Abstract:
A method for recovering a CO 2  stream free of highly reactive volatile organic compounds from an ethylene oxide production process that employs a carbonate/bicarbonate mixture to separate CO 2  and form a carbonate concentrate that is treated in a carbonate flash unit followed by a CO 2  stripper, wherein at least one of 1) an additional flash unit is employed on the carbonate concentrate upstream of the CO 2  stripper, and 2) a side draw CO 2  stream is taken from the CO 2  stripper.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to the production of carbon dioxide (CO 2 ) that is essentially free, if not completely free, of highly reactive volatile organic compounds (HRVOC). More particularly, this invention relates to the production of CO 2  from an ethylene oxide (EO) production plant wherein at least a portion of the CO 2  produced is essentially free, if not totally free, of HRVOC.  
         [0003]     2. Description of the Prior Art  
         [0004]     For sake of clarity, this invention is described using descriptive terms for operating process units that are routinely used in a typical EO production plant.  
         [0005]     In a conventional ethylene oxide production plant, ethylene and oxygen are reacted at an elevated temperature of from about 500° F. (F) to about 550 F under slight pressure in the presence of a catalyst to form EO. The reaction is fast, on the order of about 1 second, and high yielding, approaching 90%. The EO reaction product is normally gaseous and contains newly formed EO, unreacted ethylene, and by-products, mainly CO 2 .  
         [0006]     The EO is separated from the ethylene and by-products in a water-wash column (EO scrubber) in the manner of a solvent recovery process. The vast majority of the EO is absorbed by the water, and the ethylene and by-products are not. The resulting EO/water solution is then steam stripped and purified by fractionation (thermal distillation). The ethylene and by-products are split with the ethylene being recycled to the reactor that forms EO aforesaid, and the by-products and very minor amounts of ethylene being separately recovered for other processing and disposition. This process yields about 1.4 pounds of EO per pound of ethylene feed at high yields, e.g., about 89%. Yields can, however, vary widely from plant to plant worldwide.  
         [0007]     EO as a liquid boils at 56 F to form a colorless gas at room temperature. EO is traded commercially as a high purity, e.g., 99.7%, technical grade chemical. Because of its volatility under normal conditions, care must be given in the storage and transportation of EO to keep it out of the ambient atmosphere. EO is an intermediary chemical useful for making a number of derivatives of commercial value. For a full and complete description of an EO production plant see U.S. Pat. No. 6,727,389 to Viswanathan.  
         [0008]     An EO production plant is a high volume process. For example, a typical plant can produce, just as by-product, tens of thousands of pounds of CO 2  per hour that must be disposed of in an environmentally acceptable manner. This is so because the CO 2  can carry with it small amounts, parts per million (ppm) levels, of HRVOC materials such as ethylene, EO, and methane. A substantial portion of this CO 2  is sold to CO 2  vendors who process the CO 2  they purchase in an environmentally sound manner. However, such CO 2  vendors do not always purchase the full amount of CO 2  being produced at a given EO plant, thereby leaving the EO plant management with the need to dispose of the CO 2  not purchased by the CO 2  vendors. CO 2  can be vented to the ambient atmosphere, but it should not contain any HRVOC. Accordingly, it is highly desirable to be able to remove all or essentially all of the HRVOC from CO 2  produced by an EO plant and not sold to a CO 2  vendor.  
         [0009]     This invention has non-environmental considerations as well, e.g., the employment of CO 2  in a high quality application when the environment is less of an issue.  
       SUMMARY OF THE INVENTION  
       [0010]     In accordance with this invention there is provided a method for splitting the CO 2  product of an EO plant in a manner such that the CO 2  that is not sold to a CO 2  vendor, or otherwise similarly disposed of, is processed to remove the HRVOC there from before EO plant disposition thereof.  
         [0011]     This is accomplished by employing an additional flash unit at a strategic point in the plant process, taking a side draw stream from a special location on the existing CO 2  stripper unit in the plant, or doing both. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows a block diagram of the EO reactor and EO absorber and their interrelation in a conventional EO plant, including the CO 2  containing recycle stream loop that flows between the absorber and the reactor.  
