Abstract:
An aromatics complex producing one or more xylene isomers offers a large number of opportunities to conserve energy by heat exchange within the complex. One previously unrecognized opportunity is through providing two parallel distillation columns operating at different pressures to separate C 8  aromatics from C 9 + aromatics. The parallel columns offer additional opportunities to conserve energy within the complex.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to an improved apparatus suitable for energy savings in the distillation of hydrocarbons. More specifically, the present invention concerns an apparatus providing energy conservation within an aromatics-processing complex producing xylene isomers. 
       BACKGROUND OF THE INVENTION 
       [0002]    The xylene isomers are feedstocks for a variety of important industrial chemicals. The most widely produced and used of the xylene isomers is para-xylene, the principal feedstock for polyester, which continues to enjoy a high growth rate from large base demand. Ortho-xylene is used to produce phthalic anhydride, which supplies high-volume but relatively mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but is usually considered a less-desirable component of C 8  aromatics. 
         [0003]    Among the aromatic hydrocarbons, the overall importance of xylenes rivals that of benzene as a feedstock for industrial chemicals. Xylenes and benzene are produced from petroleum by reforming naphtha but not in sufficient volume to meet demand, thus conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene is de-alkylated to produce benzene or selectively disproportionated to yield benzene and C 8  aromatics from which the individual xylene isomers are recovered. 
         [0004]    An aromatics complex flow scheme has been disclosed by Meyers in the Handbook of Petroleum Refining Processes, 2d. Edition in 1997 by McGraw-Hill, and is incorporated herein by reference. 
         [0005]    Aromatics complexes producing xylenes are substantial consumers of energy, notably in distillation operations to prepare feedstocks and separate products from conversion processes. The separation of xylenes from heavy aromatics in particular offers substantial potential for energy savings. Energy conservation in such processes would not only reduce processing costs but also would address current concerns about carbon emissions. 
       SUMMARY OF THE INVENTION 
       [0006]    A broad embodiment of the present invention is a distillation apparatus comprising two distillation columns for separating C 8 -aromatic components from C 9 -and-heavier aromatic components, comprising a first distillation column adapted to operate at a first pressure and having a bottom portion in fluid communication with a reboiler, a second distillation column adapted to operate at a second pressure and having a top portion in fluid communication with an overhead conduit, the overhead conduit from the second column providing fluid communication to the reboiler of the first column. 
         [0007]    A more specific embodiment is a distillation apparatus comprising two distillation columns for separating C 8 -aromatic components from C 9 -and-heavier aromatic components, comprising a first distillation column adapted to operate at a first pressure and having a bottom portion in fluid communication with a reboiler, a second distillation column adapted to operate at a second pressure and having a top portion in fluid communication with an overhead conduit, the overhead conduit from the second column providing fluid communication to the reboiler of the first column and the overhead conduit adapted to supply a heat source to the reboiler. 
         [0008]    More preferably, the second pressure is at least 400 kPa higher than the first pressure. 
         [0009]    Additional objects, embodiments and details of this invention can be obtained and inferred from the following detailed description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  schematically illustrates an aromatics complex in which energy-savings concepts could be applied. 
           [0011]      FIG. 2  illustrates an aromatics complex in which energy conservation is applied. 
           [0012]      FIG. 3  shows the application of energy conservation in the distillation of C 8  aromatics from heavy aromatics. 
           [0013]      FIG. 4  illustrates examples of specific units within an aromatics complex in which direct heat exchange could achieve energy savings. 
           [0014]      FIG. 5  illustrates an aromatics complex in which some of the energy-savings concepts described herein are applied as a supplement or substitute for other energy savings. 
