Patent Application
20080039671
The appended claims set forth those novel features which characterize the present invention. The present invention itself, as well as advantages thereof, may best be understood, however, by reference to the following brief description of preferred embodiments taken in conjunction with the annexed drawing, in which:
The FIGURE depicts the fractional recovery of propylene to the liquid phase following partial condensation of high-RHI and low-RHI propane dehydrogenation reactor effluents.
This invention relates to improving the downstream processability of the effluent of an olefins-producing reactor. In all commercial olefin-producing technologies, including steam cracking and alkane dehydrogenation, a mixture of products is formed so that the desired olefin product must be recovered from the reactor effluent and purified from other components which exist in the reactor effluent. Essentially all of the commercial olefin recovery and purification systems in use today involve at least both the compression and partial condensation of the reactor effluent. This invention identifies a particular reactor effluent composition range within which the energy required for the subsequent compression and partial condensation of the reactor effluent is reduced. This reactor effluent composition range is quantified by the “Relative Hydrogen Index” of the effluent, which is defined as a ratio of a deference between the total flow of hydrogen atoms and the flow of hydrogen atoms in the form of water in the effluent stream to the same difference for the feedstream. The Relative Hydrogen Index is represented by the equation:
RHI=(HT−HW)E/(HT−HW)F
where RHI is the Relative Hydrogen Index, the E subscript refers to the flow reactor effluent before any cooling or other processing, the F subscript refers to the reactor feed, HT is the total flow of hydrogen atoms in the stream, and HW is the flow of hydrogen atoms in the form of water vapor in the stream. For process for chemical conversion of volatile organic compounds to value added products according to the invention, membrane reactor effluents have an RHI of less than 1.0, and beneficially less than 0.75, by which savings of a significant amounts of energy are obtained in subsequent product recovery steps.
Such low-RHI reactor effluents can be produced by membrane reactors that transport oxygen ions, hydrogen ions, or both oxygen and hydrogen ions. Examples are given which demonstrate that current technologies which convert alkanes to olefins, including steam cracking and dehydrogenation, produce effluents with an RHI equal to or greater than 1.0, while such membrane reactors can produce effluents with an RHI of less than 1.0.
As an example, consider a pure dehydrogenation reaction. With this reaction, for every mole of carbon-carbon double bonds produced in the reactor from alkanes, one mole of hydrogen is produced. In addition, for every mole of carbon-carbon triple bonds produced in the reactor from alkanes, two moles of hydrogen are produced. In this case the nature of the reaction stoichiometry requires that the RHI of the reactor effluent to be at or near 1.0. Slight departures from an RHI of 1.0 in particular instances could be attributable to, for example, the formation of small amounts of coke in the dehydrogenation reactor.
The Relative Hydrogen Index can be measured for any olefin-producing reactor. For example, the RHI of the effluent of a steam-cracking furnace, the most common type of reactor for the commercial production of light olefins, can be measured. We have found that, over a wide range of reactor conditions, conversion levels, and feedstocks the RHI for steam cracking is relatively constant and very near 1.0.
A reactor effluent with a significantly different RHI is produced by membrane-based olefin production reactors, in particular those which transport oxygen ions, hydrogen ions, or both oxygen and hydrogen ions. In the case of an oxygen-transport membrane, oxygen transported to the reaction zone is allowed to react preferentially with hydrogen within the reaction mix, converting it to water and thereby removing it as dihydrogen from the reactor effluent. In the case of a hydrogen-transport membrane, hydrogen is transported from the reaction mix to the other side of the membrane, thereby removing it from the reactor effluent. In both cases the removal of dihydrogen from the reactor effluent will reduce the RHI of the reactor effluent to a value below 1.0.
A similar hydrogen removal effect can be obtained if molecular oxygen is added to the reactor feed so that hydrogen produced in the production of alkenes is removed through reaction with oxygen to form water. Such a process is known as oxidative dehydrogenation. While oxidative dehydrogenation offers the same hydrogen removal benefits as the membrane reactors described above, the reaction of oxygen is not completely selective to hydrogen removal. As a result some of the molecular oxygen reacts with alkane feed or alkene products to form carbon oxides such as carbon monoxide or carbon dioxide. Production of such carbon oxides is detrimental in three ways. First, non-selective oxidation converts at least some of the feed to relatively low-value carbon oxide products. Second, production of carbon oxides complicates the olefin purification process in that extra steps, such as carbon dioxide removal, may be required. Finally, the presence of carbon monoxide in particular is detrimental to the chilling and partial condensation of the reactor effluent in a manner similar to hydrogen itself, the detrimental effect of which is discussed hereunder.
