Vacuum Ejector System - A Process Engineer's Perspective
Sandeep Kadam
Assistant Manager
Process and SAE
Aker Solutions

Dynyaneshwar Kinhalkar
Assistant Manager
Process and SAE
Aker Solutions

Pankaj Motharkar
Assistant Manager
Process and SAE
Aker Solutions

Mithu Saha
Manager
Process and SAE
Aker Solutions

Ejector systems are used in refineries to maintain the desired vacuum level in vacuum distillation columns. The desired vacuum level in a column permits the fractionation of reduced crude oil into its various products such as vacuum diesel, light vacuum gas oil and heavy vacuum gas oil. If the required vacuum level is not maintained, it affects the yield and also impacts the downstream unit operations in the refinery. Hence it is necessary to have a clear understanding of various factors that affect the performance of the ejector system. Although the design and manufacturing of the system is carried out by specialized ejector suppliers, it is the responsibility of the process engineer to correctly specify the vacuum system requirements. This article provides an overview of various components of the ejector system, factors affecting system performance and process design aspects of the system.


Figure 1: Typical Three Stage Vacuum Ejector System

The major components of ejector systems are described in the following sections.
  • Ejector
    Ejector is a static piece of equipment with no moving parts, consisting of a converging/diverging motive steam nozzle, suction chamber and converging/diverging diffuser. Figure 2 illustrates basic ejector components

    Figure 2: Basic Ejector Components and Energy Conversion

  • Condenser
    Condensers are shell and tube heat exchangers with special internal configurations. These condensers are manufactured as per Tubular Exchanger Manufacturers' Association(TEMA) or API 660 guidelines, typically in three basic configurations: fixed tube sheet, u-tube or floating head bundle. The configuration selection is generally dependent on capacity needs, maintenance requirements and capital cost.
  • Hotwell
    The hotwell vessel acts as a three phase separator for hydrocarbon liquid, steam condensate and non-condensable gases. A mixture of condensed hydrocarbon and steam condensates from condensers are collected in the hotwell through properly designed tailpipes. Since the operating pressure of condensers is sub-atmospheric, tailpipes are dipped into the hotwell for sufficient liquid sealing. Noncondensable gases from the last stage of the ejector are also typically routed to the hotwell.
Ejector Operating Principle
Steam jet ejectors fall into two categories:
  • Non-critical: Discharge to suction pressure ratio < critical pressure ratio
  • Critical: Discharge to suction pressure ratio > critical pressure ratio
Critical pressure ratio is defined as the ratio of discharge pressure to suction pressure at which fluid velocity in the diffuser throat is sonic. The critical pressure ratio generally ranges from 1.7 to 2, depending on the type of fluid being handled. In vacuum ejector systems, the ratio of discharge to suction pressure is in the range of 3-4 for each stage. Therefore refinery vacuum ejector systems fall under the critical category and this article focuses on these critical ejectors.

It is an inherent property of compressible fluid that whenever the flow passes through a converging section, fluid velocity increases and pressure decreases if the flow is subsonic, whereas for a supersonic flow, velocity decreases and pressure increases. In the steam nozzle, motive steam pressure energy is converted into very high velocity, creating a low pressure region in the suction chamber which pulls in process gases.

These gases are combined with the motive steam and the mixture flows at supersonic conditions in the converging section of the diffuser. Furthermore , a sonic condition with shock wave is attained in the diffuser throat. In the diverging section of the diffuser, this mixture gets deaccelerated to subsonic conditions with considerable pressure recovery. Based on the principle above, motive steam compresses the column overhead gases to the required ejector discharge pressure. Figure 2 illustrates typical energy conversion in the ejector across its various components.

The minimum motive steam pressure required for stable ejector operation is termed as pickup pressure. At a fixed suction and discharge pressure, if motive steam pressure reduces further, a point termed as break point will be reached where ejector capacity reduces rapidly and the system loses the vacuum. The ejector will not resume operations unless pick-up pressure is attained. This is primarily because the motive steam pressure is not sufficient enough to achieve the desired low pressure in the suction chamber .

