Abstract:
Liquid hydrocarbon samples from a mainstream are diverted through a sample cell in cooling chamber. An optical cloud point detector signals a microprocessor-based control system which controls the charging and cooling of the cell. Cloud point reference level is continuously updated. After each measurement cycle, the control system readjusts the cooling power so that the time interval between the end of the flush cycle and cloud point detection is brought within a predetermined range. The control system switches between several distinct modes of operation automatically.

Description:
REFERENCE TO MICROFICHE APPENDIX 
     Incorporated herein by reference is a microfiche appendix consisting of one card containing 22 pages of software listings. 
     BACKGROUND OF THE INVENTION 
     The invention relates generally to analysis of the properties of liquid hydrocarbons, and more particularly to refinery process control and test apparatus for determining the temperature at which wax crystals first appear in a liquid hydrocarbon sample. 
     In the refining of certain liquid hydrocarbons such as diesel and fuel oil, wax molecules are found to be present in varying concentrations and forms at different stages of the refining process. Wax can be removed to a large extent by cooling the product to the point where the wax crystallizes and precipitates. The point at which wax crystals begin to form is referred to in the art as the cloud point, characterized by a rapid change in the optical characteristics of the liquid. In order to control the amount of wax remaining in a liquid hydrocarbon product to meet industry specifications for a given type of fuel oil for example, it is important to know the precise cloud point of the liquid hydrocarbon being produced. However, cloud points vary widely with the type of liquid hydrocarbon and are incapable of precise prediction without actually sampling the feedstock and testing its cloud point. Cloud point temperature is used in process control. The amount of wax remaining in the product may affect its quality and indirectly the price it can command in the marketplace. Inadvertently precipitating out far more wax than is required by the specifications for a given fuel oil, for example, can result in selling a very high quality fuel oil at the wrong price. Cooling large quantities of refinery product to remove wax is a costly undertaking requiring huge refrigeration units and tankage requirements. By increasing the accuracy and frequency of cloud point measurement, the refinery can control the product more closely with respect to sales specifications, thus optimizing quality control and maximizing the refinery&#39;s more profitable fractions. 
     In the past, cloud points have been analyzed in refineries by laboratory technicians. In many cases, samples are taken only once per shift. Meanwhile process control variables can range far from the optimum level. One of the major problems facing refinery companies is the measurement of cloud point of straight run products produced from a number of different crudes with differing characteristics. With frequently varying feedstock properties, the cloud point may change abruptly. Moreover, because of the varying optical properties of the feedstock, the threshold for optical detection of cloud point may also vary abruptly. 
     SUMMARY OF THE INVENTION 
     The general purpose of the invention is to sample a liquid hydrocarbon stream repeatedly to monitor cloud point temperatures with an updated detection threshold in a system with automatic control of the sample charge, dump and flush modes. 
     These and other objects are achieved in the microprocessor-controlled system according to the invention. Liquid hydrocarbon samples from a mainstream are diverted through a test cell in a Peltier cooling chamber. An optical cloud point detector signals a microprocessor-based control system which controls the charging and cooling of the cell. Cloud point threshold is continuously updated. In one mode, after each measurement cycle, the control system readjusts the cooling power so that the time interval between the end of the flush cycle and cloud point detection is brought within a predetermined range. The control system switches between several distinct modes of operation automatically. In another mode, the cooling rate is progressively increased to a point where in the absence of a cloud, the sample is automatically dumped. If a cloud is detected in this mode, the same cooling rate can be applied in the first mode. Onstream cloud point monitoring in applications ranging from hydro skimming to hydro conversion permits continuous updating of process control variables based on changes in cloud point resulting in more cost effective quality control of a wide range of products with varying specifications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view illustrating the exterior components of an embodiment of the onstream cloud point analyzer constructed according to the invention. 
     FIG. 2 is a front view of the face of the control box for the analyzer of FIG. 1. 
     FIG. 3 is a schematic flow diagram of the cloud point analyzer apparatus contained in the lower analyzer box of the unit of FIG. 1. 
     FIG. 4, comprised of FIGS. 4A-4F, is a wiring diagram of the electrical portion of the cloud point analyzer of FIG. 1. 
