superheated to 490°C before entering the prereformer R-10.
Operating temperature of the reactor is 490°C at the inlet and 421°Cc at the outlet; the temperature drop is due to the steam-methane reaction.
The mixed feed/steam at 421oc is sent to reformer feed superheater CC-11 to be superheated to 490°C before entering the reformer H-1
The reformer consists of 10 parallel tubes placed in a firebox. The feed/steam mixture flows downwards through the 10 tubes which are filled with reforming catalyst. Burners placed in the arch of the firebox, heat the tubes to provide the steam reforming heat of reaction and to raise the temperature of the process gas to 865°C in order to obtain the desired conversion at the tube outlet. Pressure at the reformer outlet will be 16.6 Barg.
High temperature shift conversion
The reformer effluent is cooled down in the process gas boiler E-1 and is fed to the high temperature shift converter R-2. The process gas boiler is equipped with an internal by-pass which permits adjustment of the outlet temperature at the design level of 330°C. This system allows a correct operation of H.T.S. independently of catalyst age and plant capacity.
The H.T.S. converter provides the conversion of the major part of the CO present in the reformer effluent to CO2 by reduction of water to hydrogen. The heat of reaction (the shift conversion reaction is exothermic) will produce an outlet temperature of 408°C.
The converter effluent is cooled to 35°C in the exchanger E-10, E-2, E-3 and cooler E-4 which causes most of the unreacted steam to condense.
The condensate is separated from the gaseous phase ('raw hydrogen') in separator V-2.
Raw hydrogen from separator V-2 is fed to the PSA system where high purity hydrogen is produced by adsorption of the impurities on active carbon and molecular sieves. The adsorbents are placed in 4 vessels (V-5 A/B/C/D) one of them is in the adsorption stage while the other three are in different stages of regeneration. The impurities and some hydrogen are recovered in the purge gas drum.
Combustion air -The burners are designed with forced draft.
Normal air flow will be 12% above the stoichiometric quantity; increasing the excess air percentage will result in a slightly higher fluegas temperature at the convection section inlet and an increase of the flue gas flow; the effect is an increased heat transfer in the convection section.
Combustion air is supplied by forced draft fan F-1 and controlled in ratio with fuel gas/purge gas via pressure control loop PIC-3505.
Fluegas - the generated fluegas leaves the firebox at 980°C and is cooled down to 160°C in the convection section after which it is sent to the atmosphere via I.D. fan F-2 and a stack.
The flue gas I.D. fan provides the necessary draft in the furnaces by pressure control PIC-3506 that gives action on a damper at the suction of flue gas fan at the base of the stack.
Waste heat from the combustion system and the process system is used to generate steam; part of the steam is used as process steam and the remaining is sent to B.L.
( Process condensate from separator V-2 is sent to degasifier (V-3) where dissolved process gases are stripped from the water by steam entering at the bottom of the stripping tower. Make-up degasified water is imported from B.L. at 40°C and 4.0 Barg.
The degasified water is sent to the steam drum V-1 via BFW pumps P-1A/1B through preheater E-2 where the temperature is raised from 1osoc to 190°C.
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