Crude Tower Overhead Corrosion Problem

Excerpt taken from the report based on internship experience at ORC-I, Byco Petroleum Pakistan Limited

Detailed explanation can be accessed here:  Full Report

What’re the important parameters to consider in desalter?

This is a very important question to raise as there are many but some certain compulsory parameters to consider that can depict the conditions and predicts the hurdles in further processing means. Following are the important points to consider must:
Water Quality: Dilution water that’s mixed with the oil in the desalter should be salt free to maximum extent as well as has pH to the prescribed level, means neither acidic nor basic. As the dissolved salts ay contribute in fouling of heat exchangers and further cause corrosion problems. At Byco, they are using Boot Water* (overhead product) from accumulator which ensures maximum possible low salt content.
Water Solubility in Crude: This is the most important parameter to counter. At elevated temperature, as rule of thumb, around 0.4% of water dissolve in oil and as the problem of this solubility become more severe when the crude passes through a pre – heat train just before entering the pre – flash fractionating tower. As the temperature rises, water tends to leave the residual salts in the oil which can cause severe damage to equipment. At Byco, they are maintaining the level of two phases inside the desalter in such a way to minimize this possibility.

*Boot Water: An important overhead component
One important factor shall be discussed in the next part i.e. Crude Tower Overhead Corrosion and the parameter that is connected with it is the Overhead water content. At Byco, this water is tested frequently to check the amount of Iron or any traces of contaminated components as well as due to the hydrolysis in desalter HCl vapors are produced which comes out as an overhead side product from the crude tower and pre – flash tower causing severe corrosion to the upper section as well as to the condenser tubes. Thus, the importance of testing is really significant to ensure safety as well as prevention from damages.

Crude Tower Overhead Corrosion Problem, How can it be prevented?
This is one of the most common problem usually encountered in Petroleum Refineries. The problem actually originates from the desalter, the three main salt content present in crude are NaCl, MgCl2, CaCl2 and out of these the heat stability are as under:

                          NaCl > CaCl2 > MgCl2

Hence, Sodium Chloride remains unaffected while Magnesium Chloride tends to hydrolyze at elevated temperature usually at crude tower’s flash zone releasing HCl vapors and the usual severe outcomes are in the form of:
o Loss of atmospheric distillation tower’s tray material/plugging
o Corrosion of condenser tubes and reflux drum

Since, at Byco, a single stage desalter is using currently which approximately removes around 90% of the salt content as compared to 99% removal from double stage removal. Here, the question arises, why don’t Byco utilize this to prevent corrosion? – The answer is – Magnesium Chloride is a troublesome salt and its removal is not that easy whether to use a single stage or multi stage. The HCl attack is continuously regenerated by reaction with H2S because usually there’s an excess of H2S inside the crude tower.

 First, HCl & Fe react to yield FeCl3 followed by a series reaction with FeCl3 & H2S react together to give FeS2 + HCl

This unfortunately leads to another factor to consider, HCl liberated out from the crude tower has usually a greater affinity for water and as long as no water is present the HCl vapors are non – corrosive in nature but as the overhead water droplets tend to condense, it dissolves all HCl and become highly corrosive in nature. The pH of HCl vapor without mixing with water condensate is about 2 while after dilution it increases to about 5.5 – 6.5.
On the similar manner, naturally crude oil contains Naphthenic Acid which is highly corrosive in nature whether in vapor form or condense form hence, at Byco, they frequently observe the acid content present in crude and for this they perform TAN (Total Acid Number) testing.