Optimising preflash for light tight oil processing
For a crude unit handling light tight oil feedstock, a semi-preflash column can deliver significant advantages in energy saving, capacity gain and revamp economics.
Soun Ho Lee
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The basic function of a crude distillation unit is to provide the initial separation of crude oil feed into the desired fractions to feed downstream units. Now, newly introduced light tight oil feed processing may limit existing crude distillation unit capacity. The addition of a feed preflashing option may improve capacity and/or energy efficiency for light tight oil processing.
Two preflash options which are commonly implemented for crude distillation units are a preflash drum and a preflash column. Each has strengths and weaknesses.
This article discusses an improved semi-preflash column design that optimises the traditional preflash drum and preflash column options. A case study includes revamp economics for the arrangement.
Light tight oil
Light tight oil describes petroleum fractions produced from low permeability formations such as shale or tight sandstone.1 Improved hydraulic fracturing technology with directional drilling has increased light tight oil production in North America, where processing light tight oil can be a game-changer for refiners. Light tight oil slates produced in North America have typically light and sweet natures. However, its characteristics vary significantly.
Adapting light tight oil as a new feedstock may require a significant retrofit in a refinery layout which was originally configured with traditional crude oil feedstock. Capacity imbalance in downstream conversion units and insufficient overhead train processing capacity are already well recognised issues with refiners. This imbalance issue is more exaggerated in refineries which are designed with heavy crude slates and a high conversion ratio. These mismatches between new light tight oil feedstock and current refinery configurations are initiated in the crude distillation unit which provides initial separation of feedstock for entire downstream refinery processing units.
There has been extensive research to try and identify ‘magic juice’, blending ratios of light tight oil and traditional heavy crude oil slates to fit existing refinery layouts. Meanwhile, it has been discovered that blending between light tight oil and traditional heavy crude oil can cause asphaltene destabilisation due to the paraffinic nature of light tight oil.2 This destabilisation can cause unexpected fouling issues.
This article focuses on debottlenecking and enhancing crude distillation unit capacity and energy consumption using various preflash options. Other retrofit strategies for light tight oil processing are not discussed.
Crude distillation unit preflashing
Implementing crude feed preflashing has been one of the common options to debottleneck crude distillation units. Switching feedstock to a lighter crude slate can cause undesired feed vaporisation at the feed furnace pass control valve and limit capacities in the crude atmospheric column and overhead condensing circuits. Adding a preflash drum or column can reduce the charge rate to the feed furnace. In addition, transfer line vibration due to capacity limitation can be resolved through adapting feed preflashing.
Two preflash options that are commonly implemented in a crude distillation unit are the preflash drum and preflash column. Another preflash option, GT-IPS is a semi-preflash column design that may be optimised between traditional preflash drum and preflash column options.
Preflash drum option
The preflash drum is the simplest option for crude preflashing. Complex modification of equipment and instrumentation is not required for this option compared to the preflash column, making it the most economical option with regards to capital expenditure (Capex).
One weakness of the preflash drum is that this option does not have the capability to control preflashed vapour quality in foaming prone service. Therefore, the preflash drum is prone to have challenges with entrainment. Entrainment can be accelerated in a foaming environment. Improper location of preflashed vapour feeding and/or undesirably entrained heavy oil boiling range materials can contaminate rundown product qualities.
Introducing preflashed vapour to a flash zone is the safest configuration to prevent product contamination. However, lower temperature of preflash vapour containing entrainment can quench the main feed vapour and can reduce feed lifting.3
Although recent technology such as Vortex Tube Cluster (VTC) helps to discourage the chances of foaming, a preflash drum should be sized large enough to prevent foaming. 4
Preflash column option
The preflash column option produces preflashed products. Although it requires higher Capex than a preflash drum, this option is especially effective when the column top section and overhead condenser capacities are limited.
A rectifier is a common configuration for a preflash column. Crude feed is introduced at the preflash column bottom and preflashed vapour is rectified by reflux liquid. Overhead vapour can be condensed through a crude atmospheric column overhead condenser if the condenser has ample capacity. An exclusive preflash column overhead condenser needs to be installed if a crude atmospheric column overhead condenser is limited. Some deluxe preflash columns have a bottom stripping section to control the front end of the crude atmospheric column top product.
High Capex is a major drawback of a preflash column. Moreover, a large plot space is generally needed to add the equipment.
GT-IPS semi-preflash column option
GT-IPS (Improved Preflash System) has a ‘semi-preflash column’ arrangement compared to the traditional preflash column. Like the preflash drum option, the semi- preflash column does not produce any independent product. This column functions as a preflash column but the additional overhead condenser and receiver are not equipped for the preflash column.
Maintaining a desired preflash vapour quality is one of the most critical issues. To wash out the entrained components, some liquid is withdrawn from the crude atmospheric column and routed to the semi-preflash column as reflux. Then controlled preflashed vapour is safely fed to the middle of the atmospheric column.
One of the benefits of the GT-IPS concept is its flexibility. The preflash column reflux source and the preflashed vapour routing can be varied depending on process conditions, including crude slate and target performance, and refinery layout.
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