Electrostatic Purification of Gas from Dust and Tar

S. I. Smol’yaninov, K. K. Stramkovskaya, A. P. Smirnov,
N. F. Olitskii, S. A. Kvashnin

(Presented by the scientific seminar of the Section of Organic Chemistry)

The most modern method for fine purification of gas from tar and dust should be considered the electrostatic method. The most important advantages of electrostatic gas purification, in comparison with other methods, are: simplicity of design, ease of maintenance, low consumption of electric power, the possibility of complete automation of the process, a high degree of gas purification (90–99%), and the lowest purification cost among all known methods [1, 2, 3, 4]. However, this method is still insufficiently used in the practice of purifying small quantities of gas, perhaps because of the absence of simple designs of electrofilters.

In the local supply of water gas to electric-lamp plants that do not have installations for fine purification of gas from tar and dust, the presence of the latter causes certain difficulties in the operation of automatic lines.

When water gas is burned in the burners of automatic machines (with an orifice of 0.8–1.8 mm), coking of the burners occurs. Thus, for example, the burners of the automatic machines of the assembly line of one electric-lamp plant have to be cleaned twice per shift. In this case the automatic machine stands idle for from 0.5 to 1.5 hours. And bulb-shaped automatic machines of the glass shop stand idle for up to 0.5 hour per day.

Purification of water gas from tar is becoming a most important necessity, and first of all for automatic machines most sensitive to clogging.

The purpose of the present work was:

1) determination of the parameters for design, and
2) design of an installation for electrical purification of water gas for servicing the burner automatic machines of assembly lines.

As the necessary parameters it was required to determine:

1) gas flow rate, its composition, and the content of dust and tar in it;
2) determine certain characteristics of the tar;
3) determine the optimal parameters of electrical purification of water gas from tar and dust.

The total gas consumption for all automatic machines of the assembly line, reduced to normal conditions, was 39 nm³/hour.

The results of analysis of the composition of water gas, carried out on the VTI-2 instrument, are given in Table 1.

The gas contains: tar in an amount of 200 g/nm³ (determined by the Feld method); moisture—10.4 g/nm³ (determined by the absolute-weight method); hydrogen sulfide—2.5 g/nm³ (determined by the precipitation method) [5].

Table 1

Characteristics of the tar

Certain analyses of the tar, necessary for design, were carried out. The analysis data are given in Table 2.

In order to determine other parameters for design, experiments were conducted on a laboratory model of an electrofilter, consisting of a glass tube 20 mm in diameter and 0.5 m long.

Table 2

The corona electrode was a nichrome wire with a diameter [[unclear: omitted in image]]; the collecting electrode was a glass tube with a silvered inner surface.

From the data obtained, it was possible to outline the optimum operating regime of the electrofilter. Thus, at a voltage on the corona electrode of 4000 V and a gas velocity in the electrofilter of 0.83 m/sec, a purification degree of 99% is achieved. When the gas velocity is increased, the degree of purification decreases, reaching 50% at a velocity of 2.05 m/sec. By increasing the voltage on the corona electrode to 5200 V, it is possible to obtain a higher degree of purification, equal to 97%, at a considerable gas velocity of 1.3 m/sec.

On the basis of the optimum parameters found, we designed an electrofilter with a capacity of 50 nm³/hour.

Design of the electrofilter (Fig. 1)

The electrofilter consists of three main parts: a pan, a cover, and tubes, inside which corona electrodes made of nichrome wire with a diameter of 0.6 mm are positioned strictly along the center. With the aid of hooks, washers, and springs, the corona electrodes are fastened to corona frames, which are insulated from the upper and lower tube grids by porcelain insulators.

Fig. 1. Electrostatic precipitator: 1 — upper cover, 2 — feed-through insulator, 3 — upper corona frame, 4 — insulator, 5 — gas outlet fitting, 6 — steam inlet fitting, 7 — collecting tube, 8 — lower corona frame, 9 — corona electrode, 10 — tray, 11 — gas inlet fitting, 12 — resin and condensate drain fitting.

The gas enters the electrostatic precipitator from below, passes through the tubes, exits, and then goes to the automatic units after passing through the droplet separator. When high-voltage direct current is supplied to the corona electrodes, gas ionization and charging of the particles contained in the gas occur between the corona electrodes and the tubes (collecting electrodes); under the action of the electric field these particles move toward the electrodes, where they give up their charge and are deposited. The tar then flows down from the electrodes and is periodically removed.

