Patent Publication Number: US-2018050348-A1

Title: Quench water hydrocyclone

Description:
BACKGROUND OF THE INVENTION 
     The present embodiments relate to chemical processing plants such as for example ethylene plants employing a light feed stock, wherein a quench water tower is used to cool furnace effluent and remove hydrocarbons having high boiling points. 
     Referring by way of example only to a known ethylene plant using a light feedstock, it is common practice to use a quench water tower  10  to cool furnace effluent  12  and remove hydrocarbons therein with high boiling points. In such an arrangement shown for example in  FIG. 1 , the incoming furnace effluent  12 , after being cooled in heat exchangers (not shown), enters the quench water tower  10  where the gas is further cooled by direct contact with quench water. In the quench water tower  10 , water contained in the furnace effluent is largely condensed. In addition, some high-boiling hydrocarbons are also condensed. Liquids  14  at a bottom  16  of the quench water tower  10  include the quench water and condensed water, with condensed hydrocarbons present therein. The liquid is separated, with approximately 95% being cooled and recycled to the quench water tower  10  as quench water. The remaining portion goes to an oil/water separator  18  (or separator  18 ) where the hydrocarbons are drawn off. The water from the separator goes to water processing  20  where the water is purified before being recycled into the process as a dilution stream or sent to waste water treatment. The furnace effluent  12  also contains a small quantity of solids, typically coke particles. These solids and heavy oils are removed in a stream  21  after settling in the separator  18 . 
     The heavy oils and the solids must accumulate in the circulating quench water because only 4% to 5% of the liquid from the quench water tower  10  goes to the separator  18 . Therefore, the concentration of the heavy oils and solids in the circulating quench water is 20 to 25 times the feed concentration to the quench water tower. This high concentration of solids and heavy hydrocarbons causes fouling in the quench water tower  10 . The fouling reduces the tower performance and can causes premature shut-down of the plant for which the quench water tower  10  is associated. 
     Referring also to  FIG. 2  and by way of example only to a known hydrocyclone  22  (also referred to herein as a “cyclone”), such is a device to separate particles in a liquid suspension based upon a ratio of the particles centripetal force to fluid resistance. This ratio is high for dense particles (where separation by density is required) and coarse particles (where separation by size is required), and low for light and fine particles. Hydrocyclones also find application in the separation of liquids having different densities. A hydrocyclone will usually be constructed with a cylindrical section  24  at the top into which a liquid  34 , such as a waste stream, is fed tangentially through an inlet  26 , and a conical base  30 . The angle, and hence length, of the conical base  30  plays a role in determining operating characteristics of the hydrocyclone. A feed (of waste, for example) is introduced into the hydrocyclone through the inlet  26  such that cyclonic action of the cyclone produces a high density stream  38  and a low density stream  36 . 
     Referring again to  FIG. 1 , a pump  40  is provided to deliver the liquids  14  from the bottom  16  of the quench water tower  10  to a heat exchanger  42  to cool the liquids prior to return of same to the quench water tower. As shown in  FIG. 1 , pipes or lines of known construction can connect the bottom  16  with the pump  40  and the separator  18 , the pump  40  and the heat exchanger  2  with the quench water tower  10 . A top  17  of the quench water tower  10  includes an ullage space  11  from which gases in the space can be removed, vented or exhausted from the quench tower for subsequent use such as for example to charge a gas compressor (not shown). 
     Heavy hydrocarbons contained in the quench water tower  10  can become solid at temperatures present in heat exchangers that cool the quench water. The solidified hydrocarbons reduce the ability of these heat exchangers to provide cooling, thereby reducing both plant efficiency and capacity. Fouling by such cooling in a conventional quench water system has been reported by operating companies. 
     The furnace effluent  12  also contains a small quantity of solids, typically coke particles. These solids and heavy oils are removed by settling out in the separator  18 . 
     The heavy oils and the solids accumulate in the circulating quench water because only 4% to 5% of the liquid from the quench water tower  10  goes to the separator  18 . Therefore, the concentration of the heavy oils and solids in the circulating quench water is 20 to 25 times the feed concentration to the quench tower. This high concentration of solids and heavy hydrocarbons causes fouling in the quench water tower  10 . The fouling reduces the tower performance and can causes premature shut-down of the plant for which the quench water tower  10  is associated. 
     SUMMARY OF THE INVENTION 
     There is therefore provided herein a system for processing liquid from a quench water tower, comprising a quench water tower; a first pipe for removing the liquid from the quench water tower; a hydrocyclone in fluid communication with the first pipe and in which the liquid is separated into particulate and liquid constituents by centrifugal force; and an oil-water separator downstream of and in fluid communication with a first outlet of the hydrocyclone. 
