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EFFECT OF LIQUID FLOW RATE AND GAS FLOW RATE ON A PACKED ABSORPTION TOWER AND REPAIR OF EQUIPMENT

ABSTRACT

The purpose of the project is to study the effect of liquid flow rate and gas flow rate on the pressure drop in a packed absorption tower and also to repair and restore the packed absorption tower to good working condition. Trouble shooting revealed that the exit drain value was broken due to carelessness on the part of former user of the equipment.  We had to carry out glassblowing operations using borosilicate glass to construct another value.  Once this was done pressure build up within the tower was restored for absorption operation to take place. A plot of log DO/H against log a superficial gas velocity gave 2.0774 2.3037 and 1.4940 respectively for slopes at varied superficial liquid velocity. We concluded that increase in gas flow rate lead to increase in pressure drop since the graph obtained is linear.  There was a drop in slope from lower liquid rate to higher as operation neared flooding condition.  This means that industrially more cost is alluded to pumping operation to be successful.

CHAPTER ONE

1.0    INTRODUCTION

The removal of one or more selected components from a mixture of gases by absorption into a suitable liquid is the second major operation of chemical engineering that is based on inter phase mass transfer controlled large by rates of diffusion. Gas absorption defined  by Perry is a unit operation in which a soluble components of a gas mixture are dissolved in a liquid.  The inverse operation is called stripping or description.

In gas absorption a soluble rapour is absorbed  from its mixture with an inert gas by means of a liquid in which the solute gas is more or less soluble.  The washing of ammonia for a mixture of ammonia and air by means of liquid water is a typical example.  The solute is subsequently recovered from the liquid by distillation and the absorbing liquid can be either discarded or reused.  An acetone air mixture passed through a  gas stream can recover acetone by its dissolving in the gas and allow the air to pass out.  In each of the example given only physical process take place with no chemical affect appreciable. However when oxides of nitrogen are absorbed dioxide is absorbed in a solution of sodium hydroxide, a chemical reaction occurs, the nature of which influences the actual rate of absorption

In considering the design of equipment to achieve gas absorption, the main requirement is that gas be brought into intimate contact with the liquid and the effectiveness of the equipment will largely be determined by the success with which it promotes contact between the two phases. In absorption, the feed is a gas and is introduced at the button of the column, and the solvent is fed to the top 1 as a liquid  the absorbed gas and solvent leave at the bottom and the unabsorbed components leave as gas from the top.

A common apparatus used gas absorption and certain other operations is the packed tower.  The  device consists of a cylindrical column or tower  equipped  with a gas inlet and distributing space at  the bottom! A liquid inlet and distributing at the top gas and liquid out lets at the top bottom, respectively! And a supported mass of inter solid shapes, called tower packing.  The packing support is typically a screen, corrugated to give it strength with a large open area so that flooding does not occur at the support.  The inlet liquid, which may be pure solvent or a dilate solution if solute in the solvent and which is called the weal liquid is distributed over the top of he packing by the distributor and in ideal operation, uniformly wets the surface of the packing.  The solute-rich gas enters the distributing space below the packing and flows upward through the interstices in the packing countercurrent to the flow of the liquid.  The packing provides a large area of contact between the liquid and gas and encourages intimate contact between the phases.  The solute in the rich gas is absorbed by the fresh liquid entering the tower and dilute, or lean gas leaves the top the liquid is enriched in solute as it floes down the tower and concentrated liquid called strong liquor, leaves bottom of the tower through the liquid outlet.

In order to obtain a good rate of transfer per unit volume of the tower, a packing is selected which will promote a high interfacial area between the two phases and a high degree of the rebalance in the fluid usually increased  area and turbulence are achieved at the expense of increased capital cost and/or pressure drop and a balance must be made between these factors when arriving at an economic design.

Since the purpose of this project is to study the effect of variations in the liquid and gas flow rates on the pressure drop in a countercurrent packed absorption tower suing water and air, we do know that total pressure drop occurrence in the unit can have significant effect on compressor running costs.  There is appreciable loss of energy due to function and this shows itself as a fall in pressure between the gas inlet and outlet.  Increase in the gas flow rate will obviously increase the frictional loses. However, gas and liquid are both competing for the free cross-sectional area available for gas flow will also increase the functional losses.  Increase in the gas or liquid flow rate will therefore increase the pressure drop.

The relevance of air/ water testing to the unit process of absorption is seen from two perspectives; one is using fundamental approaches to assess the importance of all relevant forces and whether they are adequately modeled by air/water tests. Two by carrying out tests with a ranted of different fluids (or systems) with different physical properties if similar results are found or fundamentally sound method  that pull together the air/ water results and those for other systems cam be identified, then air/water can be used as acceptable model system.  If the air/water result and non-air/water results differ and cannot be pulled together with a fundamentally sound approach, then the value of air/ water studies for the process is less and conclusion drawn should be questioned if  applied to actual absorption systems.

1.2   SCOPE/OBJECTIVE OF THE PROJECT

The project  deal first and fore most with the repairs and restoration of the packed absorption tower to a good working condition and the performance of a laboratory experiment using it. The experiment want to measure the relationship between the liquid flow rate and gas flow rate with pressure in the packed absorption tower.