Controlling LPG/propylene quality

Online analysers can minimise costly upsets in FCC LPG/propylene quality and realise savings in caustic wash operations.

Mangalore Refinery and Petrochemicals Limited

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Article Summary

Continuous evolution of FCC process technology has changed the unit yield pattern from one centred on middle/light distillates to a LPG/propylene pattern. This gradual shift in the yield pattern is depicted in Figure 1. The primary driving force of this change has been the increase in propylene demand, advancements in FCC catalyst and process technology and the operational flexibility a FCC process offers. Annual demand for propylene is estimated to grow to 130 million tonnes worldwide by 2023,2 rising from the current value of around 90 million tonnes.2 This incremental demand is expected to be met by on-purpose propylene production by methods such as propane dehydrogenation, metathesis, coal-to-olefins and methanol-to-olefins, in addition to the traditional sources of propylene such as steam crackers and refinery production.    

The high olefin FCC (HOFCC) units operate at very high severity. This pertains to a riser outlet temperature of more than 560-570°C, a catalyst to oil ratio of 12-18 and high rates of C4 and naphtha recycle in the riser, thus increasing the cracking of gasoline fractions, leading to higher yields of C3-C4 fractions. A block flow diagram of a typical HOFCC unit is shown in Figure 2.

The use of appropriately tailored zeolite catalysts also plays a key role in this. HOFCC units are often designed to process low sulphur feeds in order to check gasoline and LPG range sulphur contents. The feed sulphur is restricted to the 1000 ppm range. A typical LPG production and caustic treatment process in FCC is shown in Figure 3. The wet gas compressor discharge containing the products from fuel gas to gasoline range goes via a high pressure separator stripper for removal of C1/C2 and H2S components. This separation is critical to maintain the quality of LPG as most of the H2S is stripped out here, thus minimising the load on the caustic treatment section. The stripper bottom stream consists of C3, C4 and gasoline range components, which are separated in the debutaniser column. The overhead from the debutaniser consists of C3 and C4 components, called sour LPG, and the bottom is stabilised gasoline or naphtha.

Sour LPG is treated in the caustic wash section for the removal of mercaptans and residual H2S. The typical composition at upstream and downstream of the caustic section is shown in Table 1.

Stripper operation and caustic treatment are the two most critical processes for the smooth production of on-spec LPG and polymer grade propylene given the conditions remain steady in the reactor-regenerator section.

Factors affecting HOFCC LPG quality
The two most critical factors impacting the LPG quality from a HOFCC unit are stripper column operation and the caustic treatment system. The stripper column is many times operated close to flooding condition owing to the high liquid and vapour traffic through the column. As a result of this the column bottom temperature required for adequate stripping is kept lower than the design value. With the ensuing flooding, the stripper column pressure drop may go higher than the maximum allowable value. When the column overhead temperature drops below 47°C, H2S slips to the column bottom stream. This scenario is depicted graphically in Figure 4. The slipped H2S appears after a process time lag in the treated LPG, resulting in a quality upset.

H2S slippage from the stripper
The stripper column is operated sometimes close to its flooding point due to significant production of lighter components, resulting in a high liquid:vapour ratio in the column and higher pressure drop. As a result, the bottom reboiling and top temperature are kept below the normal operating values to prevent the column from flooding. It has often been observed that, even with a steady temperature profile, H2S is slipped in the stripper bottom stream, leading to costly LPG failures. This happens due to deteriorating separation efficiency when the column is operated close to its flooding point as the number of ideal stages required exceeds the actual separation stages, and separation performance deteriorates. The bottom stream will have more than the normal operating value of H2S, which is called slippage. An H2S analyser is generally provided only in the treated LPG line downstream of the caustic treatment section. So, in the absence of any online indication of stream components upstream of the caustic section, H2S slippage may go unnoticed until it is detected by the H2S analyser in the sweet LPG stream.

Thus, there is a need for an online H2S analyser in the stripper bottom line to debutaniser column, which will provide a timely alert on H2S slippage as soon as it happens. Several such types of analyser systems are available on the market, offering C1, C2 and CO2 measurements in addition to H2S. With the help of this H2S analyser, the stripper operating parameters can be altered to minimise H2S slippage. This will also lead to suitable corrective actions by a plant operator in the downstream caustic treatment section, such as increasing the caustic circulation rate and fresh caustic dosing. The following is a list of corrective actions that should be initiated:

Reducing column reboiling
If flooding is due to high vapour flow rates, then stripper reboiling should be reduced to control the vapour flow in the column. This scenario is referred to as hiccups and may happen due to gradual accumulation/recycling of lighter components in the column. Unfortunately, this option cannot be exercised very often as the operating reboiling is usually kept much lower than the design and there is a chance of further slippage of H2S and C2 components on any fluctuation of bottom temperature or column reboiling.

Reducing the stripper liquid load
If flooding is due to high liquid loading, then the feed to the stripper can be reduced a little. The indication of liquid loading is the ratio of stripper feed to bottom flow, shown as L1/L2 in Figure 5. It can be termed the column ‘liquid loading ratio’.

In the study case, the design value of the liquid loading ratio is 1.14 and as the operating value goes beyond 1.25 flooding ensues. The increasing value of L1/L2 signifies a reduction in stripper bottom flow as liquid is accumulated on the trays. Thus, reducing the liquid feed to the column offers a temporary and reliable solution to come out of flooding and avoid the resulting H2S slip. However, this option is limited by the upstream stripper feed drum surge volume.

Supplementary lean oil reduction
Debutanised (stabilised) gasoline, also known as supplementary lean oil, serves to enhance the propylene yield of the unit by absorbing the propylene slipped in the stripper off-gas. It contributes to around 20% of the stripper feed and gives the option of reducing the liquid load by compromising a little on propylene yield.

Increasing the caustic circulation and fresh caustic make-up
This is done to maintain a healthy percentage of free sodium hydroxide (NaOH) in the circulating caustic. This is discussed further in the article.

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