Abstract:
In an HF alkylation process where an acid catalyst regeneration column separates HF acid from ASO and water to provide regenerated catalyst for a catalyst circulation stream, and where buildup of ASO in the catalyst circulation stream is encountered, an improved control scheme uses an on-line analyzer/controller to maintain desired concentrations of HF acid, or ASO or water in the catalyst circulation stream. In one embodoment, a continuously flowing catalyst slip stream is piped to the analyzer/controller for simultaneously determining concentration of HF acid, ASO and water, and in a preferred embodiment the ASO concentration is maintained at a desired low level by manipulating temperature of a stripping fluid charged to a lower section of the column. An alternate manipulated variable is temperature of the acid catalyst feed to the regenerator column. Alternate controlled variables are the HF acid, and water concentration in the catalyst circulation stream.

Description:
This application is a continuation of provisional application Ser. No. 60/069,866 filed Dec. 17, 1997. 
     The present invention relates to chemical process control using near infrared (NIR) spectroscopy instrumentation, and more particularly to predicting and controlling acid catalyst strength to improve a hydrocarbon alkylation process. 
    
    
     BACKGROUND OF THE INVENTION 
     Hydrogen fluoride (HF) alkylation is an important refinery process in which isobutane is reacted with olefins to produce highly-branched isoparaffins as illustrated in FIG. 1 for use in gasoline blending. In this process, hydrofluoric (HF) acid functions as the catalyst and recirculates through the reactor. The recirculating HF acid catalyst is not pure; it contains a small amount of water and a reaction byproduct called acid-soluble oil. The catalyst is also saturated with the hydrocarbons involved in the process (e.g., alkylate and isobutane). In the HF alkylation process, it is important to monitor and control the purity of the catalyst since excessive amounts of water and acid-soluble-oil (ASO) have deleterious consequences: Excessive water, for example, can cause rapid corrosion of the carbon steel reactor. 
     Controlling the composition of the catalyst requires knowing the concentrations of HF acid, water, and ASO in the recirculating catalyst. Therefore, operators must take samples of the catalyst periodically and have these components measured by classical analytical techniques. There are several problems associated with this approach: First of all, HF acid will cause serious burns if it contacts skin. Because of this hazard, collecting and analyzing these samples carries potential for injury. Another problem is that the analytical methods used for these measurements lack precision, especially the method for ASO. This often makes it difficult to determine if the composition of the catalyst has changed from sample to sample. Finally, samples are drawn from the reactor only once or twice a day, and the analyses require several hours. This makes it difficult to follow the composition of the catalyst when processing changes do occur. 
     In the past few years, there has been a great deal of interest in on-line monitoring of various refinery process streams. In part, this interest has been spurred by advances in analytical technology that have greatly expanded the capabilities for process monitoring. 
     Accordingly an object of this invention is to continuously analyze process streams containing HF acid catalyst. 
     A more specific object is to use improved control schemes in acid catalyst processes, which result in tighter process controls, higher productivity and improved product quality. 
     Yet another object of this invention is to reduce exposure of refining personnel to hazardous process chemicals. 
     SUMMARY OF THE INVENTION 
     According to this invention, the foregoing and other objects and advantages are achieved in a method and apparatus for controlling an HF acid regenerator column, which employs an isobutane stripping charge in effecting separation of HF acid, ASO and H 2 O. The HF acid regenerator, which is one column in an HF alkylation process which also includes a reactor, a settler vessel, a source of fresh HF acid, and a suitable hydrocarbon stream, employs an NIR triple or quadruple component analyzer/controller configured to control acid catalyst strength. In a first embodiment of a regeneration control system the ASO/H 2 O output of the NIR triple-component analyzer maintains a desired ASO/H 2 O concentration in the catalyst recirculating through the reactor by manipulating the temperature of the stripping isobutane charge to the regeneration column. This scheme allows more ASO/H 2 O to be withdrawn in the regenerator bottoms stream as the stripping fluid temperature is reduced. In another embodiment of the control scheme, HF/ASO output values from the NIR analyzer/controller manipulate temperature of the regenerator feed heater to increase HF/ASO content in the regenerator bottoms stream as the spent acid catalyst feed temperature is lowered. In a third embodiment of the regenerator control scheme, which would be employed when sulfolane additive is present in the process acid catalyst, the H 2 O output from the NIR analyzer/controller manipulates flow rate of a side draw stream such that an increased draw rate reduces H 2 O levels in the process catalyst. 
