Computer controlled system for the automatic neutron activation

A californium-252 source is used for neutron activation of the samples. Under computer control, the system is capable of analyzing 48 samples in an 8-...
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Anal. Chem. 1980, 52,233-239

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Computer Controlled System for the Automatic Neutron Activation Analysis of Vanadium in Petroleum with a Californium-252 Source Joaquin A. Lubkowitz” and Hector D. Buenafama INTEVEP, Apartado 76343, Caracas 107, Venezuela

Victor A. Ferrari Reactor Experiments, 963 Terminal Way, San Carlos, California 94070

A pneumatic system interfaced to a digital computer has been designed, programmed, and tested for the unattended and continuous determination of vanadium in aqueous solutions, petroleum, and cokes. A californium-252 source is used for neutron activation of the samples. Under computer control, the system is capable of analyzing 48 samples in an 8-h period. Three different calibration curves are required for the three types of materials analyzed because of different thermalization effects in the different types of samples. The system requires 5-10 g samples to achieve a detection llmk of 10 ppm f 30%. Increased precision is obtained by reanalyzing aliquots of the sample. Accuracy studies obtained with prepared samples showed differences of up to 4 % .

Neutron Activation Analysis (NAA) has been used t o determine vanadium in petroleum on numerous occasions (1-3). It has been shown t o be a n effective technique due to t h e nuclear properties of 52V. A high cross section (4.3 barns), a short half-life (3.8 m), and a 100% isotopic abundance define the 51V(n,-y)5qreaction. It is a relatively easy isotope to count because of its 1434.4-keV (100%) y transition, as well as the 3.8-m half-life. T h e recent discovery of one of the world largest heavy crude oil (15-8 API gravity) deposits named the “Orinoco T a r Belt” has necessitated t h e design of a system capable of analyzing about 10000 samples per year in order to support pilot plant studies for t h e demetalization of this petroleum. Vanadium removal is important because of its interference in catalytic refinery processes. On the other hand, such removal generates streams which have high vanadium content. Vanadium recovery processes may become important in t h e overall economic aspects of petroleum treatment because of t h e increasing use of vanadium in t h e manufacture of alloys. Californium-252 sources have been used extensively as neutron sources owing t o their relatively high neutron flux. A continuous vanadium analyzer has been designed utilizing a 100-pg 252Cfsource. However, t h e system requires a continuous sample and has been designed for crude oils of high gravity (30-40 API) ( 4 ) . T h i s paper describes a computer controlled pneumatic assembly and multichannel analyzer capable of analyzing six samples per hour. The system is capable of a lower detection limit of 10 ppm with a single precision estimate of &30%. The system has been designed with t h e concept of complete unattended operation. EXPERIMENTAL I r r a d i a t i o n Assembly. The irradiation assembly is shown in Figure 1. It consists of a 1.6 m3 stainless steel tank, which is placed in a concrete lined hole, except for the tank’s last 5.0 0003-2700/80/0352-0233$01 .OO/O

