pH Control System Research Articles

Design of fault diagnosis filters and fault-tolerant control for a class of nonlinear systems

This paper presents a set of algorithms for fault diagnosis and fault tolerant control strategy for affine nonlinear systems subjected to an unknown time-varying fault vector. First, the design of fault diagnosis filter is performed using nonlinear observer techniques, where the system is decoupled through a nonlinear transformation and an observer is used to generate the required residual signal. By introducing an extra input to the observer, a direct estimation of the time-varying fault is obtained when the residual is controlled, by this extra input, to zero. The stability analysis of this observer is proved and some relevant sufficient conditions are obtained. Using the estimated fault vector, a fault tolerant controller is established which guarantees the stability of the closed loop system. The proposed algorithm is applied to a combined pH and consistency control system of a pilot paper machine, where simulations are performed to show the effectiveness of the proposed approach.

Performance Enhancement of SBR Applying Real-Time Control

This paper presents the applications of a real-time oxidation reduction potential (ORP) and pH control system to enhance the system performance of a continuous-inflow sequencing batch reactor (SBR). The real-time controlled SBR showed better performances than sequential control regarding substrate removal efficiency and cost reduction. In substrate removal efficiency, good COD, total Kjeldahl nitrogen, and NH+4 removals were achieved by real-time and conventional sequential controlled systems. However, the poor removal of total nitrogen caused by incomplete denitrification in the sequential controlled SBR was improved significantly to 91% by real-time control. Moreover, in contrast with sequential controlled SBR, the real-time controlled SBR can save 14–32% of aeration energy and 11–29% of reactor capacity. Observation of nitrogen compound variations showed nitrites accumulated during the aerobic phase of the real-time controlled SBR. In addition, the kinetics of nitrification and denitrification also pro...

Simultaneous Computer Control of pH in Multiple Environmental Laboratory Bioreactors

This paper describes the design, construction and operation of a computerized pH control system for environmental laboratory reactors. Since pH is often a key environmental parameter in environmental process reactors, it is important to have a dependable control system. The design criteria for this system were that it must accurately control pH in 16 different reactors with the additional flexibility of allowing each reactor to be controlled and graphed independently at any selected pH. Sampling intervals, mixing intervals, pump-on time, feed volume injection and tightness of pH control (e.g., ± 0.02 to 0.2) measurement were also possible for each reactor individually. The pH, pump-on time intervals and feed injection volumes were logged into a spreadsheet so that output graphs could be generated automatically for all 16 reactors for up to three years of operation. The supplies, model numbers and costs of main components will be described as well as the detailed instructions on how it was built. Examples are given of operating conditions and output.

Demonstration of pH control in a commercial immobilized glucose isomerase

The synthesis of a variety of important biochemicals involves multistep enzyme-catalyzed reactions. In many cases, the optimal operating pH is much different for the individual enzymatic steps of such synthesis reactions. Yet, it may be beneficial if such reaction steps are combined or paired, allowing them to occur simultaneously, in proximity to one another, and at their respective optimal pH. This can be achieved by separating the micro-environments of the two steps of a reaction pathway using a thin urease layer that catalyzes an ammonia-forming reaction. In this article, the pH control system in a commercial immobilized glucose (xylose) isomerase pellet, which has an optimal pH of 7.5, is demonstrated. This system allows the glucose isomerase to have near its optimal pH activity when immersed in a bulk solution of pH 4.6. A theoretical analysis is also given for the effective fraction of the immobilized glucose isomerase, which remains active when the bulk pH is at 4.6 in the presence of 20 mM urea versus when the bulk pH is at its optimal pH of 7.5. Both theoretical and experimental results show that this pH control system works well in this case. (c) 1996 John Wiley & Sons, Inc.

Inhibition of Apatite Formation by Vitronectin

This study investigated the concentration-dependent effect of vitronectin (VN), a glycoprotein of the bone matrix, on apatite formation and growth. Precipitation trials in metastable solution and in a pH-controlled solution system showed an inhibition of apatite microcrys-tal formation by VN. In the presence of biphasic calcium-phosphate ceramic, transmission electron microscopy showed a reduction of precipitated microcrystal size: precipitates were significantly smaller than in ionic simulated body fluid without proteins or in the presence of type I collagen as a negative control. Moreover, the size of the precipitated microcrystals was reduced in a dose-dependent manner. Two indirect methods showed that calcium-phosphate precipitation was inhibited by VN. It would appear that VN prevents apatite formation by inhibiting the growth of apatite crystals rather than by secondary nucleation, as in the case of osteopontin, a bone-specific protein.

