RU2459161C2 - Method of drying carbon black - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: proposed method comprises feeding moist granulated carbon black into drying drum equipped with outer chamber with its outer surface heated by high-temperature smoke fumes generated in fuel combustion, said fumes including off gas of carbon gas formation to be fed inside said drum. In compliance with this invention, extra airflow is fed into smokes fumes before its contact with granulated carbon black.

EFFECT: high efficiency highly intensified process, reduced consumption of natural gas.

2 cl, 1 dwg, 2 tbl

Description

The invention relates to chemical-technological processes and can be used for drying wet granular soot.

The technological scheme for the production of soot includes the process of incomplete combustion of mainly liquid hydrocarbons, followed by separation of the gaseous reaction products and soot. The formation of soot granules occurs with intensive mixing of dusty soot with water in a ratio of ~ 1: 1, followed by drying of wet granular soot in a drying drum [1]. Wet granular soot is dried by high-temperature flue gas generated during the combustion of hydrocarbon fuels and most often natural gas with a heat of combustion of about 8000 kcal / m ³ , the flow rate of which is 120 ÷ 130 m ³ per 1 ton of dry granular soot. In order to reduce costs for drying wet granular soot, gaseous products of the soot formation process with a calorific value of 500 ÷ 700 kcal / m ³ are used as fuel. Usually, a combined fuel supply is carried out, in which natural gas is used as a backlight to increase the combustion efficiency of a relatively low-calorific off-gas.

There is a method of drying wet granules of soot, in which drying is carried out by high-temperature flue gas generated by burning pre-dried gaseous products of soot formation [2]. Moisture removal from the exhaust gaseous products is carried out in a scrubber by irrigation with circulating water.

To obtain 4 ÷ 6 t / h of dry granular soot, it is necessary to dry 6000 ÷ 8000 m ³ / h of gaseous soot products with a temperature of about 200 ÷ 220 ° C, containing CO ₂ , SO ₂ , H ₂ S, CS ₂ and the remains of uncoated soot . For this purpose, 25–30 t / h of cooling water is required, which, when circulated through the scrubber, turns into a hot pulp with a pH of 4–5. The costs of circulating, cooling, neutralizing and disposing of the pulp, together with the costs of acid-resistant equipment of the drying system, are many times greater than the economic effect of using dried gas as fuel, which is a drawback of this drying method.

There is a method of drying wet granular soot, in which fuel is burned with air, including gaseous products of the soot formation process, with the subsequent separation of the formed high-temperature gases into 2 streams, the first stream being supplied to the external drum heater, and the second after dilution with cooled moist gas recirculated from the drying drum, it is fed into the drum for contact with the wet granules of soot [3].

The disadvantage of this method is the low drying efficiency due to the high humidity of the flue gas sucked through the dryer drum, since the moisture content of the recirculated cooled gas supplied to the inlet of the dryer drum reaches 60 ÷ 65% and this leads to a decrease in the rate of evaporation of moisture and to a decrease in the productivity of the dryer drum and the entire processing line.

A known method of drying granular soot, selected as a prototype, in which the outer surface of the drying drum is heated by high-temperature gas generated during the combustion of fuel, including gaseous products of the soot formation process, followed by its supply into the drying drum for contact with wet granulated soot, and the air for combustion fuel is supplied in excess of 5–10% [4].

The disadvantage of this method is the low drying efficiency due to the high humidity of the flue gas sucked through the dryer drum, which reduces the mass transfer rate between the wet granular soot and flue gas and reduces the performance of the dryer drum.

The aim of the proposed method for drying wet granular soot is to increase the productivity of the drying drum by increasing the rate of removal of water vapor from the layer of wet granules of soot when dried by flue gas generated from the combustion of fuel, including gaseous products of the soot formation process.

This goal is achieved in that the drying of granular soot, including the supply of wet granular soot to a drying drum equipped with an external chamber, in which the outer surface of the drum is heated by flue gas generated during the combustion of fuel, including the exhaust gas of the soot formation process, followed by the supply of flue gas into the drum , characterized in that the flue gas before contact with granular soot is supplied with an additional stream of air. In addition, an additional air stream is supplied in an amount at which the ratio of the oxygen concentration in the fuel combustion products to the oxygen concentration in the flue gas at the outlet of the dryer drum is within 0.07 ÷ 0.77, and the oxygen concentration in the fuel combustion products is determined from the following ratio:

where [O ₂ ] _T is the oxygen concentration in the fuel combustion products,%;

Q _B , Q _T , Q _ОГ - volumetric flow rates of air, hydrocarbon fuel and soot-forming exhaust gas, respectively;

K _T , K _ОГ - specific air consumption for the complete combustion of 1 m ^{3 of} hydrocarbon fuel and soot formation waste gas, respectively, m ³ / m ³ ;

[H ₂ ], [CO], [H ₂ O] is the concentration of hydrogen, carbon monoxide in the dry exhaust gas of soot formation and the moisture content of the exhaust gas, respectively,%.