Maximum discharge pressure is the highest discharge pressure that an ejector can achieve for a fixed motive steam condition and fixed suction pressure. If the pressure in the downstream system exceeds this maximum discharge value, the ejector will become unstable and cease operations.

For any given ejector system, the manufacturer provides a performance curve wherein the suction/discharge pressure is plotted against the dry air equivalent (DAE) of suction load on a mass flow rate basis at a fixed motive steam condition and discharge pressure. The DAE load is used to define a process stream in terms of an equivalent amount of dry air load. It is not practical for a manufacturer of steam ejectors to maintain facilities for testing ejectors with all the numerous suction gas mixtures and temperatures for which ejectors are used.

Therefore, a method has been stated in the Heat Exchange Institute (HEI) standard to permit the design and testing of the ejectors using air at normal room temperatures or air and steam at any temperature convenient for the manufacturer. DAE is estimated using temperature and molecular weight entrainment ratio curves as provided in the HEI standard. Figure 3 shows a typical performance curve for a single stage ejector at designed suction pressure, designed discharge pressure and motive steam flow at fixed operating conditions.


Figure 3: Typical Performance Curve of Single Stage Ejector

Condenser Arrangements
There are various arrangements of ejectors with pre-, inter- and after -condensers depending on the vacuum column design. Pre-condensers are used in the ejector system if hydrocarbons are able to condense at the given column pressure and available coolant temperature. Inter-condensers are used to condense the steam/condensable hydrocarbons from an ejector, reducing the inlet quantity of vapor mixture for the following stage. This helps to increase the steam economy. After-condensers normally operate at atmospheric pressure. They do not affect the steam economy or ejector performance, but they allow steam to be recovered. Any upset in the condenser performance will affect the downstream ejectors and thereby the vacuum level achieved. In multistage configurations, the first inter-condenser is the largest and most critical condenser.

The condensing of the steam and hydrocarbons takes place on the shell side of the condenser and the cooling water runs through the tube side. The inlet stream enters through the top of the condenser and spreads out along the shell. A major portion of the condensable vapor contained in the inlet stream will change phase from vapor to liquid as it passes over the tube bundle. The liquid falls by gravity and exits via the tailpipe to the hotwell. The remaining noncondensables are then removed through the vapor outlet.

Thermal performance of the condenser is dependent on the effectiveness of the heat transfer achieved based on the following parameters:
  • Proper inlet stream distribution
  • Internal configuration to aid effective condensate separation and proper vapor removal without any entrainment
  • Exchanger configuration to maintain allowable pressure drop
  • Operating condition of cooling media
Normally, the type of shell configuration used is TEMA type E or X.

The following section describes the various arrangements to remove the vapor from the condenser.

Side Vapor Outlet Nozzle:
In this configuration, a longitudinal baffle (long air baffle) is used and a vapor outlet is provided from the side of the shell. If required for proper distribution of vapor, a horizontal perforated plate distributor may be provided at the shell inlet. The proper shielding of the baffle through the length of the shell ensures no bypassing of vapor from the side outlet nozzle and the achievement of proper separation between liquid and vapors. The full support plates need to have proper cutouts at the bottom to aid condensate drainage and the side to avoid restriction to vapor flow. Figure 4 indicates a typical sketch of such an arrangement for a TEMA type X shell.


Figure 4: Vapor Outlet at Shell Side

Top Vapor Outlet Nozzle
In this configuration, the up and over baffle arrangement is used to maximize the vapor distribution in the bundle. This arrangement restricts vapor bypassing and improves vapor contact with tubes. These configurations are normally used for smaller units. Figure 5 indicates a typical sketch of such an arrangement for a TEMA type E shell.


Figure 5: Vapor Outlet at Shell Side

Vapor Outlet from the Condenser Boot
In this configuration separation of the condensed liquid and the non -condensed vapor occurs at the boot provided at the bottom of the condenser. Figure 6 indicates a typical sketch of such an arrangement for a TEMA type X shell.



