System Backpressure Research Articles

Energy consumption during transient operation of coal-fired generating units with cold end system back pressure regulation

The dynamic characteristics of key parameters were investigated in this study, which reflected the operational flexibility and efficiency of thermal power units during load regulation. A refined model of the cold end of the unit was proposed. Simultaneously, an innovative exploration was conducted into the coordinated achievement of flexible load regulation and the promotion of energy-efficient operations. Two variables, the variable load rate and cooling water flow, were selected, and the continuous dynamic characteristics of unit energy consumption during load adjustment were determined. The ratio of total variable load coal consumption to load regulation and variable load time was defined as the average regulated coal consumption BZ, which was introduced to enhance the intuitive evaluation of flexibility and efficiency. It was shown that with the increase in variable load rate, there was a decrease observed in the total coal consumption during the variable load process. This trend was consistent with earlier publications. Concurrently, a reduction in cooling water flow had resulted in an increase in power generation coal consumption and a decrease in coal consumption for circulating cooling water pumps. The elevated rate of load variation was attributed to the increased flow of cooling water. A decrease of 14.9 % in back pressure was noted with the increase in the flow of the cooling water. As a result of this reduction in back pressure, the coal consumption rate for power generation was decreased by 1.334 g/kWh, reflecting a decrease of 0.43 %. Additionally, a reduction in total coal consumption of 0.16 t during variable load operation was witnessed, while a 1.97 % increase in the variable load rate was recorded. The optimal flexibility and coal-saving effect were achieved under the conditions of 30 MW/min and 60000 t/h, with ΔBZ = −50 g/(MW·s). Finally, the feasibility of the proposed scheme was verified through the case of a 350 MW unit.

Dynamic modeling and experimental validation of a low frequency piezoelectric vibration energy harvester via secondary excitation of pressured fluid

A low frequency piezoelectric energy harvester using secondary excitation of pressured fluid is proposed in this paper to scavenge low frequency vibration energy in ambient environments. This harvester was characterized by an indirect vibration-to-electric energy converter via transferring the pressure energy of pressured liquid medium. A series of simulations and experiments were conducted to prove the good performance and the feasibility of this novel structure in low-frequency high-intensity vibration. The results showed that the diameter of damping orifice, initial system backpressure and proof mass brought strong influences on the resonance frequency and output voltage. Meanwhile, according to experiments, it was found that the resonance frequency was decreased with the decreasing liquid backpressure. In this case, lowest resonance frequency of 19 Hz was achieved under the condition of 10-kg proof mass and 0.2-MPa liquid backpressure. Also, there were the optimal proof mass and backpressure to maximize the generated voltage, respectively. Moreover, in the impedance matching experiments, the maximum output power reached 1.35mW at the matching load resistance of 18kΩ.

Separation and determination of clenbuterol enantiomers by ultra-performance convergence chromatography

建立了基于超高效合相色谱技术(UPC2)的克伦特罗对映体拆分方法,并对所建立的方法进行了方法学考察及应用。实验考察了两种克伦特罗对映体标准溶液的稳定性,并对手性色谱柱、助溶剂、系统背压、色谱柱温度等色谱分离条件进行了优化。采用Acquity Trefoil AMY1 (150 mm×3.0 mm, 2.5 μm)手性色谱柱进行分离,以超临界CO2-含0.5%(v/v) 10 mol/L醋酸铵的甲醇溶液为流动相,以流速2.0 mL/min梯度洗脱,检测波长为241 nm,进样体积为10 μL,系统背压为13.8 MPa,柱温为40 ℃时,两种克伦特罗对映体分离效果最好。两种克伦特罗对映体的线性范围为1.0~20.0 mg/L,相关系数均大于0.9997,仪器检出限(S/N=3)均为0.5 mg/L。10.0 mg/L混合标准工作溶液重复进样6次,(+)、(-)克伦特罗对映体峰面积的相对标准偏差(RSD, n=6)分别为0.65%和0.76%。应用该方法对市售克伦特罗外消旋体标准品进行拆分,采用外标定量法计算克伦特罗外消旋体标准中间溶液10.0 mg/L中两种克伦特罗对映体含量,其中(+)-克伦特罗的含量为5.6 mg/L, (-)-克伦特罗的含量为5.5 mg/L。该计算结果与文献报道的工业品克伦特罗外消旋体中(+)-克伦特罗与(-)-克伦特罗的比例为1.02:1.00基本相符。该方法具有分析速度快、分离效果好、有机溶剂消耗少等特点,适用于克伦特罗对映体的拆分,为其他手性化合物的拆分、药效精细分析和产品质量评定提供了可靠的技术支持。

