Patent Application
20090166258
The present invention relates to a process for enhancing various streams based on hydrogen and on hydrocarbons so as to produce a high-purity hydrogen stream.
Refineries or petrochemical plants use large amounts of hydrogenated streams, preferably of high purity. In many industrial cases, the performances of the units, and especially the grades of the products produced, are limited by the purity of the supplied hydrogen. Furthermore, in these units, it is sometimes necessary to eliminate gases that are still rich in hydrogen under the effect of the recycle gas purges. This results in a cost premium for the operation of the unit. In order to avoid these problems, processes for recovering the hydrogen from streams of relatively low purity in order to produce high-purity hydrogenated streams have been proposed, especially in Application WO 2005/042640 which proposes using a pressure swing adsorption unit to treat these various hydrogenated streams and to obtain a high-purity stream. This type of pressure swing adsorption unit may represent a significant investment in terms of adsorbent cost and compressor cost.
The objective of the present invention is to propose a process for enhancing various streams based on hydrogen and hydrocarbons so as to produce a high-purity hydrogen stream, a liquefied petroleum gas and a waste gas, the investment cost of which is reduced by increasing the high-purity hydrogen production yield.
Another objective of the present invention is to propose a process for enhancing various streams based on hydrogen and hydrocarbons so as to produce a high-purity hydrogen stream, a liquefied petroleum gas and a waste gas, of which the energy consumption is optimized.
For this purpose, the invention relates to a process for producing a hydrogen-enriched gas stream and a liquefied petroleum gas from at least one gas stream mainly comprising hydrogen at pressure P and a gas stream comprising hydrogen and hydrocarbons, in which the following steps are carried out:
The process according to the invention allows the treatment of two gas streams comprising hydrogen at various concentrations so as to produce a hydrogen-enriched gas stream, a liquefied petroleum gas (or LPG) and a waste gas. The first gas stream treated is a gas stream that mainly comprises hydrogen, that is to say having a hydrogen concentration between 50 and 99 vol %. This stream also comprises hydrocarbons having from 1 to 8 carbon atoms and compounds such as CO, CO2, H2S, etc. The second stream treated is a gas stream comprising hydrogen and hydrocarbons having a hydrogen concentration at least 10% lower, preferably at least 15% lower and more preferably still 15 to 50% lower, relative to the value of the hydrogen concentration of the gas stream mainly comprising hydrogen. This second stream also comprises hydrocarbons having 1 to 8 carbon atoms, and also compounds such as H2O, H2S, etc. Generally, this second gas stream has a hydrocarbon concentration between 10 and 50 vol %. According to the invention, the pressure of this second gas stream comprising hydrogen and hydrocarbons is adjusted so that it is close to 2, either by compression, or by pressure drop depending on the origin of this stream. It is also possible that this stream already has a pressure P, in this case it is used as is without a pressure adjustment. These various streams may be derived from the purge of the gas loop of a hydrotreatment unit such as a hydrodesulfurization or hydrocracking unit of a refinery, or else such as a hydrodealkylation or isomerization unit of a petrochemical site. The first and second gas streams treated in the present invention may be derived from mixtures of various purges of such units. These mixtures may make it possible to attain the pressure P desired by combining the streams having a pressure below P with streams having a pressure above P.
By treatment of these various streams, the invention makes it possible to enrich the gas stream mainly comprising hydrogen. This enrichment is obtained by hydrogen depletion of the second gas stream comprising hydrogen and hydrocarbons. The unit thus produces the hydrogen-enriched gas stream generally having a hydrogen purity greater than 99 vol %, and the unit also produces a waste stream of low hydrogen purity and of low pressure which may be conveyed to the fuel gas network. The pressure and hydrogen concentration of the waste stream are respectively below the pressure and hydrogen concentration values of all the effluents going into the unit (U). This waste stream also comprises hydrocarbons (C1 to C8) and compounds such as H2O, H2S, CO, CO2, etc.
