Abstract:
A process for recovering an alkanolamine from a used gas scrubbing stream wherein a dynamic reaction system is employed to maximize conversion of bis-urea compounds typically formed in the gas scrubbing operation into the alkanolamine and minimize the formation of amino ethers which irreversibly convert the alkanolamine. A method of removing waste products from the system by the use of a wiped film evaporator.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the recovery of alkanolamines from wash or scrubbing streams used to remove acid gasses from gas streams. More particularly, the present invention relates to such a process in which carbamates formed during the gas scrubbing operation are converted back to the alkanolamine. 
     BACKGROUND OF THE INVENTION 
     The removal of acid gases, e.g. hydrogen sulfide, carbonyl sulfide, and carbon dioxide, from industrial and natural gas streams is an important and frequently encountered operation in the process industry. It is known that in these processes, certain of the alkanol amines used react with carbon dioxide to form carbamate precursors. The formation of the carbamate precursors is detrimental to the scrubbing process since they can cause corrosion, have no acid gas removal properties and reduce solution capacity. 
     Historically, thermal reclaimers have been used to remove nonvolatile contaminants from the used scrubbing or wash solution. However, these thermal reclaimers pose difficulties in that high temperatures, long residence time, and dehydrating environment are typically encountered in such reclaimers. Moreover, there are many problems with conventional reclaimer designs and operations. In particular, in the high temperature dehydrating environment encountered in conventional thermal reclaimers, the carbamate precursors can be converted into amino ethers with permanent, concomitant loss of the alkanolamine. This presents a large problem since major loss of the alkanolamine translates into significant chemical replacement costs in the overall gas scrubbing operation. It is known that the production of these amine ethers in conventional recovery processes is accelerated by increased temperature and exposed residence times in the reclaimer. 
     Using diglycolamine (DGA) as an example, in the scrubbing operation, the DGA reacts with CO2 to form a carbamate precursor. The carbamate precursor can react with DGA to form N,N,bis(hydroxyethoxyethyl)urea (BHEEU). The BHEEU, at the high temperatures in a typical thermal reclaimer, can irreversibly degrade to morpholine, an amino ether, and DGA. Accordingly, there is a net loss of DGA from the system. 
     The equations below show the reaction sequence: 
     
       
                 
         
             
             
         
      
     