Anti-slug control design: Combining first principle modeling with a data-driven approach to obtain an easy-to-fit model-based control

The limit cycle is an unexceptional problem in the oil industry that may cause significant losses in production. Also called slug flow or slugging, the unsteady flow can be handled by feedback control, although nonlinear issues must be considered. As an oil well production valve is opened, its transfer function gain tends to decrease until it reaches zero, meaning that the valve actions lose effect against the system backpressure. Notwithstanding, this sensitivity loss can be compensated by adapting a suitable tuning according to the well operating point. In this work, a methodology to generate this control policy is proposed based on combining first principle modeling with a data-driven approach. The method aims at improving closed-loop performance through a gain scheduling curve resulting from an easy-to-fit model to plant data. A systematic procedure is defined and validated through an actual deployment in a Petrobras ultra-deepwater oil rig. As a result, it was possible to suppress unsteady flow and increase oil production by more than 9%. Although the method has been validated in a satellite offshore well, one expects that feedback control can be used in different scenarios successfully, regardless of the slugging mechanism.

Development of Compression Spring Variable Valve for Exhaust Mufflers

<div class="section abstract"><div class="htmlview paragraph">The primary function of exhaust muffler is to reduce noise from the internal combustion engine without affecting its performance due to the impact of higher back pressure. The exhaust system back pressure is directly related to the engine fuel efficiency. The consumption of back pressure by the emission control system in BS IV regulation is about 30% from the total permissible engine limit, whereas in BS VI consumption is about 70%. The combination technologies used in BSVI and forthcoming RDE regulations such as TWC, GPF, DOC, DPF and SCR increases significant back pressure in exhaust system, hence the engine performance decreases. This demand robust method to control the exhaust back pressure for better fuel efficiency. Emission, noise and back pressure are the non-complimentary parameters in exhaust system development. The variable valve technology introduction in muffler is one method to optimize the above parameters. In general, mechanical variable valve in muffler containing torsional spring is used to improve the acoustics in lower rpm and to reduce exhaust backpressure in higher rpm. This paper deals with development of variable valve with compression spring as an alternative of torsional spring valve by using GT-POWER 1D tool. The conceptual design and working progress of new valve followed by comparative performance evaluation in acoustics and back pressure are dealt with in detail through simulation and validation. The new valve functioning with compression spring achieved 15% improvement in back pressure at higher rpm and 7% improvement in exhaust order noise at lower rpm.</div></div>

Modeliranje i evaluacija efekata visokog povratnog pritiska na sistemima baklji u rafineriji

Safety and reliability of the flare system are often affected by High Back-Pressure (HBP). The primary aim of this study is to simulate a steady-state model of flare system using Aspen Flare System Analyzer and with the aid of real plant data generated from Kaduna Refining and Petrochemical Company (KRPC) Ltd, Nigeria. Three credible scenarios (normal operation/surplus fuel, cooling water failure and power failure) were considered. The results showed that the steady-state model of flare system for normal operation (Surplus Fuel), cooling failure and power failure scenario and the flare system meet performance requirements at a system back pressure of 1.01325 bar, except for cooling water failure scenario which showed the occurrence of high fluid velocity and momentum (rhoV 2). Also, flare operation at normal back pressure, for all three scenarios does not exceed design and operational limits. The work recommends that KRPC managers should review options for reducing HBP particularly for cooling failure and power failure scenarios such that the back pressure would not exceed 10% of set pressure for the conventional valves.

Experimental study on the inlet behavior of CO2 foam three phase displacement processes in porous media