     Other objects and advantages of this invention will be apparent from the foregoing brief description of the invention and the appended claims as well as the detailed description and the drawings which are briefly described as follows: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a chemical formula for producing an alkylate. 
     FIG. 2 is a graphical form used to illustrate the NIR region of the electromagnetic spectrum. 
     FIG. 3 is a schematic diagram illustrating key components used for NIR spectroscopy. 
     FIG. 4 is a graph showing NIR absorbance spectrum of HF process acid catalyst. 
     FIG. 5 is a schematic diagram illustrating an NIR analyzer interfaced to a process. 
     FIG. 6 is a view similar to FIG. 5 illustrating an alternate configuration. 
     FIG. 7 is a view similar to FIG. 5 illustrating a second alternate configuration. 
     FIG. 8 is a simplified process flow diagram illustrating operational units of an HF alkylation process, and incorporating a control scheme according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Near-Infrared Spectroscopy 
     The method and apparatus described in this specification involves a process control technique based on near-infrared (NIR) spectroscopy, which uses electromagnetic radiation in the region shown in FIG. 2 to provide concentration outputs for triple components. This region of the spectrum lies between the visible region where our eyes function and the mid-infrared region where conventional infrared spectroscopy is performed. 
     In NIR spectroscopy the radiation from a halogen lamp is caused to pass through a sample, which in on-line analyzers is continuously flowing through a cell. After the radiation passes through the sample, it is dispersed into its various wavelengths. Finally, the various wavelengths are detected and a spectrum produced in which the amount of radiation absorbed is plotted as a function of wavelength. 
     The key components of NIR spectroscopy are illustrated in FIG.  3 : As in mid-IR spectroscopy, NIR spectra reflect the chemical structure of the compound(s) measured. In other words, each different chemical will have a unique absorption spectrum. NIR measurement times are fast, with results typically updated every 1-3 min., and NIR spectroscopy is inherently very precise. This is very important in process monitoring where detecting small changes in the process and following the associated trends is often of primary interest. NIR instrumentation and optics are very rugged, and instruments often have only one moving part, and also robust optic materials such as quartz and sapphire can be used. Compatibility with quartz optics allows optical fibers to be used to convey NIR radiation from the spectrometer to a remote sample point. This provides a great deal of flexibility in how the analyzer is interfaced with the process. Optical multiplexing can be used in conjunction with fiber optics to monitor several sampling points with the same spectrometer. The combination of all of these features makes NIR spectroscopy one of the very best analytical techniques for on-line process monitoring. A suitable on-line process analyzer for Fourier transform—IR application, is available from a company called Applied Automation Inc. (AAI), Inc. Bartlesville, Okla. 74004. This analyzer includes software that controls a sampling system, cell washing etc. in addition to obtaining spectral data for triple or quadruple components. 
     Analyzer Configuration 
     As mentioned above, NIR analyzers can be interfaced with a process in a wide variety of configurations. One approach, shown in FIG. 5, involves running a slip stream from the process pipe to the analyzer. This approach is analogous to that used with gas chromatographic analyzers where the sample is brought to the analyzer. There are significant design limitations in this approach: The analyzer needs to be close to the sample point and the analyzer design must provide appropriate isolation between the sample cell and the electronics. On the other hand, this design offers some significant advantages: It allows sample conditioning such as filtering or thermostating as well as automated cell washing. In addition, the spectra produced are of the highest quality since they are free of any spectral artifacts that may be caused by optical fiber. 
     Another interface configuration is shown in FIG.  6 . Here a spectroscopic probe is inserted directly into a process stream. This configuration has the advantage of simplicity. It also allows the analyzer to be located away from the measurement point through the use of optical fiber. The major disadvantage is that it does not allow any sample conditioning. The probe must also be removed periodically for cleaning, which can also present a disadvantage. 
