June 2019

Process Control and Instrumentation

The benefits of automatic batch distillation

Process control methods have been applied to batch distillation of crude oil and intermediate oils performed in the Chevron DPST labs in Richmond, California.

Process control methods have been applied to batch distillation of crude oil and intermediate oils performed in the Chevron DPST labs in Richmond, California. The method is based on following the cumulative yield curve as a “soft” variable and proportional-integral-derivative (PID) controls on the single variables and pot temperature. This combination significantly reduces the time and complexity required to complete the atmospheric distillation process (commonly called D-2892).

Following the cumulative volume curve allows for safe, steady distillation of crude oil without the need for volume/mass tracking or differential pressure control to limit the boil-up rate.

The standard method for lab-scale batch crude oil distillation calls for manual adjustments to the column overhead pressure and power rate to the pot heater. These two input adjustments are carried out in a series of steps requiring significant time periods for stabilization cooling between changes. The Chevron lab typically pushes to a vacuum of 2 torr and pot temperatures up to 315.56°C (600°F). The rate of distillation (e.g., boil-up) will be limited by the capacity of the overhead condenser and the flow resistance the condensed liquid has in returning to the pot. Liquid boil-up that exceeds these limits will result in column flooding. In the extreme case, changing pot conditions too rapidly may lead to flashing of super-heated pot liquid, thereby blowing pot liquid throughout the column and receiver, effectively ending the distillation experiment.

The applicability of the new distillation control method to liquids in addition to crude oil has been demonstrated on vacuum gasoil intermediates and atmospheric residual material. The method can be applied to any liquid where the cumulative volume vs. temperature curve is known or where it can be approximated. The same control scheme can be used in state-to-state (new pressure, new temperature) changes without cumulative curve guidance.

Control methods covered in the literature use mechanical volume/mass tracking or limits in pressure differential between pot and vapor line.1–4

The lab

The Chevron distillation lab in Richmond, California  contains 30 pot stills, ranging in capacity from 50 ml to 5,000 gal, and 6 continuous units ranging from 30 l/hr to 300 gal/hr (FIG. 1). Crude oil assays, samples from eight refineries, products from pilot plants and custom oil distillations from outside organizations are in play every week at the lab. Recognizing the need to reduce the time and complexity of batch distillation, the organization started a program to tailor-fit distillation control schema with the oil’s unique cumulative yield curve (cumyld)to more efficiently drive the distillation process to completion. The goal was to find a set of control parameters that would control the liquid boil-up rate by simultaneously increasing the pot to higher temperatures, while lowering system pressure, staying within the limits of the distillation column’s overhead condenser.

FIG. 1. A view of Chevron’s DPST lab in Richmond, California.
FIG. 1. A view of Chevron’s DPST lab in Richmond, California.

To achieve the goal of simultaneous variable control, two improvements have been implemented. The first is utilization of a PID algorithm in single variable control loops. The second is the use of the oil cumyld, vol% vs. temperature for real-time generation of setpoints for the individual PID loops. The PID control targets are adjusted on the fly to keep the cumulative volume of the oil on track, thus keeping the rate of distillation within the operating capacity of the equipment—namely the ability of the condenser to completely condense the overhead vapor without flooding.

The distillation rate is accumulated over time into a cumulative volume (comvol). This cumvol%, intersected with the cumyld curve, generates a current atmospheric equivalent temperature (AET) setpoint for the controls system. In practice, both the pressure and pot temperature are adjusted to match the target AET delivered by the oil cumyld curve. In this way, the two primary driving forces used in distillation—pressure and temperature—can be moved simultaneously to maintain a constant rate of distillation (boil-up).

The Maxwell-Bonnell5 equations are used to calculate the AET from the VLT at pressures below atmospheric. The control system is tasked to keep the calculated AET on the AET from the cumulative volume curve, while following a preset “rate of production (boil-up)” as displayed by the vol%/min curve (FIG. 2).

FIG. 2. Following the cumulative yield curve.
FIG. 2. Following the cumulative yield curve.

The time required for distillation is approximately one-third that of established procedures. Reducing pressure at the beginning of the distillation process significantly reduces initial pot temperatures, protecting thermally-labile components in the oil. Operator actions such as dropping heating mantles, manual pressure settings and pot heater adjustments, are eliminated. The use of a dry ice trap maintains the overall loss at approximately 0.25 wt% of the charge total.

