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Oct-2011

Fast turnaround for a SRU tail gas scrubber

Reuse of groundwork and equipment, as well as previous experience in meeting permitting regulations enabled an 18-week turnaround of a tail gas scrubber

Steven Meyer and Andrea Trapet, MECS
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Article Summary
When a lubricants producer experienced a catastrophic failure of an existing SRU tail gas scrubber, it was faced with a very costly and possibly extended shutdown of its facility. Under pressure to meet permitted emissions regulations within six months of the incident, the company was offered a 72-week (18-month) replacement schedule from the original scrubber vendor. Working with MECS DynaWave, the producer was able to complete the project from start to finish in 18 weeks and at substantial cost savings. This article reviews the incident and lessons learned, presents the project timeline and actions that took place to be able to achieve the short turnaround as well as the cost savings realised, and reviews DynaWave technology and its application in SRU tail gas treatment.

Plant background

The lubricants producer has a sulphur recovery unit (SRU) in operation for the recovery/removal of sulphur compounds. The SRU is a cold bed adsorption (CBA) process followed by conventional thermal oxidisers. Thermal oxidisers are, in turn, followed by a SO2 tail gas scrubber. The thermal oxidisers convert H2 S, COS and CS2  compounds to SO2. The SO2 is then absorbed in the SO2 tail gas scrubber and converted to Na2 SO4 using caustic as a reagent.

In early 2010, an explosion occurred within the SRU tail gas scrubber as a result of leaking vents and relief valves. The explosion caused a blowout of the upper head on the scrubber vessel. The internals inside the vessel were also damaged, as well as the ducting exiting the scrubber and stack.

The company contacted us for a quotation for a replacement scrubber, for which delivery time was critical. We evaluated the design conditions as well as the site location and provided a quotation that met the customer’s performance requirements and, more importantly, their schedule of 20 weeks or less. Consequently, an order was placed to provide a basic engineering 
package. Polaris was selected to perform the detailed engineering. Finally, the customer elected to manage the purchase and installation of equipment.

Scrubber
The DynaWave scrubber (see Figure 1) is a type of wet gas scrubber that offers an overall scrubber size that is significantly smaller in diameter and height than conventional spray towers or packed towers. This particular feature of the technology translated into a shorter fabrication time and lower cost, as will be explained later.

One unique feature of the scrubber is the gas-liquid contact zone that is located in the gas inlet duct going into the scrubber. The scrubber liquid is injected counter-current to the incoming gas stream. A highly energised zone, or “froth zone”, is developed in the inlet duct at the point of gas-liquid contact. In the froth zone, the gas is immediately quenched and the acid gas is absorbed into the circulating liquid. The froth zone is established by the proper selection of liquid-to-gas ratio, liquid nozzle pressure, gas velocity in the inlet duct and gas-side pressure drop.

The liquid is injected into the gas stream through a full bore, full cone liquid injector or nozzle, which is another unique feature. The nozzle is referred to as the Reverse Jet Nozzle and, contrary to other designs, the nozzle is very large to avoid pluggage or wear.

The gas and liquid both then pass into the scrubber vessel. The scrubber vessel is essentially a gas-liquid separation device, collecting the circulating liquid in the vessel sump and sending the gas out of the top of the vessel after first passing through demisting chevrons. Since the main purpose of the scrubber vessel is to separate the gas from the liquid, the vessel dimensions are significantly smaller than technologies that use the scrubber vessel also for gas-liquid contact and absorption/reaction.

Another significant design benefit of the scrubber is its ability to both quench the gas and absorb/remove SO2 from the gas stream all in one stage or vessel. A spray tower or packed tower requires a separate gas quenching stage with additional nozzles and equipment.
The scrubber was sized for an inlet gas flow rate of 36 000 lb/hr (quenched 15 000 acfm) and an inlet SO2 loading of 500 ppm. The inlet gas temperature is 1400°F (760°C), and the design SO2 outlet is less than 10 ppm.

Schedule
The turnaround time for the complete project was 20 weeks or less, from purchase order to start-up of the scrubber (see Table 1).

Critical project parameters Reuse of existing equipment
Due to the extremely tight time constraint of the project, the customer decided to reuse existing equipment wherever possible. This included the main circulation scrubber pumps, as well as some instrumentation, piping and valves.

Main circulation pumps
The original scrubber had been designed as an FCC scrubber and converted to an SRU scrubber, so the existing scrubber system was oversized, including the circulation pumps. For this reason, the pumps were down-rated in flow and discharge pressure to the new design conditions. The original circulation pumps were manufactured by Warman and belt driven. By simply changing the belts and sheaves, the speed of the pumps could be reduced. It was discovered that the pump flow and pressure had to be further reduced by the installation of a restricting orifice in the circulation piping, to reach the required pump flow and pressure for the new scrubber.

By leaving the pumps on their existing foundations, it was possible to avoid pouring new foundations for the pumps.

Piping and valves
Since the original pumps were being reused, it was also possible to retain sections of piping and valves around the pumps. The material of construction for these items was 316 stainless steel and would have had a long lead-time if purchased new.
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