     The third configuration, shown in FIG. 7, represents a compromise approach with design features common to the previous two configurations. In this approach, the sample cell is located in a slip stream, and thus sample conditioning is possible. Using an optical fiber interface maintains the ability to locate the analyzer in a remote location. 
     Feedback control systems are widely used to achieve efficient operation in HF alkylation processes by controlling the perturbations normally encountered in the operation of various units. Such perturbations occur for example due to upsets in the operation of certain equipment in the plant, adjustment of operating conditions by plant operators, changes in production rates, and the like. In these feedback control systems a plurality of parameters such as pressures, temperatures, flow rates, concentrations and liquid levels at specific locations in the process are controlled at desired set points by measuring each parameter, determining the deviation of each parameter from its set point and using the value of the deviation to manipulate a final control element such as a valve located somewhere in the process that will minimize the deviation of each measured parameter from its set point. 
     A specific control system configuration for an acid catalyst regenerator, which employs an NIR analyzer/controller  130  is set forth in FIG. 8 for the sake of illustration. However, the invention extends to different types of control system configurations which accomplish the purpose of the invention. Lines designated as signal lines, which are showing as dash lines in the drawings, are electrical or pneumatic in this preferred embodiment. Generally the signals provided from any transducer are electric in form. 
     The invention, however, is also applicable to mechanical, hydraulic or other means for transmitting information. In almost all control systems some combination of electrical, pneumatic, or hydraulic signals will be used. However, the use of any other type of signal transmission compatible with the process and equipment in use is within the scope of the invention. 
     The PID controllers  124  and  126  shown in FIG. 8 can utilize the various modes of control such as proportional, proportional-integral or proportional-integral-derivative. In the preferred embodiment a proportional-integral mode is utilized. However, any controller having capacity to accept two or more input signals and produce a scaled output signal representative of the comparison of the two input signals is within the scope of the invention. 
     The scaling of an output signal by a controller is well known in the control systems art. Essentially, the output of a controller can be scaled to represent any desired factor or variable. An example of this is where a desired temperature and an actual temperature are compared by a controller. The controller output might be a signal representative of a flow rate of a “control” gas necessary to make the desired and actual temperatures equal. On the other hand, the same output signal could be scaled to represent a pressure required to make the desired and actual temperatures equal. If the controller output can range from 0-10 units, then the controller output signal could be scaled so that an output having a level of 5 units corresponds to 50% percent or a specified flow rate or a specified temperature. The transducing means used to measure parameters which characterize a process in the various signals generated thereby may take a variety of forms or formats. For example the control elements of this system can be implemented using electrical analog, digital electronic, pneumatic, hydraulic, mechanical, or other similar types of equipment or combination of such types of equipment. 
     The specific hardware and/or software utilized in such feedback control systems except for the previously described NIR analyzer is well known in the field of process plant control. See for example Chemical Engineering&#39;s Handbook, 5th Ed., McGraw-Hill, pgs. 22-1 to 22-147. 
     Referring now to FIG. 8 which is a simplified schematic illustrating an acid regenerator column  100 , a reactor  102 , and a settler vessel  104 . These vessels are used in an HF alkylation process where a recycle acid catalyst stream recirculates through the reactor  102  and the settler vessel  104  via conduit  110 . A portion of this recirculating stream is withdrawn and sent to the regenerator column via conduit  112  where HF acid catalyst, ASO, and H 2 O are separated. FIG. 8 also illustrates three possible control loops contemplated for association with the regenerator column. The acid regenerator feed stream in conduit  112 , which contains HF acid catalyst, ASO, H 2 O and optionally an additive such as sulfolane, is supplied to the regenerator column  100  where a substantial portion of the HF acid catalyst in the feed is removed from the regenerator  100  in an overhead stream via conduit  114 . A substantial portion of the ASO and H 2 O contained in the feed stream is removed from the regenerator in a bottoms stream via conduit  118 . Optionally, a side draw stream in conduit  118  is provided for withdrawing HF acid, H 2 O and isobutane. A preheated isobutane stripping fluid is supplied to the lower portion of the regenerator column via conduit  120  for effecting the separation. The analyzer output control signals  132 ,  133  and  134 , which are illustrated as dashed lines in FIG. 8, refer to component concentrations of HF acid, ASO and H 2 O in the circulating acid catalyst. 
