EXTRACTION PROCESS

Abstract: Extraction process comprising the following steps: (i) contacting a feed (e.g. a heavy hydrocarbon oil) with an extractant (e.g. propane and/or butane) in an extraction zone, (ii) removing a product (e.g. asphalt) from a lower part of the extraction zone, (iii) heating at least part of a fluid removed from an upper part of the extraction zone, (iv) separating heated fluid in a separation zone into fluids of different density, and (v) introducing at least part of a fluid of higher density into the upper part of the extraction zone. The invention further relates to an extraction apparatus comprising a housing (1) having feed- and extractant-inlet means (2, 19), first product outlet means (5) in its lower end part (e.g. comprising a settling section and mixing means for feed and extractant) and in its upper end part fluid outlet means (7) communicating via heat exchange means with a settling vessel (11) having in its upper section second product outlet means (13) and in its lower section reflux outlet means (15) communicating with the upper end part of the housing.

Description: The invention relates to an extraction process and apparatus. It is known to contact a feed with an extractant in an extrac­ tion zone, optionally heating the upper part thereof, forming reflux and remove products from both the lower- and the upper-part thereof. Such an extraction process has a number of drawbacks i.e. the amount of reflux generated by heating the upper part of the extrac­ tion zone increases in downward direction as a result of which the reflux flow rate is less than optimal in most of said upper part of the extraction zone; consequently the mass exchange between the reflux stream (having a relatively high density) and the upwardly flowing extractant stream (having a relatively low density) is not optimal. Moreover, the reflux thus formed may be present as very small droplets, entrainment of which by the extractant stream appears to hamper the separation efficiency of such a process. Surprisingly it has now been found that the aforementioned drawbacks can be overcome by removing fluid from the upper part of the extraction zone, heating at least part of said fluid, sep­ arating heated fluid in a separation zone into fluids of different density and introducing at least part of a fluid of higher density into the upper part of the extraction zone. The invention therefore relates to an extraction process which comprises the following steps: (i) contacting a feed with an extractant in an extraction zone, (ii) removing a product from a lower part of the extraction zone, (iii) heating at least part of a fluid removed from an upper part of the extraction zone, (iv) separating heated fluid in a separation zone into fluids of different density, and (v) introducing at least part of a fluid of higher density into the upper part of the extraction zone. In the process according to the invention the fluid of higher density than the fluid(s) of lower density (comprising a substan­ tial amount of extractant) removed from the separation zone, acts as reflux stream when introduced into the upper part of the extrac­ tion zone (step (v)). This reflux stream, which is produced outside the extraction zone, is very effective because the mass flow rate thereof is optimal right from the point of introduction (preferably near the top of the extraction zone) downward; consequently the mass exchange between the (extractant-lean) reflux stream of higher density and an (extractant-rich) upwardly flowing stream of lower density will be much improved in the upper part of the extraction zone which serves as rectifying zone, compared with an extraction process in which a reflux stream is internally generated. Moreover, with the present process the reflux stream can be distributed in the rectifying zone in the form of droplets having a controlled size, thus avoiding entrainment thereof with the up­ wardly flowing stream in said rectifying zone. Furthermore, by employing a separation zone in step (iv) located separately from the extraction zone, entrainment of the (reflux) fluid of higher density with the removed fluid(s) of lower density will be substantially prevented. Preferably, at least part of the extractant (which comprises one or more selective solvents for particular components of the feed) is introduced as a stream into the lower part of the extraction zone wherein it will flow upwardly and countercurrently with the feed- and reflux streams, thus attaining optimal mass- and heat-exchange conditions. Optionally, part or all of the extractant is used for pre-diluting of the feed in particular when an asphalt-­ containing heavy hydrocarbon oil feed is to be deasphalted by means of the process according to the invention. The process according to the invention is particularly suited for feed/extractant combinations for which the solubility of the dissolved component(s) in the extractant is reduced at increasing temperature; such combinations are found in super-critical extraction processes (e.g. employing carbon dioxide as extractant) and in deasphalting a heavy hydrocarbon oil feed such as a short residue obtained by distillation at reduced pressure of a long residue (which in turn is obtained by distillation