Propane-Propylene (C3) splitters are used in Petrochemical industries to distill propylene from propane prior to export. C3 Splitter columns are traditionally operated with low pressure steam (LPS) passed to the column’s reboiler to heat up the C3 mixture thereby causing the splitting operation. However the use of Low Pressure steam is energy-demanding. A C3 splitter is equipped with a cooler and flash tank at the column top that condenses the column top vapours but in doing so, loses precious heat of condensation. In recent years, the concept of using a heat pump, such as a compressor has become a standard practice that eliminates the necessity of using low pressure steam.
Vapour compression methods for C3 stripping operations are ideal for compounds that have low relative volatilities. Below is a schematic of a C3 splitter with vapour compression of product (propylene). Low pressure steam that was used to vaporize the bottoms propylene product is replaced by installing a column top compressor and routing a portion of the discharge to vaporize the bottoms propane via the reboiler.
As per Ref , for a C3 splitter to operate effectively, the temperature difference between the column top and bottom should not be more than 250F (13.90C). Additionally, the bottom heat of vaporization should be close to overhead vapour’s heat of condensation with a pressure drop less than 15 psi (1 bar) across the column internals. The heats of vaporization for propylene and propane are nearly close at 157.6 Btu/lb and 151.7 Btu/lb respectively. The excess energy required to be supplied by the compressor is around 11%-12% of reboiler duty [Ref 2] which represents a high energy savings. Low pressure strippers also offer the advantage of fewer trays, shorter column height and lower column wall thickness that represents a capital savings with higher relative volatilities to effectuate product separation.
Low pressure C3 Stripper columns operate between 90 psig (6.2 barg) to 110 psig (7.6 barg) depending on the Technology Licensor. For the heat pump, the choice of compressor used can be centrifugal type with a typical pressure ratio of 1.8 [Ref 2] and effectively regulates the column top pressures when the throughput varies. The operation of the centrifugal compressor can be fixed type or variable speed type with the latter representing greater control but with higher installation costs.
The Suction throttling method involves using a control valve (e.g. butterfly valve) placed at suction side of the centrifugal compressor. But these are suitable only for fixed speed drives like Asynchronous Induction Motors where the driver speed cannot be manipulated. Below is a process schematic of a C3 column top compressor that works on the principle of suction throttling. A cooler on the anti-surge line cuts down the inline pressure loss in the compressor discharge and also reduces the compressor discharge side equipment and piping volumes, contributing to the fast response of the anti-surge system. The PIC on the compressor discharge receives discharge pressures from discharge side pressure transmitter (PT) to alter the suction throttle valve opening.
For C3 column operation, when the operating pressure at the column top increases, the suction throttle valve is altered based on the compressor discharge pressure. The operational advantage would be as follows,
As an alternative, variable speed motor eliminates the need for a suction throttle valve and can cater to the tower top propylene vapours during column fluctuations by altering the motor speed based on the discharge pressure of the compressor. The discharge side pressure controller PIC cascades its output (OP) to assign a set point to the speed controller (SC) and controls the motor speed. In doing so, both output flow and pressure are regulated.
In real situations, C3 splitter columns can also experience fluctuations in operating pressures due to any changes in the upstream side of the C3 splitter. This would also mean the C3 splitter column would take a while to again attain equilibrium across trays and the vapour compressor at the tower top would also need to synchronize itself with the column operating pressures.
Vijay Sarathy holds a Master’s Degree in Chemical Engineering from Birla Institute of Technology & Science (BITS), Pilani, India and is a Chartered Engineer from the Institution of Chemical Engineers, UK. His expertise over 16 years of professional experience covers Front End Engineering, Process Dynamic Simulation and Subsea/Onshore pipeline flow assurance in the Oil and Gas industry. Vijay has worked as an Upstream Process Engineer with major conglomerates of General Electric, ENI Saipem and Shell.