In relation with the potential applications of CO2 foam technology on Enhanced Oil Recovery and CO2 geological storage, the foam assisted CO2 displacement process in a surfactant solution and oil pre-saturated porous medium is experimentally investigated with special attention on the dynamic foam propagation behavior in the entrance region of porous media. Computational Tomography (CT) imaging is employed to visualize the transient CO2 foam sweep process and the dual-energy CT measurement is performed to obtain the dynamic three phase saturation distributions along the sample core. Studying parameters include the comparison with N2 foam, system backpressure and the porous medium permeability. It is found foam could push most of the liquid phase in the latter part of the porous media but leaves the forepart of the sample less flooded, showing a clear entrance effect of foam flooding process. Compared to CO2 foam, N2 foam displacement process shows higher liquid phase saturation in the entrance section and presents less oil recovery rate. Elevating system backpressure to 1.0 MPa leads to less pressure drop for CO2 foam flooding process without compromising the oil phase sweep efficiency. The CO2 foam flooding process in lower permeable medium shows higher pressure drop and higher oil recovery rate. The mechanisms for the inlet behavior of the foam displacement processes have been scrutinized based on good qualitative and quantitative agreement of the experimental and numerical results with the stochastic bubble population balance model.

Effective filter screening approach with fractionation for enabling inline filtration of protein A eluate.

Filtration of protein A eluates inline with a chromatography column is a common challenge for monoclonal antibody (mAb) purification due to the high system backpressure during elution, which can result in system shut down or require a decreased elution flow rate. The inability to filter inline not only poses a risk for process deviations, but can also lead to tank constraints and microbial ingress risk. Here, we evaluated and described a novel approach for identifying filters for inline filtration of protein A eluates at pilot scale. We fractionated the protein A eluates into 0.25 column volume fractions to screen filters under constant pressure or constant flow conditions. We observed that filtration properties for eluate fractions are significantly different from the offline eluate, and the conventional filter sizing study using elution pool is not able to predict inline filtration behavior. Through the submicron particle counts and size distribution in pre- and post-filtration samples, we determined that both attributes contribute to the high pressure across the filters. A successful proof-of-concept experiment on a column 10cm in diameter inline with the filter train selected validated this fractionation method, and the approach was applied to a different mAb molecule to confirm effectiveness.

Fingerprinting Cynara scolymus L. (Artichoke) by Means of a Green Statistically Developed HPLC-PAD Method

The process to develop a chromatographic method for fingerprinting complex matrices should be performed through a multiparameter approach that could lead to the desired separation and save environmental resources such as organic solvents and energy. In other words, this process should be pursued by employing an optimized experimental design and having a response function which takes into consideration separation parameters together with environmental parameters. Green Analytical Chemistry principles should be pursued during all steps of the research. This work presents a heuristic approach to develop a high-performance liquid chromatography method for fingerprinting an extract from leaves of Cynara scolymus L., a food plant consumed worldwide. A fractional factorial design was used to identify relevant chromatographic variables followed by a comprehensive design for optimization purposes (Doehlert design). A response function called green chromatographic fingerprinting response was employed to obtain a compromise between fingerprint quality and low environmental impact of the method. This optimized approach led to the development of a robust and green method for fingerprinting C. scolymus by HPLC-PAD. This method proved to be greener than the reference method reported in literature and compatible even with no state of art HPLC instruments because the system backpressure did not exceed 15 MPa and the column temperature was 35 °C.

Effects of pressure and frictional heating on protein separation using monolithic columns in reversed-phase chromatography

Pressure is not typically controlled or adjusted independently of flow rate during method development in reversed-phase LC (RPLC). However, it has been shown that pressure has an effect on analyte molecular molar volume, and the magnitude of this effect is greater for proteins and ionizable compounds than neutral small molecules. This phenomenon has received attention recently in the context of porous sub-2-micron particle packed columns. The present study surveys the effect of pressure and frictional heating on RPLC separations using commercially-available monolithic columns at constant flow rate and with controlled external temperature. Because the current monoliths cannot be operated at high pressures, all experiments were conducted with pressures at or below 200bar. Nonetheless, substantial changes in retention were still observed; for example, an increase in pressure of 75bar shifted the retention factor for bovine insulin from 1.27 to 1.78, a 40% increase, while a similar experiment with the neutral small molecule, toluene, showed no change in retention. Results are presented from investigations of model peptides and proteins ranging in size from 1kDa to 30kDa, as well as experiments performed with a silica-based C18 monolith and a polystyrene divinylbenzene monolith functionalized with a phenyl stationary phase. This work indicates that protein separations in monoliths are highly pressure sensitive, and pressure should therefore be considered as an additional parameter in method development for optimizing retention and selectivity. Given these findings, and the ever-increasing importance of chromatographic separations of proteins in both industrial and academic laboratories, improved instrumentation and mechanisms for directly controlling system backpressure could be of great practical value.

