(79f) Saving Potentials by Use of Dividing-Wall Columns for Natural Gas Liquid Separations

Recovery of natural gas liquid (NGL) typically uses three or four fractionation columns to separate the natural gas feedstocks into different product streams, i.e. methane(C1), ethane(C2), Propane (C3) or Propane/butanes (C3&C4), C4 and heavier hydrocarbons (C5+). The number of distillation columns required for separating N-products shall be N-1. This is because one conventional column can separate only two products. Different from the conventional columns, a dividing-wall column (DWC) can be used to separate three or more products in a single column and could have significant savings (up to 30%) in both capital and energy costs. However, currently no commercial installation of DWC for NGL fractionations has been reported in the literature.

This paper analyzed applicability of DWC for NGL separations related with liquefied natural gas (LNG) processes. Feed compositions, relative volatilities and product specs have significant impact to DWC’s feasibility, constructability and/or savings. DWC appeared not suitable for demethanizer and deethanizer columns, primarily due to high cost of refrigerant utilities for overhead condensers. Nevertheless, when an NGL unit has both depropanizer and debutanizer columns, such as where a propane stream is recovered as product or refrigerant, DWC can have good potential for savings in energy and/or capital costs, depending on the richness of the feed gases. A step-by-step flowchart has been introduced for determination of DWC’s applicability. Some considerations for DWC’s operation pressures, hydraulic balances, column internals, reflux splitters and control strategies are also discussed.