RU2315251C1 - Drying method with fully closed cycle of drying agent - Google Patents

Abstract

FIELD: drying processes of dispersed materials, possibly in microbiological, food making, chemical branches of industry.

SUBSTANCE: method comprises steps of condensing evaporated moisture due to feeding waste gas into cooler for cooling and drying by liquid vapor condensation; feeding dried gas into heater and then to drier; at first cooling gas at Xo = const till dew point; then condensing moisture vapor and performing further cooling of gas till temperature of initial mixture; determining cooling agent flow rate through surface type condenser with use of heat balance equation and determining total balance of mass and energy during process of cooling and vapor condensation according to calculation formula.

EFFECT: enhanced efficiency of drying process.

4 dwg

Description

The invention relates to techniques for drying dispersed materials and can be used in microbiological, food, chemical and other industries.

The closest technical solution to the claimed object is a method of drying as. USSR No. 553424, F26В 17/10, 1975, which consists in the fact that the evaporated moisture is condensed by supplying the exhaust gas to the refrigerator, where it is cooled and dried due to condensation of liquid vapor, and then the dried gas is fed into the air heater and then to the dryer (prototype).

The disadvantage of the prototype is the relatively low productivity of drying the final product.

The technical result is an increase in drying performance.

This is achieved by the fact that in the drying method with a completely closed cycle of the drying agent, which consists in condensing the evaporated moisture by supplying the exhaust gas to the refrigerator, where it is cooled and dried due to condensation of liquid vapor, and then the dried gas is fed into the air heater and after that to the dryer, while first the gas is cooled at X _o = const to the dew point (φ = 1), and then moisture vapor is condensed and the gas is further cooled (along the line φ = 1) to the temperature of the initial mixture, while the flow rate of the refrigerant through the condensate a surface type sator is determined from the following heat balance equation:

M = L (I ₀ -I _K ) + Q _pt / s _chl (t _{chl. K} -t _chl.o ),

where M is the flow rate of the refrigerant; Q _pt is the heat expended in the heater to heat the drying agent; with _chl - heat capacity of the refrigerant; t _chl.o and t _chl.K - the temperature of the refrigerant at the inlet to and exit from the condenser; I ₀ and I _K is the enthalpy of the refrigerant at the inlet and outlet of the condenser, L is the flow rate of dry gas,

and the full balance of mass and energy in the process of cooling and condensation of steam is determined by the formula:

I _K = [s _g t _K Δ + (r ₀ + s _p t _K ) (X ₀ Δ-I ₀ )] / [Δ- (r ₀ + s _p t _K )],

where I _K is the enthalpy of the refrigerant at the outlet of the condenser, t _K is the temperature of the vapor-gas mixture at the outlet of the condenser, r ₀ is the specific heat of vaporization at 0 ° C, with _g and s _p are the specific heat of gas and steam, respectively, X ₀ is the moisture content dry air at 0 ° С, Δ is the change in the enthalpy of the drying agent in the process of its cooling and steam condensation.

Figure 1 shows a schematic diagram of a device for implementing the proposed method of drying with a fully closed cycle of a drying agent using a mixing capacitor, figure 2 is a schematic diagram of a device for drying with a fully closed cycle of a drying agent using a capacitor of surface type, figure 3 the image of the process of cooling and drying gas on the diagram IX is shown when using a surface condenser, figure 4 presents the image of the process of cooling and drying gas on a di gram I-X using the mixing condenser.

A device for drying with a completely closed cycle of a drying agent using a mixing capacitor (Fig. 1) contains a fan 1, a heater 2, a feeder 3, a dryer 4, a cyclone 5. Fan 1 is installed in a circuit between the mixing condenser 6, made in the form of a scrubber, and by a heater 2 in such a way that the output of the mixing condenser 6 is connected to the suction line of the fan 1, and the discharge line of the fan is connected to the air heater 2. The hopper part of the scrubber 6 is connected to the condensate collector 7, from which the pump 8 is supplied condensate to the surface type refrigeration machine 9, for example a shell-and-tube heat exchanger.

A device for drying with a completely closed cycle of a drying agent using a surface type condenser (Fig. 2) contains a fan 1, air heater 2, feeder 3, dryer 4, cyclone 5, surface condenser 10 in the form of a shell-and-tube heat exchanger (recuperative heat exchanger), inlet which is associated with the outlet of the cyclone 5, and the output with the condensate collector 11, from which the drying agent is sucked off by the fan 1 and enters the air heater 2.

The drying method with a completely closed cycle of the drying agent using a surface condenser is as follows.