Through experience with the 3 × 2 (3 ft of packing by 2 in. wide) Chevron column system (FIG. 3), the typical rate of distillate production (boil-up) was set at about 400 ml/hr. This boil-up rate limit is converted to vol%/min by dividing the boil-up rate by the pot charge volume—typically 17,000 ml
(17 liters) at about 0.04%/min.

FIG. 3. Typical batch distillation equipment.
FIG. 3. Typical batch distillation equipment.

The typical Chevron assay includes the separation steps shown in FIG. 4. Nominal AET cutpoints are set and the program drives toward these targets. The pot temperatures are preset for each cut, automatically advancing to the next pot setpoint. In this scenario, the pot is under its own PID control, while pressure adjusts instantaneously to keep the AET on the cumyld boiling point curve. As the pot heats so will the VLT, sometimes erratically. However, the pressure changes respond to the VLT, keeping the AET on track. Even with the onset of boiling, the pressure controller maintains a smoothly changing AET.

FIG. 4. Sequence of cuts for a full distillation. This work goes on to DSL.
FIG. 4. Sequence of cuts for a full distillation. This work goes on to DSL.

The distillation procedure starts by lowering the system pressure without heating the pot. The cumyld tracker moves at the preset rate (in this case, 0.04%/min), sending an AET setpoint value where system pressure is calculated algebraically from the inverted Maxwell-Bonell equation. The PID controls drive the pot temperature to pre-set values for each cut.

FIG. 5 shows the essential features of this method. Starting from the top of the graph are AET, pot temperature, vapor line temperature and pressure (°F and Torr). The numbers in bubbles are the targeted cutpoints. Note that the wavy appearance of pot T and VLT are compensated by pressure to maintain the AET calculated from the VLT on the AET of the cumulative yield curve.

FIG. 5. AET vs. pressure, vapor line and pot temperature.
FIG. 5. AET vs. pressure, vapor line and pot temperature.

Other features programmed into the protocol are pressure bumps combined with a pause in the cumulative volume for “cut taking.” When the laser level indicates a full receiver, the pressure instantly increases by 50 torr and the pot temperature holds at its current level (these are the upward ticks in the yellow pressure curve). This allows for safe switching of the full receiver with an empty, while maintaining vacuum. After reset, the volume accumulator starts up, once again sending new AET setpoints to the AET pressure calculator. In this way, the system follows the cumyld within the condenser limits of the equipment.

FIG. 6 shows the crude assay true boiling point curves for two distillation runs, one using the standard methods employed at Chevron, the second using automatic batch distillation (ABD). The two curves are collinear up to 800°F, separating at this point only because a vacuum distillation was not carried out on the more than 650 ABD run.

FIG. 6. Results for regular vs. ABD.
FIG. 6. Results for regular vs. ABD.

The controls have been enhanced so that different state points (pressure and temperature) can be set independently, without the cumyld. In this way, custom distillations can be achieved without the worry of burping.


Combining PID controls with the cumulative volume curve of oil allows for smooth transitions to higher pot temperatures and lower pressures. Absent expensive volume/mass tracking equipment or complex differential pressure monitoring, these straightforward techniques can reduce the time needed to complete a batch distillation. Just as important is increased safety by reducing operator interaction with hot equipment, which is no longer necessary since the transitions to higher temperature and lower pressure are completed automatically. HP


The authors would like to thank their colleagues at the Chevron Research and Technology Center in Richmond, California. These include Patrick Andrews, Matthew Blickle, Francisco De La Torre, Reckaraido Lcasiano and Andrew Santori.


  1. Distillation Column Flooding Diagnostics with Intelligent Differential Pressure Transmitter, https://www.emerson.com/documents/automation/white-paper-distillation-column-flooding-diagnostic-dp-transmitter-en-87404.pdf
  2. ROFA laboratory and process analyzers, http://rofa-products.com/products/TBP.php
  3. Luyben, W. and E. Quintero-Marmol, “Inferential model-based control of multicomponent batch distillation,” Chemical Engineering Science, 1992.
  4. “Selecting the sensor locations for inferential control of high-purity batch distillation column,” IFAC Advanced Control of Chemical Processes, Pisa, Italy, 2000.
  5. Maxwell, J. B. and L. S. Bonnell, “Derivation and precision of a new vapor pressure correlation for petroleum hydrocarbons,” Industrial Engineering Chemistry, 1957.

The Authors

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