           [0015]      FIG. 6  illustrates the generation of steam from specific units within an aromatics complex. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The feedstream to the present process generally comprises alkylaromatic hydrocarbons of the general formula C 6 H (6-n) R n , where n is an integer from 0 to 5 and each R may be CH 3 , C 2 H 5 , C 3 H 7 , or C 4 H 9 , in any combination. The aromatics-rich feed stream to the process of the invention may be derived from a variety of sources, including without limitation catalytic reforming, steam pyrolysis of naphtha, distillates or other hydrocarbons to yield light olefins and heavier aromatics-rich byproducts (including gasoline-range material often referred to as “pygas”), and catalytic or thermal cracking of distillates and heavy oils to yield products in the gasoline range. Products from pyrolysis or other cracking operations generally will be hydrotreated according to processes well known in the industry before being charged to the complex in order to remove sulfur, olefins and other compounds which would affect product quality and/or damage catalysts used in processing such feedstocks. Light cycle oil from catalytic cracking also may be beneficially hydrotreated and/or hydrocracked according to known technology to yield products in the gasoline range; the hydrotreating preferably also includes catalytic reforming to yield the aromatics-rich feed stream. If the feed stream is catalytic reformate, the reformer preferably is operated at high severity to achieve high aromatics yield with a low concentration of nonaromatics in the product. 
         [0017]      FIG. 1  is a simplified flow diagram of a typical aromatics-processing complex of the known art directed to the production of at least one xylene isomer. The complex may process an aromatics-rich feed which has been derived, for example, from catalytic reforming. Usually such a stream will have been treated to remove olefinic compounds and light ends, e.g., butanes and lighter hydrocarbons and preferably pentanes; such removal, however, is not essential to the practice of the broad aspects of this invention. The aromatics-containing feed stream contains benzene, toluene and C 8  aromatics and typically contains higher aromatics and aliphatic hydrocarbons including naphthenes. 
         [0018]    The feed stream is passed via conduit  10  via a heat exchanger  12  to reformate splitter  14  and distilled to separate a stream comprising C 8  and heavier aromatics, withdrawn as a bottoms stream in conduit  16 , from toluene and lighter hydrocarbons recovered overhead via conduit  18 . The toluene and lighter hydrocarbons are sent to extractive distillation process unit  20  which separates a largely aliphatic raffinate in conduit  21  from a benzene-toluene aromatics stream in conduit  22 . The aromatics stream in conduit  22  is separated, along with stripped transalkylation product in conduit  45  and overhead from para-xylene finishing column in conduit  57 , in benzene column  23  into a benzene stream in conduit  24  and a toluene-and-heavier aromatics stream in conduit  25  which is sent to a toluene column  26 . Toluene is recovered overhead from this column in conduit  27  and may be sent partially or totally to a transalkylation unit  40  as shown and discussed hereinafter. 
         [0019]    A bottoms stream from the toluene column  26  is passed via conduit  28 , along with bottoms from the reformate splitter in conduit  16 , after treating via clay treater  17 , and recycle C 8  aromatics in conduit  65 , to xylene column  30 . The fractionator  30  separates concentrated C 8  aromatics as overhead in conduit  31  from a high-boiling stream comprising C 9 , C 10  and heavier aromatics as a bottoms stream in conduit  32 . This bottoms stream is passed in conduit  32  to heavy-aromatics column  70 . The heavy-aromatics column provides an overhead a stream in conduit  71  containing C 9  and at least some of the C 10  aromatics, with higher boiling compounds, primarily C 11  and higher alkylaromatics, being withdrawn as a bottoms stream via conduit  72 . 
         [0020]    The C 9 + aromatics stream from heavies column in conduit  71  is combined with the toluene-containing overhead contained in conduit  27  as feed to transalkylation reactor  40 , which contains a transalkylation catalyst as known in the art to produce a transalkylation product comprising benzene through C 11 + aromatics with xylenes as the principal focus. The transalkylation product in conduit  41  is stripped in stripper  42  to remove gases in conduit  43  and C 6  and lighter hydrocarbons which are returned via conduit  44  to extractive distillation  20  for recovery of light aromatics and purification of benzene. Bottoms from the stripper are sent in conduit  45  to benzene column  23  to recover benzene product and unconverted toluene. 
