SU1746184A1 - Method of cooling chemical reactor walls - Google Patents

Abstract

Use: chemical industry. Summary of the invention: Before the flow of the coolant flow into the outer surface of the reactor wall, the non-condensing gas is dissolved in the latter, and the pressure in the coolant is periodically reduced during the flow process, which causes gas evolution in the boundary layer near the surface of the reactor walls. 1 ep f-ly, 1 silt

Description

The invention relates to the field of the chemical industry and can be used in cooling the walls of chemical reactors, inside of which processes are carried out, accompanied by the generation of heat.

There is a known method for cooling a chemical reactor wall, which includes an adjustable feed to the outer surface of the reactor wall and withdrawal of a heat carrier stream from it.

The disadvantage of this method is low heat removal rate, which is caused by a decrease in the heat transfer coefficient from the wall due to losses in the resulting thermal boundary layer. The consequence of this decrease in the heat transfer coefficient is to limit the productivity of the reactor, since an increase in the flow rate of the reactors and an increase in the total thermal coefficient of the reaction is possible to a limited in the way the heat sink capacity.

A known method of cooling a chemical reactor wall to a predetermined temperature by means of a controlled supply to its outer surface of a coolant stream containing heterogeneous inclusions, such as bubbles of non-condensing gas, and its subsequent removal.

Despite the achievable partial destruction of the structure and flow turbulization, the known method has limited opportunities for heating and requires additional costs for compensating the hydroresistance associated with the loss of energy for the turbulization of the entire flow. In general, the disadvantages of the method reduce the cooling efficiency.

The aim of the invention is to intensify heat exchange and reduce energy costs.

This goal is achieved by that according to the method of cooling the chemical reactor wall to a predetermined temperature by controlled supply to its outer

SP

WITH

Vj

ABOUT

with

the surface of the flow of the coolant containing bubbles of non-condensing gas and its subsequent removal, the gas is pre-dissolved in the coolant, and in the process of supplying the flow to it, the pressure in the coolant is periodically reduced. The magnitude of the periodic decrease in pressure Pmin is chosen from the condition

0.5 (+1),

0)

PST 1 Gzt

where Cp is the concentration of the dissolved gas;

K is the solubility of the gas;

PST is the saturated vapor pressure of the coolant at a given wall temperature.

The concentration of the dissolved gas Cp is provided in this case from the condition

0.01 СР- (Р ° -РМИИ) - about 75 (2)

AT

where Ro is the initial pressure of the flow;

Рmin - minimum pressure while decreasing;

rp is the density of the non-condensing gas in the stream.

The period of pressure reduction in this case is chosen from 0.1 to 10 seconds.

Pre-dissolving the gas and periodically reducing the pressure of the flow of the resulting solution when it is applied to the surface of the cooled wall leads to periodic formation near the wall in the phase of reducing the pressure of the liquid layer supersaturated with gas relative to its equilibrium thermodynamic parameters (pressure and temperature). In this case, a periodic evolution of the dissolved gas in the form of free bubbles occurs in the above layer.

Since such bubble formation occurs directly inside the thermal boundary layer, it effectively increases the heat transfer rate from the wall to the flow by means of effective periodic destruction of the boundary layer.

At the same time, the bubbles do not block up the entire channel section, and their formation only in the boundary layer does not lead to an increase in hydraulic resistance losses.

Under optimal process conditions (the formation of bubbles only in the boundary layer), the apparent viscosity of the boundary layer decreases, which leads in accordance with known dependencies to a decrease in friction losses on the cooled wall.

The choice of the values of pressure reduction and initial concentration in accordance with dependencies (1) and (2) allows to obtain a positive effect.

-

0

(defines the mode limits for using the method).

The limitation of the right-hand side of dependence (1) is determined theoretically from known dependencies. For example, the equilibrium conditions for gas-saturated

liquids are determined by the equation

R

Equal PST + -Ј.

/WITH

The formation of bubbles in a gas-saturated heated flow is possible based on this condition at a pressure

ROMIN PST +

CE

TO

15 or

0

(1 +

PST V CRP.

As the experiments show, the restriction to the left in dependence (1) in terms of the intensity of the heat sink or wall actually occurs and can be characterized by

RMIN

0.1

25

thirty

35

40

50

ST

From a physical point of view, the result obtained in the experiments can be explained by the fact that, starting with a slight decrease in pressure, it no longer leads to an increase in the amount of liquid boiling on the wall (the process is transferred from the boundary layer to the flow).

In addition, while limiting heat transfer occurs due to the transition to film boiling (in experiments, starting with Pmin -0.5 PST).

Relationship (2) determines the volumetric content of the gas phase in the boundary layer.

