Thermal Dehydrogenation

Under pyrolytic conditions at temperatures above 300°C, generally within 500-800°C, the pyrolysis reaction forms alkenes by carbon-hydrogen bond scissions. An early experiment, where propane was heated to 575°C for 4 min in a silica flask, yielded propylene by dehydrogenation [Eqs. (2.19)-(2.21)] at a somewhat slower rate than it yielded methane and ethylene by cracking 54 

Similar observations were made in a study of pyrolysis53 in the temperature range 700-850°C. Of the propane that decomposed, 60% did so by cracking and 35-39% by dehydrogenation. 

Dehydrogenation is the chief reaction when ethane is pyrolyzed 53 The reaction begins at about 485°C and is quite rapid at 700°C. At this temperature about 90% of the reacting ethane is converted to ethylene and hydrogen. Minor byproducts include 1,3-butadiene and traces of liquid products. 

The major industrial source of ethylene and propylene is the pyrolysis (thermal cracking) of hydrocarbons.137-139 Since there is an increase in the number of moles during cracking, low partial pressure favors alkene formation. Pyrolysis, therefore, is carried out in the presence of steam (steam cracking), which also reduces coke formation. Cracking temperature and residence time are used to control product distribution. 

Because of their ready availability, natural-gas liquids (mixtures of mainly ethane, propane, and butanes) are used in the manufacture of ethylene and propylene in the United States.140 141 In Europe and Japan the main feedstock is naphtha. Changing economic conditions have led to the development of processes using heavy oil for olefin production.138 142 

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Tetrahydronaphthalene dehydrogenates to naphthalene at 200—300°C in the presence of a catalyst thermal dehydrogenation takes place at ca 450°C and is accompanied by cracking to compounds, such as toluene and xylene. 

The sodium formate process is comprised of six steps (/) the manufacture of sodium formate from carbon monoxide and sodium hydroxide, (2) manufacture of sodium oxalate by thermal dehydrogenation of sodium formate at 360°C, (J) manufacture of calcium oxalate (slurry), (4) recovery of sodium hydroxide, (5) decomposition of calcium oxalate where gypsum is produced as a by-product, and (6) purification of cmde oxahc acid. This process is no longer economical in the leading industrial countries. UBE Industries (Japan), for instance, once employed this process, but has been operating the newest diaLkyl oxalate process since 1978. The sodium formate process is, however, still used in China. 

The conjugated diene 1,3-butadiene is used in the manufacture of synthetic rubber and is prepared on an industrial scale in vast quantities. Production in the United States is currently 4X10 Ib/yearc One industrial process is similar- to that used for the preparation of ethylene In the presence of a suitable catalyst, butane undergoes thermal dehydrogenation to yield 1,3-butadiene. 

Tetralin has been shown to undergo thermal dehydrogenation to naphthalene and rearrangement to methyl indan in either the absence or presence of free radical acceptors [ 1, 2]. The presence of free radical acceptors usually accelerates the rearrangement reaction. Even with alkylated Tetralins>... 

If alkyl groups having (3-hydrogens are present on platinum cis to an open site, (3-H-elimination will indeed occur, reversibly sometimes, and it can occur both from Pt(II) and Pt(IV) (52,97,213-219). Catalytic dehydrogenation of an alkane using a soluble platinum complex has been reported in an early study on acceptorless thermal dehydrogenation. At 151 °C, cyclooctane was catalytically dehydrogenated (up to 10 turnovers)... 

Diphenyl, however, is prepared by thermal dehydrogenation of benzene. (A current of benzene vapour is passed through a red-hot iron tube.)... 

Table 1.5 The temperatures of peaks connected with various processes during thermal dehydrogenation of as-received LiAlH investigated by DSC...

Diels-Alder reactions of bis(trimethylsilyl)acetylene.1 A catalyst obtained from TiCl4 and (C2H5)2A1C1 (1 20) effects Diels-Alder reactions of this acetylene with butadiene and methyl-substituted derivatives to form l,2-bis(trimethylsilyl)-cyclohexa- 1,4-dienes in 70-78% yield (equation I). The yield is low (15%) only when R, R4 = CH3,R2,R3 = H because of polymerization of the diene. The products undergo thermal dehydrogenation at 240° to form l,2-bis(trimethylsilyl)ben-zenes in almost quantitative yield. This cycloaddition has been effected in low yield with an iron-based catalyst. 

The author [6] has shown that the secondary process represents a new type of reaction— conjugated oxidative dehydrogenation, and contrary to usual thermal dehydrogenation [8], may be performed at lower temperatures and give higher yields and selectivity [9],... 

In chapter 8 a new project has been formulated for the use of membrane reactors for the thermal dehydrogenation of H2S. Compared to the conventional Claus process, the application of a membrane reactor in the thermal H2S might have some large advantages. 

In this chapter, a basis is provided for a project proposal on thermal dehydrogenation of H2S in a membrane reactor. Such a proposal uses the results of this thesis as a starting point. 

After the absorber/stripper unit, in conventional operations the pure H2S is fed to a Claus unit where the H2S is converted to elemental sulphur and H2O. The Claus unit can be equipped with an after-treatment to enhance conversions. Another method to decompose H2S to less harmful compounds is the thermal dehydrogenation of H2S to hydrogen and sulphur. Both processes will be treated in detail in the remainder of this chapter. 

Recovery of the hydrogen, contained in the H2S. In the Claus process this is impossible, because all hydrogen is converted to water, but in the thermal dehydrogenation of H2S, hydrogen is produced according to [6] ... 

As shown in chapter 6, silica membranes can nowadays be prepared at temperatures as high as 825°C while state-of-the-art steam-stable y-alumina membranes are prepared at 1000°C. This enables the use of such membranes in the high-temperature range needed for thermal dehydrogenation of H2S under conditions used in literature. The permselectivity of H2/CO2 of the silica membranes prepared at 825°C was >100 with a hydrogen permeance of... 

Although silica membranes have not been studied for a process like the thermal dehydrogenation of H2S, we believe, that silica membranes make a good change to be highly suitable for the process, based on the knowledge obtained on membrane behaviour under steam reforming... 

On the basis of the results obtained in the project in chapter 8 a concept proposal is provided for the use of the developed membranes for the thermal dehydrogenation of H2S in a membrane reactor. 

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