         [0021]    The C 8 -aromatics overhead provided by fractionator  30  contains para-xylene, meta-xylene, ortho-xylene and ethylbenzene and passes via conduit  31  to para-xylene separation process  50 . The separation process operates, preferably via moving-bed adsorption using a desorbent, to provide a mixture of para-xylene and desorbent via conduit  51  to extract column  52 , which separates para-xylene via conduit  53  from returned desorbent in conduit  54 ; the para-xylene is purified in finishing column  55 , yielding a para-xylene product via conduit  56  and light material which is returned to benzene column  23  via conduit  57 . A non-equilibrium mixture of C 8 -aromatics raffinate and desorbent from the separation unit  50  is sent via conduit  58  to raffinate column  59 , which separates a raffinate for isomerization in conduit  60  from returned desorbent in conduit  61 . 
         [0022]    The raffinate, comprising a non-equilibrium mixture of xylene isomers and ethylbenzene, is sent via conduit  60  to isomerization reactor  62 . The raffinate is isomerized in reactor  62 , which contains an isomerization catalyst to provide a product approaching equilibrium concentrations of C 8 -aromatic isomers. The product is passed via conduit  63  to deheptanizer  64 , which removes C 7  and lighter hydrocarbons with bottoms passing via conduit  65  to xylene column  30  to separate C 9  and heavier materials from the isomerized C 8 -aromatics. Overhead liquid from deheptanizer  64  is sent to stripper  66 , which removes light materials overhead in conduit  67  from C 6  and C 7  materials which are sent via conduit  68  to the extractive distillation unit  20  for recovery of benzene and toluene values. 
         [0023]    There are many possible variations of this scheme within the known art, as the skilled routineer will recognize. For example, the entire C 6 -C 8  reformate or only the benzene-containing portion may be subjected to extraction. Para-xylene may be recovered from a C 8 -aromatic mixture by crystallization rather than adsorption. Meta-xylene as well as para-xylene may be recovered from a C 8 -aromatic mixture by adsorption, and ortho-xylene may be recovered by fractionation. Alternatively, the C 9 -and heavier stream or the heavy-aromatics stream is processed using solvent extraction or solvent distillation with a polar solvent or stripping with steam or other media to separate highly condensed aromatics as a residual stream from C 9 + recycle to transalkylation. In some cases, the entire heavy-aromatic stream may be processed directly in the transalkylation unit. The present invention is useful in these and other variants of an aromatics-processing scheme, aspects of which are described in U.S. Pat. No. 6,740,788 which is incorporated herein by reference. 
         [0024]    The separation of C 8  aromatics from heavy aromatics in fractionator  30  is a situation in which the distillation process of the invention generally is effective. A distillation apparatus of the present invention is represented by two or more xylene columns each effecting substantially the same separation between C 8  and C 9 + aromatics contained in two or more internal or external-feed streams of the aromatics complex designated respectively as a first and a second feed streams. Preferably the two streams comprise a first feed stream which is higher-boiling and a second feed stream which is lower-boiling, wherein the higher-boiling first feed stream has a higher content of C 9 + hydrocarbons than the second feed stream. The invention comprises distilling the first feed stream in at least one first fractionation column at a first pressure to separate a first C 8 -aromatics stream from a first C 9 -and-heavier aromatics stream, distilling the second feed stream in a second fractionation column at a second pressure to separate a second C 8 -aromatics stream from a second C 9 -and-heavier aromatics stream, and circulating an overhead stream from the second column to provide heat to a reboiler of the first column. The first pressure is relatively low, typically between 100 and 800 kPa and the second pressure is elevated to enable heat transfer from the first column to the second and typically is at least 400 kPa above the low pressure. This concept of different pressures in parallel columns is particularly valuable when the heavy components present in the higher-boiling feed stream are subject to degradation at reboiler temperatures needed to separate the light and heavy components. 
         [0025]    The second fractionation column processes a second feed stream with a lower concentration of heavy materials subject to decomposition than the feed to the first column, and the second pressure thus may be raised higher in order to effect energy savings through heat exchange between the first and second columns without loss of product yield or risk of equipment fouling. This feed preferably comprises isomerized C 8  aromatics from the isomerization reactor following deheptanization, but may also comprise other C 8 -aromatic streams with low concentrations of heavy aromatics. This stream to the second column typically contains less than about 10 weight-% C 9 + aromatics, more often less than about 5 weight-% C 9 + aromatics, and frequently less than about 2 weight-% C 9 + aromatics. Effectively, the process comprises operating the second column at a second pressure that would enable the overhead to provide heat to a reboiler of the first column and, preferably, a reboiler of at least one other column and/or steam generator in an associated processing complex. 