       FIG. 3  is a graph taken from a paper entitled Saudi Arabian Experience with DGA Units and Related Sulfur Plants, by Lewis G. Harruff, Saudi Arabian Oil Co. It shows the relationship between morpholine make, temperature and residence time. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a continuous process for recovering an alkanolamine from a wash or scrubbing stream used to remove acid gases from gas streams. 
     In another aspect, the present invention provides a dynamic process which maximizes recovery of an alkanolamine from a wash or scrubbing stream used to remove acid gases from gas streams. 
     In yet another aspect, the present invention provides a combined reaction/reclaiming process for the recovery of an alkanolamine from a scrubbing or washing solution used to remove acid gases from gas streams. 
     These and further features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic flow sheet of one embodiment of the process of the present invention. 
         FIG. 2  shows a schematic flow sheet of another embodiment of the process of the present invention. 
         FIG. 3  is a graph taken from a paper entitled “Saudi Arabian Experience with DGA Units and Related Sulfur Platns”, Lewis G. Harruff Saudi Arabian Oil Co., (1998) on showing the effect of morpholine formation as a function of temperature. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     While the invention will be described with respect to the recovery of DGA from a wash solution used to scrub acid gases from gas streams, particularly hydrocarbon streams, it will be understood that it is not so limited. 
     The process of the present invention can be used to recover any alkanolamine used in a gas scrubbing operation to remove acid gases from gas streams wherein in the reclaiming process to recover the alkanolamine for further use, the alkanolamine can react with carbamate precursors formed during the gas scrubbing operation to irreversibly produce compounds e.g., amine ethers which consume the alkanolamine. Non-limiting examples of alkanolamines include monoethanolamine (MEA), diethanolamine (DEA), diglycolamine (DGA), methydiethanolamine (MDEA) as well as mixed amines, e.g., mixtures of MDEA and DEA or MEA. It is well known for example that MDEA based mixtures are used to increase the CO 2  pickup in cases where the MDEA is allowing too much CO 2  to slip overhead in the absorber. Accordingly, spiking the MDEA with MEA or DEA provides advantages. 
     Referring now to the figures and particularly  FIG. 1 , a spent gas scrubbing medium comprising DGA, water, carbamate precursors and other contaminants and byproducts removed from the scrubbing operation is introduced via line  10  and pump  12  into reaction vessel  14 , the feed stream in line  10  passing through an exchanger  16  where it is preheated prior to entering the reaction zone in vessel  14 . Exchanger  16  can be heated in any suitable fashion well known to those skilled in the art. 
     The reaction zone in vessel  14  is conducted at a positive pressure which can range up to 250 psig and at a temperature of from about 250° F. to about 400° F. depending upon the composition of the feed stream. A product stream is removed from the reaction zone in vessel  14  via line  18  and is pumped by pump  20  through an exchanger  22  into a product tank  24 . A portion of the product stream in line  18  is recycled via line  26  to the incoming feed stream in line  10  prior to heat exchanger  16 . 
     An overhead stream of CO 2 , water and some DGA is removed from vessel  14  and transferred via line  28  and exchanger  30  into product tank  24 . In exchanger  30 , carried over DGA is condensed and in this regard the cooling medium can comprise the feed stream being introduced via line  10 . This feed stream can also be used as the cooling medium in exchanger  22 . CO 2  is vented from product tank  24  via line  32  while product in product tank  24  is removed via line  34  and pumped via line  36  into storage for further use or back into the gas scrubbing process. 
     As noted above from this description, the feed stream is being recirculated in the reaction zone in vessel  14  by virtue of the recycle loop comprised of line  18 , pump  20 , and line  26 . It is this recycle loop which controls the residence time of the feed mixture in the reaction vessel  14  and which ensures that the amount of amine ethers being made is minimized and that there is maximum recovery of the DGA. It will be recognized that the residence time is also controlled by the composition of the feed mixture. In this regard, the incoming feed can be monitored to determine the amount of BHEEU. Alternatively, or concomitantly, the product stream exiting reaction vessel  14  via line  18  can also be monitored to determine its composition so as to increase or decrease temperature and the recycle time as necessary. 
     Accordingly, temperature, recycle rate and hence residence time in the reaction zone in reaction vessel  14  are optimized to, in the case of the use of DGA, minimize the make of morpholine. 
     In this regard, the reactor provides sufficient residence time, e.g. from about 0.5 to 1 hour, to allow the reaction of BHEEU to DGA to go to completion. As was noted above, the reaction of BHEEU to form DGA and morpholine is favored at high temperatures. Accordingly, it has been found that one of the better methods to control temperature in the reaction zone is by the use of steam at about 100 to 175 psig to ensure that temperatures of 400° F., preferably 365° F., or higher are avoided. If higher pressure (hotter) steam or alternate heat mediums are used then the temperature must be carefully controlled. All in all, the system is operated such that overheating in the reaction zone is avoided. 
     Referring now to  FIG. 2 , there is shown another embodiment of the present invention wherein reaction vessel  14  is used as a pressurized flash tank. The feed stream comprising DGA, water, carbamate precursor and other contaminants from the gas scrubbing operation is introduced via line  38 , pump  40  and exchanger  42  into flash tank  46 . As noted, flash tank  46  is operated under pressure and the residence time of the circulating feed mixture in vessel  46  will generally be as described above with respect to the embodiment shown in  FIG. 1 , i.e., residence times will again be maintained to be within 0.5 to 1 hour, as discussed above, vessel  46  being operated in a temperature range of from about 330° F. to about 380° F. to ensure best conversion and hydrolysis of the BHEEU. Heat exchanger  42  requires either 250° F., 250 psig steam or some alternate heating medium, such as hot oil, and will operate at about 388° F. (in general greater than 330° F.) to provide the additional heat of vaporization required in flash vessel  46 . In operation, the feed stream in flash vessel  46  is flashed and is therefore held at a substantially constant temperature of from about 330° F. to about 375° F., preferably about 360° F. 
     The overhead flashed stream from flash vessel  46  is removed via line  48  and transferred through cooling condenser  50  into product tank  52 . To maximize DGA recovery and handle residue, a bottoms stream is removed from flash vessel  46  via line  54  and transferred via pump  56  into a wiped film evaporator  58 . A portion of the stream in line  54  is recycled via recycle line  60  into feed stream  38  and then into flash vessel  46 . 
     It may be necessary to introduce a caustic solution into flash vessel  46 . To this end, there is a line  41  having a valve  43  such that the amount of caustic or alkaline solution introduced into flash vessel  46  can be controlled. 
     It will be appreciated that the bottoms stream leaving flash vessel  46  via line  54 , although containing primarily unwanted residue, also contains significant amounts of DGA. Accordingly, the liquid from the flash tank  46  leaving via line  54  is treated in wiped film evaporator  58 . Wiped film evaporator  58  is operated under vacuum, e.g., from about 760 to 1 TOR. In wiped film evaporator  58 , the residue is further concentrated and removed as a waste stream  62  and sent for suitable disposal. In most cases, about 20% of the feed to flash tank  46  is slip streamed into wiped film evaporator  58  to provide additional concentration of residue. Vacuum conditions in wiped film evaporator  58  are provided by a vacuum system, shown generally as  64 , via line  66 . It will be appreciated that the vacuum can be provided by steam jets, a vacuum pump, etc. and that vacuum system  64  can include the usual vacuum train to recover materials removed from wiped film evaporator  58 . 
     Since wiped film evaporator  58  has an integral condenser, the DGA is condensed and transferred via line  68  and pump  70  into product tank  52 . Again, product tank  52  is provided with a vent  72  for CO 2  and any other uncondensable gases. Product from product tank  52  can be transferred via line  72  and pump  74  back into the gas processing operation or for storage and later use. It will be appreciated that the vacuum system  64  may recover waste products which can be transferred to waste stream  62  for appropriate disposal. It will further be appreciated that various valves, gauges, etc., commonly used have not been shown for simplicity&#39;s sake. 
     As can be seen from the above, the present invention provides a dynamic reaction process which avoids loss of DGA by maximizing the conversion of BHEEU and the subsequent recovery of the DGA. Additionally, the present invention provides such a dynamic reaction system in conjunction with a waste recovery system, using a wiped film evaporator, to further recover DGA and remove waste from the system for proper disposal. 
     Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.