Apr-2009

Optimising crude unit design

Real retrofit examples demonstrate how crude units can be successfully optimised with the crude slates currently being processed. Process design strategies are discussed in detail and highlight how retrofit targets are achieved

Soun Ho Lee and Ian Buttridge, GTC Technology
Jay J Ha, GS Caltex Corporation

Article Summary

The basic function of a crude distillation unit (CDU) is to provide initial separation of the crude oil feed mixture into the desired fractions to be further processed in the downstream units. The crude unit’s quality of performance impacts heavily on the downstream unit’s performance. A lot of crude units currently operate with different feed slates to their original feed specifications. This change in feed composition often results in inferior crude unit performance and reduces the unit’s run length. Re-optimising the design and operation of the crude unit with current feed slates is essential to maximise a refiner’s economics. In addition, recent crude oil price fluctuations and increased economic pressure further emphasise the importance of optimising crude unit performance.

Crude atmospheric column
The crude atmospheric column is the CDU’s core piece of equipment. In a typical crude unit design, crude oil is heated and introduced to the crude atmospheric column’s flash zone. The light products are typically recovered as distillates from multiple liquid product draws and the remaining crude is discharged at the column’s bottom. The original process arrangement relied on a single top reflux flow. The column top reflux provided condensation for all the required product draws, plus the overflash. This approach created high variations in the internal vapour-liquid traffic throughout the column (from column top to flash zone), with a maximum reflux loading at the top and the lower wash section receiving only a small amount of liquid, wash oil. The columns were then sized according to the greatest load, top section internal traffic, which resulted in an oversized column diameter. Moreover, the required size of the overhead condenser was substantially increased.

To minimise these liquid traffic variations, inter-condenser design philosophy was adapted in the crude atmospheric column design. Inter-condensers can be configured as either pumpback or pumparound circuits. Figure 1 compares typical internal reflux rate variations through the crude atmospheric column (from column top to flash zone) among three reflux methods.1 The pumparound reflux method achieves more uniform liquid balancing through the column than the other two reflux methods. This uniformity of liquid enables the column to be sized at a smaller diameter for reduced investment cost. The higher pumparound draw temperatures increase the opportunity for heat recovery for lower energy consumption. In addition, the overhead condenser size is reduced. The main trade-off is that the pumparound circuit design requires more trays and/or packing for heat transfer performance. In summary, the advantages of the pumparound reflux arrangements far outweigh any disadvantages, and as a result it has replaced the pumpback reflux method in most modern crude atmospheric column designs.

The presence of a top pumparound circuit depends on overhead distillate yield/fractionation requirements, column top temperature control and overhead condenser size/limitations. Typical crude atmospheric column overhead configurations are depicted in Figure 2 for cases A-C. Case A shows that the top section reflux is provided by a top pumparound circuit only. In this configuration, the column top temperature is relatively high and the chance of water condensing at the column top can be minimised. In addition, the overhead condenser size can be minimised due to a lack of top reflux stream. However, the top pumparound trays and/or packing do not contribute towards fractionation. An additional fractionation section is required to achieve the desired fractionation between the overhead distillate and the first side product, which increases the overall column height.

Case B depicts a crude atmospheric top section with a top reflux without a top pumparound circuit. This top reflux temperature is usually lower than the reflux through the top pumparound circuit. In this case, fractionation performance between the overhead distillate and the first side product can be maximised at the given column height. However, the required overhead condenser duty is higher and the column top temperature is lower than for Case A.

Case C is somewhat of a compromis-ing design between Cases A and B. This configuration has a top reflux irrigation line as well as a top pumparound circuit. The amount of cold reflux (from top reflux) and hot reflux (from top pumparound) can be controlled at given processing conditions. This configura-tion is suitable for the crude atmospheric column, which faces high variations in overhead distillates and yields.

Crude column internal vapour and liquid traffic rely heavily on pumparound circuit locations. The number and location of pumparound circuits are determined by crude slate structures, product yield patterns, fractionation requirements, overhead condenser size and other factors. The crude atmospheric column is designed to provide the best performance for specific ranges of crude slates and product yields. Therefore, a large change from design conditions may induce performance downgrading in the crude unit. Fractionation performance between adjacent products requires specific design internal reflux at a given number of fractionating trays or packed bed depth. Crude overhead condenser duty is also determined by design heat balances.

In most cases, the actual crude slate structures processed in the crude unit deviate from the original design ranges. To maintain desired unit performances at changed feed conditions, most refiners adjust and rearrange the pumparound balances. These operation parameter changes shift the column traffic through the crude atmospheric column. The pumparound rate change impacts neighbouring fractionation section internal reflux rates, so fractionation performance is affected.2 Unbalanced column traffic often results in unit capacity limitations. The crude atmospheric column design should be re-evaluated with current operating blends to ensure the best performance possible.

The following actual retrofit case demonstrates how a crude unit can be successfully optimised, considering a more typical crude blend used by the refinery.

Retrofit background
The crude unit under discussion was originally commissioned in the early 1970s. Charged crudes are heated through two parallel preheat trains and furnaces, and then introduced into the crude atmospheric column. This column separates crudes to intermediate products: unstabilised naphtha, kerosene, light gas oil (LGO), medium gas oil (MGO), heavy gas oil (HGO) and reduced crude (R/C). Unstabilised naphtha is then fed to the naphtha stabiliser to separate the LPG and naphtha. The kerosene stream is transported to the hydrotreating unit. LGO, MGO and HGO are combined to the diesel pool after hydrotreating. Reduced crude is either transported to conversion units or blended to fuel oil.