cm. The tank houses two concentric and centrally located stainless steel tubes. The outer one rests in a lead shield located a t the bottom of the tank and it has been perforated to allow water circulation. The inner tube is closed at the lower end and it houses the 252Cfsource, handle, and a 3.0-mm stainless steel wire attached to the source. The upper end of the inner tube has a threaded cap with two holes which hold a 3.0-mm stainless steel cable. Thus, there are two possibilities of raising the source: either by raising the source directly or by raising the inner holding tube. This is an important consideration owing to the cost and difficulty of handling the source. The lead circular shield houses the source when it is not in use and thus background y radiation is reduced. During irradiations the source is raised 25.0 cm from the bottom of the tank. The outer tube serves as a guide for two removable stainless steel crosses which hold the aluminum irradiation tubes a t different distances from the source ranging from 2.0 to 17 cm. The entire tank, crosses, tubing, and irradiation tubes were painted with a polyamide resin paint (Epomon-794, Mobil-Oil Co, Baton Rouge, La.) as an additional precaution against corrosion of the aluminum tubes. Pneumatic Assembly. A schematic diagram of the pneumatic assembly is shown in Figure 2. The pneumatically air driven valves Vl-V9 are used to introduce short pulses of air into the tubing to drive the sample at a speed of 108 km/h. The limit switches (LS1-LS9) are contact switches which are mechanically energized when a valve is pneumatically activated. The two exceptions are LS3 and LSlO which are activated by the sample capsule and by the lack of compressor pressure, respectively. The dual 2-way diverter is an assembly of two valves (V7, V8) and four limit switches (LS4-LS7) which allows the sample two forward paths (DB and DA) leading to either irradiation tube and also allows two return pathways (AC, BC) leading from either irradiation tube toward the counter end. The two-way diverter is similar to the dual-2 way diverter but it allows only one fomard path (AC) and two return pathways (CR, CA). The photodetectors P1-P4 are used to signal that the capsule has reached a selected destination. Photodetector P 5 is used to signal that no sample is left in the autoloader. Photodetectors P1, P2, and P4 also serve to determine the duration of the air pulse of the air cushion valves. The air cushions are used to soften the landing of the capsule at the irradiation tubes, counter end, and a t the drop out station. The air cushion pneumatic valves inject a short pulse of air in the direction opposite to the capsule motion thereby softening the landing. The air motion in the opposite direction is possible due to the fact that the irradiation tubes and counter end are concentric tubes. The pneumatic assembly accepts 33-mm 0.d. capsules made of high density polyethylene. Plastic sample vials (Durham Aircraft Services, Hicksville, N.Y.) of 7.0 mL or 14.0 mL were used as sealed sample holders. The loading unit as shown in Figure 2 is attached to 90 feet of polyethylene tubing capable of holding 175 samples. When V4 is actuated, a sample is loaded (LS1 depressed) and subsequently V5 is energized and the sample is sent to irradiation tube 1 or 2. When the sample arrives at the irradiation tube, P1 or P2 signals the computer to start counting the irradiation time, to energize V1 or V2, which will cushion the (C 1980 American Chemical Society

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Flgure 1. Schematic drawing of the irradiation assembly. (1) Pneumatic tube, (2) stainless steel wires, (3) outer-tube, (4) inner-source container, (5) ballast for irradiation tube, (6, 8) stainless steel cross, (7) irradiation tube, (9) source handle, (10) Cf source, (11) lead shield

sample landing, and to de-energize V5. When the selected irradiation time is completed, V1 or V2 is actuated to send the sample to the counter end. The triggering of P3 serves to start the counting, to de-energize V1 or V2 and to actuate V3. During the counting time, the next sample is introduced into a selected irradiation tube. After counting time is completed, V3 is actuated until P 3 is triggered. Subsequently, V6 is opened and LS3 is energized indicating that the cycle is complete. Six samples can be analyzed in 68 min. Counting Assembly. A multichannel analyzer (MCA) of 4096 channels calibrated at 0.5 kevlchannel (Model 8180, Canberra, Meriden, Conn.) was used for measuring the sample activity. The MCA was coupled to a PDP 11/04 computer (Digital Equipment Corporation, Maynard, Mass.) provided with 28 K of core memory, real time clock, ROM bootstrap, TTY interface, and a D L l l D communications interface. Programs and accumulated data were stored in a DEC RX-11 Dual Drive Floppy Disk. A horizontally mounted Ge(Li) detector of 1.99 keV resolution (fwhm) at 1332 keV was used to count the sample. The corresponding efficiency and peak/Compton ratio were 15.4% and 37.1, respectively. Two Canberra motion control interface cards model 2221 were used to interface the control unit of the pneumatic assembly and the PPD 11/04 computer. The card occupies positions C, D, and E of the Small Peripheral Controller (SPC) slot in the computer backplane. An additional backplane is required for the second card. Each card is capable of sensing 8 inputs and commanding 8 outputs. Pneumatic Electronic Control Unit. This unit interfaces the pneumatic assembly system with the PDP 11/04. A schematic diagram of the control unit is shown in Figure 3. The 24-V dc power supply is used for activating the solenoid valves and the 5-V dc power supply is used for internal logic use. The inputs and outputs are optically isolated to prevent spurious interfering

