Page 295 - Hydrocarbon Exploration and Production Second Edition
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282                                                     Oil and Gas Processing


                        dry gas
                         outlet

                                                                    glycol

                                                                    mist mat
                                         lean glycol
                                         inlet
                                                                    inlet
                                                                    separator

                                                    gas


                 wet gas
                    inlet



                                         rich glycol
                                         outlet
          Figure 11.15 Glycol contacting tower.


          11.1.3.3. Heavy hydrocarbon removal
          Condensable hydrocarbon components are usually removed from gas to avoid liquid
          dropout in pipelines, or to recover valuable NGLs where there is no facility for
          gas export. Cooling to ambient conditions can be achieved by air or water heat
          exchange, or to sub-zero temperatures by gas expansion or refrigeration. Many
          other processes such as compression and absorption also work more efficiently at
          low temperatures.
             If high wellhead pressures are available over long periods, cooling can be
          achieved by expanding gas through a valve, a process known as Joule Thomson ( JT)
          throttling. The valve is normally used in combination with a liquid gas separator
          and a heat exchanger, and inhibition measures must be taken to avoid hydrate
          formation. The whole process is often termed ‘low temperature separation’ (LTS)
          (Figure 11.16).
             If gas compression is required following cooling, a turbo-expander can be used. A
          turbo-expander is like a centrifugal compressor in reverse, and is thermodynami-
          cally more efficient than JT throttling. Pressure requirements and hydrate
          precautions are similar to those of LTS. When high pressures are not available,
          refrigeration can be used to cool gas. Propane or freon is compressed, allowed to cool
          and then expanded across a valve, cooling the gas as it passes through a chiller.
          Temperatures as low as  401C can be achieved. Gas dehydration or glycol injection
          must precede the operation to avoid hydrate formation.

          11.1.3.4. Contaminant removal
          The most common contaminants in produced gas are carbon dioxide (CO 2 ) and
          hydrogen sulphide (H 2 S). Both can combine with free water to cause corrosion and
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