In-line mixer for feedforward control and adaptive feedback control of pH processes

Components and concentrations of the influent stream are often varying in pH processes and changes of components and concentrations alter the nonlinear characteristics of pH processes severely. Hence adaptation coping with such influent stream changes is required for most pH control systems. As in linear adaptive control systems, bursting phenomenon of the estimator is a serious problem in the adaptive control of pH processes. One reliable method avoiding this problem is to excite the process to some extents intentionally. However, it may not be obvious how much the input should be perturbed. Too large perturbation can deteriorate control performances much. To avoid the bursting phenomenon of the estimator without deteriorating control performances, we introduce a small in-line static mixer which is easily implemented and whose output can be excited persistently for the reliable estimation of process parameters. Furthermore, it can be used for the feedforward action to compensate effectively the influent stream changes, which are the major disturbances. Improvements in control performances will justify the additional investment of the in-line static mixer.

PH-Control System Based on Artificial Neural Networks

A control system based on a combination of two artificial neural networks (ANN's) was designed for the pH-neutralization of acidic liquid streams. The first ANN is a plant neural model that predicts future pH values from past/present values of pH and valve stem position and future values of valve stem position. The second ANN is a plant inverse neural model that calculates the future valve stem positions from present/past values of pH and valve stem position and future values of set point. The performance of the controller was studied first by numerical simulation. The controller was further implemented in a continuous stirred tank reactor in which the neutralization of acetic and propionic acids with sodium hydroxide was performed. The controller robustness and adaptive performance were tested under different perturbations of flow, composition, and set point and several buffering changes.

Continuous Ethanol Production from Cheese Whey Fermentation by Candida Pseudotropicalis

Three pilot-scale continuous mix reactors of 5-L volume each were used to study the effects of retention time (18–42 hours) and initial substrate concentration (50–150 g/L) on the cell yield, lactose consumption, and maximum ethanol concentration during continuous fermentation of cheese whey using the yeast Candida psuedotropicalis. A microaeration rate of 480 mL/min and a nutrient supplement (yeast extract) concentration of 0·1% vol/vol were used. The results indicated that the dissolved oxygen concentration, temperature, cell concentration, lactose utilization rate, and ethanol concentration were affected by hydraulic retention time and initial substrate concentration. The highest cell concentration of 5·46 g/L and the highest ethanol concentration of 57·96 g/L (with a maximum ethanol yield of 99·6% from the theoretical yield) were achieved at the 42-hour hydraulic retention time and the 150 g/L initial substrate concentration, whereas the highest cell yield was observed at the 50 g/L initial substrate concentration and the 36-hour hydraulic retention time. Lactose utilizations of 98, 91, and 83% were obtained with 50, 100, and 150 g/L initial substrate concentrations at the 42-hour hydraulic retention time. A pH control system was found unnecessary.

A Gravity-Feed System for Controlling pH, in Flow-Through Aquatic Toxicity Experiments

A standard pH control system was modified to provide more reliable and precise control for acute and chronic aquatic toxicity tests using flow-through exposures. System components were developed which reduced both the lag time associated with standard control systems, and the loss of control function which occurs during power failure. These modifications produced a system which exhibited little deviation around the pH setpoint during acute experiments and may be amenable to longer-term studies in which the likelihood of power failure is high. Diagrams of system components are included, along with an example application of the system.

5466344 Method and apparatus for controlling water-based liquid waste

A pollution control apparatus (10) for controlling water-based liquid waste includes a vacuum distillation vessel (14) for separating water-based liquid waste into a distilled water component and a pumpable sludge component. A pH control system (12) allows the vacuum distillation vessel (14) to operate at maximum efficiency, and a purification system (22) allows the distilled water component to be purified to a level capable for reuse in chemistry, or else suitable for direct discharge into a municipal sewer. A microprocessor allows the apparatus (10) to operate automatically.

Mole fraction control of poly(3-hydroxybutyric-co-3-hydroxyvaleric) acid in fed-batch culture of Alcaligenes eutrophus

The aims of this study were to investigate the reaction kinetics of production of poly(3-hydroxybutyric-co-3-hydroxyvaleric) acid, P(HB-co-HV), to control its mole fraction in a fermentor, and to develop an optimal copolymer production system using fed-batch cultures of Alcaligenes eutrophus ATCC 17699. In this study, butyric and valeric acids were utilized as carbon sources. In order to keep the concentrations of these fatty acids constant, the well-organized fed-batch experiments were performed using a 5 l jar fermentor equipped with a temperature and pH control system. Mole fraction and sequence distribution of monomeric units in a copolymer, 3-hydroxybutyric acid (HB) and 3-hydroxyvaleric acid (HV), were determined by 1H NMR and 13C NMR spectroscopy, respectively. The specific production rate of P(HB-co-HV) was lower than that of PHB when the total fatty acid concentration in the medium was 3.0 g/ l, and it was constant and independent of the mole fraction of valeric acid at total fatty acid concentrations less than 0.3 g/ l. The mole fraction of HV units in copolymers (0–40%) was linearly related to the mole fraction of valeric acid in the medium. Thus, the mole fraction of HV units in copolymer can be controlled within the range of 0–40 mol% by adjusting the mole fraction of butyric acid in the feeding medium. Moreover, for P(HB-co-HV) with a given mole fraction of HB and HV units, the most economical feeding strategy in terms of achieving the maximum reaction rate involved adjusting the total fatty acid concentration. From the sequence distribution of diads and triads of HB and HV units, P(HB-co-HV) with a 0–40% mole fraction of HV was concluded to be a random copolymer. The weight average molecular weight of P(HB-co-HV) measured by the GPC method changed from 0.77 × 10 6 to 3.4 × 10 6. This range is of the same order of magnitude as the homopolymer, PHB.

