WO2013068934A1

WO2013068934A1 - Evaporator arrangement incorporating a condenser

Info

Publication number: WO2013068934A1
Application number: PCT/IB2012/056217
Authority: WO
Inventors: David John Love; Leon Smith; Stephen David Peacock
Original assignee: Tongaat Hulett Limited
Priority date: 2011-11-11
Filing date: 2012-11-07
Publication date: 2013-05-16
Also published as: ZA201304119B

Abstract

This invention relates to an evaporator arrangement, and more particularly but not exclusively, to an evaporator arrangement for use in removing water from sugar juice. The invention furthermore relates to an evaporator arrangement that incorporates a condenser for selective condensing of specific vapours. The evaporator arrangement includes an evaporator vessel for receiving the sugar juice, a heat transfer arrangement for transferring heat to the sugar juice inside the evaporator vessel; and a vapour outlet where a vapour stream evaporated from the sugar juice exits the evaporator arrangement. The arrangement is characterized in that a condensing arrangement for condensing specific vapours from the vapour stream is located upstream of the vapour outlet.

Description

EVAPORATOR ARRANGEMENT INCORPORATING A CONDENSER

BACKGROUND TO THE INVENTION

THIS invention relates to an evaporator arrangement, and more particularly but not exclusively, to an evaporator arrangement for use in removing water from sugar juice. The invention furthermore relates to an evaporator arrangement that incorporates a condenser for selective condensing of specific vapours.

During the sugar manufacturing process, after sugar juice has been extracted from cane, but before crystal growth commences, the clear juice must be concentrated to syrup by the removal of water by evaporation. This process takes place in an evaporator, where heat is transferred to the sugar juice in order for the water to evaporate. To improve the efficiency of the water removal step a process known as multiple effect evaporation is used. Multiple effect evaporation is a scheme where juice is boiled in series in several vessels, with steam fed to a first vessel only. Vapour from the first vessel boils the juice in a second vessel, vapour from the second vessel boils the juice in the third vessel, and so on until vapour from the final vessel goes to waste. Vapour bleeds streams generated during evaporation, whether single or multiple effect, are also used extensively for heating applications in other parts of the sugar manufacturing plant. The use of the vapour bleed streams are very important form an energy efficiency point of view, which will become even more important as the cane sugar industry, and indeed industry as a whole, is moving towards more efficient operation.

Volatile acids, such as for example acetic acid, may be present in the vapour bleed streams referred to above. These acids can be volatilised at any stage of the evaporation process where the required temperature, pressure and concentration are present. However, these conditions are mostly present in the second effect vessel of a multiple effect evaporator station that has been designed for high levels of energy efficiency. The effect if this is that the acids present in the sugar juice are volatilised into the vapour stream.

When a vapour stream containing a volatile acid is subsequently condensed within downstream heat transfer equipment, it leads to a high rate of corrosion, because the acid has a higher boiling point temperature than water and it therefore condenses in a localised fashion on the first portion of the heat transfer surface.

Although the dosing of corrosion-inhibiting chemicals into the vapour stream has been practiced in the past, this has proved expensive and results have been inconsistent. Consequently there is currently no cost effective way to avoid the high rates of corrosion caused by the volatilised acid.

It is accordingly an object of the invention to provide an evaporator incorporating a condenser that will, at least partially, alleviate the above disadvantages. It is also an object of the invention to provide an evaporator which will be a useful alternative to existing evaporators.

SUMMARY OF THE INVENTION

According to the invention there is provided an evaporator arrangement for use in the evaporation of sugar juice, the evaporator arrangement including:

an evaporator vessel for receiving the sugar juice;

a heat transfer arrangement for transferring heat to the sugar juice inside the evaporator vessel; and

a vapour outlet where a vapour stream evaporated from the sugar juice exits the evaporator arrangement;

characterized in that a condensing arrangement for condensing specific vapours from the vapour stream is located upstream of the vapour outlet.

There is provided for the condensing arrangement to be located in an outlet of the evaporator vessel, or inside a vapour/liquid separator vessel of the evaporator arrangement.

The condensing arrangement may include a cooling means located in a flow path of the vapour stream, so as to condense a small portion of the vapour stream.

The cooling means may be in the form of at least one elongate heat transfer tube arranged about the vapour outlet. The tube may be arranged to form a plurality of U-shaped convolutions that are circumferentially spaced about the vapour outlet.

Preferably the tube is configured to form two concentric, but angularly offset, rings around the vapour outlet when seen in cross-section.

There is provided for a condensate tray to be located operatively below the heat transfer tubes.

The condensate tray is positioned adjacent lower ends of the heat transfer tubes.

