Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce aromatic
Reexamination Certificate
1999-07-15
2001-05-01
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce aromatic
C585S315000, C585S251000, C585S252000, C585S254000, C585S266000, C585S448000, C585S455000
Reexamination Certificate
active
06225516
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the production of linear alkylaromatic hydrocarbons.
More specifically, the present invention relates to a process for the preparation of linear alkylbenzenes (LAB) containing from 10 to 14 carbon atoms in the alkyl chain.
2. Discussion of the Background
Linear alkylbenzenes, for example containing from 10 to 14 carbon atoms in the alkyl chain, are generally intermediates which are used in the detergent industry.
Processes for the synthesis of linear alkylbenzenes are known in the art. U.S. Pat. No. 5,276,231 describes a process for the preparation of alkylaromatic derivatives, such as LAB, which consists in dehydrogenating a C
10
-C
15
paraffine stream and alkylating an aromatic compound, for example benzene, with this dehydrogenated stream. Hydrofluoric acid is used as alkylation catalyst.
The by-products obtained in the first dehydrogenation step, essentially of an aromatic nature, are removed by adsorption on molecular sieves or by liquid-liquid extraction, as their presence reduces the activity of the alkylation catalyst and therefore the selectivity to LAB.
SUMMARY OF THE INVENTION
The objective of the present invention is to increase the selectivity of the formation reaction of alkylbenzenes obtained by the continuous alkylation of benzene with mono-olefins coming from the dehydrogenation of n-paraffins and containing, as well as non-converted n-paraffins, the aromatic by-products produced during the dehydrogenation itself. A further objective of the present invention is to increase the selectivity of the dehydrogenation reaction.
The Applicant has now found that these objectives, and others, can be reached by reducing the content of aromatic by-products present in the dehydrogenated stream to values of less than 1.2% by weight of the total in the stream, preferably less than 0.9%, without resorting to particular physical extraction treatment. This result can be obtained by effecting a hydrogenation reaction on the recycled paraffinic stream coming from the alkylation and essentially consisting of paraffins and aromatic by-products as well as, possibly, non-reacted olefins (present in traces).
The hydrogenation reaction of the recycled stream is carried out under particular conditions, as described hereunder, to transform the aromatic by-products into cycloparaffins.
These cycloparaffins are subsequently partly dehydrogenated to cyclo-olefins and act as agents for alkylating the benzene in the alkylation and produce non-linear LAB (iso-LAB). The remaining cycloparaffins that are retransformed into aromatics in this phase, re-enter the cycle but their concentration in the feeding stream of the alkylation reactor is reduced under stationary conditions to values of less than 1.2%, generally less than 0.9%.
U.S. Pat. No.5,276,231 discloses the possibility of hydrogenating the recycled paraffins. This hydrogenation however substantially has the purpose of eliminating the non-reacted olefins during the alkylation of benzene, as their presence is considered harmful for the dehydrogenation catalyst.
The present invention therefore relates to a process for the production of linear alkylaromatic hydrocarbons, containing from 10 to 14 carbon atoms in the alkyl chain, comprising the following operating cycle:
(a) dehydrogenating C
10
-C
14
n-paraffins to the corresponding n-olefins obtaining a mixture also comprising diolefins and aromatic by-products as well as light cracking products and hydrogen;
(b) selectively hydrogenating the diolefins formed during step (a) into mono-olefins, obtaining a mixture essentially consisting of mono-olefins and n-paraffins, in addition to the aromatic by-products formed in step (a);
(c) feeding the stream coming from step (b), together with a stream consisting of an aromatic hydrocarbon, to an alkylation unit in which an alkylation catalyst is present;
(d) feeding the alkylation product to a distillation section for the recovery of the excess aromatic hydrocarbon, a paraffinic stream essentially consisting of C
10
-C
14
n-paraffins and a romatic by-products, and the alkylated aromatic product, respectively;
(e) subjecting the paraffinic stream coming from (d) to a hydrogenation step to transform the aromatic by-products into cycloparaffins;
(f) recycling the stream coming from step (e) to the dehydrogenation unit of step (a).
The C
10
-C
14
n-paraffins in the charge are preferably introduced into the cycle before step (e).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Step (a), for the dehydrogenation of n-paraffins, is carried out according to procedures well-known to experts in the field. The reaction is thus effected in the presence of a catalyst comprising a noble metal supported on porous material. The catalyst generally comprises an element of the platinum group in a ratio of 0.01-2% by weight, with respect to the total (catalyst+carrier), an alkaline or earth-alkaline metal in a ratio of 0.1-5% by weight, and it may optionally also contain a constituent selected from one or more of the following metals:
tin: from 0.1 to 1% by weight;
indium: from 0.1 to 1% by weight;
thallium: from 0.1 to 1% by weight.
In the case of the contemporary presence of indium and thallium, these are present in such quantities that the ratio In/Tl is ≧0.3 whereas the ratio Pt/In+Tl is ≧1.5.
In addition, the dehydrogenation reaction of step (a) is carried out at a temperature ranging from 400 to 500° C., at a pressure within the range of 0.1 to 10 kg/cm
2
and with a space velocity (LHSV) ranging from 0.1 to 100 h
−1
.
The dehydrogenation reaction preferably takes place in the presence of hydrogen, with a molar ratio hydrogen
-paraffins ranging from 0.5 to 20, and only partially to reduce secondary cracking and isomerization reactions and the formation of by-products such as diolefins and aromatic hydrocarbons.
At the end of the dehydrogenation reaction the stream essentially consists of linear mono-olefins (10-20% by weight), small quantities of non-linear olefins (generally less than 3% by weight), aromatic by-products (0.1-0.7% by weight), diolefins (0.5-3% by weight) and non-reacted n-paraffins.
During the above step (a) for the dehydrogenation of n-paraffins to olefins, significant quantities of diolefins are therefore formed. Their extent is entirely linked to the conversion and conditions under which the dehydrogenation is carried out. Their presence subsequently leads, during the alkylation step (c), to the formation of impurities such as, for example, tetralines in the alkylbenzenes and heavy, high-boiling products such as, for example, diphenylalkanes, tetralines and indanes with a higher molecular weight.
In order to reduce the disadvantages mentioned above, a selective hydrogenation of the diolefins, step (b) of the process of the present invention, to mono-olefins, is consequently effected.
Step (b) for the selective hydrogenation of the diolefins is carried out on a fixed-bed catalyst based on nickel supported on alumina, partially poisoned, or on a catalyst based on noble metals such as palladium supported on carbon or alumina. In any case, the ratio H
2
/diolefins is maintained at more than 1 and, generally, between 1.1 and 5, depending on the catalyst used and the process conditions selected.
The above step (b) can be carried out at a temperature ranging from 50 to 250° C. depending on the type of catalyst used and at a pressure ranging from 1 to 20 kg/cm
2
, whereas the space velocity of the flow can vary from 0.5 to 20 h
−1
. In this way, conversion yields of the diolefins of up to 100% are obtained with a selectivity of up to 90%.
The alkylation reaction, step (c) of the process of the present invention, is carried out after mixing the reagents with the alkylation catalyst. Any aromatic hydrocarbon can be used in the alkylation process of the present invention even if benzene and toluene are preferred.
Catalysts which can be used for the purpose are those traditionally used in this type of reaction, for example HF or A
Cozzi Pierluigi
Ontano Rosanna
Radici Pierino
Zatta Agostino
Condea Augusta S.P.A.
Dang Thuan D.
Griffin Walter D.
Oblon, Spivak, McClelland, Maier & Neustadt, P. C.
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