Article pubs.acs.org/EF
Rapid and Environmentally Friendly Three-Dimensional-Printed Flow Injection Analysis System for the Determination of the Acid Number in Thermal Conductive Oil Jingjing Wang, Chunhua Xu, and Qijun Song* The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, People’s Republic of China ABSTRACT: A flow injection analysis (FIA) method was developed for the rapid and environmentally friendly determination of the acidity of thermal conductive oil. The acid number (AN) of the oil sample was obtained by online measurement of the concentration change of the standard KOH solution before and after the interaction with the oil sample. To ensure the adequate reaction of KOH with the acid in oil, the oil sample and KOH solution were segmented into small droplets in the confluence tube with the help of a coiled wire. The parameters, such as the mixing ratio of KOH with oil, the length of mixing tube, and the flow rate of the two mixing streams, were optimized. Then, the FIA manifolds was integrated to a three-dimensional (3D)printed FIA system (3DFIA), which was used for the determination of the AN in real oil samples. The results showed that the AN values in the range of 0.20−6.4 mg of KOH/g oil can be accurately determined with a limit of detection of 0.177 mg of KOH/g and a sampling frequency of 10 h−1. Also, the results obtained with the 3DFIA system agreed well with that obtained from the conventional FIA system as well as the standard titration method. The new method not only exhibited good precision and rapidity of analysis but also completely eliminated the use of organic solvents.
1. INTRODUCTION Thermal conductive oil is widely used in industry as heattransfer fluids.1,2 Under high temperature and prolonged running time, the oil could be oxidized and hydrolyzed and most oxidation and hydrolysis products are acidic substances. These acidic products shorten the life of thermal conductive oil and even affect the safety of the system.3,4 Therefore, the determination of the acidity in thermal conductive oil is often required for the evaluation of the heat-transfer process. By definition, the acid number (AN) is expressed as the milligrams of KOH that is needed for titration of acids contained in 1 g of oil.5 The AN is a measure of the degree of oxidation and hydrolysis in the oil and is one of the most important indices for oil production, storage, and application.6 The standard methods for AN determination are usually based on colorindicator titration or potentiometric titration, which are conducted in non-aqueous systems.5 These methods are often non-automated, laborious, and time-consuming and use a large amount of toxic solvents. Furthermore, it is often difficult to visualize the end point of the titration because of the dark color of the used oil. Potentiometric titration also suffered from the difficulty in the end point judgment, which often needs to refer to the electrode potential of non-aqueous buffer solution. To overcome these drawbacks, many instrumental methods were proposed for the AN determination, including gas chromatography (GC),7 high-performance liquid chromatography (HPLC),8,9 capillary electrophoresis (CE),10 and Fourier transform infrared spectroscopy (FTIR)11,12 methods. These methods generally have high sensitivities; however, the tedious and time-consuming pretreatment procedures, such as liquid− liquid extraction and solid-phase extraction, were still required before the analysis. Also, the use of expensive and often © XXXX American Chemical Society
cumbersome instruments are not suitable for in situ analysis. In contrast, the flow injection analysis (FIA) was a rapid, accurate, and simple method, which can incorporate multiple solution manipulations in a fairly simplified and readily automated flow system. In a typical FIA procedure, a sample solution is injected into a carrier solution, which mixes through radial and convection diffusion with a reagent for a period of time before the sample passes through a detector. New variants of the FIA technique use computer-controlled pumps that generate the flow that is precisely choreographed to the needs of the assay protocol. In this manner, the highly reproducible results may be obtained with a dramatic decreased sample and reagent consumption and waste generation.13 The automated methods based on flow injection (FI) have been developed for the determination of oil acidity.14−16 Combinations of the FI with HPLC,17 ultraviolet−visible (UV−vis),18 pH detector19,20 were reported for the AN analysis with good results. However, these methods still need to use organic solvents and large sample volume, and some require a long time to complete the analysis.21 For example, in the method using a pH electrode as the detector, the premixing of the oil sample with the alkaline solution in isopropanol (off-line) was conducted to ensure the completion of the reaction. Moreover, it could be problematic when the conventional pH electrode was used in such semiorganic solution, because the oil adsorbed on the electrode surface could have an adverse impact on the electrode response. In this paper, a much simpler and straightforward method was developed to deal with the above-mentioned problems. The oil sample and standardized KOH aqueous solution were Received: November 6, 2014 Revised: January 19, 2015
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DOI: 10.1021/ef502484f Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels directly segmented by means of confluence flow. To ensure the fully mixing of the oil phase and alkaline solution, a piece of coiled wire was inserted into the mixing tube to increase the reaction area. On the way to the phase separator, the acid in oil sample reacted with KOH solution also in the mixing tube. After the reaction, KOH in the aqueous phase was accumulated in the bottom of the separator because of its greater density in comparison to the oil phase. Then, the KOH stream was injected into a carrier stream. A flat-end glass pH electrode was used to sense the output voltage of every element of the mixture, and the signal was recorded as a typical FIA peak. On the basis of the KOH consumption, the AN of the oil sample can be calculated. Three-dimensional (3D) printing is a very versatile technique that can be used in 3D object manufacture. Typically additive processes are used, in which successive layers of material are laid down under computer control; thus, objects of almost any shape or geometry may be produced from a 3D model or other electronic data source.22,23 The rapid development in the printing technology and printing materials makes the 3D printing technology applicable and has border applications, including architecture, construction, industrial design, automotive, aerospace, military, engineering, dental, and medical industries etc.24−26 Thus far, however, the reports on the application of the 3D printing technology in FIA analysis were not seen. Therefore, in the present work, an integrated 3D model was designed on the basis of a previously optimized conventional FIA manifold, which included an oil/base segmentor, a mixing tube, a separator and a pH detection chamber. Thus, the AN of an oil can be rapidly and environmentally friendly determined in a miniaturized and robust system with increased precision and repeatability.
