Dehydrogenation of a dehydrogenatable hydrocarbon using alkali metal liquid to remove hydrogen and supply heat of reaction

Abstract: Dehydrogenatable hydrocarbons are dehydrogenated by contacting the hydrocarbon with a liquid comprising an alkali metal in a dehydrogenation zone to produce a dehydrogenated hydrocarbon and an alkali metal halide. The resulting alkali metal hydride is removed from the zone and heated to produce a heated liquid alkali metal and hydrogen. The heated liquid alkali metal is recycled to the dehydrogenation zone to provide heat and elemental metal for hydrogen removal and the dehydrogenated hydrocarbon is recovered.

Description: The field of art to which this invention pertains is the production of olefinic hydrocarbons by the dehydrogenation of dehydrogenatable hydrocarbons. This invention relates more specifically to a process for the dehydrogenation of a dehydrogenatable hydrocarbon by contacting the dehydrogenatable hydrocarbon with a liquid comprising an alkali metal in a dehydrogenation zone to produce a dehydrogenated hydrocarbon and an alkali metal hydride. The resulting alkali metal hydride is heated to produce a heated liquid alkali metal and hydrogen. The heated liquid alkali metal is recycled to the dehydrogenation zone to provide heat and to remove hydrogen. There is a steadily increasing demand for technology which is capable of producing olefins from dehydrogenatable hydrocarbons containing from 2 to 18 carbon atoms. Dehydrogenating hydrocarbons is an important commercial hydrocarbon conversion process because of the great demand for dehydrogenated hydrocarbons for the manufacture of various chemical products such as detergents, high octane motor fuels, pharmaceutical products, plastics, synthetic rubbers, polymerization and other products well known to those skilled in the art. Processes for the dehydrogenation of light acyclic hydrocarbons are well known to those skilled in the hydrocarbon conversion arts. US-A-4675465 discloses a process for the dehydrogenation of hydrocarbons in which a thermodynamically unfavourable dehydrogenation reaction is driven by means of a hydrogen-dissolving metal, alloy, or intermetallic compound having a negative standard free energy of formation for the material hydride MHy. In the specific example, the dehydrogenation catalyst is platinum/alumina which is used as a mixture with titanium as the hydrogen-dissolving metal. EP-A-0332479 discloses a process for the cyclodehydrogenation of polyalkyl 1,1-dinaphthalenes to form corresponding polyalkylperylenes. The reaction uses alkali metals for the dehydrogenation. In the embodiment specifically described, the alkyl 1,1'-dinaphthalene is heated under reflux with a suspension of the alkali metal (e.g. lithium) in a solvent (e.g. tetrahydrofuran), followed by cooling, blowing oxygen through the mixture, and filtering off the excess alkali metal. Other prior art processes for the dehydrogenation of paraffins suffered under several disadvantages including poor olefin product yields and poor catalyst life caused by the relatively high catalyst inlet temperature required to supply the essential heat of reaction and the relatively high cost of the required multi-stage reactors and their attendant interheaters. The present invention provides a process for the dehydrogenation of a dehydrogenatable hydrocarbon by contacting said dehydrogenatable hydrocarbon with a hydride-forming metal in a dehydrogenation zone at dehydrogenation conditions effective to produce a dehydrogenated hydrocarbon and metal hydride. According to this invention, the hydride forming metal is an alkali metal, and (a) said alkali metal in liquid form is contacted with said dehydrogenatable hydrocarbon in said dehydrogenation zone to produce dehydrogenated hydrocarbon and alkali metal hydride; (b) at least a portion of said alkali metal hydride is removed from said dehydrogenation zone and heated to produce a heated liquid alkali metal and hydrogen; (c) at least a portion of said heated liquid alkali metal [9] is recycled to said dehydrogenation zone; and (d) said dehydrogenated hydrocarbon is recovered. The present invention provides a convenient and economical process for the production of olefinic hydrocarbons. Important elements of the process are the facile removal