Friday, March 23, 2012

Optimize hydrogen management for distillate production

Moving more distillate streams to diesel production and/or fuel oil (FO) production is a major refining activity. The distillate qualities and quantities, hydrogen price and consumption, product demand and prices, and refinery configuration constraints are key factors for these decisions.
Producing higher-quality products from poorer-quality crudes requires more hydroprocessing. These distillate streams, e.g., heavy kerosine (HK), gasoils (GOs) and light cycle oils (LCOs), etc., are also used as cutter stocks to upgrade vacuum residues (VRs) and downgraded as low-value FO products. FO and VRs are major constrains if no resid-upgrading facilities are available.
The heavy distillate streams from the fluid catalytic cracking unit (FCCU) must be hydrotreated to upgrade todiesel specifications. Hydrotreating consumes substantial hydrogen quantities, and this process adds more costs. When adding new hydrogen-consumer streams, hydrogen demand can exceed available refinery supplies. In such scenarios, optimizing existing hydrogen supplies is the key to improving the total profitability of the refinery.
A simple reliable optimization tool for routing intermediate distillate streams maximizes benefits while meeting all constraints of hydrotreating capacity, hydrogen, VR utilization, product specifications and prices. The Excel-based calculator is generic for hydrogen management, and it also fits into various refinery configurations where different resid-evacuation options are practiced.

Hydrogen management.

Processing opportunity crude oils and meeting critical product specifications of EURO III, IV and V are the real challenges for refiners. The quality of straight-run (SR) distillate streams obtained from the processing of high sulfur (S) and low API crude oils is considered inferior. Thus, refining higher-quality products from poorer-quality crudes has increased hydrogen addition. Conversely, LCOs (from the FCCU) are such intermediate distillate streams; they are also routed through the diesel hydrodesulfurization (DHDS) unit to meet product specifications. These streams (especially LCO) consume substantial hydrogen quantities, thus increasing processing costs to meet final diesel specifications. Under these conditions, optimizing hydrogen consumption is the key to total profitability.
Planning tool. To fully optimize the routing of intermediate distillate streams and hydrogen management, additional requirements for these streams must be added to existing planning and optimization tools. These tools must not only determine hydrogen and hydrocarbon routings, but also accommodate individual unit capacities and refinery configuration constraints.
These intermediate distillate streams, in general, are also routed to residue evacuation when no modern resid-upgrading facilities such as coker/visbreaker/solvent deasphalting (SDA) are available. Adding intermediate distillate streams as cutter stock with residues produces FO, when capital investment may not be required. In the presented study, an optimization tool evaluates the routing of intermediate distillate streams to the diesel pool and/or resid-upgrading while meeting the refinery configuration constraints.
More hydrogen demand. Hydrogen consumption for intermediate distillate streams has grown significantly. Hydrogen-addition processes, in general, are preferred due to two factors. First, new environmental regulations over transportation fuels require higher-quality refinery products. Second, the differential prices for light- and heavy-crude oils continue to increase as light-crude reserves are declining, and supplies of heavy-crude oils are increasing. Refiners are taking advantage of these spreads; they are incorporating more lower-cost, heavier, sour, opportunity crudes into the feedslate.
Under these conditions, it is essential to understand the crude oils and their hydrogen content. Increased hydrogen consumption is an additional cost to process these crude oils. Therefore, to produce the same yields of transportation fuels either carbon rejection and/or hydrogen-addition processes must be selected. In actuality, even with incremental new carbon-rejection process capacity, additional hydrogen consumptions and, thus, their enhanced process capacities are preferred. This processing scheme enables optimizing hydrogen management for the refinery. The presented optimization tool can help facilitate efficient hydrogen usage in various resid-upgrading scenarios.

Problem definition.

While processing high-sulfur crude oils at a crude distillation unit, the processed distillate qualities were considered inferior. These intermediate distillates and LCO (from the FCCU) streams are routed through the DHDS unit to meet diesel-product specifications. These streams (especially, LCO) consume additional hydrogen to meet final diesel specifications. However, there are capacity limitations for hydrogen (maximum of 35 tpd) and hydraulic limits for DHDS capacity (maximum of 6,000 tpd). This is a common problem for any refinery. The presented study can be replicated to any other refineries with many commonalities and/or additional constraints.
In the presented case, the VR is being evacuated as FO, where it consumes distillates as cutter stock to meet the final product specifications. Two grades of FO (180 cst and 380 cst) are produced when the cutter profiles are different. Thus, the available distillate streams are being used either for diesel production, which has a higher value and/or routed to FO production, which is needed for upgrading VR.
The minimum VR production is approximately 3,500 tpd while processing 18,000 tpd of crude oils (6 MM tpy crude oil processing basis). The distillate streams available at this refinery are HK (high sulfur), GO (high sulfur) and LCO (high sulfur), etc. Fig. 1 shows the processing flow diagram for routing distillate streams. The cutter requirement depends on the quality of the produced distillate products. However, while optimizing the overall FO production, one must consider the total hydrogen consumption for upgrading both SR and cracked feedstocks.

A simple optimization tool was developed for optimal routing of the distillate streams to diesel and/or FO production. The study considered all constraints of meeting DHDS capacity, hydrogen capacity, VR utilization and product specifications, prices, etc.

METHODOLOGY

Diesel production is always the first choice due to its higher value over FO production. When distillate streams are routed through the DHDS unit, hydrogen is consumed to meet diesel-product specifications and, of course, increases processing costs. However, FO production needs no additional cost, but FO demand is declining. In this scenario, VR upgrading is one of the limits when equivalent distillates are downgraded. Thus, an optimal decision must be made between additional hydrogen consumption vs. producing more low-value FO products.

Hydrogen consumption.

In diesel hydrotreating, hydrogen consumption is governed by feed properties and product specifications. The affecting variables are carbon/hydrogen (C/H) ratio, S, basic nitrogen (N) and metal content, etc. To estimate the C/H ratio, a correlation as a function of specific gravity is applied. In this study, C, H and impurities (I) are evaluated in a balanced approach, especially across the DHDS unit to determine H2 consumption while upgrading distillates. The estimated H consumption is based on these assumptions:
·         Data for distillates are from refinery test runs
·         H2 consumptions are based on the distillate quality
·         All components, other than C and H, are considered as impurities for the calculations
·         Estimated cost for H2 is $2,150/ton.
In this approach, the Excel-based optimization tool was developed to maximize benefits by optimum routing of intermediate streams to diesel (via DHDS) and/or FO production. The spreadsheet is enabled with macro, where input and output are linked with a single click button, as shown in Fig. 2. The model has provision to enter all inputs for the total distillate quantities available to routing and qualities, product specifications and prices, cutter profiles to meet product specifications and all process limits, e.g., DHDS capacity, hydrogen and VR upgrading, etc. These input data are treated as the Base Case, which is normally being practiced. The output data are reported as H2-consumption profiles for each stream, capacity utilization, optimum routing of intermediate distillates, final product profiles and overall benefits.


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