     FIG. 5, comprised of FIGS. 5A-5D, is a schematic circuit diagram of the central processing unit (CPU) printed circuit board (PCB) of FIG. 4. 
     FIG. 6, comprised of FIGS. 6A-6D, is a schematic diagram of the input/output (I/O) PCB of FIG. 4. 
     FIGS. 7A and 7B is a read only memory table of machine code in hexadecimal digits. 
    
    
     Microfiche Appendix 1 consists of a complete assembler listing of all software instructions in the standard format for Intel 8085 microprocessors used in conjunction with the microprocessor control system according to the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The automatic onstream cloud point analyzer shown in FIG. 1 includes a control box 10 and analyzer box 12 mounted on a rugged free-standing frame 14 designed to be located on the refinery premises near the process line to be sampled. The system is designed to monitor sample stream cloud point temperatures in correlation with ANSI/ASTM (American National Standards Institute/American Society for Testing and Materials) D-2500 and IP (Institute of Petroleum)-219 tests. The instrument&#39;s flexible mode automatic operation allows it to handle frequent changes in feedstock characteristics. Thus, the operator can select highspeed trend monitoring (mode 1), simulate laboratory testing (mode 2) or combine the two measurement modes (mode 3). Boxes 10 and 12 form explosion proof housings which can be pressurized if desired to meet Class 1 Group D Division 1 standards for hazardous area applications. The microprocessor controlled system shown in FIG. 1 is designed for use on products as varied as diesel fuel, cycle oil and gas-oil streams with cloud point temperatures as low as -30° C. (-22° F.) with 20° cooling water. The sample measuring apparatus is housed in the lower box 12 separately from box 10 which houses the electronics of the control system as well as the front display panel mounted on the hinged bolted door or cover 10A shown in FIG. 2. The display/control panel includes start and stop pushbuttons S10 and S11 on one side and on the other side pushbuttons S12 and S13 for automatic and manual flush. Buttons S10 through S13 are equipped with self-contained indicator lights. In the central portion of the cover 10A, an array of light emitting diodes (LED&#39;s) D10 through D15 are visible through a glass window 16. Below the window 16 is an explosion-proof vent 18 for a pressure transducer for atmospheric sensing. 
     The liquid hydrocarbon sample is cooled in a test cell 20, as shown in FIG. 3, which is thermally isolated and insulated inside the analyzer box 12 (FIG. 1). Test cell 20 includes a vertical glass tube 22 in an aluminum block 24, to which two PELTIER-effect thermoelectric (T/E) cooling modules 26 and 28 are clamped. Liquid coolant is recirculated from coolant inlet line 30 via T/E modules 26 and 28 and out through the outlet line 32. The sample is fed through the test cell 20 from a sample inlet line 34 via a three-way solenoid 36 physically connected to the control box 10 in FIG. 1 and up through the bottom of the vertical glass tube 22 through the test cell and out through a spent sample outlet line 38. Cloud point analyzer sampling valve 36 is a three-way solenoid valve normally deenergized in the bypass mode for recirculating sample. A thermocouple 40 is operatively positioned with respect to the glass tube 22 to monitor the precise temperature of the sample as it passes through the center of the test module. Light rays emanating from an incandescent lamp 42 pass through a first polarizing film 44 which limits their vibration to one plane. The rays then pass through a hole through the test cell 20 intersecting the glass tube 22 at a point of measurement 46 as shown in FIG. 3. The rays pass through the walls of the glass tube, through the sample, through a second polarizing film 48 to an aligned photocell 50. The second polarizing film 48 lies perpendicularly with respect to the first film 44, thus limiting passage of non-refracted light rays into the photocell 50. The electrical resistance of the photocell varies with the amount of light to which it is exposed. Adjusting the sensitivity level of the resistance detection circuits, as explained below, permits the photocell to ignore lesser amounts of light, thus reacting selectively to that refraction attributable to the presence of wax crystal growth in the sample trapped in the tube 22. The sample does not flow continuously through the test cell 20. Rather, a new sample is introduced via the solenoid 36 which thereupon closes to stop further sample liquid from flowing into the test cell during the test. Meanwhile, when the line 52 extending from the solenoid 36 to the test cell is static, the sample withdrawn from the process line is recirculated via line 54 back to the mainstream (not shown). 