because of its greater density, which is then propelled to the pH detector along with the carrier stream by the peristaltic pump (P2). A 16-port valve (V) was used for the base injection, and the sample loop was 150 μL. When required, the mixture ratio and flow rate of the oil and base stream could be changed by changing the rate of the pump (P1). The change in the base concentration was monitored by a flatend pH-sensitive electrode (D) fitted in a specially designed flow cell (Sensorex, Garden Grove, CA). The flow cell has a thin channel with the dead volume of 90 μL, which allowed for a trace amount of solution to pass through the bottom of the pH electrode. In the conventional FIA setup, the flow cell with the pH electrode (D) was connected to the injection valve via a 40 cm long Teflon tube (inner diameter of 1 mm). The signal of output voltage was recorded as a typical FIA peak in the HW-2000 software (Shanghai Qianpu Software Limited Company, China). After the optimization of the experimental conditions, the part within the dotted line of Figure 1 was printed on the SPS 250E 3D printer (Shanghai Aowei Digital Technology Limited Company, China). The 3D printing material is photosensitive material (Somos 14120, Royal DSM), and the printed product is anticorrosive and transparent. A standard method was used to validate the proposed method.5 It is a non-aqueous titration method, where 0.1000 M KOH solution prepared in isopropyl alcohol was used as the titrant, the oil sample was dissolved in a mixture of toluene and isopropyl alcohol containing a small amount of water, and the indicator solution was prepared by dissolving 1 g of α-naphtholbenzein in 100 mL of mixture solution.
3. RESULTS AND DISCUSSION 3.1. Determination of the Base Concentration in the FIA System. On the basis of the principle described in the Introduction, we can divide the overall analysis into two parts: one part is a typical FIA system to measure the change in the base concentration, and the other part is to carry out the interaction between the oil and base phases. As the concentration of carrier, the volume of the injected sample and the length of the mixed tube affect the peak area and the sensitivity of the FIA system in the first part. Hence, these parameters were initially optimized to obtain a high sensitivity. It was found that the fast sampling rate and highest sensitivity can be obtained when the flow rate was set at 0.92 mL/min, the volume of the sample injection was 150 μL, and the concentration of the carrier was chosen as 9.08 × 10−4 mol/ L. Thus, these conditions were chosen for the subsequent evaluation of the oil/base interaction study. 3.2. Mixing of the Oil and Base Phases and Their Subsequent Separation. The reaction of acid in the oil sample with the base solution was mainly taken place in the mixing tube (Figure 1); therefore, the length and geometry of the mixing tube are critical for the completion of the oil/base reaction. In the case of a straight hollow mixing tube, the change in the length of the mixing tube from 20 to 100 cm almost has no influence on the FIA peak area and the obtained AN values were lower than that determined by the titration method, suggesting that the acid in oil did not completely react with the base solution. As shown in Figure 2a, the segmentation was inadequate in a straight tube and the contacted surface between the oil and base was very limited, leading to the incompletion of the acid−base reaction. In this case, the mere change of the oil/base flow rate or the length of the mixing tube cannot effectively increase the contact area. When a piece of coiled copper wire was inserted into the mixing tube, however, the oil sample was well-segmented into fine slices in the mixing tube, as shown in Figure 2b. The effect of the length of the coiled copper wire on the AN detection was examined. With the increase of the wire length,
2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals used were of analytical reagent grade, and high-purity deionized water was used throughout the experiment. The stock solution of KOH (0.46 mol L−1) was standardized against potassium biphthalate. The HCl solution was standardized with the KOH standard solution. 2.2. Method and Instrumentation. Figure 1 depicts the FIA manifold used for the determination of the AN of oil, with an oil/base segmentor and a separator configuration included. The oil sample and base solution were delivered to the Y-branch tube by a dual channel syringe pump (P1, Zhejiang Medical Equipment Limited Company, China), where the oil sample reacts with the base in the mixing tube and is, subsequently, separated in the phase separator. After reaction, the base phase is settled in the bottom of the separator chamber
Figure 1. Schematic diagram of the FIA system used: C, carrier; P, pump; o, oil sample; b, base solution; MR1, oil−base mixing reactor; MR2, mixing reactor; V, valve; D, detector; and W, waster. B
DOI: 10.1021/ef502484f Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
the FIA peak area, and the typical FIA peaks and the standard curve were shown in Figure 4. The calibration equation is S =
Figure 2. Segmentation of oil sample obtained from (a) straight hollow tube and (b) tube inserted with a coiled wire.
