"Method for online cleaning of fouled PVC fills of cooling towers exposed to air/water borne dust/bio-fouling."
FIELD OF INVENTION
This invention relates to a method for online cleaning of fouled PVC fills of cooling towers exposed to air/watpr borne dust/bio -fouling.
BACKGROUND OF INVENTION
Many of the power stations are using recircultaing type cooling water system for condensation of steam. In the recircultaing system the cooling water is passed through condenser tubes to condense the steam coming from low pressure turbines. The heated water is taken to cooling towers where the hot water is converted into small droplets and the air is allowed to take away the heat of the water droplets. There are different types of cooling towers such as natural draft, forced draft, etc. since its first introduction to the market in the 1970s, cooling tower film fill technology has significantly increased thermal performance and reduced the size of cooling towers. However, the narrow spaces between film fill sheets make them susceptible to fouling. Without proper chemical treatment, deposits can accumulate within the film fill resulting in reduced tower efficiency, increased fouling and plugging of the fill. These phenomena could eventually lead to collapse of the tower structure. This document describes a new approach to remedy the high efficiency film fill fouling problem in power plants. The plants have a long history of fill fouling problems due to a very complex make-up water chemistry and
dusty environmental conditions. Normal cleaning methods were not able to remove the fouling from the fills and thus restore the efficiency of cooling towers. Unique chemical cleaning procedures have been developed that can remove the dust based foulants during operation of the units effectively and improve the performance of the cooling towers and also remove calcium carbonate based deposits. The procedure developed also helps in cleaning the cooling water system of suspended material & biofilm.
Cooling towers are important part of power plants & many other industries where the water used for cooling the process fluid such as steam is again cooled by forming a spray and the forced/natural draft of air cools the droplets of water. The cooled water is again sent back to the heat exchanger/condenser for cooling the process fluid. Fills help in proper cooling of circulating water. The PVC Film type fills are very efficient in cooling the circulating water, but they are equally prone to fouling from air/water borne dust particles and microbes. Once fouling starts in the fills the dust/suspended particles stick to the wet surface to the fills and with time start clogging the narrow corrugated space of the fill packs. This results in reduced flow of water through the fill packs and thus reduced flow of water through the fill packs and thus reduced efficiency of the cooling towers. This also results in excessive loads on the cooling tower structures and damages them. In extreme cases it can result in collapse of the cooling towers. Normal mechanical cleaning methods are tedious, time consuming and damage the fills.
Film fill is a densely packed plastic, often PVC. It creates a large wetted surface that increases evaporation but is prone to deposit buildup and plugging. Plugging can be due to microbiological organisms, oils, scale, mud, corrosion products and water treatment inhibitors. Film fill that is partially plugged can reduce cooling tower efficiency. Severely fouled film fill has been known to collapse into the cooling tower basin.
OBJECTS OF INVENTION
The main object of this invention, is to develop a method for online cleaning of fouled PVC fills of cooling towers.
The other object is to develop a method for online cleaning of fouled PVC fills of cooling towers explosed to air/water borne dust/bio fouling.
Another object is to develop a suitable cleaning method that can remove the foulants from the cooling tower fills without removing the fill packs.
Yet another object is to develop a suitable cleaning method that can remove the foulants in unit operating conditions.
STATEMENT OF INVENTION
This invention relates to a method for online cleaning of fouled PVC fills of cooling towers exposed to air/water borne dust/bio-fouling using a combination of an oxidizer like hydrogen peroxide at a concentration range of 0.1 to 0.2% by weight; surfactant like Linear alkyl benzene sulphorate 5-20ppm and dispersant like Polyacrylate, 2-15ppm wherein
fouling material is bacteria and silt/clay/suspended dust and using Ethylenediamine tetra acetic acid in the range of 0.05% to 0.2% weight by volume at a pH range of 9.5 to 11 in the presence of corrosion inhibitor Rodine 2000 (Henkel Chemicals) or equivalent inhibitor (having similar composition as Rodine 2000, which is a proprietary chemical of Henkel chemicals) for dissolution of calcium carbonate based scale deposit from PVC fills.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
A potion of the fouled PVC Film type fill pack was obtained from site and different chemicals were used to clean the fouled fills. The first photograph shows the condition of the as-received fouled fill pack. The second photograph shows the condition of the cleaned fill pack. The cleaning was effected using the claimed cleaning chemicals/ procedure and more than 90% foulants were removed from the closely packed fills.
DETAILED DESCRIPTION OF INVENTION
Two online chemical methods have been developed to clean the fouled fills of the cooling chemical towers. 'One method consists of treatment of cooling towers with hydrogen peroxide, surfactant and a polymeric dispersant to remove dust/silt & microbiological based fouling. The other method consists of treatment of cooling towers with EDTA at higher pH (above 9.5 to 11) in presence of corrosion inhibitor to remove calcium carbonate based scale/fouling deposits. Both methods are for online cleaning but can also be used off-line. In situations where both type of
foulants are present, than first the system is to be treated with EDTA
solution followed by washing with Hydrogen
peroxide/surfactant/dispersant. Subsequently periodic washing with Hydrogen perodixe/surfactant/dispersant can help in preventing further fouling of PCV film type fills of cooling towers.
