Abstract:
A method and apparatus for quenching the coke drum vapor line from a coke drum to the main fractionator in a coker unit whereby the volume of quench liquid prevents the drum vapor line from plugging with carbon-based deposits. A differential pressure control technique is utilized to quench the drum vapors being delivered to the fractionator as opposed to a temperature, delta temperature, uninsulated vapor line, or fixed flow rate control as used in the prior art. Vapor line quench control by differential pressure prevents over-quenching of the vapor line during a coke drum switch, unit startup, or slowdown as well as under-quenching during drum warm-ups. It improves the fractionator recovery time from a drum switch and overall liquid product yield during the drum cycle which can be produced by over-quenching. It also prevents the vapor line from drying out at anytime, an under-quenched condition, as long as the quench oil quality and conditions do not vary significantly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is related to coker units and their operation, particularly in the quenching of the vapor line running from coke drums to a fractionator in a coker unit. 
     2. Description of Related Art 
     Flow rate in a coke drum vapor line is influenced by several factors including quench injection rate, quench oil properties, coke drum temperature, vapor rate and pressure drop from the coke drums to the fractionator. In prior systems, the actual rate of liquid flowing out of the vapor line into the coker main fractionator varies during the coking cycle and can go to zero liquid flow, a dry vapor line condition which can eventually lead to plugging of the vapor line. Prior systems result in either of two undesirable conditions: (1) overquench, which reduces yields and possibly reduces unit feed rates, OR (2) underquench, which leaves a vapor line without any liquid to flush the line out into the main fractionator and which will eventually shut down the coker as the vapor line cokes. Once the line cokes to the point of causing enough pressure drop from the coke drums to the main fractionator such that all the liquid evaporates, only a short time remains until the coker must be shut down—a very expensive event. In the prior systems, the quench cannot generally be adjusted to target its contribution to the recycle ratio. One prior method, the delta temperature control technique, could possibly target a contribution of the recycle ratio; however, the downstream temperature indicator (TI) must be located in the common part of the vapor line near the fractionator in order for this to work correctly. The problem with putting a TI in this location is that, in all likelihood, it will foul and become inaccurate. As described in the present disclosure, a TI located at the coke drum vapor line outlet into the fractionator is not accessible during operation but is easily cleaned while decoking a drum. Prior quench techniques do not consider pressure differential between the coke drum and the fractionator. 
     SUMMARY OF THE INVENTION 
     The invention is a method and apparatus for quenching the coke drum vapor line which runs from the coke drum to the main fractionator in a coker unit. The unique part of this improved quench system is that it uses both pressure differential and unit feed rates to control quench rates for a given quench oil and unit feed quality. If the composition of the coker feed or the quench oil changes significantly, a new set of quench curves should be generated to ensure proper quenching of the coke drum vapor line. The purpose of quench is to prevent the drum vapor line from plugging with carbon-based deposits. Plugging of the vapor line causes a restriction in coker unit feed rates and ultimately leads to severely limiting coker feed rates until the plug is removed. In order to remove the vapor line plug, shut down of the unit is required which results in lost coker capacity, due to the gradual slowdown and subsequent shutdown of the coker unit, and in significant economic loss. A differential pressure control technique is utilized to quench the drum vapors going to the fractionator as opposed to a temperature, delta temperature, uninsulated line or fixed flow rate control technique as used in prior systems. Vapor line quench control by differential pressure prevents over-quenching of the vapor line during a coke drum switch, unit startup, or slowdown as well as preventing under-quenching during drum warm-ups. It improves the fractionator recovery time after a drum switch and the overall liquid product yield during the drum cycle which can be reduced by over-quenching. It also prevents the vapor line from drying out at anytime, an under-quenched condition, as long as the quench oil quality and conditions do not vary significantly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of a coker unit which incorporates the instant invention. 
     FIG. 2 is a graph showing quench flow vs. pressure differential for the minimum and maximum feed rates for a typical coker unit and coker feed quality. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The root cause of a coker vapor line plug is drying out of the vapor line. In particular, during coke drum warm-up, the vapor line may dry out due to the increased pressure drop from the coke drum to the fractionator if there is no increase in quench rate to prevent drying. This added pressure drop can cause all of the liquid to flash off inside the vapor line which leaves a layer of carbon residue with entrained coke fines. To reduce the risk of plugging the vapor line, the quench technique disclosed herein adjusts quench rates based upon pressure drop and unit feed rate. This delta pressure quench control technique greatly reduces the potential of the vapor line drying out and maintains a constant flow of liquid flowing out the end of the vapor line into the fractionator. It will generally increase yields vis-a-vis the prior art delta temperature quench control (if the vapor line temperature