Abstract:
A method for forming multiple tripropylene glycol products of acrylate grade comprising forming a composition of dipropylene glycol, tripropylene glycol, tetrapropylene glycol and heavier, and at least one aldehyde, separating from said composition tripropylene glycol containing aldehyde, mixing with the thus separated tripropylene that contains aldehyde an aldehyde controlling additive to form a first individual tripropylene glycol product that contains glycol controlling additive and has an aldehyde content below that required for acrylate grade tripropylene glycol, separating from the remainder of the composition a tripropylene glycol concentrate, adding an aldehyde controlling additive to the concentrate, and separating from the concentrate a second individual tripropylene product that has an aldehyde content below that required for acrylate grade tripropylene glycol and is free of aldehyde controlling additive.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the production of tripropylene glycol (TPG) and, more particularly, to the production of acrylate grade TPG. 
     2. Description of the Prior Art 
     Acrylates, including methacrylates, are valuable chemical building blocks that find many commercial applications such as surface coatings, adhesives, leather and textile finishing, paper coating, and the like. Many commercial acrylate uses require good clarity (transparency) of the final article, e.g., transparent containers. 
     Urethanes also have valuable chemical uses in the formation of flexible foams, rigid foams, and laminates. Polyurethanes have a wide range of commercial uses such as soft cushioning applications in furniture, vehicles, and the like; laminate padding in clothing; rigid foam insulation; and finishes/varnishes, particularly for surfaces that undergo abusive and/or abrasive wear. 
     Producers of acrylates and urethanes that go into the making of polyacrylates and polyurethanes often use TPG in their manufacturing processes. 
     TPG is typically made by hydrolyzing propylene oxide in known manner, but, in so doing, monopropylene glycol (MPG), dipropylene glycol (DPG), tetra propylene glycol (TTPG) and heavier glycols; and one or more aldehydes, including propionaldehyde and aldehydes derived from DPG, TTPG, and the like, are formed in admixture with the desired TPG. Essentially pure TPG must be extracted from this mixture before it can be used in the commercial manufacture of acrylates and urethanes. 
     Aldehydes mentioned here in above tend to be carried along with the TPG into the acrylate and urethane manufacturing processes. Such aldehydes can have an adverse affect on the clarity of acrylate products that is not tolerable in certain commercial applications. Urethane manufacturing requirements as to the aldehyde content of the TPG used in such manufacture is not as rigid as acrylate manufacturing requirements since, for example, polyurethane product clarity is not important in most polyurethane uses. Therefore, TPG used in urethane manufacturing can have a larger aldehyde content than TPG used in acrylate manufacturing. 
     Accordingly, there is what is known in the art as acrylate grade TPG and urethane grade TPG. At present, the dividing line between the two grades of TPG is 20 parts per million (ppm) of total aldehyde content in the TPG in question. Acrylate grade TPG can contain no more than 20 ppm of total aldehyde content. Urethane grade TPG can contain more than 20 ppm of total aldehyde content. Both ppm contents are reported as ppm “CHO.” 
     Therefore, it is important to control the aldehyde content of the final TPG product when the TPG is produced for use in the manufacture of acrylates and urethanes, and particularly acrylates. 
     The aldehyde content of a TPG product is typically controlled by the use of at least one additive (agent) which reacts with and neutralizes most, if not all, of the aldehyde present in that product. Such aldehyde controlling (reducing) additives are well known in the art and include alkali borohydrides, particularly sodium borohydride. The borohydride, at present, reduces the aldehyde content of TPG to the lowest levels achievable. By use of such additives, the aldehyde content is substantially reduced, if not eliminated, but the additive itself can be left in the TPG product. 
     Depending on its individual processes and desires, an acrylate manufacturer may be willing to use a TPG product that contains such aldehyde controlling additives, or may, on the other hand, require that the TPG product it buys be essentially free of such additives. In either case, to be acrylate grade, the TPG product must contain no significant amount, no more than 20 ppm, total aldehyde, and often no more than 10 ppm aldehyde. 
     Accordingly, it is desirable to be able to provide an acrylate grade TPG product either with or without the presence of an aldehyde controlling additive or additives. This invention does just that, and does so with significant cost savings over the practice of the prior art. 
     The prior art practice, which will be discussed in greater detail here-in-after, has been to make in a single TPG product that is essentially free of aldehyde controlling additive. This has necessitated the transport of large volumes of TPG to a final processor (toiler) which, for a fee, separates out an essentially pure, single, final TPG product that contains essentially no significant amount, as defined above, of such additive. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, two separate final TPG products can be produced. The first such product is acrylate grade and contains aldehyde controlling additive, but it has the lowest aldehyde content of the two products. The second such product also is acrylate grade, but it has a larger aldehyde content than the first product, and is essentially free of aldehyde controlling additive. 
     Thus, this invention has a distinct processing advantage in flexibility. It can provide a TPG product that can meet the requirements of acrylate manufacturers that demand the lowest possible aldehyde content (the first product above), and at the same time provide a concentrate that is a source of raw material for a TPG product that is also acrylate grade but which is essentially free of aldehyde controlling additive (the second product above). This raw material can then readily be made into the second product aforesaid in the same (single) process or a subsequent independent process as shown in  FIG. 3  hereof. 