Fast Chromatography in The Regulated Bioanalytical Environment: Sub-2-µM Versus Fused-Core Particles

Increased throughput and rapid data turnaround have necessitated the need for faster LC separations in the regulated bioanalytical environment; sub-2-μm and fused-core chromatography are two viable approaches to accomplish this. Sub-2-μm columns offer high efficiency and resolution separations at the cost of high system backpressure and are best suited for preclinical sample analysis, whereas fused-core columns offer slightly lower efficiency and resolution, but are more tolerant to dirty extracts (such as those seen in clinical samples) that may clog the inlet filter. For clinical sample analyses, sub-2-μm columns should be restricted to sample extracts obtained via LLE or SLE, whereas fused-core columns can be used with sample preparations techniques ranging from protein precipitation to SLE.

In-Line-Water-Separation Prototype Development and Testing

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 137967, ’Inline Water Separation (IWS) Field Prototype Development and Testing,’ by J.J. Xiao, SPE, and Ramsey White, Saudi Aramco, and Shoubo Wang and Luis Gomez, Multiphase Systems Integration, originally prepared for the 2010 SPE Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, 1-4 November. The paper has not been peer reviewed. A compact multiphase in-line-water-separation (IWS) field prototype system has been developed and tested. The system consists of a gas/liquid cylindrical cyclone (GLCC), a liquid/liquid pipe separator (LLPS), a liquid/liquid cylindrical cyclone (LLCC), and two-stage liquid/liquid hydrocyclones (LLHCs). The function of the system is to separate a significant portion of the produced water from a production stream while the remaining production fluids are sent to existing processing facilities. Introduction As oil fields mature, more water is produced with the oil and gas production. This is especially true when water injection is used to maintain reservoir pressure and increase oil recovery. A common production-water-handling strategy is centralized processing. In this scheme, production from individual wells is gathered to manifolds through flowlines and is transported to the gas-/oil-separation plant (GOSP) through large-diameter trunklines. At the GOSP, separation and processing of oil, gas, and water take place with large conventional vessels. The processed water is either disposed of at nearby water-disposal wells or sent back to the field for water reinjection into the reservoir. With increasing water production, several issues can develop: Water volume exceeds water-handling capacity of the GOSP, which can cause processed water to be off specification. The off-specification water in turn can lead to a decline in water injectivity, which may require regular stimulation treatments. Upgrade or expansion of GOSP water-handling capacity with conventional bulky separation systems can be costly. Existing gathering systems can become a bottleneck to oil production. To maintain oil production, the total fluid produced will increase as the water cut increases, and the gathering-system capacity will become a limiting factor. Laying extra flowlines or trunklines can be expensive, especially offshore. Wells can cease to produce because of increasing system backpressure as a result of the increasing water volume being transported through the gathering systems. Higher water production leads to higher chemical use (demulsifers and corrosion inhibitors) and, hence, higher processing cost. In addition, energy consumption will increase to transport large quantities of water from the field to the GOSP and then from the GOSP back for reinjection into the wells in the field. A number of methods are used to reduce the quantity of water produced to the surface. These methods include restricted production, cyclic production, mechanical or chemical water shutoff, and short-radius horizontal drilling. For new wells, the use of production equalizers and smart completions also can be viewed broadly as a way to control water production. The next-best place to handle water production is in the wellbore. However, downhole oil/water separation and reinjection still remain a challenge even after 20 years of industry research and development.

High pressure electroosmotic pump based on a packed bed planar microchip

Abstract A high pressure electroosmotic flow (EOF) pump was fabricated within a glass substrate and tested to characterize its fluid dynamic performance. The EOF pump was constructed on a compact, planar microchip platform with (L × W × D) dimensions of 10 cm × 3.6 cm × 0.3 cm. The pumping region itself consisted of a straight channel (3 cm × 160 μm × 62 μm) uniformly packed with 3.38 μm silica microspheres held in place by a microfabricated weir structure. Characterization of the microchip EOF pump included pressure and flow rate measurements as a function of voltage and system backpressure for a buffered aqueous fluid containing 10 mM cyclohexylamino alkyl sulfonate CHES. Measurements indicate a maximum pressure of 25 atm and a maximum measured flow rate of 85 nL/min. Linear relationships of pressure and flow rate with voltage were confirmed. Current measurements show linear profiles with voltage for flow rate and maximum pressure measurements, indicating minimal effects due to Joule heating under either test condition. Design and performance considerations relating to fluid dynamic considerations are discussed. The large pressures generated electrokinetically by the microchip EOF pump show the potential for a compact motive source that allows for easy integration with applications requiring high pressures.