indicator (TI) is not located near the fractionator), or the constant vapor temperature quench flow technique, at a much reduced risk of plugging the vapor line. These latter two prior art techniques rely on over-quenching for most of the drum cycle in order to prevent drying of the vapor line during drum warm-up. Or, if the temperature indicator (TI) is placed in an inaccessible portion of the vapor line, the TI can foul with coke and produce unreliable data, resulting in under-quenching. If the delta temperature quench control technique is to be reliable, accurate vapor line temperatures near the coker main fractionator are necessary; however, temperature indication in this portion of the vapor line is inherently unreliable since it is in this common portion of the vapor line where the vapor line will Likely foul, producing unreliable temperature data. The fixed-quench rate vapor temperature control may result in under-quenching and a dry vapor line whenever a drum switch occurs, and this can lead to the formation of a plugged vapor line. 
     The present invention overcomes three limitations of the quenched vapor temperature control technique used in prior systems: (1) the possibility of drying out the coke drum vapor line; (2) the inferior reliability of temperature indication in a coking environment to control the quench rate, and (3) the essential over-quenching necessary during most of the drum cycle if adequate quench is to be supplied during drum warm-up, when the pressure drop is usually at its highest. Also, the accuracy of the drum pressure indicator is easily verified during every drum cycle because the inactive drum is opened to the atmosphere, therefore the pressure indicator will read zero psig if working properly. However, the temperature transducer can certainly foul with coke, such that its accuracy is not easily verified between drum cycles, due to the metal not having time to cool to ambient verifiable conditions between cycles. Or if the TI is located in the common portion of the vapor line, one will not know if the TI is fouled, thus producing unreliable data to control quench rates. 
     In the following discussion, two coke drums are illustrated and described. It will be appreciated that a coker unit may comprise more than two coke drums. Referring now to FIG. 1, a typical coker unit comprises two coke drums  10  and  20 , two coker furnaces  30  and  40 , a main fractionator  50 , a light gasoil stripper  60 , a heavy gasoil stripper  70  and possibly a rectified absorber  80 , all of which are known to those skilled in the art. In the instant invention, a computer controller  90  is additionally required to receive input data from the coke drums  10 ,  20 , the fractionator  50  and the input feed rate indicator  100  and to generate control signals for controlling quench flow rate as will be subsequently described. Each of the coke drums  10 ,  20  contain pressure transducers  11 ,  21 , respectively, which monitor the pressure inside the respective drums at all times and relay such data to the controller  90 . It will be appreciated that, at any given time, one of the coke drums will be “active” (on-line) and the other will be off-line undergoing decoking and cleaning in preparation for the next cycle, as is well known to those skilled in the art. Likewise, the main fractionator  50  also includes a pressure transducer  51  for constantly monitoring the pressure therein and relaying such data to controller  90 . 
     In operation, a cold feed heavy oil such as 6-Oil at about 180° F. is fed through flow meter  102  and line  104  to fractionator  50 , via line  104   a  to grid tray/spray unit  59  or via line  104   b  to the bottom of the fractionator  50 . Concurrently, a hot feed, such as hot pitch at about 500° F., is fed through flow meter  103  and line  105  into the bottom of fractionator  50 . Flow meter signals from flow meters  102 ,  103  are relayed through data lines  106 ,  107  respectively to the unit feed flow indicator  100 . The resulting flow signal is relayed over data line  101  to the controller  90 . The hot fractionator bottom stream is fed through line  54  to furnaces  30 ,  40 , after injecting velocity steam at  33 ,  43 , respectively, where it is circulated through tubes  31 ,  41 , respectively, and heated up to about 910° F. The bottoms must be severely thermally cracked, otherwise it will not coke, and will, instead, form tar. The hot fractionator bottoms exit the furnace tubes  31 ,  41  at  32 ,  42 , respectively, at about 910° F. and are directed to the active coke drum, either  10  or  20 . In the usual manner, the active coke drum  10  or  20  catches and retains carbon matter while hydrocarbons evaporate. It will be appreciated that this described apparatus is called a “delayed coker” since it requires a combination of residence time and temperature to form coke in the coke drums  10 ,  20 . Pressure transducers  11  and  21  relay data over lines  11   a  and  21   a  respectively to the controller  90 . Vapor from the active coke drum  10  or  20  is passed through one of the valves  18 ,  28  to the overhead coke drum vapor line  29 . A quench liquid is also injected into vapor line  29  through inputs  12  or  13 , flow meter  14  and valve  17  to form a mixture of quench oil and vapor in vapor line  29 . Quench liquid  12  may be slop oil while quench liquid  13  may be a coker gasoil. Quench liquid flow rate through vapor line  29  is set by the quench flow indicator controller  15  which regulates valve  17  in response to a signal received from the controller  90  over control line  91  as will be subsequently explained. 
