Depropaniser

Polimer Grzegorz Grzesik carried out a turnkey project for the supply of Hydrofluoric acid alkylation plant equipment for an oil refinery end-user. A part of the scope was design, produce and assembly Depropaniser.  

Utilisation

The Depropaniser is responsible for separating propane from the feed stream and reclaiming isobutane through a side draw for reintroduction into the reactor. The residue from the main fractionator is directed to a Debutaniser, where butane is extracted as an overhead product, while the alkylate product is obtained from the bottom. In a modified setup tailored for higher-capacity operations, the effluent from the reactor is divided between a main fractionator and a stripper. Another fractionation arrangement, commonly utilized in conjunction with the time tank water-cooled reactor process, employs a sizable stripper to recover alkylate product as a bottom stream, before partitioning the light products and recycle streams.

Construction

As equipment, the Depropaniser is a two-diameter vertical column, performing the role of a fractionator in Hydrofluoric acid alkylation. The unit contains 36 distillation trays, with a diameter of Ø1250 mm at the bottom and Ø950 mm at the top. The lower section of the column contains a shell and tube heater that provides heat for the distillation process.

The design has been carried out in accordance with the requirements of the end user and UOP licensor, pressure vessels design codes ASME BPVC Sec. VIII Div. 1 and TEMA R, as well as good practice from the National Association of Corrosion Engineers and the American Petroleum Institute.

HF alkylation equipment operates under harsh conditions, including high pressures of up to 24.6 bar. Hydrocarbons and Hydrofluoric acid are toxic and corrosive media, in process temperatures up to 135°C, cause both liquid and vapour.

Materials 

The materials used are SA516, Gr. 70, SA106 Gr. B, SA350 Gr. LF2, S235JR. For this part of the installation, it was decided to use special carbon grades of steel, showing satisfactory resistance to concentrated HF at the design temperature. However, the matériel was carefully selected based on standards (including NACE 5A171) and additional requirements compliance confirmed by certificates. 

Additional requirements for carbon steels contain a controlled chemical composition without impurities, produced with special treatments and chemical control to ensure lower inclusion content are referred to as HIC-resistant steels. During production, the steel should be ultrasonically inspected and heat treatment.

Scope of project

The Depropaniser column was fitted with platforms to facilitate maintenance. Complex design of such special structures is part of our service to end users and is carried out by our design department.  Specialised engineers prepare technical designs, including strength calculations, finite element method simulations and fatigue analyses of structures subjected to dynamic loads.

The column is equipped with additional safety measures during operation.  Instrumentation, controls and automation are selected and installed to measure liquid level, pressure and temperature. The electronics is programmed to give alarm signals in response to certain events, approaching certain indicators, the exceeding of which can affect the operation of the entire installation.  The analogue component of the safety systems are the pressure relief valves. To protect the vessel from being subjected to pressures exceeding its design limits, the relief valves are designed to open at a predetermined set pressure.

In this project, Polymer was able to assume a role beyond that of an EPC, undertaking material procurement and inspection, design, fabrication and installation. The scope of work also included the creation of pipework linking the column to other units in the plant.  The installation was carried out during a scheduled refinery shutdown in accordance with the API 751 requirements. To maintain consistency with the existing units, a laser 3D scan of plant layout was conducted with points accurate to ± 2 mm. Surveying and foundation works were conducted.