Jul-2016

Optimization of an existing Habshan II amine sweetening unit

As part of its ongoing optimisation effort, Abu Dhabi Gas Industries (GASCO) is working with the Petroleum Institute (PI) and Bryan Research and Engineering Inc. (BR&E) to identify opportunities for optimisation of the Habshan II amine sweetening unit for a wide range of gas throughput.

A. Alkasem and Y.A. Al Zarouni, Abu Dhabi Gas Industries Ltd - GASCO
J.C. Slagle, Bryan Research & Engineering
A.S. Berrouk and D. Satyadileep, The Petroleum Institute

Article Summary

In a previous study, ProMax®, a process simulation package, was first verified by comparing the model results to operating data for about 300 days and was then used to optimise the facility operating near maximum capacity. Recently, most of the gas has been diverted to another facility resulting in Habshan II operating in the 30-40% capacity range.

In the present study, ProMax, is used to re-optimise the facility at the current low throughput. The model results for the new optimum are implemented in the plant and compared to operating data to confirm the predictions. In addition, the plant plans to increase throughput in the coming months. Therefore, the plant has been optimised at various gas flowrates to provide operators with set points at any given gas flowrate. In addition to preparing operators for throughput fluctuations, the results show a reduction of operating costs that amount to roughly 800,000 USD/yr.

Introduction
GASCO operates several methyl diethanolamine (MDEA) units, such as the Habshan II unit shown in Figure 1. Unlike primary and secondary amines, MDEA is commonly used for selective removal of H2S while slipping some CO2 into the sweet gas.

The Habshan II amine sweetening unit may treat up to 500 MMSCFD of sour gas containing roughly 12% acid gas. The plant has been optimised to meet the sweet gas specifications of 9 ppm H2S and 2% CO2 at 230 MMSCFD, as described in a previous work (1). However, the plant conditions have changed and the sweet gas specification has been updated. Now the plant is meeting a sweet gas specification of 20 ppm H2S and 3.3% CO2 while operating near 200 MMSCFD.

Conveniently, the model infrastructure is already in place to optimise the plant once again. This work addresses each of the variables optimised previously and updates the optimised case to represent current and future operations. The results from the ProMax(2) model are applied in the plant and observed.

Model Validation
Before optimising the plant, it is important to have confidence the model is predictive and robust. If the model is able to predict the plant performance, it should be able to match past operating conditions. A robust model will be able to accurately represent a wide range of values. To validate the model, data from 2013-2015 were modelled using the ProMax Scenario Tool. If the model accurately represents the data from these dates, it can be said the model is predictive and robust due to the wide range of operating conditions.

There are two distinct ranges for data. The first 300 days are from 2013 when the plant had relatively high flow. The average results for the first 300 days are shown in Table 1.

The next 180 days are from 2014 and 2015 when the plant reduced its gas flow to less than half the previous 300 days. The average results for the last 180 days are shown in Table 2.

The model matches the data very well at both high and low gas flow rates. Therefore, the ProMax model may be seen as predictive and robust, as confirmed in many other studies and publications (1) (3) (4) (5) (6).

Optimisation
The optimisation of this plant starts with the previous work completed at higher gas flow rates. The variables used to optimise the plant previously are seen in Table 3.

Since this study is focusing solely on optimising the operating costs, optimisation that includes capital expenditures are avoided. It is important to note that while the lean amine temperature is often a variable to optimise, Abu Dhabi’s ambient temperature is over 40°C for more than 40% of the year. Therefore, as with most amine sweetening plants in the Middle East, the lean amine temperature is as low as an air cooler can achieve. For this reason, the lean amine temperature is not included in the optimisation.

Upon deeper evaluation, the inlet temperature and pressure to the regenerator are constrained due to the plate and frame lean/rich heat exchanger design. Therefore, the latest study considers these variables constant, to be revisited at a later date when capital cost expenditures are evaluated.

The optimisation of the plant is now primarily considering the solvent concentration, circulation rate and steam consumption.

Currently, the plant is operating near 200 MMSCFD. The model shows good representation of the plant data at these conditions, as shown in Table 4.

Using methods described in previous work, the plant is optimised (1) (7). As demonstrated in the previous work, CO2 absorption decreases as the MDEA concentration increases. Due to project constraints, the maximum MDEA concentration the operators are able to achieve is 48 wt% in this plant. Therefore, the MDEA concentration is set at 48% to maximise CO2 slip into the sweet gas.

The model will calculate the solvent circulation rate and steam rate based on maintaining less than 3.3% CO2 and 20 ppm H2S in the sweet gas. The optimised case is compared to the plant operating conditions prior to optimisation in Table 5.

The steam rate may be reduced by roughly 25%, or 14 tonnes/hr, when compared to the plant’s current steam consumption. To do so, CO2 slip is maximised by reducing the solvent circulation rate. If this level of steam reduction is maintained throughout the year, the optimisation may amount to a savings of roughly 800,000 USD/yr (8).

Application
While it is beneficial to perform the above exercise, the operators see a wide range of inlet gas flowrates. Therefore, the optimisation needs to be done at various throughputs. To optimise the plant in a way that is easy for an operator to control, the model is executed at hundreds of conditions with constraints placed on the process to always maintain less than 20 ppm H2S and less than 3.3% CO2. The cases traverse operating conditions between 150 and 450 MMSCFD at step sizes of 5 MMSCFD.