Revamp study of a mothballed sour water stripper for a new service.
SOUR WATER STRIPPER REVAMP STUDY
The client, an independent petroleum refiner, contracted Process Engineering Associates, LLC (PROCESS) to study the revamp of an out-of-service sour water stripper for use in a new service. Project tasks included:
- Process simulation development
- Preliminary cost estimates for new equipment
- Operational guidelines development.
The client provided the sour water rates and compositions from the various refinery sources as the design basis. The client initially requested that PROCESS evaluate four different options. The first three options required injecting sulfuric acid in order to selectively strip hydrogen sulfide (H2S). The first three options differed from each other in the tower operating pressure and the requirements to compress the stripped gas to feed downstream equipment. The fourth option required stripping both H2S and ammonia (NH3) and feeding the stripped gas to an Ammonium Thiosulfate (ATS) Unit.
PROCESS used its licensed commercial process simulation software, CHEMCAD, to develop the mass and energy balances for the four options. PROCESS also evaluated other processing options, such as the choice of direct steam or reboiled stripping, the choice to include or not include heat integration, the choice to include or not include various overhead condensing and reflux schemes. PROCESS submitted a preliminary report to the client with the details of multiple processing schemes for each of the four options.
The client requested that PROCESS narrow the evaluation down to Option 4, anticipating the construction of an ATS Unit. The client also stated its preference to minimize capital costs over optimizing the tradeoff between capital and operating costs. The client added design high limits for total dissolved solids and pH of the stripped sour water stream.
PROCESS determined that the existing sour water stripper tower was inadequate for the required service. PROCESS evaluated other out-of-service towers and heat transfer equipment for use in the new service. PROCESS worked with the ATS Unit engineering firm to ensure that the stripped gas stream met the ATS Unit feed requirements. The final design utilized an existing tower and specified new packing and a new plate and frame heat exchanger. The tower operates at an elevated pressure in order to feed the stripped gas to the ATS Unit without compression. The elevated pressure creates a low differential temperature between the tower bottom and low pressure steam. PROCESS recommended the plate and frame exchanger given the low differential temperature.
At this point in the project, PROCESS was ready to complete the equipment, piping, and process control specifications and make final cost estimates. However, the client reconsidered its choice to install an ATS Unit. The client described the fundamental refinery regulatory problems that it needed to solve. Near-term, the client needed to lower the NH3 content of the stripped sour water. Longer-term, the client needed to also lower the sour water sulfide content. Currently, the NH3-removal bacteria in the treatment pond were not meeting the waste water NH3 limit. Aeration used to convert the sulfides to sulfates was lowering the pH below the tolerance of the bacteria. PROCESS gave opinions to the client on all of the options known to be available. The client chose to pursue a limestone filtration system in conjunction with the treatment pond. The client put on hold the project to install the ATS Unit and sour water stripper.
- Petroleum Refining
- Alternatives evaluations
- Hydrogen sulfide removal process design
- Ammonia removal process design