Calculation packed absorber

INTRODUCTION

CALCULATE ABSORPTION UNIT

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BASIC SYMBOLS

ɑ - specific surface area, m²/m³;

D – coefficient of diffusion, m²/s;

d – diameter, m;

F – surface mass transfer, m²;

G – inert gas flow rate, kg/s;

g – acceleration of gravity, m/s²;

H, h – height, m;

K – coefficient of mass transfer;

L – absorber consumption, kg/s;

M – material weight, transmitted through the surface of the mass transfer per unit time, kg/s;

M_amm – the molar mass of ammonia, kg/kmol;

m – distribution factor;

P – pressure, mPa;

T – temperature, K;

U – irrigation density, m³/(m²∙s);

ω – gas velocity, m/s;

x – concentration of liquid;

y – concentration of gas;

ΔẌ_av – the average driving force for absorption of the liquid phase, kg/kg;

ΔῩ_av – the average driving force for absorption of the gas phase, kg/kg;

β – coefficient of mass transfer;

ℰ - free volume, m³/m³;

λ – coefficient of friction;

μ – viscosity, Pa∙s;

ξ – coefficient of resistance;

ρ – density, kg/m³;

σ – surface tension, N/m;

ψ – wettability factor;

Re – Reynolds criterion;

Fr – Froude criterion;

Гс – water resistance test;

Nu^/ - diffusion Nusselt number;

Pr^/ - diffusion Prandtl number.

Index:

fin – the final option;

in – the initial option;

x – liquid phase;

y – gas phase;

av – average value;

0 – under normal conditions;

w – water;

^* - equilibrium composition.

Scope of absorption processes in industry is very extensive; getting the finished product by absorption of gas by a liquid; the separation of gas mixtures into their constituent components; the purification of gases from harmful impurities; the capture of valuable components from gas emissions.

Distinguish physical absorption and a chemisorption. At physical absorption dissolution of gas in liquid isn't followed by chemical reaction or influence of this reaction to the speed of process it is possible to neglect. As a rule, physical absorption doesn't accompanied by essential thermal effects. If at the same time initial streams of gas and liquid slightly differ on temperature, such absorption can be considered as isothermal. We will begin absorption process calculation consideration with this simplest case.

The main complexity in case of design of absorbers consists in a right choice of estimated regularities for determination of kinetic coefficients from bigger number different, couple contradictory, the dependences provided in technical literature. Calculations for these equations, usually fair for special cases, result often to differing, and sometimes in certainly incorrect results. The equations recommended here are selected after the careful analysis and comparative calculations from a wide interval of variables, checks of adequacy of calculated data experimental, received on real systems.

This chapter provides examples of calculations of packed absorbers on the basic kinetic mass transfer equation.

Fig. 3.1. The concept of absorption of the installation:

1 - ventilator (gas blower); 2 - absorber; 3 - spray wedge; 4, 6- distributor; 5 - fridge; 7 - adsorber; 8 - cubic desorbera; 9, 13 - tanks for absorbent; 10, 12 - pump; 11- heat exchanger – recuperator.

Purified gas

In Figure 3.1 is a diagram of the absorption unit. Gas absorption is fed to blower 1 to the bottom of column 2, which is uniformly distributed before entering the contact element (or nozzle plate). Absorbent capacity of the intermediate 9 by the pump 10 is fed to the top of the column and is distributed uniformly over the cross section of the absorber via sprinkler 4. The column is performed countercurrent gas - liquid interaction. Gas absorption after passing spray wedge 3, exits the column. Absorbent flows through the water seal in the intermediate tank 13, where pump 12 is sent for regeneration to the stripper 7, after preheating in exchanger - recuperator 11. Running out from the absorbent absorbed component is produced in the cube 8, heated, generally saturated water vapor. Before serving irrigation absorber column, having a heat exchanger - heat exchanger 11, is further cooled in the refrigerator 5. The regeneration can be carried out also by other methods, for example by stripping absorbed component stream of inert gas or steam, decreasing pressure, increasing the temperature. The choice of regeneration method significantly affects the technical - economic indicators of the absorption unit as a whole.

Design assignment. Calculate ammonia absorber to absorb water from its mixtures with air under the following conditions:

1) gas performance under normal conditions V₀=0,5m³/s;

2) absorbed ammonia concentration in the gas at the inlet to the column y=10%;

3) the degree of absorption C=98%;

4) pressure in the column P=0,3mPa;

5) absorption of temperature t=20⁰C.

To simplify the calculations, the following gas mixture and the absorber considered to be a binary consisting of a distributed component (ammonia) and inert part (media); the physical properties of their average taken.

For linearization of the working line absorption phase compositions of the equation is expressed as the relative concentrations of distributed component and load in phases – in an inert carrier consumptions. In the following concentration expressed in calculations relative weight proportions distributed component and load – a mass flow of carriers.

The geometric dimensions of the column mass transfer apparatus are mainly determined by the mass transfer surface required for this process, and phase velocities.

Mass transfer surface can be obtained from the basic equation of mass transfer [1]:

F = M/(K_xΔX_av) = M/(K_yΔY_av) (3.1)

where K_x, K_y – the mass transfer coefficients, respectively, the liquid and gaseous phases, kg/(m²∙s).

Дата добавления: 2017-01-14