Anal. Chem. 1988, 60, 1965-1968
1965
Electronic Alternative to the Reciprocating Piston for Pumping in Liquid Chromatography Theodore E. Miller, Jr.,* and Charles M. Davis The Dow Chemical Company, Central Research Laboratories, 1712 Building, Midland, Michigan 48674
Pumping In llquld chromatography Is conventionally based upon the prlnclple of a reciprocating piston and slldlng seal. Pressure and flow control efforts are thus limited by the inherently mechanical nature of the system. Furthermore, slldlng seals tend to wear and leak, resulting In undesirable maintenance requirements. A novel "electrothermal" pumping method avoids pistons and seals by ohmically heating solvent wlthln steel tublng to create pressure In a two-channel alternating arrangement. Intake occurs upon cooling. Electronic control uslng slgnals from a pressure transducer and a thermal pulse timeof-flight flowmeter results In stable flow rates. Flow rates are reported wIth a precision of 0.1% (a). The pumped fluid is Isolated from the heated fluld by use of dlaphragms. I n Its present form, the pump operates reliably for extended perlods and is capable of delivering flow rates up to 2 mL/min at pressures ranglng to 400 atm (6000 pslg). Electrical in nature, the approach is amenable to complex pressure and flow programmed appllcatlons.
Liquid chromatography is today's single most predominant method of analysis and its continued growth seems assured. Fundamental advances in LC are occurring principally in the areas of column packing materials and detection technology, while the most popular solvent delivery system by far continues to be based upon the reciprocating piston (1). There are problems inherent t o this mode of pumping: its output is pulsatile by nature and requires fairly elaborate mechanisms to smooth the flow (2); because pressure is generated mechanically, control based upon flow metering necessarily involves moving links in the feedback loop which are prone to wear and hysteresis that impair precision (3);and as the most mechanical component of an LC system the pump requires some maintenance, particularly checking sliding seals for incipient leaks (4). Piston pumps can also be surprisingly inaccurate in producing a flow rate that agrees with the flow setting (4). The alternative pumping method reported here uses a thermal cycling technique controlled by a precise flowmeter, permitting electronic actuation to replace the conventional reciprocating piston and sliding seal. Patents on the flowmeter have issued and a case has been filed on the subject of the pump on behalf of The Dow Chemical Co. (13).
EXPERIMENTAL SECTION Principle of Operation. When confined, heated liquids develop pressure according to expansion and vapor/liquid equilibria relationships as discussed below. A pair of stainless steel tubing coils, arranged with check valves as shown in Figure 1, act as both electrical conductors and conduits for solvent. Appropriate ohmic power dissipation within the tubing heats solvent, develops pressure, and produces flow as directed by the check valves. A small fan continuously cools both coils with ambient air. Thus when the applied electrical current is periodically removed, its radiating surface cools the tubing and
contents, forming a partial vacuum due to expelled solvent. This spontaneously draws replacement liquid through the inlet check valve. The two channels alternate so that solvent output is virtually constant, controlled by feedback based on signals from a pressure transducer and a precision flowmeter. The operating cycle period is about 60 s. The flow-through mode (Figure 1)was employed exclusively in early evaluationsbut has since been replaced with a diaphragm isolation alternative. In neither form of the pump is the effluent liquid temperature above ambient, due to the low flow rate and check valve mass. Some approximate calculations were used to match thermodynamic with electrical power requirements. Assuming the pumping of water, a relatively high boiling point solvent among LC mobile phases, a pressure of 3000 psig requires heating to no greater than 370 "C (see below). An estimate can be made by using the relationship A Q = cMAT
(1)