Online chemical methods have been developed based on the principle of oxidizing the biomass and then removing the fouled particles using a combination of anionic surfactant and polymeric dispersant from the fill packs (applicable for foulants based on dust/silt and microbiological species). For foulants consisting of calcium carbonate scale and dust/silt along with microbiological species, initial washing with EDTA (ethylenediamine tetra acetic acid, a chelant) at higher pH with corrosion inhibitor has also been developed. The methods developed can be used for cleaning the fouled fill packs without stopping the unit or cooling towers. The method is safe for the other materials used in the cooling water system of the power plants. The method utilizes oxidizing agents such as Hydrogen Peroxide supplemented by Chlorine, Chlorine dioxide, Sodium hypochlorite, etc for killing the biomass followed by removal of particulate matter sticking to the fill surfaces by utilizing the surfactant and the polymeric dispersant which help the dislodged suspended matter to remain in suspension till they are taken out of the system by blow down. EDTA helps in dissolving calcium carbonate based scales.
Fill design has changed dramatically in recent years from wood or plastic splash-type to plastic film-type. Film fill is a density packed plastic often PVC. It creates a large wetted surface that increases evaporation but is prone to deposit buildup and plugging. Plugging can be due to
microbiological organisms, oils, scale, mud, corrosion products and some water treatment inhibitors. Film fill that is partially plugged can reduce
cooling tower efficient. Severely fouled film fill has been known to collapse into the cooling tower basin.
Since its first introduction to the market in the 1970s, cooling tower film fill technology has significantly increased thermal performance and reduced the size of cooling towers. However, the narrow spaces between film fill sheets make them susceptible to fouling. Without proper chemical treatment, deposits can accumulate within the film fill resulting in reduced tower efficiency, increased fouling and plugging of the fill.
These phenomena could eventually lead to collapse of the tower structure.
Many of the power stations where PVC film type fills have been used reported fouling of the fill packs results into loss of heat rate and in some cases damages to the support structures of cooling towers. The weight of many fill packs had gone up (almost to 180 Kg against the virgin weight of PVC fill pack of 20 Kg). The efficiency of cooling towers had come down substantially resulting into loss of heat rate 85 loss to the station since due to loss of efficiency of cooling towers, the temperature of cooling water also increases causing problems in condenser vacuum.
Investigation team members visited the stations and analysed the problem. Based on the analysis it was observed that improper water treatment of cooling and water and suspended particles/bacteria are mainly responsible PVC fill pack fouling.
Considering the various factors and causes identified, it was decided that R&D should develop a comprehensive non-proprietary chemical treatment for cooling water systems for such to resolve the problem of CT fill fouling and other related problems in CW system. Accordingly R&D had taken up the development of chemical treatment program and studies to develop a procedure to clean the fouled PVC fill packs online.
R&D developed a chemical procedure for online cleaning of fouled PVC fill packs and a chemical treatment program was also developed to prevent further fouling of the fill packs. The cleaning procedure consists of oxidizing the adherent biofilm on Fill packs by a strong oxidizer like
Hydrogen peroxide (0.1 - 0.2%) and then detaching the surfactant (Linear alkylbenzene sulphonate, 5-20 ppm) and a dispersant
(Polycrylate, 2-15 ppm). Using this procedure satisfactory cleaning of the fouled CT fills could be achieved (Photographs as given above). Periodic cleaning of the cooling water System can help in controlling further fouling of CT fills and thus improve efficiency of the cooling towers.
Further it was observed that with prolonged operation with fouled fills, local evaporation of water takes place resulting in deposition of calcium carbonate based scale, which is quite adherent and reduces the efficiency of the cooling towers. An online chemical cleaning method using EDTA (Ethlenediamine tetra acetic acid) at higher pH and using a
corrosion Inhibitor was developed for this application. Combination of both the methods can help in removing the fouling deposits from PVC film type fills of cooling towers as well as from the CW system, thereby improving the overall performance, life and efficiency of cooling towers/cooling water systems.

We Claim
1. A method for online cleaning of fouled PVC fills of cooling towers
exposed to air/water borne dust/bio-fouling using a combination
of an oxidizer like hydrogen peroxide at a concentration range of
0.1 to 0.2% by weight; surfactant like Linear alkyl benzene
sulphonate 5-20ppm and .dispersant like Polyacrylate, 2-15ppm
wherein fouling material is bacteria and silt/clay/suspended dust
and using Ethylenediamine tetra acetic acid in the range of 0.05%
to 0.2% weight by volume at a pH range of 9.5 to 11 in the
presence corrosion inhibitor Rodine 2000 (Henkel Chemicals) or
equivalent inhibitor having similar compositions as Rodine 2000
for dissolution of calcium carbonate based scale deposit from PVC
fills.
2. A method for online cleaning of fouling deposits consisting of
dust/silt, microbial species and calcium carbonate scale deposit
as described herein.