smaller segments of the oil were obtained, resulting in a more complete acid−base reaction; hence, the determined AN values were approaching that of titration values (Figure 3a). With the Figure 4. FIA outputs and the calibration curve for KOH standards. The concentrations from “a” to “h” correspond to 1.850, 3.699, 5.549, 7.418, 9.248, 11.09, 14.79, and 18.49 mmol/L KOH, respectively.
1.44572 × 106 log C + 5.11421 × 106 (R = 0.9992), where C stands for the concentration of KOH standard solution and S is the FIA peak area. With this equation, the concentration of KOH solution after the interaction with oil can be calculated; also, the AN of an oil sample can be obtained on the basis of eq 1: (C0 − Cr) × V × 56 (1) m where the AN is the acid number of the oil sample (mg of KOH/g), C0 is the initial concentration of KOH (mol/L), Cr is the concentration after reaction with acid in the oil (mol/L), V is the volume of KOH (mL), and m is the quantity of the oil sample (g). To test the practical applicability and precision of the proposed methods, the oil sample collected from local factories was analyzed with the proposed method and the results were compared to that obtained by the conventional titration method. As seen in Table 1, the results obtained with our AN =
Figure 3. (a) Effect of the length of the copper wire and (b) flow rate of the oil/base stream on the AN analysis.
further increase of the wire length, however, the flow resistance also increased, which would affect the operation of the syringe pump. Thus, a 30 cm copper wire was chosen, because, in this case, the obtained AN value was very close to that obtained from the standard titration method. To obtain the regular and sufficiently small oil/base segments, the flow rates of the base and oil phases were preliminarily examined, because they could affect the size of oil droplets.27 As shown in Figure 3b, the measured AN values decreased with the increase of the flow rate from 0.2 to 1.7 mL/ min. The reduced contact time between the oil and base should be responsible for the low AN value in the cases of fast flow rates. On the other hand, when the flow rate was too slow, it would take a long time to complete the analysis; hence, a flow rate of 0.5 mL/min was found to be appropriate for the balance of rapidity and the accuracy of the analysis. In relation to the flow rate of the individual phase, the ratio between the two flow rates also has a significant impact on how well the two phase can be mixed and how readily they can be separated after the interaction. When the oil has a small AN value, the large flow rate ratio (i.e., the flow rate of the base is substantially greater than that of the oil phase) will lead to the change in the base concentration too minimal to be determined accurately.14 Thus, a flow rate ratio of 1:1 was chosen for the rest of the experiment. 3.3. Analytical Features of the Optimized FIA System. Under the optimized experimental conditions, a series of KOH standard solutions were injected into the FIA system and corresponding FIA peaks were obtained. The logarithm of the concentration of KOH standard solution was proportional to
Table 1. Comparison of Different Methods for the AN Analysis oil sample 1 2 3 4 5 6 7 8
reference method 0.70 0.92 1.33 2.30 3.30 4.54 5.45 6.56
± ± ± ± ± ± ± ±
0.05 0.03 0.05 0.05 0.08 0.08 0.07 0.09
FIA method 0.74 0.90 1.31 2.31 3.25 4.51 5.43 6.49
± ± ± ± ± ± ± ±
0.02 0.01 0.02 0.03 0.03 0.02 0.04 0.05
3DFIA method 0.73 0.91 1.29 2.27 3.27 4.49 5.41 6.46
± ± ± ± ± ± ± ±
0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.03
method were in good agreement with the results obtained by the reference method. Furthermore, the precision obtained by the our method was very satisfactory; the relative error of the proposed method was less than 4%; and the measurement had a mean relative standard deviation (RSD) of