     There is also provided herein an apparatus downstream of and for processing liquid from a quench water tower, comprising a first pipe for removing the liquid from the quench water tower; a hydrocyclone in fluid communication with the first pipe and in which the liquid is separated into particulate and liquid constituents by centrifugal force; and an oil-water separator downstream of and in fluid communication with a first outlet of the hydrocyclone. 
     There is further provided herein a method of processing liquid from a quench water tower, comprising removing the liquid from the quench water tower; exerting a centrifugal force on the liquid with a hydrocyclone for separating said liquid into particulate and liquid constituents; and further separating the particulate and liquid constituents downstream of the exerting centrifugal force. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference may be had to the following description of exemplary embodiments considered in connection with the accompanying drawing Figures, of which: 
         FIG. 1  shows a schematic of a known quench water tower system to cool furnace effluent; 
         FIG. 2  shows a perspective view of a known hydrocyclone; and 
         FIG. 3  shows a schematic of a quench water hydrocyclone apparatus and system embodiment according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before explaining the inventive embodiments in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, if any, since the invention is capable of other embodiments and being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. 
     In the following description, terms such as a horizontal, upright, vertical, above, below, beneath and the like, are to be used solely for the purpose of clarity illustrating the invention and should not be taken as words of limitation. The drawings are for the purpose of illustrating the invention and are not intended to be to scale. 
     As used herein, the term “ullage” refers to an amount of a tank or container not being full. 
     Referring to  FIG. 3 , and in general, the total “bottoms” or the liquids  114  from the quench water tower  110 , which include quench water, process water, heavy oils and solids, are delivered from an outlet  123  of the tower into a pipe  125  in fluid communication with a pump  140  and through a line  134  to the hydrocyclone  122  by a pump  140 . The quench water tower resembles a tank or a vessel. The stream  50  moving through the line  134  contains the quench water, the process water and all heavy oils and solids, and is separated by centrifugal force in the hydrocyclone  122 . The heavy stream  138  leaving an outlet of the hydrocyclone  122  contains all the process water, all heavy oils and solids, and is sent through a line  52  to the oil/water separator  118 . The remaining water from the hydrocyclone  122  is the quench water, which is now free of heavy oils and solids and can therefore be recirculated through a line  56  as a quench water stream  54  to the heat exchanger  142  and thereafter to the quench water tower  110 . The embodiment of  FIG. 3  can be used for example in an ethane-cracking ethylene plant. 
     Separation of the liquid stream  50  in the hydrocyclone  122  into particulate and liquid constituents results in a particulate laden heavy stream  138  directed in the line  52  to the oil-water separator  118  from an outlet of the hydrocyclone, and the quench water stream  54  which is directed through a line  56  to the heat exchanger  142  and quench water tower  110 . 
     At least one and in certain applications a plurality of quench water heat exchangers  142  or coolers coact with the line  56  to cool the quench water from the hydrocyclone  122  before the quench water is fed into the tower  110 . 
     Cooled furnace(s) gases in the ullage space  111 , which now have a reduced water content and are without solids and heavy hydrocarbons, are removed or exhausted from the upper portion of the column through a nozzle  58 , for example. The gases from the ullage space  111  are delivered to a charge gas compressor  60  and then to an ethylene plant  62  for further processing. 
     The present embodiments can be applied to quench water towers that use multiple quench water loops. The hydrocyclones can serve to concentrate the heavy oils into the hotter sections of the loops to avoid fouling cooling exchangers. 
     Some gas crackers use either a separate vessel or the lowest section of the quench water tower  110  to saturate the incoming furnace effluent with water. The water used in these systems is separate from the quench water used to cool the furnace effluent. A large portion of the circulating water from the saturator is vaporized by the furnace effluent and is condensed by the circulating quench water. In these plants, a hydrocyclone could be used on the quench water. The hydrocyclone would remove heavy oils and solids from the quench water and return same, along with the condensed water from the saturator, to the saturator system. This would prevent fouling of quench water cooling exchangers. 
     While the quench water hydrocyclone  122  can be used in new constructions, it has advantages for use as a retrofit to an existing plant. For example, the process water effluent from the hydrocyclone can be sent to an oil/water separator  118  which is disposed at a remote location from the quench water tower. Most conventional plants place the oil/water separator next to the quench water tower so the separator can be fed by gravity. 
     The integration of a cyclone improves significantly the quench water quality because i) the quench water can be cooled without concern that the heavy oils will solidify and foul the exchangers, ii) the oil content is significantly reduced in the quench water bottoms, which reduces the possibility of fouling of the quench water tower bottoms, and iii) solids are voided which would otherwise foul the quench water coolers and the quench water tower. 
     It will be understood that the embodiments described herein are merely exemplary, and that a person skilled in the art may make variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as described herein and defined in the appended claims. It should be understood that the embodiments described above are not only in the alternative, but can be combined.