of a mineral- or synthetic crude oil). Preferably an alkane(s)-comprising extractant is employed which most preferably comprises propane and/or butane and/or pentane depending on the intended use of the deasphalted product e.g. as feed for a lubricating oil plant in which case a propane-comprising extractant is preferred or as feed for a cata­ lytic cracking operation (employing a butane- or pentane- com­ prising extractant). The present process can be carried out within a wide range of temperatures (e.g. 10-300 °C) and pressures (e.g. 2-100 bar abs.); preferably, it is carried out at temperatures form 50-150°C and pressures from 10-60 bar abs. In a particularly preferred embodi­ ment the process is carried out at such an elevated temperature and pressure that the fluids present in the extraction zone are main­ tained just below the critical conditions; at such conditions an increase in the temperature of the fluid removed from an upper part of the extraction zone (step (iii)) will lead to the formation of two distinct fluids which can be easily separated due to their different densities. Heating and separating of the fluid removed from an upper part of the extraction zone (steps (iii) and (iv)) can be carried out in a single zone, if desired. However, it is preferred to carry out step (iii) and (iv) of the process according to the invention in separate (preferably indirect) heating and separation zones in order to attain optimal separation of said fluid into a reflux- and at least one product stream. In a particularly preferred embodiment of the present process at least part of the (reflux) fluid of higher density obtained in step (iv) is cooled before being introduced into the upper part of the extraction zone in order to be able to regulate the temperature in the extraction zone independently from the temperature in the separation zone which is determined by heating step (iii) of the process, thus attaining optimal separation of the various components present in the feed. Moreover, the aforementioned droplet size of the reflux stream can be regulated by the design of the distributor therefor. If desired, substantially all of the product (e.g. deasphalted oil) contained in the fluid removed from the upper part of the extraction zone can be separated off from the extractant by appro­ priate heating and cooling of said streams, thus providing an extractant stream which can be at least partly recycled to the extraction zone. In this case part of the product obtained is employed as reflux (step (v)). Without loss of efficiency, the part of the extraction zone below the feed inlet (which serves as a stripping zone) may be replaced, in particular in deasphalting processes, by a mixing/­ settling zone, which is advantageous both from an operational and from a capital expenditure point of view. The invention further relates to an extraction apparatus which comprises a housing having feed- and extractant-inlet means, first product outlet means in its lower end part and in its upper end part fluid outlet means communicating via heat exchange means with a settling vessel having in its upper section second product outlet means and in its lower section reflux outlet means communicating with the upper end part of the housing. Preferred embodiments of the apparatus according to the invention are described hereinafter using Figures 1 and 2 in which reference numerals relating to corresponding parts are the same. In Figure 1 an apparatus according to the invention is schema­ tically depicted in an elementary from. Figure 2 represents a particularly preferred embodiment of the apparatus. The apparatus shown in Figure 1 comprises a housing (1), feed inlet means (2) above which the rectifying section (3) and below which the stripping section (4) are situated in the housing. First product (e.g. asphalt) outlet means (5) are arranged in the lower end part (6) and fluid outlet means (7) are arranged in the upper end part (8) of the housing. Fluid outlet means (7) are in communication with settling vessel (11) via indirect (e.g. tubular) heat exchanger (9) and line (10). A heating medium such as steam is led via line (12) through heat exchanger (9). Settling vessel (11) is provided with second product (e.g. a relatively low density mixture of deasphalted oil and solvent) outlet means (13) in its upper section (14) and reflux outlet means (15) in its lower section (16). Reflux outlet means (15) are connected to the upper end part (8) of housing (1), whereas extractant inlet means (19) are connected to the lower end part (6) thereof. In Figure 2 a particularly preferred embodiment of the appara­ tus according to the invention is depicted which in addition to the elements described in relation with Figure 1 comprises a settling section (17) in housing (1) which is in communication with mixing means (18) (e.g. turbulence coils or, preferably, an in-line mixer). Feed is introduced through line (2) and extractant through line (19) into mixing means (18) wherein both fluids are thoroughly mixed with each other. Preferably, the housing (1) comprises a column provided with mass transfer means (20) (e.g. baffles as shown in Figure 2 or perforated trays or packings in the form of rings or other bodies known in the