Process for purifying naphthalene by selective...

Details Process for purifying naphthalene by selective... Process for purifying naphthalene by selective...

1998-01-16

2001-04-17

Griffin, Walter D. (Department: 1764)

Mineral oils: processes and products

Refining

Sulfur removal

C208S212000, C208S2160PP, C208S25400R, C208S255000

Reexamination Certificate

active

06217750

ABSTRACT:

The invention concerns a process for purifying a naphthalene cut, generally from coal derivatives.
Coal tar distillation products include a naphthalene fraction containing essentially naphthalene but the presence of sulphur-containing products such as benzothiophene, nitrogen-containing products such as quinoline, oxygen-containing products such as phenolic derivatives, and also unsaturated hydrocarbons such as indene can also be detected, thus being a non limiting list.
Two types of naphthalene currently exist on the market. The first type is a “technical naphthalene” with a purity of over 98%. The second type is a “pure naphthalene” mainly used for synthesis and manufacture of insecticides (“Naphthalin”). Since the latter product must be completely colourless, it has to be pure. It must be over 99.95% pure and the sulphur content must be practically zero (a few ppm by weight).
The naphthalene cut which can be used in the process of the invention can contain more than 50% by weight of naphthalene, advantageously more than 75%, and most often more than 85% by weight. It also contains sulphur-containing compounds representing up to 5% by weight of sulphur but usually less than 1%, nitrogen-containing compounds in the form of quinoline, for example (up to 1% by weight), mono-olefins such as indene (up to 1% by weight but usually less than 0.5%) and oxygen-containing compounds such as phenols (up to 1% by weight but usually less than 0.5% by weight).
The present invention concerns a process for purifying a naphthalene cut, in particular a naphthalene cut from coal derivatives, to obtain a “technical naphthalene”.
Some processes for treating naphthalene cuts already exist.
Japanese patent application JP-05-017,376 describes a process for low pressure (0-20 bars) hydrogenation of a naphthalene cut in which impurities are hydrogenated as well as a portion of the naphthalene and a portion of the tetralin formed.
JP-05-085,960 describes a process for purifying a naphthalene cut comprising a first hydrogenation step in the liquid phase at a low or medium pressure (0-20 bars), at 100-300° C., using a catalyst selected from the group formed by Ni—Co—Mo, platinum on coal, Pt—Ni—Mo, Pd-alumina, or CoMo-alumina catalysts. In a second step, the effluent obtained is degassed to eliminate H
2
S, NH
3
and ethylbenzene. Then the effluent is washed with an inorganic acid (third step) and then separated (fourth step). The effluent obtained is dehydrated by azeotropic distillation with ethylbenzene (fifth step). The residual impurities are adsorbed (sixth step) on clay. The effluent can then be distilled (seventh step) and pressed (eighth step) to obtain purified naphthalene.
We have sought a process for treating naphthalene cuts which is simple to carry out, in contrast to JP-05-085,960 which requires numerous treatment steps, and which can produce higher naphthalene yields than in JP-05-017,376.
The proposed invention is based on the use of a particular catalyst which can carry out prior deep hydrotreatment of the feed to eliminate the major portion of the sulphur-containing, nitrogen-containing, oxygen-containing and olefinic impurities in a single step while limiting hydrogenation (reduction in the quantity of tetralin and decalin) which constitutes selective hydrotreatment so as to be able to separate purified naphthalene simply by crystallisation. More precisely, the invention provides a process for treating a naphthalene cut containing sulphur-containing and/or nitrogen-containing and/or oxygen-containing and/or olefinic impurities in which, in a first step, the naphthalene cut is brought into contact in the presence of hydrogen with a catalyst containing at least one matrix, at least one group VIII element and at least one group VI element. It is characterized in that the catalyst used in the first step contains 5-40% by weight of oxides of metals from groups VIII and VI, with a ratio of the oxide of the group VI metal to the oxide of the group VIII metal which is in the range 1.25 to 20 by weight, said catalyst having a BET specific surface area of at most 220 m