Figure 6: Vapor Outlet at Shell Side

All three configurations specified above involve complexity in design and are of critical importance. These designs are proprietary in nature and offered by ejector manufacturers. Due to unconventional internal configurations, generic programs do not properly model the flow distributions. Hence, thermal performance and pressure drop estimations from these generic programs need to be revalidated based on engineering judgement .

Performance Factors
  • Process Conditions
    Condensable and non-condensable load from a vacuum column varies with the type of crude oil being processed. The designer also needs to specify the process conditions that define the capacity of the ejector. A suitable design margin is also considered based on capacity to address the variations in the suction load.
  • Non-Condensable Loading: Noncondensable loading consists of light end hydrocarbons, cracked gases generated in the fired heater and air leakage into the system. An increase in the non -condensable loading from the column increases the load on the downstream ejector, making it operate above its design point. Since the downstream ejector is not designed to handle this increase in load, it creates higher backpressure on the upstream ejector. An increase in ejector discharge pressure beyond its maximum discharge pressure causes the ejector operation to break down, resulting in column overpressure .
  • Condensable Hydrocarbons: Variations in the crude type or an upset in the fired heater/vacuum column operation may result in increased condensable load to the ejector system. Higher condensable hydrocarbon loading increases the oil film thickness on condenser tubes. This results in a reduced heat transfer coefficient, and ultimately raises the operating pressure of the condenser/ejector discharge pressure. An increase in ejector discharge pressure beyond its maximum discharge pressure causes the ejector operation to break down resulting in column overpressure.
  • Liquid Entrainment with Suction Load:Liquid entrainment can lower the capacity of an ejector, which can then increase suction pressure resulting in reduced product yield. Hence, it is necessary to ensure provisions are made to arrest any entrainment from the column.
Utility Conditions
Motive Steam Condition: The motive steam supply condition is one of the most critical variables affecting ejector performance. In refinery units, steam pressure and temperature conditions may fluctuate. Therefore, the ejector nozzle should always be designed considering the lowest available steam pressure. Also, a pressure control valve is normally provided on motive steam line to address the pressure fluctuations and maintain constant steam supply pressure.

For economical ejector design, the motive steam should be saturated. To supply saturated steam, de-superheater can be provided on steam line at the inlet of ejector. It is also to be ensured that the steam is dry. If the steam is wet, the moisture droplets in high velocity steam erode the steam nozzle and might decrease the energy available for compression. Hence, a certain degree of superheat is generally maintained to avoid erosion. Generally, an adequate number of steam traps are provided on steam supply lines to arrest condensed water. In addition, the provision of a condensate separator with a trap can be considered to arrest any excessive boiler feed water flow during control valve failure of the de-superheater. To avoid condensation in steam lines, proper insulation of these lines is required.

If the operating steam pressure and temperature parameters differ from the values used for design of the ejector, it has an impact on steam consumption and ejector performance. Tables 1 and 2 provide details about the system performance if steam conditions differ from the design point.

Cooling Water: Condenser heat duty is fixed by the amount of steam being used by the upstream ejector and the suction load available from the vacuum column. If cooling water supply temperature is above its design value, the available logarithmic mean temperature difference (LMTD) decreases and condenser performance deteriorates. This leads to the carry -over of more saturated vapors with noncondensables to downstream ejectors. As the downstream ejector load increases, system backpressure to the upstream ejector rises. Similarly, if cooling water flow rate falls below the design value, the condenser duty suffers resulting in similar consequences. Cooling water with a temperature lower than the design point performs favorably and helps to improve performance. Hence, vacuum column performance improves during the winter due to lower cooling water temperature.

