Coalescers

Filters

LOW pH UNDERDEPOSIT CORROSION ON A179 CARBON STEEL

Ms. Cheah Phaik SimTitan Petrochemicals Sdn. Bhd.

Pasir Gudang, Johor.

ABSTRACT In a quench system of an ethylene plant, water is separated from hydrocarbons and carbon solids via filters and coalescers and further stripped off of dissolved hydrocarbons before being vaporized into steam. It is not uncommon corrosion occurs in this system comprising of carbon steel equipment and piping. The type of corrosion that has been encountered is low pH under deposit at the reboiler heat exchangers. Chemicals such as amines are added to control corrosion, thereby giving rise to either a change in the bulk corrosiveness of the environment. In this paper, corrosion problems that arise from contaminants in the quench water are discussed and some chemical solutions are also presented.

INTRODUCTION

Process Description In an ethylene plant, direct contact of water with the pyrolysis furnace effluent from thermal cracking is the stage of cooling. This step not only cools the gases, but also condenses the dilution steam and some of the hydrocarbon portion of the gas. The resultant water from the quench water bottoms is a mixture of circulating water, condensed dilution steam and condensed hydrocarbon. Part of the hot quench water is used for low level heating services throughout the plant. Before the water can be reused for dilution steam, it must be cleaned for hydrocarbon and solids. Equipment for the clean up of the quench water includes oil/water separators, filters, coalescers and stippers. The quench water form the oil separator is split with some being sent back to the quench water recirculating loop and some going to dilution steam make up. This is sent to a second more effective water hydrocarbon phase separation via filters and coalescers prior to being sent to a water stripper for further removal of dissolved hydrocarbon and gases. The “clean” quench water is then fed directly to the dilution steam generator and vaporized to steam via reboilers using medium pressure steam as heating medium. Figure 1 shows a typical flow diagram of a quench system.

FIGURE 1 Typical Quench Flow Diagram

Chemical Treatment Program Applied

Blowdown

DSG Reboiler

Quench Water Tower

Water Stripper

Quench Water Exchangers

Quench Water Pump DSG Feed

Pumps

Furnace

Dilution Steam Generator (DSG)

Blowdown Cooler

The quench water contains high levels of both soluble and insoluble oils. The soluble oils are removed at the separation equipment upstream of the dilution steam generator while the insoluble oils which are typically aromatic oils cannot be separated. Acids from the cracked gas condensed in the quench water causing low pH condition. Therefore, emulsion breakers are injected to allow the insoluble oils to be removed better in the oil/water separator and coalescers. Amine is injected in the quench system to neutralize the acids and bring up the pH of the quench water.

Corrosion Encountered The primary corrosion encountered is the low pH under deposit type. Many failures associated with this attack were experienced on the DSG reboilers. The upper half of the reboiler tube bundle was found to be subjected to attack in the form of ditches with rough rolling contour. The thick layer of deposits were essentially corrosion products. The deposit sample was found to be mostly of iron oxide (98.8%), with trace amounts of silicon, aluminum, phosphorous and sulfur species. The iron oxide is either coming from the tube itself due to corrosion or a carryover from the upstream equipment and process. Presence of oil was also noted.

FIGURE 2 Reboiler Tube Failure FIGURE 3 Deposit Sample

Materials of construction The selection of tube bundle material for the DSG reboilers is ASTM A179. This carbon steel has a chemical composition of Carbon 0.06 – 0.18%Manganese 0.27 – 0.63%Phosphorus, max. 0.035%Sulfur, max. 0.035%

CORROSION MECHANISM The definition of corrosion is the undesired attack of a material as a result of electrochemical reactions with components in the environment. Electrochemical corrosion is resulting from reactions between a metal surface and an ion-conducting environment such as water. There is at least two reactions involved, namely anodic reaction during which a metal dissolves while releasing electrons Fe = Fe2+ + 2eand a cathodic reaction, in which the electrons formed are removed 2H+ + 2e = H2

Organic contaminants decompose under high pressure and temperature to form organic acids, thus reducing pH in the process water to 5.0 and lower. Acid promotes the iron corrosion reactions by supplying a reactant, H+. The overall reaction is: 2H+ + 2RCOO- + Fe = Fe(RCOO)2 + H2

corrosionproduct

Corrosion of ferrous metal piping results in the formation of iron oxides products many times the volume of the metal loss from the pipe wall. Acid attack is usually in the form of thinning of all surfaces.

Under deposit corrosion occurs when corrosion products accumulate on a metal surface. This is localized corrosion caused by oxygen concentration cells. As deposits form on metal surfaces, oxygen levels under the deposit are reduced and an electrochemical corrosion cell is set up. This can cause the metal under the deposit to corrode rapidly, leading to severe localized corrosion and perforation of the metal surface.

DISCUSSION The contaminants in the “clean” quench water contain organic acids (majority acetic, some propionic and butyric), ammonia and some carbonates from the cracked gas. Typical acetic acid level sampled from the blowdown is 300 to 600 ppm. Ammonia at stripper outlet is about 20 to 50 ppm. The corrosion protection rendered by the chemical treatment program uses a low volatility neutralising amine to neutralize all acid species and elevate the water pH to 8.0 to 9.0. Ammonia, an alkaline compound will have the same effect as the amine. It is however, much more volatile, higher neutralising capacity and basicity than the amine. pH measured of the bulk water were averaging 75% within the range of 8.0 to 9.0. Only 23% were below pH of 8.0 and 2% above pH of 9.0. Figure 4 shows the operating pH from April 2000 to June 2001. At pH below 8.0, the passive magnetite film that protects the carbon steel would be destroyed and as such, significantly increases general corrosion rate.

