Corrosion Assessment of a Process

A corrosion assessment of a process typically focuses on the feed and products. However, the impact of reaction intermediates must also be considered. Let’s consider a plug flow reactor in which the series reaction A -> B -> C takes place. Assuming the reactions are first order and irreversible, then the following set of equations models the reaction system [1]:


Ca/Cao = exp (-k1 * η)
Cb/Cao = k1/(k2-k1) * [exp (-k1 * η) - exp (-k2 * η)]
Cc = Cao – Ca – Cb


η is defined as V/νo. According to this scheme, Ca decreases along the length of the reactor, while Cb reaches a maximum and then decreases and Cc increases along the length of the reactor. The maximum concentration of “b” in the plug flow reactor is determined as:


Cbmax = (k1/k2) k2/(k2-k1)


The maximum concentration of “b” depends on the k2/k1 ratio. In assessing the corrositivity of a catalytic system, the focus is typically on the feed and the products. However, depending on the reaction system the impact of the intermediate product(s), in this case product “b,” must also be considered, especially if the process is operated at 100% conversion and intermediates are not detected in the product stream.


In the conversion of triacylglycerides in vegetable oils several investigators [2, 3, 4] have concluded that the triacylglycerides first undergo hydrogenolysis over a variety of catalysts to form the corresponding fatty acids and propane. The fatty acids then undergo decarboxylation, decarbonylation or hydrodeoxygenation further down the reactor to form paraffins, carbon oxides, and/or water. Fatty acid free triacylglycerides and these products have total acid numbers (TAN) of 0 mg KOH/gr. The fatty acid intermediates are carboxylic acids that have TANs of ~200 mg KOH/gr (C18 fatty acid) and could be potentially corrosive. While these reactions are conducted in trickle bed reactors, the plug flow analysis is still applicable. These results emphasize that in the selection of the metal used to build the reactor, the reaction kinetics and the corrosion properties of intermediates must be taking into account.


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References:


1. Levenspiel. O., Chemical Reaction Engineering, John Wiley & Sons, New York, 1972.
2. Morgan, et al., Topics in Catalysis, Vol. 53, No. 11-12, July 2010 (abstract retrieved from Internet on 7/18/2010)
3. Boda, et al., Applied Catalysts A: General, Vol. 374, No. 1-2, Feb., 2010, pp. 158-169.
4. Guzman, et al., Catal. Today, In press (2010).