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Climbing Film Evaporator

Objective

To determine the most economic operating conditions for a climbing film evaporator using experimental and theoretical techniques.

Introduction

Climbing film evaporators are primarily used in industry to concentrate a solution. These evaporators are normally operated under a vacuum, so as to lower the boiling point of the solution and aid its passage through the evaporation column. This characteristic, combined with the high heat transfer rate at low temperature and low contact time makes the climbing film evaporator ideal for concentrating heat sensitive materials. Climbing film evaporators are most commonly used to concentrate fruit juice,blackliquor in the Kraft pulping process, dairy products, and in desalination applications. An important feature of this type of evaporator is its low cost. The climbing film evaporator is the cheapest evaporator per unit capacity available. A basic goal of chemical and process engineering is to develop designs and operating conditions for chemical processes that yield a maximum return on capital and operating investment. Some methods of optimizing the operating conditions have been achieved through mathematical techniques: linear programming and response surface experimental design methods are two such techniques. The optimum operating conditions will be determined using the “Simplex” self-directing Evolutionary Operation (EVOP).The EVOP method can rapidly find an optimum operating point in the presence of experimental error. It is a straight forward technique, readily handled by manual calculations, and has been used to improve process operations based on actual plant data. The climbing film evaporator used for this experiment is a QVF pilot-plant climbing film evaporator. It is located at the east end of the Denham semi-scale laboratory. The Climbing Film Evaporator in Detail:

A Brief Description of the Process

The climbing film evaporator has two distinct flows. The first of these is the steam stream. The steam flow rate is controlled by a control valve that is regulated by valve top pressure (VTP).This in turn is controlled by a signal current that varies between 1.5mA and 4mA.The steam flows through the jacket surrounding the outside of the vertical feed column. As it condenses, it releases its heat of vaporization, then passes out of the evaporator through a steam trap at the bottom. The feed stream is injected into the column by a solenoid pump, which is regulated by a 4-20mA control current. The feed flows up the centre of the evaporating column, where it is heated by the steam. A portion of the feed boils to vapor and flows up the evaporation column. Another portion of the feed is forced up the column by the plugs of vapor beneath it and the vacuum maintained from the top of the column. At the top of the evaporation column there is a cyclone that separates the sugar solution and the watervapor into separate streams.
The liquid phase, which is the concentrated product, passes down through a series of air locks before passing out of the system. The vapor stream passes through a condenser, before passing through a series of air locks and out of the system. The entire tube side is operated at a significant vacuum to reduce the boiling point of the solution and aid its travel up the evaporating column. By this process, the sugar solution is concentrated from a concentration range of 7-9% to a concentration of 25% or more.

Sensing Equipment

All the electronics of the climbing film evaporator are operated from the electronic board to the right of the evaporator. The power switch, which powers all the equipment electronics, is located at the top left of the board. To the right of the power switch are the switches for the stirrers of the sugar and water tanks. The other parts of the sensing equipment which are of interest are the steam and feed flow rates and the cycle time. The feed flow rate is determined by an instrument current which controls the speed of the solenoid pump. One may alter this current via a variable resistor; an ammeter dial which provides a readout in milli-amps (mA).The steam flow rate is varied by a steam valve controller which is likewise set by a variable resistor, with the current also indicated on the electronic board. It also has a milli-amp readout to provide an indication of the steam flow rate. The cycle time refers to the time between successive operations of the solenoid sequence that operates the air lock system.Itgoverns the amount of time that the product is allowed to accumulate before it is discharged. A longer cycle time will provide more product per cycle, but less cycles.

The Vacuum

A partial vacuum is preserved in the section of the climbing film evaporator after the evaporation column. This is to aid the flow of the sugar concentrate up the evaporator and to reduce the boiling point of the solution. The vacuum is maintained by a vacuum pump situated next to the LPG boiler. The vacuum line is attached to all parts of the equipment. This is to prevent pressure gradients developing between pieces of equipment, which could lead to damage. The vacuum is operated from the electronic board. The vacuum line from the vacuum condenser can either be directly connected to the equipment, or the line closed and another line to atmosphere opened. This allows the pressurization and depressurization of the equipment when desired.

 

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