15N-Allocation of15NH4-N and15NO3-N to Nitrogenous Compounds at the Vegetative Growth Stage of Potato Plants

Abstract Effect of NH4 + and NO3 −on 15N-aIIocation to organs and nitrogenous compounds in potato plants (Solanum tuberosum L.) was examined in hydroponic culture using an automatic pH control system, and the results obtained were as follows. 1. Growth of upper leaves was stimulated by NH4 + because 15NH4-N was preferentially distributed into upper leaves compared to 15NO3-N which was mainly distributed to lower leaves. 2. At the vegetative growth stage, 15NH4-N and 15NO3-N distribution among organs was almost similar except for roots, whereas at the tuber swelling stage, 15NH4-N was distributed to stems and roots, and 15NO3-N was distributed to leaves. 3. 15N amino acid metabolism was affected by NH4 + or NO3 −, for example, in leaves grown in 15NH4-N, the contents of aspartic acid and asparagine (Asx) increased with growth, while those of glutamic acid and glutamine (Glx) decreased with growth. In leaves grown in 15NO3-N, the Asx content remained constant, but the Glx content increased with growth. In t...

Robust nonlinear control with neural networks

A new method for robust nonlinear control of single-input single-output systems is presented. The control law utilizes the universal approximation characteristic of neural networks augmented with the ability for adaptation. The presence of neural networks obviates the need for a mechanistic model for control law computations and the difficulties associated with model-based approaches become irrelevant. The new control law called N-RNCL incorporates the ability for adaptation through an adjustment of bias neurons and ensures offset-free performance in the presence of load and unmeasured disturbances. The performance of N-RNCL is demonstrated using the examples of a strong-acid strong-base pH control system and a nonlinear heat exchanger system. The state-of-the-art controller shows excellent servo and regulatory performance.

Influence of pH on the toxicity of ammonia to Chironomus tentans and Lumbriculus variegatus

AbstractThe U.S. Environmental Protection Agency water‐quality criteria establish permissible levels of ammonia in the nation's fresh waters. These criteria are based on accumulated research suggesting that, for most aquatic species (primarily fishes), the toxicity of un‐ionized ammonia predominates over that of the ammonium ion. The development of a sediment‐quality criterion for ammonia requires evaluation of the relative toxicity of the two ammonia forms to benthic and epibenthic macroinvertebrates to determine whether the water‐quality toxicity model can be applied to sediments. Flow‐through ammonia toxicity tests were conducted over 10 d with the oligochaete Lumbriculus variegatus and the larval midge Chironomus tentans at four pH values (6.3, 7.2, 7.8, and 8.6) using a unique pH control system. Total ammonia was more toxic at elevated than at low pH to both species, suggesting that un‐ionized ammonia (more prevalent at high pH) is important in determining the toxicity of ammonia to these two species. Hardness or alkalinity differences in the range of 30 to 200 mg/kg (as CaC03) did not appear to affect the toxicity of ammonia to the two species in separate 4‐d tests. Based on results of the 10‐d tests, the joint toxicity/pH model that establishes the water‐quality criterion value for ammonia appears to be sufficiently protective of L. variegatus and C. tentans, by factors of at least 3 and 10, respectively. However, this study did not address potential differences in exposure of benthic organisms to ammonia in sediments vs. that in the water column; nor were the chronic effects of ammonia on these species measured in this study.

