DEPRESSURIZATION CATALYST (NICKEL- MOLYBDENUM CATALYST)

 WONDERFUL DISCUSSION BY SHRI V. SHYAM SUNDER SIR AND SHRI JKP SIR.

DEPRESSURIZATION CATALYST (NICKEL- MOLYBDENUM CATALYST)

    The natural gas feed stock supplied to NFCL contains no H2S, but it is anticipated that future supplies may contain sulphur compounds which have to be removed in order not to poison the reforming catalysts and the LT shift catalyst. Natural Gas from battery limit is heated to 385 deg.C in the Feed stock preheater F-203, and is passed through the Hdrogenator. A bed of Nickel-Molybdenum catalyst is provided to catalyse the hydrogenation of organic sulphur compounds to hydrogen sulphide.  There are two types of organic sulphur compounds that may be present in the feed stock.  One is called 'Normal Sulphur' containing H2S, COS, CS2 and Mercaptans and the other is called 'Less Reactive Sulphur', containing Thiophenes, Thioethers etc.  In case of normal sulphur except Mercaptan Hydrogen recycle gas is not consumed where as for less reactive sulphur, recycle hydrogen is consumed as per the following hydrogenation reactions:

                   RSH + H2                               RH + H2S

                   (Mercaptans)

                   R1SR + 2H2                            RH + R1H + H2S

                   (Thioethers)

                   R1SSR + 3H2                          RH + R1H + 2H2S

                   (Thiophenes)

                

If sulphur is present, natural gas is mixed with recycle gas from synthesis gas compressor first stage discharge with flow of recycle gas around 1306 NM3/hr., in order to avoid Carbon deposition on the catalyst due to catalytic cracking of higher hydrocarbons if any. After preheating to 385 deg.C,the gas mixture passes to Hydrogenator Reactor R-201 and reacts to produce H2S.  The above reactions are exothermic but insignificant (which depends on the type of Sulphur that determines the number of moles of hydrogen taken up).  H2S produced in R-201 and that already present in Natural Gas is then removed in H2S Absorbers R-202 A/B, thereby the gas will be free of H2S.  Each absorber contains one bed of Zno catalyst to absorb the sulphur.  The absorbers are operating in series with the second vessel acting as guard.  When the Zno in the first vessel is getting exhausted, a break through of H2S from the first vessel may be observed.  The operation will then continue with the second vessel in service, while the first vessel is being reloaded with fresh catalyst.  The sulphur content at the exit of R-202B shall be less than 0.1 ppm on dry volume basis at all times which is tolerant to reforming catalyst.

    The sulphur removal reaction in Zno bed takes place as follows:

                  

                   Zno  +  H2S                             ZnS  +  H2O

                   Zno  +  COS                             ZnS  +  CO2

    Zno reaction with 'S' depend on :

    1.   Type of sulphur compounds.

    2.   Temperature:  Increase in temperature will generally increase the ability of Zno to remove sulphur.

    3.   Capacity   :   As Zno reacts with sulphur it gets saturated with sulphur and looses its activity.  Normal life of Zno catalyst depends on the H2S and sulphur concentration in the natural gas.

Reactor	Catalyst  Type	Size mm	Volume M3	Bulk Density	Bed ht. mm	composition	Life time (exp)
R-201	TK-251	5X2.5	8.44	480	2220	NiO:2-3% MoO:10%	>5
R-202A	HTZ-3	4	10.64	1300	2800	ZnO:99%
R-202B	HTZ-3	4	10.64	1300	2800	ZnO:99%	Guard