Design Fract Condense Systems

1. Table of Contents

2. Purpose

This procedure describes the guidelines to be followed in selecting and designing fractionator overhead condensing systems using water cooled and/or air cooled condensers.

3. Partial Condensing Systems

These systems always have a net flow of vapor from the overhead receiver. The parameters, which determine the selection of the fractionator overhead pressure and receiver temperature, are shown below.

3.1 Fractionator Overhead Pressure

a. Net Gas Destination

Generally, the destination of the net vapor, along with a reasonable pressure drop across the control valve, sets the lowest level of this pressure. Employ gas compression if special situations occur.

b. Relative Volatility

In order to obtain the highest possible relative volatility and improved fractionation, keep this pressure as low as possible.

c. Reboiler Temperature

The reboiler outlet temperature sets the highest level of this pressure. Choose this temperature to prevent thermal decomposition or to avoid an approach to critical conditions.

d. Reboiler Heating Media

The temperature of the available heating media to meet a suitable LMTD in the reboiler can influence the choice of pressure.

e. Product Losses

Consider increasing the receiver pressure in order to reduce the net overhead vapor product while increasing the net overhead liquid product from the receiver.

f. Pressure Control Location

The pressure control is almost always based on regulating the amount of net vapor from the receiver. The location of the PI for pressure control is based on the presence or absence of composition control, a TIC.

If the column does not have composition control, a TIC, the PI should be on the receiver or the net vapor line from the receiver.

If the column has composition control, a TIC, the PI should be on the overhead vapor line from the column.

3.2 Receiver Temperature

The parameters, which determine normal operating temperature of the receiver temperature, are shown below.

a. Economic Approach Temperature

The economic approach temperature of a condenser is affected by the temperature of the available condensing medium and the added product value that results from condensing material from the net overhead vapor into the net overhead liquid.

The highest cooling water inlet temperature likely to be encountered is around 90F (32C). An approach temperature of 7F (4C) is reasonable for a water cooled condenser. Hence, a receiver temperature of 100F or above will allow the use of a water cooled condenser. Inflection Point Engineering will typically default to a receiver temperature of 100F (38C) with a water cooled condenser in the absence of guidelines from the Owner. (IPE-TM-400-09 also touches on this topic.)

Similarly, the highest air temperature likely to be encountered is around 95F(35C). An approach temperature of 20 to 25F (11 to 14C) is reasonable for an air cooled condenser. Hence, a receiver temperature of 115F or above allows the use of an air cooled condenser. Inflection Point Engineering will typically default to a receiver temperature of 120F (49C) with an air cooled condenser in the absence of guidelines from the Owner.

The Basic Engineering Design Questionnaire, Tool ” provides additional guidance. This guidance is in the Air Cooled Exchanger section 5.5. If available the completed BEDQ for the project of interest should be reviewed.

b. Product Losses

Select the receiver temperature to minimize net overhead vapor product and hence minimize the chance of condensing vapor in downstream equipment or piping.

c. Product Rundown Temperature

A suitable rundown temperature for the net overhead liquid product must be met. If the receiver temperature is higher than the required rundown temperature, then a separate net overhead liquid cooler is required. Generally, this is a more economic design than cooling the total overhead stream.

4. Total Condensing Systems

These systems totally condense overhead vapor from the fractionator and have no net vapor product. The parameters, which determine the selection of the fractionator overhead pressure and receiver temperature, are shown below.

4.1 Fractionator Overhead Pressure

Parameters 3.1.b,c, and d in the preceding section also apply to total condensing systems.

a. Relief Header

Set the lowest level of the fractionator overhead pressure by the requirement to vent non-condensables or to allow a flow of purge gas to the relief header. For this reason, the receiver pressure is frequently assumed to be 1 psig, when there is an open line from the receiver to the relief header. Check with the appropriate Technology Specialist for the number for a particular process. For fractionators making high purity products, consider a separate vent header direct to the knockout drum and flare to avoid the effect of pressure variations in the relief header.

b. Condenser Cold End Temperature

The selected pressure shall allow an economical approach temperature at the cold end of the condenser. Refer to Section 3.2.

c. Pressure Control

In order to maintain stable fractionator pressure the preferred control method is either with the receiver floating on the relief header or with a positive pressure controlled with a gas blanketing system. This latter system is commonly known as a "push-pull" system. If there are significant disadvantages in the application of these methods of pressure control to particular systems, then consider the use of a hot vapor bypass pressure control. The hot vapor bypass control system can be used whether the condenser is located at grade or is elevated. One possible disadvantage of gas blanketing or "push-pull" systems would be the resultant presence of non-condensables in the overhead liquid product.

