Reform of liquid nitrogen interval heat exchanger

- Aug 10, 2018-

Reform of liquid nitrogen interval heat exchanger


The design parameters of the methane fraction: flow rate 10609m3 / h, pressure 01135MPa (A) (before compression), temperature 35e, volume composition CO13188, H22138, CH478121, N2 Ar5156. In the main heat exchanger, the medium that participates in the heat exchange in the upper part is the syngas after washing with liquid nitrogen and the liquid nitrogen injected into the syngas, and is finally reheated to -65e. Most of the media that participate in the heat exchange in the lower part are liquid tail gas fractions.


The liquid nitrogen washing machine was designed by the French Air Liquide Company. The main heat exchanger was designed and manufactured by Sumitomo, Japan, using a plate-fin heat exchanger. Reasons for the transformation 

1) The methane fraction composition deviated significantly from the design value. The liquid nitrogen washing machine was driven from the original to March 1999, and the running load fluctuated between -143 and 155e, which did not reach the design-167e. The first separator never produced liquid and lost the function of separating methane. 

All methane is concentrated in the second separator. The methane fraction has a low concentration of methane and a high concentration of carbon monoxide, which deviates significantly from the design value. 

2) The ammonia production system has high output and low output. Compared with the design value, in 1998, the ammonia production system gas production capacity was 101, and the output of synthetic ammonia was only 82. In addition, 19 was wasted.

3) Methane conversion work is difficult to operate and maintain. In actual operation, the content of carbon monoxide in the methane fraction exceeds the standard, which is more than twice the design content, causing the methane conversion work pre-conversion furnace to be severely overloaded. The furnace temperature is high and the carbon monoxide output often exceeds the standard. After entering the methane reformer, carbon deposition will inevitably occur, resulting in a short service life of the conversion catalyst, which is only 6 months on average.


The reason for the analysis caused the methane fraction to fall short of the design value. In actual operation, T2 is only -151e, and methane cannot condense here. The high-concentration methane fraction is not separated, and the high-concentration methane liquid and the low-concentration methane liquid are all separated in the second separator, and the methane content in the methane fraction sent after mixing is low. Because the temperature at the bottom of the main heat exchanger is too low, the amount of carbon monoxide condensation increases, resulting in an increase in the concentration of carbon monoxide in the methane fraction.


The reason for the large cold loss is that the pumping temperature T2 and the bottom temperature T3 in the main heat exchanger both deviate from the design value. The composition of the methane fraction changed, the evaporation temperature changed, and the operating conditions of the whole liquid nitrogen washing device changed. The heat exchangers could not be operated according to the design, and the temperature difference at the hot end increased. The increase in gas outflow is one of the main reasons for the increase in cold loss.


It was determined that the methane fraction at the location where the problem was present was all condensed in the main heat exchanger. Looking at the operation of the main heat exchanger, the outlet temperature of the bottom purge gas (-188e) and the composition of the purge gas can reach or exceed the design value. The components that should be condensed have all been condensed in the main heat exchanger, and the reheated syngas can reach -65e. It can be considered that the hot and cold materials are basically balanced, and the overall heat exchange capacity can meet the process requirements. The problem is that the temperature distribution of each section of the heat exchanger is unreasonable, and the reason should be found from the inside of the main heat exchanger. 


Analysis of the main heat exchanger problem It can be seen from the (before the transformation) that the main heat exchanger can be divided into three sections: In the first stage, the purge gas is cooled and a large amount of methane condensation zone is passed. After the purge gas passes through the section, the temperature is lowered to -167e (design). The second stage is the deep gas purification zone after the first separation of methane. The purge gas continues to exchange heat with the synthesis gas having a high grade of refrigeration, and the temperature is further reduced to about -177e. The third stage is the purified gas and then the deep cooling zone, and the medium that exchanges heat with it is a tail gas fraction mixed by flashing gas and liquid.


Before and after the transformation, the main heat exchanger is shown as A1) purge gas inlet; A2) medium extraction outlet; A3) medium extraction inlet; A4) purification gas outlet; B1) synthesis gas inlet; B2) synthesis gas outlet; BL, BLL) liquid nitrogen inlet C1) tail gas fraction inlet; C2) tail gas fraction outlet. The passage of the section of the ò section is equivalent to that of the section of the ó section, which is also blocked, which will inevitably affect the heat transfer of The section ó, and the problem of insufficient back-up of the section cannot be solved. Because of the special structure, there is no choice but to block the passage, so it is impossible to reform the original heat exchanger. Based on factors such as the comprehensive construction period, it was decided to re-create the main heat exchanger. The basic situation of the re-made main heat exchanger (after the transformation).