         [0026]    In another embodiment, the process comprises operating the second fractionation column at a pressure that would enable the overhead to provide heat to generate steam useful in an associated processing complex. Further, the C 8 -aromatics fractionator may comprise three or more columns comprising additional heat exchange between overheads and reboilers in an analogous manner to the above description. 
         [0027]      FIG. 2  is an energy-efficient aromatics complex employing a number of concepts of the invention. For ease of reference, a parallel numbering system is employed to those of  FIGS. 1 and 2 . The feed stream is passed via conduit  110  via heat exchangers  112  and  113 , which raise the temperature of the feed stream, to reformate splitter  114 . The heat exchange is supplied via conduits  212  and  213  respectively from the net para-xylene product and the recovered desorbent from the para-xylene separation process as discussed later in this section. 
         [0028]    As in  FIG. 1 , C 8  and heavier aromatics are withdrawn as a bottoms stream in conduit  116  while toluene and lighter hydrocarbons recovered overhead via conduit  118  are sent to extractive distillation process unit  120  which separates a largely aliphatic raffinate in conduit  121  from a benzene-toluene aromatics stream in conduit  122 . The aromatics stream in conduit  122  is separated, along with stripped transalkylation product in conduit  145  and overhead from para-xylene finishing column in conduit  157 , in fractionator  123  into a benzene stream in conduit  124  and a toluene-and-heavier aromatics stream in conduit  125  which is sent to a toluene column  126 . Toluene is recovered overhead from this column in conduit  127  and may be sent partially or totally to a transalkylation unit  140  as shown and discussed hereinafter. 
         [0029]    A bottoms stream from the toluene column  126  is passed via conduit  128 , along with bottoms from the reformate splitter in conduit  116 , after treating via clay treater  117 , and a purge stream of heavy aromatics in conduit  138 , to low-pressure first xylene column  130 . The feed stream to this column is characterized as a higher-boiling feed stream, as it generally contains more than about 5 weight-% C 9 + aromatics and often more than about 10 weight-% C 9 + aromatics. Other C8-aromatics streams having significant contents of C 9  and heavier aromatics, including streams obtained from sources outside the complex, also may be added to this higher-boiling feed stream; a portion of deheptanizer bottoms in stream  165  also may be included depending on overall energy balances. The low-pressure xylene column separates concentrated first C 8 -aromatics stream as overhead in conduit  131  from a high-boiling first C 9 -and-heavier stream comprising C 9 , C 10  and heavier aromatics as a bottoms stream in conduit  132 . 
         [0030]    Simultaneously, an isomerized C 8 -aromatics stream is passed via conduit  165  to a high-pressure second xylene column  133 . This is characterized as a lower-boiling feed stream which contains a lower concentration of heavy materials subject to decomposition than the feed to column  130 , and the second column pressure thus can be increased in order to effect energy savings. Other C 8 -aromatics-containing streams having similarly low contents of C 9 -and-heavier aromatics, including streams obtained from sources outside the complex, also may be contained in the feed stream to this column. The second xylene column separates a second C 8 -aromatics stream as overhead in conduit  134  from a second C 9 -and-heavier stream in conduit  132 . At least a portion of overhead vapor from the high-pressure xylene column in conduit  134  preferably is employed to reboil low-pressure xylene column  130  in reboiler  135 , leaving as a condensed liquid to the xylene-separation process  150  in conduit  136  as well as reflux (not shown) to column  133 . In addition, the overhead in conduit  134  may be used to provide energy to the reboiler of extract column  152  or other such services which are described later or will be apparent to the skilled routineer. 