where AQ is heat input (calories),c is specific heat, M is the mass heated, and AT is temperature change. Each 2-m length of 1.6 mm (1/16in.) o.d., 1.0 mm (0.040 in.) i.d. stainless steel tubing with an internal volume of 1.6 mL employed in the pump has a specific heat of about 0.1 cal (g OC)-l and a mass of about 18 g. Thus, to heat the tubing adiabatically 350 "C above an ambient 20 OC requires approximately 630 cal, according to eq 1. Assuming 4.18 J cal-l, 600 J s-* (W) is sufficient power to achieve 3000 psig (AT = 350 "Cy within about 4 s. These electrical requirements are quite reasonable. A toroidal transformer with 120-V ac primary and dual 33-V ac secondaries powers both pump coils (Toroid Corp., Bowie, MD, part 160.332). The tubing electrical resistance exhibits a positive temperature coefficient, the 2-m length offering about 1.3 Q at 370 "C; thus 600 W calls for 25.4 A of current from the 33-V ac supply with a 72% duty cycle. During the pump operation described below and shown in Figure 5, current/voltage measurements showed the transformer to be delivering an average of 650 W. The rise in volume of the tubing due to a 350 "C temperature increase amounts to a negligible 0.8%, assuming a linear coefficient of thermal expansion for steel of 1.1 X 10-5/0C. Apparatus. The pumping system is shown in Figure 2. The timing cycle allows more than adequate time for coil cooling and refill along with preventing either coil from emptying more than 60% of its volume during the delivery phase. The system must be primed initially because no more than the coil is heated, and from Boyle's law, any residual air if compressed to 50% of its initial volume produces a maximum pressure of only Vi Vi Pf = Pi- = 15 psia= 30 psia Vf 0.5Vi
N
15 psig
(2)
This is avoided simply by adequate priming prior to starting the system. The check valves in the system can be less complex than the multiple ball/seat assemblies of conventional pumps because delivery rate is determined by pressure and flowmeter feedback control rather than piston displacement. A satisfactory choice has been the check valve shown in Figure 3, containing a gravity-assisted poppet equipped with an O-ring as a seal (SSI part 02-0129, SSI, Inc., State College, PA). Suitably low forward pressures are sufficient to open this valve so that each refill cycle is complete. Several months of continuous pump operation have
0003-2700/88/0360-1965$0 1.50/0 0 1988 American Chemical Society
1966
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988 Table I. Pump Flow Rate Data syringe time,
low
s
pressure
HPLC single piston
2.4 0.9549 1.0424 4.8 0.9761 1.0689 7.2 0.9769 1.1000 9.6 0.9781 1.0176 1.0304 12.0 0.9682 14.4 0.9438 1.0760 1.0463 16.8 0.9701 19.2 0.9684 1.1134 1.0257 21.6 0.9626 \ 24.0 0.9610 1.0437 1.0972 26.4 0.9238 28.8 0.9453 1.0467 31.2 0.9762 1.0882 33.6 0.9821 1.0503 1.0510 36.0 0.9227 38.4 0.9046 1.1065 1.0264 40.8 0.9949 43.2 1.0229 1.0518 1.0668 45.6 1.0197 48.0 1.0221 1.0691 1.1128 50.4 1.0080 Flgure 1. Electrothermal pump operation. 52.8 0.9855 1.0295 55.2 1.0307 1.0520 ELECTROTHERVAL PUMP 57.6 0.9887 1.1086 CO8l 1 1.0417 60.0 0.9761 _-___ 62.4 1.0131 1.0393 CONI 2 -30 V a i _____._____ ;; 9.1097 64.8 0.9842 67.2 0.9935 1.0335 CONTROL SYSTEM 0 OS I O M8n 1.0710 69.6 0.9784 t 7 . 72.0 0.9500 1.0610 I 74.4 1.0429 1.0569 I 76.8 1.0373 1.1202 79.2 1.0123 1.0350 81.6 1.0196 1.0565 84.0 1.0063 1.0823 1.0398 86.4 1.0222 88.8 1.0403 1.1271 d s 91.2 1.0203 1.0413 93.6 1.0390 1.0510 Flgure 2. Solvent delivery system overvlew showing diaphragm iso96.0 0.9975 1.1073 lation. 98.4 0.9915 1.0694 100.8 1.0262 1.1242 " 103.2 1.0074 1.0574 105.6 1.0152 1.0609 1.0376 108.0 1.0069 110.4 0.9723 1.1285 112.8 1.0461 1.0554 115.2 1.0078 1.1046 117.6 0.9967 1.0799 120.0 0.9787 1.0511 122.4 1.0169 1.1305 124.8 0.9949 1.0587 127.2 1.0144 1.0884 129.6 0.9642 1.0849 132.0 1.0178 1.0557 134.4 1.0082 1.1400 136.8 0.9735 1.0387 139.2 0.9914 1.0811 1.0877 141.6 0.9846 144.0 0.9578 1.0555 146.4 0.9686 1.1317 Figure 3. Check valve internal view. The gravityassisted poppet/O148.8 0.9766 1.0396 ring appears in the center of this sectional view. 151.2 0.9748 1.0752 153.6 0.9835 1.1043 revealed no observable adverse effecta on the performance of these 156.0 0.9584 1.0610 check valves. Wetted surfaces are stainless steel, and the Kalrez 158.4 0.9731 1.1320 O-ring is available in many other solvent-resistant elastomeric 1.1338 160.8 0.9460 forms. 1.0533 163.2 1.0024 Preferably, diaphragm assemblies isolate the pumped fluid from 165.6 1.0163 1.0520
I
On
Off
- 2
_--- _-
the heated working fluid, avoiding thermal degradation of the pumped fluid. These simple dual high pressure units contain 3.2 cm diameter planar sheets of elastic material such as Kalrez that mediate the pressure responses as they alternatively move into
3.04%" a
3.03%"
Relative standard deviation.