High-speed liquid chromatography by simultaneous optimization of temperature and eluent composition.

Clearly, a major trend in liquid chromatography is to increase its speed to make it faster. Improving throughput for routine analysis of pharmaceutical release samples and stability assays are all key motivations for improving separation speed. Recent work has focused on the theoretical benefits of temperature on speed in liquid chromatography. We have shown that the 5-10-fold decrease in eluent viscosity that comes from a temperature increase of 175 degrees C over ambient, and the concomitant increase in analyte diffusitivity, act to dramatically decrease the time needed to generate a theoretical plate. Lower viscosities at elevated temperatures decrease the pressure drop across the column and allow the use of higher linear velocities as the pump pressure limit is approached. Simultaneously, faster analyte diffusion at higher column temperature improves efficiency at high eluent velocity conditions compared to the efficiency at lower temperatures at the same velocity. We find that higher temperature plays a central role in improving speed. In this work, we show that when the percent organic modifier in the eluent and column temperature are adjusted to keep retention factors fixed, highly efficient, subminute separations can be routinely achieved when a hot column is used at the maximum system back pressure. We find that the best way to facilitate such an optimization, assuming constant selectivity, is to use a very retentive column so that one can work at both high temperature and high volume fraction of organic modifier to achieve the lowest possible eluent viscosity. We have also analyzed the effect that key extracolumn contributions have on column selection and system design.

Electrophoretic Deposition (EPD) Coatings of Sol-Gel Solutions and Suspensions

question, ∼300 m 2 /g, is made available by the mesopores. Because the total porosity of the monolithic silica matrix is higher than 80%, the user is able to perform his chromatography using a much lower back pressure than with conventional particles that have a total porosity of only app. 65%. By optimising the ratio of throughpores to total porosity and the silica gel skeleton thickness, separations become possible at higher flow rates. This is due to accelerated adsorption and desorption of the substances to be separated by the silica gel surface. Due to better mass transfer properties of a monolithic skeleton over distinct particles, highspeed separation is possible without noticeable impact on resolution. For example, five β-blockers were separated at high α values (α = k‘Substance2/k‘Substance1) and with excellent peak symmetry. At a flow rate of 1 ml/min, the 5 substances were separated within 5 minutes. Under these conditions, the total system backpressure was 13 bar. At a flow rate of 9 ml/min., the 5 β-blockers can be baseline separated within

Integrating biofiltration with SVE: A case study

An integrated soil vacuum extraction/ biofiltration system has been designed and installed at a gasoline‐contaminated leaking underground storage tank (LUST) site in southern Delaware. The system remediates contaminated moisture entrained in the air stream, employs automatic water level controls in the filters, and achieves maximum vapor extraction and VOC destruction efficiency with an optimum power input. In addition, the valving and piping layout allows the sequence of air flow through the filters to be reversed at given time intervals, which minimizes biofouling, thereby increasing efficiency by decreasing system backpressure. This integrated system achieves VOC destruction rates of up to 69%. The modular design allows for easy mobilization, setup, and demobilization at LUST sites throughout Delaware.

Vortex flow filtration of mammalian and insect cells.

The use of vortex flow filtration for harvesting cells or conditioned medium from large scale bioreactors has proven to be an efficient, low shear method of cell concentration and conditioned medium clarification. Several 8-10 L batches of the human histiocytic lymphoma U-937 cell line (ATCC CRL 1593) were concentrated to less than 1 L by vortex flow filtration through a 3.0 microns membrane. An aggressive filtration regimen caused a 17% loss of cell viability and a 32% loss of IL-4 receptor binding capacity when compared to a batch centrifuged control. A reduction of the rotor speed from 1500 to 500 RPM and reduction of system back pressure from 10 to 0 PSIG resulted in cell viability and IL-4 binding capacity comparable to the control. Several 10 L batches of baculovirus infected Sf-9 cells were also concentrated to less than 1 L by vortex flow filtration through a 3.0 microns membrane. SDS-PAGE analysis of filtrate samples showed that aggressive filtration caused cell damage which led to contamination of the process stream by cellular lysate. When rotor speed was reduced to 500 RPM and system back pressure was reduced to 0 PSIG, the amount of contaminating lysate proteins in filtrate samples was comparable to a batch centrifuged control.