Preferably, the gas separation unit (U) is a pressure swing adsorption (PSA) unit combined with an integrated compressor, in which, for each adsorber of the unit, a pressure swing cycle is carried out comprising a succession of phases which define the phases of adsorption, decompression, purge and pressure rise, such that:
So as to obtain an effective purification, the adsorbent of the PSA beds should, in particular, allow the adsorption and desorption of impurities. The adsorbent bed is generally composed of a mixture of several adsorbents, said mixture comprising, for example, at least two adsorbents chosen from: activated carbons, silica gels, aluminas or molecular sieves. Preferably, the silica gels should have a pore volume between 0.4 and 0.8 cm3/g and a specific surface area greater than 600 m2/g. Preferably, the aluminas have a pore volume greater than 0.2 cm3/g and a specific surface area greater than 220 m2/g. The zeolites preferably have a pore size below 4.2 Å, an Si/Al molar ratio below 5 and contain Na and K. The activated carbons preferably have a specific surface area greater than 800 m2/g and a micropore size between 8 and 20 Å. According to one preferred embodiment, each PSA adsorbent bed is composed of at least three layers of adsorbents of different natures. Each PSA adsorbent bed may comprise: in the bottom part, a protective layer composed of alumina and/or silica gel surmounted by a layer of activated carbon and/or of carbon-based molecular sieve and optionally in the upper part a molecular sieve layer. The proportions vary as a function of the nature of the gas mixture to be treated (especially as a function of its percentages of CH4 and of C3+ hydrocarbons). For example, a water-free gas mixture comprising 75 mol % of H2, 5% of C3+ and 20% of light (C1-C2) hydrocarbons, CO and N2 may be treated by an adsorption unit whose beds comprise at least 10 vol % of alumina and 15 vol % of silica gel in the bottom bed, the remainder being obtained from activated carbon.
During the decompression phase of the PSA, the waste gas is produced. This production of the waste gas may be obtained by countercurrent decompression initiated at a pressure below P. This waste gas comprises impurities and has a hydrogen content lower than all the streams introduced into the PSA unit. This waste gas may be discharged from the process and burnt or reused as a recycle gas in the PSA unit as indicated previously.
The low pressure of the cycle being attained, a purge phase is carried out to finalize the regeneration of the adsorber. During the purge phase, a gas is introduced countercurrently into the adsorber and a purge gas is produced. The gas introduced countercurrently into the adsorber during the purge phase is a gas stream derived from one of the steps of the decompression phase. The purge gas is generally used as a recycle gas after recompression.
During the pressure rise phase, the pressure of the adsorber is increased by countercurrent introduction of a gas stream comprising hydrogen such as the gas produced during the various steps of the decompression phase.
According to the main feature of the invention, between step a) of adjusting the pressure of the gas stream comprising hydrogen and hydrocarbons and step b) of treating this stream via the gas separation unit (U), the temperature of the gas stream comprising hydrogen and hydrocarbons is lowered so as to condense the hydrocarbons, preferably to less than 0° C., more preferably still to less than −20° C. Preferably, this temperature drop is carried out by means of a cryogenic device, such as cryogenic (compression/expansion) loops using liquid ammonia or propane.
This stream 22 and the stream 1 are treated by the gas separation unit 5 so as to produce: a hydrogen-enriched stream 6, a waste gas 7 and a purge gas 8 which is mixed with the stream 2 comprising hydrogen and hydrocarbons before compression of the latter. The process also makes it possible to treat another stream 9 comprising hydrogen and hydrocarbons that is equivalent to the stream 2 but originates from another purge. This stream 9 already has a pressure P or a pressure that is slightly greater than P; it is therefore directly treated by the device 4 with the mixture 21 resulting from the compressor.
By implementation of the process as described above, it is possible to reduce the size of the pressure swing adsorption unit and to reduce the compression costs of the unit (U) while increasing the hydrogen yield. The process additionally makes it possible to produce, at the outlet of the temperature-lowering device, a liquefied petroleum gas, which may be a stream of high added value, that can be recovered as a fuel.
The process of the invention also has the advantage of allowing a natural energy integration between the cryogenic device enabling the temperature to be reduced and the compressor of the pressure swing adsorption unit.
Two gas streams comprising hydrogen were treated by means of a pressure swing adsorption unit. These two streams had the following concentrations:
These streams were treated by a pressure swing adsorption unit according to the invention as illustrated in
It is observed that by the implementation of the invention, it is possible:
Furthermore, the stream of liquefied hydrocarbons may be recovered as a commercial product within the plant.
The liquefied petroleum gas resulting from the cryogenic device is a product of high added value which compensates for the additional energy used by the cryogenic device of the process according to the invention relative to the process of the prior art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0552972 | Sep 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2006/050921 | 9/21/2006 | WO | 00 | 8/7/2008 |
