CO2 Removal

Carbon Dioxide (CO2) gas must be removed and recovered from ammonia process gas to make it fit for the synthesis reaction and later used for urea production. The removal of CO2 gas in ammonia plant is considered as a key stage in ammonia production. Organic amines (Primary, Secondary & Tertiary) have the ability to perform alone, or in combination with an inorganic salt the CO2 removal from gas mixture. The analysis of these organic amines in the scrubbing solution is usually performed by a recommended traditional procedure supplied by manufacturer, where the concentration of the concerned amine is determined by spectrophotometric or potentiometric methods.

These methods suffer from many disadvantages like interference with degraded amine products & carbon dioxide. Furthermore, these methods are not accurate and are unsuited to distinguish between different amines. The method gives the specific determination of each amine like (DEA, ACT-1, MDEA & Piperazine). The method is simple, accurate and allows the determination of amines even in CO2 loaded solution. Profitability and reliability in ammonia plants depends heavily on the efficiency of the CO2 removal from process gas. New technologies have dramatically improved the absorption rate efficiency, reduced CO2 slip to a few parts per million by volume, lowered energy requirements for CO2 regeneration and minimised corrosion in plant equipment. In addition, emerging technologies now apply non-toxic scrubbing solutions. Primary and secondary Organic amines, Monoethanol amine (MEA) and Diethanolamine (DEA) are currently employed alone and or in combination with hot potassium carbonate solution to catalyse the CO2 removal Process[1]. They achieve this catalysis by increasing the absorption rate of the chemical reaction between CO2 gas and the solution. Tertiary amines, Methyldiethanol amine (MDEA) on the other hand do not have a hydrogen atom attached to the nitrogen. The CO2 reaction can only occur after the CO2 dissolves in the water to from a bicarbonate ion. The bicarbonate formation is slow and only occurs in the liquid phase. Thus to effectively use MDEA for bulk CO2 removal, the liquid phase residence time should be high so that the CO2 reaction occurs efficiently. The disadvantage of using MDEA alone was solved by addition of an activator, namely Piperazine a cyclic amine, which has revolutionised the technology for CO2 removal. CO2 absorption by pure MDEA is quite slow but Piperazine enhances the absorption rate. The variation of the activator concentration can shift the thermodynamic behaviour in terms of process economics, reliability, energy consumption, corrosion control etc. Unlike MEA and DEA, the inability of tertiary amine to react with CO2 to form amides and subsequently amine carbamate may be the reason why these are less corrosive. Amines are usually prone to foaming but such tendency is not pronounced with MDEA solution. A mild dose of antifoam agent controls containment-induced foaming. Activating Agent in CO2 Removal :

1. Diethanolamine (DEA) is a secondary amine : HO-CH2-CH2
2. ACT-1 used in Ammonia -2 is a ploy alkyl amine. It is a proprietary chemical supplied by UOP Incorporated, USA
3. Methyl Diethanol amine (MDEA) a tertiary amine : HO-CH2-CH2 NCH3 MDEA HO-CH2-CH2
4. Piperazine is a cyclic amine which as an activator in scrubbing solution.

Each scrubbing solution has its own amine to promote the efficiency of the CO2 removal. Licensors supplied method to test their solution. Hence, Lab has been adopting three methods to quantify the amine content in its scrubbing solutions. Many times the inaccuracy was obvious. Samples of the same solution at different stages of the same system gave different results on the same amine. Specification check results on new chemical consignment lead to dispute with suppliers. Several check samples were done during CO2 slip to ascertain the amine content. Apart from involving three different methods there were many problems associated with these determinations.