     The quench oil/vapor mixture in vapor line  29  is injected at the bottom of fractionator  50  at  29   a , where, in prior systems, a thermocouple may have been placed to detect and relay temperature data and to possibly be used for controlling the flow rate. As has been explained, this temperature tended to be unreliable since the thermocouple became coated with coke and became inaccurate. Main fractionator  50  includes a heavy gasoil pump-around exchanger  53  for cooling vapors and removing heat from the system. A circulation reflux unit also includes a pump-around exchanger  52  for cooling vapors and removing heat from the system further up the column  50 . Exchanger  52  receives hot circulating reflux oil through line  52   b  and sends cooled circulating reflux oil back to fractionator  50  through line  52   a . Exchanger  53  receives hot unstripped heavy gasoil through line  53   b , and part of the hot heavy gasoil can possibly go back to the spray  59  through line  53   c  to prevent entrained coke fines from escaping into the overhead vapors. Cooled heavy gasoil from exchanger  53  is sent back to the fractionator  50  via line  53   a  where it is flowed onto tray  53   d  as part of the pumparound heat removal system. Heavy gasoil stripper  70  receives unstripped heavy gasoil from the fractionator  50  through line  74  and steam is injected through line  72  to form stripped heavy gasoil which is withdrawn by line  71 . Steam and stripped-out heavy gasoil is recirculated to the fractionator  50  via line  73  where it flows onto tray  53   d . Line  53   c  is an alternate source of liquid for spray  59  which, if used, reroutes the cold feed flowing in line  104  to the bottom of the fractionator  50  via line  104   b  along with the hot pitch through line  105 . Spray unit/contacting trays  59  prevent entrained coke fines from escaping into the overhead vapors. 
     Light gasoil stripper  60  may be used for receiving light unstripped gasoil through line  64  and steam through line  62 . Light stripped gasoil is produced and is withdrawn through line  61  while the remaining vapors are sent back to the fractionator  50  through line  63 . The overhead vapors in fractionator  50  are passed on to the overhead condenser  54  which removes heat from the overhead vapors. The condensed liquid passes to an accumulator  55  and wet gas compressor  56  compresses the wet gasses, such as methane, ethane, propane, and butane. The output of wet gas compressor  56  is transported through line  57  to the rectified absorber (RA)  80  where fuel gas is withdrawn at  82  and coker naphtha at  84 , the latter being sent to a hydrotreating unit. The absorber  80  receives a lean oil input  83  which assists in the separation of ethane from propane. Line  81  contains the overhead liquid hydrocarbons that have been condensed in the overhead condenser  54 . These liquids are either sent back to the main fractionator  50  as reflux or to the  80 . Pressure transducer  51  continuously transmits the pressure inside fractionator  50  to the controller  90  over line  51   a.    
     As noted, the controller  90  receives continuous pressure signals from pressure transducers  11 ,  21  in coke drums  10 ,  20 , respectively, and from pressure transducer  51  in fractionator  50 , even from the off-line drum being decoked. The  16  controller  90  also receives an input feed rate signal  101  (in barrels per day) from unit feed flow indicator  100 . Controller  90  senses which of the drums  10 ,  20  is active (on-line), since the pressure in the off-line drum is lower than the pressure in the on-line drum. It then calculates the difference in pressure (DP) between the active drum ( 10  or  20 ) and the fractionator  50  pressure transmitted by pressure transducer  51 . This DP is used by the controller  90 , along with the feed flow rate  101 , to calculate the quench flow rate which is required to be injected at  12 ,  13  in order to maintain a selected fresh feed liquid flow percentage of, say 5 vol %, in vapor line  29  at point  29   a  where the vaporline  29  intersects the main fractionator  50 . This is a very important area of the vapor line to understand. If one does not understand what influences the amount of liquid in the vapor line at this point, one could potentially (1) overquench, i.e., too much liquid, which reduces liquid yields and increases coker unit recycle to the main fractionator bottoms and potentially could reduce coker unit throughput OR (2) underquench, i.e., too little liquid, resulting in a dry, non-irrigated, vapor line which will foul with coke and eventually shut down the coker unit. Either one of these conditions is. undesirable. A signal is sent over line  91  to the quench flow indicator controller  15  and valve  17  is automatically adjusted to maintain such selected flow rate. 
     Quench rates needed to maintain a wetted line at various vapor line pressure differentials, and unit feed rates required to ensure a constant liquid rate flowing out of the lo vapor line  29  into the coker main fractionator  50  were calculated. A PRO/II® general purpose process and optimization software by Simulation Sciences, Inc. was used to generate the data. This data is presented in Tables 1 and 2 below. 
     Tables 1 &amp; 2 were obtained via computer simulation of the coke drum vapor line thermodynamics. Based upon the measured coker feed product yields and quench liquid properties, a simulation was run to determine the quench rate needed to produce a constant percentage of unit recycle from liquid flowing out of the coke drum vapor line into the bottom of the main fractionator. The vapor line pressure drop was varied to determine the quench rate needed to maintain constant liquid flow into the main fractionator, while at premeasured product yields and quench oil properties. 
     From Tables 1 &amp; 2, the curves shown in FIG. 2 were produced. Differential pressure drop (psi) from the active coke drum to the main fractionator is used as the X axis and quench rate (bpd) as the Y axis. Once the curves are prepared for a particular coker, (for a given set of unit yields and quench oil properties) such information is used to control quench flows via computer control thereafter. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Quench Flow Calculation for 5 Vol % Recycle based on 
               