signals from reaching the computer. The unit controls ten limit switches, one pressure switch, and five photodetectors. Nine drivers are used for solenoid valves and latch resets. The signals from the photodetectors and the limit switches are sensed by the computer through sensor buffers which facilitate coupling and noise elimination. The latch circuits act as two-state memories indicating whether photodetectors Pl-P4 and limit switch LS3 have been activated. The computer will read the latch circuit, and reset the latch. Limit switches LS1-LS9 are not latched since they are continuously on or off depending on whether they are depressed or not by the piston of the pneumatic valve. P 5 is also not latched since it is used to indicate the absence of samples in the loader. Insufficient pressure from the compressor is read by the computer from LS10. The unit has LED indicators that display the conditions of all latches, limit switches, solenoid valves, and photodetectors. Toggle switches enable manual operation for troubleshooting. Computer Control of the Pneumatic Assembly. The PDP 11/04 computer uses the floppy disc tw systems device and utilizes the RT-11 operating system (Digital Equipment Corporation, Maynard, Mass.). Canberra’s Laboratory Automation Software System (CLASS) was used as the programming language. Five subroutines were written in order to send capsules to either irradiation tube, to send them from either tube to the counting room, and to send them to the drop out air cushion. A typical subroutine to send the sample to irradiation tube $2 is discussed as follows. The irradiation time is entered and the computer is asked to verify that the following initial starting conditions are met: LS10, LS1, and P5 = 0, while V7, LS5, LS7, P2 = 1. This statement checks that the compressor, exit valve, photodetector 2, tube selection, and sample loader are ready for operation. Next, V4 and V5 are energized until P2 = 1. When the computer clock is equal to the preset time, V2 will be actuated. This subroutine

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J lLb AUTO L M D E R

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Figure 2. Schematic diagram of the pneumatic assembly. 3 electrical connector; 0 capsule photodetector Pl-P5; 0 air connection, 2.5 kg/cm2; A air connection, 7 kg/cm2; 0 air connection, 1 kg/cm2;(-) polyethylene tubing, 38 mm; ( - - - - ) polyethylene tubing, 32 mm; V l - V 9 pneumatic actuating valves; EXH = Exhaust; LS1-LS9 limit switches; M1, manual capsule retriever; W 1-W3 pressure regulators

can be concatenated with other subroutines that control the motion of the capsule or with data reduction subroutines. Detailed copies of the programs are available from the authors. Californium-252 Source. The 252Cfsource had a t the time of installation 2.9 mg and an activity of 1.6 Ci. The cermet wire encapsulated source was of the SR-Cf-100 type and it was obtained from the Monsanto Research Corporation, Dayton, Ohio. The maximum permissible cermet wire length was used in order to uniformly irradiate large sample volumes. The encapsulated source was screwed into a stainless steel rod having a diameter of 30 mm and a length of 150 mm. The rod served as additional weight to keep the source wire taut and it served as shielding for the exposed area of the inner tube of the irradiation assembly. The radiation level at the tank surface when the source is raised 30 cm from the lead casket is 1.0 pGy/h. The source is raised by a synchronous stepping motor (Superior Electric Co., Bristol, Conn.) capable of moving 200 steps per revolution. The source position can be reproduced within A0.8 mm. The thermal flux was estimated by performing irradiations of gold foils bare and covered with cadmium. The activity of the foils was determined by counting the 411.8-keV y ray from "'Au and by determining the detector efficiency with an 15'Eu standard source. The maximum thermal flux obtained with water in the tank was a t 2.0 cm and was estimated at 14.8 X 10' n cm2 s-l f 30%. The uncertainty in the measurement is due to lack of precise corrections for flux perturbation of the monitor. Sample Preparation. Preparation of Oil Standards. Standards of V in diesel oil were prepared in the range of 10-4500 ppm (w/w). Typically, for the preparation of a 10-ppm standard, a 0.2-g aliquot of a 5000-ppm Conostan (Continental Oil Co. Ponca City, Okla.) V standard solution was weighed in a 200-mL high density polyethylene bottle. Subsequently, about 100 g of Co-