         [0031]    The C 9 + bottoms stream passing to reboiler  137  may provide energy via one or both of the stream before the reboiler in conduit  270  and the heated stream from the reboiler in conduit  259  for reboiling respectively one or both of heavy-aromatics column  170  and raffinate column  159 ; the bottoms stream after heat exchange would be sent to the heavy-aromatics column  170 . Other similar heat-exchange services will be apparent to the skilled routineer. The net bottoms stream in conduit  138  usually is passed through column  130  or may be in conduit  139  combined directly with the stream in conduit  132  to heavies column  170 . The heavies column provides an overhead a stream in conduit  171  containing C 9  and at least some of the C 10  aromatics, with higher boiling compounds, primarily C 11  and higher alkylaromatics, being withdrawn as a bottoms stream via conduit  172 . This column may be reboiled by xylene column bottoms in conduit  270 , as discussed above. Overhead vapor from columns  130  and  170  also may generate steam respectively via conduits  230  and  271  as indicated, with condensed liquids either serving as reflux to each column or as net overhead respectively in streams  131  or  171 . 
         [0032]    The C 9 + aromatics from heavies column in conduit  171  is combined with the toluene-containing overhead contained in conduit  127  as feed to transalkylation reactor  140  to produce a transalkylation product containing xylenes. The transalkylation product in conduit  141  is stripped in stripper  142  to remove gases in conduit  143  and C 7  and lighter liquids which are returned via conduit  144  to extractive distillation  120  for recovery of light aromatics following stabilization in isomerate stripper  166 . Bottoms from the stripper are sent in conduit  145  to benzene column  123  to recover benzene product and unconverted toluene. 
         [0033]    The first and second C 8 -aromatics streams provided by xylene columns  130  and  133 , containing para-xylene, meta-xylene, ortho-xylene and ethylbenzene, pass via conduit  131  and  136  to xylene-isomer separation process  150 . The description herein may be applicable to the recovery of one or more xylene isomers other than para-xylene; however, the description is presented for para-xylene for ease of understanding. The separation process operates via a moving-bed adsorption process to provide a first mixture of para-xylene and desorbent via conduit  151  to extract column  152 , which separates para-xylene via conduit  153  from returned desorbent in conduit  154 . Extract column  152  preferably is operated at an elevated pressure, at least about 300 kPa and more preferably about 500 kPa or higher, such that the overhead from the column is at sufficient temperature to reboil finishing column  155  via conduit  256  or deheptanizer  164  via conduit  265 . Heat supplied for reboiling duty via conduits  256  and  265  results in the condensation of the extract in these streams which is either or both refluxed to column  152  (not shown) or sent as a net stream in conduit  153  to finishing column  155 . The para-xylene is purified in finishing column  155 , yielding a para-xylene product via conduit  156  and light material which is returned to benzene column  123  via conduit  157 . 
         [0034]    A second mixture of raffinate, as a non-equilibrium blend of C 8  aromatics, and desorbent from separation process  150  is sent via conduit  158  to raffinate column  159 , which separates a raffinate to isomerization in conduit  160  from returned desorbent in conduit  161 . The raffinate column may be operated at higher pressure to generate steam via conduit  260  or to exchange heat in other areas of the complex; condensed liquids from such heat exchange either serve as reflux to the raffinate column or as net overhead in conduit  160 . Recovered desorbent in conduits  154  and  161  and net finishing column bottoms may heat the incoming feed stream in conduit  110  via conduits  213  and  212 , respectively. 
         [0035]    The raffinate, comprising a non-equilibrium blend of xylene isomers and ethylbenzene, is sent via conduit  160  to isomerization reactor  162 . In the isomerization reactor  162 , raffinate is isomerized to provide a product approaching equilibrium concentrations of C 8 -aromatic isomers. The product is passed via conduit  163  to deheptanizer  164 , which removes C 7  and lighter hydrocarbons and preferably is reboiled using overhead in conduit  265  from extract column  152 . Bottoms from the deheptanizer passes via conduit  165  to xylene column  133  to separate C 9  and heavier materials from the isomerized C 8 -aromatics. Overhead liquid from deheptanizer  164  is sent to stripper  166 , which separates light materials overhead in conduit  167  from C 6  and C 7  materials which are sent via conduit  168  to the extractive distillation unit  120  for recovery and purification of benzene and toluene values. Pressures of deheptanizer  164  and stripper  166  are selected to exchange heat or generate steam in a manner analogous to the xylene columns discussed elsewhere in this specification. 
