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Correcting vacuum column design flaws

Conventional design practices have been proven inadequate when maximising diesel recovery and gas oil cut point

Recon Management Services, Inc.
Article Summary
To achieve a good return on investment (ROI) from a project requires developing minimum cost designs regardless of whether it is a grassroots or revamp project. However, low cost designs do not necessarily guarantee a good ROI. In recent years, a number of new fuels vacuum units have been designed to produce diesel off the top of the vacuum column (see Figure 1). The vacuum units are commonly designed for low pressure damp or wet operation to enable maximising HVGO cutpoint.1 These design conditions, in addition to higher LVGO pumparound duties associated with maximising diesel production, lead to large diameter LVGO pumparound sections. The operating conditions associated with the design make the possibility of excessive overhead slop oil production much more likely. Design practices of the past are inadequate and must be re-evaluated to obtain an acceptable design.

This article is based on a revamp design to fix the operation of a large scale Gulf Coast vacuum column with excessive liquid entrainment into the overhead system. The LVGO pumparound section of this vacuum column is 38ft (11.58m) in diameter and typically has a diesel product yield of 17500 b/d (115.9 m3/hr). The vacuum column is designed for damp operation, meaning that it has a resid stripping section using stripping steam and the charge heaters utilise coil steam. Spray header distributors produce a range of droplet sizes and essentially all LVGO pumparounds designed with these distributors have some overhead entrainment. This unit typically operates at about 1700 b/d (11.26 m3/hr) of slop oil yield, with approximately 40% of this from equilibrium oil and the remainder from entrainment. While this may seem high, in actuality it is worse. Due to problems with the overhead ejector system design, the column operating pressure is normally higher than its original design value. In addition, the operators limit the LVGO pumparound circulation rate to limit overhead entrainment. During operation with increased LVGO pumparound circulation or during the winter when the conditions enable lower pressure operation, the vacuum column overhead slop oil make has been in the range of 2750 b/d (18.22 m3/hr) and at times higher. Due to economic yield incentives, the refiner would prefer to operate at the lower column pressure and higher LVGO pumparound rate if these problems did not exist. At the lower column pressure and higher pumparound rate, the liquid entrainment into the overhead is in excess of 2000 b/d.

Slop oil make is detrimental to the plant’s bottom line. At minimum, it adds to the operating costs to reprocess the oil and in the worst case it consumes plant capacity to process additional crude. To reduce the excessive slop oil make at lower pressure operation, the operators must adjust the heat balance on the column. It is modified to lower the LVGO pumparound heat duty. This reduces the required LVGO pumparound rate and consequently reduces the entrainment rate to a somewhat more acceptable overhead slop oil yield. This however significantly reduces the diesel yield.
LVGO pumparound
The typical design of a vacuum column LVGO pumparound section includes a product/pumparound collector tray, a packed bed, and a spray header for pumparound return liquid distribution. A spray header has been the industry distributor of choice because it is relatively cheap and it provides for good heat transfer. All spray headers produce a range of liquid droplet sizes. Normally, the majority of droplets are large enough and the vapour velocity low enough that the entrainment is minimal. When using spray headers, a damp vacuum column design always makes the entrainment problem worse because of the corresponding higher top of the column vapour velocity. The problem becomes more of an issue with higher coil and stripping steam rates. Dry columns have very little vapour compared to damp columns when operating at the same column overhead temperature and pressure. Also, the more spray nozzles the higher the potential entrainment. This LVGO pumparound distributor has 61 nozzles while most vacuum units have 7 to 19.

Spray header design
Spray header distributors, as shown in Figure 2, are good distributors for packed bed heat transfer service. This is due to the high surface area contact between liquid and vapour due to atomisation of the liquid. Heat transfer between the liquid and the vapour phase is occurring prior to entering the packed bed. However, in general, the liquid distribution to the bed is only roughly uniform and every nozzle has different distribution characteristics.

While spray headers are relatively simple distributors, many engineered designs that have been produced are very poor. Common design mistakes include: layouts that produce excessive liquid overspray onto the vessel wall; too small an nozzle orifice size that is not practical and leads to fouling; and designs that do not take into consideration turndown requirements or, as addressed in this article, that ignore the effects of liquid entrainment.

Equilibrium and non-equilibrium oil in overheads
High vacuum column overhead slop make can occur from operating problems in the vacuum column or from light material carried over from the atmospheric crude column. A case study of the latter is provided in the literature.2
Table 1 lists the vacuum column overhead slop oil make distillation curve determined by the plant laboratory during one set of operating conditions. The column top pressure and temperature measured were 38 mmHg and 125°F (51.7°C) respectively. Proper evaluation and fixing of the problem requires good test run data.3 Using a full set of test run data to model the vacuum unit yields a much lighter overhead slop oil composition based on equilibrium calculations at the measured operating conditions. The measured overhead slop oil make on this day of operation was 1726 b/d (11.43 m3/hr) with a 38.1 API gravity. To match the model results with actual operation, 978 b/d (6.48 m3/hr) of entrained LVGO pumparound liquid had to be added into the column overheads to match the material balance and slop oil composition.

Equilibrium oil in the column overhead vapour is a function of the column overhead operating pressure and temperature. Non-equilibrium oil is heavier oil that should not be in the vapour phase at these operating conditions. In this case, the heavier oil is from the smaller atomised oil droplets formed from the pumparound distributor spray nozzles that are entrained by the rising vapours. The vacuum column also has temperature indicators measuring the vapour temperature in the top head of the vacuum column and in the column overhead vapour line. There was a temperature drop of 21°F (11.7°C), which is further indication of entrainment. As the rising vapours from the LVGO pumparound bed exit the column, they are mixed with the cold LVGO 
pumparound liquid that is entrained into the overheads, resulting in a reduction of the measured vapour temperature in the overhead vapour line.

Mist eliminators
In some applications, mist eliminators located at the top of the column can be used to remove or minimise the entrained liquid. However, years of bad experiences by the industry have proved that mesh pads should not be used for this application. Experience has proved that they will foul, leading to high pressure drop and a corresponding decrease in gas oil yields.

Phase Doppler interferometry
To properly design a spray header requires knowing the estimated entrainment rate for the selected nozzles. The tendency for the atomised drops to be entrained is a function of the drop size distribution, the vapour velocity of the up-flowing vapours, and the vapour and liquid physical properties. There are three forces that act on the liquid droplets from the spray nozzles that determine if entrainment of liquid will occur. The forces are gravitation, the buoyant force which acts in the opposite direction from the gravitational force, and a drag force due to the relative motion between the particles and the rising vapours. The drag force acts to oppose the motion of the liquid droplets in 
the opposite direction of their movement.

In a revamp case such as this, operating data to determine the overhead vapour rate and composition is available from plant meters and lab data. However, if inadequate meters are available or if this is a new design then air leakage, cracked gas, and so on must be estimated. Guidelines regarding estimates for these values are available in the literature.4
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