Revamps 2023 Issue

Today’s owners/operators are also frequently faced with the challenge of processing additional gas supply while minimising capital expenditures. A retrofit can provide a relatively inexpensive alternative to building a new plant to achieve higher production targets. Once the goals of a retrofit are identified, a study is rec - ommended to select the best process to achieve the stated goals. This study should include a simulation of the original process design and a simulation of the current plant that may include updates to equipment or possible deficiencies that may or may not be corrected with the retrofit. When an accurate representation of the equipment per- formance has been completed, simulations of various retro- fit alternatives can be developed. These simulations should consider all limitations of the current equipment, such as exchanger performance, rotating equipment capabilities (using available power and performance curves), and rat- ing of separators and columns at the proposed retrofit conditions. Finally, additional external restrictions must be consid - ered. These may include the available plot space and loca- tion for new equipment, time allotted for a plant shutdown to make tie-ins and install equipment, and available fund- ing for the project. A detailed analysis will identify the best technology for the plant retrofit. Retrofit options Retrofitting a cryogenic NGL recovery plant is not a new concept. The procedures applied more than 40 years ago to upgrade simple expander plants to GSP have not changed and are just as applicable to upgrading a standard GSP unit to an enhanced retrofit technology. Several papers have previously covered different retrofit options and the pro - cess of accomplishing retrofits. 4,5 One example is upgrading the GSP unit to the recycle split vapour (RSV) process. For the purposes of this article, we will re-introduce a simple GSP-to-RSV retrofit as a starting point and expand to more recent retrofit technology options specifically developed to upgrade a GSP plant. Recycle split vapour retrofit When increased product recovery is of key importance, converting the standard GSP plant to the RSV process can provide rapid returns with minimal downtime. Figure 2 shows a typical RSV retrofit applied to the gas subcooled process. New equipment and piping are shown in red, with tie-in points indicated by circles. As a means of extending the existing demethaniser with - out any welding or structural concerns, a new absorber column (packed or trayed) is added to improve fractiona- tion. Overhead vapour from the existing demethaniser is routed to the bottom of the absorber. Absorber bottoms pumps transport the bottom liquids to the top of the exist - ing column. The top reflux for the absorber is provided by a recycle stream of lean residue gas that has been compressed. This high-pressure gas is cooled, condensed, and sub- cooled in a new heat exchanger. The high-pressure liquid is then flashed to provide reflux for the top of the absorber,

New heat exchanger

Absorber

Subcooler

Residue gas

Pump

Residue gas compressor

Expander

Inlet gas

Demethaniser

Bottom product

Figure 2 Standard RSV retrofit

The GSP technology was a great improvement and remained the best available NGL recovery technology for many years, but it was not without limitations. In particular, the source of the top reflux stream was still essentially feed gas, containing significant fractions of ethane and propane. In ethane recovery mode, ethane recovery is limited to approx - imately 90-94%, with a propane recovery level of around 99%. Recoveries beyond these levels are technically achiev- able, but the required additional compression is not econom- ically viable. In addition, market conditions in recent years have, at times, made ethane more valuable in the residue gas product for the reasons previously mentioned, forcing operators to switch to ethane rejection mode. Unfortunately, the ‘standard’ GSP process typically loses 5% to 15% of the propane to the residue gas stream when configured for full ethane rejection, i.e., less than 2% ethane recovery. When an accurate representation of the equipment performance has been completed, simulations of various retrofit alternatives can be developed Process retrofit feasibility The limited performance of the ‘standard’ GSP technology provides many opportunities for retrofitting an older facil - ity with an enhanced, more efficient technology, especially given the large number of GSP installations worldwide. The goals for any retrofit will depend on the specifics of each project. As previously mentioned, potential benefits include higher product recoveries, improved operational flexibility, and increased plant throughput. These enhanced retrofit technologies can achieve greater than 99% ethane recovery and 100% propane recovery in ethane recovery mode or maintain greater than 99% propane recovery while rejecting ethane, with equal or only slightly more compression power.

Revamps 2023