     The sample should be withdrawn from process lines at a point and in a manner conforming to ASTM (American Society for Testing and Materials) procedure D-270. A fast loop sample pump may be installed in the sample inlet line 34 to generate the recommended flow velocity (ideally two to five feet per second) or to establish pressure conditions as demanded by the system. It is also recommended that between the solenoid 36 and the sample pump, a sampling condition system be included with means to filter, coalesce and regulate the sample entering the analyzer. A back check valve and throttling valve are recommended to prevent recirculation of spent sample. Some provision may be made to introduce standardization samples into the analyzer for calibration purposes with suitable pipe fittings and valves. In general use, filtered dry instrument air or nitrogen should be introduced into the analyzer box 12 to prevent condensate accumulation on the super cooled components in the vicinity of the test cell 20. 
     The heart of the electronic portion (FIGS. 4-7 and Microfiche Appendix I) of the control system for the cloud point analyzer is an Intel 8085 8 bit multiplexed bus microprocessor IC220 shown in the central processing unit (CPU) schematic drawing of FIG. 5. The machine code of FIG. 7 is stored in electronically programmable read only memory (EPROM) IC201 through IC206, representing a total capacity of 16K bytes only a portion of which is actually used by the concise program of Microfiche Appendix I. The EPROM&#39;s are addressed via 8 bit transfer latch IC215 which forwards the address lines AD0 through AD7 under the control of the address load enable (ALE) output of the CPU IC220. Additional address lines AD8, 9 and 10 are employed to furnish the complement of the 11 bit address for the EPROM&#39;s. The individual EPROM&#39;s IC201, IC202, etc. are selected by decoder IC214 under the control of outputs A11-13 of the CPU. The CPU board in FIG. 5 also includes ample random access memory IC207-210 selected in cooperation with decoder IC213. The address lines are passed to the I/O printed circuit board (PCB) shown in FIG. 6 via hex inverter IC211 and the data bus is also furnished to the I/O board via bus drivers IC216 and 217. The CPU IC220 is clocked by a crystal Y201 at 4 MHz. The I/O PCB circuit of FIG. 6 includes three 8 bit latches IC112, 113 and 114 selected by decoder IC103. Latch IC112 is read by port 3800 from decoder IC103 and determines which one of three modes, (1) normal, (2) ramp or (3) combination mode, has been selected. The system output signal from the apparatus described above is the output of the thermocouple 40. The thermocouple output terminals TB3-13 and 14 are designed to be converted to a 4 to 20 milliamp DC signal with a conventional signal converter (not shown). As the thermocouple output is cyclic, representing the alternate heating and cooling sequences of the measuring cycle, a sample and hold circuit may be required in certain situations, for example, for closed loop control of process variables. Otherwise, the thermocouple output can be plotted on a standard X/Y recorder. The peaks on the recorded graph will reliably indicate the cloud point temperature. Independent timer IC115 issues an output signal every second which forms interrupt request RST7.5 to the CPU. The output of the timer IC115 also lights LED D14 on the display panel. 
     Specific external actions are taken in response to output commands from the CPU via the multiplexed bus. For example, individual bits of latch IC113 are set by the computer at the appropriate time to energize the DC solenoid, for example, via solenoid control in the wiring diagram of FIG. 4, the detect solenoid K40 in FIG. 4 or the auto dump solenoid K30 of FIG. 4. These solenoid control signals are passed by individual bits of IC113 via Darlington switches IC105, 106 and 109. Auto dump and detect output commands also energize lines P and N from the I/O PCB to drive the indicators D12 and D13. Another output bit from IC113 lights the photocell indicator D11. Latch IC114 drives digital to analog converter IC111 which produces a prescribed cooling power rate for the T/E modules 26 and 28. The photocell outupt is passed to analog to digital converter IC101 equipped with voltage reference IC102. The digital output of the analog to digital converter IC101 is placed on the data bus by the decoder IC103 as shown in FIG. 6. 