nostan base oil and 0.11 g of Conostan stabilizer were added and weighed. The sample was stirred magnetically for 8 h. These standards were used for calibration of the system for the analysis of petroleum. Preparation of Aqueous Standards. Analytical reagent grade vanadium pentoxide was dissolved in 3 N H2S04. The resulting solution containing 5000 ppm vanadium was used as stock solution for calibrations in the range of 10-5000 ppm. Preparation of Coke Standards. About 0.5 g of dried V 2 0 5 was added to 9 g of activated charcoal. The resulting 4% V mixture was used as stock suspension to prepare either more dilute or more concentrated standards by either weighing additional charcoal or V205respectively. The standards were prepared in the range of 0.01-10%. Either 5 g or 10 g of standards (aqueous, petroleum or cokes) were weighed depending on the concentration level of the samples. All samples were irradiated for 15 min and counted for 10 min. Calibration and Data Analysis. A linear regression analysis is used for the 50-point calibration curve (5). Mandel's method of determining the uncertainties of the calibration curves by the use of a hyperbola is applied in the mathematical interpolation of sample analysis (6). A flow chart of how the system operates under computer control is shown in Figure 4. The time for performing the calculation is 2.20 min and thus the limiting throughput is the irradiation step. The programs used for peak integration, smoothing operations, area computation, and variance calculations of the counting rate are those contained in Canberra's SPECTRAN 111 programs (7).

RESULTS AND DISCUSSION Calibration Curves. Our first attempt t o compare t h e counts of aqueous standards with those obtained from

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Figure 3. Schematic diagram of the pneumatic interface

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standard synthetic oil samples, both containing the same vanadium concentration, failed to yield a correlation. Thus, it was necessary to perform three different calibration curves covering the three types of samples normally analyzed. Figure 5 shows the calibration curve obtained for aqueous solutions. The aqueous solution curve was constructed with five different samples a t each point shown in the graph. The slope for this curve is 0.8951 count/fig and it has a correlation coefficient of 0.9999. I t can be observed that the relationship is linear up to 55 mg of vanadium. This linear relationship deteriorates for higher vanadium content. The calibration curve obtained for synthetic oil samples and cokes is shown in Figure 6. The relationship was studied in the range of 100-60000 fig of vanadium. This range covers the highest concentration of vanadium found in petroleum (8). The calibration curve for crude oil has a slope of 0.9387 countlfig and a correlation coefficient of 0.9977. The calibration curve for cokes is shown also in Figure 6 and it has a slope of 0.8576 countlfig and a correlation coefficient of 0.9971. This curve is similar to that obtained for aqueous solutions in that it has linear and nonlinear regions. The three calibrations curves have different slopes which decreased in the order crude oil > aqueous > coke. T h e neutron energy distribution of the z52Cfspontaneous fission is different than that of the induced 235Ufission. Furthermore, the sample weight used in this work is in the range of 5-12 g. This larger weight is necessary to cover the

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possible range of concentrations with a thermal flux of IO8 n cm2 s-l. Thus, in our irradiation assembly, the large sample mass serves as an additional neutron moderator whose thermalizing effect is added to that produced by the available water path. Infinitesimal sections of the sample are exposed to different thermal fluxes. T h e thermalization of the flux will depend on the atomic composition of the samples and consequently it will depend on the effective mean 2 number. This explanation is consistent with the relative decreasing order of the slopes of the three calibration curves. The fact that the aqueous and coke calibration curves have nonlinear ranges a t high vanadium concentration is due to a selfshielding effect of the vanadium. Samples of this high V concentration are rare and can be diluted with water or activated carbon respectively. Since the average half-life for =%f is 2.26 years, it is necessary to perform the calibrations every two months so that the 4% decrease in flux in this time period will not affect the analysis. Reproducibility Studies. Table I shows the analyses of nine aliquots of deasphalted crude oil samples performed two weeks apart. Different standards were used for each calibration curve. The relative standard deviation of 2.92% and 2.82% obtained is good. The reproducibility in this case is a measure not only of operator controlled variables but also it includes the reproducibility of operating limit switches, reproducible capsule speeds, counting time, reproducible