         [0036]      FIG. 3  shows in more detail the heat exchange of the invention between parallel xylene distillation columns  130  and  133 . Feed to the low-pressure xylene column  130  comprises bottoms from the toluene column via conduit  128 , clay-treated bottoms from the reformate splitter in conduit  116 , and purge C 8  aromatics in conduit  138  and may comprise other C 8 -aromatics-containing streams not suitable for processing in the high-pressure xylene column as well as a portion of the deheptanized stream  165  if appropriate for energy balances. The combined feeds of heavy reformate and toluene-column bottoms may contain heavy aromatics which are susceptible to degradation at high temperatures, and operating at a pressure lower than 800 kPa permits temperatures to be maintained in the bottom of the column and reboiler which avoid such decomposition. The low-pressure xylene column separates concentrated C 8  aromatics as overhead in conduit  131  from a high-boiling stream comprising C 9 , C 10  and heavier aromatics as a bottoms stream in conduit  132 . The overhead stream from column  130  may be used at least partially via conduit  230  of  FIG. 2  to generate steam or reboil other columns as discussed previously and thus be condensed to provide reflux to the column as well as the net overhead to xylene separation in conduit  131 . 
         [0037]    Simultaneously, an isomerized C 8 -aromatics stream is passed via conduit  165  to high-pressure xylene column  133 ; this stream contains a lower concentration of heavy materials subject to decomposition than the feed to column  130 ; the column pressure is elevated with respect to that of the low-pressure xylene column according to the invention, as discussed previously, in order to effect energy savings through concomitantly higher temperatures which may be employed to exchange heat at useful levels. The temperature of the overhead vapor from the high-pressure xylene column  133  therefore is sufficient to provide useful energy to other services in an aromatics complex. As shown, the temperature of the overhead vapor is sufficient to reboil the low-pressure xylene column  130  in reboiler  135 , providing reflux to column  133  and a net stream in conduit  136 . A small net bottoms stream in conduit  138  preferably is sent to low-pressure column  130  for recovery of remaining C 8  aromatics. 
         [0038]    Alternatively or in addition, the temperature of overhead vapor from high-pressure xylene column  133  is sufficient to generate steam useful for heating services or to reboil columns in other processing units. Such steam is generated usually at a pressure of in excess of about 300 kPa, preferably at least about 500 kPa, and most preferably about 1000 kPa or higher. The overhead stream may be indirectly heat exchanged with a water circuit which feeds a steam drum. Most usually, boiler feed water is heated in heat exchangers decoupled from the steam drum. Multiple water circuits serving different exchangers are arranged in parallel with each other and feed a single steam drum to provide a steam product of a desired pressure for which only one set of instrumentation is needed. Such steam systems are well known, and details can be added through such teachings as found in U.S. Pat. No. 7,730,854 which is incorporated herein by reference. 
         [0039]    Energy recovery according to the present invention, often involving close temperature approaches between process fluids, is improved through the use of exchangers having enhanced nucleate boiling surface. Such enhanced boiling surface can be effected in a variety of ways as described, for example, in U.S. Pat. Nos. 3,384,154, 3,821,018, 4,064,914, 4,060,125, 3,906,604, 4,216,826, 3,454,081, 4,769,511 and 5,091,075, all of which are incorporated herein by reference. Such high-flux tubing is particularly suitable for the exchange of heat between the overhead of the second high-pressure xylene column and the reboiler of the first low-pressure xylene column or for the generation of steam from the xylene-column overhead. 
         [0040]    Typically, these enhanced nucleate boiling surfaces are incorporated on the tubes of a shell-and-tube type heat exchanger. These enhanced tubes are made in a variety of different ways which are well known to those skilled in the art. For example, such tubes may comprise annular or spiral cavities extending along the tube surface made by mechanical working of the tube. Alternately, fins may be provided on the surface. In addition the tubes may be scored to provide ribs, grooves, a porous layer and the like. 
