Cracked distillates, selective hydrogenation

CDTECH Butadiene C4+ from naphtha cracking Selective hydrogenation of C4 acetylenes in a distillation column to produce low acetylene feed for butadiene extraction 2 1998... 

A further opportunity for process optimisation involves the judicious selection of reaction conditions (temperature, pressure and hydrogen / hydrocarbon ratio) so as to promote the transfer of primary middle-distillate product into the vapour phase with a view to reducing its residence time in the catalyst bed and, consequently, preventing distillate loss via secondary cracking. This approach to improved overall distillate selectivity has previously been referred to by Filers et al. [2] and provides the incentive to optimise catalyst performance within the constraints of the operating window so defined. 

The C3+-fraction of the C2 /C3+ splitter enters the C3/C4+ splitter that separates propane, propene, propadiene, and propyne from all heavier products. The C3 stream undergoes a selective hydrogenation step in a fixed bed reactor that converts propyne and propadiene mainly into propene. Propene and propane are separated in a very similar way as ethane/ethene. Again, distillation columns with more than 100 trays are applied, making these separation units very costly in investment and energy consumption. The bottom fraction of the C3/C4+ splitter is transferred to C4/C5+ splitter. The C4 fraction leaving this column at the top contains mainly butadiene, isobutene, 1-butene, 2-butene, and butane. The further use of this crack-C4 mixture is described in detail in Section 5.3. 

KLP [Dow K Catalyst Liquid Phase] A selective hydrogenation process for removing acetylenes from crude C4 hydrocarbons from ethylene cracking, with no loss of butadiene. The catalyst is based on either copper metal or alumina. Developed by Dow Chemical Company from 1983 and first commercialized at its plant in Terneuzen, the Netherlands. Bight units were operating in 2012. The KLP licensing business was sold to UOP in 1991. It is now used in conjunction with the BASF butadiene extraction process that is an extractive distillation process using N-methyl pyrrolidone. 

The feedstock, mixed with recycle stock from the fractionator, is injected into the cracker is immediately absorbed into the pores of the particles by capillary force and is subjected to thermal cracking (Figure 8-7). In consequence, the surface of the noncatalytic particles is kept dry and good fluidity is maintained allowing a good yield of, and selectivity for, middle distillate products (Table 8-3). Hydrogen-containing gas from the fractionator is used for the fluidization in the cracker. 

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