The essence of the proposed technical solution is as follows. In the General case, the performance of the drying drum depends on the rate of evaporation of water from the surface of the wet material and transfer into the gas stream and is determined by the following relationship [5]:

where U is the amount of moisture evaporated in units. time from 1 m ² surface;

W is the gas velocity above the material;

ΔP = (P _NAS- P _G ) - the pressure difference of saturated water vapor above the surface of the material and in the passing gas.

As can be seen from equation (1), at a constant gas velocity W and saturated vapor pressure above the surface of the wet material P _NAS, the drying rate U increases in direct proportion to the decrease in the partial pressure of water vapor in the gas.

When moving hot gas along the drum, the temperature decreases due to the expenditure of heat on the evaporation of moisture with a simultaneous increase in its moisture content. Moreover, in accordance with equation (1), the drying speed decreases due to a decrease in the transfer parameter — the partial pressure difference ΔP. The moisture content of the gas can reach a critical value at which the drying speed tends to zero, limiting the performance of the drying drum, even with significant excesses of thermal energy. In other words, a necessary condition for the high efficiency of the drying process is to maintain the maximum value of ΔP. Although the use of gaseous products of the soot formation process can significantly save hydrocarbon fuel, their low calorific value and high moisture content (35 ÷ 50%) creates at least 2 main problems: ensuring high completeness of combustion for maximum thermal effect and maintaining high speed steam transfer from wet granules to flue gas with high moisture content.

Since the amount of heat transferred from the flue gas to the outer surface of the drum is proportional to the difference between the temperature of the flue gas and the wall of the drum, the combustion of low-calorie and wet exhaust gas from the soot formation process is carried out using the enrichment of the fuel mixture with natural gas and with an excess of air that is minimally sufficient for complete combustion 5 ÷ 10 %, as shown in the prototype [4]. With an excess of air of 10%, the humidity of the flue gas during the combustion of natural gas will be 17% at the inlet to the drum, and when burning off the exhaust gas of the soot formation process, ~ 35%. With an increase in the excess of air supplied to the combustion of the fuel, the moisture content of the flue gas can be reduced by increasing the total volume of the flue gas, but at the same time its temperature will decrease, and consequently, the heat supply through the wall of the drum to the layer of moist soot granules in the drum. This will ultimately lead to a decrease in the productivity of the drying drum, since in the heat balance of the drying process of wet granules, the heat consumption for heating and evaporating water and heating its vapor to a flue gas temperature exceeds 94%. The supply of an additional air stream to the dryer drum allows you to maintain a high temperature of the flue gas in the heating chamber and the rate of heat transfer to the wet soot granules and at the same time ensure the moisture content of the flue gas in the dryer drum and, therefore, increase the rate of steam removal from the layer of wet soot granules ultimately increase the productivity of the drying process.

Thus, the additional air supply to the dryer drum is an essential distinguishing feature, which ensures the elimination of contradictions in the optimization of 2 processes - heat transfer during heat supply, which evaporates the water, and heat removal from the wet granular soot layer to the flue gas. If the temperature of the flue gas and the amount of heat transferred through the drum wall to the layer of wet granular soot are equal to the prototype, the additional air supply inside the dryer drum provides, due to a larger total volume of gas of lower humidity, a higher driving force of the moisture transfer process ΔР during the entire drying process, which in accordance with equation (1) increases the rate of evaporation of moisture U, and therefore the performance of the drying drum. It is also obvious that the proposed drying method has an advantage over the patent [3], in which the high-temperature flue gas is diluted before being fed into the drum with recirculated cooled moist (60 ÷ 65%) flue gas that has passed the dryer drum.

In the prototype drying method, the flue gas passing through the dryer drum is saturated with water vapor, but the dry composition of this gas, as determined, for example, by gas chromatography, does not change. In the proposed drying method, the composition of the dry part of the flue gas after mixing with an additional air stream will change, which will most significantly manifest itself in an increase in oxygen content, since the air contains oxygen 21%, and in the dry part of the fuel combustion products does not exceed 1 ÷ 6%, depending on the ratio of the flow rate of air, natural gas and exhaust gas from the soot formation process, which allows reliable control and management of the flue gas moisture content, which has a significant impact on the intensity of ca water vapor from the moist granular layer of soot in the gas flow.

The average humidity of the exhaust gas from the soot formation process is ~ 45%, and Table 1 shows the average composition of the main components of the dry part of the exhaust gas.

The average composition of the exhaust gas of the soot formation process (% vol.)