Typically, the receiver pressures are as shown in the table below. Remember that fractionator pressure is higher than receiver pressure and it is the fractionator pressure that is the controlled variable or desired value in almost all cases.

Receiver Pressure psig	System
1-5 5 + (1) 5	Pressure floating on the relief header Push-pull (2) Hot vapor bypass (2)

Notes: 1 The upper limit of the push-pull pressure range is usually limited by the supply pressure of the blanketing gas.

2 If either push-pull or hot vapor bypass can be applied, chose push-pull before hot vapor bypass for both cost and ease of use advantages.

4.2 Receiver Temperature

a. Enclosed Systems

Set the receiver temperature at the bubble point of the net overhead liquid. Extra surface is normally specified in the condenser design to give a design margin to ensure there is enough surface for the following contingencies:

To be able to condense additional volatile components in the net overhead

To provide enough capacity to handle changes in reflux rate from the control system

This margin is provided by specifying the correct thermodynamic bubble point in the process and project specifications and adding a note to request some oversizing; e.g., “Design for 110% of the flows and duty shown above”. The Technology Specialist for a particular process can advise the amount of extra surface required. Normally this value is 10%.

Sometimes there are extra lighter components in the net overhead that, under certain circumstances, should be condensed. It may then be necessary to calculate the lower bubble point and then back calculate the extra surface required. If water may be present, consider this in calculating the bubble point. The topic of condensing curves for non-linear condensing systems is covered in Procedure .

b. Open System

If the receiver floats on the flare pressure, some subcooling is required to avoid loss of receiver material to the flare. Refer to ” for more guidelines.

c. Product Rundown Temperature

Satisfy the required rundown temperature of the overhead liquid product. If the bubble point temperature is higher than the required rundown temperature, refer to Section 3.2.c.

5. Condenser Instrumentation and Hydraulics

5.1 Water Cooled Condenser - Partial Condensing

Figure 5.1

Elevations are for example purposes only.

See section 6.10 for elevation standards.

a. Instrumentation

The PRC controls the net vapor line control valve and regulates the receiver pressure.

b. Hydraulics

P1 = 150 psig

P2 = P1 + Phead + Pline = 150 + (13×0.433×0.08)***

+ (100×(0.25/100))** = 151

P3 = P2 + Pexchanger = 151 + 3.0 = 154

P4 = P3 + Pline -Phead = 154 + (400×(0.5/100))** - 0 = 156

** = (equivalent length × P/100 ft)

*** = (head × conversion factor × condenser outlet two phase density)

For a water condenser that is located at grade, the elevation, when an actual value is not know, can be determined by the method in Section 5.10 of this memo.

c. Comments

The condenser outlet line has two phase flow. Generally locate the condenser below the receiver for lower cost and easier maintenance. Size the condenser outlet line to prevent slug flow and to provide sufficient velocity to carry an immiscible liquid phase into the receiver.

Locate the condenser above the receiver if the fractionator pressure is to be minimized or if the net vapor volume is extremely low or variable. If an air cooled condenser was used, locate it above the receiver. Instrumentation would be identical.

5.2 Non-Elevated Water Cooled Condenser or Steam Generator - Total Condensing (Enclosed)

Figure 5.2

Elevations are for example purposes only.

See section 6.10 for elevation standards

a. Instrumentation

The PRC controls a hot vapor bypass control valve which varies the receiver pressure and changes the liquid level in the condenser. The condenser surface available for condensing regulates the fractionator pressure.

b. Hydraulics

P1 = 15 psig

P2 = P1 - Pline + Phead = 15 - (400×(0.25/100)) + 0 = 14

P3 = P2 - Pexchanger = 14 - 3.0 = 11

P4 = P3 - Phead - Pline = 11 - (13×0.433×0.78) - (100×(0.25/100) = 6

For a water condenser that is located at grade, the elevation, when an actual value is not know, can be determined by the method in Section 6.10 of this memo.

c. Comments

The condenser must be below the receiver to maintain the liquid seal with the rundown line inlet into the bottom of the receiver. When the condenser is a steam generator the minimum elevation difference between the centerline of the tube bundle and the bottom of the receiver is to be shown on the P&ID. This dimension is to ensure that

1) the steam generator is located below the receiver so that there will be a liquid seal for the rundown line going into the bottom of the receiver, and

2) to provide sufficient static head to ensure adequate delta P across the hot-vapor bypass control valve.

