White Paper: Impact Of HTA Dosing On Vent Stream Acidity Of An Acid Dosed MSF Plant

By Dorothy Neu and Keith Minnich
Aqua-Chem ICD Division of Aquatech International Corporation
Milwaukee, Wisconsin

WEB Aruba N.V. operates seven Aqua-Chem ICD desalination plants at their power and water utility. The desalination plants are long tube, brine recirculation Multistage Flash (MSF) plants. These MSF plants have capacities of 6000 TPD (1.6 USMGD) each with a design Gain Output Ratio (kg of distillate produced/kg of steam input) of 10.7. The plants were designed to have sulfuric acid injection for scale control. In November 2001, WEB Aruba began a trial using high temperature additive (HTA), BetzDearborn's HT-20, to replace the current scale control method.

By injecting sulfuric acid into the make-up seawater flow, calcium carbonate scale is controlled by converting the bicarbonate ion into carbon dioxide. The carbon dioxide is removed from the system through a deaerator. Because the residual m-alkalinity of the seawater make-up stream is very low, there is very little bicarbonate remaining to decompose into carbon dioxide in the plant.

In the case of HTA injection, bicarbonate in the recirculating seawater stream should be released as carbon dioxide as the bicarbonate decomposes. Previous analytical models and field measurements have shown this to be true [Ref 1]. If the carbon dioxide released would become redissolved in the condensing vapors forming carbonic acid, corrosion could take place. Corrosion that is attributed to carbon dioxide has been reported in MSF plants in the Middle East [Ref 3]. For this reason, even though the capital cost is higher, modern HTA dosed MSF plants are made of stainless steel shells. In the case of the plants at WEB Aruba, the presence of carbonic acid could affect the 90-10 Copper Nickel tubes and tubesheets in the vent section, or more significantly, the carbon steel walls, roof, and vent manifold. The vent and vacuum systems might also have to be redesigned to accommodate release of carbon dioxide throughout the stages rather than primarily in the deaerator.

As a part of the HTA dosing trial, a vent and distillate sampling system was designed to monitor acidity due to carbon dioxide in the MSF plant. This paper will discuss the results of the HTA dosing trial with respect to the effect on venting and corrosion.