原油蒸餾工場脫鹽後注鹼操作改善

The Performance Improvement of Downstream Caustic Injection in Crude Distillation Unit

陳兆泰(Chao-Tai Chen) ； 賴宗成(Chong-Chien Lai) ； 洪正宗(Cheng-Tsung Hong) ； 陳源德(Yuan-Der Chen) ； 呂盆(Pen Liu) ； 柯光明(Huang-Ming Ko)

《石油季刊》 50卷4期 (2014/12) Pp. 61-69

摘要

原油蒸餾工場利用注鹼（Caustic Injection）來控制主塔頂部HCl的腐蝕是一已超過60年的老技術。此方法便宜又有效，不過若注入不恰當或過量，將會產生一些不良的副效應，因此需做正確、有效且又適量的注鹼，才能取其利而避其害。本文將介紹一修改脫鹽後注鹼位置並調整注鹼濃度來改善注鹼效率以降低塔頂氯離子量偏高的案例。A蒸餾工場在2007年10月底至2010 年11月19日這一操作週期有脫鹽不佳、塔頂氯離子含量升高及空氣冷卻器破管增加的問題。至2009年10月脫鹽已改善，脫鹽後鹽含量大多可≦1 ptb，但塔頂氯離子含量卻仍有增加的趨勢，且脫鹽廢水的pH有段期間偏低，A蒸餾工場擬恢復注鹼來改善。因過去曾發生疑因注鹼造成閃蒸塔（Flash Drum）底泵進口管線腐蝕洩漏的問題，建議先稀釋鹼液的濃度，再利用2010年底歲修期間，在第二段脫鹽槽出口管線上增設一鹼液注入點來取代原設在閃蒸塔底部的注鹼點。如此，除可加強「脫鹽後注鹼」對降低塔頂氯離子濃度的效果外，也可避免因注鹼造成上述管線腐蝕洩漏的問題。由改善前、後兩個操作週期（即2007年10月至2010年11月與2011年1月至2013年8月）的長期化驗數據來做比較與解析，以瞭解修改後是否有達到預期的效果？我們是根據在「相同的脫鹽後鹽含量」的基準上，來觀察修改脫鹽後注鹼位置對降低塔頂HCl生成量的效果。上週期後半段（2009年3月至2010年3月）的脫鹽後鹽含量與修改後之週期前11個月（即2011年1月至11月）的脫鹽後鹽含量差不多，平均值分別為0.85ptb及0.83ptb；而相對應期間的塔頂氯離子量的平均值則分別為133.03ppm及82.78ppm，降幅約為三分之一。因此，修改注鹼位置對降低HCl的產生量是有效果的，且未再發生過去因注鹼而造成管線鹼脆裂腐蝕的問題。正確、適量的脫鹽後注鹼可有效減少原油蒸餾工場主塔頂部HCl的生成量，降低塔頂腐蝕，而對塔底重油鈉離子含量的增加及後續重油加工觸媒活性的影響是有限的。塔頂HCl的生成量減少後，塔頂pH容易穩定控制，塔頂中和胺與防蝕劑的用量及費用可減少，塔頂設備的腐蝕也減輕，有助於降低蒸餾工場的操作與維修成本。

關鍵字

原油蒸餾 ； 注鹼 ； 塔頂腐蝕控制 ； HCl腐蝕

並列摘要

Caustic injection used to control the crude tower overhead corrosion caused by hydrogen chloride (HCl) is a more than 60 years old technology. It is a cheap and effective method to reduce the amount of HCl produced by hydrolysis of residual chloride salts, but also have certain problems if it is used improperly. To minimize these side effects, caustic injection in crude distillation unit (CDU) must be used efficiently and at a suitable rate. In this paper, a modification of caustic injection point and rate after the desalters of a CDU was illustrated, then the chloride ion (Cl^-) contents of overhead condensate water before and after the modification were compared.Although the desalting of CDU A could reach to 1ptb or less salt in desalted crude after optimizing desalter operation, however, it still remained both problems of high overhead HCl content and severe air fan tubes corrosion during the operation cycle from October, 2007 to November, 2010. The pH of desalter effluent water was also low sometimes. Therefore, the restore of caustic injection was considered again.The original caustic injection point after the desalter of CDU A was designed at the bottom outlet of flash drum, and corrosion problem occurred in the inlet pipe of hot crude charge pump due to the caustic injection in the past. A new injection point at the outlet of 2nd stage desalter was set up to replace the original in the turnaround at the end of 2010. The caustic solution was also diluted to 3~4 fold with water to get a better and more uniform dispersion in desalted crude oil.The Cl^- amounts of overhead condensate water before and after the caustic injection modification were compared based on the same salt contents of desalted crude. The averaged salt amounts in desalted crude were 0.85ptb and 0.83ptb for the periods of Mar. 2009 to Mar. 2010 (before modification) and Jan. 2011 to Nov. 2011 (after modification) respectively, almost the same, and the corresponding chlorides contents in the overhead condensate water were 133.03ppm and 82.78ppm respectively. The decrease ratio of overhead Cl^- amount is more than one-third. The corrosion in the inlet pipe of hot crude charge pump caused by caustic injection does not happen again.Appropriate caustic injection into desalted crude is effective in lowering HCl formed from hydrolysis of residual chloride salts and subsequent crude tower overhead corrosion rate. Accordingly, it is responsible for a fairly low additional contamination of the resid (ATB) with sodium and producing a catalytically processable resid. It can improve crude tower overhead pH control and reduce the amount of organic neutralizers and corrosion inhibitors, lowering down the operating and maintenance costs of CDU.

並列關鍵字

Crude Distillation ； Caustic Injection ； Overhead Corrosion Control ； HCl Corrosion