     In addition to its processing flexibility advantages, this invention has multiple manufacturing cost advantages. As explained in detail here-in-after, by producing the aforementioned concentrate of this invention, the transportation of a very substantial volume of TPG containing material to a toiler for separation of an additive-free TPG product is materially reduced. By virtue of such concentrate, this invention reduces not only the cost of transporting a very large volume of material to the toiler, but also the toller&#39;s charge for making the final separation from the concentrate of a TPG product free of aldehyde controlling additive. Finally, the bottoms material remaining after the second product of this invention is recovered is of a substantially smaller volume than that of the prior art. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a block-flow diagram of the prior art process mentioned above. 
         FIG. 2  shows a block-flow diagram of a process within this invention. 
         FIG. 3  shows a block-flow diagram of one independent process for producing the second TPG product aforesaid. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an exemplary and typical prior art process used commercially at present wherein the mixture of glycol and aldehyde formed from reacting propylene oxide with water, as discussed above, is used as feed  1  to a conventional distillation tower (column)  2  for the removal of an overhead MPG stream  3  from feed  1 . The remainder of the MPG depleted feed comprises MPG and heavier glycol molecules plus one or more aldehydes (MPG plus), and it is passed by way of conduit or line  4  to a separate distillation tower  5 . In tower  5 , a mixture of MPG and DPG is removed overhead at line  6 , and an intermediate DPG stream is removed medially at  7 . The remainder of stream  4  comprises DPG and heavier glycol molecules plus at least one aldehyde (DPG plus), and it is passed via line  8  to a separate distillation tower  9  wherein an overhead stream  10  composed essentially of the remaining DPG and a minor amount of TPG is removed. The remainder of stream  8  is substantially depleted in MPG and DPG and, as represented by the combination of side stream  11  and bottom stream  12 , is stream  14 . Stream  14  comprises TPG and heavier glycol molecules plus at least one aldehyde (TPG plus remainder). This TPG plus remainder stream  14  is now ready for processing to produce the desired prior art TPG product. 
     The prior art practice has been to ship this TPG plus remainder stream  14  which contains most of the TPG present in the original feed to an independent toiler for the extraction of the desired TPG product. For sake of simplicity, the transport by truck, train, ship, or a combination thereof, to a toiler is represented by line  14 , although, in reality, such transportation is not as straight forward nor inexpensive as is represented by line  14 . 
     For example, in a typical TPG plant, stream  14  can vary from 14 to 18 million pounds per year (mpy). At a typical 2 cent per pound transportation cost, the transportation fees are substantial for this size of volume. Add to these transportation costs the toller&#39;s fee of from 10 to 25 cents per pound, and substantial total costs are realized in forming from stream  14  a single product  17 , which product is essentially free of aldehyde controlling agent. As shown later, this invention reduces both the transportation and toiler costs. In the toller&#39;s plant, stream  18  can be as large as 2 mpy which is a substantial non-product stream to be disposed of in some form or fashion. 
     At the toller&#39;s plant, stream  14  is mixed via line  13  with at least one aldehyde controlling additive such as sodium borohydride (NaBH4). The amount of additive used can vary widely, but it is that which is sufficient to reduce the aldehyde content of stream  14  to a level that would be acrylate grade as aforesaid, a straight forward determination for one skilled in the art. Generally, from about 50 to about 2,000 parts per million (ppm) weight percent additive(s) are used based on the total weight of the mixture. The mixture resulting from streams  14  and  13  is passed into a conventional distillation tower  15 . An overhead fraction  16  is removed from tower  15  which is composed essentially of remaining DPG and a minor amount of TPG. The sole prior art TPG product is separately removed as an intermediate stream  17 . This product is essentially free of additive. The tower bottoms  18  contain TPG and heavier glycol molecules (TPG plus) and the additive remaining from line  13 . 
     Thus, this prior art process produces a single acrylate grade TPG product  17  that contains essentially no aldehyde controlling additive. Because product  17  contains no additive, this product does not have the lowest possible aldehyde content, and may not be acceptable to acrylate manufacturers who demand the lowest possible aldehyde content in the TPG they purchase for their manufacturing purposes. 
     For sake of ease of comparison only,  FIG. 2  shows the same towers  2 ,  5 , and  9  as  FIG. 1 , and the same streams  3  through  10  of  FIG. 1 . This invention may use such a processing arrangement or a different arrangement known in the art and is not limited to this particular arrangement. Where this invention departs from the sample prior art process of  FIG. 1  is in tower  9 . 
     In tower  9  feed  8  comprises a mixture of DPG, TPG, TTPG, glycols heavier than TTPG, and at least one aldehyde compound. A blend of mostly DPG and a minor amount of TPG is removed via stream  10  in normal fashion. After this removal, the material remaining in tower  9  contains a major amount, at least about 85 weight percent (wt %), TPG. The material also contains minor amounts of DPG and lighter molecules (less than 1 wt %), TPG and heavier glycol molecules (less than about 13 wt %), and one or more aldehydes (less than 1 wt %). All wt % are based on the total weight of such material. The prior art, as shown in  FIG. 1 , stream  14 , transports in one form or fashion the entirety of this material to a toller for separate processing as described above. 