HPLC dual piston type A
HPLC dual piston type B
electrothermal
0.976 0.991 1.000 1.000 1.000 0.992 0.972 0.967 0.973 0.982 0.998 1.001 1.000 1.000 0.979 0.966 0.968 0.977 0.994 1.000 1.000 1.001 0.989 0.970 0.966 0.974 0.984 0.998 1.002 1.000 0.999 0.976 0.965 0.970 0.979 0.996 1.000 1.001 1.000 0.984 0.968 0.966 0.974 0.988 1.000 1.001 1.001 0.997 0.974 0.965 0.971 0.980 0.998 1.001 1.000 1.001 0.981 0.967 0.967 0.977 0.989 0.999 1.000 1.000 1.000 0.980 0.967 0.969 0.977
1.0181 1.0136 1.0188 1.0165 1.0214 1.0150 1.0130 1.0137 1.0256 1.0149 1.0173 1.0110 1.0104 1.0185 1.0197 1.0154 1.0203 1.0156 1.0245 1.0078 1.0125 1.0136 1.0143 1.0149 1.0208 1.0208 1.0202 1.0168 1.0160 1.0199 1.0174 1.0115 1.0233 1.0218 1.0124 1.0109 1.0115 1.0210 1.0116 1.0161 1.0168 1.0123 1.0153 1.0185 1.0176 1.0136 1.0082 1.0184 1.0221 1.0179 1.0181 1.0122 1.0131 1.0166 1.0190 1.0133 1.0083 1.0209 1.0242 1.0117 1.0187 1.0129 1.0157 1.0228 1.0124 1.0127 1.0212 1.0155 1.0135
1.0047 1.0109 1.0116 1.0081 1.0136 1.0093 1.0066 1.0093 1.0054 1.0067 1.0111 1.0110 1.0102 1.0042 1.0076 1.0094 1.0082 1.0066 1.0144 1.0109 1.0149 1.0130 1.0137 1.0123 1.0123 1.0124 1.0112 1.0122 1.0124 1.0161 1.0180 1.0104 1.0117 1.0098 1.0097 1.0111 1.0142 1.0146 1.0105 1.0121 1.0096 1.0187 1.0194 1.0191 1.0064 1.0154 1.0093 1.0122 1.0165 1.0096 1.0114 1.0233 1.0117 1.0075 1.0152 1.0125 1.0160 1.0153 1.0181 1.0123 1.0197 1.0135 1.0108 1.0101 1.0117 1.0102 1.0144 1.0138 1.0125
1.37%"
0.41%"
0.37%"
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
1967
WRK COIL'.C
I
+ I
~
FLUID IN
Flgure 4. Power MOSFET pump control (20 kHz).