Table 1 CO ₂ CO H ₂ N ₂ four fifteen fifteen 66

The minimum required amount of air oxygen is determined from the stoichiometric combustion equations for natural gas (methane) and the combustible components of the soot formation exhaust gas:

According to the above equations, for burning 1 m ^{3 of} natural gas it is necessary K _T = 2 / 0.21≈9.52 m ^{3 of} air, and for burning 1 m ^{3 of} exhaust gas K _OG = (0.5 * 15 + 0.5 * 15) / 100 * (1-45 /100)/0.21≈0.393 m ^{3 of} air.

For known values of Q _B , Q _T , Q _OG , stoichiometric equations (2) determine the excess of air and oxygen in it, the volume of flue gas and taking into account the water vapor coming from the soot-forming exhaust gas and generated during combustion, and the oxygen concentration in dry flue gas:

By comparing the oxygen concentration [O ₂ ] _T with the oxygen concentration in the flue gas at the outlet of the dryer drum [O ₂ ] _d , we can obtain an estimate of the ratio of the flow rates of flue gas and additional air supplied to the dryer drum, and thus control the moisture content of the mixed gas stream. For example, for flue gas generated during the combustion of exhaust gases of soot formation with an excess air coefficient of _KI = 1.1, the content of free oxygen in dry gases is 1%, and the moisture content is 40%. When diluting flue gas with air in a ratio of 1: 1, the oxygen content in the dry part of the mixed stream will be 13.5%, and the moisture content will decrease to 20%.

The ability to determine the moisture content of flue gas at the inlet of the drum by the ratio of [O ₂ ] _T / [O ₂ ] _D is especially significant in conditions where an excess amount of fuel combustion products is ejected from the heating chamber through the chimney, and its part entering the drum unknown.

Figure 1 shows a schematic flow diagram of wet granulation and drying of soot, which includes the following series-connected devices:

- storage hopper 1 with gateway feeder 2;

- granulator 3;

- air pipe 4 with adjustable damper;

- a drying drum 5 with an intake pipe 6;

- an external heating chamber 7 with a chimney 8 equipped with an adjustable damper 9;

- fan 10 with adjustable damper 11;

- bag filter 12;

- fan 13;

- cyclone 14 with a gateway feeder 15.

Dusting soot in an amount of 3-5 t / h after separation with gaseous products of the soot formation process (exhaust gas) enters the storage hopper 1, from which it is fed to the granulator 3 from a gate feeder 2, where ~ 1: 1 occurs with vigorous stirring with water the formation of wet granules of soot. From the granulator 3, the wet granulated soot flows by gravity to the dryer drum 5. In the outer chamber 7, the dryer drum 5 is heated with high-temperature flue gas, which is formed by burning fuel with soot formation exhaust air. Natural gas is used as hydrocarbon fuel. From the outer heating chamber 7, the flue gas through the intake pipe 6 enters the dryer drum 5 due to the vacuum created by the fan 10 in the dryer drum 5 and the heating chamber 7. In addition to the flue gas, an additional air stream is drawn from the atmosphere through the annular gap between the chamber body heating 7 and dryer drum 5. If necessary, the flow rate of the additional air flow can be increased using the pipe 4, equipped with an adjustable damper. The amount of rarefaction inside the drum 4 and in the heating chamber 6, and therefore the flow rate of the additionally sucked-in air, is regulated by a shutter 10 in front of the fan 9, after which flue gas is sampled to determine its composition.

Fan 9 moist flue gas at a temperature of 180 ÷ 250 ° C and a moisture content of 45 ÷ 65% is sent from the drum 4 to the bag filter 11 to separate the dusty soot and small fragments destroyed during the drying of the granules, which are fed by a fan 12 to the cyclone 13 through gas transport and through the gate feeder 14 are discharged into the storage hopper 1. Dry granular soot with a temperature of 120 ÷ 140 ° C and a moisture content of not more than 0.5% is sent for cooling and storage.

Table 2 shows the experimental data of comparative tests of the prototype and the proposed method for drying wet granular soot. During the experimental work, the coefficient of excess air, determined according to the equation below, was maintained equal to 1.1:

To evaluate the efficiency of the drying process, we used the indicator of the specific fuel consumption per calorific value for natural gas per 1 ton of water.

