Feedback Control of Electrification in Powder Pneumatic Conveying Process

In a powder pneumatic conveying process, powders are remarkably charged due to the collisions and frictions between themselves or against the conveying pipe, which sometimes causes explosion and fire. In order to prevent these troubles and hazards, development of a reliable system for monitoring and control of electrification (electrostatic charge) is strongly required. In this study, an electrostatic charge control system composed of corona discharge neutralizer, electrostatic filed strength sensor and computer control system was newly developed and applied to the powder pneumatic conveying process. Dynamic characteristics of electrostatic charge and its elimination by the corona discharge neutralizer were analyzed. Based on the characteristics, a simplif ied transfer function composed of first order lag element including dead time was proposed and optimal control parameters for the digital PID control was determined. Performance of the control system was also investigated experimentally under various control parameters. It was found that the electrostatic charge during powder pneumatic conveying process was favorably self-controlled by means of the newly developed control system.


Introduction
Recently, electrostatic disasters and troubles have become a serious problem in powder handling processes because they sometimes cause explosion and fire [1][2][3].For example, in the process of powder pneumatic transportation, powders are tremendously charged due to their collisions between themselves and against transportation pipe.In the case such charged powders are fed into a silo directly, a lightning discharge (atmospheric discharge) in the upper space of the silo or cone discharge along with the accumulated powder surface are frequently observed, which sometimes induce explosion and fire [4][5].
In addition to the progress in the powder handling technologies, size of powder becomes much smaller in order to increase the functionality.However, the minimum ignition energy also decreases awfully with a decrease in powder size.Among such fine powders, some of them have ignition energy smaller than 1mJ, which is as small as the flammable gas.The surface area also increases with a decrease in powder size, leading to increase the electrification.In order to prevent these hazards beforehand, development of continuous motoring and practical control system of powder electrification is strongly required.So far, we have developed a novel electrostatic detecting system based on the measurement of electrostatic field strength, and tried to apply to fluidized bed drying and pneumatic conveying processes to continuously monitor the powder electrification [6][7].We also have developed a corona-discharge type electrostatic neutralizer to remove electrostatic charge of powders during pneumatic conveying of powders into a silo [8].However, it is impossible to remove electrostatic charge completely and sometimes electrifica-tion to the opposite pole due to the too much ion supply has been observed.In order to remove electrostatic charge completely without having the opposite electrification, accurate measurement and control of electrostatic charge is required.
In this study, we have tried to develop an electrostatic removal system composed of electrostatic field strength sensor, corona-discharge type neutralizer and computer control system.We also have applied this system to powder pneumatic conveying system and try to analyze the dynamic characteristics of electrification and its neutralization process.Base on the analysis, an optimum feedback control system of electrification has been developed and performance of the system was also confirmed.

Equipment
Figure 1 describes a schematic diagram of a pneumatic conveying system.The system consists of a transportation pipe (i.d.28mm҂2m) made of stainless seamless pipe (SUS304), an electrostatic filed strength sensor, a corona-discharge type neutralizer, and a Faraday cage.This Faraday cage is modified to have a stainless mesh at the bottom of the inner cylinder and a bag filter inside it, so that the only air passes through them and powders remain inside the bag filter.In the conveying experiment, powders are fed into the conveying pipe through a feeder and conveyed by air supplied from an induced blower.Powders are mainly charged due to their collisions with conveying pipe and are continuously measured its electrostatic field strength by an developed electrostatic field strength sensor and finally measured its total charge by a Faraday cage.The air used for conveying powders were heated up to 333K to maintain its relative humidity around 45% R.H.
Figure 2 illustrates measurement principle of a developed electrostatic field strength sensor [6,7].The sensor is installed inside a stainless cylinder (i.d.8mm) and measures electrostatic field strength as an alternating voltage, which is induced at an electrode by periodically chopping the electrostatic field.
Assuming that the chopping cycle is ω (҃500Hz), area of the electrode where electrostatic field f lows in is S 0 , and the one which changes periodically due to the electrode vibrating is S 1 , then the effective area of the electrode S can easily be written as: The Gauss's law calculates an electrical charge q, which is induced by the electrostatic field periodical change, where E shows an electrostatic field.A current I s running through an electric resistance R s which connects the electrode and ground is calculated as, The voltage of the electric resistance V s is thus: Finally, the electrostatic field strength E can be detected by measuring the voltage V s of the electric resistance R s .
Figure 3 describes a schematic diagram of a developed corona-discharge type neutralizer.The neutralizer composes of a needle electrode and nozzle electrode (grounded metal cover).Between the both electrodes, corona-discharge is occurred, leading to ionize the air supplied from a compressor.The ionized compressed air is then sprayed to charged powders to neutralize their charge.In this study, DC type is applied in order to increase the neutralize efficiency as much as possible.As shown in Fig. 1, the nozzle is installed perpendicular to the pipe and horizontal level of the nozzle extremity and the inside pipe surface is set at the same.For the needle material, Ni-Cr composite is used to prevent corrosion and erosion.