Design Aspects

Specification of Ejector System: While preparing the process data sheet, it is important to specify the following key details:
  • Vacuum level required and the discharge pressure expected at the package outlet
  • Specify suction load as per heat and material balance. For estimation of air leakage, the rate guidelines available in the HEI standard can be used.
  • Fluid composition, operating temperature, molecular weight and other details of the condensable and non-condensable load
  • Corrosive constituents in the process fluid such as hydrogen sulfide , ammonia and phenol
  • Available utility conditions
  • Material of construction details for each component of the system
  • The fouling factor and the allowable pressure drop for condenser design
  • Project specifications covering any specific design aspects to be considered by the vendor
Piping Arrangement: The ejector may be installed either vertically or horizontally based on space availability. The suction line size shall be driven by estimating the total pressure drop from the vacuum column overhead to the ejector suction nozzle. If there are multistage ejectors and each stage has two or more elements, it needs to be ensured that the total cross-sectional area of each element supply line is equal to or higher than the crosssectional area of the main supply header. Each supply line shall have a provision for isolation with either a manual or motor-operated valve, depending on the sizes and the project specific requirement.

Considering the low pressure system, utmost care should be taken while carrying out hydraulic calculations for the inter-connecting piping between different stages of ejectors. Available industry guidelines can be referred to for the criteria of the allowable pressure drop and velocity.

Liquid from the condensers is drained to the hotwell through tailpipes. In the event of varying plant operation or an increase in motive steam flow, there is a possibility of an increased condensable load. To prevent condensate back-up in the condenser, a properly sized tailpipe is required. Tailpipes are sized for gravity flow considering the maximum possible liquid load with a suitable design margin. Consideration should be given to the relative elevation of the condenser and the hotwell to ensure that the static head in the tailpipe is greater than the pressure difference. Typical guidelines to estimate the height of the tail pipe is available in the HEI standard. It is preferred to have tailpipes vertically connected to the hotwell but 45º elbows can be considered if unavoidable. Horizontal runs with excessive elbows and fittings should be avoided as they are susceptible to gas pockets and other drainage problems.

The ejector manufacturer needs to be consulted for any hardware provision that may be required during the field test run. Normally the ejector test run is done by isolating the vacuum system. Based on the manufacturer’s feedback, the necessary isolation facility and a test connection on suction piping need to be provided.

During start-up, ejectors and condensers may experience higher operating temperatures(motive steam) due to the unavailability of suction load. Hence, the designer must take this scenario into account while deciding the mechanical design temperature of the system.

Column Pressure Controls: Vacuum column performance and product quality depends on the column flash zone pressure. This makes pressure control necessary, which is achieved mainly with the controlled recycle of ejector discharge to the ejector suction. There are various recycling configurations possible based on the tap-off location of the recycle stream. The preferred method is recycling from the first stage of ejector discharge to the suction. The typical arrangement can be seen in Figure 1. When the recycle is taken from either the second or third stage discharge, there is a possibility of an increase in non-condensable load at the first stage ejector. This would result in increases in the first stage ejector discharge pressure above its maximum value and have a negative impact on downstream ejectors. To maintain a steady column overhead pressure during turndown scenario, the recycle control valve is sized for the difference between DAE at design load and turndown load.

There are other methods to control column pressure, either with injection of external gas stream (e.g. fuel gas, steam, nitrogen etc.) into the first stage or throttling of the ejector suction load depending on the capacity of the unit or column vacuum level.

In case of parallel ejector trains, one or more trains can be isolated to achieve turndown requirement if adequate isolation facilities are available on the motive steam and process side lines.

Conclusion
Ejector systems support the vacuum column operation. There are various factors that impact the operation of the vacuum ejector system. Therefore it is necessary to have clarity on the significance of each parameter and its impact on the system performance. It is important to pay attention to design aspects like the selection of the motive steam condition, capturing all probable variations in suction load and utility conditions, proper condenser design and correct piping arrangement along with a suitable control scheme to address all operating scenarios.

References
1. Ernest E. Ludwig, Applied Process Design for Chemical and Petrochemical Plants, Volume 2, Third Edition
2. Heat Exchange Institute Standard for Steam Jet Vacuum Systems, Fourth Edition