(25a) Carbonyls Removal in Steam Crackers & FCC Units

A large proportion of propylene is produced by two sources: (1) Steam Cracking (SC) of naphthas and/or gas oils and, (2) Fluid Catalytic Cracking (FCC).  The global propylene production contribution of SC accounts for about 60–65% of the world’s production while refinery configurations utilizing FCC accounts for 30%.  The remainder of the world’s production is produced by on-purpose propylene technologies such as metathesis or propane dehydrogenation.  With on-purpose propylene production technologies, such as propane dehydrogenation and metathesis being touted as possible alternatives, the cost associated with these technologies remains less competitive relative to SC and/or FCC.

By reconfiguration of the FCC unit and using enhanced catalyst formulations to maximize the production of propylene and light olefins, two technologies have emerged which have been developed and commercialized by SINOPEC are Deep Catalytic Cracking (DCC) and Catalytic Pyrolysis Process (CPP) using continuous fluid bed reactor/regenerator systems.

DCC and CPP use more steam than conventional FCC, refer to Table 1; therefore, their operation may be better termed as steam cat­alytic cracking (SCC).  SCC is a process of cracking hy­drocarbons to light olefins in mild temperatures in the presence of steam over a catalyst. SCC combines mild thermal cracking combined with catalysis using an acid promoted zeolite-based catalyst, and can provide very high yields of light olefins (with the possibility of varying the propylene-to-ethylene ratio) while operating at temperatures much lower than those used in SC.

Table (1)   FCC/SC Key Operating Parameters

	FCC	DCC	CPP	SC
Reaction temp/ °C	500–550	530–590	560–670	760–870
Reactor pressure/barg	1–3	1, 2	1	1–0
Residence time/s	1–5	1–10	1–3	0.1–0.2
Cat./oil ratio (wt/wt)	4–8	10–15	15–25	–
Dilution steam (%)	1–3	5–30	30–50	30–80

Hydrocarbons in the presence of steam at prevailing high temperatures during the SC Process undergo decomposition to C and H₂ whereby some of the carbon, as C, converts catalytically to CO and H₂ on the radiant coil surfaces made containing Ni and Cr at operating temperature.  The similar reaction mechanism is true within the FCC Reactor Unit (FCCU) over its catalyst. 

Also oxygenates as carbonyls are catalytically formed in SC and FCC including organic acids, alcohols and other carbonyls in concentrations ranging from 10-1000 ppm depending on the type of feedstock, catalyst, ratio of dispersion steam-to-hydrocarbon (feed) at operating temperature.

In SC plants, all the acids in the cracked gas are highly soluble in water and are removed in the quench water tower; the alcohols are mostly removed with the condensate streams in the knock out drums within the cracked gas compressor. Aldol (Red Oil) polymer is formed in Amine Absorber(s) and/or Caustic Tower(s) within the multistage cracked gas compressor as a result of carbonyls reacting by Aldol Condensation reaction mechanism catalyzed by the presence of alkaline solutions; the Red Oil polymer cause fouling of the towers and contaminates the spent caustic stream.

In FCCU’s when particularly operated in high olefins mode (DCC & CPP), relatively large concentrations of oxygenates are formed.  Virtually all the carboxylic acids in the FCC reactor outlet gas are removed with the condensate from the overhead main fractionator accumulator, alcohols and portions of other carbonyls are mostly removed with the condensate streams in the knock out drums within the wet gas compressor.  The remainders of carbonyls follow the C3/C4 LPG and foul the amine unit by forming Red Oil Polymer, and those carbonyls reaching the RSH Extractor would foul the extractor and the downstream caustic regeneration unit.

A new, patent pending, effective removal method of carbonyls from a gas stream comprises feeding the gas stream containing the carbonyls to a gas-liquid absorber and is contacted with an inorganic metal salt in aqueous solution where the carbonyl is transferred from the gas phase to the aqueous phase. The carbonyl reacts chemically with the inorganic metal salt and forms a solid adduct that is soluble in solution.  The treated gas leaving the absorber will be free of all the very reactive aldehydes and most of the less reactive ketones. The treated gas stream will not form Red Oil polymer and will not foul both the caustic or amine units and their associated equipment.