         [0013]      FIG. 2  shows a block diagram of one embodiment within this invention, and more particularly shows both the additional flash unit and the side draw embodiments of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]      FIG. 1  shows the first two units of a typical EO production plant  1 . The first unit is EO reactor  2  wherein an ethylene feed and a separate oxygen feed are reacted to form EO with the consequent formation of by-product as aforesaid. Unit  2  forms a first, normally gaseous, reaction product stream  3  that contains a major amount of newly formed EO, a substantial amount of free CO 2  (CO 2 ), and a minor amount of unreacted ethylene.  
         [0015]     The second unit in  FIG. 1  is the EO absorber  4  which is a water wash scrubber that operates in known manner as a solvent extractor by absorbing (dissolving) EO out of stream  3  to form a principally EO/water stream  5 . Stream  5  is normally at a temperature of from about 75 to about 105 F under a pressure of about 220 psig, and is primarily a water stream that can contain from about 3 to about 5 weight percent (wt. %), based on the total weight of stream  5 , EO dissolved therein. Stream  5  is then further processed in a manner that is not shown here for sake of brevity, but is shown in its entirety in U.S. Pat. No. 6,727,389. Such further processing recovers, as products of the EO plant, purified EO and/or derivatives thereof such as one or more glycols. For example, in some processes the EO is not purified, but rather is converted into ethylene glycol.  
         [0016]     EO absorber  4  also produces a normally gaseous by-product stream  6  that is made part of the plant recycle gas loop. The recycle gas in stream  6  can contain from about 55 to about 97 wt. % methane, from about 20 to about 40 wt. % unreacted ethylene, from about 3 to about 5 wt. % CO 2 , and a trace of EO, all wt. % being based on the total weight of stream  6 . Stream  6  is recycled to EO reactor  2  by way of the plant recycle gas loop for reuse of the unreacted ethylene in EO reactor  2 , but in so doing all or a substantial part of recycle stream  6  can be processed for the removal of CO 2  there from. This is accomplished, in one embodiment within this invention, as shown in  FIG. 2 .  
         [0017]     In  FIG. 2 , typical EO plant recycle loop  6  is processed, using conventional EO plant terminology, in a CO 2  absorber  8 , followed by a carbonate flash unit  11 , and finished in a CO 2  stripper  18  to produce CO 2  product  19  that carries with it small amounts of ethylene, EO, and methane (HRVOC). This is the CO 2  product that is normally sold at least in part to CO 2  vendors. The remainder of this product that is not sold to CO 2  vendors and that contains HRVOC is what this invention addresses.  
         [0018]     Recycle  6  is normally passed into CO 2  absorber  8 , which is a packed column reactor. In absorber  8 , recycle  6  is typically contacted with a mixture of alkali metal carbonates such as potassium carbonate and potassium bicarbonate. Carbonate mixture  7  contains a majority, e.g., more than about 50 wt. %, of alkali metal carbonate (carbonate), and a minority, about 50 wt. % or less of alkali metal bicarbonate (bicarbonate), both wt. % being based on the total weight of the mixture. This mixture, having less bicarbonate relative to the carbonate, is called “lean carbonate.” Lean carbonate  7  can be composed of from about 76 to about 80 wt. % water, from about 20 to about 24 wt. % of the aforesaid carbonate/bicarbonate mixture, and no carbon dioxide.  
         [0019]     Absorber  8  is operated under conditions that favor the conversion of CO 2  to bicarbonate which conditions are, generally, a temperature of from about 195 to about 220 F under a pressure of from about 290 to about 300 psig. In absorber  8  one mole of CO 2  reacts with one mole of carbonate and one mole of water to form two moles of bicarbonate thereby removing one mole of free CO 2  from stream  6 , while increasing the amount of bicarbonate present with a corresponding decrease in the amount of carbonate present. In this manner all the CO 2  present in unit  8  is eventually converted to and tied up in bicarbonate, and, thereby effectively removed from recycle  6  as free CO 2 , so that the relative proportions in the carbonate/bicarbonate mixture is changed from “lean” to “rich” in unit  8 . Accordingly, the rich carbonate  10  removed from CO 2  absorber  8  has more bicarbonate than carbonate, e.g., a major amount of bicarbonate and a minor amount of carbonate. Rich carbonate  10  is essentially all carbonate/bicarbonate mix but carries with it minor amounts of ethylene, EO, and methane dissolved therein.  
