November 2020

Process Optimization

Unconventional improvement of propylene recovery yield at the PP splitter

Thermocompression-equipped propane-propylene (PP) splitters are used to upgrade large quantities of refinery-grade propylene (RGP) to polymer-grade propylene (PGP).

Capra, M., Aggreko

Thermocompression-equipped propane-propylene (PP) splitters are used to upgrade large quantities of refinery-grade propylene (RGP) to polymer-grade propylene (PGP). This configuration does not rely on reboiling heating mediums, and it relies very little on utilities conditions—yet, deterioration of the cooling water temperature is the cause of lower propylene recovery yield.

The following case study includes process conditions, recovery yield, final setup and economic evaluation of a condensation improvement project for rent that exploited industrial refrigeration equipment (mobile chillers) applied to distillation (i.e., PP splitters).

In crude oil refineries, propylene is obtained as a byproduct of thermal or catalytic treatment of heavy oil fractions (specifically coking and visbreaking)—primarily through fluid catalytic cracking (FCC), where olefins are produced due to exposure of hydrocarbons to high temperature, with reactions directly driven by specialized catalysts. Propylene can be used directly in the alkylation unit to produce alkylate—a blending stock of high octane and low Reid vapor pressure (RVP)—for naphtha to correct its volatility and match the anti-knock requirements of gasoline. A more profitable route is to produce PGP (99.5 wt%) and chemical-grade propylene (CGP) (92 wt%) out of RGP (65 wt%–70 wt%).

More than half of the global propylene production is obtained via steam crackers, while 30% of the world’s production is generated directly by refineries—where FCC is the main producer—that can sell their RGP to petrochemical plants specialized to produce CGP or PGP. To utilize high-value propylene grades, large refineries are equipped with PP splitters, especially if they are integrated with petrochemical productions.

The premium price paid by CGP and PGP justifies investments in PP splitters to produce higher-grade propylene, as higher grades are utilized in the synthesis of different chemicals: polypropylene (67%), acrylonitrile (7%), oxo alcohols (8%), propylene oxide (8%), cumene (6%) and acrylic acid (4%).

PP splitter operations

Refineries with onsite separation facilities to recover high-purity propylene may operate conventional distillation equipment, which includes a reboiler utilizing low-pressure steam and water-cooled condensers—while gas-cracking plants may use quench water to reboil the PP splitter.

The top condensation temperature is set by the available cooling water temperature. To limit the condenser size, the ΔT approach should be approx. 5°C–10°C. Consequently, the top pressure of the PP splitter will be very close to the resulting propylene vapor pressure at the corresponding condensation temperature—typically around 20 barg–23 barg.

For example, the higher the temperature of the available cooling water, the higher the top pressure of the PP splitter. When considering a condensation at 50°C, the propylene vapor pressure is 2,071 kPa abs, while the propane vapor pressure is 1,720.4 kPa abs, and the resulting relative volatility is 1.2. The higher the operating pressure of the PP splitter, the lower the relative volatility will be, increasing the required trays to achieve the desired grade. Typically, PP separations by conventional distillation require a high number of trays—between 150 and 200.

Conversely, operating the distillation at low pressure increases the separation efficiency, as the relative volatility will increase. This will also allow reduction of the liquid-vapor internal traffic, along with the number of trays.

CASE STUDY

The problem

Modern PP splitters perform condensation and reboiling operations by thermocompression (FIG. 1). As the distillation is conducted at low pressure (8.3 barg), the gross overhead dewpoint is increased from 24°C to 46.6°C by increasing the pressure up to 18.2 barg. The column bottom is reboiled at 38.5°C and 11.6 barg. This allows the reboiling and condensing operations to be performed in a single heat exchanger by using the condensing vapors as a reboiling medium.

FIG. 1. Schematic of a PP splitter.

As a rule of thumb, when total condensation is achieved, the water-cooled condenser removes the equivalent thermal power introduced by the compressor. The splitter shown in FIG. 1 produces PGP (99.5%) out of RGP, granting a recovery yield of 99.7% of the propylene feed when the cooling water temperature at condenser E-02 is lower than 36.9°C. At a higher water temperature, vapors are vented from the reflux accumulator D-01. Material balances are detailed in TABLES 1 and 2.

Historical records showed cooling water temperature supplied at E-02 during summer daytime to be as high as 39.5°C, with recovery yield dropping down to 60%. This forced the operator to reduce the PP splitter capacity. At €910/t (trading price of propylene at project time), losing 4,228 kg/hr of PGP meant flaring €3,800/hr for at least 7 hr/d. A 100-d project totaled approximately €2.7 MM (nearly $3.2 MM).

Alternatively, the pressure at the condenser E-02 can be increased up to 18.5 barg, as this is the condition for total condensation when the cooling water temperature hits 39.5°C (TABLE 3). The compressor flowrate will drop 5%, along with the reboiler duty. The purified propylene concentration will be off-spec, as the column internal liquid-vapor traffic cannot sustain the separation.

The implemented solution

Cooling 900 m3/hr of water from 39.5°C to 36.5°C engaged a maximum 3,160 kW of refrigeration capacity, which was reduced during night hours to grant stable operations of the PP splitter.

The cooling water feed line to E-02 had to be modified to extract 110.5 m3/hr of water at a maximum temperature of 39.5°C (FIG. 2), refrigerated in four parallel mobile, air-cooled chillers at 15°C and reinjected in the feed line to be mixed with the remaining hot cooling water and to produce a stream at 36.5°C. Mechanical modifications consisted of two 6-in. valve nozzles.

FIG. 2. Schematic of the temporary cooling solution.

The chillers’ set point was fixed at 15°C, while the three-way valves equipping each chiller provided control of the cooling water flowrate refrigerated to keep the final cooling water stream at a constant temperature or 36.5°C (day and night).

As the maximum refrigeration delivered by four chillers was 3,216 kW and the solution was modularly configured, the possibility to add a fifth chiller was maintained. To avoid any modifications of the refinery’s electrical installation, the solution was powered by two mobile diesel generators (1,250 kVA) and a fuel tank with remote monitoring. All working parameters—mainly water flowrate, inlet/outlet temperatures and final cooling water temperature—and the motors’ status were constantly recorded and remotely monitored.

The implemented solution was fully automated and its process is intrinsically safe. It did not require an operator’s presence. Each single electrical motor failure driving the chiller compressor motors resulted in a temperature increase of 0.37°C of cooling water routed to E-02. Each pump electric motor failure resulted in 0.75°C of the same cooling water, providing plenty of time for maintenance and/or repairs.

Takeaway

Propylene recovery yield, and specifications and condensation capacity, were enhanced. The solution (FIG. 2) was engineered, mobilized, commissioned and started up in 8 wk, with no capital expenditures involved—and this solution was a leased engineered temporary solution, which is a new and unconventional approach to temporary process enhancement.

The improved delivery time schedule, project cost structure, benefits and performances make this approach an innovative alternative to capital expenditures, as it ensures a high attention to safety, performance and reliability to match the typical demands of the hydrocarbon processing industry.

Furthermore, the risks of economic and technical failures are mitigated, as costs and benefits are synchronized and the project duration is flexible. This new approach provides a new class of projects that allows customers to take advantage of limited in-time market opportunities or to mitigate utilities upsets, even though the relevant capital expenditures do not surpass the economic test for approval. HP

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