In experiments 1 ÷ 3, performed in accordance with the prototype, the flow rate of water for granulation increased sequentially with a constant weight ratio of 1: 1 with the consumption of soot. With constant values of the moisture content at the inlet and outlet of the drum, a sequential increase in productivity leads to a decrease in the temperature of soot at the exit of the drum and an increase in the temperature of the flue gas. In experiment 3, the temperature of soot dropped below a critical temperature, and the temperature of the flue gas exceeded the maximum permissible value for a bag filter - 250 ° C. The moisture content of granular soot at the outlet of the drum was 3.5% with an allowable rate of not more than 0.5%. An increase in the temperature difference of soot and flue gas at the drum outlet from 80 ° C in experiment No. 1 to 167 ° C in experiment No. 3 indicates a decrease in the rate of heat and mass transfer between wet granular soot and flue gases. It should be noted that with an increase in water consumption, the specific reduced fuel consumption also increased from 105.6 to 109.6 m ³ / t of water, which indicates a decrease in the rate of moisture transfer to a critical value, at which even an excess of thermal energy does not allow to obtain the required moisture content of granular soot.

The experimental data of comparative tests of the prototype and the proposed method for drying wet granular soot.

In experiments 4 ÷ 8 shows the test results of the proposed method of drying at a constant flow of water, as in experiment No. 3-4 t / h A consistent increase in the flow rate of the additional air flow supplied to the mixture with the fuel combustion products before being fed into the dryer drum from 50 m ³ / h to 6,000 m ³ / h leads to an increase in the oxygen concentration in the flue gas [O ₂ ] _D from 1.39 to 11.81% , reducing moisture content at the inlet of the drum from 33.6 to 19.4% and the output of the drum from 58.3% to 40.4%. The ratio [O ₂ ] _T / [O ₂ ] _D decreases from 0.87 to 0.1, and the value 0.77 is effective, which corresponds to an additional air consumption of 100 m ³ / h, since with it the soot temperature at the drum outlet was 110 ° C, which provided dry granular soot.

In experiments 9 ÷ 12, the productivity of the drying drum is successively increased to 4.3 and 4.5 t / h. Starting from experiment No. 9, part of the gases after heating the outer surface of the drum is discharged through the chimney, which further reduces the humidity of the gas due to greater dilution with air. In this case, the ratio [O ₂ ] _T / [O ₂ ] _D = 0.07 in experiment No. 11 can be considered effective, since a further decrease in this ratio in experiment No. 12 to 0.06 leads only to an increase in the specific reduced fuel consumption.

Thus, the effective action of the additional air flow supplied for mixing with the flue gas is achieved when the concentration in the combustion products of the fuel to the concentration of oxygen in the flue gas at the outlet of their drying drum [O ₂ ] _T / [O ₂ ] _D in the range 0.07 ÷ 0.77.

Experiments 13-14 show the modes of drying wet granular soot using only soot-forming exhaust gas as fuel. These drying conditions are the most difficult, since the moisture content of the combustion products is maximum and only dilution with air and discharge of the excess part of the flue gas allows the drying process to be carried out at high productivity.

Using the proposed method for drying wet granular soot allows you to solve the complex technical and economic problem - to ensure high performance of the drying drum while reducing natural gas consumption by using a secondary source of thermal energy: the productivity of one process stream increases from 20 to 25 thousand tons of granular soot per year while saving 3 million m ^{3 of} natural gas due to the use of soot process as an exhaust gas fuel Ania.

Literature

1. V.Yu. Orlov, A.M. Komarov, L.A. Lyapina. Production and use of carbon black for rubber. Yaroslavl, Ed. AR, 2002, p. 314.

2. Pat. US 4282199, 1981.

3. Pat. US 2952921, 1960.

4. Pat. US 3102005, 1963.

5. A.G. Kasatkin. Basic processes and apparatuses of chemical technology. Ed. chemical literature, 1948, p. 420.

Claims (2)

1. The method of drying soot, including the supply of wet granular soot to a drying drum equipped with an external chamber, in which the outer surface of the drum is heated with high-temperature flue gas generated during the combustion of fuel, including the exhaust gas of the soot formation process, followed by their supply into the drum, characterized in that an additional air stream is supplied into the flue gas before contact with granular soot. 2. The method according to claim 1, characterized in that the additional air flow is supplied in an amount in which the ratio of the oxygen concentration in the fuel combustion products to the oxygen concentration in the flue gas at the outlet of the drying drum is within 0.07 ÷ 0.77, and the oxygen concentration in the fuel combustion products is determined from the following ratio

where [O ₂ ] _T is the oxygen concentration in the fuel combustion products,%;
Q _B , Q _T , Q _ОГ - volumetric flow rates of air, hydrocarbon fuel and soot-forming exhaust gas, respectively;
K _T , K _ОГ - specific air consumption for the complete combustion of 1 m ^{3 of} hydrocarbon fuel and soot formation waste gas, respectively, m ³ / m ³ ;
[H ₂ ], [CO], [H ₂ O] is the concentration of hydrogen, carbon monoxide in the dry exhaust gas of soot formation and the moisture content of the exhaust gas, respectively,%.

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