Gas separation: processes – Liquid contacting – And degasification of a liquid
Patent
1995-02-03
1996-05-28
Spitzer, Robert
Gas separation: processes
Liquid contacting
And degasification of a liquid
95180, 95184, 95186, 95193, 95209, 95240, B01D 5314, B01D 1900
Patent
active
055207243
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for the recovery of low molecular weight C.sub.2+ hydrocarbons, in particular ethylene and ethane, from a gas mixture obtained by the cracking of hydrocarbons and having been freed of high molecular weight hydrocarbons.
In various cracking gas processes, in particular in fluidized catalytic cracking (FCC) a gas mixture is obtained which comprises, inter alia, low molecular weight hydrocarbons, in particular ethylene and ethane, but also C.sub.3 and C.sub.4 hydrocarbons. In most refineries the high molecular weight hydrocarbons (C.sub.5+ hydrocarbons) are first separated from the FCC waste gas by fractional distillation. Thereafter, the C.sub.3+ hydrocarbons are recovered by oil scrubbing, whereas the entire C.sub.2 hydrocarbons, but also a certain proportion of C.sub.3- and C.sub.4 hydrocarbons are discharged into the fuel gas grid.
In the event that C.sub.2 hydrocarbons as well are to be recovered from the FCC waste gas, this is done by partial condensation in a low temperature process. Such a process is known for example from EP-B 0 185 202. Because the FCC waste gases invariably contain traces of higher, polyunsaturated hydrocarbons, nitrogen oxides and oxygen, this process always involves a risk that explosive resins may form in the very low temperature items of equipment. A plant with partial condensation in a low temperature process accordingly always entails a certain safety risk. Thus, for example in February, 1990, such a plant exploded in France after having been in operation for eight years. Accordingly, it is an object of the present invention to provide a process of the type as set out in the introduction, wherein the C.sub.2+ hydrocarbons can be recovered from the input gas in a simple manner, but where the safety of the plant is warranted by a special mode of operation.
This object is attained according to the invention in that the low molecular weight C.sub.2+ hydrocarbons are scrubbed from the gas mixture by absorption by means of an organic, preferably paraffinic, physically acting scrubbing agent in an absorption column, the scrubbing agent having a molecular weight between 50 and 75 g/mol, preferably between 60 and 75 g/mol, the scrubbing agent being passed in a circuit to be first loaded with low molecular weight hydrocarbons, being regenerated in a regenerating column and subsequently being returned into the absorption column for renewed loading.
The process according to the invention offers the advantage that the low molecular weight hydrocarbons are removed from the gas mixture during gas scrubbing, thereby avoiding a safety risk in that the formation of explosive resins is prevented by this procedure.
Advantageously, a C.sub.4+ hydrocarbon fraction, preferably pentane, isopentane or mixtures thereof, is employed as a scrubbing agent. Such a scrubbing agent is characterized by a very high solvent power for C.sub.2 hydrocarbons, in particular ethylene and ethane, but also for other readily soluble components. Light gasoline comprising proportions within the above described molecular weight range, in particular pentane compounds are obtained during the pretreatment of the feed gas, for example in the fractional distillation, and are accordingly readily available.
In a preferred embodiment of the process according to the invention the loaded scrubbing agent, prior to its being fed into the regeneration column, is stripped with a stripping gas, resulting in particular in the stripping off of co-absorbed methane. The methane thereby recovered can be further processed or may, for example, be used as a fuel gas.
Particular advantages result if the stripping of the loaded scrubbing agent is conducted in a lower section of the absorption column. This procedure, apart from simplifications in respect of apparatus requirements, offers the advantage that methane dissolved in the scrubbing agent can even be stripped off from the scrubbing agent in the absorption region in addition to the stripping region, since the stripping gas can flow through the entire absor
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Bauer Heinz
Becker Hans
Linde Aktiengesellschaft
Spitzer Robert
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