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S E L E C T IRRADIATION TIME C O U N T I N G T I M E , T U B E No A N D S E T NUMBER OF CYCLES

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Figure 4. Computer programs flow chart

source position, etc. The good reproducibility obtained is indicative that these parameters can be carefully controlled under computer command. Unlike a reactor irradiated crude oil sample which shows 20-30% dead time, the samples irradiated in this study never exceed 5% dead time. Thus, no dead time corrections had to be applied. The fact that it may be necessary to increase sample weight so as to increase sensitivity, required the study of the effect of sample size upon the analyzed vanadium concentration. The results are shown in Table 11. The results indicate that there is no statistical

difference in utilizing about twice the sample weight. When the sample size is increased, the sample diameter is not changed but the height of the sample in the container increases. Thus, the average flux seen by the sample does not change enough to affect the results. Accuracy and Detection Limit Studies. Table I11 shows the results obtained by analyzing prepared standards and the concentrations calculated from the weights of the vanadium. In the range of 19-5000 ppni, the greatest difference obtained is 4% while the smallest is about 0.1%. In addition, a par-

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Flgure 6. Calibration curve for oils and cokes ticular bias is not observed. The results give sufficient confidence in the accuracy that can be obtained. Table I11 also shows the results obtained with the only commercially available fuel oil standard. The agreement is good. The preparation of standards of heavier crude oils having 22 API gravity or lower is difficult since we have previously shown t h a t these crudes are heterogeneous and show variable concentrations of several elements ( 3 ) as a function of time. Table I11 also shows the detection limit that can be obtained by the system. Sample S-41 having a weighed content of 18.9 ppm shows a standard deviation for a single measurement of *30%. Samples in the concentration range of 1&20 ppm were grouped and analyzed in triplicate under separate runs. Table I11 shows t h a t a sample containing 11.84 ppm showed a

standard deviation of f l l % . A t the 21.2-ppm level, the relative standard deviation was 8.9%. The corresponding relative standard deviation a t the 50-ppm level decreased to 3.8%. The system possesses the capability of generating a greater precision for the samples having a content of 10-20 ppm. The choice of reanalyzing the sample with more aliquots depends on the particular experiment. The 10-ppm level is considered as the practical detection limit of the system. Any sample having a lower content is analyzed by other techniques such as electrothermal atomic absorption.

CONCLUSIONS The system described is advantageous for analyzing a large number of samples requiring a minimum sample preparation

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Table I. Reproducibility of the Analysis of Deasphalted Crude Oils vanadium concn, ppm sample code B1 637.0' 641.8b B3 656.2 660.6 B4 657.9 661.0 B5 690.1 694.2 B6 644.7 678.5 B7 634.3 638.7 B8 672.1 676.0 B9 681.0 684.2 B10 661.6 665.1 659.1 i 19.2 666.3 + 18.7 average RSD, 7% 2.92 2.82