         [0020]     Recycle gas  6 , after processing in unit  8 , is depleted in CO 2 . It is then removed from unit  8  as stream  9 , and returned to the recycle gas loop for return to EO reactor  2 . Rich carbonate  10  is passed to carbonate flasher  11 , and therein flashed with from about 5 to about 10 psig methane primarily to remove ethylene and EO from stream  10 . The term “carbonate flasher” is a term of art in an EO plant in that it does not flash carbonate, but rather flashes off primarily ethylene, EO, and methane which are removed overhead from carbonate flasher  11  as stream  12 . Some very slight amount of CO 2  is also flashed off, but care is taken to keep this to a minimum because flashed material stream  12  next routinely goes to residual gas absorption and compression steps, and care is taken not to load up the compressors with CO 2 . Carbonate flasher  12  normally operates at a temperature of from about 190 to about 210 F at a pressure of from about 10 to about 15 psig.  
         [0021]     A rich carbonate concentrate which is reduced, but not devoid of, its ethylene, EO, and methane content, is removed as stream  13 . In a conventional EO plant, stream  13  next passes directly to CO 2  stripper  18  which is operated under conditions that favor the conversion of carbonate back to bicarbonate with the consequent release of free CO 2 , two moles of bicarbonate yielding one mole of CO 2 , one mole of carbonate, and one mole of water. Unit  18  operates at a temperature of from about 224 to about 230 F under a pressure of from about 5 to about 10 psig. CO 2  stripper  18  is a column that contains a large number of spaced apart trays along a substantial portion of the internal length thereof, as discussed in greater detail here in after, so that the material being treated in that column, as it progresses through the column, moves to the first tray it encounters after entering the interior of the column, and then from tray to tray toward the opposite end of the column. During such travel more and more bicarbonate is transformed into carbonate. Normally, such a column will contain at least about 16 trays and always more than seven trays.  
         [0022]     Absent this invention, CO 2  stripper  18  normally generates an overhead CO 2  product stream  19 , at about 4 to 6 psig, that is composed essentially of CO 2  and water, with ethylene, EO, and methane each present in very small (ppm) amounts, individually and collectively. Stream  19  normally is at a temperature of from about 230 to about 235 F and contains about 2 pounds of water per pound of CO 2 . On a water free basis, this stream is 99.9% pure CO 2 . Stream  19  is then passed to one or more coolers to condense most of the water there from and produce a pure CO 2  product that contains ethylene, EO and methane each on the ppm level, e.g., from about 200 to about 2,500 ppm collectively. This is the stream that is sold at least in part to one or more CO 2  vendors.  
         [0023]     Bottoms  20  of CO 2  stripper  18 , after flashing under an atmospheric to slight vacuum to remove the last vestiges of CO 2  present, is principally a carbonate/bicarbonate mixture which can contain more, e.g., major amount of, carbonate and less, e.g., minor amount of, bicarbonate, so that it again qualifies as lean carbonate suitable for return to and reuse in CO 2  absorber  8 .  
         [0024]     In accordance with this invention, at least one of two modifications (improvements) in apparatus and operation are employed. For sake of brevity, these are both shown in  FIG. 2 , however, it is to be understood that this invention does not require both modifications to be used at the same time. One or the other of such modifications can be employed by itself in an EO plant and still fall within the scope and spirit of this invention.  
         [0025]     The first of such modifications is the use of a separate, additional steam flashing column (unit)  14 . In this improvement, the rich carbonate concentrate  13  passes in whole or in part to additional flasher  14  wherein, by the use of steam  15 , CO 2  is flashed off as a first separate CO 2  stream product  16  which contains HRVOC, and can be sold to a CO 2  vendor. Additional flasher  14  is operated under conditions that favor the removal of all the ethylene, EO, and methane from rich carbonate concentrate  13  along with the CO 2  in stream  16 . This can leave a rich carbonate concentrate stream  17  which is at least substantially depleted in or free of HRVOC. Accordingly, if, for example, stream  17  is treated in unit  14  so as to be free of ethylene, EO, and methane, stream  17 , after passage by way of line  27  to CO 2  stripper  18  and processing in unit  18 , yields a second, separate CO 2  product stream  19  that is free of HRVOC and can be safely vented to the ambient atmosphere. Additional flasher  14  can be operated at a temperature of from about 190 to about 230 F under a pressure of from about −12 to about 0 psig to either severely deplete or entirely remove HRVOC there from.  
