S. Chakraborty, Engineers India Ltd., New Delhi, India
Petroleum refining is an energy intensive process. Although the level of energy efficiency, in general, is relatively higher in oil refining compared to the steel or cement sectors, there is still a scope for improvement. Given the quantum of energy used in refining, even a minor increase in efficiency to the process can have a significant impact on refining economics. This is especially true in the present geopolitical scenario, where man-made and natural calamities are impacting the profit per barrel of crude processed.
Energy efficiency is recognized as the first step to decarbonization. The underlying takeaway of the COP28 meeting—United Nations Climate Change Conference held in Dubai, UAE in December 2023—stressed the need for greater energy efficiency and better energy utilization processes. Such declarations are a positive development for the energy industry, especially considering that energy-efficient solutions were historically not prioritized due to the possibility of low return rates. The introduction of carbon costs (carbon incentives) can play a positive role in improving the overall economics.
In this article, a classic case study is presented whereby the feasibility of introducing pressure recovery turbines (PRTs) into an existing process scheme was evaluated.
PRTs vs. let-down valves. With tightening vehicular emission norms worldwide, reducing sulfur in fuel has become mandatory. As a result, almost all grassroots and brownfield refinery expansion projects are associated with either new hydrotreating units or the capacity enhancement of existing hydrotreating plants. Hydrotreating diesel or hydrocracking vacuum gasoil is invariably carried out at high pressure, assisting the process of removing sulfur from oil. Therefore, a very high pressure of 160 kilogram-force per square centimeter gauge (kg/cm2g)−190 kg/cm2g are commonplace in such units which is achieved by heavy-duty compressors. The energy used to pump the fluid to such a high pressure is seldom recovered in profit and, in most cases, wasted and released through valves. PRTs help in recovering this high-pressure fluid energy and using it to generate electrical energy, which can be effectively connected to an existing grid power. Thus, through this process, a part of the grid power requirement can be offset, improving the economics of operation and reducing the dependence on external power sources.
FIG. 1 provides a scheme of a PRT vs. a let-down valve. The energy efficient credentials of the PRT are greater than the let-down valve. Then, why are they not the standard choice?
CASE STUDY: BASELINE DEFINITION
A standalone refinery with primary processing, secondary processing and bottom upgradation units was examined for this case study. The feasibility of installing PRTs as clean energy solutions in the high-pressure streams available at the hot high-pressure separator (HHPS) of hydrocracker and hydrotreater units was evaluated. The three primary streams were targeted to determine if high-pressure fluid energy could be recovered and transformed into electrical energy. The streams were:
The above streams are available at a pressure of 150 kg/cm2g−160 kg/cm2g and with a flow of 180 tons per hour (tph)−370 tph. Of the total flow available, 80% is at the separator bottom, routed through the PRT; the remaining 20% is routed to the existing let-down valves on level control.
In this case study, the power demand of the refinery was partly met from the boiler and steam turbine generator (STG) combination, and the rest was imported from the grid. A substantial quantity of fuel firing was used in the existing boilers to meet the steam and power demands of the refinery. The PRT can generate a significant amount of the power that was generated from the boiler and STG. The power generated through PRTs can reduce the fuel firing in captive power plants (CPPs), which will ultimately lead to a reduction in greenhouse gas (GHG) emissions and reduce the carbon footprint of the complex. This will be in accordance with government net-zero policies, decarbonization financing and environmental, social and governance (ESG) principles. This is a minor but remarkable step toward achieving a refinery’s corporate objective of achieving net zero.
Projected benefits of retrofitting a refinery to use PRTs. To evaluate the maximum benefit being accrued from the above three proposals, extensive vendor interaction was carried out. Based on the technology offerings, the projected benefit in electrical power was in the range of 1.5 megawatts (MW)−2 MW. TABLE 1 shows a summary of the benefits from the three PRTs.
Therefore, the net benefit accrued is approximately 2 MW of additional power generation, which is a significant offset. The additional power generated here helps in offsetting the load of the CCP by an equivalent amount, thereby, reducing the amount of fuel firing in the boilers. Due to reduced fuel firing, there is an overall benefit of approximately 1.4 tph of carbon dioxide (CO2) reduction during operation. If a facility operates for 8,000 hr/yr, more than 11,000 tpy of CO2 emissions will be avoided.
Economics of the proposal. An in-depth economic analysis of the benefits accrued vs. investment required was carried out. The total investment included budgetary cost estimates from reputed PRT suppliers. The overall cost of the project included vital project execution considerations such as:
There are some important inferences from the above summary. First and foremost, the sales revenue or the benefit accrued in power saving seems to have a high payback period, making the proposal difficult to adopt from a financial point of view. However, with the inclusion of carbon cost, there is a significant improvement in the project economics. The same is evident when considering the simple payback of the project, which was reduced substantially when the carbon cost is calculated.
Takeaways. As the third-largest consumer of energy in the world, India has shown tremendous determination and commitment to international communities’ efforts to combat climate change.
Although a complete transition to renewables or zero-carbon sources is the goal, in the interim (as such technologies mature and become cost competitive), it is prudent to focus on opportunities to improve existing technologies. Today, an emphasis is being placed on more effective energy usage in the processing industry, and consequently, areas in which energy is wasted are being closely monitored and methods for energy recovery are being investigated.
A fluid under high pressure must be throttled to suit subsequent process steps. Typically, conventional pressure reducing valves are used to dissipate and waste hydraulic energy. Hydraulic power recovery turbines (HPRTs) can convert the excess pressure into mechanical shaft energy and increase the overall process efficiency. It is being recognized increasingly that replacing throttling valves with turbines can effectively recover a large percentage of the available energy with an acceptable prime cost.
PRT installation can help convert the potential hydraulic energy to electrical energy and further generate power which would ultimately be exported to the outside battery limits for internal usage inside the refinery. This means that the power produced from these PRTs would reduce the power requirements for the complex, resulting in less fuel gas firing in the CPP. A decrease in the fuel gas quantity used in firing would reduce CO2 emissions. Therefore, PRTs can be a useful tool to align operations with net-zero policies, decarbonization financing and ESG principles.
It is evident from the earlier sections that the financials of the project may not sound lucrative with respect to money and time, but considering CO2 emission reductions and the capital expenditure involved, retrofitting PRTs in existing refineries instead of let-down valves may gain more attention in the future, especially with carbon cost projected to be a contributing factor in energy economics. Thus, PRTs are an effective clean energy solution with proven credentials and can act as a catalyst in a refinery’s journey to decarbonization and ESG goals. HP
SUBHOSREE CHAKRABORTY is the Assistant General Manager in Engineers India Ltd.’s (EIL’s) process department. She has more than 16 yr of experience in refinery and petrochemical configuration, utility system design and energy benchmarking. Chakraborty earned her B.Tech degree in chemical engineering from Kolkata. Email: s.ghosh@eil.co.in