' Analysis based on calibration on date 11-22-78using standard Conostan I. Analysis based on calibration on date 12-5-78 using standard Conostan 11. Table 11. Irradiation of Crude Oil Samples sample code weight, g vanadium concn, ppm 5.674 18.6 I3.7' 40 11.072 18.1 t 4.2 41 5.899 23.4 i_ 5.4 1181P 12.197 21.2 i 4.9 1181PP 5.697 205.6 i 1 0 . 4 43 11.412 209.3 t 11.8 43s ' Square root of variance of a single measurement. Table 111. Accuracy and Detection Limit Studies of Prepared Standards vanadium concentration, ppm sample sample calcd code weight, g analyzed from weight S41 11.072 19.2 L 3.2 19.80 S42 10.840 107.5 i 6 . 1 103.76 s43 11.412 218.7 i_ 9.8 209.85 s44 11.553 300.0 t 17.0 300.08 s45 10.985 2574.4 i 116.2 2499.9 S-41s 10.726 4827.9 i 201.6 5000.0 s-45s 11.156 404.2 = 19.6 409.4 NBS sTD 299.0 i_ 5.7 320 i 15" S-lb 21.20 + 1 . 8 9 21.34 50.20 S-L2b 50.20 1 . 9 1 S-L3b 11.84 i 1.31 10.97 ' Value reported b y National Bureau of Standards. Analvzed in tridicate. +_

beyond that of weighing. Since large samples are used, weighing is simplified by using top loading balances having a precision of fl mg. Although larger samples are required, it is not disadvantageous to use 10-g samples. As a matter of fact, inhomogeneity effects of heavy crudes ( 3 ) and the distribution of the metal porphyrinates in several solid phases make it more amenable to use larger weight samples. A more

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representative sample is taken than in the case of reactor irradiations which require milligram samples. Thus, the vanadium concentrations obtained with this source yield better mass balances in pilot plant studies (8). The system as described in this work, would be difficult to set up in a research reactor. The sole purpose of the 252Cf source is to achieve a spontaneous fission and thus provide a steady unperturbed thermalized neutron flux. I t does so continuously. On the other hand, reactor operation requires continuous power adjustments which render flux variations as a function of time (9) and such irradiations require continuous comparisons with standards. Moreover, the continuous operation of research reactors around the clock is costly. The lo8 n cm2 s-' flux seems sufficient for the activation of the 51V(n,y)52Vreaction and thus other potentially high dead time causing isotopes present in the crude oil are not as easily activated. The result is a relatively interference-free V spectrum. The fact that a decision must be made on selecting a proper calibration curve for three types of materials is not a burden since sample weighing renders an inspection of the type of samples. One of the drawbacks a t present lies in the time required in weighing a sample of crude oil of 8-15 API gravity. Considerable care of sample attachment to a balance or other equipment is required. The cost of the system is less than t h a t of X-ray fluorescence, which is the next most viable technique for a large number of samples. The higher y energy of 52Vused in neutron activation analysis is less affected by matrix effects than the corresponding V radiation used in X-ray fluorescence.

ACKNOWLEDGMENT The authors thank Carlos Inguaneo and J e s h GonzPlez for their electronic and mechanical help in developing the system. The authors thank Carmen Pacheco and Edward Fisher for computer programming. Thanks are also due to Mirtha JimBnez for sample preparation and helpful discussions during the initial phases of the project.

LITERATURE CITED (1) Zaghloul, R.; Obeid, M.; Staerk, H. Radiochem. Radioanal. Lett. 1973, 15, 363. (2) AI-Shahristani, H.; AI-Atyia. J. M. J. Radioanal. Chem. 1973, 14, 401. (3) Buenafama, H.; Lubkowitz, J. A. J. Radioanal. Chem. 1977, 39, 293. (4) Californium-252 Progress Reports, United States Atomic Energy Commission, No. 15, May 1973, p 16. (5) Natrella, Gibbons M. "Experimental Statistics", National Bureau of Standards, Washington, D.C., October 1966, Handbook 91, p 20. (6) Mandel, J.; Linning, F. J. Anal. Chem. 1957, 29, 743. (7) Spectran 111, Users Manual, Canberra Industries Inc., Meriden, Conn., 1977. (8) Villoria, D. A,; Krasuk, J. H.; Rodzguez, 0.: Buenafama, H.; Lubkowitz, J. A. Hydrocarbon Process. 1977, 56, 109. (9) Heurtebise, M.; Lubkowitz, J. A. Anal. Chem. 1971, 4 3 , 1218.

RECEIVED for review July 11, 1979. -4ccepted September 17, 1979.