         [0026]     The second of such modifications is the recovery of a CO 2  side draw stream  21  from CO 2  stripper  18  in addition to CO 2  stream  19 . This side draw improvement can be employed alone without the use of additional stripper  14  at all, in which case stream  13  is passed directly to unit  18  by way of lines  25  and  27 . This side draw can be used in combination with additional stripper  14  if stream  17  is not completely free of HVROC.  
         [0027]     In this second improvement, side draw stream  21  is removed from the interior of stripper  18  at least “three trays down,” including the first tray to be encountered, as explained more fully here in after. Side draw stream  21 , when removed in accordance with this invention, has been found to be essentially pure CO 2  and water, contain no HRVOC, and be suitable for venting. Pursuant to this invention, side draw stream  21  is drawn from below the inlet for stream  27  along the length of unit  18  at a location that is from “three to seven, inclusive, trays down,” including the first or upper most tray encountered after entry into the interior of unit  18 . Side draw stream  21  can contain a substantial amount of water, and is passed through at least one cooler  22  to remove water  23  and produce a third separate CO 2  stream that is essentially pure CO 2  on a water free basis, and that is HRVOC free. Cooler  22  can be one or more fin fan cooler (not shown) that already exists in the EO plant for the purpose of cooling and dewatering stream  19 .  
         [0028]      FIG. 2  shows the first or upper  5  trays ( 30  through  34 , inclusive) that are in the upper half of upstanding (essentially vertical) stripper  18 . Tray  30  is the first tray that stream  27 , upon entering the interior of unit  18 , encounters as it falls downwardly inside unit  18  as shown by arrow  29 . Trays  31  through  34 , inclusive, are spaced apart from one another below the top (first or initial) tray  30 . Tray  31 , therefore is “two trays down,” i.e., the second tray stream  27  encounters after entering unit  18  or one tray below first tray  30 .  FIG. 2  it shows side draw  21  being taken at a location “four trays down.” This is three trays below first tray  30 .  
         [0029]     In this invention additional stripper  14  can be used alone, and without side draw  21 . Or side draw  21  can be employed by itself without additional stripper  14 . Or both can be used in combination as shown in  FIG. 2 . When additional stripper  14  is used by it self, side draw  21  not being used at all, first separate CO 2  product stream  16  can be made to contain all the HRVOC present in stream  13 , while second separate CO 2  product stream  19  will be free of HRVOC. When additional stripper  14  is not used at all, and side draw  21  is used by itself, stream  19  can be made to contain all the HRVOC present in stream  13 , side draw stream  21  then being HRVOC free. If additional stripper  14  does not remove, by way of line  16 , all the HRVOC present in line  13 , then side draw stream  21  can be used in combination with additional stripper  14  to produce an HRVOC free CO 2  stream  21 . Stream  19 , in such a case, could have minute quantities of HRVOC. Other combinations will be obvious to those skilled in the art.  
         [0030]     Accordingly, if an EO plant  1  of  FIG. 1  produces 50,000 pounds per hour (50 kpph) total CO 2  in stream  13  of which 30 kpph can be sold to a CO 2  vendor, 20 kpph needs to be disposed of else where. If only additional flasher  14  is employed pursuant to this invention, the plant can be operated so that stream  16  is tailored to a volume of 30 kpph which contains all the HRVOC in stream  13 . This would leave stream  19  at a volume of 20 kpph and HRVOC free. If only side draw  21  is employed pursuant to this invention, the plant can be operated so that stream  19  has a volume of 30 kpph and contains all the HRVOC in stream  13 , while side draw  21  has a volume of 20 kpph, all HRVOC free.  
       EXAMPLE I  
       [0031]     An EO plant as described here in above provides a recycle gas loop having a volume of about 2,000 kpph and containing about 4 wt. % CO 2  based on the total weight of the loop gas. About 1,000 kpph of this loop gas, stream  6 , is split off from the recycle loop and passed to CO 2  absorber  8 . Stream  9  from absorber  8  returns about 950-970 kpph of recycle gas to the loop, stream  9  having a composition consisting essentially of a mixture of ethylene and methane, up to about 1 wt. % CO 2 , and a trace of EO, all wt. % being based on the total weight of stream  9 .  