As it is known from the condition of maximum packing of spheres, the maximum gas content of a bubble liquid layer can not exceed 0.75. Part of gas that is released per unit volume with decreasing pressure is determined by the expression

D Svyd Wed - (Ro - Pmin) K

45 and occupies part of the volume A svyd // Ep

Therefore, the condition for limiting the right-hand side of dependence (2) can be established theoretically:

ASvyd / At 0.75.

As experiments show, a significant improvement in heat removal with insignificant losses of hydroresistance is provided at

O 01 SR (Ro Rmin) K

Al Maximum heat transfer intensity

with some decrease in hydroresistance obtained at

min

 0.95

PST Wed (Re - RMIN) -.

The need to limit the range of periods of pulsations (decreases) in pressure is determined by general physical reasoning. An excessive increase in the period leads to a decrease in the cycles of periodic disruption of the boundary layer by bubbles and a decrease in the intensity of the heat sink, and an excessive decrease in the period may lead to a subsequent decrease in pressure if the wall has bubbles that are not removed from it. In this case, the contact overheating phase of the boundary layer drops out of the process, and the intensity of gas release (and turbulization) decreases sharply.

The minimum time required to remove bubbles from the wall to the thickness of the boundary layer (minimum period) depends on the viscosity of the fluid, flow rate, wall condition, and the like.

It has been experimentally obtained that the efficiency of the method is ensured by choosing a period of pressure reduction of 0.1-10 seconds.

It should be noted that with a decrease in pressure with a period of 0.1-0.01 seconds, the intensification of heat transfer continues to remain at a certain level (below the optimum), which is obviously due to the pulsating mechanism of heat transfer intensification.

The drawing shows an installation diagram implementing a method for cooling a chemical reactor wall.

The plant contains a chemical reactor for producing n-xylene with temperature sensors connected to the control unit.

The walls of the working tank 1 of the chemical reactor (during operation must have a temperature of 100 ° C) are equipped with an external cooling casing 2 with supply and removal pipelines for the heat carrier (water) connected to the circulation pump.

In the supply pipeline there are flow controllers connected to the control unit and an ejector-mixer connected to the gas (air) supply pipeline to the throttle washer.

A rotary pulsator with a drive is placed in the discharge pipe providing pressure reduction (increase in the flow area of the pulsator) with a period T of 1.0 s. A water cooler is installed at the end of the discharge pipe,

A drain from which is connected to the inlet of the circulating pump. The installation is equipped with fittings for supplying water from an external source and its drain.

The installation works as follows.

After loading the reagents into the reactor and starting an exothermic reaction in the reactor volume, according to the sensor readings, water is supplied to the housing from the pump (or fitting) under a pressure of 4 MPa.

During the movement of water in the mixer-ejector (at a pressure in the diffuser of the mixer 1 ati and pressure in the suction zone 1 eta), the air supplied through the pipeline is dispersed, which leads to saturation of the water with air. The ratio of the flow rate of the fluid and air into the mixer is automatically provided due to the insignificant change in the ejection coefficient in the operating range of the mixer ejector.

The drive, rotating the rotary pulsator, provides an increase in the area of the pulsator flow area once per second from 0.8x10 m2 to m, which leads to a drop in pressure in the casing volume from 1 bar to 0.1 bar. Since in working condition the temperature of the wall of the reactor is 110 ° C, this decrease in pressure causes periodic gas evolution in the boundary layer at the outer surface of the walls of the reactor and intensifies the cooling process. In this case, it is possible to increase the consumption of reagents in the reactor (due to the possibility of a large heat removal at a given water flow).

Heated water after the pulsator enters the cooler (cooling tower), then into the pump.

With the circuit open, water is supplied from the nozzle and drained through the nozzle.

The regulation of the water flow is carried out by the controllers from the control unit according to the sensor signals.

The use of the invention in the chemical industry by intensifying the heat sink without increasing the flow rate of the cooler will improve the performance of chemical apparatuses (for example, in the chlorination of n-xylene) while reducing costs.

Claims (2)

1. A method of cooling a chemical reactor wall to a predetermined temperature by means of a controlled supply to its outer surface of a coolant stream containing bubbles of non-condensing gas, and its subsequent removal, characterized in that, in order to intensify heat exchange and reduce energy consumption, Before being supplied to the wall, the gas is pre-dissolved in the coolant, and in the process of being fed to the flow wall, the pressure in the coolant is periodically reduced, while the magnitude of the pressure drop and the concentration of dissolved gas are chosen according to dependencies min + 1): РЗт К PST Ср (Ро - Рмин) К рп where Pmin is the minimum pressure when it decreases; Cf is the concentration of the dissolved gas; PST is the saturated vapor pressure of the coolant at a given wall temperature; AC - gas solubility; Ro is the initial pressure in the flow; rp is the gas density in the stream. 2. The method according to claim 1, characterized in that the period of pressure reduction is chosen equal to 0.1-10.0 s.