Drying schemes for products with a completely closed drying agent cycle are based on the condensation of evaporated moisture and the use of a surface type condenser or a mixing condenser (scrubber). The exhaust gas enters the refrigerator 9 or 10, where it is cooled and dried due to condensation of liquid vapor. The dried gas is supplied to the air heater 2 and then to the dryer 4.

First, the gas is cooled at X _o = const to the dew point (φ = 1), and then moisture vapor is condensed and gas is further cooled (along the line φ = 1) to the temperature of the initial mixture (Fig. 3). The lower the temperature of the refrigerant, the higher the degree of drying of the recycle gas. The flow rate of the refrigerant through a surface-type condenser can be determined from the heat balance equation:

where M is the flow rate of the refrigerant; Q _pt is the heat expended in the heater to heat the drying agent; with _chl - heat capacity of the refrigerant; t _chl.o and t _chl.K - the temperature of the refrigerant at the inlet to and from the condenser.

The gas temperature at the outlet of the condenser t _K (the initial state in front of the air heater) is usually set, and the moisture content X _K and the enthalpy I _{K of the} gas-vapor mixture are calculated according to traditional dependencies. The required heat exchange surface of the condenser is calculated by known methods.

When using a mixing condenser, the gas leaving their dryers is in contact with a refrigerant, which is usually used as a cooled condensate of an evaporated liquid (solvent or diluent of the starting material to be dried). In the process of interaction with a cold irrigation liquid, the gas-vapor mixture is cooled, its parameters change along a certain curve A ₀ A _K (Fig. 4). At the same time, for an infinitely small period of time of contact, steam condenses in an amount determined by the material balance:

Integrating equation (2) in the range from M ₀ to M _K and from X ₀ to X _K , we obtain:

We denote

where σ _W is the specific fluid flow rate for irrigation (irrigation density), 1 kg of fluid per 1 kg of dry gas, L is the flow rate of dry gas.

Then the mass of evaporated moisture

According to the heat balance of the gas cooling process in the mixing condenser (excluding losses)

Reducing the terms of equation (6) to the specific heat consumption by dividing by W according to equations (3) and (5), after the conversion we get:

where Δ is the change in the enthalpy of the drying agent during its cooling and vapor condensation; q _W - specific heat of the irrigating liquid;

From equation (7) we have:

The gas enthalpy at the exit of the scrubber can be found by graphical construction on the diagram IX or calculated using the basic equation for the enthalpy of a vapor-gas mixture

where I _G , I _P - respectively, the enthalpy of gas and vapor; t is the temperature of the vapor-gas mixture, r ₀ is the specific heat of vaporization at 0 ° C, with _g and c _p are the specific heat of gas and steam, respectively, X is the moisture content of dry air.

Combining equations (10) and (9), we obtain:

Equations (10), (4), (5), (7) - (11) express the full balance of mass and energy in the process of cooling and condensation of steam.

A completely closed gas cycle in drying technology is usually used when a valuable liquid evaporates from the product and needs to be returned to production, or when the material or solvent to be dried is toxic and cannot be released into the atmosphere. In addition, it is advisable to use the closed cycle when drying fire and explosive products, when expensive inert gas is used as a drying agent.

Claims (1)

A drying method with a completely closed cycle of a drying agent, which consists in condensing the evaporated moisture by supplying the exhaust gas to the refrigerator, where it is cooled and dried due to condensation of liquid vapor, and then the dried gas is fed into the air heater and then into the dryer, characterized in that the first gas is cooled when X ₀ = const to the dew point, and then condensed moisture vapor and carried further cooling the gas to a temperature of the feed mixture, wherein the flow of refrigerant through the condenser surface type definition elyayut heat balance of the following equation: M = L (I ₀ -I _K ) + Q _pt / s _chl (t _chl.K -t _chl.o ), where M is the flow rate of the refrigerant; Q _pt is the heat expended in the heater to heat the drying agent; with _chl - heat capacity of the refrigerant; t _chl.o and t _chl.K - the temperature of the refrigerant at the inlet to and exit from the condenser; I _about and I _To - the enthalpy of the refrigerant at the inlet to the condenser and the exit from it; L is the dry gas flow rate, and the full balance of mass and energy in the process of cooling and condensation of steam is determined by the formula: I _K = [s _g t _K Δ + (r ₀ + s _p t _K ) (X ₀ Δ-I ₀ )] / [Δ- (I ₀ + s _p t _K )]. where I _K is the enthalpy of the refrigerant at the outlet of the condenser; t _K is the temperature of the gas-vapor mixture at the outlet of the condenser; r ₀ - specific heat of vaporization at 0 ° C; c _g and c _p are the specific heat of gas and steam, respectively; X ₀ - moisture content of dry air at 0 ° C; Δ is the change in the enthalpy of the drying agent during its cooling and steam condensation.

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