Moreover, the mixing of oxygen with the hydrocarbon feed and subsequent heating of the mixture entails significant safety concerns since oxygen and hydrocarbons can form flammable and explosive mixtures. This concern is relevant in normal plant operation, but particularly so during upset conditions where the flowrates of oxygen and hydrocarbon may be difficult to control. For these reasons it would be desirable to achieve the hydrogen removal effect provided by oxidative dehydrogenation without resorting to the addition of molecular oxygen to the reactor feed. This desirable hydrogen removal effect can be achieved without oxygen addition through the use of the membrane reactors described above.
The primary benefit of a reactor effluent with a lower RHI is that it requires less energy to recover a purified olefin product from it than a reactor effluent with a higher RHI. This benefit is manifested both in the compression of the reactor effluent, and in the chilling and partial condensation of the compressed reactor effluent.
Other things being equal, for a given molar flow of the desired olefin product, a reactor effluent with a lower RHI will have less hydrogen and therefore a higher molecular weight than a reactor effluent with a higher RHI. As is well known to those skilled in the art of hydrocarbon processing, a gas with a higher molecular weight has a higher compressibility than a gas with a lower molecular weight. Put another way, for a given molar flow of the desired olefin product, a reactor effluent with a lower RHI will require less compressor power than a reactor effluent with a higher RHI. In addition, if in addition to the olefin flow rate the reactor effluent pressure and temperature are also constant, a reactor effluent with lower RHI will have a lower volumetric flow rate than a reactor effluent with higher RHI. In such a case the compressor will be smaller for the low-RHI reactor effluent than the high-RHI reactor effluent, resulting in lower compressor capital cost for the low-RHI reactor effluent.
In addition to the benefits arising when the reactor effluent is compressed, a relatively lower RHI reactor effluent will save energy when the compressed reactor effluent is chilled and partially condensed. The presence of light gases, such as dihydrogen and oxides of carbon, decreases the temperature at which liquid can condense. It is believed that a presence of dihydrogen lowers the temperature at which a hydrocarbon vapor condenses in the same way that it lowers the temperature at which a hydrocarbon liquid vaporizes (See U.S. Pat. No. 5,711,919 and EP 840,079 where dihydrogen was added to a vessel in which liquid hydrocarbon was being vaporized in order to reduce the temperature of the hydrocarbon vaporization.)
From this is becomes apparent that a lower-RHI reactor effluent, with less hydrogen present, will condense at a higher temperature than a higher-RHI reactor effluent. Put another way, at a given reactor effluent pressure a lower-RHI reactor effluent will begin to condense at a higher temperature and will have a greater fraction condensed at any given temperature than a higher-RHI reactor effluent. It is well known to those skilled in the art of olefin recovery that relatively more energy is required to provide refrigeration at a lower temperature than at a higher temperature. Therefore an olefin-rich product can be obtained from a lower-RHI reactor effluent at a higher temperature and therefore with less energy than from a higher-RHI reactor effluent.
Those skilled in the art will recognize that there is the opportunity to optimize these two benefits of a lower-RHI reactor effluent gas. In particular, compressing the reactor effluent to a higher pressure will both increase the required compressor power and reduce the required refrigeration energy needed in the subsequent chilling step or steps. Alternately, the reactor effluent can be compressed to a lower pressure, consequently saving on compressor power while increasing the required refrigeration energy. The optimum combination of compression and refrigeration power will be determined by the cost of energy, the capital cost of the compressors, drivers, and refrigeration equipment, and other factors. It is a benefit of this invention that the processing of a lower-RHI reactor effluent will have lower total energy (defined as the sum of the reactor effluent compressor energy and the refrigeration energy) than the processing of a higher-RHI reactor effluent.
The following examples will serve to illustrate certain specific embodiments of the herein-disclosed invention. These examples should not, however, be construed as limiting the scope of the novel invention, as there are many variations which may be made thereon without departing from the spirit of the disclosed invention, as those of skill in the art will recognize.