     Software instructions in assembly listing form are provided in Microfiche Appendix I. The program consists of the following declarations and routines. 
     
                       TABLE I______________________________________RAM           RAM Storage DeclarationsINIT          Cloud Point Initialize RoutineRST 75        Interrupt RST 7.5 Service         RoutineFLUSH         Flush RoutineDETEK         Detect RoutineNORCLD        Normal Cloud Routine (mode 1)RMPCLD        Ramp Routine (mode 2)COMCLD        Combination Routine (mode 3)RTEST         RAM Test RoutineCKSUM         Check Sum Routine______________________________________ 
    
     Microfiche Appendix I is a copy of a printout created by an ISIS-II 8080-8085 macroassembler. The memory location in the left-hand column begins at zero for each routine. The order of routines prescribed by the assembler determines the beginning ROM address for each routine in Table I. The term &#34;PUBLIC SYMBOLS&#34; refers to locator symbols for routines or subroutines which are used elsewhere in the program. The term &#34;EXTERNAL SYMBOLS&#34; refers to routines or subroutines external to the subject routine, for example, the interrupt routine is &#34;jumped to&#34; at relative ROM location 3C in the cloud point initialize routine. The assembly language and address of the operand are given in the columns labelled Source and Statement, respectively, to the right of which are annotations which describe the function. 
     The cloud point initialize routine initializes the cloud point hardware and software to known states and runs a self-diagnostic test of EPROM&#39;s and RAM&#39;s. 
     Each measuring cycle begins with the introduction of a sample into the test cell via the solenoid valve 36. The test cell is first flushed clean of any previous residue for 30 to 60 seconds, depending on whether mode 1 or mode 2 operation has been selected. Then the solenoid valve is deenergized, trapping fresh sample in the measuring cell and initiating a cooling sequence. In mode 1 operation, the microprocessor initially controls power to the cooling modules 26 and 28 via the D to A converter IC111 of FIG. 6 to achieve the maximum cooling rate. This quickly establishes the preliminary cloud point temperature. The photocell 50 signals the first appearance of wax crystals by a significant change in its electrical resistance. The analog to digital converter IC101 of FIG. 6 is read via an output command. Upon detection of this change in electrical resistance, the old sample is flushed via the solenoid 36 and a new sample is trapped. After each measurement cycle, the microprocessor readjusts the cooling power so that the time interval between termination of the flush cycle and detection of cloud point is brought back toward a nominal value of 90 seconds. This feature allows the analyzer to react to changes in feedstock. The reference level or baseline to which the photodetector output level is compared is automatically stored and updated to reflect changes in feedstock properties. 
     Should full power be applied without detection of a cloud point, the microprocessor automatically initiates an auto dump sequence, flushing the cell before repeating the test. An auto dump counter in software counts the number of auto dumps. Should three cycles occur without detection, a nine minute flush cycle will commence in order to remove any deposits that have accumulated on the inner walls of the glass tube. 
     In mode 2, the microprocessor gradually increases the cooling rate to simulate laboratory test conditions. If a cloud point is not detected, full power and minimum temperature, and 20 minutes elapses, the analyzer dumps the sample, flushes for three minutes and repeats the cycle. 
     Mode 3, a combination of modes 1 and 2, consists of 120 cycles of mode 1 operation followed by a single mode 2 cycle. The power required to reach cloud point in mode 2 is sensed by the microprocessor and this setting is used for the testing in mode 1. 
     Using the above described system, refineries can realize dramatic savings. Capacity for self-adjustment to sample characteristics and self-diagnosis has been incorporated into the analyzer&#39;s fully automatic operation. The system&#39;s unique design provides automatic shutdown in the event of coolant system malfunction or loss of case pressurization. LED displays keep the operator informed of the analyzer status, including flush, detection, auto dump, internal timing cycles and detection of sample cell component failure. 
     The foregoing description is of a preferred embodiment and is given by way of illustration. Many variations, adaptations, additions or omissions of specific components can be made by those skilled in the art without departing from the spirit or scope of the invention as indicated by the appended claims and the equivalents thereto.