And I'm doing it again.
Yes, I'm doing it again.
Oh, I'm doing it again.
I said it would end but here it goes again.

("Again," by J.Stephens aka John Legend, 2006)

 

PTOA Segment 37: ENDO VS. EXO...ALL GREEK TO ME!

The focus on temperature-decreasing process equipment began.

Endothermic reactions decrease process temperatures because they absorb all the heat around them to continue the reaction.

Therefore, endothermic chemical reactions directly lower the process temperature while rearranging chemical bonds.

Endothermic reactions are associated with forming bonds, not breaking them as happens most frequently in exothermic reactions. The thermal energy absorbed during the chemical reaction is stored into the new bonds that are formed.

Not exactly spelled out in this PTOA Segment but rather implied is the fact that forming carbon-carbon bonds from carbon-hydrogen bonds means that hydrogen is displaced in some chemical reactions.

Hydrogen gas (H2) can be a by-product of endothermic reactions that involve hydrocarbons.

Therefore, PTOA Readers and Students have learned that hydrogen gas (H2) can be made two ways:

• As a by-product of an endothermic chemical reaction.
• By the SMR and Hydrogen Shift Reactions that take place in a Hydrogen Plant.

Characteristics of endothermic processes are:

• The outlet temperature of the reactor is less than the inlet temperature. The temperature profile of the catalyst bed will indicate colder and colder temperatures as the process stream flows from the top of the bed to the bottom.
• Fired heaters are interstaged between reactors for endothermic chemical reactions that occur in liquid process streams.The staging of heaters between reactors returns the process stream temperature to the targeted reaction temperature.

In a stew of competing chemical reactions, the most highly endothermic reactions tend to occur first.Therefore, the catalyst beds and reactors in an endothermic chemical process will become gradually taller and the fired heaters will become gradually smaller.

A typical endothermic process flow scheme would be:

biggest heater→first reactor with smallest catalyst bed length→mid-sized second heater→ mid-sized second reactor containing mid-sized catalyst bed length→ third and smallest heater→third and final reactor which has the most catalyst and is the tallest of the set.

A simplified PFD for a catalytic Naphtha Reformer process depicted interstaged heaters and reactors.

Naphtha is reformed to make a product with a higher octane that is suitable for blending into gasoline. Catalytically reforming medium-sized hydrocarbons like naphtha is more cost effective than using steam to reform them.

This PTOA segment concluded with an introduction to two-phase (gas and liquid) separation which is logically performed in vessels called "Separators."

Recycling a process gas stream with a centrifugal compressor was also introduced in this PTOA segment.PTOA Readers and Students were introduced to the ISA symbol for a centrifugal compressor.

Centrifugal compressors and their industrial uses will be featured in a future PTOA Rotating Equipment focus study.

 

PTOA Segment 38: THAR SHE BLOWS!

This PTOA segment introduced the components and operation of a fin fan condenser via associating this temperature-lowering equipment with the more familiar radiator and cooling system in an automobile.

The purpose of both the car radiator and fin fan condenser is to indirectly collect the heat from a process stream and then inject the heat into the atmosphere. Fin fan condensers are also called Air Cooled Condensers and/or Air Cooled Heat Exchangers.

Neither the car radiator or the fin fan condenser is concerned with conserving energy; their purpose is to remove and get rid of thermal energy (aka heat) from a process stream pronto before the stream flows to the next processing step or flows into tankage.

PTOA Readers and Students learned that a fin fan condenser is a horizontal version of a vertically-mounted car radiator. The two devices share many common hardware components.

The thin-metal fins on the tubes of the car radiator and fin fan condenser increase the surface area for heat transfer to take place between the hot process fluid and colder air.

The fan on a car radiator inducts ambient air to flow through the radiator. The fan on a typical fin fan condenser forces air to flow through the layers and rows of finned tubes.

The fluid flowing through and/or condensed in the tubes of a radiator and/or fin fan condenser exits at a much lower process temperature.

In the fin fan condenser, the process fluid exiting the equipment is totally in the liquid state having indirectly transferred the Heat of Condensation into the cool air that flows around the nooks and crannies of the finned tubes.

 

PTOA Segment 39:  COOL   CLEAR   WATER

PTOA Readers and Students were introduced to an induced-air cross-flow Cooling Tower.

The PTOA classifies Cooling Towers and Cooling Water Systems as temperature-decreasing process equipment because their ultimate purpose is to supply the cold water that extracts the heat from hot process streams in shell and tube HExes.

The Cooling Tower is an example of direct heat transfer because the hot water contacts cold air while in the process of transferring heat.

The heat is eventually injected into the sky above the Cooling Tower as Evaporative and Drift losses. These losses are expected because they make the cooling process work!

Eighty percent of the temperature drop between the hot water flowing to the Cooling Tower in the Return Water Header and the cold water flowing out of the Cooling Tower in the (Cold Water) Supply Header is attributable to the Heat of Evaporation.

Just 1% of water evaporated reduces the temperature of the remaining cooling water by 10°F (5.6°C) because it takes a lot of heat to change the state of liquid water into water vapor via evaporation.

PTOA Readers and Students noticed the cross-wise process flow paths of the cool air and hot water that enter the Cooling Tower.

PTOA Readers and Students focused on the hardware  components that enhance decreasing the process temperature of the water.

The terms Fresh Make Up Water and Blow Down were introduced.

 

PTOA Segment 40: THE HEEBIE JEEBIE BLUES

PTOA Readers and Students learned that all circulating gas or liquid systems have similar problems and similar solutions related to the accumulation and removal of "Heebie-Jeebies."

The specific species of Heebie Jeebies formed is unique to the circulating system that generated it. However, all Heebie Jeebies are alike in the fact that... if not removed... their accumulation will eventually make the recycling process stream ineffective in performing its intended function.

Left unchecked to accumulate, Heebie Jeebies will require a circulating system to be taken offline, drained, cleaned, and refilled with fresh fluid (Hey! that's exactly why the oil in a car must be changed every few thousand miles).

Removal of Heebie Jeebies requires blowing down a small portion of process fluid to sewer on a regular basis either manually or with an automatic Blow Down valve.

Process Operators must wear PPE when operating a Blow Down Valve.

Heebie Jeebies can also be removed by having a small stream (sometimes called a "slip strip") continuously diverted out of the system.

The removal of Heebie Jeebies must be made up with the addition of fresh Make Up process fluids.

The Fresh Make Up line for a liquid stream is typically piped to flow into the suction of a recycle pump.

The Fresh Make Up line for a gas stream is typically piped to flow into the suction of a recycle compressor.

In the case of a Cooling Tower, the Fresh Make Up flows into the basin from which the recycling Supply Pump draws suction.

The volume of Fresh Make Up Water must be sufficient to replenish water losses due to evaporation, drift, and blow down.

An animated video from Terlyn Tech helped illustrate how a Cooling Tower works.

 

©2015 PTOA Segment 00049
PTOA Deja Vu Review 2-4

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