         [0041]    Generally, the more efficient enhanced tubes are those having a porous layer on the boiling side of the tube. The porous layer can be provided in a number of different ways well known to those skilled in the art. The most efficient of these porous surfaces have what are termed reentrant cavities that trap vapors in cavities of the layer through restricted cavity openings. In one such method, as described in U.S. Pat. No. 4,064,914, the porous boiling layer is bonded to one side of a thermically conductive wall. An essential characteristic of the porous surface layer is the interconnected pores of capillary size, some of which communicate with the outer surface. Liquid to be boiled enters the subsurface cavities through the outer pores and subsurface interconnecting pores, and is heated by the metal forming the walls of the cavities. At least part of the liquid is vaporized within the cavity and resulting bubbles grow against the cavity walls. A part thereof eventually emerges from the cavity through the outer pores and then rises through the liquid film over the porous layer for disengagement into the gas space over the liquid film. Additional liquid flows into the cavity from the interconnecting pores and the mechanism is continuously repeated. Such an enhanced tube containing a porous boiling layer is commercially available under the trade name High Flux Tubing made by UOP, Des Plaines, Ill. 
         [0042]      FIG. 4  illustrates examples of specific units within an aromatics complex in which direct heat exchange of overhead from one or more higher-temperature columns to reboilers of one or more lower-temperature columns could achieve energy savings, using numerical designations of processes from  FIG. 2 . Overhead in conduit  134  from the high-pressure xylene column  133  has a temperature sufficient to provide energy to reboil extract column  152  via reboiler  235 , condensing the xylene overhead in conduit  236  for return to  133  as reflux or net overhead. The extract column may be pressurized such that overhead in conduit  256  has a sufficient temperature to reboil finishing column  155 , which preferably operates at vacuum pressure, via reboiler  257 , condensing extract column overhead in conduit  258 . As before, the product para-xylene is recovered in conduit  156 . 
         [0043]      FIG. 5  summarizes a number, not exhaustive or exclusive, of direct heat-exchange possibilities related to  FIG. 2 . High-pressure xylene column  133  may provide heat to reboil one or more of low-pressure xylene column  130 , extract column  152 , and raffinate column  159 . The low-pressure xylene column  130  may provide heat to reboil extractive distillation column  120 . A pressurized extract column  152  may provide heat to reboil one or more of benzene column  123  and finishing column  155 . A pressurized raffinate column  159  may provide heat to reboil one or more of reformate splitter  114 , toluene column  126 , and deheptanizer  164 . 
         [0044]      FIG. 6  summarizes nonexhaustive examples of indirect heat-exchange possibilities through the generation of medium-pressure steam. Overhead streams  230  ( FIG. 2 ) from the low-pressure xylene column  130  and  260  ( FIG. 2 ) from the pressurized raffinate column  159  may generate medium-pressure steam in header  100  at 0.6 to 2 MPa, and preferably 0.7 to 1.5 MPa which can be used to reboil one or more of reformate splitter  114 , extractive distillation column  120  and toluene column  126  with the added potential of exporting steam to other units. Such generation and usage of steam can be considered as a supplement or substitute for other energy savings such as those described in  FIG. 5 . For example, the high-pressure xylene column  133  may provide heat to reboil the low-pressure xylene column  130  and extract column  152 , which in turn reboils the benzene column  123  and finishing column  155 . 
       EXAMPLE 
       [0045]    The combination of steam generation and direct heat exchange described above in  FIG. 6  was evaluated in terms of payback on investment. The base case is the facility described in  FIG. 1  and the case of the invention is the  FIG. 6  case as applied to the flow scheme in  FIG. 3 . The relative key parameters for the production of para-xylene are as follows: 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Base Case 
                 Invention 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Fuel consumption 
                 1.0 
                 0.922 
               
               
                   
                 Net Steam consumed 
                 1.0 
                 0 
               
               
                   
                 generated 
                 1.0 
                 1.35