Jun-2015

Optimization and throughput opportunities at PTT PLC’s amine plant

Removal of CO2 from natural gas is a necessary treating step before cryogenic processing. At the PTT Public Limited Company Gas Processing Plant 5, the wellhead gas has CO2 concentrations ranging from 19 to 23 mol%.

Sittiwat Kitsatienkun, PTT Public Company Limited
Martin Pieronek, Peter Krouskop and Barry Burr, Bryan Research & Engineering

Article Summary

This gas feeds an amine sweetening unit where most of the CO2 is removed. The sweet gas product is dried before entering a cryogenic demethaniser where ethane and heavier natural gas liquids are recovered. The demethaniser overhead reaches temperatures as low as -100 to -120 C. Thus to prevent CO2 freeze-out, the CO2 concentration in the sweet gas must be less than 900 ppm. This study focuses on optimisation of the amine sweetening unit to increase throughput, provide adequate cold protection, and avoid corrosive operating conditions in the amine regenerator.

Introduction
Carbon Dioxide (CO2) is a major impurity in natural gas wells that causes corrosion in transportation pipelines, may form a solid “hydrate” when in the presence of water, and can freeze by itself (forming “dry ice”) at cryogenic gas plant conditions. The required CO2 level to prevent solids formation in the cryogenic NGL recovery process is in the hundreds of ppm range.

In the past, primary and secondary amines were used to sweeten natural gas to such low CO2 levels. Lately, MDEA has become a popular solvent because it is less corrosive and needs less heat for regeneration. However, MDEA by itself is slow to absorb CO21. Within typical amine absorbers, there is insufficient contact time for the gaseous CO2 to complex with the aqueous MDEA cations. Thus, MDEA is usually incapable of sweetening gas to the ppm levels demanded by cryogenic gas processing. However, blends of MDEA with certain activating agents has been found to hasten CO2 absorption so that gas can be suitably treated for subsequent cryogenic processing.

These activators are added in small amounts to the MDEA solution to enhance the CO2 absorption while mostly maintaining the desirable qualities of MDEA^2-3. The primary reactions for an amine process are

H2O <-> H+ + OH-                                   Rxn 1
CO2 + OH- <-> HCO3 -                            Rxn 2
MDEA + H+ <-> MDEAH+                        Rxn 3

The second equation represents the hydrolysis of CO2. The reactions for the activator are ⁴

AM + CO2 <-> AM(CO2)                          Rxn 4
AM(CO2) + H2O <-> AMH+ + HCO_³ -      Rxn 5
AMH+ + MDEA <-> MDEAH+ + AM         Rxn 6

AM represents different activators available on the market such as DGA, MEA, DEA, and Piperazine. The reactions show the activator cation reacts directly and quickly with CO2. Then another very fast reaction occurs where the CO2 flips from the activator cation to the MDEA cation. This combination of two very fast reactions replaces the slow reaction sequence occurring when CO2 is absorbed by MDEA alone. The activating agent does have its own small amount of absorption capacity which comes with a high regeneration energy comparable to other primary or secondary amines¹. The activated MDEA blend’s activating energy increases proportionally to the amount of activating agent in the blend. Since the activating agent is present in small amounts, the low regeneration energy benefits of MDEA are largely achieved.

PTT PLC, a public owned company in Thailand, has such an activated MDEA sweetening unit. Our study of the sweetening unit was undertaken to maximise plant throughput, minimise operating expenses, reduce corrosion, and maintain adequate CO2 removal. The study was accomplished by first creating a model in the ProMax⁵ process simulation program and comparing it to plant operating data to ensure a good match.

Then scenarios covering several key operating parameters were run to examine alternatives and find optimum operating conditions.

Current Plant Operation - Plant configuration
The process flow diagram for the PTT Gas Separation Plant (GSP) no. 5 amine sweetening unit is shown in Figure 1. GSP#5 consists of two identical amine trains of which one is shown in the figure. Sour gas is split equally by flow controllers to each packed bed absorber where it contacts the amine solution. The sweet gas is then dried before entering the Ethane Recovery Unit. Rich amine solution leaves the absorber bottom and proceeds to a high pressure flash tank where most light hydrocarbons and some acid gas are flashed. The rich amine from the high pressure tank proceeds to a lower pressure column where it contacts regenerator acid gas to scrub and recover any residual amines. The rich amine is then regenerated in a hot oil reboiled stripper. Figure 1 shows the current plant configuration of the Amine Unit.

Comparing ProMax to Plant Operating Data
Operating data for the unit from July 1st to August 19th 2014 were used in ProMax to calculate plant performance. The average operating conditions are shown in Table 1 below.

The absorber and regenerator are modelled using the proprietary Electrolytic Property packaged developed by Bryan Research & Engineering, Inc. The excellent agreement between ProMax predictions and plant measurements of sweet gas CO2 are shown in Figure 2 for several typical days.

Process Optimisation
Since ProMax accurately represents the plant performance, it can be used to carry out plant optimisation. During the optimisation study, the desire is to most profitably utilise the process equipment without violating any of the product quality, reliability, or equipment constraints. One requirement is to keep the treated gas below the 900 ppm CO2 spec while reliably operating the amine unit. Optimisation will consider opportunities to reduce reboiler duty. Also, avoidance of corrosive conditions in the reboiler will be monitored within the ProMax simulation strategy.

Optimisation Input Variables
The following adjustable input parameters (or manipulated variables) are considered in this study.

Reboiler Duty: The reboiler duty will be optimised primarily to avoid corrosion in the regenerator and assure constraint variables are within limits. If there is additional flexibility, reboiler duty will be reduced to save energy.

Amine Ratio (mass rate of activator/mass rate of MDEA): The amine ratio describes the proportion of activator relative to base amine, MDEA, in the custom amine blend. Too low of a ratio may reduce the effectiveness of the solvent in absorbing CO2 while too high of a ratio will increase the required duty for regeneration. The optimisation will determine the optimal amine ratio for meeting CO2 spec at minimum reboiler duty.