This invention is described by way of a number of relevant examples. Comparative examples show that conventional commercial olefins-producing technologies produce reactor effluent compositions with an RHI equal to or greater than 1.0. Examples of the invention demonstrate that suitable membrane-based reactors produce reactor effluent compositions having values of RHI less than 1.0. These Examples of the invention demonstrate that reactor effluents with lower RHI values advantageously require less energy to compress effluent and partially condense value added products than for the reactor effluents with higher RHI values.
This comparative example demonstrates that the effluent from a conventional catalytic dehydrogenation reactor has an RHI value of at least 1.0, which values exceeded RHI values of this invention. Data was taken from U.S. Pat. No. 6,313,063 that demonstrated operation of conventional, catalytic, propane dehydrogenation at RHI values greater than 1.0. A magnesium hydrotalcite catalyst containing platinum (Pt), and tin (Sn) was evaluated under dehydrogenation conditions of 1 bar absolute pressure, 600° C., Gas Hourly Space Velocity of 2100 hr-1 with a feedstock consisting of 35 NmL/min propane, 5 NmL/min H2, 25 NmL/min N2, and 41 NmL/min steam (see Example 7). This resulted in a RHI of 1.0 for this example, and a propane-to-dihydrogen mol ratio of 7. The data given in the patent shows that this catalyst is stable over 25 hrs under these conditions resulting in 58 percent propane conversion at greater than 93 percent selectivity. While this example is typical for this particular catalytic dehydrogenation technology, propane-to-dihydrogen ratios of between 1 and 10 have been used in other catalytic, propane dehydrogenation technologies. In all cases however, the RHI was greater than or equal to 1.0, which values exceeded RHI values of this invention.
This example demonstrates that the effluent from a conventional stream cracking furnace had an RHI of value of at least 1.0, and that this was true for several furnace feedstocks of commercial interest. In theory the RHI of a commercial steam cracking furnace effluent can be measured directly by sampling both the furnace feed and the furnace effluent. However, in practical terms is difficult both because the furnace effluent is very hot and difficult to handle, and because the furnace effluent composition is very complex and difficult to analyze accurately, particularly the heavier hydrocarbon components. The data for this example, therefore, is taken from the results of an accurate proprietary kinetic model of a commercial steam-cracking furnace. This model rigorously models the free radical reactions occurring in the cracking furnace and provides detailed furnace effluent compositions that are readily analyzed. This model has been validated against numerous commercial furnace tests and can therefore be considered a reliable predictor of actual furnace performance.
The model was used to simulate the performance and yields of steam cracking furnaces processing pure ethane, pure naphtha, and pure propane. The steam-to-hydrocarbon mass ratios were 0.28, 0.5, and 0.3 in the feeds to the ethane, naphtha, and propane furnaces, respectively. Furnace feed and effluent compositions are given in Table I, along with the calculated molar hydrogen flows in these streams. The molar atomic hydrogen flows were calculated for each component through the use of the formula
MolH=(MassC/MwtC)(MolH/MolC)
Where the term MolH is the molar atomic flow of hydrogen, MassC is the mass flow of the component, the term MwtC is the molecular weight of the component, and the ratio MolH/MolC is the number of hydrogen atoms in the component molecule. Over 100 distinct and lumped components were tracked by the model, and these are simplified to the categories shown in Table I.
Using the formula for RHI given above, the RHI for the effluents from the ethane, naphtha, and propane furnaces are 1.000, 1.003, and 1.001, respectively. These are all very near to 1.0, demonstrating that over a wide range of feedstocks, conventional steam cracking furnaces produce an effluent with an RHI of values of at least 1.0, which values exceeded RHI values of this invention.
This example demonstrates that effluent from membrane-based reactors of the invention have values of RHI that are significantly less than 1.0.
A multiphasic, solid state electron, hydrogen, and oxygen transport membrane was fabricated from cerium gadolinium oxide and palladium (CGO/Pd) using the following method:
a) A batch of cerium gadolinium oxide powder, obtained from Rhodia, was heated in air to 1000° C. and held at that temperature for one hour. The powder was then sifted with a 60-mesh filter.
b) 6.6 g of the sifted cerium gadolinium oxide powder was mixed with 5.5 g of palladium flake, obtained from Degussa Corporation, for 30 minutes in a mortar and pestle.
c) Approximately 6 g of the mixture was loaded into a cylindrical dye (1.25 inch diameter) and compressed to 26,000 lbs. using a Carver Laboratory Press (Model #3365).
d) The CGO/Pd disc was heated in air to 1300° C. and held at that temperature for 4 hours.
