原油蒸餾工場注鹼效率改善的案例討論

「一脫四注」是防止原油蒸餾工場主塔頂部系統腐蝕的綜合性措施。其中利用注鹼(Caustic Injection)來降低HCl 的生成量以降低塔頂腐蝕是一已超過60年的老技術。此法便宜又有效，然而若注入不恰當或過量，將會產生一些不良的副作用，因而有所爭議，有些煉油廠也因而停止蒸餾工場的脫鹽後注鹼。蒸餾工場是否不宜注鹼？如何做正確且又適量的注鹼，才能棄其短而留其長？本文先簡述注鹼的原理，並說明注鹼的利與弊及如何避免鹼脆裂(CausticEmbrittlement)，再討論兩個修改脫鹽後注鹼位置與注鹼濃度來改善注鹼效果的案例。案例一：A 蒸餾工場在民國96 年10 月底至99 年11 月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。至民國98 年10 月底，脫鹽已改善，脫鹽後鹽含量大多可≦1ptb，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，建議利用民國99 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過去設在閃蒸塔底部的注鹼點，並稀釋鹼液濃度，以加強「脫鹽後注鹼」對降低塔頂氯離子濃度的效果，並避免因注鹼造成上述破管洩漏的問題。由改善前、後兩個操作週期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氯離子生成量的效果。上週期後半段(在民國98 年3 月至99 年3 月)的脫鹽後鹽含量與修改後週期初的前11 個月(即民國100 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，降幅超過三分之一。因此，修改脫鹽後注鹼位置及調整鹼液濃度對降低其塔頂HCl 的產生量是有效果的。追蹤至今，不僅塔頂氯離子量未再升高，也未再發生之前閃蒸塔底泵入口管線鹼腐蝕脆裂的問題。案例二：B 蒸餾工場過去其脫鹽後的注鹼點是設在加熱爐前，由於該處的溫度較高，加上鹼液濃度較濃，一操作週期中發生過3 次破管洩漏。因第3 次破管時離該操作週期的歲修僅剩3 個多月，而在其兩段脫鹽槽的中間管線上已設有注鹼點，因此建議先改為中間注鹼(Interstage Injection)，並關閉其他的注鹼點，同時將鹼液的濃度減半。調整後，總注鹼量大幅降為2.61ptb，為調整前的1/4。主塔頂部回流槽冷凝水氯離子含量的平均值則由過去的15ppm 上升至37.2ppm，仍在合理範圍內。不過為了使主塔回流槽水腳的pH 與調整前接近，平均值仍維持在6.9 左右，塔頂中和胺的用量較過去增加約30％。塔頂腐蝕指標T-Fe 的平均值為0.192ppm，與調整前的0.184ppm 接近。原油脫鹽槽排水的pH 值則由過去的6.3 上升至7.7。正確、適量的注鹼可有效減少原油蒸餾工場主塔頂部HCl 的生成量，而對塔底重油鈉離子含量的增加是有限的，同時兼顧蒸餾工場塔頂設備的防蝕及後續RDS、RFCC 的觸媒。它仍是一可用來控制原油蒸餾主塔頂部腐蝕的方法。塔頂HCl 的生成量減少後，塔頂的pH 容易穩定控制，不僅減輕塔頂設備的腐蝕，也減少塔頂中和胺與防蝕劑等化學處理藥劑的用量及費用，有助於降低原油蒸餾工場的操作與維修成本。

關鍵字

塔頂腐蝕控制；注鹼；鹼脆裂；原油蒸餾

並列摘要

Desalting, caustic addition, neutralizing amine, filming inhibitor and water washing are the common measures for reducing the crude tower overhead corrosion caused by hydrogen chloride (HCl). The use of caustic addition to reduce the amount of HCl produced by hydrolysis of residual unstable chloride salts is a more than 60 years old technology. This method is cheap and effective, but also has certain caustic-related problems if it is used improperly. Many refineries stopped using caustic addition due to a number of problems eventually revealed. To minimize these side effects, caustic addition in crude distillation unit (CDU) must be used efficiently and at a suitable rate. This article describes both cases of modified caustic injection after the desalters, then the results before and af ter the modification were compared. Case 1: Although the desalting of CDU A could reach to 1ptb or less salt in desalted crude after optimizing desalter operation, but two problems were still remained: 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 a leakage 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. According to our trackings, so far the overhead chloride content has not significantly increased, nor the caustic stress corrosion at the inlet pipe of hot crude charge pump has occurred. Case 2: The downstream caustic injection point of CDU B was located in front of the furnace, where the temperature is higher and coupled with concentrated caustic, it led pipe cracked 3 times during an operation cycle. Because the scheduled turnaround was about 3 months later at that time, so the inter-stage injection between the desalters was suggested and closed the other injection points. In the meantime the concentration of caustic was diluted by half. The main results after the adjustment were summarized as follows: 1. The amount of caustic addition was reduced significantly to 2.61ptb, about the quarter of the previous one. 2. The average chloride ion contents of the crude tower overhead increased from 15 ppm to 37.2ppm, but still in the acceptable range. 3. In order to maintain the overhead pH as before, the monthly average amount of overhead amine neutralizer increased to 30％. The averaged T-Fe contents in the overhead condensing water were 0.184ppm and 0.192ppm before and after the adjustment, respectively, which were almost the same. 4. The pH of the crude desalter effluent increased from 6.3 to 7.7. 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 is still an available means for controlling crude tower overhead corrosion. While it does not eliminate the need for proprietary chemicals, it can significantly 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

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

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原油蒸餾工場注鹼效率改善的案例討論

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