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Oct-2009

Stepwise simulation of vacuum transfer line hydraulics

A stepwise hydraulic calculation determines the pressure profile of a vacuum transfer line by linking the hydraulic model to process simulation results

Harry Ha, Fluor Canada
Matthew Reisdorf, Fluor Enterprises
Abdulla Harji, Fluor Canada
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Article Summary
When designing a vacuum transfer line, a robust hydraulic model that predicts velocity and a corresponding pressure drop is crucial. A stepwise approach to hydraulic modelling of vacuum transfer lines increases accuracy and enhances the under-standing of two-phase fluid behaviour. Vacuum gas oil yield, reliability and operability depend on correct design of the vacuum transfer line.

With the depletion of global conventional oil, refinery feedstocks are becoming heavier and often rely on unconventional heavy oil. Canadian oil sands-derived bitumen — a heavy, unconventional oil — is being increasingly processed in Canadian upgraders to produce synthetic crude oil as part of the crude slate in US refineries. Processing unconventional heavy oil like bitumen is challenging. Unconventional heavy oil is usually unstable and prone to coking at high temperature. To maintain reasonable run lengths, a temperature limit is applied to the heater outlet, which indirectly puts a limit on the heater tube’s inside 
film temperature.

Design objectives
A typical objective in the design of a vacuum unit is to maximise the yield of vacuum gas oil to improve a refinery’s profitability. The vacuum overhead system, column flash zone, vacuum transfer line and the charge heater have to be optimised as a single system to ensure that design objectives are met during unit operation.

Based on the steam and cracked gas loads, the vacuum overhead system is configured so that a low absolute pressure is ensured in the flash zone to allow the unit to reach target oil vapourisation at an acceptable heater outlet temperature (HOT). A pressure drop in the vacuum transfer line sets the heater outlet pressure (HOP), which in turn determines the HOT by vapour-liquid equilibrium.

The HOT is limited to an acceptable value to avoid coke formation inside the heater coils. A high pressure drop in the vacuum transfer line increases the temperature difference from the heater outlet to the flash zone. Given a fixed HOT, an increased pressure drop in the vacuum transfer line decreases the vacuum gas oil lift in the flash zone, resulting in a lower yield of vacuum gas oil. Therefore, the vacuum transfer line’s hydraulics plays a crucial role in achieving the desired product yields and operational reliability.

A study1 showed that one extra kPa added to the total pressure drop of a transfer line reduces the gas oil yield by about 0.2 vol%. For a refinery with a 100 000 bpd throughput, each kPa of pressure drop in the vacuum transfer line implies a significant loss of revenue. From a process design point of view, the pressure drop in the vacuum transfer line should be as small as possible to maximise the yield of vacuum gas oil. This usually leads to a large transfer line and increased heater passes, resulting in significantly increased capital costs. Therefore, it is essential to select the most cost-effective design, which meets both process design and mechanical requirements.

The vacuum transfer line is a large, elevated line that routes the vacuum unit feed from the charge heater outlet to the vacuum tower flash zone. Depending on capacity, the line’s diameter can range from 48–84 inches inside diameter and its length is typically 40–70 ft. A typical piping layout for a transfer line includes either individual heater pass outlet piping discharges into the main line routed to the vacuum tower, or half the heater passes discharge into a manifold and the two manifolds discharge into the main line. The piping design group should be consulted to establish the preliminary transfer line routing, including approximate lengths and allowance for thermal expansion. Since transfer lines have a low allowable pressure drop, pressure loses due to fittings should be minimised.

The number of parallel heater tube passes is determined by the required cross-sectional area at the heater outlet to accommodate the large volume of two-phase flow. At the heater outlet, there are typically four to eight separate heater tube passes from one or more cells. While cost-effective heater design favours using fewer tube passes, the need to stay below the critical velocity necessitates an adequate number of tube passes.

A limiting factor in minimising the vacuum transfer line’s size is the bulk HOT. For a state-of-the-art and deep- cut vacuum unit, the maximum recommended oil temperature in a heater is usually 365–415°C to avoid excessive cracking and coking within the heater coils. To reduce cracking and coking, the HOT must be set low enough to ensure the film temperature does not exceed the maximum recommended oil temperature.

The pressure in the vacuum transfer line keeps decreasing from the HOP 
to the pressure of the column flash zone, which is normally set at 20–30 mmHg absolute for a deep cut. A typical pressure drop in a vacuum transfer line is about 100–150 mmHg to achieve a deep cut point. Considering the low pressure at the flash zone, the pressure drop of the vacuum transfer line is quite significant. Corresponding to this pressure change, the temperature also changes isenthalpically from the HOT to the flash zone temperature, as the feed goes through adiabatic flashes in the transfer line. Depending on the total pressure drop, a temperature difference of 10–20°C can be expected between the heater outlet and the flash zone. As a result, the vapourisation, density of fluid, volumetric flow and the transport properties all change simultaneously along the transfer line. Therefore, a stepwise, equilibrium simulation is necessary to reflect the continuous changes in a vacuum transfer line.

An important consideration in transfer line design is the two-phase critical velocity, which raises concerns about vibration in the line, especially acoustic-induced vibrations. Field measurements of the vacuum transfer line and calculations using theoretical hydraulic models confirm the existence of critical velocity and its influence on the pressure profile inside a vacuum transfer line.

This phenomenon is difficult to predict for the two-phase flow system. Two-phase critical velocity is much lower than the sonic velocity of the gas phase alone. Therefore, many transfer lines, designed to run under sonic velocity, actually operate at critical velocity, especially near the heater outlet and at the column entrance. The potential impacts of critical velocity are not trivial. The vibration of shock waves could result in failure of the vacuum transfer line.
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