hemispherical 1.5-mL volumes. The diaphragms are durable and do not observably impair pressure control characteristics. The pressure transducer shown in Figure 2 monitors solvent at the common outlet and elicits corrective responses from the control system to maintain steady pressure (Entran Model EPX15-10W-5000G, T = 11 ps, Entran Devices, Inc., Fairfield, NJ). As shown in Figure 2, flow rate information to the control system originates from a thermal pulse time-of-flight flowmeter described elsewhere (5-7) and obtained from Molytek, Inc., Pittsburgh, PA. Flow rate information, measured with a precision of 0.1% (u), not only assures regulated volumetric pumping but allows for an external data system to correct for any residual flow fluctuations. Control Systems. The purpose of the controller is to proportion electrical power to the pump coils in an alternating, timed fashion as shown in Figure 2 while maintaining desirable pressure and flow rate of solvent. Several analog implementations were evaluated, the simplest being a bang-bang controller (a comparator with hysteresis). This drives a pair of solid-state relays which in turn control the alternating current from the transformer secondaries through the pump coils. Because solid-state relays are actuated or deactuated at the zero crossings of the ac waveform, energy is available only in discrete packets and their multiples. This causes a control problem in that if the hysteresis band is tight enough to afford theoretically good pressure regulation, repeated pressure overshoot may occur due to this discrete energy effect. This is particularly troublesome at low pressure flow rate conditions where the energy required to drive the system is low. A secondary problem is that the power transformer must have a much higher rating in this service than it would appear to require from the average power consumed. This is due to core saturation effects. A second analog system uses a proportional-plus-integral control loop with phase fired triacs acting as the power-controlling elements. Phase firing alleviates many of the problems associated with the original system but still suffers from the fact that suf-
ficient power is not continuously available over the course of the sine wave to optimize dynamic pressure response. A third and preferred method of analog control is overviewed in Figure 4. Note that power MOSFETs replace the triacs or silicon-controlled rectifiers used in previously mentioned systems. The ac from the transformer secondary is rectified and filtered to form a dc power source. The filter capacitors in the power source store the energy required for optimum dynamic pressure control over the complete cycle of line frequency. Power to the coils is controlled by turning the power MOSFETs on and off at a %-kHz rate while varying the duty cycle (ratio of the on time to the total time of the period). This is known as pulse width modulation (PWM) and is noted for its high efficiency. Although a much lower switching rate would be more than sufficient for control purposes, acoustical noise generated at the switching rate dictates operation at ultrasonic frequencies. The amplified pressure signal is combined in a summing junction with set point and flow rate signals. This amplified combination is the error signal (Ve) and is summed with its integral and derivative in a second summing junction. The output of the summing amplifier is the control signal (Vc). The control signal is compared to a reference voltage (Vr); if the control voltage exceeds the reference for a period greater than the time it takes for the counter to clock through, the latch is set and the system is shut down. This protects the coils from over-temperature in case of a malfunction. The control voltage (Vc) is also applied to the power decrement circuitry to smooth the on to off transitions of the two coils. The two outputs of the power decrement circuit, Vcd and Vcd', are applied to the PWM comparators of each pump channel. The outputs of the PWM comparators are rectangular waveforms whose duty cycles are directly proportional to Vc as modified by the sequence logic. These waveforms are first buffered and then transformer coupled to the power MOSFETs. Heat generated in the coils raises their internal fluid pressure which is sensed by the pressure transducer thus closing the control loop. Pressure is the primary controlled variable in this system
1968
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988 Pressure (PSIA)
1
1
5000 I
4000
3000
1
/
Tetrahydrofuran1
1
'Methanol 12-
I
Elecra "e", 0-
51-
HFLC3.rlP#iCn TiueA
0 100
200
I
I
I
300
400
500
I
600 700
Temperature OC Figure 6. Vapor temperature/pressure behavior at equilibrium.
because of the transducer's speed of response. Since this system is meant to replace positive displacement pumps, flow rate control is of primary importance to its prospective users. The thermal pulse liquid flowmeter is thus used as a secondary control element that modifies the pressure set point as needed to maintain the desired flow rate in the manner described above.
