ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 1 Part 6 potx

LTX Units and Line Heaters 111

decrease to well below the hydrate point. Hydrates form, but they fall

into the bottom of the separator and are melted by the heating coil. The

hydrates do not plug the choke because the choke is inside the separator.

The gas, condensate, and free water are then discharged from the ves￾sel through backpressure and liquid dump valves. The gas leaving the

separator is saturated with water vapor at the temperature and pressure of

the top of the low temperature separator. If this temperature is low

enough, the gas may be sufficiently dehydrated to meet sales specifica￾tions. Dehydration is discussed in greater detail in Chapter 8.

The low-temperature separator acts as a cold feed condensate stabiliz￾er. A natural cold reflux action exists between the rising warmed gases

liberated from the liquid phase and cold condensed liquid falling from

the stream inlet. The lighter hydrocarbons rejoin the departing gas stream

and the heavier components recondense and are drawn from the vessel as

a stable stock tank product. This process is discussed in more detail in

Chapter 6. The colder the temperature of the gas entering the separator

downstream of the choke, the more intermediate hydrocarbons will be

recovered as liquid. The hotter the gas in the heating coil, the less

methane and ethane there will be in the condensate, and the lower its

vapor pressure. In some cases, it may be necessary to heat the inlet gas

stream upstream of the coil, or provide supplemental heating to the liquid

to lower the vapor pressure of the liquid.

In summary, a colder separation temperature removes more liquid from

the gas stream; adequate bottom heating melts the hydrates and revaporizes

the lighter components so they may rejoin the sales gas instead of remain￾ing in liquid form to be flashed off at lower pressure; and cold refluxing

recondenses the heavy components that may also have been vaporized in

the warming process and prevents their loss to the gas stream.

LTX units are not as popular as they once were. The process is diffi￾cult to control, as it is dependent on the well flowing-tubing pressure and

flowing-tubing temperature. If it is being used to increase liquid recov￾ery, as the flowing temperature and pressure change with time, controls

have to be reset to assure that the inlet is cold enough and the coil hot

enough. If the coil is not hot enough, it is possible to destabilize the con￾densate by increasing the fraction of light components in the liquid

stream. This will lower the partial pressure of the intermediate compo￾nents in the stock tank and more of them will flash to vapor. If the inlet

stream is not cold enough, more of the intermediate components will be

lost to the gas stream.

112 Design of GAS-HANDLING Systems and Facilities

From a hydrate melting standpoint it is possible in the winter time to

have too cold a liquid temperature and thus plug the liquid outlet of the

low temperature separator. It is easier for field personnel to understand

and operate a line heater for hydrate control and a multistage flash or

condensate stabilizer system to maximize liquids recovery.

LINE HEATERS

As shown in Figure 5-2, the wellstream enters the first coil at its flow￾ing-tubing temperature and pressure. Alternatively, it could be choked at

the wellhead to a lower pressure, as long as its temperature remains

above hydrate temperature.

There is typically a high-pressure coil of length Lj, which heats the

wellstream to temperature, T\. The wellstream at this point is at the same

pressure as the inlet pressure, that is Pj = Pin. The wellstream is choked

and pressure drops to P2. When the pressure drops there is a cooling

effect and the wellstream temperature decreases to T2. This temperature

is usually below the hydrate temperature at P2. Hydrates begin to form,

but are melted as the wellstream is heated in the lower pressure coil of

length L2. This coil is long enough so that the outlet temperature is above

the hydrate point at pressure, P2. Typically, a safety factor of 10°F higher

than the hydrate temperature is used to set Tout.

In fire tube type heaters, the coils are immersed in a bath of water. The

water is heated by a fire tube that is in the bath below the coils. That is,

the fire tube provides a heat flux that heats the water bath. The water bath

Figure 5-2. Schematic of line healer.