Characterization of Water Soluble Polymers by Ion Chromatography. I. Poly(Acrylamide-co-Sodium Acrylate)

Abstract The sodium counterion contents of a series of poly(acrylamide-co-sodium acrylate) copolymers of varying composition have been investigated by Ion Chromatography (IC). Sodium ion contents obtained by IC are comparable with values based on either elemental analysis or titration techniques. Therefore, IC results can be used to determine the degree of polymer hydrolysis. Since sodium ion can be quantified at ppm levels, this analysis is possible with solutions containing =20 ppm polymer. At such concentrations, there is no evidence for column plugging or excessive system backpressure. Ionic contamination from glass storage vessels can be significant at these dilutions, however. The results of polymer dialysis and sonication studies are included. This study has explored the effects of key polymer and IC variables on the determination of Na+ ion content of aqueous HPAM solutions. Good agreement with the results based on elemental analysis or an ion-exchange/titration technique was established. A good correlation was also obtained between the sodium ion content of replicated samples of HPAMs versus the degree of hydrolysis of these polymers. Thus the feasibility of using IC for compositional analyses of these water soluble polymers has been demonstrated. Refinement of the method should lead to improved precision and several useful applications. Based on the inherent attributes of ion chromatography (e.g. small sample size requirements, high precision, quick analysis times, etc.) IC should have wide applicability in studies of ionic water soluble polymer systems. Potential applications include investigation of polymer composition, polymer concentration, polymer adsorption, and effects of processes such as dialysis.

40] High-performance liquid chromatography of porphyrin methyl esters

Publisher Summary This chapter discusses the high-performance liquid chromatography (HPLC) of porphyrin methyl esters. Separation of porphyrin esters by HPLC is performed on silica columns using mixtures of organic solvents. The porphyrins elute from the column with increasing polarity. Chromatographic systems are therefore designed and can be modified predictably for highly specific applications. Samples of dried porphyrin esters are dissolved completely in a known volume of CHCl 3 . A measured portion is injected onto the column and chromatographed. The volume of CHCl 3 used for dissolving the sample must be empirically determined and depends on porphyrin concentrations, the type of injector used, the column dimensions and resolution capabilities, and the type of detector used. The amount of porphyrin ester represented by a chromatographic peak is directly proportional to both the area under the peak and the peak height over a broad range, using either fluorescence or absorbance detection. Reliable separation and quantification of porphyrin esters require rigorous control of chromatographic conditions. For this reason, the following should always be observed: (1) a standard mixture of porphyrin esters should be chromatographed at least once a day to monitor instrument and column performance and to adjust chromatographic conditions if necessary, (2) all solvents should be HPLC grade or better. Solvent mixtures should be thoroughly mixed and then filtered and degassed by vacuum prior to use, (3) all samples should be rendered free of particulate material which could block the column by filtration through 0.45- μ m filters. Regenerated cellulose or PTFE filters are most effective. After filtration, the samples should be blown dry, then redissolved in a known volume of filtered CHCl 3 (stabilized with 1-hexene) prior to chromatography, and (4) system backpressure should be routinely monitored.

Chromatographic reproducibility in high-performance liquid chromatographic gradient elution

Normal bonded phase chromatography in a syringe pump gradient system satisfies a basic requirement for high-performance liquid chromatography of retention time reproducibility within half a peak width. Reversed bonded phase chromatography is characterized by a retention time variance in excess of this goal in certain cases. The source of the variance was traced to fluid compressibility effects and ambient thermal fluctuations. Fluid compressibility effects, due to the change in system backpressure with changing mobile phase viscosity, generated a retention time variation of the order of a peak width for solutes eluting in the 20–50% B portion of a 0–100% acetonitrile (B) in water (A) gradient. However, the retention of solutes eluting beyond 50% acetonitrile was not significantly affected by fluid compressibility. Thermal effects, due to ambient temperature variation, generated retention time variations of the order of a peak width for solutes eluting throughout the gradient program. A constant backpressure valve was developed which successfully eliminated fluid compressibility effects during gradient elution. Thermal effects were eliminated by use of a water-jacketed column and circulating constant-temperature bath. The dual syringe pump gradient system operating with these modifications reproduced retention times to better than 6 sec out of 1200 sec (σ rel values below 0.5%).