Powder sample
For powder sample, spherical PMMA particles, which had been sieved to have size range between 100 and 200µm were used.Before the experiment, the PMMA particles were sufficiently dried in a shelf drier and initial charge was removed by ground.The particle feed rate was 1kg/min and airf low velocity was set at 33m/s.

Performance of the neutralizer
Figure 4 investigates the relationship between supply current to the neutralizer and ion current generated by the neutralizer under various supplied air velocities.In this experiment, the ionized air was sprayed into the Faraday cage directly to measure the electric charge.Seen from the figure, the ion current increased with an increase in the supply current, and the larger ion current was observed when the supplied air velocity was larger for both negative and positive charge.This implies the possibility of practical control of neutralizing performance by means of the supply current.

Dynamic characteristics of the neutralizing process
In order to control the electrostatic charge in powder pneumatic conveying process, its dynamic characteristics should be intensively understood beforehand.Thus the step response of the neutralizing process was investigated by using developed equipment shown in Fig. 5.The equipment composes of two metal plates (0.2҂0.2m) located at the opposite side having distance of 0.01m, electrostatic filed strength sensor and corona-discharge type neutralizer.Stepwise input difference was generated by a personal computer then its output response was investigated.This equipment imitates the process in Figures 6 and 7 illustrate the method for determination of step input response and result of output response against the unit step input, respectively.Seen from the figure, the output response converged to a constant value at t→ȍ.From this phenomena, the dynamic characteristics of this process was assumed to be described by a simplified transfer function G(s) composed of first order lag element including dead time as Eq.( 5).

G(s)҃
(5 where K, L and T mean gain, dead time and time constant, respectively.By curve fitting, each parameters in Eq.( 5) can be determined as K҃1, L҃0.34s and T҃0.95s.The inverse Laplace Transform gives the numerical output response against the unit step input, which is also described in the Fig. 7. Since adequate correlation was estimated between the both output responses, it is safe to assume that the dynamic characteristics of the neutralizing process is described by the Eq.( 5).In the following, a feedback control of the powder charge is attempted by using the transfer function G(s).Assuming that the difference between the desired and output values is e(t), the manipulated output value of the PID controller, u(t), is described as Eq.( 7).u(t)҃K P Ά e(t)ѿ ͐ t 0 e(t)dtѿT D · (7) In the case that the transfer function is described by the order lag element including the dead time, each parameters of PID controller can de determined by a method proposed by Ziegler and Nichols ( Table 1) [10].
Figure 8 shows the results of control by three different feedback control methods, P, PI and PID.Here, the desired value of each control was set at 0V/m and the sampling interval was 20Hz (0.05s).
From Fig. 8, it was confirmed that the powder charge was awfully decreased in each control  method.However, in the P control, the powder charge did not converge to the desired value (0V/m) and offset was still remained.By contrast, PI control could remove the offset by introducing the integral (I) element.In addition, PID control could increase the response speed by applying the derivation (D) element.
Figure 9 indicates the total charge of particle measured by the Faraday cage located at the downstream of the conveying pipe.By using the feedback control, the total charge could be greatly reduced.It was also found that the total charge of particles could be reduced almost 0 by using the optimized PID control.

Conclusion
electrostatic charge control system composed of corona discharge neutralizer, electrostatic filed strength sensor and computer control system was newly developed and applied to the powder pneumatic conveying process.Dynamic characteristics of electrostatic charge and its elimination process through the corona discharge neutralizer were analyzed.Based on the characteristics, a simplified transfer function composed of first order lag element including dead time was proposed and the optimal control parameters for digital PID control was determined.Performance of the control was also investigated experimentally under various control parameters (P, PI and PID controls).It was found that the electrostatic charge during the powder pneumatic conveying process was favorably self-controlled and powder charge was awfully reduced by each control.The optimum PID control was also found to reduce the powder charge almost completely without having any offset.This control method can be used to any kind of powder handling processes and is expected to be very useful and reliable device to prevent troubles originated by the electrostatic charge.

Fig. 1
Fig. 1 Schematic diagram of experimental apparatus used

Fig. 4 Fig. 5 Fig. 6
Fig. 4 Relationship between supply current and ion current generated by neutralizer under various supplied air velocities

Fig. 8 Fig. 7
Fig. 8 Results of control and behaviors of supply current for neutralizer under various control methods

Fig. 9
Fig. 9 Total charge of particle measured by Faraday cage

Table 1
Optimal parameters for PID Control