A scraped surface exchanger is a type of double pipe heat exchanger designed to cool a process fluid to crystallize some component out of the fluid by use of a cooling fluid housed in a jacket surrounding a shell. The SSE geometry takes a warm process fluid on the shell side and runs it counterflow to a cooling fluid located on the jacket side, causing heat transfer through the shared shell pipe wall and condensation of various process fluid components into wax or crystals in the shell. A series of internal continuously rotating scraper blades located in the shell then scrape the wax or crystals off the shell's internal wall where they are separated by downstream filters from the rest of the process fluid. The pipe and jacket design are simple, but the rotating internals have a complex proprietary design including scraper blades, springs, drive housing, sprockets, and shear pins that connect the internals to a motor to run the equipment. These are often supplied as 'stands' of multiple double pipe sections connected in series to meet the process requirements. The scraper shells and jackets are a pressure vessel and are typically designed to ASME Section VIII and carry a u-stamp.
SSE's offer a scalable solution for high purity crystallization through a continuous wall scraping process, with crystallization up to 99.9% and avoiding a time consuming batch crystallization approach. They can easily scale from a single crystallizer shell in a pilot plant proof of concept to a commercial sized installation of up to 14 shells per stand with no upper limit on the number of SSE stands installed in parallel or series operation. Crystallization typically requires less energy input than distillation to purify a process fluid (up to 80% less), offering cost savings for end users. The rotating internal scrapers offer a few benefits as well: the rotational fluid movement increases turbulent flow in the inner pipe which greatly enhances heat transfer, and the blades scrape the crystals formed on the cold inner pipe wall which reduces fouling.
The scraper design allows for easy access to the shell side rotating internal components, so end users are able to swap out old or damaged components for spare replacement parts relatively quickly during routine maintenance.
The continuous operation, high crystal purity, and low operating costs offer an appealing solution to crystallizing or dewaxing needs.
A table of historic process fluids is below. "Waxes" typically refers to removing paraffins from hydrocarbon feedstocks as part of the production of lubricating oil. Deoiling is a similar process with the primary goal of obtaining wax to purify a process fluid.
SSE's are suited for operating temperatures from -100°F to 480°F [-73°C to 245°C] and process fluid viscosities in excess of 10,000 cP.
Typical MOC is carbon steel, 304/304L stainless steel, or 316/316L stainless steel for both the inner and outer shells. Depending on the process fluid and specific design conditions, a combination of the above materials can be used for the inner or outer shells. Alternate materials can be analyzed for specific process cases. We once used Titanium for both the inner shell and jacket.
The scraper's rotating internals will also vary depending on the process fluid, and the scraper blade material is of particular importance as it must be softer then the inner shell material to avoid damaging the pipe as it scrapes along the inner surface but stronger than the formed crystals to knock them loose as they build up along the inner surface. We recommend blade material based on historic data and will offer to send a sample to customers so they can place the material in their process fluid to confirm compatibility.
The scraper size is determined by the cooling duty and flow rates required for each job. Inner shell nominal pipe sizes of 6", 8", 10", or 12" are available in stands from 1 to 14 shells per stand and scraped lengths up to 42 ft [13 m].
