Jul-2009

Passivation of ethylene cracker transfer line heat exchangers

Case study based on fours years of applying DEHA-based passivation treatment technology to the TLEs of Egyptian Petrochemicals’ ethylene crackers

Abbass A Ezzat
Egyptian Petrochemicals Company

Article Summary

Diethylhydroxylamine (DEHA) has proven to be an effective oxygen scavenger for transfer line heat exchangers (TLEs) in the production of super high-pressure steam. After four years’ experience using DEHA in the TLE systems of Egyptian Petrochemicals’ ethylene crackers, the replacement of hydrazine with DEHA has led to both outstanding protection from oxygen corrosion and excellent feed water passivation in the TLE systems. DEHA contributes no dissolved solids or ammonia to the system, and has been shown to be a strong metal passivator in both laboratory works and plant applications. Measurements of the polarisation characteristics of mild steel in the presence of these compounds reveal enhanced passive film formation between pH ranges of 8–9.

Operating experience
In an ethylene cracker’s TLEs, super high-pressure steam is produced utilising heat generated from the steam cracking of petroleum feedstocks. In replacing hydrazine with DEHA, three parts of DEHA per part oxygen dose were envisaged, corresponding to a DEHA-to-hydrazine replacement ratio of 1.5.

Super high-pressure steam is also produced utilising heat generated from pyrolysis reactions. TLE water treatment programmes are employed to prevent scale deposition and corrosion. If these problems do occur, TLE efficiency decreases, slowing down production while increasing maintenance and energy costs. Deposition, commonly known as scale formation, can become critical, hindering heat transfer and affecting steam production.

Scaling can lead to overheating of tubes and, in extreme cases, tube rupture. Once deposits have formed, the environment under them is ideal for corrosion. Metal loss from under-deposit corrosion, usually in the form of pitting, can shorten tube life. Corrosion byproducts are likely to deposit elsewhere in TLEs, causing the cycle to repeat itself.

Corrosion occurs in the steam condensate system due to carbonic acid attack and oxygen pitting. Carbonic acid attack occurs due to CO2, which is the breakdown product of carbonate alkalinity in the TLEs, condensing with water to form H2CO3. This results in grooving of the condensate piping. Oxygen pitting occurs as steam condenses and the vacuum created pulls air into the system. Due to the localised nature of oxygen pitting, it can cause relatively quick failure in the condensate system. 

The most common method of addressing carbonic acid attack is the use of neutralising amine. These chemicals neutralise the carbonic acid and increase the pH of the condensate. Therefore, corrosion inhibitors for both scavenging oxygen and enhancing passivation are required for the proper treatment of TLE steam condensate and feed water systems, to attain the following objectives:
•  Protection of plant equipment integrity
•  Effectively securing plant continuous operations
•  Minimising the amount of condensate corrosion byproducts returned to the TLE condensate return system.  

Chemical treatment system selection is critical to secure continuous operations in ethylene cracker TLE systems. Loss of steam production can lead to ethylene cracker shutdowns, product degradation and equipment failure. Treatment options should cover oxygen scavenging and passivation of the TLE condensate return and feed water systems.

With regards to oxygen scavenging, its purpose is to minimise oxygen corrosion in steam circuits. In the Egyptian Petrochemicals case, it is used as a supplemental oxygen control agent with applied mechanical dearators to control corrosion and pitting attacks. However, the term passivation also refers to the transformation of a corroding, unstable surface to an inactive and stable surface.

A number of treatment options are available for retarding TLE system corrosion, each with their own advantages and disadvantages.

Historical background
Traditionally, sulphite and hydrazine in the absence or presence of a catalyst have been used for retarding TLE system corrosion. Catalysed sulphite is considered an effective oxygen scavenger. It minimises corrosion by reducing the oxygen level in condensate water to trace levels. This effectively limits the cathodic half-cell reaction of the electrochemical cell. However, it is generally recognised that is does not enhance passivation, since it is non-volatile and does not leave the TLE with steam.

In the Egyptian Petrochemicals facilities, hydrazine has been applied to alleviate oxygen attack in the pre-TLE systems. Hydrazine is used as an oxygen scavenger, but it is not as effective as catalysed sulphite in removing oxygen. However, it has been shown to enhance passivation and does not produce corrosive gases in steam generation systems. Since it is not considered volatile, it does not leave the system to scavenge oxygen and passivate metals throughout the system. At high temperatures, it can degrade to ammonia and volatilise with steam, which in turns leads to corrosion in the presence of oxygen. It has also been included in the Occupational Safety and Health Administration (OSHA) and National Institute for Occupational Safety and Health (NIOSH) lists as a suspect carcinogen. Several new oxygen scavengers have been developed, which reportedly enhance passivation.

Research work has concentrated on reaction rates with oxygen and the ability to reduce hematite to magnetite. From the evaluation of such studies, it has been concluded that DEHA is considered the most available oxygen scavenger in TLEs. The reported main advantages of DEHA are summarised as follows:
•  It is not temperature dependent and does not add to the dissolved solids. Hence, blow down is reduced relative to sulphite and hydrazine
•  It shows excellent reactivity (eg, rate of reaction) with oxygen compared with other materials
•  It has advantages over other oxygen scavengers, such as high volatility and metal passivation characteristics, and can be used more safely than hydrazine
•  Less DEHA is required compared with hydrazine.

Table 1 summarises the main properties of the major oxygen scavengers applied in high-temperature steam generation systems.