of hydrogen from the dehydrogenation zone which minimizes chemical equilibrium constraints and simplifies the recovery of the resulting olefinic hydrocarbons and the supply of heat to the dehydrogenation zone without the need to heat the dehydrogenatable hydrocarbon reactants to reaction temperature prior to entering the reaction zone. One preferred embodiment of the present invention may be characterized as a process for the dehydrogenation of a paraffin hydrocarbon having 2 to 18 carbon atoms which process comprises: (a) contacting the paraffin hydrocarbon with a liquid comprising an alkali metal in a dehydrogenation zone at dehydrogenation conditions effective to produce an olefin hydrocarbon and an alkali metal hydride; (b) removing and heating at least a portion of the alkali metal hydride from the dehydrogenation zone to produce a heated liquid alkali metal and hydrogen; (c) recycling at least a portion of the heated liquid alkali metal to the dehydrogenation zone to provide the heat of reaction and to remove hdyrogen; and (d) recovering the olefin hydrocarbon. The drawing is a simplified process flow diagram of a preferred embodiment of the present invention. The present invention provides a process for the dehydrogenation of a dehydrogenatable hydrocarbon. The dehydrogenatable hydrocarbon is contacted with a liquid comprising an alkali metal in a dehydrogenation zone at dehydrogenation conditions to produce a dehydrogenated hydrocarbon and an alkali metal hydride. The dehydrogenated hydrocarbon is preferably removed from the dehydrogenation zone in gaseous phase and the alkali metal hydride is removed from the dehydrogenation zone in a liquid phase. The alkali metal hydride is subsequently heated to remove hydrogen thereby providing heated metal which may be recycled to serve as a hydrogen sponge and to provide heat for the endothermic dehydrogenation reaction in the dehydrogenation zone. Paraffin dehydrogenation is an endothermic reaction and the heat of reaction for the formation of a mono-olefin is approximately 30 kilocalories/gram mol for a feed that may vary from C 2 (ethane) to C 18 paraffins. Therefore, when olefins are produced from paraffins, the heat of reaction must be supplied from an external source. In accordance with the present invention, the dehydrogenatable hydrocarbon charge stock may contain from 2 carbon atoms to 18 carbon atoms. Representative members of this class are ethane, propane, butane, pentane, hexane, heptane and mixtures thereof. A particularly important class of charge stocks include propane, butane, pentane and mixtures thereof and which are readily prepared by the fractionation of relatively low boiling point hydrocarbon fractions. The dehydrogenatable hydrocarbon feedstock is introduced into a dehydrogenation zone and contacted with a liquid comprising an alkali metal at dehydrogenation conditions effective to produce a dehydrogenated hydrocarbon and an alkali metal hydride. Preferred dehydrogenation conditions include a pressure from 101 to 3549 kPa (atmospheric to 500 psig), a temperature from 200 to 710°C (392°F to 1310°F), and a metal to hydrocarbon mole ratio from 1 to 20. A resulting hydrocarbon stream containing olefin hydrocarbons and typically unreacted hydrocarbons is removed from the dehydrogenation zone and recovered. In a preferred embodiment, the resulting hydrocarbon stream is separated to recover the olefin hydrocarbons and to produce a stream of unreacted hydrocarbons which may then be recycled to the dehydrogenation zone to produce additional olefin hydrocarbons. A liquid stream containing alkali metal hydride is removed from the dehydrogenation zone and is heated to produce a heated liquid alkali metal stream and hydrogen. In order to regenerate the alkali metal hydride stream, it is preferably heated in a heating zone to a temperature in the range from 400°C to 850°C (752°F to 1562°F). The circulation rate of the heated liquid alkali metal stream is preferably selected to ensure that the required heat of reaction is subsequently supplied to the dehydrogenation zone to maintain the desired dehydrogenation reaction temperature. In accordance with the present invention, the alkali metal may be selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and admixtures thereof. In one embodiment