For supplying the electrostatic precipitator, a half-wave current-rectification circuit was used, as being more effective and economically advantageous (Fig. 2).

Fig. 2. Electrical circuit of the electrostatic precipitator: PR — fuse, BD — block contacts, K — contacts, RT — current relay, RN — voltage relay, RP — intermediate relay, ZV — bell, LK — red indicator lamp, LO — colorless lamp, LZ — green lamp, RZ — discharger, TN — filament transformer, TR — heating element.

Current–voltage characteristics

Before startup and during testing of the electrostatic precipitator, current–voltage characteristics were taken in nitrogen, in water gas, and a current–voltage characteristic in water gas at different diameters of the corona electrodes. The data are given in Table 3.

As can be seen, the volt-ampere characteristics of the electrostatic precipitator differ for different gases. The dependence of the volt-ampere characteristic on the gas composition is explained by the different mobilities of ions for gases. Thus, for example, for air saturated with water vapor, at

Table 3

26°C, with ion mobility equal to \(K = 1.58\ \mathrm{cm^2/sec}\); for nitrogen, \(K = 1.84\ \mathrm{cm^2/sec}\); for hydrogen, \(K = 8.13\ \mathrm{cm^2/sec}\). This is why, at the same values of the potential difference on the electrodes of the electrostatic precipitator, the current strength in the electrostatic precipitator differs from one to another depending on the composition of the gas. Even in the case of one and the same gas composition, the current value is also affected by the diameter of the corona electrode. The current strength in the electrostatic precipitator is determined by the following formula:

\[ I=\frac{2K V_{\mathrm{p}}-V_{\mathrm{kp}}\cdot V_{\mathrm{p}}} {R_1^2 \ln \frac{R_2}{R_1}}, \]

\(V_{\mathrm{p}}\) and \(V_{\mathrm{kp}}\) are the operating and critical corona voltage in the electrostatic precipitator, in kV;

\(R_1\) and \(R_2\) are the radii of the collecting and corona electrodes, in cm;

\(K\) is the ion mobility, in \(\mathrm{cm^2/sec}\).

Thus, for example, when the product \((V_{\mathrm{p}} - V_{\mathrm{kp}})\cdot V_{\mathrm{p}}\) for one diameter of the corona electrode is equal to the product \((V_{\mathrm{p}} - V_{\mathrm{kp}})\cdot V_{\mathrm{p}}\) for another diameter of the corona electrode, at one and the same mobility

gas ions the current strength will be determined only by the ratio \(\dfrac{R_2}{R_1}\).

Undoubtedly, the current strength will be higher in the case where the diameter of the corona electrode is larger.

The use of an electrostatic precipitator gives the following effect.

1. The need for gas workers to service the automatic machines of the assembly line is eliminated.

2. It is possible to reduce the time required for preparing the automatic machines of the assembly line.

3. Losses due to rejects are reduced by 13.5%. On one of the assembly lines, losses due to rejects amount to 8.4%; of these, 1.135% is attributable to poor gas, according to the following types of defects (Table 4).

Table 4

During operation of the electrostatic precipitator, the degree of purification was determined (the data are given in Table 5).

Table 5

In addition, the voltage–current characteristics were determined (Table 6).

As can be seen from Table 5, a sufficiently high degree of gas purification is achieved even at a capacity exceeding the design capacity, and with a considerable operating time without cleaning (45 days).

Conclusions

1. Electrostatic purification of gas from resin and dust has been investigated at one of the electric-lamp plants.

2. A low-capacity electrostatic precipitator has been designed, distinguished by simplicity of manufacture, including the electrical part.

3. The use of fine purification of water gas from resin and dust in automatic lines makes it possible to reduce downtime of automatic machines and to reduce rejects.

Table 6

REFERENCES

1. Purification of Industrial Gases. Goskhimizdat, 1947.
2. G. M. Gordovich and I. A. Peisakhov. Dust Collection and Gas Purification. Metallurgizdat, 1955.
3. B. L. Shneerson. Electrostatic Precipitators. Gosenergoizdat, 1950.
4. L. D. Gluzman, I. I. Edelman. Control of Coke-Chemical Production. 1951.