The minimum delta P across the hot-vapor bypass control valve, assuming little or no friction loss in the exchanger and piping is 2 psi. Therefore, the minimum elevation difference should be 1400 mm/ flowing sp. gr. Due to requirements for the blowdown drum and piping the contractor must set the elevation of the steam generator and this elevation need not be shown on Inflection Point Engineering’s P&ID.

5.3 Elevated Condenser - Total Condensing (Enclosed)

Figure 5.3

Elevations are for example purposes only.

a. Instrumentation

The PRC controls a valve which regulates the flow rate to the air cooled condenser. The flow rate from the fractionator controls the fractionator pressure. The PDIC is necessary to keep the PRC valve in its operating range under the wide range of cooling conditions expected from an air cooled condenser. The PDIC regulates the flow of hot vapor to obtain a temperature at the surface of the receiver liquid to give a vapor pressure to satisfy the PDIC.

Hydraulics

P1 = 15 psig

P2 = P1 - Pline + Phead = 15 - (400 × (0.25/100)) + 0 = 14

P3 = P2 - Pvalve = 14 - 2.0 = 12

P4 = P3 - Pexchanger = 12 - 2.0 = 10

If rundown flow is two phase:

P5= P4+ Phead - Pline =

10 + (9 x 0.433 x 0.08) - 100 x (0.25/100) = 10

If rundown flow is liquid phase (which does not occur when an equalizing line is installed):

P5= P4+ Phead - Pline =

10 + (9 x 0.433 x 0.75)- 100 x (0.25/100) = 13

c. Comments

The rundown line design should be a minimum distance, have no traps, and be kept two phase to eliminate the static head. A water cooled condenser may be used in place of the air cooled condenser if desired.

Elevated Condenser - Total Condensing (Open Vent)

Figure 5.3

Elevations are for example purposes only.

a. Instrumentation

There is an open line from the receiver to the relief header. Introduce a purge into the line to the relief header to maintain a positive flow to the relief header.

b. Hydraulics

P1 = 1 psig

P2 = P1 - Phead + Pline = 1 - (9 x 0.433 ×0.08) + 100 x (0.25/100)) = 0.9

P3 = P2 + Pexchanger = 0.9 + 3.0 = 3.9

P4 = P3 + Pline - Phead = 3.9 + (400 × (0.17/100)) -0 = 4.6

c. Comments

Cooling of the material going to the relief header may cause some liquid to form. The relief header can vary in pressure, typically from a minimum of 1 psig to a maximum of 5 psig. Use the lowest pressure to calculate the condenser outlet temperature. The rundown line design should be a minimum distance, have no traps, and be kept two phase to eliminate the static head. A water-cooled condenser may be used in place of the air-cooled condenser if desired.

See Procedure and for design of the purge gas.

5.5 Push-Pull Total Condensing

Figure 5.5

a. Instrumentation

For this system a blanketing gas flows into the receiver when its pressure is low. Gas is vented from the receiver when the receiver pressure is high.

b. Hydraulics

The receiver pressure must be between the blanketing gas pressure and the vent gas destination pressure with sufficient pressure drop for each of the control valves. See Section 4.1.c for more discussion on this.

c. Comments

One of the difficulties with this system is that some of the blanketing gas will dissolve into the receiver liquid.

5.6 Contact Condensing

Figure 5.6

The use of contact condensing will lower a column’s operating pressure by dramatically reducing the pressure drop needed for the condenser. Six to twelve feet of random packing is normally used with a resulting pressure drop of less than 30 mm Hg.

5.7 Air Cooled Followed by Water Cooled Condenser (Enclosed)

With a partial condensing system, the combination of an air cooled condenser followed by a water cooled condenser is acceptable. For a total condensing system, avoid this combination whenever possible.

Figure 5.7a

The above combination will work in a total condensing system but only when a liquid level is retained in the water-cooled condenser. If large changes in ambient air conditions are expected or if the water-cooled condenser duty is less than 30 per cent of the total condensing duty, then very close operator attention will be required.

Figure 5.7b

The above control system will work but there is difficulty in specifying conditions for the condenser rundown lines in order to maintain two phase flow conditions at the air cooled condenser outlet under all operating conditions. Total condensing in the air cooled condenser can occur in cool weather. Subsequent subcooling in the water cooled unit will cause pressure variations, so operator attention is required to reduce the fan pitch or to shut down fans. The Inflection Point Engineering design practice for this situation is to specify the amount of material condensed in the air cooled condenser to be a maximum of 70 per cent of the total mass flow rate.

