Nov-2019

Revamping an air separation unit

Revamp of a high pressure column with structured packing, reducing operating costs and avoiding discharging finished products to atmosphere.

YANG QUAN and DONG JIAO-JIAO, Sulzer Chemtech
XU FENG-JIE and ZHANG XING-DONG, China National Air Separation Engineering

Article Summary

Since its commission in 2004, the operating flexibility of an air separation unit (ASU) situated in East China had long been constrained by the high pressure (HP) column, which was equipped with aluminum trays. In order to meet the requirement of product purity, the unit had to be run at close to its full capacity even when demand for product oxygen and nitrogen was low. Revamping the column with structured packing would reduce the operating cost significantly and avoid discharging finished products to the atmosphere at turndown of operation. However, the revamp faced challenges due to the limited column height available to accommodate packing and internals, as well as difficulty in installation via the only manhole to the column. The final revamp proposals including the Sulzer VSR redistribution system, the latest customised solution for ASUs, were executed in the unit in late 2018 and early 2019. The commission data show that revamp objectives were successfully achieved.

Background
The gaseous oxygen (GOX) process ASU located in East China has a capacity of 20000 Nm3/h and was built in 2003. That was the period in China when, because of much lower pressure drop, structured packing already commonly replaced conventional aluminum trays for the low pressure (LP) column while the HP column still utilised trays with low tray spacing. The reasons for that were that adoption of structured packing for the HP column brings insignificant energy savings compared with its adoption for the LP column, and trays with low tray spacing helped to have a reasonable overall height for the cold box.

The 3m diameter HP column in the unit had 72 circular one-pass trays with a tray spacing of 160 mm. It is well known that the operating range of a tray is closely linked to its tray spacing, and the lower the tray spacing, the narrower its flexibility.1 While tray spacing is typically around 600 mm for other industries, it ranges from 60-250 mm for an ASU.2 Thus, the operating range of the unit targeted at 75-105% of capacity of 20000 Nm3/h. Unfortunately, due to the oversized open area percentage of the sieve hole, the GOX unit was able to be run satisfactorily only in the range 95-105%.

Due to fluctuations in market demand, the unit needed to be operated at lower capacity. However, because of the narrow flexibility of the unit, the compressed air feed could not go below 95% of design, otherwise the purities of product oxygen and nitrogen would not be met, and the performance of the downstream crude argon column and pure argon column would be also negatively affected. As a result, the main compressor, the most energy intensive equipment in the unit, had to be operated at a higher load than necessary, and over-produced oxygen and nitrogen were simply discharged to atmosphere during market downturns. In 2018, driven by energy saving, the plant requested to explore revamping the HP column with structured packing, which is well recognised for its broad operational flexibility.

Even though structured packing has been commonly used in modern HP columns over the past two decades, the revamp option of structured packing would be viable only when the constraints posed by the existing HP column’s configuration were overcome.
 
Revamp objectives and challenges
The unit was operated in the range 19000-21000 Nm3/h in terms of gaseous oxygen product. In this range, correspondingly, the pressure drop of the existing HP column varied from 18-22 kPa. As per operators’ experience, when the pressure drop was below 18 kPa, the products of the unit became out of specification. Apparently, this was due to poor interaction between vapour and liquid on the sieve trays. One of the most important specifications for the HP column was the oxygen molar concentration in the liquid nitrogen generated by the main condenser, which is used as reflux for both the LP and the HP columns.

The objectives of the revamp using structured packing were to broaden the operating range of the unit and to avoid compressing air feed more than required, especially at operational turndown.

For the lower limit, it was required to keep the oxygen concentration in the reflux below 2 ppm (molar) at 75% of design.

Meanwhile, the pressure drop in the HP column was expected to decrease by 15 kPa with structured packing. The optimisations proposed by China National Air Separation Engineering (CNASEC) to external piping as well as flow restricting instruments connected to the pipes, such as flow meters, could bring an additional 15 kPa reduction in pressure drop. An overall reduction of 30 kPa between the main compressor and the HP column was sufficient to unleash the potential of the existing main compressor. Therefore, the upper operating limit of the revamp aimed at 110% of the design case.

Table 1 lists the guaranteed purity of liquid nitrogen and the pressure drop of the HP column for the revamp.

As Figure 1 shows, the HP column was accessible only via a 500 mm manhole at the top of the column. Hence, the packing and internals had to be designed segmentally so that they could be passed through the manhole and assembled inside the column. Additionally, to eliminate liquid and vapour leakage through the gap between the column inner wall and new internals, the existing tray support rings had to be ground off (see Figure 2), which required a tremendous amount of work inside the column.

Process simulations were conducted by CNASEC and Sulzer to rematch the operational data of the unit, and then to determine the minimum height of structured packing required for the revamp.

As Figure 3 shows, the column would consist of a single packing bed only, equivalent to 39 theoretical stages. From the process point of view, for such a tall single bed it is general practice to use a liquid redistribution system in the middle of the bed to collect liquid and redistribute it. Otherwise, the efficiency of structured packing would be compromised due to liquid maldistribution generated along the long packing bed.3 Typically, a standard redistribution system itself stands up to 2m.

The height between the manhole at the top and the air feed nozzle at the bottom of the column is fixed, and a new top liquid distributor, a new liquid redistribution system as well as structured packing would compete for the limited space available in the column. Sulzer’s latest customised redistribution system for ASU proved to be a good choice here, as it requires short height, yet it has full function in liquid hydraulic equalisation. More importantly, unlike the other short solutions long existing in the ASU market, the new redistribution system also has almost full liquid mixing functionality, regardless of column diameter. Details regarding the system can be found elsewhere.^4,5

In the last quarter of 2018, the cold box was opened and the existing tray supports were ground off. Subsequently, the new internals and packing were installed in the HP column, and eventually the cold box was refilled with perlite for thermal insulation. At the beginning of 2019, CNASEC and Sulzer assisted in commissioning the HP column and operating data were collected.