     One problem encountered in an HF alkylation process is building up of excessive levels of ASO in the HF acid catalyst stream that is circulated through the reactor in conduit  110 . The only way to cause more of the light ASO to drop out of the recirculating acid catalyst in the alkylation process, for removal through the regenerator bottoms stream in conduit  116 , is to decrease the temperature of this bottoms stream. A preferred way to decrease this temperature is to decrease the temperature of the stripping isobutane entering the lower portion of the tower in conduit  120 . Alternately, decreasing the temperature of the regenerator catalyst feed stream in conduit  112  will decrease the temperature of the bottoms stream, thus increasing the amount of ASO withdrawn in the bottoms stream in conduit  116 . 
     Another benefit for manipulating the temperature of the regenerator bottoms stream in conduit  116  involves maintaining water content in the recirculating catalyst at a desired value. Significant economic benefits can be obtained if the water content of the recirculating catalyst is maintained at 1-2 wt. %. Many refiners, however, do not realize this upgrade because serious corrosion problems occur if the water content should unintentionally rise much above 3 wt. %. 
     As illustrated in FIG. 8, an output signal  132  from the triple component NIR analyzer/controller  130  is used to control the temperature of the stripping isobutane. In this case, the analyzer output signal used could be representative of either the measured concentrations of ASO or H 2 O of the recirculating acid catalyst. As illustrated, however, with connection to temperature controller  126 , control signal  132  would be scaled to be representative of the temperature of the isobutane stream in conduit  120  that would maintain the desired concentration of ASO or H 2 O in conduit  110  represented by set point signal  144 . 
     Also as illustrated in FIG. 8, the HF/ASO output signal  134  from the NIR analyzer  130  could be used to control the temperature of the catalyst feed to the regenerator in conduit  112  since reducing the catalyst feed temperature also reduces the bottoms temperature so as to increase the amount of HF/ASO withdrawn in the bottoms stream. As illustrated signal  134  would be a scaled control signal. 
     Another useful control loop in the HF alkylation process, which is also illustrated in FIG. 8, includes manipulating the side draw flow rate in conduit  118  responsive to the measured H 2 O concentration in the acid recycle stream, such that a higher side draw flow rate will reduce the H 2 O content in the recirculation catalyst stream in conduit  110 . This loop would be useful when an additive such as sulfolane is present in the alkylation catalyst. 
     This on-line concentration measurement and control allows rapid and very close control of critical HF alkylation variables, which results in increased production with close to specification products. 
     Still referring to FIG. 8 one of the three illustrated candidate control loops will be described in more detail. A temperature transducer such as a thermocouple operably located in conduit  120  provides an output signal  140  which is representative of the actual temperature of liquid flowing in conduit  120 . Signal  140  is provided as a first input to the temperature controller  126 . Temperature controller  126  is also provided with a set point signal  132 , which originates in the NIR analyzer  130 , and is a control signal scaled to be representative of the temperature of the stripping isobutane stream in conduit  120  required to maintain the concentration of a component such as ASO in the circulating acid catalyst stream in conduit  110 , substantially equal to the desired concentration of the circulating acid stream component represented by set point signal  144 . 
     Responsive to signals  140  and  132  the temperature controller  126  provides an output signal  146  which is representative of the difference between signals  132  and  140 . Signal  146  is scaled to be representative of the position of control valve  148 , which is operably located in conduit  150 , required to maintain the actual temperature in conduit  120  substantially equal to the desired temperature represented by signal  132 . Control valve  148  is manipulated responsive to signal  146  so as to adjust steam flow to heat exchanger  152 . 
     In another preferred alternate embodiment NIR analyzer output signal  134  can provide a set point signal for temperature controller  124  which in turn controls the temperature of the acid catalyst feed in conduit  112  so as to maintain a desired concentration of a component in the circulating acid catalyst stream flowing in conduit  110 . 
     Also illustrated in a similar alternate embodiment NIR analyzer output signal  133  could provide a set point signal for pressure controller  135  which is operably located in conduit  118 . 
     While the invention has been described in terms of the presently preferred embodiments, reasonable variations and modifications are possible by those skilled in the art and such modifications and variations are within the scope of the described invention and the appended claims.