RESULTS AND DISCUSSION Tetrahydrofuran (THF), methanol, methanol/water mixtures, and water are the solutions employed in most tests and evaluations to date. Thermal isolation using the diaphragms is necessary at certain pressures; for example at about 100 atm (265 "C) T H F begins to show evidence of degradation to propylene and formaldehyde, in the form of effluent bubbles that persist at ambient pressure. Water shows no such evidence of degradation in normal operation. When the isolating diaphragm assemblies are employed, water is a suitable working fluid. The absence of electrolytic decomposition of water is apparently due to the applied voltage developing an electrical potential gradient uniformly along the 2-m tubing length rather than at discrete interfaces; electrical current is conducted through the tubing rather than the solution. An electrothermal pump in the flow-through mode (without diaphragms) replaced a dual piston HPLC pump in a size exclusion chromatograph (8)to compare the elution behavior of a monodisperse polystyrene standard eluted at 1 mL/min with T H F a t 75 atm (1100 psig). The chromatographic behavior was seen to be equivalent (8),indicating that the eluting characteristics of the T H F mobile phase were preserved during the thermal pumping process. However, an attempt to carry out ion chromatography with the electrothermal pump using M orthosulfoindirect photometric detection (9) and a benzoate eluent at 1000 psig was frustrated by an extremely variable base line, suggesting that the eluent composition was altered within the pump. Diaphragm isolation (Figure 2) corrected the problem and has become a standard provision in the current design of the system. Experiments were conducted to compare the flow rate performance of the electrothermal pumping system (25 kHz power MOSFET control) to commercial HPLC pumps under identical conditions. Two common dual piston high-performance liquid chromatography (HPLC) systems, a singlepiston HPLC pump, and the electrothermal unit each delivered methanol at 950-1025 psig and 1.0 cm3/min into a 62-ft length of 0.007 in. i.d. stainless steel tubing without pulse dampeners. A 6000 psig range (Alltech 2D430) pressure gauge was located between the pump and the capillary. Flow rate measurements were performed with 0.170precision (7) using the thermal pulse time-of-flight flowmeter (5, 6). The comparative data are shown in Table I and plotted in Figure 5 . The electrothermal delivery system has operated for 2
months continuously without degradation in performance or signs of wear. The temperatures required to produce various pumping pressures depend upon the thermal expansion properties of the heated solvent. Assuming that the vapor phase obeys the ideal gas law, and neglecting the volume of a mole of the liquid solvent compared to a mole of its vapor, it has been shown that log P =
-A&por
2.303RT
(3)
+
where P is absolute vapor pressure, AHvapor is heat of vaporization, R is the ideal gas constant, T is absolute temperature, and C is an integration constant (IO). The relationship in eq 3 is borne out by vapor/liquid equilibrium data (11) up to the critical temperature, beyond which solvents are assumed to behave as dense ideal gases. The critical pressure is not an upper limit to pumping pressure since gas pressure continues to rise with temperatur9 above the critical point. These temperature vs pressure properties are plotted for water, T H F , and methanol in Figure 6. These relationships set an upper boundary on the thermal conditions required for pumping. I t has been shown that expansion of the liquid phase rather than vaporization is sufficient to account for the pumping process under the conditions reported here (12). A recent survey indicates that more than 90% of LC is operated below 200 atm (3000 psig) and half a t less than 100 atm ( 4 ) , so that in these applications the electrothermal pump would operate a t relatively low temperatures.
LITERATURE CITED (1) Barth, Howard G.; Barber, William E.; Lochmuller, Charles H.: Majors, Ronald E.; Regnier, Fred E. Anal. Chem. 1986, 5 8 , 21113-250R. (2) Snyder, L. R.; Kirkland. J. J. Inhoduction to Modern Liquid Chromatography, 2nd eU.; Why: New York, 1979; pp 92-103. (3) Schwartz, H. E.;Brownlee, R. G. J . Chromatogr. Sci. 1985, 23(9), 402-406. (4) Dolan, John W. LC Mag. 1985, 3(11), 956. (5) Miller, Theodore E., Jr.; Small, Hamish Anal. Chem. 1982, 5 4 , 907-910. (6) Chamberlin, Thomas A.; Tuinstra, Hendrick E. Anal. Chem. 1983, 5 5 , 428-432. (7) Davis, Charles M.; Steinbrunner, James E.; Miller, Theodore E., Jr. Am. Lab (Fairfield, Coon.) 1987, 19(7), 60-65. (8) Chamberlin, Thomas A., unpublished data, Central Research-Organic Specialtles Laboratory, The Dow Chemical Go., May 21, 1984. (9) Small, Hamish; Miller, Theodore E., Jr. Anal. Chem. 1982, 5 4 , 462-469. (10) Daniels, Farrington; Alberty, Robert A. Physical Chemistry, 3rd ed.; Wiley: New York, 1966; p 126. (11) Weast, Robert C. Handbook of Chemistry and Physics, 60th ed.; CRC Press: Boca Raton. FL, 1980; pp D-197, D-217. (12) Syverud, Alan, unpublished data, Thermal Laboratory, The Dow Chemical Co., Dec 1987. (13) United States Patents 4491 0 2 4 4532811: 4628743 case filed, Ser. no. 859 175.
RECEIVED for review March 27,1987. Resubmitted December 29, 1987. Accepted April 29, 1988.