FIGURE 4 Operating pH from April 2000 to June 2001

However, visual inspection revealed the tube leaks were not related to tube wall thinning as a result of general pH corrosion. The failures were related to localized under deposit corrosion and the leaks were located at the upper section of the reboiler where bulk of the evaporation is expected to take place. The under deposit corrosion developed as a result of concentration of organic acids (mostly acetic) underneath the porous deposit. When the bulk water at alkaline pH enters the porous deposit and with high percentage of evaporation taking place at the upper section of the bundle, the condition for setting up concentration cells under the deposits like a miniature boiler prevails. The high volatility ammonia will vaporize and travel with the steam upsetting base/acid ratio in the water phase. This could result in a localized low pH environment that is highly corrosive towards magnetite and carbon steel and ultimately leading to tube leak. Figure 5 illustrates this phenomenon. The conditions favorable for under deposit corrosion effects to take place are: bulk water temperature is approaching boiling point

high heat flux (heat absorbed per unit area)

FIGURE 5 Illustration of Under Deposit Low pH Corrosion

Both of these conditions prevail when the percentage evaporation in the bulk water is high. This explains why the tube leaks are found only at the upper section of U-tube bundle and in case history presented in Table 1 that the medium pressure steam reboilers with low evaporation did not have any tube leaks with the amine program.

TABLE 1 Case History of DSG Reboilers

Plant Reboiler Heating Medium % Evaporation Remarks

A Quench Oil 7.6 No corrosionB MP Steam 5.8 No corrosionC MP Steam 35 CorrosionD MP Steam 30 CorrosionE Quench Oil 8.2 No corrosion

Condensate modeling system (CMS) simulated water & steam chemistries in steam generators in the presence of treatment chemicals and common contaminants. A simulation of the impact of concentration effect (50% evaporation or 2 times concentration) of blowdown water is shown in Table 2. It can be seen that the amine product concentrates more than the acid. That is desirable as it helps to maintain the water pH. Ammonia on the other hand due to its high volatility shows significant drop in concentration. The lost in ammonia from the bulk water cause the pH to drop from 8.0 to 5.9, thereby creating a localized low pH environment. It is important to note that even with consistently good pH control of 8.0 to 9.0, the possibility of under deposit localized low pH corrosion still exists. Obviously, when the bulk water pH is low, it would be easier to develop under deposit low pH.

TABLE 2 Concentration Effect Using AmineBlowdown (100% water) Blowdown (50% evaporation) Concentration factor

pH = 8.0 pH = 5.9Neutralising amine = 280 ppm Neutralising amine = 551 ppm 1.97

Acetic acid = 400 ppm Acetic acid = 656 ppm 1.64

MP Steam at 260 C

pH=8.5 Bulk Water

pH<7

Fe3O4

Steam out

Amine/AmmoniaR-NH4/NH4OH

CH3COOH

Deposit

H2O NH4+OH-

CH3COO-H+

“Free” Caustic Region

NH3 = 40 ppm NH3 = 21 ppm 0.53 In order to maintain the blowdown bulk water pH at 8.0 to 9.0, non-volatile base chemicals such as products with a caustic/phosphate (Na/PO4) ratio blend need to be used. Caustic is a stronger alkaline than amine but can itself be corrosive causing gauging and embrittlement to metals. Phosphate buffers the water reducing the chance of large pH change due to the development of high caustic concentrations. The under deposit concentration effect will not create an acidic localized pH as shown in Table 3. The same acetic and ammonia are present in the bulk water. The bulk water pH is elevated to 9.6 by 135 ppm of Na and 10 ppm of orthophosphate and only 50 ppm of amine instead of 280 ppm previously as shown in Table 3. The amine feed is much lower as it is designed to elevate the dilution steam pH. To control blowdown bulk water at 9.6 using amine alone will require 5 times of current amine feed rate. Figure 6 shows the pH/PO 4 recommended to control reboiler corrosion.

TABLE 3 Concentration Effect Using Na/PO4Blowdown (100% water) Blowdown (50% evaporation) Concentration factor

pH = 9.6 pH = 9.1Neutralising amine = 50 ppm Neutralising amine = 94 ppm 1.88

Acetic acid = 400 ppm Acetic acid = 656 ppm 1.64NH3 = 40 ppm NH3 = 21 ppm 0.53Na = 135 ppm Na = 270 ppm 2.0

O-PO4 = 10 ppm O-PO4 = 20 ppm 2.0

FIGURE 6 Coordinated pH/PO4 Control Zone

CONCLUSION Low pH under deposit corrosion is aggravated by the high evaporation rate (30%) at the reboiler causing organic acids to concentrate in the quench water particularly underneath the deposits. Amines used may be sufficient to neutralize the bulk water but not at the evaporation zone. Addition of pH/PO 4 chemical

Coordinated pH/PO4 Control Box

8.2

8.4

8.6

8.8

9

9.2

9.4

9.6

9.8

10

1.0 10.0

Orthophosphate as PO4 (ppm)

pH at 25°C

2.0 4.0 6.0 8.0

CONTROL ZONENa/PO4 = 2.8

Na/PO4 = 2.2

treatment program will help elevate the pH at the evaporation zone but must be controlled to prevent formation of free caustic.

REFERENCES1. Marilyn W. Blaschke, Quench Water Chemistry and Downstream Concerns, Ethylene Forum (2001).2. Matcor Technology & Sevices Pte. Ltd., Failure Analysis of Reboiler Tube and Analysis of Deposits

(1995).3. ASTM Standard Specification Designation A179, Seamless Cold-Drawn-Low Carbon Steel Heat

Exchanger and Condenser Tubes (1993).4. Corrosion Atlas Vol. 1, A1-A6 (1991).5. Betz Handbook of Industrial Water Conditioning, 82-83 (1991).