               
                 28,500 bpd Fresh Feed Rate 
               
             
          
           
               
                   
                   
                 Drips (Liquid 
                 Quench 
                   
               
               
                 DP - 
                 Quench 
                 Flowing out of) - 
                 Temperature 
                 Drum 
               
               
                 Differential 
                 Flow 
                 Vapor Line into 
                 at Main Frac - 
                 Pressure 
               
               
                 Pressure, psi 
                 BPD 
                 Main Frac - BPD 
                 ° F. 
                 Psig 
               
               
                   
               
             
          
           
               
                 0 
                 1200 
                 1425 
                 811 
                 25 
               
               
                 5 
                 1633 
                 1425 
                 811 
                 30 
               
               
                 10 
                 2025 
                 1425 
                 811 
                 35 
               
               
                 15 
                 2383 
                 1425 
                 811 
                 40 
               
               
                 20 
                 2714 
                 1425 
                 811 
                 45 
               
               
                 30 
                 3307 
                 1425 
                 811 
                 55 
               
               
                 40 
                 3831 
                 1425 
                 811 
                 65 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Quench Flow Calculation for 5 Vol % Recycle based on 
               
               
                 14,500 bpd Fresh Feed Rate 
               
             
          
           
               
                   
                   
                 Drips (Liquid 
                 Quench 
                   
               
               
                 DP - 
                 Quench 
                 Flowing out of) - 
                 Temperature 
                 Drum 
               
               
                 Differential 
                 Flow 
                 Vapor Line into 
                 at Main Frac - 
                 Pressure 
               
               
                 Pressure, psi 
                 BPD 
                 Main Frac - BPD 
                 ° F. 
                 Psig 
               
               
                   
               
             
          
           
               
                 0 
                 602 
                 725 
                 810 
                 25 
               
               
                 5 
                 818 
                 725 
                 810 
                 30 
               
               
                 10 
                 1014 
                 725 
                 810 
                 35 
               
               
                 15 
                 1193 
                 725 
                 810 
                 40 
               
               
                 20 
                 1356 
                 725 
                 810 
                 45 
               
               
                 30 
                 1656 
                 725 
                 810 
                 55 
               
               
                 40 
                 1918 
                 725 
                 810 
                 65 
               
               
                   
               
               
                 Note: Quench Oil temperature is assumed to be 100-150° F. and of a light gasoil boiling range hydrocarbon. If the available quench oil is significantly different, another set of tables may need to be produced.  
               
             
          
         
       
     
     Referring now to FIG. 2, Tables 1 and 2 have been displayed in graph form for the maximum (28.5 MBPD) and minimum (14.5 MBPD) feed rates for a typical coker unit.