2
/g, a pore volume which is in the range 0.35 to 0.7 ml/g and an average pore diameter of at least 10 nm, and in that the naphthalene cut is brought into contact with the catalyst at a temperature of 150-325° C., at a total pressure of 0.1 MPa-0.9 MPa, at an hourly space velocity of 0.05-10 h
−1
and with a hydrogen
aphthalene molar ratio which is in the range 0.1 to 1.3, such that the tetralin yield is less than 10% by weight and in a second step, at least a portion of the effluent from the first step is separated from H
2
S, NH
3
and water, and in a third step, at least a portion of the effluent from the second step undergoes treatment to separate naphthalene and tetralin from the effluent. If the tetralin is recycled, the tetralin
aphthalene ratio is in the range 0.005 to 0.08 by weight.
The process comprises three successive steps. The first step is intended to reduce the sulphur content present in the form of sulphur-containing molecules to the desired value. The sulphur-containing molecules are transformed into desulphurised molecules and H
2
S. In this first step, nitrogen-containing molecules are mainly transformed into denitrogenated molecules and NH
3
. It can also hydrogenate olefinic molecules and dehydroxylate molecules containing an OH group. During this step, the catalyst and the operating conditions are selected so as to carry out deep hydrotreatment while limiting hydrogenation of naphthalene to tetralin. The tetralin can be considered to be a by-product of the reaction which is thus unwanted. Practically no tetralin is formed (generally less than 5% by weight in the effluents, preferably less than 3%). The term “deep hydrotreatment” means at least 70% desulphurisation, preferably at least 90%, and more preferably at least 98% desulphurisation, at least 50% denitrogenation, preferably at least 80% denitrogenation, at least 80% olefin hydrogenation, preferably at least 95% hydrogenation, and at least 75% dehydroxylation, preferably at least 90% dehydroxylation.
The total pressure is in the range 0.1 MPa (1 bar) to 0.9 MPa (9 bars), preferably in the range 0.2 MPa (2 bars) to 0.9 MPa (9 bars)) and more preferably in the range 0.2 MPa (2 bars) to 0.8 MPa (8 bars). The reaction temperature is in the range 150° C. to 325° C., preferably 200° C. to 320° C., more preferably in the range 220° C. to 300° C. The hourly space velocity (HSV) is in the range 0.05 h
−1
to 10 h
−1
, preferably in the range 0.1 h
−1
to 5 h
−1
and more preferably in the range 0.15 h
−1
to 2 h
−1
. The hydrogen
aphthalene molar ratio at the reactor inlet is in the range 0.1 to 1.3, more preferably less than 1.
The catalyst used for the first hydrotreatment step is a catalyst containing at least one matrix, advantageously based on alumina, preferably containing no zeolite, and at least one metal having a hydro-dehydrogenating function. The matrix can also comprise a silica-alumina, boron oxide, magnesia, zirconia, clay or a combination of these oxides. The hydro-dehydrogenating function is assured by a combination of at least one group VIII metal or metal compound, in particular nickel or cobalt, and at least one metal or metal compound (in particular molybdenum or tungsten) from group VI of the periodic table. The total concentration of oxides of group VI and VIII metals is in the range 5% to 40% by weight, preferably in the range 7% to 30% by weight, and the weight ratio (expressed as the metallic oxide of the metal) (or metals) from group VI over the group VIII metal (or metals) is in the range 1.25 to 20, preferably in the range 2 to 10. Further, the catalyst can preferably contain phosphorous. The phosphorous content, expressed as the concentration of phosphorous oxide P
2
O
5
, is below 15% by weight, preferably less than 10% by weight.
The preferred catalyst is of the type CoMo deposited on alumina.
The catalyst has very particular characteristics:
The BET specific surface area is at most 22

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