A dealkalizer is a pre-treatment device commonly seen in boiler rooms which is responsible for removing carbonate alkalinity in the feed water before it reaches the boiler. In many cases, the water chemistry parameter which hits its limit first is alkalinity. Thus, by reducing the alkalinity in the feedwater, we can decrease blowdown volume, increase the cycles of concentration, and effectively decrease energy consumption.
The following blog will touch on one of the many challenges when operating a steam plant, condensate corrosion. We will review some dealkalization technologies which address these challenges – how they work, what to expect and some design considerations for each.
The first step in boiler water treatment is to analyze the untreated raw water to determine the type and amount of impurities present. The solution for your plant depends on the quality of your incoming water. What may be right for one plant won’t necessarily be right for the same plant in a different geographic location. Great Lakes Water is considered surface water and makes up some of the purest water in Canada. This surface water typically “goes further” than ground water from an operating cost perspective as we can operate systems at higher cycles of concentration, blow down less and require less water treatment chemicals. Ground or well supplied water is typically higher in dissolved solids (calcium, magnesium, carbonates, bi carbonates, etc.). In short, not all water sources are created equally and boiler water treatment programs will vary depending on the source and quality of the feed water.
Naturally occurring alkalinity comes in the form carbonate and bicarbonate. When alkalinity enters the boiler, it breaks down into OH– and CO2. CO2 (gas) exits with the steam and forms carbonic acid as the steam condenses (pH < 6.0). Left untreated, low pH water can potentially rot out the condensate network.
Some physical indicators of carbonic acid attack are the thinning of carbon steel piping along the bottom curvature of the pipe, leakage of condensate at points of low wall thickness (ie. threads) and erosion at control points like traps or control valves.
A common chemical treatment is to add neutralizing amines to the boiler feedwater or steam header. When the steam condenses, the neutralizing amine will neutralize the effect of the carbonic acid and maintain the pH at an acceptable level to prevent corrosion. The degree of alkalinity in the boiler feedwater determines amine requirement (ie. the required application rate for volatile amine is likely significantly greater for the boiler operating on rural ground water than the same boiler operating on great lakes surface water).
This leads us into the focus of our blog – two different technologies which effectively remove alkalinity before water enters the boiler – significantly reducing or even eliminating the need for neutralizing amine condensate treatment and offering water and energy savings.
A quick review of ion exchange is required to understand dealkalization and we’ll use the water softening process as an example, as most boiler operators are very familiar with this. Water softeners use strong acid cation (SAC) resin for ion exchange. SAC resin has an affinity for divalent ions (Calcium, Magnesium) meaning that the resin wants to grab a hold of these divalent ions as they’re passing through the bed and exchange them with the sodium ions. Once resin is saturated and there are no more available free resin beads for ion exchange, a brute force wash of the SAC bead with sodium chloride (salt) brine is required.
A chloride cycle dealkalizer works just like a water softener but a strong anion base (SAB) resin is used for ion exchange. In this case, the resin has an affinity for divalent anions (carbonate, bicarbonate). Carbonate and bicarbonate are exchanged for chloride ions. Once resin is saturated, a brute force wash of the SAB bead with sodium chloride (salt) brine and caustic is required. Caustic is utilized to enhance resin efficiency and boost effluent pH.
The easiest way to understand this process as three steps:
The first step in the weak acid cation (WAC) dealkalization process is the stoichiometric regeneration with acid (either H2SO4 – sulfuric acid, or HCl – hydrochloric acid). Both water softening and chloride cycle dealkalization are saturation reactions and not stoichiometric meaning they are “less efficient” reactions. In the case of WAC dealkalization, stoichiometric means if we have 1000 ion exchange cites, we only need 1000 hydrogen ions. WAC resin has an affinity for H+ ions. As such, H+ ions are loosely bound to WAC resin following the regeneration step. The resin is said to be in “hydrogen form” at this point. With the resin in hydrogen form following regeneration, WAC is ready for service.
If anyone has ever dropped a small volume of acid on a piece of calcium carbonate scale, you’ll remember that “fizz” you observed. The “fizz” is the result of the reaction of the carbonate and the hydrogen – the product of this reaction is gaseous carbon dioxide.
The same thing happens in the WAC resin bed. The H+ ion from the resin bead being in “hydrogen form” essentially destroys the carbonates and bicarbonates which are passing through the bed. The product of this reaction is water and carbon dioxide.
As soon as the hydrogen is relieved of duty having destroyed the carbonates/biocarbonates, this frees up the ion exchange site. The ion exchange site is “hungry” and the system is unbalanced. The WAC resin bead now has a negative ion exchange site open and we have calcium and magnesium (hardness) ions stranded with nowhere to go. The negative exchange sites want to take on something so they grab on to the calcium and magnesium.
Once the WAC bed has reached its capacity, the regeneration cycle is initiated once again. The resin bed is regenerated with acid. Since the WAC resin has an affinity for hydrogen and not calcium/magnesium, the calcium/magnesium ions readily wash off and are flushed down the drain as the H+ ion loosely binds to the resin bead again. The resin is now back in its hydrogen form and ready to go back into service.
So as we said before, the hydrogen is destroying the carbonate and bicarbonate and the product is water and carbon dioxide. The next step is we need to get rid of that carbon dioxide such that it doesn’t get to the boiler. This is accomplished with a degasifying tower and this is installed downstream of the WAC. The carbon dioxide and water are separated – the gaseous carbon dioxide is sent out to atmosphere while the water is sent to the next step.
Some important design considerations for the chloride cycle dealkalizer are:
Some important design considerations for the WAC dealkalizer are:
We’ve gone over two different technologies which will effectively remove alkalinity and decrease water, chemical and energy spending. Below is a case study which illustrates the differences in savings between both technologies and furthermore, how this changes with different feedwater sources (great lakes water vs. well water).
The food plant operating on great lakes water runs at 10 cycles of concentration initially and this is increased to 15 COC with the installation of a chloride cycle dealkalizer. The plant operating on well water runs at 4 COCs initially and this is increased to 12 COCs with the installation of a chloride cycle dealkalizer. The approximate ROI for this project was 5 years for the great lakes customer and 1 year for the well water customer.
For the WAC dealkalizer savings breakdown, both options result in an increase in COCs in the boiler and similar ROIs to the chloride cycle dealkalizer (approximately 6-7 years for great lakes water and 1 year for well water).
Again, not all water sources are created equally. Operating costs for a plant in Toronto will be very different for that same plant in rural Ontario which draws from a well. Neither dealkalizer option for the Great Lakes water steam plant offered a payback which would justify the project, but this was very different for the rural water customer.
Whenever possible, we should strive to “remove rather than treat for” and in order to formulate a proper pre-treatment solution, a thorough understanding of incoming water quality is required.
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