5.8 Vapor Only Overhead Product

No Composition Control

Figure 5.8a

Figure 5.8a shows the typical control scheme used for columns that have only a vapor product from the overhead and do not have composition control.

Composition Control

Figure 5.8b

Figure 5.8b shows the typical control scheme used for columns that have only a vapor product from the overhead and do have composition control. This type of control is difficult and should be avoided if possible. The main concern is that the Tray 1 vapor product is always needed. To insure this, the Tray 1 vapor product should be at least 20 mol % of the total overhead product and the condenser outlet temperature should be controlled.

6. Design Guidelines

Attachment 1 shows a P&ID representation of an enclosed, air cooled condensing system for reference purposes.

6.1 Subcooling

Subcooling at the receiver liquid is generally avoided, as subcooled reflux has more disadvantages than advantages. Subcooling the reflux requires a larger condenser and could cause hammering and pressure control problems. Two cases that justified the use of subcooling are when it is desirable to lower the water solubility and when an open pressure control scheme is used.

A condenser that is designed to provide significant subcooling is normally designed to have a liquid full section. Figures 6.1a and 6.1b show some possible configurations. At higher that design capacities these liquid full sections can cause high pressure drops due to vapor flow into them.

a. Water Cooled Condensers

Figure 6.1a

Air Cooled Condensers

Figure 6.3

The static head between the condenser and the receiver is HL×0.433×S.G. If the pressure drop in the condenser rundown line is greater than the static head, the rundown line will be liquid full which will reduce the condenser outlet pressure. If the pressure drop in the condenser rundown line is less than the static head, a siphon is alternately created and broken which will cause pressure fluctuations that may upset fractionator operation.

6.2 Pressure Drop

Use the typical pressure drops shown below in the tables to determine the hydraulics of fractionator condensing systems. Use the minimum pressure drops shown when pressure drop has to be low.

a. Overhead Equipment

		Pressure Drop psi	Pressure Drop psi
		Minimum	Typical
Water Cooled Cooled Condenser	1 shell	2	3-5
Condenser	2 shell	4	6-8
Air Cooled Condenser	Air Cooled Condenser	2	3-5
Control Valve	Control Valve	1	3-5

b. Overhead and Condenser Rundown Lines

	Maximum Pressure Drop psi/100ft
Overhead Vapor and Bypass Lines
P > 300 psia	1.25
150 < P <300 psia	0.75
14.7 < P <150 psia	0.30
3.75 < P < 14.7 psia	0.15
P < 3.75 psia	4% P
Condenser Rundown Lines
Two Phase	0.25
Liquid Phase (free draining)	0.25
Liquid Phase (with seal)	1.0

If the possibility exists of a heavy liquid phase such as water being present and the condenser is located below the receiver, size the rundown line for a minimum velocity of 350 ft/min.

6.3 Vent Condensers for Total Condensing Systems

For systems designed with a purge gas vent to the relief header, where the receiver temperature is above ambient temperature, or if there are inerts in the fractionator feed necessitating continuous venting, a vent condenser can be beneficial to recover net product.

6.4 Receivers

a. Distributor

All condenser rundown line inlet nozzles, where the condenser is elevated, have a slotted distributor with a top entry into the receiver. To size and specify distributors, see Section 5 of Procedure ”. On systems with a hot vapor bypass control system and a slotted top inlet distributor, locate the hot vapor bypass inlet line nozzle and the slotted distributor at opposite ends of the receiver from each other.

b. Inlet Baffles

Systems such as those shown in Figure 5.2, with the water cooled condenser at grade, have a baffle on the condenser rundown line inlet. Refer to Standard Drawing No. 3-185. The hot vapor bypass inlet line also has a baffle to prevent disturbing the liquid surface. Neither of these baffles shall be shown on the PFD per Reference Procedure ."

c. Receiver Level

The hydraulics program does not recognize the level in horizontal receivers. Therefore, in the design of systems containing non-elevated water cooled condensers, it is necessary to set the elevation of the destination of the condenser outlet line at the normal liquid level of the receiver to simulate this elevation.

6.5 Non-Condensable Vents for Total Condensing Systems

Provide a 1" diameter non-condensable vent from the condenser. This vent line connects to the rundown line a minimum distance from the receiver inlet.

6.6 Rundown Piping for Total Condensing Systems

Minimize the static head in the rundown line from the condenser to the receiver. This can be ensured by adhering to the following guidelines:

a. Equalizing Line

If the condenser is located above the receiver, an equalizing line is required between the condenser outlet header and the receiver. A valve is required in this line for hot vapor bypass systems while no valve is required in open or push-pull systems. Size the equalizing line to be 1/3 to 1/4 of the diameter of the condenser rundown line, as shown below.