of the present invention, the circulating liquid stream containing alkali metal and/or alkali metal hydride may be transferred to and from the dehydrogenation zone and the heating zone by means of pumps, gravity or thermal siphon. In the drawing, the process of the present invention is illustrated by means of a simplified flow diagram in which such details as total number of reaction zones, heating zones, pumps, instrumentation, heat-exchange and heat-recovery circuits, compressors and similar hardware have been deleted as being non-essential to an understanding of the involved invention. With reference now to the drawing, a dehydrogenatable hydrocarbon is introduced into the dehydrogenation zone 2 via conduit 1 and contacted with a heated liquid stream containing alkali metal which is introduced via conduit 9 into dehydrogenation zone 2. A resulting gaseous hydrocarbon stream containing olefin hydrocarbons is removed from dehydrogenation zone 2 via conduit 3 and recovered. A liquid stream containing alkali metal hydride is removed from dehydrogenation zone 2 via conduit 4 and introduced into heat exchanger 5. A heated effluent from heat exchanger 5 is transported via conduit 6 and introduced into vapor-liquid separator 7. A gaseous stream containing molecular hydrogen is removed from vapor-liquid separator 7 via conduit 8. A liquid stream containing alkali metal is removed from vapor-liquid separator 7 via conduit 9 and introduced into dehydrogenation zone 2 as described hereinabove. EXAMPLE A pilot plant autoclave was charged with lithium and heated to 500°C (932° F). A pure isobutane feedstock was introduced below the surface of the liquid lithium and the pressure in the autoclave was maintained at 446 kPa (50 psig). A gaseous product was continuously withdrawn from the autoclave and analyzed. The characteristics of the gaseous product are presented in Table 1. PRODUCT ANALYSIS Isobutane, weight percent 94.5 Propane, weight percent 0.5 Isobutylene, weight percent 4.5 Hydrogen, weight percent 0 From Table 1, it can readily be seen that when isobutane is contacted with liquid lithium at a temperature of 500°C and a pressure of 446 kPa, 4.5 weight percent isobutylene is produced. Moreover, no hydrogen is seen in the gaseous product as it remains in the liquid as lithium hydride. Therefore, the conclusion is drawn that two atoms of lithium react with a molecule of isobutane to produce two molecules of lithium hydride and one molecule of isobutylene.

Claims: A process for the dehydrogenation of a dehydrogenatable hydrocarbon by contacting said dehydrogenatable hydrocarbon [1] with a hydride-forming metal in a dehydrogenation zone [2] at dehydrogenation conditions effective to produce a dehydrogenated hydrocarbon and metal hydride; characterized in that said hydride forming metal is an alkali metal, and (a) said alkali metal in liquid form [9] is contacted with said dehydrogenatable hydrocarbon in said dehydrogenation zone [2] to produce dehydrogenated hydrocarbon and alkali metal hydride; (b) at least a portion of said alkali metal hydride is removed [4] from said dehydrogenation zone [2] and heated [5] to produce a heated liquid alkali metal [9] and hydrogen [8]; (c) at least a portion of said heated liquid alkali metal [9] is recycled to said dehydrogenation zone [2]; and (d) said dehydrogenated hydrocarbon [3] is recovered. A process according to Claim 1 characterized in that said alkali metal is selected from lithium, sodium, potassium, rubidium and cesium. A process according to Claim 1 or 2 characterized in that said dehydrogenation conditions include a pressure from 101 to 3549 kPa (atmospheric to 500 psig), a temperature from 200 to 710°C (392° to 1310°F) and a hydrocarbon to alkali metal mol ratio from 1 to 20. A process according to any one of Claims 1 to 3 characterized in that said alkali metal hydride is heated in step (b) to a temperature in the range from 400 to 850°C (752° to 1562°F) to produce sad reated liquid alkali metal and said hydrogen. A process according to any one of Claims 1 to 4 characterized in that said dehydrogenatable hydrocarbon is a paraffin hydrocarbon having from 2 to 28 carbon atoms.

Inventors: OROSKAR ANIL R, OROSKAR, ANIL R.
Applicants: UOP INC, UOP
Application year: 1995
Main classification: C07C5
IPC codes: C07C5