The sintered membrane was placed between two gold rings and heated to 900° C. at 0.5° C./minute. The sintered membrane was sealed with gold rings into a two-zone flow reactor. While in this example a disc reactor was used, the principles of operation are the same for numerous reactor geometries including tube or cylindrical shaped reactors.
One side of the membrane was exposed to a flow air and the opposite side was exposed to a hydrocarbon mixture of steam and alkane feed. Product from the hydrocarbon side was analyzed by gas chromatography. Feeds and effluent compositions are given in Table II. Membrane reactor molar hydrogen flows and RHI were calculated using the formulas described above.
The RHI for the effluents from the membrane reactors for ethane, propane are butane feeds were 0.94, 0.96, and 0.98, respectively. These values of RHI are all significantly below 1.0, demonstrating that over a wide range of different feedstocks, processes of the invention produce membrane reactor effluents with an RHI less than 1.0.
Benefits that reactor effluents of relatively lower RHI values have for downstream processing of the effluent to recover value added products are demonstrated by the examples. In particular, the process according to the invention demonstrate that less compressor horsepower is required to compress the reactor effluent over a given pressure range, and demonstrate that a greater fraction of the desirable olefin can be recovered into a liquid condensate product at a given temperature and pressure.
This example was prepared using available thermodynamic and physical property data and a commercially available process simulation package for the relevant thermodynamic and physical property calculations.
Reactor effluent from a reactor for the dehydrogenation of propane was used to represent current practice olefin production methods that result in a relatively high-RHI reactor effluent. Pure propane feed was used and the reactor provided 60 percent conversion of the propane to propylene. In addition, a minor fraction of the propane thermally cracked to produce ethylene and methane. The composition and flow rate of the reactor effluent for this system is given as the High-RHI effluent in Table III. The reactor effluent flow rate in Table III corresponds to the production of approximately 250,000 metric tons of propylene per year. A low-RHI effluent was produced by removing 20 percent of the dihydrogen (molecular hydrogen) formed in the reactor, for example through the use of a hydrogen ion transport membrane reactor as described in Example 1. The Low-RHI effluent composition is also given in Table III. The RHI value of the effluent is also provided in Table III, assuming no dihydrogen or olefins existed in the reactor feed. The low-RHI effluent has an RHI of 0.8 while the high-RHI effluent (the one with no hydrogen removed) has an RHI of 1.0. Note that this example uses a low-RHI effluent with an RHI of 0.8 in order to provide a valid comparison case with other examples herein. In practice the RHI of a membrane reactor effluent may be higher and will depend on the hydrogen removal characteristics of the membrane material, the design of the reactor, and the heat balance requirements of the process.
The commercial process simulation package was used to determine the power required to isentropically compress the high- and low-RHI reactor effluents from the reactor outlet pressure of 50 psia to a final pressure of 250 psia. For the high-RHI effluent, a compressor power of 4140 HP was required, while for the low-RHI effluent only 3654 HP was required. This demonstrates that for a given olefin production rate, the 0.8 RHI reactor effluent saves approximately 12 percent in effluent compressor horsepower. More efficient membrane reactors (i.e. reactors which produce streams with RHIs lower than 0.8) would provide higher energy savings than shown in this example.
The same process simulation package was then used to calculate the fraction of propylene recovered to the liquid phase after chilling the compressed reactor effluent to various final temperatures between 37.8° C. and negative 73.3° C. The results are represented in the FIGURE, which shows the fraction of the propylene in the reactor effluent that is recovered to the liquid phase after chilling to a given temperature as a function of the final temperature. In this FIGURE the high-RHI effluent is represented by open squares and the low-RHI effluent is represented by closed circles. It is clear that for a given fractional propylene recovery, a lower temperature is required for the high-RHI effluent. For example, to recover 75 percent of the propylene in the reactor effluent, the high-RHI effluent requires chilling to approximately negative 10° C. while the low-RHI effluent requires chilling to only negative 3.9° C. It is well known that less refrigeration system energy is required to chill to negative 3.9° C. than to chill to negative 10° C. Therefore in addition to the reactor effluent compressor energy savings detailed above, the low-RHI effluent also reduces the refrigeration energy that is required to recover the desired olefin product. Once again, more efficient membrane reactors (i.e. reactors which produce streams with RHIs lower than 0.8) would provide a larger shift in the condensing curve and therefore higher energy savings than shown in this example.