Upgrading a low-cost physicochemical wastewater treatment plant to solve operational problems

A comprehensive programme of jar tests utilizing lime to treat municipal wastewater in northeast Brazil showed that a supernatant could be achieved that attained the WHO (1989) guidelines for effluent reuse for unrestricted irrigation (≤ 1000 faecal coliforms per 100 ml; ≤ 1 intestinal nematode egg per litre). In the light of these findings a pilot-plant was built to investigate the continuous treatment of wastewater with lime for effluent reuse in irrigation. Bearing in mind the problems often encountered in less-developed countries with the operation and maintenance of complex wastewater treatment systems, it was decided to base the lime treatment technology on that used successfully throughout the world (in both industrialised and non-industrialised countries) to treat potable water; namely non-mechanised coagulation and flocculation, and settlement in a horizontal-flow sedimentation tank. Although the prototype pilot-plant achieved a final effluent that attained the WHO (1989) guidelines for unrestricted irrigation, a number of operational and maintenance problems were encountered. Effluent quality data has been reported previously. This paper summarises the data and describes the practical problems associated with the operation and maintenance of the lime treatment plant, and the upgrading techniques used to overcome these difficulties. The prototype plant consisted of a rapid-mix basin in which raw municipal wastewater was combined with lime slurry at a ratio that achieved a pH of greater than 11.0 after mixing; a sedimentation tank, for solid-liquid separation; and a recarbonation basin for tertiary treatment. Constant monitoring of the lime-wastewater mix pH, using a pH controller to regulate the flow of lime slurry, proved difficult. Frequent cleaning of pipework was needed to avoid clogging by the lime slurry. In a second phase of the research work, the pilot-plant was upgraded to tackle these and other operational and maintenance problems. Plant geometry was improved to prevent the sedimentation of lime solids within the delivery pipework. All pipework carrying lime slurry was set up in duplicate so that plant operation could continue during cleaning operations. An improved pH control system, which linked lime slurry flow to the pH of the lime-wastewater mix, was also installed. The design of the sedimentation tank was altered to aid the removal of sludge and the tank inlet and outlet were upgraded in order to minimise short-circuiting. Plant operation and maintenance were simplified by the design improvements and the frequency of mechanical breakdown was reduced. The pH control system was difficult to operate effectively because the pH probe became rapidly fouled by lime solids. However, gel-filled, low sodium-error pH electrodes calibrated in pH 11.0 buffer proved significantly more robust in the aggressive high-lime environment than did conventional combined glass electrodes. Options for further improving the design of a lime treatment plant are considered.

Relay feedback method for tuning of nonlinear pH control systems

For easier implementation of nonlinear pH controllers, we propose an on-line tuning method using the relay feedback with hysteresis. Since just two or three parameters are sufficient to characterize pH processes for practical applications (Williams et al., 1990; Lee et al., 1992), we parameterize pH processes with three parameters and then identify them with the relay feedback testing. The relay feedback method is very popular in on-line tuning of PID controllers (Astrom and Hagglund, 1984; Seborg et al., 1989). This on-line tuning method would reduce the efforts to obtain titration curves with off-line experiments

A computer-based ph measurement and control system for fermentation processes

The measurement and control of pH is essential in most fermentation processes. The highly nonlinear response of pH to the addition of acid or base makes pH control by conventional means very difficult. This paper presents a description of the hardware and software of a computer-based pH measurement and control system. The system is applicable to any fermentation process requiringin situ pH measurement and control and is capable of controlling the pH within ±0.1 U with a very short response time. It has a proportional metering device capable of adding the required amount of acid (or alkaline) solution with high accuracy, thereby preventing overshooting.

Start-up in anaerobic treatment of natural-rubber effluent

Laboratory experiments were carried out to compare the controlled and uncontrolled start-up of the anaerobic treatment of natural-rubber effluent. It was found that a pH-controlled start-up was much more promising than an uncontrolled one. The biogas production was higher and a shorter period for stabilization was required in pH-controlled systems.

The pH-static enzyme sensor: Design of the pH control system

The pH-static enzyme sensor offers a solution to the buffer dependency of ISFET-based enzyme sensors. A continuous coulometric titration of the reaction products keeps the pH in the enzymatic membrane at a constant level. This paper presents an automatic system to control the compensating current that is a direct measure for the substrate concentration.

Measurement of enzyme reaction rates using advanced pH control system: Application of repetitive PF system

The aim of this article is to develop the on-line measurement system of enzyme reaction rates, using an advanced pH controller, which is called a repetitive PF(Programmed Controller/Feedback Compensator) System. In the esterolysis reaction of the N-acetyltyrosine ethyl ester by alpha-chymotrypsin, the enzyme reaction rate could be calculated from the amount of base required for keeping the pH constant. The proposed controller has a learning mechanism in which the knowledge is obtained from the former results of the repetition, and pH was controlled much more successfully by the proposed repetitive PF System than by the conventional on-off controller, PI controller, and the adaptive controller. The enzyme reaction rate could be evaluated each time as accurately as possible based on the result of the controlled batch reaction. Using Lineweaver-Burk plots of the estimated reaction rates, kinetic parameters, such as the enzyme activity and Michaelis-Menten constant, could be estimated more accurately and from fewer experiments by the repetitive PF system than by initial reaction rates using on-off pH-stat.