Condenser Rundown Line Size	Equalizing Line Size
36” 34” 32” 30” 28” 26” 24" 20" 18" 16" 14" 12" 10" 8" 6" or less	10” 10” 10” 8” 8” 8” 6" 6" 6" 4" 4" 3" 3" 2" 1-1/2"

The purpose of the equalizing line is to prevent the condenser rundown line from becoming liquid full. A liquid full rundown line is bad as the liquid static head in this line will reduce the operating pressure of the condenser and hence the available temperature difference. A valve is placed in the equalizing line for enclosed systems. Non enclosed systems include those using push pull control or having the receiver floating on the relief header. The equalizing line valve should normally be full open. This valve should only be closed when high vapor flow through this line causes a noise or vibration problem. For non enclosed systems this problem will not occur due to the presence of non condensable that can enter through the vent line.

b. Rundown Line

The rundown line shall be free draining from an elevated condenser to the receiver and as short as possible. The P&ID shall carry the following notes on the rundown line; "Free draining (no pockets)" and "Minimum run of horizontal pipe." See Standard Specification 9-51 “Plot Plan Design Criteria for Process Units” for clarification on the minimum run of horizontal pipe.

6.7 Hot Vapor Bypass Line Sizing

Size the hot vapor bypass line and control valve for 6 per cent of the total fractionator overhead vapor flow rate and in accordance with the table of pressure drops in Section 6.2.

6.8 Overhead Pumps

a. NPSH Requirement

In calculating the NPSH requirement for the overhead pumps the overhead receiver liquid shall be considered to be at its bubble point. Even in gas blanketed receivers the liquid is in equilibrium with the inert gas even though the hydrocarbon may be considered to be subcooled. Refer to Procedure ”.

b. Effect of Subcooling

Subcooling may occur in the condenser in cold weather particularly if the condenser is oversized for any reason. It is not necessary to provide extra head on the pump to account for a possible lower suction pressure. The receiver must operate close to the normal operating pressure for the fractionator to operate properly. Inflection Point Engineering practice provides a valve in the cooling water inlet line and fan pitch control on air cooled condensers so that operators can take corrective action.

6.9 Vacuum Design

Consider if it is possible for the fractionator, receiver and associated equipment to be exposed to a vacuum if the fractionator heat input is turned off. If the system under consideration has nitrogen blanketing, or a push-pull system, do not take any credit for this to avoid designing for vacuum. Design for vacuum if the possibility exists. Refer to Procedure .

6.10 Water-Cooled Condenser Elevation

a. Water Cooled Condenser at Grade

An elevation is required on the P&ID’s for a water -cooled condenser located at grade in a totally condensing overhead service. The purpose of a fixed condenser elevation is to insure that the contractor does not elevate the condenser to a point that the static head pressure drop in the condenser outlet line is significantly reduced, and the pressure drop available for the PRC disappears.

For totally condensing systems with a water-cooled condenser located at grade, the following table should be used to determine the condenser elevation. This table is based on the nozzle and piping layout criteria as shown in Procedure , in the section on minimum reboiler elevation. The 2” allowance for insulation is excluded, as insulation is not needed.

A non-totally condensing system with a water-cooled condenser located at grade does not require an elevation on the P&ID, as no PRC control valve is required.

Condenser Outlet Line Diameter	Condenser Elevation Above Grade	Condenser Elevation Above Grade
Up to 6”	4’-0”	1200mm
8”	4’-6”	1300mm
10”	4’-6”	1400mm
12”	5’-0”	1500mm
14”	5’-6”	1600mm
16”	5’-6”	1700mm
18”	6’-0”	1800mm
20”	6’-6”	1900mm
22”	6’-6”	2000mm
24”	7’-0”	2100mm

b. Elevated Water Cooled Condenser

Water cooled condenser are sometimes located above and free draining into the overhead receiver. Two advantages of this configuration are that less plot area is needed and less pressure drop is needed between the top tray and the receiver. Normally specify the condenser to be 10 ft (3000 mm) (max) above the top of the receiver. It is expected that the condenser will be on the next deck directly above the receiver. This rule can be relaxed if the contractor reports a serious problem meeting this specification.

6.11 Non-Elevated Steam Generators – Total Condensing (Enclosed)

This topic is discussed in Section 5.2.C

Attachment 1