In a preferred embodiment the evaporator of the evaporator arrangement is a falling film evaporator, and the condensing arrangement is located inside a liquid / vapour separator vessel of the evaporator arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described by way of a non- limiting example, and with reference to the accompanying drawings in which:

Figure 1 is a cross-sectional side view of an upper end of a liquid / vapour separator of an evaporator arrangement in accordance with the invention, with a condensing arrangement located inside the liquid / vapour separator; and

Figure 2 is a cross sectional plan view through line A-A' of Figure 1. DETAILED DESCRIPTION OF INVENTION

Referring to the drawings, in which like numerals indicate like features, a non-limiting example of part of an evaporator arrangement in accordance with the invention is generally indicated by reference numeral 10. In this particular example, the part that is focused on is a liquid / vapour separator vessel 20 of the evaporator arrangement 10.

The liquid / vapour separator arrangement of the evaporator arrangement receives vapour and possibly some entrained liquid from the evaporator vessel, and assists in the separation of such entrained liquid from the vapour. A vapour outlet 21 is provided at the top of the vessel, with liquid typically falling down into the vessel under the influence of gravity. At this time a vapour stream leaving the vessel 20 may however still include undesired vapours, for example that of volatilized acids such as acetic acid. A condensing arrangement 30 is therefore provided to aid in the selective removal of such undesired vapours from the vapour stream.

The condensing arrangement 30 in this example takes the form of a continuous heat transfer tube 31 that is shaped to form a plurality of U- shaped, serpentine convolutions that are circumferentially spaced about the vapour outlet. When viewed in cross-section (as is seen in Figure 2) it will be seen that each convolution is disposed at an angle relative to a radius of the vessel, so as to form an inner ring 31.1 of heat transfer tubes, and a concentric outer ring 31.2 of heat transfer tubes. The reason for this is to ensure that vapour flowing across the heat transfer tubes 31 , in the direction of arrow B, is exposed to the largest possible heat transfer area, so as to optimize condensation.

A condensation tray 35 is located immediately below the heat transfer tube(s) 31 , and collects condensate (i.e. the volatile acid) that condenses from the vapour stream. An outlet 36 is provided to remove the volatile acid from the condensation tray 35, and thus the vessel 20. A gap 37 between the tray 35 and bottom ends 32 of the serpentine heat transfer tube 31 is relatively small, which aids in ensuring that the vapour stream ("B") flows across the heat transfer tubes.

The heat transfer tube 31 , condensation tray 35 and outlet 36 is typically made from a corrosion resistant material, for example stainless steel, in order to withstand potential corrosion caused by the condensed acid vapour.

Due to the volatilized acid having a higher boiling point than water, the new design aims to remove the acid from the vapour by reducing the vapour stream temperature using a cooling media such as water. By carrying out this removal or stripping operation within the body of an existing vessel within the evaporator station, it is possible to protect the downstream heat transfer equipment at a relatively low cost.

More specifically, cold make-up water on its way to the boiler station is passed through the inside of the heat transfer tubes. Heat transfer will take place from the hot vapour to the cold water within the tubes, thereby heating the water stream. The process is designed in such a way as to condense approximately 1 ,5% of the vapour flow (or more). This small amount of condensation will preferentially remove the majority of the higher-boiling-point acetic acid from the vapour stream, thereby protecting any downstream heat transfer equipment from corrosion. The condensate generated on the outside surface of the tubes will be collected in the tray and discharged to the outside of the vessel via a U-leg (which avoids the blow-through of steam from the separator vessel to atmosphere). No energy is wasted as a result of this condensation of vapour, as it is being used to heat the boiler feed water make-up stream, which would conventionally be heated using another steam source. It will be appreciated that the above is only one embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention.

Claims

CLAIMS:

1. An evaporator arrangement for use in the evaporation of sugar juice, the evaporator arrangement including:

an evaporator vessel for receiving the sugar juice;

a heat transfer arrangement for transferring heat to the sugar juice inside the evaporator vessel; and a vapour outlet where a vapour stream evaporated from the sugar juice exits the evaporator arrangement; characterized in that a condensing arrangement for condensing specific vapours from the vapour stream is located upstream of the vapour outlet.

2. The evaporator arrangement of claim 1 in which the condensing arrangement is located towards an outlet of the evaporator vessel, or towards the outlet of a vapour/liquid separator vessel forming part of the evaporator arrangement.

3. The evaporator arrangement of claim 1 or claim 2 in which the condensing arrangement includes a cooling means located in a flow path of the vapour stream, so as to condense a small portion of the vapour stream.

4. The evaporator arrangement of claim 3 in which the cooling means is in the form of at least one elongate heat transfer tube arranged about the vapour outlet.

5. The evaporator arrangement of claim 4 in which the tube is arranged to form a plurality of U-shaped convolutions that are circumferentially spaced about the vapour outlet.

6. The evaporator arrangement of claim 5 in which the tube is configured to form two concentric, but angularly offset, rings around the vapour outlet when seen in cross-section.

7. The evaporator arrangement of any one of claims 3 to 6 in which a condensate tray is located operatively below the cooling means.

8. The evaporator arrangement of claim 7 in which the condensate tray is positioned adjacent lower ends of the heat transfer tubes.

9. The evaporator arrangement of any one of the preceding claims in which the evaporator of the evaporator arrangement is a falling film evaporator, and the condensing arrangement is located inside a liquid / vapour separator vessel of the evaporator arrangement.