         [0032]     CO 2  absorber  8  is a packed, as opposed to tray containing, column that operates at a temperature of about 200 F under a pressure of about 290 psig, and contacts about 2,000 kpph of lean carbonate  7  with the incoming recycle stream  6  to convert all the CO 2  in stream  6  to bicarbonate in the manner aforesaid. Lean carbonate stream  7  consists essentially of about 22 wt. % of a mixture of potassium carbonate and potassium bicarbonate (carbonate/bicarbonate), about 78 wt. % water, and no CO 2 , all wt. % being based on the total weight of stream  7 .  
         [0033]     Rich carbonate stream  10  is formed in absorber  8  and removed there from to carbonate flasher  11  from which stream  12  in the volume of about 3 kpph is removed, stream  12  consisting essentially of ethylene, EO, methane, steam, and a slight amount of CO 2 . Stream  12  is sent to residual gas absorbing and compressing operations.  
         [0034]     Carbonate flasher  11  is operated at about 200 F under a pressure of about 15 psig using 1 kpph of methane at about 10 psig as the stripping medium. This yields a rich carbonate concentrate stream  13  at about 190 F which consists essentially of carbonate/bicarbonate mixture having a major amount of potassium bicarbonate, no CO 2 , and about 50 ppm of a mixture of ethylene, EO, and methane.  
         [0035]     Rich carbonate concentrate  13  is passed to additional flasher  14  which uses steam as the stripping medium, and operates at a temperature of about 220-230 F under a pressure of about −10 psig. In additional flasher  14  a first separate CO 2  product stream  16  is formed which is recovered as overhead in the volume of about 5-10 kpph. Stream  16  contains all the ethylene, EO, and methane that were present in stream  13 . The bottoms product  17  of flasher  14  consists essentially of a rich carbonate/bicarbonate mixture which is passed to CO 2  stripper  18  by way of line  27 .  
         [0036]     CO 2  stripper  18  contains 16 spaced apart trays, and is operated at a temperature of about 225 F under a pressure of about 5 psig using steam  28  as the stripping medium. In stripper  18  a majority of the potassium bicarbonate therein is converted to potassium carbonate thereby freeing CO 2 . The ethylene, EO, and methane were removed by way of first, separate CO 2  stream  16 . Accordingly, the second, separate CO 2  stream  19  is removed from stripper  18  in the volume of about 20-30 kpph, is HRVOC free, and is ready for disposition other than sale to a CO 2  vendor, e.g., venting. Stream  19  can then be cooled in one or more fin fan coolers to remove water there from.  
         [0037]     The carbonate/bicarbonate mixture in stripper  18 , after processing as aforesaid in stripper  18 , is recovered as bottoms  20  there from. Stream  20 , after a separate vacuum steam stripping operation to remove the last vestiges of CO 2  there from, contains no CO 2 . After such processing stream  20  contains a carbonate/bicarbonate mixture whose potassium carbonate content is significantly greater than its potassium bicarbonate content so that it is suitable for use as lean carbonate. It, therefore, is returned by way of line  20  for reuse in absorber  8 .  
       EXAMPLE II  
       [0038]     The process of Example I above was carried out except that additional flasher  14  was not employed. Instead, stream  13  was passed directly to CO 2  stripper  18  by way of lines  25  and  27 , and a side draw stream  21  was removed from stripper  18  separate from, and in addition to, CO 2  product stream  19 .  
         [0039]     In this example, side draw stream  21  was removed from stripper  18  one tray below first tray  30  that entering stream  27  initially encountered inside stripper  18 , i.e., “two trays down” after stream  27  enters stripper  18 . This “two trays down” location is shown as tray  31  in  FIG. 2 . In this example side draw stream  21  contained no EO or methane, but its ethylene content was only 90 wt. % less than that of stream  27 , second, separate CO 2  product stream  19  containing the remainder of the ethylene, EO, and methane that was present in stream  13 . Side draw stream  21  was sent to fin fan coolers that already existed in the plant to remove water there from and produced a third separate CO 2  product that was 99.9 wt. % CO 2  on a water free basis.  
       EXAMPLE III  
       [0040]     The process of Example II above was repeated except that side draw stream  21  was taken “four trays down,” i.e., from tray  33 , as shown in  FIG. 2 . This was four trays down after stream  27  entered unit  18 , and 3 trays below upper most tray  30  which stream  21  encountered first. The side draw stream of this Example contained no ethylene, EO, or methane.