     This invention departs from the prior art in that it operates tower  9  in such a manner, known in the art, to effect a separate intermediate cut from this material, that cut being stream  20  of  FIG. 2 . In accordance with this invention, stream  20 , in essentially its entirety, is deliberately kept physically separate and is mixed with at least one aldehyde controlling additive  21 . This produces the first final TPG product of this invention. This product is acrylate grade. It contains additive  21  and therefore has the lowest aldehyde content possible, if not essentially zero. 
     By separately removing product  20 , the material left in tower  9  is a concentrate that contains residual TPG and aldehyde and is removed as bottoms  23 . As discussed above, prior art stream  14  is typically a volume of 14 to 18 mpy. Concentrate stream  23  is typically a volume of 4 to 6 mpy. The elimination of transporting and toiler treating costs for 10 to 12 mpy is clearly a substantial cost saving. 
     Stream  23  is a concentrated source of raw material for the second, final TPG product of this invention. Stream  23  can be processed as part of a single process to form such second TPG product, or can be processed in the manner of  FIG. 1 , i.e., transported to an independent toiler for processing. Again for ease of comparison, the formation of the second product of this invention will be discussed in the manner of  FIG. 1 , although the second product of this invention need not necessarily be formed in this manner, other ways being obvious to those skilled in the art. 
       FIG. 3  shows concentrate  23  being subjected to distillation in tower  30 , which tower can be a toller&#39;s column or another tower in the TPG manufacturer&#39;s plant. Concentrate  23  is mixed with aldehyde controlling additive  31  and then fed to tower  3  wherein it is fractionated in known manner to form an overhead  32  comprising essentially DPG and TPG and an intermediate cut  33  which is the second final TPG product of this invention. Tower  30  operating conditions are controlled in known manner so that second product  33 , unlike first product  20 , is essentially free, if not totally free, of aldehyde controlling additive. The bottoms  34  of tower  30  are concentrate  23  depleted of TPG and containing the remaining additive from line  31 . 
     Bottoms  34  are of a volume substantially less than the 2 mpy of line  18 . Thus, by the practice of this invention, substantially less bottoms product must be disposed of or otherwise handled. 
     Accordingly, by the use of this invention, two separate and distinct acrylate grade TPG products are formed, the first product with additive and the lowest aldehyde content of the two products, and the second product with no additive; along with the elimination of the transportation to and toiler treating of a large volume, e.g., 10 to 12 mpy, of material, and a smaller bottoms volume of which to dispose. 
     Any known aldehyde controlling additive can be used in this invention. The most effective additives for reacting with and neutralizing aldehydes are alkali metal borohydrides, particularly sodium borohydride. Other known additives such as methyl ether hydroquinone can be employed in this invention. All are employed by simple mixing at normal processing temperature and pressure, well known in the art, in an amount effective to reduce the aldehyde content of the stream they are mixed with to a level equal to or less than 20 ppm. 
     EXAMPLE 
     A mixture of DPG, TPG, TTPG, glycols heavier than TTPG, and at least one aldehyde is subjected to distillation conditions in tower  9  of  FIG. 2 . Tower  9  is operated at less than 50 millimeters absolute pressure, and a bottom temperature of about 430° F. to produce an overhead  10  consisting essentially of a major amount of DPG and a minor amount of TPG. The material remaining in tower  9  after removal of the overhead consists essentially of about 96.1 wt % TPG, less than 0.1 wt % DPG, about 3.8 wt % TTPG and heavier glycols, and about 300 ppm aldehyde. 
     Under the distillation conditions aforesaid, physically separate and independent intermediate stream  20  is removed and consists essentially of TPG and 270 ppm aldehyde. 
     Sodium borohydride stream  21  is mixed with stream  20  under ambient conditions of temperature and pressure in an amount of less than 50 ppm wt %, which is sufficient to reduce the aldehyde content of that stream to less than 20 ppm and form the first product of the invention. 
     Concentrate  23  is removed from tower  9 . Concentrate  23  consists essentially of about 70.0 wt % TPG, less than 0.1 wt % DPG and lighter, about 30.0 wt % TTPG and heavier, and about 270 ppm aldehyde. Concentrate  23  is fractionated in tower  30  of  FIG. 3 . Tower  30  is operated at a pressure of about 15 millimeters absolute pressure, a bottom temperature of about 400° F., and an overhead temperature of about 330° F. Intermediate stream  33  is removed from tower  30 , and contains about 50.0 wt % TPG, the remainder being DPG and TTPG. The bottoms  34  contain essentially TTPG and heavier plus sodium borohydride and less than 1 wt % TPG. 
     As can be seen from the example, two acrylate grade TPG products are formed, one with sodium borohydride additive, and one without.