art). Settling vessel (11) may be provided with internals (21) such as parallel plates or other means known in the art to enhance settling. Third product (e.g. heavy hydrocarbon oil) outlet means (22) are in communication with the reflux outlet means (15) which are also in communication with heat exchanger (23) through which a cooling medium (e.g. water) is led via line (24). Finally, cooled reflux fluid is introduced through line (25) into the upper end part (8) of housing (1) via reflux distribution means (not depicted in the Figures). The invention is illustrated by means of the following Exam­ ples. EXAMPLE 1 A feed stream consisting of a 1000 ton/day of asphalt-­ containing heavy hydrocarbon oil is pre-diluted with 500 ton/day of propane to form a mixture which enters through feed inlet means (2) the extraction apparatus as depicted in Figure 1 operating at a temperature of 70 °C and a pressure of 40 bar abs. inside the housing (1). 3500 ton/day of propane extractant is introduced through extractant inlet means (19). The fluid leaving upper end part (8) of housing (1) is heated to 85 °C in heat exchanger (9) and subsequently passed to settling vessel (11) wherein 450 ton/day of a fluid having a relatively high density containing heavy deasphalted oil and dissolved propane is separated off and recycled through line (15) as reflux into the rectifying section (3) of housing (1). The product fluid obtained via outlet means (13) from settling vessel (11) contains 250 ton/day of substantially asphalt-­ free relatively light deasphalted oil dissolved in 3500 ton/day of propane which is further separated in equipment not depicted in Figure 1. 750 ton/day of asphalt and heavy deasphalted oil is removed together with 500 ton/day of propane through product outlet means (5). EXAMPLE 2 A feed stream consisting of 1000 ton/day of asphalt-containing heavy hydrocarbon oil is fed together with 2000 ton/day of an extractant mixture containing normal- and iso-butane into mixing means (18) as depicted in Figure 2. Mixing means (18), settling section (17) and rectifying section (3) are (substantially isothermally) operated at 135 °C and 40 bar abs. Via outlet means (5) 600 ton/day of asphalt and 400 ton/day of butanes are removed, whereas a mixture of 1700 ton/day of butanes and 550 ton/day of deasphalted oil is removed via fluid outlet means (7). Said mixture is heated to a temperature of 150 °C in heat exchanger (9) and subsequently separated in settling vessel (11) into 400 ton/day of a light deasphalted oil product, which is removed via product outlet means (13) together with 1600 ton/day of butanes, and 250 ton/day of a mixture of undesired relatively heavy deasphalted oil and butanes which is cooled to a temperature of 135 °C in heat exchanger (23) and subsequently recycled as reflux into rectifying section (3).

Claims: 1. An extraction process which comprises the following steps: (i) contacting a feed with an extractant in an extraction zone, (ii) removing a product from a lower part of the extraction zone, (iii) heating at least part of a fluid removed from an upper part of the extraction zone, (iv) separating heated fluid in a separation zone into fluids of different density, and (v) introducing at least part of a fluid of higher density into the upper part of the extraction zone. 2. Process according to claim 1 wherein a heavy hydrocarbon oil feed is deasphalted. 3. Process according to claim 1 or 2 wherein an alkane(s)-com­ prising extractant, most preferably comprising propane and/or butane and/or pentane, is employed. 4. Process according to any one of the preceding claims wherein the feed and the extractant are mixed and the resulting mixture is introduced into the lower part of the extraction zone comprising a settling zone from which a product is removed (step (ii)). 5. Process according to any one of the preceding claims wherein at least part of the fluid of higher density obtained in step(iv) is cooled before being introduced into the upper part of the extraction zone. 6. Process according to any one of the preceding claims which is carried out at such an elevated temperature and pressure that the fluids present in the extraction zone are maintained at substan­ tially critical conditions. 7. Process substantially as described hereinbefore with reference to the Examples. 8. An extraction apparatus which comprises a housing having feed- and extractant-inlet means, first product outlet means in its lower end part and in its upper end part fluid outlet means communicating via heat exchange means with a settling vessel having in its upper section second product outlet means and in its lower section reflux outlet means communicating with the upper end part of the housing. 9. Apparatus according to claim 8 wherein the lower end part of the housing comprises a settling section which is in communication with mixing means for feed and extractant. 10. Apparatus according to claim 8 or 9 wherein the housing comprises a column provided with mass transfer means. 11. Apparatus substantially as described hereinbefore with ref­ erence to the Figures.

Inventors: CORNELISSEN, ANTON EDUARD
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1987
Main classification: C10G21
IPC codes: B01D11, C10C3, B01D3, C10G21